Isomerization of Allylbenzenes - Chemical Reviews (ACS Publications)

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Isomerization of Allylbenzenes Mohammad Hassam,† Abu Taher,† Gareth E. Arnott,† Ivan R. Green,† and Willem A. L. van Otterlo*,†,‡ †

Department of Chemistry and Polymer Science, Stellenbosch University, Private Bag X1, Matieland 7602, South Africa School of Chemistry, University of the Witwatersrand, Braamfontein, Johannesburg 2000, South Africa

‡

5.1.3. Summary Concerning Palladium-Mediated Methods 5.2. Rhodium-Catalyzed Isomerizations 5.2.1. Homogeneous Rhodium Catalysts 5.2.2. Supported Rhodium Complexes 5.2.3. Rhodium-Mediated Hydroformylations and -borations and Related Isomerization Catalysis 5.2.4. Summary Concerning Rhodium-Mediated Isomerizations 5.3. Platinum-Catalyzed Isomerizations 5.3.1. Homogeneous Platinum Complexes 5.3.2. Summary of Platinum-Mediated Methods 5.4. Ruthenium-Catalyzed Isomerizations 5.4.1. Homogeneous Ruthenium Complexes 5.4.2. Supported Ruthenium Catalysts 5.4.3. Green Approaches with Ruthenium Catalysts 5.4.4. Cationic Ruthenium Complexes 5.4.5. Ruthenium Hydrides 5.4.6. Isomerizations and Metathesis 5.4.7. Isomerization during C−H Activated Allylations 5.4.8. Summary Concerning Ruthenium-Mediated Isomerizations 5.5. Iridium-Catalyzed Isomerizations 5.5.1. Homogeneous Iridium Catalysis 5.5.2. Supported Iridium Systems 5.5.3. Summary Concerning Iridium-Mediated Isomerizations 5.6. Iron-Catalyzed Isomerizations 5.6.1. Homogeneous Iron Catalysts 5.6.2. Supported Iron Catalysts 5.6.3. Summary Concerning Iron-Mediated Isomerizations 5.7. Cobalt-Catalyzed Isomerizations 5.7.1. Homogeneous Cobalt Catalysis 5.7.2. Summary Concerning Cobalt-Mediated Isomerizations 5.8. Titanium-, Zirconium-, and Hafnium-Catalyzed Isomerizations 5.8.1. Summary Concerning Titanium, Zirconium, and Hafnium Catalysts 5.9. Nickel-Catalyzed Reactions 5.9.1. Summary Concerning Nickel Isomerizations 5.10. Gold-Catalyzed Isomerizations

CONTENTS 1. Introduction 1.1. Naturally Occurring 1- and 2-Propenylbenzenes and Their Value 1.2. The Importance of Transforming 2-Propenylbenzenes into 1-Propenylbenzenes 1.3. Scope of This Review 2. Isomerization Mechanisms 2.1. Base-Mediated Mechanism 2.2. Transition Metal-Mediated Mechanisms 2.2.1. Alkyl Mechanism 2.2.2. Allyl Mechanism 3. Isomerization Methods − General 4. Base-Mediated Isomerizations 4.1. Hydroxide/Alkoxide Ion-Mediated Isomerizations 4.1.1. Hydroxide/Alkoxide Ions in Alcohol (Methanol, Ethanol, n-Butanol) or without Solvent 4.1.2. Hydroxide/Alkoxide Ions in Dimethyl Sulfoxide 4.1.3. Hydroxide Ions and Utilization of Phase Transfer Catalysis (PTC) 4.1.4. Supported Hydroxide Reagents 4.2. Potassium tert-Butoxide-Mediated Isomerizations 4.3. Bases Used Less Frequently for Allybenzene Isomerizations 4.3.1. n-Butyllithium 4.3.2. Phosphazene Bases 4.3.3. 1,8-Diazabicyclo[5,4,0]undec-7-ane (DBU) 4.3.4. Sodium Hydride 4.3.5. Metal−Ammonia 4.3.6. Organic “Super-Electron-Donor” as Base 4.4. Solid Bases as Isomerization Catalysts 4.5. Summary Concerning Base-Mediated Methods 5. Transition Metal-Mediated Isomerizations of Allylbenzenes 5.1. Palladium-Catalyzed Isomerizations 5.1.1. Homogeneous Palladium Complexes 5.1.2. Supported Palladium Catalysts

B B D D E E F F F G G G

G K M P P Y Y Y Z AA AB AB AB AE AE AE AE AQ

AS AS AS AW

AZ BB BB BB BC BC BC BF BF BG BH BJ BP BQ BQ BQ BR BS BS BS BU BU BU BU BV BV BX BX BZ BZ

Received: February 4, 2015

© XXXX American Chemical Society

A

DOI: 10.1021/acs.chemrev.5b00052 Chem. Rev. XXXX, XXX, XXX−XXX

Chemical Reviews 5.10.1. Summary Concerning Gold Isomerizations 5.11. Miscellaneous Transition Metal Catalysts 5.11.1. Rhenium-Catalyzed Isomerizations 5.11.2. Niobium- and Tantalum-Catalyzed Isomerizations 5.11.3. Yttrium-Catalyzed Isomerizations 5.11.4. Chromium-, Molybdenum-, and Tungsten-Catalyzed Isomerizations 6. Miscellaneous Methods for Isomerizing Allylbenzenes 6.1. Electrochemical Methods for Isomerizing Allylbenzenes 6.2. Allylbenzene Isomerization by Way of Flash Vacuum Pyrolysis 6.3. Photochemical Isomerization of Allylbenzenes 7. Acids as Isomerization Catalysts 8. Biochemical Isomerization of Allylbenzenes 9. Conclusions Author Information Corresponding Author Notes Biographies Acknowledgments Dedication References Note Added in Proof

Review

CA CA CA CA CA CB CB CB CB CC CC CD CD CD CD CD CE CF CF CF DD

1. INTRODUCTION 1.1. Naturally Occurring 1- and 2-Propenylbenzenes and Their Value

The “phenylpropenoids” are a well-known set of naturally occurring compounds obtained mainly from plant sources.1,2 Of this group, the generalized 2-propenylbenzenes 1 and 1propenylbenzenes 2, as shown in Figure 1, are very important compounds, having found application in the flavor and fragrance3 industry, cosmetic, pharmaceutical, materials chemistry, and also as intermediates in synthetic sequences for the construction of complex products.1

Figure 2. Figure 1.

are important members of this group of bioactive compounds,5 and specific (i.e., noncomprehensive) examples of the biological activity of 2-propenylaryl compounds (Figure 2) include the following: (a) Antipesticidal activity:6,7 For example, safrole 6 and its isomerized regioisomer, isosafrole 19, have been identified as natural pesticides for the beetles Sitophilus zeamais (Coleoptera: Curculiondae) and Tribolium castaneum (Herbst) (Coleoptera: Tenebrionidae).8,9 It should also be noted that asaricin (also described as β-asarone) 9,10−12 isoeugenol 18,13−16 and anethole 2017,18 have all been shown to possess insecticidal activity. Research into the larvicidal activity of phenylpropenoids, which include isomerized variants, particularly affecting crop pests, has seen much recent investigation.19,20

Figure 2 illustrates the importance of natural products related by a simple alkene isomerization. Compounds based on the allylbenzene scaffold 3 are shown above the line, while the generic isomers based on the 1-propenylbenzene scaffold 4 are shown below the line in Figure 2. The 2-propenylbenzenes 1 include many well-known alkenyl aromatics, such as eugenol 5, safrole 6, estragole 7, myristicin 8, and others, while a number of the corresponding isomeric 1-propenylbenzenes 2 are shown below the line, viz., isoeugenol 18, isosafrole 19, anethole 20, isomyristicin 21, and others. Many plant constituents are known to have effective antibiotic activities,4 which include inhibitory abilities against fungi, bacteria, and insects. In this regard, the arylpropenoids B


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major component in several essential oils, viz., anise seed oil (80−90%), star anise oil (>90%), and sweet fennel oil (80%).59 It should also be realized that arylpropenoids have found uses in many different areas, including the flavor, fragrance, and pharmaceutical industries. For example, isoeugenol 18 and its methyl ether have been regularly used in the perfumery industry to mimic blossom compositions.60 In addition, the World Health Organization has recognized the value of arylpropenoids as food additives, both for human and for animal61 consumption, and has subsequently developed an online database containing information pertaining to the acceptable use of these compounds (database contains results pertaining to a series of monographs titled “WHO Food Additives”).62 As a final example, anethole 20 has been used for the synthesis63,64 of the pharmaceutical anethole trithione [5(4-methoxyphenyl)-3H-1,2-dithiole-3-thione; trade names: Sulfarlem, Mucinol], a drug used for the treatment of xerostomia (also known as “dry mouth”) and other conditions.65 Because of the increasing demand on these compounds from natural resources, new synthetic approaches to arylpropenoids have become increasingly necessary. In addition, the understanding of the importance of obtaining valuable synthetic compounds from biomass is gaining global momentum.66,67 Examples of natural propenyl compounds being used in other transformations,68 for instance, the chemical conversion of isosafrole 19, isoeugenol 18, and anethole 20 into piperonal, vanillin, and p-anisaldehyde, respectively, have been the focus of intensive research,69 and recent approaches to these synthetic derivatives include microbial,70−76 electrochemical,77 and chemical methods.78−85 Among the microbial approaches, an investigation into the organisms involved in arylpropenoid isomerization and oxidation has resulted in identification of the enzymes able to effectively promote oxidation of the naturally occurring propenylbenzenes.86−88 Other chemical-based oxidative transformations performed on the substituted phenylpropenoids include epoxidations89,90 and ozonolytic cleavage.91 In particular, production of vanillin, an important flavor component, has seen much investigation due to its value,70,92−94 and the production of “biovanillin” has become a specific focus.95 It should further be realized that application of isomerization for the beneficiation of naturally occurring alkenes is experiencing serious investigation, and in addition to the propenylbenzenes, other isomerizable substrates exemplified by natural oils96 and fatty acids97 have also being used. Last, application of genetic engineering to modify metabolic processes for the production of plant allyl- and propenylphenols, as well as lignin pathways, has also been considered in terms of the future supply of biofuels, polymer monomers, and specialty chemicals.98 1-Propenylbenzenes 2 have additionally been used as starting materials in many multistep syntheses, and numerous examples of this will be illustrated in this Review. Many naturally occurring 1-propenylbenzenes 2 have therefore been directly isolated from essential oils in which they occur and subsequently synthetically transformed to afford a wide range of diversified products, most of which display interesting bioactivities. Examples include the synthesis of the antimalarial diaryldienones by Sinha and co-workers who performed allylic oxidations on naturally occurring isosafrole 19, followed by condensations and esterifications to eventually afford analogues 26 with potential antiprotozoal activity (Figure 3),99,100 to the use of anethole 20 by Dinh and co-workers, for the synthesis of bioactive 1,3-thiazolidin-4-ones and indoles (not shown).101

(b) Antiparricidal activity: Dillapiole 11 and its isomerized isomer 23 have been shown to exhibit modest antileishmanial activity.21 Furthermore, dietary anethole 20 has been shown to have a positive effect in mitigating Eimeria acervulina coccidiosis infections in poultry.22 (c) Antifungal activity: Anethole 20 has been shown to have synergistic antifungal effects against Saccharomyces cerevisia and Candida albicans when combined with dodecanol.23 The same authors involved in this research then showed that the same compound’s effect on Mucor mucedo was most likely due to the induction of cell wall fragility.24 In other recent evaluations, anethole 20 was found to be active against Candida spp. and Microsporum canis,25 as well as a range of plant pathogenic fungi.26 In addition, “38 clinical isolates of Candida albicans” were tested with eugenol 5 and eugenol-containing essential oils, including isoeugenol 18 among other constituents27 (for a related study, see ref 28). (d) Antialgal activity: Pollio and co-workers tested 27 phenylpropenoids in terms of their ability to inhibit the growth of the green alga Selenastrum capricornutum.29 It was found that a number of the compounds showed activity in the (1−5) × 10−4 M concentration range. In addition, the number and position of methoxy groups on the allylbenzene core seems to affect the activities of the compounds tested. For another paper investigating the effect of α-asarone 22 on S. Capricornutum and Ankistrodesmus braunii, see ref 30. (e) Anti-inflammatory31 and antioxidant32 activity: Dillapiole 11, safrole 6, and their isomerized isomers 23 and 19, respectively, have been shown to exhibit modest antiphlogistic properties in a carrageenan-induced rat paw edema model.33 It should also be noted that anethole 20 has been intensively tested for its anti-inflammatory activity34−37 and its effect on wound healing.38 Of further interest is that researchers concluded that the effect of anethole 20 on mammalian tissue is probably by way of the voltage-dependent Ca2+ channels in the vascular tissue.39,40 The gastroprotective ability of anethole 20 has also recently been investigated.41 (f) Cytotoxicity: The impact of eugenol 5 and isoeugenol 18 on mammalian tissue has also been investigated,42−44 as well as the effect these compounds have on the growth of cancerous tissue45 and the possible mechanism by which these effects are manifested.46−48 At this point, it should also be noted that because of the ubiquitous nature of these compounds in plant material, and the subsequent incorporation of phenylpropenoids into our diet and living environment, the toxicity of these compounds has additionally been investigated.42,49−52 It should further be noted that a number of 1-propenylbenzenes have been tested as anticarcinogenics; see, for example, the references on the activity of carpacin 24 and its corresponding β-isomer,53 and the potential of anethole 20 as a spice-derived phytochemical with cancer preventing properties.54 Finally, recent studies on the toxicity of phenylpropenoids such as methyleugenol 17 have identified certain metabolites of these compounds to include variants with isomerized allyl groups, and have proved that these metabolites can be converted into mutagens.55−58 Because of the significant challenges in terms of resource management on Earth today, scientists have been striving to find “renewable” options in terms of the chemicals used by the human race. In this regard, the phenylpropenoids have an important part to play, because many of the 2-propenylbenzenes 1 are available from natural sources or cultivated crops. For example, the trans isomer of anethole 20 is found as a C


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its 1-propenylbenzene derivative, followed by conversion into other functional groups, will thus also be described. In this Review, the chemical processes used to transform 2propenylbenzenes 1 into their corresponding 1-propenyl isomers 2 (Figure 5) will thus be described in depth. Care has been taken to include the most important strategies used over roughly the last 30 years and to summarize this significant research into distinct chapters. We will also seek to contextualize the rearrangement with regards to the original researchers’ strategies toward more developed synthetic targets. Rather surprisingly, there has been no recent attempt to comprehensively review this particular area of chemistry, apart from some book chapters and sections in general review articles:111−115 (for an older Chemical Reviews article on this subject matter, see the ref 116); other examples of these types of brief and rather general overviews will be referred to in the appropriate sections that follow. This Review therefore aims to summarize advances made in the isomerization of arylallyl substrates and thus provide an overview of possible synthetic applications and useful reagents to bolster the synthetic arsenal of the organic chemist, as well as give representative examples of where these methods have been applied.

Figure 3.

It should be mentioned that 1-propenylbenzenes have also been very popular substrates for the development of new synthetic methods, requiring, in particular, the synthesis of pure cis and trans isomers of these compounds (for example, work involving the asymmetric hydroboration of β-substituted vinylarene, which required pure 1-propenylbenzenes102). Finally, in terms of motivation for the development of efficient isomerization methods as described in this Review, it should be noted that the 2-propenylaryl motif occurs in a large number of natural products. Examples include licarin A 27103 (previously synthesized from 2-propenylaryl precursors104−106) polysphorin 28,107 fumimycin 29,108 and nigerloxin 30109 (Figure 4). As a direct consequence, approaches to (some of)

1.3. Scope of This Review

In terms of the chemical transformations covered, this Review will aim to mainly discuss the conversion of 2-propenylbenzenes 1 into 1-propenylbenzenes 2 (Figure 5). This means that, with respect to the structural transformations covered in this Review, the following will not be covered in any great detail: (a) examples involving the isomerization of longer homoallyl compounds to 1-propenylbenzenes (see, for example, work performed in refs 117 and 118); (b) the isomerization of allylic double bonds on other important heterocyclic scaffolds such as indoles,119,120 pyridines,121 or cyclopentadienyl systems;122 (c) research involving the isomerization of substituted allyl groups attached to benzene rings, for example, (2-methylallyl)benzene,123−125 but-2-en-1-ylbenzene,126,127 but-3-en-2-ylbenzene,128,129 or that of “internal” allylic systems, for instance, in substituted dihydronaphthalenes130−136 or more complex systems;137 (d) isomerization of systems involving phenylsubstituted allylic alcohols (see, for example, ref 138); (e) allylic C−H activation of allylarenes with transition metal complexes, resulting in 1-propenylbenzenes with a nucleophile on the 3position (see, for example, papers139−142 and references cited therein); and (f) the synthetic transformations of allylbenzenes that result in apparent “isomerization” and functionalization at the terminal methyl group (Scheme 1), which illustrates the ready conversion of propenylbenzenes 3 or 4 into propenaldehyde 31.143 Finally, the time period covered in this Review will be from the early 1980s until the end of 2014. Older references will be

Figure 4.

their synthesis have involved methods designed to generate the allylaryl functional groups and their subsequent isomerization. Further appropriate examples will be described in this Review. 1.2. The Importance of Transforming 2-Propenylbenzenes into 1-Propenylbenzenes

The specific focus of this Review is to highlight the different isomerization protocols employed to produce isomeric versions of the 2-propenylbenzenes 1, that is, the 1-propenylbenzenes 2, because these compounds have seen so much application in academic and industrial chemistry, and are in fact readily available from their 2-propenylbenzene 1 equivalents by way of a relatively simple isomerization (Figure 5). In synthetic chemistry, the allyl group has also been considered a potential “protecting group” for a variety of functional groups.110 Examples of where the allyl group has been isomerized into

Scheme 1

Figure 5. D


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reaction. Rather, the authors invoked a discrete intermediate II that is formed when the base removes the proton from the benzylic position. Here, the three carbon atoms of the resulting anionic allylic system are coplanar, and the hydrogen atom of the alcohol product is hydrogen bonded to both terminal carbon atoms. Collapse of this intermediate would result in the conjugated product III-h being formed. What is important is that the hydrogen atom in II is on the same face that it was abstracted from in the transition state. This answers the question of both the retention and the inversion of stereochemistry for the simple exchange reaction giving back I (which it should be noted is highly disfavored because k2 ≫ k−1). Retention of configuration would occur by first noting that the potassium ion would also be coordinated to the same face as the abstracted hydrogen atom because it was originally coordinated to the oxygen of the base. A molecule of deuterated alcohol can then approach this same face through coordination to the potassium. Rotation of these ligands around potassium would result in the deuterated alcohol now hydrogen bonded to the allylic anion (IV-top), which following collapse would return the chiral alkene I-d with the same configuration. On the other hand, inversion of stereochemistry would occur via the deuterated solvent approaching from the face of the allylic anion opposite to that hydrogen bonded, and exchange with the alcohol giving (IV-bottom). This could collapse and give chiral alkene I-d with inversion of configuration. The E/Z ratio of 60 for product III-h is also important because this value is extremely high as compared to the equilibrium value of 4.2 for the diastereoisomers, and compares to other systems as well. For example, the isomerization of allylbenzene into 1-propenylbenzene has an E/Z ratio of 44 when using potassium t-butoxide in DMSO.145 The reason for this higher-than-equilibrium ratio is explained in Scheme 3. Two “conformations” of the chiral alkene I are suitable for deprotonation and will lead to either the E or the Z allylic anions as shown. The rate of “conformational rotation” must be many orders of magnitude greater than the rates for

infrequently used to add insight where required. However, any references from pre-1980 serve only as examples and should not be viewed as being a comprehensive part of this Review.

2. ISOMERIZATION MECHANISMS The mechanisms for the isomerization of 2-propenylbenzenes 1 into the more thermodynamically favored 1-propenybenzenes 2 have been carefully studied due to the importance of this transformation to the various industries already mentioned in the introduction. These mechanisms are obviously dependent on the reagents and conditions used and have by no means been exhaustively studied under all possible circumstances. Nevertheless, we will look here at some of the main mechanistic themes that have emerged as a foundation to this Review’s exposition of the many varied methods available. Common to all mechanisms, though, is the recognition that upon alkene isomerization both the E- and the Z-isomers can be obtained. 2.1. Base-Mediated Mechanism

The base-mediated mechanism has been carefully studied by Cram and co-workers some 50 years ago.144 Using reaction kinetic data and isotopic labeling experiments under a wide variety of base and solvent conditions, a mechanistic interpretation for the allylic rearrangement of 3-phenyl-1butene (I for the purposes of the mechanism discussion) was developed and is illustrated in Scheme 2. The researchers Scheme 2. Proposed Base-Mediated Reaction Mechanism144

Scheme 3. Interactions Leading to Observed E/Z Ratios

envisaged either an intra- (base facilitated movement of proton within the same molecule) or an inter- (base moves proton between different molecules) molecular pathway. Cram and Uyeda were able to show that both of these pathways were operating, but that the degree of intramolecularity depended strongly on the base and solvent used. For example, the degree of intramolecularity was as high as 56% when using I-h with potassium triethylcarboxide in triethylcarbinol-O-d, and 33% when using I-h with potassium ethylene glycoxide in ethylene glycol-O-d. The high degree of intramolecularity of the reaction suggested that a one-stage mechanism involving deprotonation by base and separate reprotonation by solvent could not be taking place. This was further supported by the fact that the ratios of the rates of intra- and intermolecular isomerizations did not appreciably change in different media with different bases, which would be expected for a one-stage intermolecular E


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Scheme 5. Allyl Mechanisma

deprotonation (kE and kZ), and thus the E/Z ratio in the product must be related to the kE/kZ ratio. The transition state leading to the Z-allylic anion must thus be higher in energy than the one leading to the E-allylic anion, presumably due to the disfavored 1,3-allylic interactions, which would make charge stabilization through conjugation more difficult. Cram has reported this in more detail, including an analysis of collapse ratios (i.e., ratio of protonation of either carbon on the allylic anion),145 the stereochemical stability of allylic and vinyl anions,146 and the kinetic and thermodynamic stabilities of olefinic products formed by protonation of allylic anions.147 2.2. Transition Metal-Mediated Mechanisms

The mechanisms for the transition metal-mediated isomerizations have in some cases been very carefully determined, while in many they are assumed from one of two generally accepted pathways (Schemes 4 and 5).148 The first involves a Scheme 4. Alkyl Mechanisma

a

M = transition metal; [L]n = bound ligand(s); [L]0 = dissociating ligand or vacant 2e− site.

thermodynamically favored because of the conjugation to the benzene ring. 2.2.2. Allyl Mechanism. The allyl mechanism, however, requires a transition metal capable of having two vacant coordination sites, and, more specifically, no metal−hydride should be present or the alkyl mechanism will take place (see Scheme 5). The first step involves coordination of the πelectrons of the allylbenzene to one of the transition metal’s vacant sites. This is followed by an oxidative addition reaction giving an η3-allyl metal−hydride complex, which can collapse to either the starting material or the rearranged and thus more stable alkene. The η3-complex may also rearrange to the η1complex as part of the reaction pathway. The difference between these two mechanisms can be determined through deuterium labeling with 32 and crossover experiments (Scheme 6).149,150 The allyl mechanism is entirely intramolecular and involves an effective 1,3-hydride shift as the only mechanistic pathway. Thus, in a crossover experiment, such as that in Scheme 6, the deuterium should (a) only be found at the 1- and 3-positions of the allylic system, that is, 34, and (b) not be incorporated into the second nondeuterated substrate. In the case of the alkyl mechanism, products similar to those of the allyl mechanism may be detected in addition to (a) the nondeuterated substrate showing some deuterium incorporation and (b) the loss of deuterium and incorporation of hydrogen on the deuterated substrate. Furthermore, deuterium incorporation at the 2-position of the allylic system, 35, is also expected because the initial metal−hydride insertion reaction may have poor regioselectivity as already explained in Scheme 2. Of course it should be noted that these two general mechanisms are just that, and the specific reaction pathway for different transition metal catalysts will depend heavily on the transition metal, ligand, solvent, and substrate combinations. More detailed mechanistic data can be obtained from kinetic studies supplemented by state-of-the-art measurements, for example, nanosecond time-resolved IR,151 NMR,152 and DFT153 calculations. These studies have invariably revealed

a

M = transition metal; [L]n = bound ligand(s); [L]0 = dissociating ligand or vacant 2e− site.

discrete transition metal−hydride active catalyst and the formation of a transition metal−alkyl intermediate, while the second involves the formation and collapse of a η3-allyl hydride complex. Each of the mechanisms described below bases its premise on the fact that each step is reversible, and thus the reactions are under thermodynamic control at equilibrium. 2.2.1. Alkyl Mechanism. For this mechanism to occur, the transition metal catalyst must have both an empty 2e− coordination site (e.g., via dissociation of a ligand) and a metal hydride bond that is typically generated in situ under the reaction conditions (see Scheme 4). The catalyst first coordinates to the π-electrons of the alkene followed by an insertion reaction to give either a primary or a secondary metal−alkyl intermediate. The primary metal−alkyl is generally formed faster for many catalysts, but is a mechanistic dead-end returning the starting material through β-hydride elimination and is thus a nonproductive pathway. The secondary metal− alkyl intermediate can however produce either the E- or the Z1-propenylbenzene on β-hydride elimination, which is clearly F


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of an allylbenzene. This method has seen much application in the past literature, and some general information has been collated in book chapters.156,157

Scheme 6. Outcomes Expected for a Typical Deuterium Labeling and Crossover Experiment To Determine the Most Probable Rearrangement Mechanisma

4.1. Hydroxide/Alkoxide Ion-Mediated Isomerizations

This particular method involves the use of fairly harsh reaction conditions in that it generally comprises heating the substrate in a protic solution (ethanol,158 methanol,159,160 or n-butanol) of sodium or potassium hydroxide (giving rise to a mixture of the hydroxide and corresponding alkoxide under equilibrium conditions). Other examples include the use of hydroxide in DMSO.161 Of interest is that the KOH/ethylene glycolmediated isomerization of eugenol 5 has actually been incorporated into the curriculum of a teaching laboratory to demonstrate how the kinetics of a reaction can be studied using NMR spectroscopic, GC, and HPLC laboratory techniques.162 It should also be mentioned here that the importance of the potassium t-butoxide system means that a separate subsection will be dedicated to this method (see section 4.2). In the first section, examples of where hydroxide-mediated isomerization has been applied will be highlighted. This will be followed by methods in which the isomerization approach has been modified, albeit with additives (phase transfer), microwave heating, or different solvent systems. 4.1.1. Hydroxide/Alkoxide Ions in Alcohol (Methanol, Ethanol, n-Butanol) or without Solvent. It should be noted that the use of sodium ethoxide has been rigorously studied, showing that the isomerization of para-substituted 3-phenyl-1propenes occurs with first-order kinetics.163 A range of electron-donating and -withdrawing substituents were incorporated onto the aryl rings, and the rate constants gave a straight line in terms of the Hammett equation. The next section will thus provide examples of where this method has been applied to address specific synthetic problems. In 1991 Saha and Nasipuri communicated their stereoselective approach to the synthesis of (±)-veadeiroic acid 38a and (±)-veadeirol 38b.159,160 An important step in their synthesis was the quantitative isomerization of substrate 36 into the (1-propen-1-yl) analogue 37 using boiling methanolic potassium hydroxide (Scheme 7). Afon’kin et al. described in 2002 the isomerization of 2-allyl6-methylaniline 39, by the action of potassium hydroxide at high temperatures (300 °C) for 1 h, to afford the trans- and cisisomers of 2-methyl-6-(1-propenyl)aniline 40 and 41 in 80% and 17% yield, respectively (Scheme 8).164 Of interest was that

a

Note: Other products would be possible, but these typify the products that would establish either mechanism.

mechanisms more complicated than those presented above, but at the same time generally holding true to them. One major mechanistic departure has been that proposed by Harvey and Lloyd-Jones154 in which they suggest a binuclear palladium complex being involved in the E/Z isomerization of alkenes. However, it should be noted that this is only applicable in some cases. More important is that the specific mechanisms themselves answer questions related to E/Z selectivity in these transition metal-mediated isomerizations. Simplistically, the reactions can be considered to be under thermodynamic control, and therefore the E/Z ratios will favor the E-isomer. The π-allyl mechanism has been linked, in a general sense, with higher E/Z ratios,155 but can by no means be used as proof for a particular mechanism. This is because the E/Z selectivity in the reaction is strongly governed not only by the thermodynamic stability of the E isomer (and intermediates leading to the E-isomer), but can be shifted through ligand and or kinetic control to high ratios of either the E or the Z isomers. The sections below detailing the specific examples in the literature will highlight these selectivities.

Scheme 7

3. ISOMERIZATION METHODS − GENERAL Perusal of the literature quickly confirms that 2-propenylaryl isomerization reactions have been promoted by mainly two classes of methods, viz., the application of bases (section 4) or transition metal complexes (section 5). In this Review, these sections will thus be discussed separately, in addition to a final section describing miscellaneous allylaryl isomerizations (section 6). In each section, substrates of particular interest will be highlighted, with particular care being taken to convey important experimental data such as yields, the cis/trans ratios obtained, and any other relevant information. 4. BASE-MEDIATED ISOMERIZATIONS In general, base-initiated reactions require at least a stoichiometric amount of base to accomplish the isomerization G


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rearrangement/arylallyl isomerization sequences have also been observed before.174 In 2006, Srikrishna and Satyanarayan demonstrated the isomerization of a substituted 2-allylphenol with potassium hydroxide in methanol during the total synthesis of (±)-herbertenediol 47 (Scheme 9).175 The isomerization

Scheme 8

Scheme 9

step, 45 → 46, was readily accomplished with methanolic potassium hydroxide and gave the isomerized product 46 in an excellent yield of 95%. A significant number of additional steps, which included ring-closing metathesis (RCM), then afforded (±)-herbertenediol 47, as well as its dimers, the mastigophorenes A−D (by way of “formal” total syntheses). Of additional interest is that Srikrishna and co-workers used a related approach employing a potassium hydroxide-mediated isomerization in the synthesis of (±)-12-methoxyherbertenediol (structure not shown).176 Another example in this section demonstrates the importance of isomerization reactions for positioning the alkenes in a desired position to facilitate an RCM reaction. In one of the first papers describing the immense potential for RCM in synthetic chemistry, Grubbs and co-workers synthesized benzofurans by using isomerization reactions, followed by olefination and RCM, as key steps.177 In the example shown in Scheme 10, describing the synthesis of an antifungal phytoalexine isolated from Sophora tomentosa L., potassium hydroxide-promoted isomerization of compound 48 in n-butanol as solvent readily afforded styrene 49 as a 2:1 mixture of E:Z isomers. An esterification (49 → 50) was followed by the generation of an olefinic enol ether 51 by using the Tebbe reagent on compound 50.178 The Schrock metathesis catalyst SI was then applied to achieve the desired metathetic transformation to afford the desired benzofuran 52, which was debenzylated into the naturally occurring bisphenolic molecule 53 by way of palladium-catalyzed hydrogenation. De Lima and colleagues also used potassium hydroxide in nbutanol to readily produce isosafrole 19 from the natural compound safrol, 6.179 After completion of the reaction, neutralization and workup, followed by distillation under reduced pressure, the desired product 54 representing a mixture of the E/Z isomers was obtained in an excellent yield of 95% and on a reasonable scale (10 g). This mixture was then converted into the indanone 55, and then eventually into the strigol analogue 56 (Scheme 11). These researchers also demonstrated the same isomerization in an earlier paper describing the synthesis of a sulindac analogue from the same starting material.180 It should be noted here that research groups from Brazil have been very effective in using safrole 6 as

this reaction was performed without the addition of solvent. In terms of allylanilines, it should also be noted that Wehrli et al. isomerized 2-allyl-N-methylaniline and 2-allyl-3,5-dimethylaniline (structures not shown), with KOH in MeOH at 110 °C for 5 h, to give the corresponding isomerized products in good yields of 94% (E:Z = 3:1) and 84% (E:Z = 2:1), respectively.165 Of possible interest to the reader may be that when oallylanilines are generated by high temperature amino-Claisen rearrangement reactions, unwanted thermal isomerization of the allyl groups is frequently encountered (see, for instance, the following example166). Heterocycles bearing allyl groups have also been successfully isomerized. Two examples include the isomerization of 7-allyl-8-aminoisoquinoline167 and 7-allyl-8hydroxyisoquinoline,168,169 which when treated with potassium hydroxide in alcohol solvents gave the products 42a and 42b in yields of 90% and 72%, respectively. Finally, 4-allyl-2,3dihydrobenzofuran-5-ol and 6-allyl-5-methoxy-2,3-dihydrobenzofuran were isomerized under similar conditions to afford 43 and 44 in good yields.170 Compound 42a was used for the synthesis of novel 1,10-phenanthrolines applied as transition metal ligands,167 while compounds 43 and 44 were converted into analogues of known hallucinogenic phenethylamines.170 In terms of the high temperature Claisen reactions mentioned in this paragraph, it should be noted that Strauss and co-workers applied high temperature autoclave heating to allyl phenyl ether in basic aqueous media (0.4 and 1.29 M sodium hydroxide, temperature 200−248 °C) and found that the isomerized products cis- and trans-2-(prop-1-enyl)phenols were obtained in good yields due to facile isomerization after the Claisen reaction.171 For a further study of this reaction, in water and under conventional and microwave heating, see the following paper172 by the same research group. Nicols and coworkers also applied an interesting one-pot “Claisen rearrangement/O-methylation/alkene isomerization” strategy during their synthesis of ortho-methylated phenylisopropylamines.173 The key step in this sequence involved the use of phenyltrimethylammonium methosulfate with potassium carbonate (DMF, reflux, 36 h) to promote the Claisen, allyl isomerization, and phenol methylation in an excellent yield of 79% over the three steps. It should be noted here that other heat-induced H


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(NaOH or NaOMe) on related substrates 59 and 61 gave much better yields of the isomerized products 60 and 62, respectively. Of interest is that the more hindered 2-methyl-2propenyl group in substrates 59 and 61 did not isomerize into the thermodynamically more stable internal alkenes and that the chloroallyl moiety in 57 was also resistant to the isomerization conditions. The Rutledge research group also investigated the basepromoted isomerization (KOH in MeOH) of the bis-allylaryl system 63.195 The products obtained after isomerization proved to be difficult to purify, and preparative layer chromatography eventually facilitated separation of the mixture of products (Scheme 13) to give the partially isomerized compound 64, the doubly isomerized compound 65, the tetracycle 66, and the pentacycle 67. Of interest is the fact that the amounts of 64, 65, and 66 increased with longer reaction times. The authors of this work were able to propose anion-based mechanisms for the formation of the latter products, involving the partially isomerized compound 64, which acted as starting material. The aromatic allyl group can also be considered as a cleverly masked carbonyl group. Pausler and Rutledge made use of this strategy when they isomerized the allyl group in substrate 68 with potassium hydroxide to give 69, which on subsequent ozonolytic cleavage of the styrene moiety afforded benzaldehyde 70 (Scheme 14).196 This same isomerization−oxidation approach to produce a benzaldehyde was also used in a related system (sequence for R = Me, 71 → 72 → 73), and methanolic sodium hydroxide was again found to be efficient for the isomerization process.197 In the case of 71 where R = H, the isomerization was also facile under basic conditions. Finally, this research group isomerized the aryl allyl group in substrate 74a (R = Me) to afford compound 75a (R = Me), thereby also demonstrating the chemoselectivity of the methanolic potassium hydroxide with respect to the allyloxy group. After removal of the allyloxy group under acidic conditions, the aldehyde was revealed after cleavage of the alkene with osmium tetroxide and followed by periodic acid. This compound was converted into the homochiral dioxolane 76 by treatment with (2R,3R)-butane-2,3-diol under acidic conditions.198 Note should also be taken of related work in which Brown et al. isomerized 74b (R = H) into 75b (R = H), which was an important part in the synthesis of some novel chloroanthracyclines (structures not shown).199 Furthermore, Rutledge and co-workers demonstrated the versatility of using allyl and vinyl alkene functional groups for the introduction of different carbonyl groups on the anthracyclinone skeleton. For example, a hydroxide ionmediated isomerization applied to anthraquinone 77 afforded 78 in quantitative yield (Scheme 15).200 The chemoselectivity of this reaction should be noted as the O-allyl ether was not affected under these conditions. A subsequent Claisen reaction involving the allyloxy group, followed by methylation, then gave bis alkene 79. The allyl group was then converted into the methyl ketone 80 using the Wacker oxidation protocol, after which the ß-methylstyrene was cleaved to afford aldehyde 81, demonstrating the orthogonality of the two alkene-containing functional groups (Scheme 15). In an alternative approach to 78, the researchers found that isomerization of the phenolcontaining allyl group 82 was very sluggish when using 2% methanolic sodium hydroxide, presumably because of the decreased acidity of the benzylic protons due to the adjacent phenoxide ion. However, addition of magnesium hydroxide, which was proposed to form a chelate with the phenoxide,

Scheme 10

Scheme 11

starting material for the synthesis of various bioactive molecules.181 In these cases, the potassium hydroxide−alcohol solvent combination was used to isomerize the safrole allyl functional group, followed by subsequent multistep synthetic procedures to afford molecules with antihypertensive,182 antiplatelet,183−186 vasodilatory,187 analgesic,184,188,189 antipyretic,188 anti-inflammatory,184,188,190 inotropism-increasing,191 and antithrombotic192 activities. For related studies on basemediated (KOH or NaOH) isomerizations of safrole and related compounds, see refs 21, 33, and 193. Over a period of two decades, the Rutledge group reported on a series of important studies toward the synthesis of anthracyclinones and associated compounds. Of particular interest to this Review is that the researchers used a variety of isomerization strategies on a number of interesting phenylallyl substrates to gain access to the desired anthracyclinone analogues.194 In the initial example shown in Scheme 12, Beauregard et al. found that the use of a palladium-mediated isomerization on the chlorine-containing substrate 57 gave the desired isomerized product 58 in a rather poor yield of ∼30% (see section 5.1 for more information on these types of catalysts). However, application of base-mediated procedures I


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Scheme 12

Scheme 13

Scheme 14

resulted in a facile isomerization reaction and afforded 83 in good overall yield. It should be noted that this additive could be a valuable synthetic “trick” for the isomerization of similar systems.200 Finally, in the same study, compounds 84 and 85 were also successfully obtained from the respective allylcontaining substrates by isomerization using the standard basemediated method. As an interesting aside, Mal and co-workers found that the addition of D-glucose to 1-allyloxy anthraquinones facilitated the Claisen reaction under lower temperatures (100 °C, instead of 150 °C) and also resulted in some of the isomerized isomer being observed.201 During further investigations by the Rutledge group, it was found that treatment of iodide 86 with ethanolic potassium hydroxide under reflux gave the pentacyclic product 87 in which the allyl group had isomerized (Scheme 16).202 Under these reaction conditions, a serendipitous cyclopropanation also

occurred, resulting in the interesting fused pentacyclic structure of 87. In the same paper, the authors also described how they synthesized compound 89 (as a mixture of the methyl and ethyl phenyl ethers) from 88 under similar conditions (Scheme 16). J


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Scheme 15

Figure 6.

or under phase transfer conditions205,206), as well as to facilitate the isomerization of other polymethoxylated aromatic compounds, viz., 91, 92, and 93 (ethanol, anhydrous KOH).207 A variety of substituted 3-(prop-1-en-1-yl)phenols have also been readily isomerized using KOH in ethylene glycol208,209 or methanol165,210,211 as solvent. Finally, halogenated substrates have also been successfully isomerized using potassium hydroxide as base. For example, 1-allyl-3-(trifluoromethyl)benzene (n-butanol),212 1-allyl-2-chlorobenzene (n-amyl alcohol),213 2-allyl-6-bromophenol,214 and 1-allyl-3-bromo-2-methoxybenzene (ethanol/water)158 were all isomerized with potassium hydroxide. The latter example is interesting as it is likely that some of the transition metal-mediated isomerizations described later in this Review might not be so tolerant of aromatic halides and, in particular, bromides. Finally, in this section, in an attempt to overcome some of the limitations of the classical isomerization procedures, involving high concentrations of strong base, high temperatures, and long reaction periods, Zucco et al. employed microwave irradiation for the isomerization of safrole 6 and eugenol 5. These isomerizations were performed in the presence of potassium hydroxide and various alcohols as solvent.215 These researchers found that the microwaveenhanced conditions increased the rate of reaction up to 13 times faster than that observed when using conventional heating. The best solvents for the isomerization of safrole 6 and eugenol 5 were found to be 1-butanol and glycerol, respectively. 4.1.2. Hydroxide/Alkoxide Ions in Dimethyl Sulfoxide. Application of the superbasic medium KOH−DMSO (H2O) in synthetic chemistry has seen increased usage216 since being popularized by Trofimov and co-workers.217 The impressive basicity of this solution (pKa = 30−32)218 means that it readily facilitates the isomerization of allylaromatics. Examples of the application of this reagent combination will be dealt with next. An interesting example of the use of a simple isomerization reaction with hydroxide ions in DMSO solvent, ultimately leading to the synthesis of a complex molecule with numerous stereocenters, was demonstrated by the Fukuyama group in their synthesis of (±)-renieramycin A 98 (Scheme 17).161 The synthesis started from base-catalyzed isomerization of allylbenzene 95, formed from the thermolysis of allyl ether 94, to afford the conjugated olefin 96. This functional group was in turn oxidatively cleaved into the aldehyde 97, after protection of the phenol, which was used to synthesize the natural product 98. This overall conversion of an arylallyl ether to an aldehyde functional group is a relatively well-known and effective synthetic strategy, albeit a rather lengthy one. Trofimov and co-workers reported the isolation of the isomerized allylbenzene, (E)-1-phenylprop-1-ene 4, during the synthesis of (1-phenylprop-2-yl)phosphine 99 and bis(1-

Scheme 16

In an investigation into the algicidal activity of a variety of phenylpropenoids, Della Greca et al. used ethanolic KOH solutions under reflux to synthesize a variety of 1-propenylaromatics as shown in Figure 6.29 It should be noted that other researchers have made use of the same strategy to obtain the 1propenyl-1-benzenes 4 and 90 (with n-butanol as solvent203,204 K


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(PH3) by way of a single or double nucleophilic addition to afford the primary and secondary organophosphines obtained. The authors of this work proposed to use these novel phosphines, among other things, as potential ligands for the synthesis of transition metal complex catalysts. In further work by Trofimov,220,221 isomerization of allylbenzene 3 was also observed when treated with red phosphorus in the presence of the superbasic medium KOH/ DMSO/H2O (for an overview regarding the halogen-free synthesis of organophosphorus compounds by the Trofimov research group, see ref 222). When the reaction was carried out at 130 °C, the main product, 1-methyl-2-phenylethylphosphinic acid 101, was obtained in 58% yield, along with 10% of isomerized product E-prop-1-enylbenzene 4 (Scheme 19).

Scheme 17

Scheme 19

phenylprop-2-yl)phosphine 100 (Scheme 18).219 These products were obtained from the reaction between allylbenzene 3, Scheme 18

Under microwave heating conditions, the secondary phosphane 100 could be obtained in yields approaching 50% (together with the “mono”-phosphinic acid 101 in 9%). It should be noted that the authors of this work tried a number of different conditions and obtained different ratios of phosphane and phosphinic acids. In addition, for this second set of examples, the researchers also postulated that the first step in the synthetic pathway involved isomerization of the allylbenzene starting material. Trofimov subsequently proved this idea when exposing 2-[prop-1(E)-enyl]anisole to red phosphorus in the presence of the same system (KOH/DMSO/H2O) and hydroquinone (as radical scavenger) to obtain the corresponding (2-aryl-1-methylethyl)phosphinic acid in a good yield of 83%.223 This research group also converted a set of substituted 1-propenylaromatics into their respective (2-aryl-1methylethyl)phosphinic acids in reasonable yields of 33−52%. Finally, Trofimov mentioned that they were able to isomerize these same 2-propenylaromatics into their 1-propenyl derivatives under the KOH/DMSO/H2O conditions (quantitative results at 100 °C for 2 h for five examples).

phosphine, and KOH when using wet dimethyl sulfoxide as solvent. Milder reaction conditions afforded (1-phenylprop-2yl)phosphine 99 in moderate yield (53%) along with 100 (30%), while using 2 equiv of allylbenzene 3 under more forcing conditions gave a good yield of the secondary phosphine 100 (80%). The authors were able to isolate the putative intermediate 4, which formed under the “super basic” KOH−DMSO conditions, that reacts with the phosphine L


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4.1.3. Hydroxide Ions and Utilization of Phase Transfer Catalysis (PTC). Modifications of the potassium hydroxide isomerization method include use of PTC205,206,224 and aprotic solvents.225 In the former adaptation, phase transfer catalysts such as adogen 464 (tricaprylmethylammonium chloride), tetrahexylammonium bromide, tetrabutylammonium chloride, and tetrapentylammonium bromide have all been used for the isomerization of allylbenzene 3 (at 70−75 °C in toluene or xylene resulting in conversions of up to 98%).205,206 In addition, aprotic solvents such as DME and dioxane were found to be suitable for fast isomerizations at moderate temperatures (40 °C). This method essentially makes use of the base suspended in a solvent (toluene, dioxane, or DME), which reportedly then affords the isomerized product in up to quantitative yields.225 Solvent-free conditions, investigated by Loupy and Thach, under microwave heating were also found to give satisfactory amounts of the isomerized product.226 In this particular research, a catalytic amount of phase transfer agent, Aliquat, as well as a number of amine additives were found to be effective transfer agents for the isomerization reactions. It should be noted that these same researchers also applied the KOH−Aliquat method for the isomerization of safrole 6 to give the desired isomerized product 19 in an excellent yield of 96%.227 Later, Thach and co-workers expanded their interests involving the isomerization of safrole 6 into isosafrole 19, and compared the yields obtained when using a number of different reaction conditions for this specific isomerization reaction.228 In this particular work, the researchers used potassium hydroxide, tert-butoxide, and potassium fluoridealuminum oxide, with or without phase transfer catalysts, for the allylaromatic isomerization reactions. The authors found that the use of microwave heating decreased the reaction time to minutes (3−15) and still gave very good conversions for the three bases used for this transformation. An extension of this work saw the solventless application of potassium hydroxide and the phase transfer catalyst, triethylbenzylammonium chloride (TEBACl), for the isomerization of safrole 6 with good results. Application of microwave heating (3 min, 420 W) afforded the desired isosafrole 19 in an excellent yield of 98%.229 In a subsequent study to investigate the insecticidal activity of safrole 6 and derivatives on the adult housefly, Musca domestica, Guerin and co-workers used the Loupy conditions (CaO/KOH, 245 °C) to readily obtain isosafrole 19 from safrole 6.230 Of interest in this respect is a particular example described by Buchanan et al., in which potassium hydroxide, with Aliquat 336 as phase transfer catalyst, was used to isomerize significant amounts of safrole 6 (20−200 g).231,232 This material was required for a study involving the isotopic mass spectrometric profiling of the drug intermediate, piperonyl methyl ketone (PMK) 102, used in the clandestine production of 3,4-methylenedioxymethamphetamine 103 (MDMA, also known as “ecstasy”) (Scheme 20). For an interesting overview of the development of these compounds by Shulgin and others, see the review.233 It should also be noted that others in this field of analysis have used this approach to obtain isosafrole 19 for studies involving the evaluation of impurities in intermediates leading to illegal substances.234 In this study, isomerized compounds arising from other allylaromatics were also studied in an attempt to “fingerprint” certain chemical routes. It should further be noted that application of isosafrole 19 has formed an important part in

Scheme 20

the investigation of the synthetic routes toward MDMA and its precursors,235−237 the identification of intermediates in these syntheses,238,239 and application to the training of chemical sniffer dogs to find caches of these drugs.240,241 Of further interest is that isosafrole 19 currently forms part of the synthesis of 3,4-methylenedioxybenzaldehyde 104 (also known as piperonal or heliotropine), which is used as a flavorant in the perfumery and pharmaceutical industry (Scheme 20). Because safrole 6 is mostly obtained from plant material, there has been concern that unsustainable harvesting practices have resulted in deforestation, particularly in developing countries. In addition to these ecological concerns, the rising price of safrole 6 has resulted in alternative synthetic approaches to 3,4-methylenedioxybenzaldehyde 104 being investigated (in this regard, also see the section detailing with the use of solid bases, section 4.4).242,243 Rabinovitz and Sasson reported a protocol that resulted in isomerization of allylbenzene 3 into trans- and cis-1propenylbenzenes 4 and 90, respectively (Scheme 21), which Scheme 21

involved use of hydroxide ions under liquid−liquid PTC conditions with tetrabutylammonium bromide (TBAB) as catalyst.205,206 The authors mentioned that the reactions were believed to proceed through a “hydroxide extraction mechanism”, as determined by a number of physical organic chemistry experiments. In a later study, Sasson and co-workers M


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A sodium hydroxide-catalyzed isomerization of allylbenzene 105 under PTC was used as one key synthetic step during the total synthesis of fuscinarin by Xie and co-workers.249 The researchers used sodium hydroxide with TBAB to isomerize the allyl group in 105, to afford 106 in an excellent yield of 96%. Reduction of the ketone, followed by ozonolytic cleavage of the 1-propenyl group, then afforded the hemiacetal 107, which was readily converted into fuscinarin 108 over two steps as shown in Scheme 24.

demonstrated that another ion pair, didecyldimethylammonium bromide (DDAB), a very stable and highly lipophilic phase transfer catalyst, was able to facilitate the isomerization of 4allylanisole (estragole) 7 into anethole 20 under basic conditions (aqueous sodium hydroxide and toluene) (Scheme 22).244 Scheme 22

Scheme 24

Neumann and Sasson used a three-phase system comprised of organic solvent, a PEG−KOH phase, and a saturated aqueous basic phase to isomerize 4-allylanisole 7 as part of a study to investigate the mechanism of base-catalyzed reactions in phase transfer systems.245 These researchers also investigated the base-catalyzed isomerization of allylbenzene, using polyethylene glycol as liquid−gas phase transfer catalyst, by using a pulse reaction technique to obtain kinetic parameters for the reaction.246 In this second paper, it was postulated that the polyethylene glycol (PEG) formed complexes with the base, this time potassium carbonate, onto which the allylbenzene was adsorbed. It was suggested that the PEG was able to transfer the inorganic carbonate anions from the solid phase into the liquid catalyst phase. Two important parameters that determined the efficiency of the isomerization were (a) the adsorption equilibrium of the allylbenzene onto the PEG− K2CO3 matrix and (b) the extent of solvation of the carbonate anions. In fact, K2CO3 dispersed on γ-alumina was found to be better as a catalyst than the 10% PEG-6000−K 2 CO 3 combination, because potassium carbonate on alumina gave rise to “non-solvated” anions. It should also be noted that Kočevar and co-workers found that a heated mixture (150−160 °C) of PEG and sodium hydroxide, complimented by additional tetramethylammonium chloride or benzyltrimethylammonium chloride, readily afforded the respective Omethylated or O-benzylated 2-propenyl isomers of eugenol 5 or 2-allylphenol.247 Another example using PTC, which does not utilize KOH as base, was published by Sasson and co-workers. They reported on the isomerization of allylbenzene 3 into a mixture of 80% trans-β-methylstyrene 4 and 19% cis-β-methylstyrene 90 (Scheme 23) at 100 °C in DMSO as solvent, this time in the

The application of PTC was also used by Semenov and coworkers, who in 2011 employed potassium hydroxide and TBAB without any solvent at 100 °C, for the isomerization of a series of substituted allylbenzenes 109 into the corresponding (1-propenyl)-substituted benzenes 110 (Scheme 25).250 This approach was applied during the synthesis of phenstatin and its derivatives and started from plant-derived allylpolyalkoxybenzenes 109. One significant advantage of using these compounds as starting material for the synthesis of bioactive derivatives is that they are available on a significant scale by way of CO2extraction followed by fractional distillation, and up to 40 kg of allylbenzenes have been obtained in this manner.251,252 After the isomerization process, the derived styrenes were oxidatively cleaved with ozone to afford the substituted benzaldehydes 111, after which a number of transformational steps gave the desired phenstatin derivatives 112. The researchers were able to demonstrate that the antimitotic tubulin-destabilizing activity of one of the derivatives was similar to that of the naturally occurring combretastatins. The same researchers used this approach to access other scaffolds exhibiting potential anticancer activities. For example, the researchers accessed differently substituted combretastatin analogues 113, derived from Wittig reactions performed on the substituted benzaldehydes 111, which in turn had been obtained after the isomerization−oxidative cleavage strategy. It should be noted that some combretastatin derivatives synthesized using this approach demonstrated impressive antiproliferative activity in a sea urchin embryo assay developed by the group.252−255 Of additional interest was that Semenov and co-workers extended the strategy described above to generate two series of polyalkoxy-3-(4-methoxyphenyl)coumarins256,257 and 4H-chromenes,258 all with antimitotic activities. Recently, Semanov and collaborators extended the use of plant-based allylpolyalkoxybenzenes 109 to synthesize a series

Scheme 23

presence of potassium phosphate (K3PO4) and 18-crown-6 as a phase transfer catalyst.248 The use of potassium phosphate in itself is rather unusual, and the researchers stress that this solid base is very potent in anhydrous systems, thus readily deprotonating the weakly acidic allylbenzene (pKa = 34) to afford the carbanion intermediate and therefore resulting in the isomerized products. N


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Scheme 25

Scheme 26

Scheme 27

of 4-oxa- 114 and 4-aza-podophyllotoxin analogues 115 (Scheme 26), making use of the same isomerization step, followed by oxidative cleavage to obtain a range of substituted benzaldehydes as in Scheme 25.259,260 The most active members of the libraries of compounds that were synthesized had the myristicin or 3′,5′-dimethoxy-substitution pattern on the E ring, and the compounds were shown to be active against a number of human cancer cell lines (a number of synthesized compounds were found to be more potent than the natural product podophyllotoxin itself, as also found in related studies261). In addition, it should be noted that ethanolic potassium hydroxide has also been applied by others to isomerize the allyl groups of myristicine 8262 and dillapiole 11.263 Liu and Zhu obtained isosafrole 19 by isomerization of safrole 6 using potassium hydroxide under solvent-free conditions and application of phase transfer catalysts PEG (polyethylene glycol) or PEG supported on polystyrene.264 The researchers obtained isosafrole 19 in quantitative yield when the reaction was performed at 90 °C for 5 h. An advantage of this approach for potential large-scale application was that the PEG-X supported on polystyrene could be used repeatedly (up to 10 times) without any noticeable loss of activity. An interesting one-pot two step reaction developed by Lasek and Makosza, involving an allylbenzene isomerization process, is illustrated in Scheme 27.265 The 2-(1-propen-1-yl)anisoles 117 were readily produced by the phase transfer assisted, basemediated isomerization of the precursor 2-allylanisoles 116, which had electron-withdrawing groups. Compounds 117 were

then reacted in situ in a Michael fashion with arylalkanenitrile carbanions to afford the substituted 4-aryl-2-phenylbutyronitriles 118 as a 1:1 mixture of diastereoisomers. Furthermore, it is of interest that Lasek and Makosza performed a detailed study describing how the phase transfer catalysis was affected by “adsorption at the liquid−liquid interface”.266 Part of this work involved the rearrangements of substituted o-allylanisole derivatives (2-allyl-4-bromoanisole, 2-allyl-4-cyanoanisole, 2allyl-3,4,6-trichloroanisole, 2-allyl-4,6-dibromoanisole, and 1methoxy-2-allyl-4-chloronaphthalene) to determine the position of the extraction equilibria of various anions with their quaternary ammonium cations. It is also interesting to note that Sasson and co-workers used the isomerization of 4-allylanisole (estragole) 7 to 4-methoxy-βmethylstyrene (anethole) 20 to study the catalytic Hofmann decomposition of quaternary ammonium salts under PTC. O


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Using this particular isomerization as a kinetic probe, the authors were able to demonstrate that “the quaternary ammonium hydroxide was able to catalyze the Hofmann degradation and isomerization faster than the corresponding alkoxide.”267,268 It was furthermore observed that at higher temperatures the initial rate was higher, but with a concomitant decrease in the final conversion of the isomerization reaction. From these results, the authors concluded that the quaternary ammonium hydroxide was thus a stronger base in the nonpolar aprotic solvents used in the PTC experiments. 4.1.4. Supported Hydroxide Reagents. Kouznetsov and co-workers employed 10% potassium hydroxide on alumina for the isomerization of eugenol 5 into isoeugenol 18. Eugenol 5 was for this study obtained from the essential oil extracted from clove buds (containing approximately 60% of this natural product).269 The isoeugenol 18 thus generated was then used without further purification in a BF3·OEt2-mediated, highly diastereoselective three-component imino Diels−Alder cycloaddition reaction with compounds 119 and 120 or 122, which elegantly afforded heterolignan-like 6,7-methylendioxytetrahydroquinolines 121 or 123, respectively, as illustrated in Scheme 28.

Scheme 29

Scheme 28

extended to enantio- and diastereo-selective variants by Masson and co-workers, again using isoeugenol 18 as a key starting reactant, and in conjunction with a chiral phosphoric acid catalyst.277 4.2. Potassium tert-Butoxide-Mediated Isomerizations

Prosser was among the first researchers to use potassium tertbutoxide for the practical isomerization of mono- and polyfunctional allyl ethers into their corresponding propenyl ethers at 150−175 °C.278 In an obvious extension of the method, isomerization of allylbenzenes followed shortly after. In particular, early researchers provided evidence that polar solvents such as dimethyl sulfoxide and dimethylformamide greatly accelerated the isomerization rates of this reagent on a variety of allyl-containing substrates.279−282 As potassium tertbutoxide has been regularly used for the isomerization of allylbenzenes, the synthetic value of this reagent is described in more detail. Arguably, one of the reasons for the broad utilization of potassium tert-butoxide for isomerizations is that yields are generally better than with other alkoxide methods. While metal hydroxides may be more readily available, potassium tertbutoxide can be readily obtained by sublimation (or synthesized in situ) and used in catalytic amounts (although it is more commonly used in excess). Furthermore, lower temperatures are generally applicable, allowing more sensitive substrates to undergo isomerization with fewer side-reactions. From the examples listed in the rest of this subsection, it can be seen that many solvents have been used for the isomerization reactions with this reagent, including THF,283 DMF, DMSO,284−286 tertbutanol,287 and polar polyether solvents.288 In the first example described in this section, de Koning et al. used potassium tert-butoxide in dimethylformamide for the isomerization of substituted allylbenzenes during their synthesis of isochromanols (such as rac-130), as exemplified by the conversion of bis-alkene 128 into the 1-propenyl derivative 129289 (for related work by this group, see ref 290). Of interest

The Kouznetsov research group amply demonstrated the value of using 1-propenylaromatics, viz., 18, in a variety of other multicomponent reactions (Scheme 29). The products synthesized by the group include dihydro(1H)indenes, e.g., 124,270 spiro-dihydroquinoline-oxindoles e.g., 125,271 substituted dihydrobenzofurans e.g., 126, and a variety of substituted tetrahydroisoquinoline scaffolds (see, for example, compound 127),272−276 demonstrating the value of using isomerized allylbenzenes as substrates. Of additional interest is that the inverse electron demand aza-Diels−Alder reaction resulting in the substituted tetrahydroisoquinoline structure has even been P


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into the isochromane analogues of the natural products, the korupensamines and michellamines (for instance, 144) (Scheme 31).295,296 For a related synthesis of the Nprotected-1,3-dimethyltetrahydroisoquinoline portion of the korupensamines and michellamines, see refs 297 and 298.

is that in this reaction only the allylaryl group was isomerized, while the allyl ester was not affected by the strong base. In a related paper, Green et al. reported a potassium tert-butoxidemediated isomerization in the synthesis of 4-oxo-benzo[c]pyran 133.291 In this work, aryl ketone 131 was isomerized into 132 in the presence of potassium tert-butoxide in tetrahydrofuran. Substituted 1-propenylbenzene 132 was then converted into the substituted 1,3-dimethylisochroman-4-one 133 in four additional steps. de Koning and co-workers also used potassium tert-butoxide in a synthesis of ventiloquinone L.292 In this approach, the bisallyl aromatic compound 134 was converted into the benzopyran 135 in excellent yield, a process involving the isomerization of both allyl groups and a subsequent cyclization to afford trans-1,3-dimethylpyran 135 (Scheme 30).

Scheme 31

Scheme 30

It should be noted that the potassium tert-butoxide-mediated isomerization of allylbenzenes has seen regular use in total syntheses. For example, Nicolaou and co-workers employed this method to obtain substituted bromo-naphthalene 145 on their way to the total syntheses of a number of kinamycins,299 with only kinamycin F 146 being shown in Scheme 32. For the synthesis of a related naphthalene, see ref 300. In terms of other antibiotics, Rizzi and Kende used potassium tert-butoxide in tbutanol to obtain compound 147,301 which was then converted into the naturally occurring aklavinone 148, the aglycon of aclacinomycin (for the synthesis of another anthracycline by the same group, which utilizes potassium tert-butoxide promoted isomerization, see ref 302). In another example, Zhang and co-workers used a similar high-yielding isomerization to obtain the 1-propenylbenzene 149, which was used to synthesize pulverolide 150 in a study dedicated to confirming the structure of this natural product (Scheme 32).303 Wang and co-workers readily isomerized 2-(2-allyl-3,4dimethoxyphenoxy)-1-(4-chlorophenyl)ethanone 151 with 1

The synthesis of substituted isochromans from allylbenzenes is an interesting synthetic transformation putatively first involving an intermediate isomerization process. In 1983, Giles and co-workers described the synthesis of trans-1,3dimethylisochroman 137 from the precursor 136, by way of potassium t-butoxide293 (also see ref 294). The authors confirmed that the process was most likely going through an initial isomerization because they were additionally able to synthesize 139 from the styrene-derivative 138 in which the double bond was isomerized. Some time later, de Koning and co-workers used this approach to synthesize the transdimethylisochromanes 141 and 143, from the precursors 140 and 142, respectively. These compounds were then converted Q


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gin.305 In their work, allylnaphthalenes 136 and 154 were readily converted into their conjugated alkene analogues 155a,b with potassium tert-butoxide in dry tetrahydrofuran as solvent. The first of these, 155a, was then converted into 1,3dimethylpentalongin 156 by way of an oxidative demethylation, followed by an oxidative cyclization and a final dehydrolysis with oxalic acid, which afforded 156 in moderate yield (Scheme 34). In related work, the same researchers converted 155b into

Scheme 32

Scheme 34

equiv of potassium tert-butoxide at 0 °C, to afford the substituted styrene derivative 152.304 When the reaction was performed with 2 equiv of potassium tert-butoxide in tetrahydrofuran at reflux, the cyclized product 153 was obtained as a mixture of cis- and trans-diastereoisomers (29% and 50%, respectively) (Scheme 33). The authors proposed that products 153 were formed by what was described as an “intramolecular carbanion-olefinic 6-endo-trig cyclization”. De Kimpe and co-workers used a strategy similar to that described above in their synthesis of analogues of two naphthoquinone antibiotics, viz., psychorubin and pentalon-

the vinyl ether and ring-closed the diene in a metathetic manner with the first generation Grubbs catalyst GI to afford the reduced and protected form of pentalogin 157.306 In a similar vein, a synthesis of substituted coumarins, using potassium tert-butoxide-mediated isomerization and RCM as key steps, has been described by De Kimpe and co-workers.307 In this work, the 2-allylphenols 158 were isomerized with 4 equiv of potassium tert-butoxide in tetrahydrofuran to give the corresponding isomeric analogues 159, followed by acylation with acryloyl chloride in triethylamine to afford the dienes 160. For an alternative synthesis of 2-(prop-1-en-1-yl)phenols, see refs 308 and 309. These dienes, viz., 160, were then ring-closed by application of the second generation Grubbs catalyst GII to afford a library of substituted coumarins 161 (Scheme 35). Another “metathesis-related” isomerization strategy was used by van Otterlo and co-workers to synthesize a number of indenones, indenols, and indanones.310,311 Scheme 36 demonstrates how potassium tert-butoxide was used to isomerize the substituted arylallyl compounds 163, obtained after Grignard reaction between the appropriate vinylmagnesium species and aromatic aldehydes 162, to afford dienol compounds 164. These were then treated with the Grubbs second generation catalyst GII to afford a number of substituted indenols 165 or, under harsher conditions, the related indenones 166. In a related paper, Beau and co-workers made use of a potassium tert-butoxide isomerization during their RCM-mediated syn-

Scheme 33

R


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Scheme 35

Scheme 37

Scheme 36

houdt and co-workers were able to obtain the unisomerized 2(1-phenylallyl)phenol scaffold 172, albeit in low yield, when they performed a related Claisen rearrangement on 171 in N,Ndimethylaniline under reflux.316 In addition, when potassium tert-butoxide was used to isomerize substrate 173, only moderate amounts of the desired isomer 174 were obtained, in addition to appreciable amounts of the trisubstituted naphthol 175 (this approach has been used before to generate the naphthalene skeleton,313,317 and recently a related palladium-mediated Wacker-type approach to naphthols starting from allylbenzaldehydes also resulted in isomerized substrate under the basic conditions used by the researchers318). A recent, related synthesis of fluorinated naphthols by Magauer and co-workers makes use of the treatment of substituted 2-allyl-3-(trifluoromethyl)-phenols with potassium tert-butoxide in sulfolane (7 equiv, 120 °C, 7 h, yields 24− 56%).319 This work has the advantage that it did not involve transition metals in the synthesis of the difficult to obtain fluoronaphthols, and it should be noted that the 1propenylbenzene intermediates were frequently obtained as side-products. Substrate 173 also proved resistant to the catalyst [RuClH(CO)(PPh3)3], used in other related work (see later sections on this catalyst), demonstrating an important point that substituted allyl groups may frequently be problem-

thesis of indenols on their way to A-ring aromatic strigolactone analogues.312 Related work that involved the use of a thermal Claisen rearrangement to generate the arylallyl functionality on a substrate with the cinnamyl group, viz., 167 → 168, resulted in the spontaneous isomerization of the terminal allyl group in 168 to the thermodynamically more favored alkene 169 under these conditions (Scheme 37).310,313−315 Alkene 169 was then converted into the bis-alkene 170 via a Grignard reaction as described in Scheme 36. On an interesting aside, BezuidenS


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atic to isomerize, particularly in the presence of certain groups (Scheme 37). In this particular case, the benzaldehyde carbonyl group can supposedly react (as with the potassium tert-butoxide reaction) or coordinate with a transition metal catalyst (as with the Ru-mediated attempt) to retard isomerization. In another example that demonstrates the ability of potassium tert-butoxide to chemoselectively discriminate between different allyl groups, van Otterlo and co-workers employed this reagent on bis-alkene 176 that resulted in only the arylallyl group being isomerized, affording the bis-alkene 177.320,321 Subsequent application of the Grubbs second generation catalyst GII then readily afforded the 1,3dihydrobenzo[c]oxepine system 178 (Scheme 38).

Scheme 40

used, indicating that the base can be used in substoichiometric amounts. ́ Rodriguez-Garci á and co-workers used sodium tert-butoxide for the isomerization of 2-allyl-4-methoxyphenol 184 into 4methoxy-2-(prop-1-enyl)phenol 185 during the total synthesis of a number of cis- and trans-pterocarpans 188,324−326 suggesting that the sodium counterion makes little difference (Scheme 41). The pterocarpans synthesized were of interest

Scheme 38

Scheme 41

Beau and co-workers also used a potassium tert-butoxide isomerization during their synthesis of the plant hormone (−)-solanocol 181.322 These researchers performed the isomerization with potassium tert-butoxide in dry tetrahydrofuran on the allylbenzene 179 to obtain 180 in 90% yield (Scheme 39). This compound was then extrapolated into the desired natural product 181 by way of a number of further synthetic steps.

due to reports that they possess a wide range of biological activities, which include antitumoral, anti-HIV, antimalarial, and, apparently, even potency against snake venom. A critical ́ part of the Rodriguez-Garci á work involved generation of the silicon-containing benzo-fused compound 187, which was generated from the silyloxydiene 186 (see ref 327 for a related synthesis of silacycles by way of RCM). Extending their application of potassium tert-butoxide toward the synthesis of small aromatic-core molecules of interest, Schühly and co-workers employed this strategy for the chemoselective “mono” isomerization of 4′-O-methylhonokiol 189, during an investigation into the cyclooxygenase activity (COX) of anti-inflammatory biphenyl neolignan derivatives.328 The isomerization protocol resulted in a mixture of E- and Zisomers 190 and 191 (Scheme 42, ratio not reported), which were separated by preparative HPLC. Demethylation of 190 then afforded honokiol analogue 192. Both isomers 190 and 191 demonstrated interesting activity against the isoenzymes COX-1 and COX-2, while 192 additionally inhibited leukotriene (LTB4) formation. Two years later, Hering and co-workers used 4′-O-methylhonokiol 189 to synthesize a further set of derivatives to be tested as γ-aminobutyric acid (GABA) modulators.329 Again, treatment of 189 with potassium tert-butoxide in tetrahydrofuran generated isomerized isomers 190/191 in which only one of the allyl groups had been isomerized.

Scheme 39

During the structural investigation of the yellow-green fluorescent fungal pigment leprocybin, by Steglich and coworkers, a potassium tert-butoxide-mediated isomerization was performed to obtain compound 183 from the allyl precursor 182 (Scheme 40).323 The propenyl group was then converted into the carboxylic acid by way of an ozonolysis, followed by a permanganate-promoted oxidation. Of interest in this example is that only a catalytic amount of potassium tert-butoxide was T


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Scheme 42

Scheme 44

compound could best be characterized as its bis-acetyl derivative and was found to be comprised of a mixture of two compounds (E,Z isomers) in a ratio of 2:1. Unfortunately, the subsequent attempted intramolecular diphenolic oxidation reactions, required to afford the dibenzocyclooctene skeleton frequently found in natural products, were less than satisfactory. As described earlier, the allyl functional group can readily be converted into a range of useful carbonyl derivatives. This was effectively demonstrated by Sargent and co-workers who isomerized substrate 200 with potassium tert-butoxide to afford 201 as only the E isomer in good yield (Scheme 45).335

An interesting isomerization reaction making use of potassium tert-butoxide was demonstrated by Reinhoudt and co-workers in 1990.330,331 These researchers showed that this isomerization protocol worked well on a bis-allyl calix[4]arene 193 and gave isomerized calixarene 194 in an excellent yield of 99%, which was subsequently converted into the bisbenzaldehyde. This functionality was furthermore chemically modified to afford other substituted calix[4]arenes with functionalized “upper rims”. Another example involves the conversion of calix[4]arene 195 into 196 by Shinkai and coworkers.332 These researchers ozonolized the isomerized 2propenyl calixarene 196 into the corresponding aldehyde and then converted this into a fluorogenic 5-benzothiazolyl calixarene 197 (Scheme 43). Finally, for an example of

Scheme 45

Scheme 43

Subsequent osmylation, followed by periodate cleavage of the intermediate diol, afforded naphthaldehyde, which could further be converted into the desired naphthoic acid 202 with sodium chlorate (details not shown in scheme). In another example involving naphthyl frameworks, Giles et al. reported that the isomerization of 1-(2-allyl-1,4,5,7tetramethoxynaphthalen-3-yl)ethanone (R = OMe) 203b and 1-(3-allyl-1,4,8-trimethoxynaphthalen-2-yl)ethanone (R = H) 203a with potassium tert-butoxide in tetrahydrofuran gave high yields of the respective trans-isomers 204a/b (Scheme 46).336 These compounds were then used to synthesize a series of naturally occurring naphtho[2,3-c]pyran-5,10-quinones (not shown). Scheme 46 isomerization on a monoallyl calix[5]arene system, see the work of Wang and Gutsche who used this method during their synthesis of calix[5]arene-fullerene dyads (not shown).333 In another biaryl system, in which the two allyl groups are in a 6,6′-relationship, Taylor and co-workers showed that the application of in situ generated potassium tert-butoxide (from potassium in tert-butanol) to substrate 198 readily afforded the corresponding bis-styryl compound 199 (Scheme 44).334 This U


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Roth and co-workers, researchers from the Wellcome Research Laboratories, used the potassium tert-butoxidemediated isomerization method during their synthesis of a series of antibacterial 2,4-diamino-5-(benzyl)pyrimidines.337 In this work, the two allylaryl groups in 2,4-diamino-5-(3,5-diallyl4-methoxy benzyl)pyrimidine 205 were isomerized by heating the compound in the presence of potassium tert-butoxide in DMSO, to afford 2,4-diamino-5-(3,5-prop-1-enyl-4methoxybenzyl)pyrimidine 206 in which both olefinic bonds were E (Scheme 47). In a similar manner, compounds 207 and 208 were synthesized from their respective allylaromatic precursors.

Scheme 48

Scheme 47

Over the past number of years, Katzenellenbogen and coworkers have been effective in applying potassium tertbutoxide-mediated isomerizations to the synthesis of a series of small molecules that interact with the nuclear receptors, estrogen α and β. A number of these approaches are graphically illustrated in Scheme 48. In the first example, the group used a potassium tert-butoxide-mediated isomerization to convert 6allyl-2,3-dichlorophenol 209 into 2,3-dichloro-6-(prop-1-enyl)phenol 210 with an E:Z ratio of 9:1. A reaction regime then transformed this compound into the corresponding diphenylsalicylaldoxime 211 (Scheme 48). This included an oxidative cleavage of the 1-propenyl compound 210 to afford the corresponding salicylaldehyde, which was converted into the oxime under standard conditions. From the subsequent biochemical evaluation, the salicylaldoxime moiety of 211 appears to be an effective stereoelectronic replacement of the aromatic A-ring of typical estrogen ligands, as the hydroxyl group of the oxime seems to interact well with the estrogen receptor.338 The researchers employed the same concept for the isomerization of substituted allylanilines such as 212 into the corresponding 1-propenylaromatics 213 (Scheme 48).339,340 Further transformation of these compounds into 3,4-diphenylaniliniumaldoximes 214 and subsequent biochemical testing showed that the ERα estrogen binding affinity of the compounds was dependent on the substitution of the amine group with potency being in the order: N−H > N−Me > N− Et. In the synthesis and testing of another set of ERβ selective monoaryl-substituted salicylaldoximes 217, Katzenellenbogen and co-workers successfully isomerized allylaryl compound 215 into 216 under the potassium tert-butoxide-dimethyl sulfoxide conditions.341 A similar approach was used in the synthesis of a related set of salicylaldoximes 222 with excellent ERβ-binding affinities, in which substituted 2-allylphenol 220 was readily isomerized into compound 221.342 The isomerized products

218, 219, and 223 were produced in the same manner (Scheme 49). An interesting paper, describing estrogen receptor (ER) agonists and antagonists, indicates how the group was able to perform the potassium tert-butoxide-induced isomerizations on compounds comprised of a larger skeleton. For instance, the isomerization of compounds 224a,b into 225a,b occurred without any reported problems, and the latter compounds were readily transformed into the estrogen mimics 226a,b (Scheme 50).343 Of particular interest was the observation that functional groups, as well as their position on the phenyl rings, strongly perturbated the binding affinities of the synthesized compounds toward ERα and ERβ. For a related study also starting from 5-bromo-2-(prop-1-en-1-yl)phenol, see ref 344. In this section involving medicinal chemistry, the Rapposelli research group also employed potassium tert-butoxide in the isomerization of 2-allyl-3-bromophenol 227 to afford 3-bromo2-(prop-1-enyl)phenol 228. This compound was further transformed through a number of steps into carbaldehyde oxime 229 (Scheme 51), which showed a modest affinity toward the AT1 receptor of Angiotensin II and as such represented a new sartan that could form the basis of a novel set of antihypertensive drugs.345 Flitsch and Langer used potassium tert-butoxide in the isomerization of substituted allyl phenol 230 into compound 231.346 This compound then readily led to the mitosene analogues 232 after a number of synthetic steps (Scheme 52). Carrying on the theme of isomerization of allylaniline compounds described earlier in this Review, the next two examples should be noted. First, Fujiwara et al. isomerized the diphenyl thioacetal 233 using potassium tert-butoxide in DMSO. The isomerized analogue 234, predominantly obtained as the E-isomer, was cyclized into the respective 2,3-dihydroV


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Scheme 49

Scheme 51

Scheme 52

1H-benzo[b]azepine 235 by way of a titanocene(II)-promoted ring-closing metathesis process (Scheme 53).347 Scheme 53

Scheme 50

In the second example, de Mayo and co-workers employed potassium tert-butoxide to isomerize 2-allylaniline 236 into both the E- and the Z-2-(prop-1-enyl)anilines, which were isolated separately as their respective acetamide derivatives 237.348 These compounds were then converted into the corresponding thioamides 238 (only shown for major E isomer in Scheme 54). The E-compound 238 was then used in a photochemical investigation toward the neat synthesis of 2,3dimethylquinoline 239. The allylbenzene isomerization strategy has been successfully applied to the synthesis of compounds with macrocyclic structures of importance in the study of ligand−metal interactions. The following examples highlight how the allyl W


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either 1,2-diaminobenzene or 1,2-ethanediamine to afford the macrocycles 247 or 248 respectively (Scheme 56). In subsequent experiments, these macrocycles were found to be able to bind two divalent transition metal cations with overall electronic neutrality.

Scheme 54

Scheme 56

isomerization−oxidative cleavage strategy has facilitated the construction of enticing macrocycles. In the first example, Moneta et al. reported the isomerization of the diallyl derivative 240 using potassium tert-butoxide in dimethyl sulfoxide with heating (Scheme 55).349 Compound 241 was obtained in good Scheme 55

In much a similar vein, Lagrenée and co-workers used potassium tert-butoxide for the isomerization of 6,6′-(1,3,4thiadiazole-2,5-diyl)bis(2-allylphenol) 249 into 6,6′-(1,3,4thiadiazole-2,5-diyl)bis[2-(prop-1-en-1-yl)phenol] 250. This transformation was required during the synthesis of a thiadiazole-based macrocycle 252.353 Of interest was that after the polyether bridge had been formed, viz., 251, the two 1propenyl groups were transformed into carboxylic acid functional groups by the action of the strong oxidant, potassium permanganate (Scheme 57). This last step is of note as the direct transformation of an isomerized allyl group into a carboxylic acid has not seen extensive use in the synthetic community.354−358 For an example of a lithium cation selective “polyether”-based carrier with an isomerized allyl group, see the following work by Hiratani and co-workers.359 Reinhoudt and co-workers applied the potassium tertbutoxide isomerization method for conversion of 2,6-bis[2methoxy-5-methyl-3-(2-propenyl)phenyl]pyridines 253 into the corresponding pyridines 254 (Scheme 58).360 These compounds were then transformed into their corresponding pyrido-hemispherands 255 in a few synthetic steps. This particular class of ligands was subsequently shown to complex well with smaller alkali cations. From the many examples in the literature, it is clear that the method of using potassium tert-butoxide in an organic solvent at an elevated temperature is broadly applicable to many organic substrates. It is thus of interest to this particular section that Loupy and Thach have studied the effect of microwave

yield (78%) and then used in the synthesis of tetraazacyclophane 242 (for related work by the same researchers, see ref 350). In a related approach, Baret and co-workers reported the isomerization of 3-allyl-[1,1′-biphenyl]-2,2′-diol 243 with potassium tert-butoxide in DMSO, to afford the isomerized compound 244.351 This compound was subsequently used for the synthesis of hexamethoxylated tripodands as potential iron sequestering sidophores. Perez and Bermejo used the same potassium tert-butoxide isomerization method for the synthesis of two multidentate 1,3,4-oxadiazole-imine- and phenol-containing macrocycles.352 The isomerization step, employing a large excess of tertpotassium butoxide, resulted in compound 246 being obtained from 245 in a reasonable yield of 67%, and was followed by ozonolytic cleavage to the corresponding bis-aldehyde. Ringclosure was subsequently performed by imine formation with X


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4.3. Bases Used Less Frequently for Allybenzene Isomerizations

Scheme 57

4.3.1. n-Butyllithium. n-Butyllithium has found limited application in the isomerization of arylallyl substrates (for indepth studies on the “protonation and alkylation of 1arylpropenyl-lithium” species, see refs 361 and 362). In fact, it has mainly been noted that isomerization occurs during the application of this base to allylbenzene substrates in multistep one-pot reactions. For instance, Beller and co-workers observed that catalytic amounts of n-butyllithium promoted the fast isomerization of allylbenzene 3 into the 1-propenyl derivative 4.363 This was followed by a regioselective hydroamination with piperidine (Scheme 60), or a range of other amines including Scheme 60

Scheme 58 morpholine, benzylamine, n-butylamine, and aniline, to afford compounds such as 256. The authors demonstrated that a range of primary and secondary amines could be used in this domino-isomerization-hydroamination protocol to provide a new route to various amphetamines. van Otterlo and co-workers modified the Beller approach to provide 3-methyl- and 3,4-dimethyl-1,2,3,4-tetrahydroisoquinolines 258 from precursors 257, by way of an intramolecular hydroamination promoted by substoichiometric amounts of nbutyllithium.364,365 To prove that an isomerization was critical to the success of the reaction, compound 259 could readily be isolated after treatment of 257a with n-butyllithium at room temperature, which was subsequently converted into the corresponding tetrahydroisoquinoline 258a with n-butyllithium at higher temperatures (Scheme 61). For an interesting aside, involving the synthesis of 1,2,3,4-tetrahydroisoquinolines from ortho-allyl arylaziridines using sec-butyl lithium in a flowmicroreactor system, see the work by Yoshida, Luisi, and coworkers.366 These researchers showed that the basic conditions used were thus able to isomerize the allyl functional group by way of a deuterium experiment. Both of the examples in this section highlight a number of important points regarding the use of n-butyllithium as an isomerization catalyst: (a) that the reactions occur at room temperature, while many other bases are used at higher temperatures, and (b) that the n-butyllithium can be applied in substoichiometric amounts, in contrast to many other bases that require at least 1 equiv of base. Both of these aspects are of considerable interest and should provide motivation for more investigations into this seldom-used method. 4.3.2. Phosphazene Bases. Less traditional bases, such as the so-called phosphazene bases, have also been used for isomerization of allylaromatic compounds. For example, in 2004 Wu and Verkade reported the isomerization of a number of allylbenzene derivatives with the strong phosphazene base, EtNP(NMe2)2NP(NMe2)3

heating on the efficacy of the potassium tert-butoxide isomerizations.226 For instance, it was found that under microwave heating conditions, eugenol 5 readily isomerized into isoeugenol 18 (Scheme 59), when potassium tert-butoxide Scheme 59

and a catalytic amount of phase transfer agent were used in the absence of solvent. According to the authors, this particular method constitutes an inexpensive and more environmentally friendly method for the large-scale isomerization of eugenol 5, and is thus all the more significant because solvent was not required. These researchers also applied their method to safrole 6 and were able to obtain isosafrole 19 in 99% yield on a 10 g scale (E:Z = 95:5).227 Y


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tetramethylguanidine (TMG) and DBU for the same types of isomerization, but were unable to generate conjugated products under the conditions attempted. Proazaphosphatranes, such as compound 261 [P(RNCH2CH2)3N with R = i-Pr, Me], represent an alternative type of base that has been reported for the isomerization of allylbenzenes.367 He and co-workers mentioned that advantages of this base include good solubility, even in less polar organic solvents and the reasonably low temperature required to facilitate the isomerization (40 °C). In addition, the catalysts resulted in high yields of isomerized product with good Eselectivity being obtained, as can be seen for the set of allylaromatics shown in Figure 7. However, the bases did not work well with 2-allylphenol, which required longer times and higher than stoichiometric base ratios to afford the desired isomerized product in generally poor yield. Later this group demonstrated that disubstituted proazaphosphatranes were even more effective isomerization catalysts.368 These researchers were able to obtain the desired isomerized products in isolated yields greater than 95% and with very good E:Z ratios. 4.3.3. 1,8-Diazabicyclo[5,4,0]undec-7-ane (DBU). Sterically encumbered and non-nucleophilic bases such as 1,8diazabicyclo[5,4,0]undec-7-ane (DBU) have had limited use in the area of allylbenzene isomerizations. Occasionally, this base has been responsible for unwanted allyl isomerizations, for instance, the DBU-mediated Horner−Wadsworth−Emmons chain extension of aldehyde 262 into 263 (Scheme 62), which resulted in significant amounts of the undesired 264.369 This problem was circumvented by the use of less base in the reaction procedure.

Scheme 61

260, in acetonitrile at 40 °C.130 As the results in Figure 7 show, the products were all obtained in good yield using 10 mol % of

Scheme 62

Omura and co-workers were the first researchers to synthesize bioactive louisianin C 266 through a sequence of synthetic steps from louisianin A 265 (Scheme 63).370 These researchers then employed DBU as base for the isomerization of the allyl moiety found on louisianin C 266, to afford louisianin D 267 in a good yield of 84%. In a later synthesis of louisianin D 267, Taylor and co-workers used 1.2 equiv of methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene (MTBD) for the same transformation, in toluene at 80 °C, to afford the desired isomerized compound 267 in a good yield of 78%.371,372 Surprisingly, the use of polymer-bound MTBD on the same substrate only gave the desired isomerized product in 10% yield, along with extensive decomposition of the substrate. Also

Figure 7.

260, with the thermodynamically trans isomers being favored as expected. Of interest was that acetonitrile turned out to be the best solvent, and the authors postulated that the base 260 (with a pKa of 32.7) deprotonates the acetonitrile to form the acetonitrile anion, which is then likely the active isomerization agent. The researchers also tried using the bases 1,1,3,3Z


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Scheme 63

Scheme 65

of interest, with respect to this set of natural products, is that Chen et al. used potassium tert-butoxide as base to isomerize the allyl group introduced in 268 by replacement of the tosyl group, in their approach to compound 269, which was eventually converted into louisianin D 267.373 Finally, Chang and co-workers performed an innovative onepot allylation−isomerization reaction to afford louisianin D 267. In their synthesis, the allyl moiety was isomerized into the propenyl derivative by DBU after the Stille coupling of 4bromo-6,7-dihydro-5H-cyclopenta[c]pyridin-5-one 270 with allyltri-n-butyltin (Scheme 64). Of interest was that when the DBU was omitted from the reaction mixture, louisianin C 266 was obtained in good yield, instead of louisianin D 267.374

caused them to postulate that it was the crown ether−sodium hydride complex that was responsible for the isomerization as it is known that this combination gives rise to a very strong base.376 It would thus be of interest to discover whether this combination of reagents would be of use for the isomerization of other allylaromatics. Finally, it was found that if cesium fluoride was used instead of the sodium hydide, compound 271 could be obtained without any contamination with allyl isomers.377 In a different type of synthetic approach involving allylaryl isomerization induced by the base sodium hydride, Chang and co-workers recently described an innovative one-pot synthesis of tetrahydrocyclobuta[a]naphthalenes, which required a “domino aldol condensation/olefin migration/electrocyclization”.378 Their reaction made use of substituted 2-allylbenzaldehydes such as 273 and cinnamyl sulfone 274 (Scheme 66). Under action by the base sodium hydride, it was postulated that the isolatable intermediate 275 was formed involving an aldol condensation as well as a base-mediated isomerization of the allyl functional group. This intermediate was ideal for a conrotatory 8π-electrocyclization to afford intermediate 276, followed by a disrotatory 6π-electrocyclic ring closure to obtain the highly substituted tetrahydrocyclobuta[a]naphthalene 277 in very good yields (75−87%). As an aside, it should be noted that the same research group also investigated the ammonium acetate-promoted cyclocondensations of 3-(o-allylphenyl)pentane-1,5-diones 279, this time synthesized by a sodium hydroxide-mediated aldol condensation involving substituted 2allylbenzaldehydes 273 and acetophenones 278, and obtained most interesting tetracyclic benzo-fused tricyclo[5.3.1.03,8]undecane structures such as 280.379 Use of an excess sodium hydroxide in the condensation between allylbenzaldehyde 281 and acetophenone 282 resulted in the concomitant isomerization of the allyl group to afford dione 283. Again, treatment of this substance with ammonium acetate gave the analogous ring-contracted 1-azahomoisotwistane molecule 284 in excellent yield by way of a cascade cyclocondensation (Scheme 66). It should be stressed that both of these synthetic examples highlight how the incorporation of allylbenzene isomerization into complex synthetic cascades can give rise to important chemical structures; this area thus certainly deserves further investigation.

Scheme 64

4.3.4. Sodium Hydride. Sodium hydride has seen little application in terms of allylbenzene isomerizations. One example of note appears in the work by Reinhoudt and coworkers who found that when making the crown ether 271 from 3,3′-diallyl-[1,1′-biphenyl]-2,2′-diol 240, the resultant macrocycle was always contaminated with isomeric compounds due to isomerization of the allyl groups.375 Furthermore, treatment of the contaminated diallyl crown ether 271 with additional sodium hydride in tetrahydrofuran then resulted in the related crown ether 272 in which all alkene moieties were now conjugated to the aromatic backbone (Scheme 65). The authors observed that the reaction of the related 3,3′-diallyl[1,1′-biphenyl]-2,2′-diol 240 with excess sodium hydride did not give any of the corresponding isomerized product. This AA


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obtained in excellent yield (Scheme 67). The researchers of this work pointed out that SEDs like 285 have been known to

Scheme 66

Scheme 67

act as bases, probably by forming the protonated species 286. It should be noted that this isomerization occurred under mild conditions (RT) and that these reagents could be the focus of more research for the isomerization of sensitive allylbenzene substrates. 4.4. Solid Bases as Isomerization Catalysts

In recent years, use of solid base catalysts for the isomerization of allylbenzenes has become increasingly important. With simple substituted (1-propen-1-yl)benzenes being very important in the fragrance and flavor industry, it comes as no surprise that large-scale isomerizations over solid basic catalysts would be industrially advantageous, particularly in comparison to the present day liquid caustic hydroxide procedures. For instance, use of aqueous potassium hydroxide has some serious drawbacks, which include the need for intensive separation and workup procedures, as well as the storage and disposal of such a corrosive base. The effective and strategic use of solid catalysts would hope to minimize such disadvantages, and, interestingly, advantages of solid base systems, particularly relevant for industrial processes, have been described in review form by Hattori,384,385 Baba,386,387 and others.388−390 The advantages listed by these authors include the fact that solid base catalysts are less likely to interact with substrate heteroatoms and that they show a lower propensity to mediate C−C bond cleavage, which is an often encountered problem during comparative acid-mediated processes. It should be realized that these catalyst systems have seen industrial use, an example being the application of a heterogeneous Na/NaOH/γ-Al2O3 catalyst system for the isomerization of safrole 6 into isosafrole 19 by the Japanese company Sumitomo Chemicals.391,392 The application of hydrotalcites as heterogeneous catalysts has enjoyed significant advancements in the past decade, and arguably could be seen as having had the most interest in terms of solid bases.393 Hydrotalcites are defined as the doublelayered hydroxides of magnesium with the general formula Mg6Al2(OH)16CO3·4H2O. In addition, numerous variants of the hydrotalcites have been synthesized, including Zn−Al hydrotalcites, and the generation of these compounds with reducible bivalent and trivalent cations has resulted in hydrotalcite oxides with a wide reactivity spectrum. A significant advantage of these solid bases is that their basicity can be moderated by the specific utilization of additives, and thus, unsurprisingly, these materials have also been used for the isomerization of allylbenzenes as described in the following examples.

4.3.5. Metal−Ammonia. It should be realized that metal− ammonia reduction protocols have been used for the isomerization of allylbenzenes. Examples include the use of lithium380 and sodium381 amides in this regard. A later study involved the application of potassium amide in ammonia, followed by protonation that afforded 1-propenylbenzene in good yields (>90%).382 4.3.6. Organic “Super-Electron-Donor” as Base. The last example in this section involves a rather unusual reagent, which facilitates allylaryl isomerization. Murphy and co-workers were investigating the use of “organic neutral super-electrondonors (SED)” such as 285 for the conversion of aliphatic and aryl triflate esters into their corresponding alcohols and phenols, respectively.383 Fairly unexpectedly, when 2-allylphenyl trifluoromethanesulfonate 287 was treated with the SED reducing agent 285, 2-(prop-1-en-1-yl)phenol 159e was AB


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zeolites comprising Cs−X (60), Rb−X (64), K−X (80), and Na−X (100), as well as impregnated KOH on alumina. Of interest was that all of these catalysts proved to be adept at facilitating the conversion of 7 into 20 (conversions ranging from 61% to 98%) with the order of results by the alkali ionexchanged zeolites following their basicity order (Cs−X > Rb− X > K−X > Na−X). Jasra and co-workers also used rutheniumcontaining hydrotalcite solids (Ru−Mg−Al) for the isomerization of allylbenzenes, viz., estragole 7, eugenol 5, safrole 6, and allylbenzene 3, this time under solvent-free condition.404 These catalysts showed excellent catalytic activity with reasonable reaction times (2 h), but again at quite high temperatures (∼210 °C). The yields were good (92−98%), with general trans selectivity being observed (>88% trans). In addition, the researchers proved that the catalyst systems could be reused four times without significant loss in activity. As an aside, these researchers discovered that isomerization was an unwanted side-reaction, during treatment of allylbenzene under hydroformylation conditions promoted by the catalyst system, HRh(CO)(TPPTS)3-hydrotalcite [TPPTS = tris(3-sodium sulfonatophenyl) phosphine].405 The group of Urbano and Marinas used allylbenzene 3 as a test substrate to evaluate the molar conversion per gram of catalyst in comparison to the basic site density. This was performed by directly measuring the isomerization ability of a number of magnesium-containing mixed oxide solid base catalysts. In the researcher’s investigations, it was found that the solid Mg−Ti and Mg−Zr mixed oxide catalysts gave good molar conversions when applied in the transformation of allylbenzene 3 into 1-phenyl-1-propene 4.406,407 Very recently, Sobczak et al. examined the isomerization of safrole 6 by way of amino-grafted mesoporous molecular sieve materials as basic catalysts.408 These amino-grafted mesoporous materials were based on the silicate material MCM-41 and included derivatives with aluminum and niobium. Onto these parent materials were readily grafted three different amino functionalized chains, 3-aminopropyl-trimethoxysilanes (APMS), [3-(2-aminoethylamino)propyl]trimethoxysilane (2APMS), and 3-[2-(2-aminoethylamino)ethylamino]propyltrimethoxysilane (3APMS), which were subsequently used for the isomerization studies. In these reactions, isomerization of safrole 6 was shown to depend on the nature of the support onto which the amine chains had been grafted and produced the following order of activity: APMS/Al-MCM41 > APMS/Nb-MCM-41 ≫ APMS/MCM-41. The nature of the amine chain also appeared to play an important role for the production of isosafrole 19 as the best conversion was observed in the order of APMS/Al-MCM-41 > 2APMS/Al-MCM-41 > 3APMS/Al-MCM-41. The researchers also demonstrated that with an increase in polarity of the solvents used, conversion increased, with DMF being the best solvent for isomerization resulting in yields of 85% (with cis/trans ratio = 1:9). Again, just as in the case of Kannan and Kishore, conversions increased ́ with higher temperatures. Martin-Aranda and Ziolek expanded the study on these materials to include isomerization of eugenol 5, this time under thermal and ultrasound activation.409 After investigating a number of modified basic mesoporous molecular sieve catalysts including Li, Na, K, Rb, and Cs-modified NbMCM-41 materials, the research team found that rubidiummodified Nb-MCM-41 was the best catalyst under the conditions employed (yields ∼90% for sonochemical activation and ∼72% for thermal conditions, and for both the cis/trans ratio was ∼1:9). Finally, the researchers were able to

The Kannan group has been particularly active in the application of hydrotalcites and modified versions thereof to the isomerization of allylbenzenes.394 In 2002, this group found that solid hydrotalcite bases could effectively isomerize eugenol 5 and safrole 6.395 The researchers proved a 4:6 ratio of Mg:Al to be the best in terms of the hydrotalcite composition, providing isomerization catalysts that were able to deliver isomerized products in yields of 73−75%, and having cis:trans ratios of 15:85. It should be noted that high temperatures (160−200 °C) were necessary to give satisfactory conversions of the substrates used. Later, Kishore and Kannan found that hydrotalcites were also effective isomerization catalysts for the conversion of safrole 6 into isosafrole 19.396 The authors found that Mg−Al hydrotalcites (Mg:Al = 6:1) promoted safrole 6 isomerizations with yields as high as 98%, and with a cis:trans ratio of 1:9 (Scheme 68). The mechanism of the isomerization was postulated to involve deprotonation of the acidic ArCH2 proton by the Brønsted basic hydroxyl groups of the hydrotalcite lattice. Scheme 68

Kishore and Kannan next performed a detailed and systematic study on the isomerization of estragole 7 over various synthesized bimetallic M(II)M(III) hydrotalcite-like heterogeneous solid base catalysts, where M(II) = Mg, Ni, Co, Cu, and Zn and M(III) = Al, Fe, La, V, and Cr, with carbonate as the interlayer anions and with varying M(II)M(III) atomic combinations.397,398 In the series of binary hydrotalcites synthesized, use of Mg2+ (up to 99% conversion) and Ni2+ (87% conversion) was shown to be the most suited for the estragole 7 isomerization, whereas the other bivalent transition metal ions showed no activity at a reactor temperature of 200 °C. In the case of trivalent transition metal ions, Al3+ displayed the best activity, followed by Fe3+ and Cr3+ transition metal ions. In the same study, it was found that the latter incorporation of ruthenium or cesium onto the hydrotalcites also gave good isomerization systems. It was further found that the solvent played an important role in isomerization yields, with highly polar solvents, such as DMF and DMSO, giving better results than THF. The catalyst loadings used in the experiments were reasonable, with a mass ratio of 10:1 being used. Finally, the researchers were able to demonstrate that their hydrotalcite catalyst systems were reusable with up to three runs being performed with the same catalyst with no observable deterioration in the yields. Further research by these researchers has included magnesium399-, nickel398-, and copper400,401-containing hydrotalcites, as well as studies that demonstrated that the allylaromatic isomerizations could readily occur under milder microwave heating (140 °C instead of 200 °C, and reaction times of 90 min).402 In 2003, Jasra and co-workers reported their efforts for the production of 1-methoxy-4-(1-propen-1-yl)benzene (transanethole) 20 from estragole 7 using hydrotalcite for the isomerization process. In the same paper, the authors reported on the application of zeolites and basic alumina for the same isomerization.403 In this work, estragole 7 was treated with the following catalysts: Mg−Al hydrotalcite, alkali ion-exchanged AC


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demonstrate the recyclability of the mesoporous molecular sieve materials as they could be reused up to four times without a noticeable decrease in activity. In 2005, Smirnov and co-workers reported the isomerization of allylbenzene 3 into trans-β-methylstyrene 4 using magnesium−anthracene cluster adducts at room temperature,410,411 based on earlier observations.410,411 The application of a magnesium allylbenzene adduct, forming organomagnesium clusters by the simultaneous precipitation of magnesium and allylbenzene vapors at low temperature (liquid nitrogen), was also investigated. Of interest was that the latter method involved a solid-phase isomerization reaction, and that, although these systems were rather unstable, their high activity made this approach an interesting isomerization option. Later, the authors investigated a related isomerization, involving organopolymagnesium hydrides, at a computational level, and were able to conclude that the isomerization mechanism involved initial insertion of a tetranuclear magnesium cluster at a C−H bond in the allylbenzene methylene group.412 The authors were able to explain the observation that cluster decomposition competes with the isomerization, at least on a computational level, and used these data to justify the relatively short activity lifetimes of the magnesium cluster hydride isomerization catalysts. For another related study, involving diethylaminomagnesium hydride and which also involved the isomerization of allylbenzene, see ref 413. Alumina, and modified versions thereof, has been successfully used for isomerization of allylbenzenes. For example, Aristoff and co-workers found that the straightforward elution of compound 287 through a column of basic alumina (Woelm, activity I, eluent 1% ethyl acetate in hexane) afforded the isomerized substrate 288 in almost quantitative yield (Scheme 69).414 This approach has definite potential because the

Figure 8.

corresponding conjugated analogues 292, followed by an oxidative cleavage facilitated by potassium permanganate− copper sulfate to afford the corresponding substituted benzaldehydes 293 by way of the intermediate 1-propenyl-1benzenes 292 (Scheme 70). The isomerization step was Scheme 70

Scheme 69

achieved by the use of solid KF/alumina under solvent-free microwave heating conditions and in relatively short reaction times ( meta > ortho, and that this order was reversed for methoxy and chloro substituents. The researchers also postulated a mechanism for the isomerization reaction based on cationic intermediates, which were thought to be stabilized by the HFIP present in the reaction mixture. In addition, steric and electronic effects were invoked to explain the order of reactivities for the substituted allylbenzene substrates. Thiery and coworkers reported that the Pd(OCOCF3)2/ [(HOCH2CH2NHCOCH2)2NCH2]2 387-catalyzed oxidation of 2-allylphenol 158e with hydrogen peroxide in a water/ methanol solution afforded a syn and anti mixture of 2-(1,2dihydroxypropyl)-phenol 389 and 2-(2-hydroxy-1methoxypropyl)phenol 390.505,506 Mechanistic experiments and ESI−MS studies supported a pathway wherein isomerization of the CC bond (to give 159e), followed by epoxidation to afford 388, with subsequent oxirane opening, gave rise to the products 389 and 390 (Scheme 92). The authors pointed out the importance of the o-phenoxy group, as applying these oxidation conditions on allylbenzene 2 or 1-allyl-

Scheme 93

Scheme 92

illustrates a wide range of examples of the isomerized compounds obtained in excellent yields and highlights the functional group tolerance of the method. An advantage of the method, as pointed out by the researchers, was the very low catalyst loadings required (0.25−1.0 mol %). In terms of the catalyst system itself, the researchers showed that the steric bulk of the P(tBu)3 ligand was critical. This Lindhardt−Skrydstrup catalytic system, as it is sometimes referred to, also demonstrated a selectivity for the allylbenzene group in the presence of an allyloxy olefin.511 This is illustrated in Scheme 94 in which the reaction conditions resulted in a 95% conversion of the diene 391 into the allylAM


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annulation reaction. In a number of further steps, this scaffold could be extrapolated into the complex natural product family, the trioxacarcins, of which trioxacarcin A 400 is shown in Scheme 96. For earlier, related work from this group also involving a potassium tert-butoxide in DMSO isomerization, see ref 513.

Scheme 94

Scheme 96

isomerized 392 with a minor amount of the di-isomerized analogue 393. The researchers also investigated the identity of the actual catalyst involved in the isomerizations and observed that the reaction was particularly mild in that it did not interfere with activated aryl triflates, as noted for conversion of a mixture of the triflates 394 and 395 into the pure isomerized 395. The authors therefore found it unlikely that any Pd(0) species were part of the catalytic cycle. On the basis of their mechanistic investigations, Lindhardt and Skrydstrup were also able to improve their precatalyst system by using the alternative phosphine ligand cataCXium PinCy, although it should be noted that this system was not used on allylbenzene substrates.511 Finally, as a result of their studies, they were able to demonstrate that the palladium hydride [P(tBu)3]2PdHCl was also efficient in the isomerization of safrole 6 into 19, resulting in turnovernumbers (TON) of over 10 000 (Scheme 95).

In their synthesis of azabicycles, Lete and co-workers observed some interesting results involving the isomerization of allyl groups during allylation reactions on 2-iodoaniline 401.514 In their work, 2-iodoaniline 401 was converted into 2propenyl aniline 403 with allyltrimethylsilane under Heck conditions using Pd(dba)2/PPh3/nBu4NOAc in DMF at 50 °C. Under these conditions, the isomerized E-2-(prop-1-en-1yl)aniline 403 was obtained in moderate yield (56%) by way of 402. When the reaction was repeated under modified conditions [allylSnBu3, Pd(dba)2/PPh3/toluene (reflux)], a mixture of the allylaromatic 402 (32%) and isomerized compound 403 (26%) was obtained. In addition, when the aniline group was protected as the methyl carbamate, isomerization of the allyl group was not observed. Aniline 403 was then converted into the dienes 404, after which RCM afforded the desired azabicycles 405 in poor to reasonable yields (Scheme 97). On the topic of “Heck” reaction conditions, Zhou and coworkers recently published work detailing the use of a Mizoroki−Heck reaction on cyclic olefins to afford products that demonstrated the excellent vinylic selectivity.515 Optimized conditions involved the use of the ligand 1,3-bis(diphenylphosphino)propane (dppp) and a palladium source, Pd(hfacac)2, for the condensation between phenyl triflate 406

Scheme 95

In an elegant synthetic journey to the trioxacarcins and their synthetic analogues, Myers and co-workers made use of Lindhardt and Skrydstrup’s isomerization conditions to convert allyl-substituted amide 396 into the respective 1-propenyl derivative 397 after a phenol protection step [elsewhere in the Supporting Information file of their work, a potassium tertbutoxide isomerization of 3-allyl-4-(benzyloxy)-N,N-diethyl-2(methoxymethoxy)benzamide was also used].512 MOMprotected amide 397 was subsequently converted into the MOM-protected cyanophthalide 398, which afforded the highly substituted naphthalene core 399 after a Hauser-type AN


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as the major product, in contrast to only a moderate amount of the desired 1-phenylpropan-2-one 410 (Scheme 99).

Scheme 97

Scheme 99

In an in-depth study in 2012, Trapp and co-workers successfully employed a number of bidentate 3,3′-bipyrazole palladium complexes (see, for instance, complex 411 shown in Scheme 100), for the isomerization of a variety of allylbenzenes, Scheme 100 and cyclohexene 407 to give the desired product 409 with 23:1 vinylic selectivity with respect to isomer 409. The authors of this work were able to show that the Heck reaction first produces the nonconjugated 3-arylhexene 408 (the alkylpalladium intermediate cannot readily eliminate to form the 1-arylcyclohexene 409 directly as the benzylic hydrogen is anti to the palladium). This compound then undergoes a palladiumhydride-mediated isomerization to afford the 1-arylcyclohexene 409 (Scheme 98). It should also be noted that this reaction was Scheme 98

viz., 412, into their corresponding isomers 413.518 Their results indicated that these catalysts generally afforded significant amounts of the thermodynamically more stable (E)-isomers. The catalysts were developed in a modular fashion where it was found that the electron-donating abilities of the ligands had an important effect on the rates of isomerization. A further important aspect that was discovered was that a 2:1 toluene− methanol solvent mixture proved to be the best. Among important features of the bipyrazole catalysts were the low loading required, the ease of purification, and that no side reactions such as hydrogenation were observed. In addition, the isomerization reactions required very mild conditions, because even at room temperature, high yields were obtained. It is of interest to note that the authors found that electron-deficient materials were more challenging to isomerize. Finally, it was found that the catalyst system was stable enough to survive multiple additions of substrate during the full time span of the reaction. During an investigation into improved approaches for the synthesis of substituted isoquinoline ring systems, Heck and coworkers prepared a series of isoquinolinium salts 415, by reaction between cyclopalladated benzaldimines 414 and various alkynes (Scheme 101).519 Of interest regarding this Review is that the researchers observed that benzaldimines 414, with R1 = 4-NO2 and R2 = Ts, were able to catalyze the isomerization of allylbenzene 3 into 1-propenylbenzene 4 at

extended to include a wide variety of aryl and heteroaryl triflates, as well as a number of cyclic alkenes. Recently, this group expanded their arylation-isomerization reaction sequence to allow for the use of aromatic bromides, with modified reaction conditions for the Heck reaction (PtBu3·HBF4, Pd(hfacac)2, iPrNEt, DMPU, 120 °C).516 A selective Wacker-type oxidation of long-chain terminal alkenes into methylketones using a Pd(OAc)2/molybdovanadophosphate (NPMoV)/O2 system was described by Yokota et al.517 These researchers examined several long-chain terminal alkenes, which were selectively oxidized to their corresponding methyl ketones in satisfactory yields, without formation of unwanted isomerized byproducts. However, in the case of oxidation of allylbenzene 3, the protocol was somewhat more difficult to perform, with 1-propenylbenzene 4 being obtained AO


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Isomerization of arylallyl groups was observed during palladium-mediated Kumada couplings with allylmagnesium halides. Applying this protocol to the synthesis of 4allylisoindoline, Zacuto and co-workers obtained up to 10% of the undesired isomerized product 419, along with the desired 4-allylisoindoline 418 (Scheme 103).522 The research-

Scheme 101

Scheme 103

ers found that use of a larger proportion of toluene (versus THF) and a moderate reaction temperature resulted in the best solubility profile and additionally limited production of the isomerized indole 419. It should be noted that this protocol was optimized for a pharmaceutical application and that the reactions were thus performed on a significant scale (100 g). This example highlights the care that needs to be taken when dealing with allylbenzene substrates, especially when palladium catalysts are involved, because certain catalysts are able to promote unwanted isomerization of the allyl chain as a secondary event. A comparison in the yields of isomerized product obtained in the presence and absence of radical traps was performed by Albéniz et al.523 These researchers demonstrated that the yield of palladium-catalyzed isomerized compounds decreased significantly when radical traps such as galvinoxyl, DPPH, and TEMPO were added. A specific example used allylbenzene 3, which isomerized almost instantaneously to 4 at room temperature when the palladium catalyst 420 was used (Scheme 104). However, when the same reactions were

100 °C, in 93% yield and with a selectivity of 96:4 in terms of E:Z ratio. Points of note were that the cyclopalladated benzaldimine catalyst 414 was used in a 1 mol % loading and that the reaction was performed under solventless conditions; that is, the allylbenzene served as solvent during the reaction. Unfortunately, no mention was made as to whether the catalyst was reisolated intact after the reaction. Heck and co-workers also demonstrated the isomerization of allylbenzene 3 into 1propenylbenzene 4 in the presence of a related azobenzene palladium-catalyst 416.520 This particular reaction was performed at ambient temperature in a sealed tube containing a mixture of bis(μ-chloro)bis(azobenzene-C2,N)dipalladium, silver tetrafluoroborate, and diisopropylethylamine. Unfortunately, the yield in this instance was very low (12%), although it appears that the researchers did not optimize the reaction conditions (Scheme 101). It should be noted that Heck had earlier investigated the general allylation of aromatic compounds with organopalladium salts, in particular those prepared in situ from arylmercuric salts and palladium(II) complexes.521 These salts were then reacted with allylic halides at room temperature to produce allylaromatic compounds in moderate yields (31−87%). It was noted by the authors that isomerization of the allylaromatics “occurred only if there was insufficient catalyst or reoxidant present or if the allylic halide concentration was below ca. 0.1 M.” Heck further commented on the fact that isomerization was unlikely to occur “unless a precipitate of palladium metal appeared in the reaction mixture,” and that the best reaction conditions, avoiding isomerization but giving good coupling yields, involved the use of 10−30 mol % of cupric chloride in an acetonitrile solution, which could act as reoxidant (Scheme 102).

Scheme 104

performed with galvinoxyl (79% conversion after 1 h) or DPPH (19% conversion after 1 h), significantly less isomerized product was obtained. The experiments demonstrated that radical traps can react with some palladium hydrides, and application of these reagents to determine mechanistic details with metal hydride species should thus be done with care. In an innovative approach using the Hoveyda-metathesis catalyst 423, Gooβen and co-workers were able to generate substituted styrenes by the application of “isomerizing ethenolysis” of the corresponding allylbenzenes.524 Styrenes are valuable starting materials for many important products, and this approach gives access to these compounds in a reliable

Scheme 102

AP


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olefin with the catalyst and promoting desorption of the olefin, and as a result increasing the reaction rates. In terms of the substrates, the allylbenzene 3 and chavicol 12 gave very similar results with yields of 43−63%. However, eugenol 5 proved difficult to isomerize under these conditions with yields being obtained below 10%. This paper was an improvement on earlier work published by the group, with the same Pd/Al2O3 catalyst system, but without the zinc and aluminum chloride additives.527 Sharf et al. synthesized a number of palladium complexes (based on Na2PdCl4) supported on silica gel and modified with tertiary amines or quaternary ammonium bases, and demonstrated the catalytic activities by, for example, the hydrogenation and/or isomerization of allylbenzene 3.528 The researchers demonstrated that the use of monodentate amine ligands resulted in the best isomerization catalysts with respect to the isomerization/hydrogenation ratios. Johnstone and co-workers developed a “lead-modified palladium on charcoal” catalyst system for the hydrogenolysis of 5-cinnamoyloxy-1-phenyltetrazoles 424 to afford 1-phenyltetrazolone 424a, a mixture of cis- and trans-1-phenylpropenes 4 and allylbenzene 3 (Scheme 106).529 When unmodified

way, particularly for the generalized allylbenzenes 1, which may be either sourced from natural products or readily made by catalytic allylations. Scheme 105 depicts how a ruthenium− Scheme 105

Scheme 106

palladium bimetallic catalytic system was applied to a number of substituted allylbenzenes 1 to afford the styrenes 2. In this case, the very effective palladium catalyst 422 promoted the isomerization to form intermediates 2, while the Hoveyda catalyst 423 then catalyzed the metathesis reaction of the resulting 1-propenylbenzenes 2 with ethylene at 10 bar to afford olefins 421. This approach was also undertaken on a preparative scale in the case of eugenol 5 (10 mmol), with excellent results (87%). It should be noted that the palladium(I) catalyst [Pd(μ-Br)(PtBu3)]2 422 was discovered by Gooβen to be a very effective isomerization catalyst for a wide variety of unsaturated systems, including that of eugenol 5 into its 1propenyl analogue (0.05 mol % 422, toluene, 50 °C, 2 h, 96%, E:Z = >20:1).525 Of interest in terms of the mechanism is that Gooβen mentioned that the interesting palladium(I) [Pd(μBr)(PtBu3)]2 catalyst system 422 readily transforms into a reactive palladium(II)−hydride species, thus accounting for its reactivity. 5.1.2. Supported Palladium Catalysts. The last examples of palladium-based catalyst systems to be illustrated in this section are those typified by being “supported systems”, that is, either on inorganic scaffolds, dendrimers, and polymers or in three-dimensional metal−organic frameworks. For instance, in 1981, Sokolskii and Trukhachova demonstrated that an increase in the isomerization yield of allylbenzenes by a factor of between 2 and 3 was obtained when zinc and aluminum chloride were added to a Pd/Al2O3 catalyst system containing 5% palladium by weight.526 The substrates isomerized under these conditions included allylbenzene 3, chavicol 12, and eugenol 5, and the authors postulated that the added Lewis acids formed ternary πcomplexes with the alkenes, thus weakening the binding of the

palladium on carbon was used, the hydrogenation of the resulting alkenylaromatics was so facile that 1-phenylpropane was the major product obtained (although it should be noted that other researchers have been able to use this isomerization system without concomitant reduction−; see the following papers530,531). In the same work, it was found that the complex Pd(PPh3)2Cl2 could also facilitate formation of the alkenes from 424. The researchers finally demonstrated that the complex Pd(PPh3)2Cl2 in dioxane, with ammonium formate as additive, was able to isomerize allylbenzene 3 into the trans-2propenyl derivative 4 in a moderate yield (55%). Recently, Karakhanov et al. demonstrated a Wacker-type oxidation facilitated by a dendrimer-based bimetallic catalyst.532,533 The researchers made use of bimetallic complexes of Pd(II) and Cu(II) with polypropylenimine (PPI) dendrimers containing terminal nitrile groups, and demonstrated that these complexes possessed significant activity in the Wackeroxidation of terminal olefins in an alcohol−water medium (Scheme 107).532,533 When allylbenzene 3 was used as substrate, the presence of the dendrimer significantly increased the ratio of isomerization product from 35% with no dendrimer AQ


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that the palladium deposits occurred with high loading of small size metal particles and with narrow size distribution. The palladium(II) species were deposited onto the titanate powder by ion exchange from aqueous PdCl2 solutions, and the researchers found that the optimum palladium loading was 8 wt %. In addition, it was found that the Pd(II) species were readily reduced to the Pd(0) form by use of an appropriate alcohol as solvent. The supported catalysts showed high selectivity for the promotion of double bond migration reactions versus hydrogenations, resulting in yields of (E)-prop-1-en-1-ylbenzene 4 in excess of 95% when allylbenzene 3 was used as substrate for the reaction. Of interest was that these double bond migrations were faster in protic solvents, such as methanol or ethanol, and that the inclusion of molecular oxygen seemed to increase the stability of the catalysts. As a final example in this section on palladium isomerization catalysts, an interesting catalyst system that involved as a strategy “molecular paneling via coordination”540,541 was used by Fujita and co-workers.542 One of the reactions tested in this system involved the isomerization of allylbenzene 3. The researchers found that treatment of allylbenzene with catalytic amounts of the square planar palladium complex 425 and the three-dimensional palladium cage structure 426 in water/ hexane as solvent afforded the isomerized product in 50% yield (Scheme 108). Of interest was that the isomerization did not occur when either 425 or 426 were omitted from the reaction mixture. In addition, trimethoxybenzene was found to inhibit

Scheme 107

ligand to 74% including the dendrimer. In addition, only a minor amount of ketone 410 ( K[Pd(C6H5SO2)2Cl(H2O)] > K2[Pd(C6H5SO2)2Br2] > K2[Pd(C6H5SO2)2Cl2]. It should be noted that all of the above complexes predominantly catalyzed the isomerization of allylbenzene 3 and that the final distribution of the trans- and cis-1-propenylbenzenes 4 was as an equilibrium mixture (2:1). In an interesting example of the application of heterogeneous organometallic catalysis for isomerization processes, Lapkin and co-workers designed and synthesized a palladium(II) catalyst supported on titanate nanotubes, specifically for the isomerization of allylbenzene 3.539 The researchers generated their own multilayered titanate nanotubes and were able to show

Scheme 108

AR


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the isomerization reaction, probably because it preferably occupied the cavity of the 426 complex. 5.1.3. Summary Concerning Palladium-Mediated Methods. Organic chemists have frequently used palladium complexes for the isomerization of allylbenzene functional groups; the most popular examples include the complexes PdCl 2 (MeCN) 2 , Pd(PPh 3 ) 2 Cl 2 , PdCl 2 (PhCN) 2 , PdCl 2 , Pd2(dba)3, Pd(dba)2, and Pd(OAc)2 (some require the addition of coreagents to generate the actual isomerization catalyst). PdCl2(MeCN)2 has probably been the palladium catalyst of choice and is frequently used at room temperature. In terms of recent research in the area, the Lindhardt−Skrydstrup catalytic system,511 comprised of Pd(dba)2, P(tBu)3, and isobutyryl chloride, and the Gooβen catalyst,525 [Pd(μ-Br)(PtBu3)]2, appear to be excellent reagents. These complexes facilitate the isomerization of a wide range of allylbenzenes, into the desired 1-propenylbenzenes, with excellent selectivities and at low catalyst loadings (below 0.5%). In terms of supported systems, palladium(II) catalysts supported on titanate nanotubes, as described by Lapkin and co-workers,539 seem to be a promising way to move forward in this area.

Scheme 109

5.2. Rhodium-Catalyzed Isomerizations

The rhodium-mediated isomerization of aryl-allyl groups has long been a favorite method among isomerization strategies. This particular section will start with the description of homogeneous approaches to the isomerization of allylbenzenes, followed by methods that were discovered when using rhodium catalysts as hydrogenation facilitators. These subsections will then be followed by examples demonstrating the use of supported/entrapped forms of the rhodium complexes and will finally include a section describing the impact of allylbenzene isomerization on rhodium-mediated hydroformylation strategies. 5.2.1. Homogeneous Rhodium Catalysts. The most straightforward isomerization protocol involves treatment of the allyl substrate with RhCl3 or RhCl3·H2O in an alcohol solution (ethanol or methanol). A representative example of this approach was published by Č ervený et al., who concluded that the most efficient way to isomerize eugenol 5 was by way of anhydrous rhodium(III) chloride (0.1%) in ethanol, at reflux and for 3 h, which gave isoeugenol 18 in essentially quantitative yield.536 These researchers compared the rhodium method to some potassium hydroxide methods (in amyl alcohol or glycerol) and found the former to be the best one. Other researchers have used the rhodium(III) chloride isomerization method on eugenol 5 or its methylated derivative 17 and obtained similar excellent results (with yields >90% and E:Z ratios >8:1).543−546 Recent examples of the use of this catalyst on different substrates include those by the group of Clive, who prepared the precursor 428 by isomerization of 427 in their synthesis of (±)-puraquinonic acid 429 (Scheme 109).547 A second example for this catalyst was found in the synthesis of 431 by isomerization of 430, an intermediate in the synthesis of the aromatic core of calicheamicin γ1 (structure not shown).548 A third example is found in the formation of a relatively small molecule, viz., lactone 432 (Scheme 109), which was required by Perdo and co-workers for the synthesis of a novel (salen)manganese(II) complex that was used in aerobic catalytic epoxidations of unfunctionalized alkenes.549 Bräse and co-workers used rhodium(III) chloride hydrate in their 2010 syntheses of (±)-fumimycin 435550,551 and its

dimethoxy analogue (not shown).552 Intermediate 434a was readily obtained by the isomerization of the allyl diphenylphosphinic amide 433a precursor (Scheme 110). Subsequent synthetic steps then afforded the naturally occurring product 435. Of interest is that the group used an alternative potassium tert-butoxide-mediated isomerization in an earlier study toward the synthesis of fumimycin (details not shown; see section 4.2 for more examples of this method).553 Conversely, in another Scheme 110

AS


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approach to fumimycin 435, Zhou and co-workers observed that Z-434b was obtained by treating lactone 433b with rhodium(III) chloride hydrate under reflux. Although the yield was only 28%, this unusual observation was confirmed by the cisoid coupling constant in the proton NMR spectrum.554 This example does not however appear to be general because the same researchers also recently reported the synthesis of racemic methoxyfumimycin ethyl ester under the same conditions, resulting in a 10:1 E:Z mixture (not shown).555 Further recent examples using the RhCl3 catalyst include that by Sachleben and co-workers who used this method in their synthesis of 13 structurally related bis(alkylbenzo)-crown ethers 438.556 Isomerization of bis-allyl compounds 436 with catalytic amounts of RhCl3 readily afforded the isomerized bis-styrenes 437, which were then further elaborated to the corresponding crown ethers 438 (Scheme 111).

Scheme 112

Scheme 111

occurring anthracyclinones (see summaries of work below). For instance, in 1983 the group described how ethanolic sodium hydroxide [or, alternatively, rhodium(III) chloride] was used for the isomerization of simple substrates.558 Despite ethanolic sodium hydroxide not working (attributed by the authors to the charge on the phenolate anion),559 the naphthol substrate 443 was successfully isomerized into 444 using rhodium(III) chloride hydrate. Unfortunately, in this instance, two undesired side-products, viz., 445 and 446, were also obtained, demonstrating that this method does have its inherent problems, at least when applied to these naphthol derivatives (Scheme 113).558 Finally, application of the rhodium- (or base-)-mediated isomerization on the acetate-, methyl-, or benzyl-protected ethers 447 afforded the desired isomerized products 448 in excellent yields (>98%). Carroll and co-workers applied rhodium(III) chloride isomerization protocols on allylbenzene derivatives in their syntheses of a series of nortropane-based serotonin transporterselective derivatives, viz., 451 and 453.560 These compounds were generated to study the physiochemical mechanisms involved in cocaine 449 addiction. First, the group observed that, after being left to stand at room temperature on a bench, light and possibly some residual palladium from the palladiummediated Stille coupling used to make 450 were responsible for the quantitative isomerization into analogue 451. Of interest was that the Z-analogue of 451 was also synthesized (not shown), but this time by hydrogenation of the corresponding prop-1-yn-1-ylbenzene with Lindlar’s catalyst. Second, compound 453 was synthesized with difficulty from precursor 452 (after subsequent removal of the N-Troc group) in moderate yield, using RhCl3 in high catalyst loadings in ethanol at reflux (Scheme 114).

Another example involving generation of macrocyclic ligands was published by Reinhoudt and co-workers in 1995.557 In this work, the bis-allyl naphthalene 439 was readily isomerized into diacetoxynaphthalene 440 by the action of rhodium(III) chloride. Of notable interest in this case was an observation by the authors that use of palladium on carbon in ethanol under reflux was less effective because the isomerization reactions stopped before the reaction was completed. Ozonolysis, followed by reductive hydrolysis, then afforded the bis-aldehyde 441, which when reacted with a barium salt, followed by a series of diamines, gave the macrocyclic ligands 442 shown in Scheme 112. Rutledge and co-workers invested extensive research time and energy into studying the conversion of aryl-2-propenyl groups into aryl-1-propenyl groups in their series of publications describing the total syntheses of naturally AT


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Scheme 113

Scheme 115

Scheme 114

in catalyst preparations, to afford the novel carbenes 458 modeled on the Hoveyda−Grubbs catalyst 423. The authors also elegantly demonstrated that they could successfully utilize the Nishida isomerization protocol, in which the Grubbs second generation ruthenium catalyst GII was converted into an isomerization catalyst with vinyloxytrimethylsilane.562 In terms of an interesting aside, Grela and Kim also demonstrated how a ruthenium metathesis catalyst could readily be generated from the commercially available 2-propenylaromatic, α-asarone 22.563 Another example linking isomerization with metathesis was published by Barrett and co-workers.564 In their studies toward the novel antifungal agent Sch 56036 459, substituted phenanthrenes 462 were readily generated by the isomerization of the allyltriflates 460 to afford the corresponding isomers 461 (Scheme 116). This was followed by generation of the biaryl system by a Suzuki−Miyaura reaction, and then a final RCM reaction with the Grubbs second generation catalyst GII to give compound 462 (for a review on this approach, see ref 565). Although 2-allylphenol 158e has regularly been used as a substrate in isomerization reactions, its aniline equivalent 402 has seen far less interest. Aresta and de Fazio disclosed some interesting research in which rhodium(I) complexes were synthesized by reactions between the complex [Rh(C2H4)2Cl]2 with 2-allylaniline 402 and N-allylaniline (not shown).566 The reaction of [Rh(C2H4)2Cl]2 with 2-allylaniline 402 in toluene or benzene at room temperature gave a dimer [Rh(2allylaniline)Cl]2 in which it was postulated that the ligand bound in a bidentate manner through the nitrogen atom and the olefin. Application of this complex to a solution of 2allylaniline 402 afforded trans-1-propenylaniline 403 in

In keeping with the isomerization-metathesis subtheme found in other parts of this Review, it is interesting to note that Arlt, Grela, and co-workers used the isomerization of allylbenzenes in their efforts to construct a small library of novel metathesis catalysts based on the commercial Hoveyda− Grubbs catalyst 423.561 These researchers used catalytic rhodium trichloride to isomerize the allylaryl functional groups (454 → 456 or 455 → 457) as shown in Scheme 115. If necessary, the phenol was then protected as an alkyl ether, followed by a ligand exchange-metathesis procedure with the Grubbs second generation catalyst GII, an approach often used AU


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amount of isomerized material was observed after 30 min (∼20% trans and 90%) illustrated in Scheme 119.574 The researchers involved were able to use deuterium transfer experiments to prove that the Rh−H species were produced in situ by interaction of the Rh−OH species with the COD ligands. In addition, using NMR spectroscopy

contained two trans-1-propenylaniline ligands (this time expected to bind in a monodentate manner through the aniline NH2). For earlier work by this group, involving these types of rhodium−allylaniline complexes, see refs 567 and 568. Birch and Rao reported on the isomerization of safrole 6 with the well-known hydrogenation complex, Wilkinson’s catalyst (PPh3)3RhCl. When applying this catalyst to the allylbenzene substrate, safrole 6, in chloroform under reflux for 2 h, a mixture of cis (40%) and trans (60%) isomerized products 19 was obtained. When the same reaction was performed in benzene as solvent, for the same period of reflux, the only product obtained was cis-isosafrole (60%) and unchanged safrole.569 The authors of this work mentioned that after 3 h, some trans-isosafrole 19 (20%) was evident in the reaction mixture. This result is of interest as methods to generate the cisisomers are in the minority. Gol’dfarb et al. reported their work on allyl arene isomerization during the hydrogenation of allylbenzene with chloro-tris(tri(phenylphosphine)-rhodium(I) (Wilkinson’s complex) or chloro-tris[tri-(2-thienyl)phosphine]-rhodium(I) complexes.570 Under standard hydrogenation conditions, the total amount of cis- and trans-isomers did not exceed 3%. However, when hydrogenation was conducted on allylbenzene that had been treated with sodium borohydride, a significant

Scheme 119

AV


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having an order less than one, while still being independent of the hydrogen pressure.579 In terms of the actual mechanisms involved, the researchers postulated that isomerization could most likely occur by way of a π-allyl complex, although the possibility of an orthometalation mechanism was also mentioned. The water-soluble rhodium(I) phosphine complex, [Rh(PTAH)(PTA)2Cl]Cl (PTA = 1,3,5-triaza-7-phosphaadamantane) illustrated in Scheme 120, was found to be an efficient

experiments, it was also shown that it was unlikely that a basemediated migration mechanism was in operation. Nolte and co-workers have developed novel cage compounds with rhodium centers, which displayed various chemical reactivities. In terms of this Review, the complexes (see an example illustrated in Figure 10) promoted the isomerization of

Scheme 120

catalyst for hydrogenation and isomerization of olefinic double bonds.580 When Darensbourg and co-workers hydrogenated allylbenzene 3 in the presence of this catalyst, hydrogenation to afford 469 (51%) was accompanied by significant isomerization of the double bond to give cis- and trans-1-propenylbenzene 4 (47%) in which the ratio of trans:cis was 20:1. When aqueous sodium formate was used as the hydrogen source, isomerization was enhanced further over hydrogenation, with isomerization becoming the dominant reaction, particularly at room temperature (yields of isomerized product >60%). Kameda and Yoneda reported on the isomerization of allylbenzene 3, eugenol 5, 2-allylphenol 158e, and estragole 7, as being catalyzed by the dihydridorhodium complex [RhH2(Ph2N3)(PPh3)2].581 These reactions were studied in a number of solvents, viz., dimethyl sulfoxide, dimethylformamide, tetrahydrofuran, acetone, toluene, and benzene, and under a hydrogen atmosphere to give the corresponding transand cis-1-propenylbenzene analogues together with the hydrogenated propylbenzenes. The isomerization rate proved to be faster under a hydrogen atmosphere than under a nitrogen atmosphere, which may be attributed to the formation of active species by reaction of the dihydridorhodium complex with hydrogen. It should be noted that the researchers performed a thorough investigation as they carefully tracked the compositions of the reaction mixtures with respect to time. 5.2.2. Supported Rhodium Complexes. Dovganyuk et al. observed isomerization of allylbenzene 3 to propenyl benzene when using the supported rhodium complexes [RhCl(cod)]2, [RhCl(PPh3)]3, and RhCl3.582,583 These rhodium complexes were prepared by immobilization on silica gel that had been modified with aminophosphine groups. Of additional interest was that the researchers performed the hydrogenation experiments using alcohol as the hydrogen source under an argon atmosphere. Under these hydrogen transfer conditions, it was found that the rate of allylbenzene isomerization increased when the reaction was carried out under an atmosphere of molecular hydrogen. The activity of the supported [RhCl(PPh3)]3 complex was found to be the best out of the three rhodium complexes evaluated. It should be noted that the researchers made use of potassium hydroxide as an essential

Figure 10. Reprinted with permission from ref 575. Copyright 1995 American Chemical Society.

a series of substituted allylbenzenes into their corresponding 1propenylbenzenes.575,576 Of particular interest was that the authors of this work postulated that the isomerization process preferentially takes place inside the cage due to substrate− catalyst π−π stacking and hydrogen-bond interactions. As support for their hypothesis, the researchers showed that the substrate 4-allylcatechol isomerizes into the corresponding Zand E-isomers (ratio ∼1:3) significantly faster than allylbenzene 3, due to additional hydrogen-bond possibilities with the “cage” catalyst. Subsequent comparative isomerization experiments with the reference rhodium complex HRh(CO)[P(OPh)3]3 supported the evidence that the isomerization was occurring within the cage structure. In addition, Nolte and co-workers were able to show that the supramolecular catalyst had kinetic properties similar to those displayed by enzymes, and that among others the kinetics of the rhodium complexes obeyed the Michaelis−Menten rate laws.575 It should be noted that the ability of transition metal complexes to facilitate isomerization of arylallyl groups is often serendipitously discovered during investigations of other desired transformations, for example, hydrogenation. This was the case when Trzeciak and Ziólkowski observed the isomerization of allylbenzene 3 into prop-1-en-1-ylbenzene 4 when using the complex HRh[P(OPh)3]4 as a stoichiometric hydrogenation reagent in an attempt to prepare propylbenzene from allylbenzene.577,578 Of interest was that van Leeuwen and co-workers had already reported on the isomerization of allylbenzene with {HRh[P(OPh)3]4} in 1995 and had noted the fascinating effect of the addition of triphenyl phosphite. In this particular work, isomerization reactions resulted in the E[3-methylstyrene] product; that is, no Z-isomer was observed. It was also determined that the isomerization reaction was zero order in both substrate and hydrogen pressure in the absence of additional phosphite ligand. However, addition of phosphite to the reaction mixture resulted in the isomerization reaction AW


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(Figure 11), were also shown to be capable of isomerizing allylbenzene 3 into the corresponding cis- and trans-βmethylstyrenes. Compound 472 appeared to provide the best results with 5% cis-1-phenyl-1-propene 90, 88% trans-1-phenyl1-propene 4, and 7% starting material being obtained under the following reaction conditions: 1 mol % catalyst, o-xylene, 120 °C.588 Of interest was that the supported arsane complexes, such as 473, typically underwent deactivation when 30−35% of starting material was still unconverted, and that this was in contrast to the phosphine-bearing supported complexes, viz., 471, which generally displayed longer periods of activity. A limiting feature of free complexes such as 470 described above is that they frequently require exclusion of oxygen to give good isomerization results. However, this sensitivity to oxygen could potentially be mitigated by supporting the dirhodium complexes on solid supports. To this end, Blum and co-workers synthesized a series of polystyrene-, silica-, and alumina-bound rhodium catalysts based on the dirhodium complexes having one bridging thiolato and one bridging chloro ligand described in the previous paragraph (see generalized structures of supported complexes in Figure 12). When tested for their

additive and that this could also have been responsible for the considerable isomerization of allylbenzene under the conditions studied. About 3 years later, the same research group found that isomerization of allylbenzene 3 to propylbenzene was accompanied by partial reduction of the allylbenzene when derivatives of the complex [Rh2(OAc)4], immobilized on γamino-propyl-modified silica gel, were employed as catalyst.584 This particular supported catalyst showed the best isomerization:hydrogenation ratio when compared to other immobilized rhodium complexes tested in the study, and the authors were able to conclude that rhodium(II) supported complexes were much more active than the trivalent variants. The Blum research group spent considerable effort on the design, synthesis, and testing of homogeneous and heterogeneous rhodium-based catalysts, often using allylbenzene isomerization as a key reaction to test their catalysts. Their contributions, together with those of their collaborators, are illustrated next. The first contribution from the Blum and Schumann groups involved isomerization of allylbenzene 3 with the homogeneous complexes, viz., cis-{[(tBu3P)Rh(CO)]2(μCl)(μ-PtBu2)} and the related chiral analogue in which the phosphine group has been exchanged by (+)-neomenthyldiphenylphosphine.585 The authors pointed out that a significant problem frequently affecting Rh(I) and Rh(III) catalysts used in homogeneous reactions such as isomerizations is that they can be deactivated before full conversions of the substrate into the product has been achieved. Of significance was that when the first dirhodium catalyst was used to isomerize allylbenzene 3, it maintained full activity until an equilibrium mixture comprised of isomerized compounds (cis 8.6%, trans 90%) and minimal starting material (1.4%) was formed. In 1984 and 1985, Schumann, Blum, and co-workers synthesized and characterized a number of μ-(alkylthio)- and μ-(arylthio)-μ-chloro-dicarbonyl-bis(tritert-butylphosphane)dirhodium complexes, for example, complex 470 (Figure 11).586 These complexes were then immobilized onto a fused

Figure 12.

ability to perform allylbenzene isomerizations, these researchers found that, in general, the silica- and alumina-supported catalysts 476 were much more reactive than the related polystyrene-bound complexes based on 474 and 475. The silica- and alumina-bound systems were also found to be more stable than the polystyrene-anchored complexes with their activities being fairly constant.589 Blum and co-workers demonstrated in 1996 that isomerization of allylbenzene with the ion pairs, RhCl3/Aliquat 336 or RhCl3/[Me3N(CH2)3Si(OMe)3]Cl, entrapped in SiO2 sol−gel matrixes was very efficient.590 The authors of this work argued that in effect their approach was to convert a traditional homogeneous catalysis system into a heterogeneous one. According to these researchers, the advantages of the immobilized ion pair catalysts included that they are “stable, leach proof, and recyclable” (for a review on this concept, see paper591), although a negative aspect was that on reuse the catalyst’s efficiency dropped and could only be restored by opening the pores with boiling water. These results

Figure 11.

silica support to give heterogeneous systems of the type 471. The homogeneous complexes and their heterogeneous counterparts were then shown to readily catalyze the isomerization of estragole 7 (for example, conditions for 471, 1 mol % catalyst, o-xylene, 100 °C; products, ∼9% cis-anethole 20, 84% transanethole 20, and ∼7% starting material). 587 For the homogeneous catalysts, a series in which three different sulfides were used indicated that the activity of the catalysts occurred in the order SnBu > SiPr > StBu. In addition, a related set of tritert-butyl arsane complexes, viz., complexes 472 and 473 AX


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with the polystyrene polymer-supported rhodium (0.57 mol % of Rh), to afford 7.7% of cis- and 92.0% of trans-1propenylbenzene within 20 min (with only 0.3% starting material left unreacted). Furthermore, it was found that the catalyst could be reused in subsequent isomerization cycles. The researchers also proved that the amount of water present in the reaction mixture was crucial for success of the reaction, with the best conversion rate being observed at levels of 8% water in ethanol. Further studies on this system indicated that the degree of cross-linking of the polystyrene support had a significant effect on the efficacy of the reactions rates as this affected the rigidity and surface area of the polymers.595 However, a balance between reactivity and physical stability led the researchers to choose Dowex 1 × 4 as their polystyrene support of choice. Finally, in the extended study, use of four differently substituted allylbenzene substrates (R = H, Me, OMe, or Cl) indicated that EWGs decreased the reaction rate, while EDGs had the opposite effect. The researchers interpreted these data to mean that stabilization of the positive charge on rhodium was important during the rate-determining step of the isomerization reactions. It should also be noted that in the early 1980s, Blum and coworkers investigated the possibilities of improving the synthetic methods with respect to transition metal-complex promoted isomerizations of allylbenzenes, in particular, the mass transfer problems typically associated with reactions involving lipophilic allylbenzene substrates, with the typically water-soluble transition metal catalyst systems. One of the first solutions proposed by the group involved using a biphasic system containing rhodium chloride hydrate (RhCl3·H2O) and Aliquat 336, a phase transfer agent.596 Under the mild reaction conditions employed (80 °C), allylbenzene 3 and estragole 7 could readily be isomerized with yields of 70−89% after 3 h (trans/cis ratio 5.5−6). In addition, recovery of the catalyst was facilitated by addition of perchlorate to the reaction mixture, resulting in the valuable rhodium complexes being partitioned into the aqueous phase again (the researchers point out that this process needs to be repeated a number of times for efficient recovery of the catalyst). It should be noted here that Alper and Hachem also reported on the use of a phase transfer-promoted isomerization facilitated by the catalyst, chlorocarbonylrhodium(I) dimer [Rh(CO)2Cl]2, in a 6 M NaOH−dichloromethane biphasic solution system.597 It was found that the phase transfer catalyst was essential to achieve good results, with benzyltriethylammonium and dodecyltrimethylammonium chloride giving the most reasonable results. It should be noted that in general the yields obtained were not spectacular (1propenylbenzene 4 38%, E:Z = 100:1; anethole 20 47%, E:Z = 100:1; 1-naphthyl-1-propene 24−59%, E:Z = 78:22−45:55), but that for a methyl-substituted 2-methyl-3-phenyl-1-propene substrate a yield of 88% was obtained with the dodecyltrimethylammonium chloride additive. Finally, in more recent work published by the Blum group, they reported an environmentally friendly procedure for the transformations of the allylarenes 1 into (E)- and (Z)-1-phenyl1-propenes 2, again using the rhodium-trichloride-Aliquat 336 ion pair, but this time in aqueous microemulsions.598 To this end, the catalyst system for these reactions was encaged within a hydrophobic silica sol−gel, to allow for the isomerization process to take place in water as medium. Under the fairly robust reaction conditions of up to 140 °C (Scheme 121), it was found that after 20 h, allylbenzenes 1 gave rise to a mixture of 91.6% trans- and 7.5% cis-isomerized product 2, along with

corroborated earlier work by this group, which showed that sol−gel encapsulated RhCl3, with or without cetyltrimethylammonium bromide, provided recyclable catalysts that were able to facilitate isomerization of allylbenzene 3 at a greater rate than “free” RhCl3·H2O.592 In addition, in terms of reaction rates, the immobilized RhCl3 system was twice as fast as the soluble cetyltrimethylammonium/rhodium chloride ion pair under phase transfer conditions. In this particular paper, Avnir and Blum synthesized a series of novel encapsulated isomerization catalysts by polymerization of (MeO)4Si in the presence of the transition metal complexes CoCl2, RhC13, and PtCl4, together with quaternary ammonium salts.592 The researchers then demonstrated the utilization of the resulting glasses as recyclable catalysts for organic processes. In terms of the isomerization of allylbenzene 3, it was found that the rhodiumcontaining glass did not reach full activity in the first catalytic run; that is, only in the second or third cycle did the reaction go to completion within the 2 h reaction time. After this time, analysis of the reactor contents showed that it was made up of an equilibrium mixture comprised of the following components: trans-1-propenylbenzene (89.3%), cis-1-propenylbenzene (8.1%), and starting material (2.6%). The researchers noted that in terms of catalyst recyclability, leaching was not a particular problem, but importantly, the pores in the transition metal-containing glasses had to be reopened by ultrasound treatment prior to reuse to maintain the efficiency of the recycled catalysts. Continuing on the topic of sol−gel rhodium supported catalysts, Blum and co-workers found that encapsulation of a rhodium−carbonyl cluster, Rh2Co2(CO)12, within the alumina sol−gel matrixes was also a successful strategy to produce heterogeneous catalytic systems.593 The researchers were able to demonstrate that the entrapped rhodium complexes maintained their activity and that the transition metal− carbonyls were solidly ensconced within the sol−gel matrixes with little leaching being observed. In terms of application, the rhodium system was successfully applied as a catalyst for the isomerization of allylbenzene 3, resulting after 3 h at 100 °C in a mixture comprised of the following components: trans-1phenyl-1-propene 4 (81%), cis-1-phenyl-1-propene 90 (17.5%), and starting material (1.5%). The researchers also mentioned that they obtained essentially the same mixture of products in three separate experimental runs, each time using the same ceramic catalyst, thus demonstrating the excellent recyclability of the rhodium-containing heterogeneous system. An important point that should be raised here is that the complex Rh2Co2(CO)12 is known to be a poor isomerization catalyst, and that it is the actual encapsulation into the alumina sol−gel matrixes that produces an active isomerization catalyst, thus demonstrating the value of this process. An interesting aspect in this instance is that the comparative Rh2Co2(CO)12 complex prepared within a silica sol−gel matrix was unable to perform the same arylallyl isomerization. An approach involving the use of polymer-supported ion pairs as recyclable isomerization catalysts has been investigated by Blum and co-workers. In a first example on this theme, the group demonstrated that polystyrene-supported ion pairs, readily generated from RhCl3 and commercially available Dowex 1 ion-exchange resin, were able to be used as highly active and fully recyclable catalysts for several hydrogen transfer reactions, including the isomerization of allylaryls.594 According to the published procedure, isomerization of allylbenzene 3 was performed by heating (50−70 °C) in 92% aqueous ethanol, AY


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viz., propylbenzene 469 (50%), trans-1-propenylbenzene 4 (35%), and cis-1-propenylbenzene 90 (15%). Recently, Jackson and co-workers investigated the ability of a 2.5% rhodium on silica system, prepared from RhCl3·H2O by way of the incipient-wetness method, to isomerize allylbenzene.603 In particular, their study focused on how the molecular structure of the catalyst affected hydrogenation and isomerization of the isomers allylbenzene, trans-β-methylstyrene and cis-β-methylstyrene, as well as mixtures of the three compounds. The authors observed that allylbenzene inhibited the hydrogenation of both trans- and cis-β-methylstyrene, but allowed for isomerization to occur (while the rate of allylbenzene hydrogenation was unaffected). They proposed that these data support the notion that hydrogenation of the internal alkenes was occurring at a type of site (terrace face) different from that for the external alkenes (edge/corner). This type of study is specifically of value in that it will potentially allow for the design of supported catalysts that facilitate isomerization reactions while not promoting other reactions like hydrogenation. 5.2.3. Rhodium-Mediated Hydroformylations and -borations and Related Isomerization Catalysis. The topics of isomerization and hydroformylation604 (and to a much lesser extent hydroboration) have been intertwined since the discovery that certain rhodium complexes were able to perform both operations on unsaturated substrates.605 The fact that competing isomerization/hydroformylation processes have an impact on the product distribution, as well as the importance of the processes in modern industrial chemistry, has seen much research energy being put into this particular topic. The next subsection will highlight some studies in which use of allylbenzenes has given rise to some interesting results due to the competing isomerization process (for examples of research including hydroformylation of allylbenzenes, see refs 606−612). It should also be noted that the related rhodiummediated hydroaminomethylations of topical substrates such as eugenol 5 are of relevance due to competitive isomerization.613 In a first example, isomerization was observed during rhodium-mediated hydroborations of various alkenes. Doyle and co-workers noticed that use of dirhodium(II) tetraacetate in a hydroboration-hydrogen peroxide oxidation sequence led to a regioisomeric mixture of alcohols, clearly indicating the propensity of the unsaturated substrates to isomerize under the reaction conditions used.614 Further research then revealed that application of catalytic amounts of Rh2(OAc)4 and catecholborane was able to isomerize allylbenzene 3 to a mixture of E:Zisomers with a ratio of 4:1, respectively. The researchers found that neither the rhodium catalyst, nor the catecholborane, were able to promote the isomerization process on their own. Doyle and co-workers postulated that the active isomerization species was most probably a rhodium hydride.615 Morrill and D’Souza extended these results and made use of a rhodium source and a hydroborating reagent to effect an alkene hydroboration with a regioselectivity reversed from that normally observed. In this work, allylbenzene 3 was treated with rhodium(III)chloride· xH2O and borane, with the substrate:catalyst mole ratio being ∼75:1.616 An oxidative workup (H2O2) then gave the 1phenylpropan-1-ol product 478 in greater than 80% yield, rather than the 90% of 3-phenylpropan-1-ol 479 obtained in an uncatalyzed reaction, indicating that the rhodium had mediated an isomerization reaction (Figure 14). However, it should be noted that the authors of this work mentioned the fact that in their opinion isomerization by the rhodium trichloride alone

Scheme 121

less than 1% starting material. Of interest is that, although not fully understood, the researchers were able to prove that under the reaction conditions employed, the rhodium had been converted into catalytically active Rh(0) nanoparticles, which appear to be solely responsible for the isomerization of the substrate. Two other factors that affected the isomerization rate were the electronic nature of the substrates (as previously determined) and the hydrophobicity of the sol−gel support. Finally, it was proved that the catalyst was readily recovered after completion of an isomerization reaction cycle, and that it could be recycled at least six times without a marked decrease in the catalytic activity. It should be noted here that Blum and co-workers also disclosed details concerning a three-phase microemulsion/sol−gel system for the rhodium-mediated hydroformylation of hydrophobic alkenes, which gave 1propenylbenzene as a side-product when allylbenzene was used as a substrate.599 A wonderful example of a polymer-supported rhodium catalyst capable of isomerizing allylbenzene 3 was reported by Tung and Brubaker. 6 0 0 The supported dichloro(cyclopentadienyl)rhodium(III) catalyst 477 (Figure 13) was

Figure 13.

generated by the coordination of the rhodium trichloride trihydrate complex to 20% divinylbenzene cross-linked polystyrene to afford a supported catalyst with 2% loading. Isomerization was then performed at 85 °C to afford the isomerized product with 90% yield after 40 h (3:1 E:Z ratio). Addition of triethylamine to the reaction mixture terminated the isomerization reaction immediately, strongly suggesting that the isomerization was initiated by oxidative addition of the rhodium center to the allylic C−H bond. At this point, it should also be mentioned that Zoran et al. generated a polystyrene-bound RhCl(PPh3)3 catalyst and compared this supported complex as an isomerization facilitator to polystyrene bound IrCl(CO)(PPh 3)2 and RuCl 2(PPh3) 3 systems.601 Because the rhodium complex lost much of its activity after recycling due to extensive leaching, it was not used further [this work is described in the “ruthenium” discussion (section 5.4)]. Kalinin et al. developed another polymeric system for the rhodium-mediated isomerization of unsaturated hydrocarbons. In this work, tris(triphenylphosphine)rhodium chloride was loaded onto a polycarborane backbone, obtained from the reaction between methyl methacrylate and 1-isopropenyl-(3)1,2-dicarbaundecacarborate salts, to afford a phosphinocarborane−polymer rhodium complex.602 When allylbenzene 3 was exposed to this supported catalyst system in ethanol and under hydrogen (atmospheric pressure), three products were formed, AZ


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complexes such as PdCl2(PPh3)2, with and without phosphine and SnCl2·2H2O, to catalyze the same type of reaction, but this time with the aim of obtaining esters by way of an alkoxycarbonylation process.622,623 Isomerization of allylbenzene was observed as a side reaction during the synthesis of 5,6-dihydronaphthalene derivatives by way of a one-pot hydroformylation-cyclization of allylbenzene derivatives using the catalyst system Rh(CO)2acac/ultranox626/CO/H2/CH2Cl 2.624,625 In addition, these same researchers investigated the rhodium-mediated hydroformylation and one-pot hydroformylation-acetalization reactions on a variety of substituted allylbenzene substrates.390 dos Santos and co-workers described an investigation in which they studied how to control the rhodium-catalyzed hydroformylation of allyl- versus 1-propenyl-benzenes, in which a variety of naturally occurring compounds, including eugenol 5, eugenol methyl ether 17, safrole 6, estragole 7, and their related isomerized compounds, were used.626 It was found that different rhodium catalysts gave different ratios of linear versus branched chains when starting from allylbenzenes,621−623,627 indicating their propensity to isomerize the allyl group. The researchers found that for various catalyst systems, the ligand bite angle of the phosphine, as well as the basicity of the phosphine ligands used, were important influences on the regioselectivity of the resultant hydroformylation. Baricelli and co-workers reported on the isomerization of the allyl group of several naturally occurring compounds, viz., eugenol 5, estragole 7, and safrole 6, in attempting a hydroformylation procedure using the water-soluble rhodium and ruthenium catalysts [Rh(CO)(MeCN)(L)3][BF4] and [HRu(CO)(MeCN)(L)3][BF4], respectively. It was obvious from the products obtained after the hydroformylation that the allyl groups had been isomerized prior to the hydroformylation reaction. In these cases, the ligand L was either msulfonatophenyldiphenylphosphine (tppms) or tris-m-sulfonatophenylphosphine (TPPTS). Of interest was that the use of a phase transfer agent, cetyltrimethylammonium chloride (CTAC), effectively inhibited the entire isomerization reaction, and actually increased the selectivity of the hydroformylation reaction. It should also be noted that under some reaction conditions, up to ∼40% of isomerized product was obtained, that is, without this compound having undergone further hydroformylation.628 It should come as no surprise that the asymmetric hydroformylation of allyl- and 1-propenyl-benzenes has also been studied. Claver and co-workers used rhodium complexed with chiral diphosphite ligands with syn gas on a set of naturally occurring allyl- and 1-propenyl-benzenes.629 The researchers noted that when hydroformylation was performed with rhodium−P(OPh)3 catalytic systems, significant amounts of isomerization of the allyl groups occurred, and this event was greatly reduced by addition of PPh3. Finally, under the topic of isomerizations occurring during formylation reactions, the work of a number of other groups should be mentioned. Despite initial problems with significant substrate isomerization, Breit and co-workers were able to optimize their catalyst system, comprised of a carbon monoxide and hydrogen atmosphere with a [Rh(CO) 2 acac]/6(diphenylphosphino)pyridine-2(1H)-one catalytic system to give less than 4% isomerization when allylbenzene 3 was used as substrate.630 This was a significant improvement because the initial use of ligands such as t Bu-XANTPHOS and BIPHEPHOS on a 1-octene substrate resulted in hydro-

Figure 14.

would be too slow and that the “mechanism likely involves multiple and reversible addition/elimination of a Rh-activated B−H species across the double bonds.” It should also be noted that other research into the ability of rhodium hydride species to promote isomerizations of unsaturated substrates (including allylbenzenes) during hydroboration-oxidation sequences has been performed.617,618 Moving on to hydroformylation, Kollár et al. have observed the isomerization of allylbenzenes during the hydroformylations of substituted allylbenzenes with [Rh(nbd)Cl]2 (nbd = norbornadiene). Estragol 7 was one of the substrates used, and the mixtures of aldehydes 480, 481, and 482 obtained indicated that the rhodium catalyst was isomerizing the alkene to give as intermediate 20, prior to the hydroformylation reactions (Scheme 122).619 It should be noted others have obtained similar types of mixtures during the rhodium-catalyzed hydroformylation of allylbenzene substrates.620 Scheme 122

At this point, it should also be noted that other researchers had previously looked at rhodium-based catalysts to produce aldehydes from allyl- and propenylaromatics derived from biomass, and that the ability of the catalysts to isomerize the former to the latter was noted as important in the distributions of aldehydes obtained. For example, Kalck and Park investigated the ability of catalysts of the general formula [Rh2(μ-SR)2 (CO)2(PA3)2] (R = tBu, C6F5; A = Ph, OMe, or OPh) to hydroformylate estragole 7, eugenol 5, eugenol methyl ether 17, and safrole 6.621 Later, Kalck and co-workers extended this investigation to include the use of palladium BA


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C24H54O3Pt2Si6, derived from the reaction between chloroplatinic acid and 1,1,3,3-tetramethyl-1,3-divinyldisiloxane)632 or platinum black, gave up to 10% of isomerized 1-propenyl compounds as side-product.633 A number of applications will next be illustrated in which the isomerized products are the major products of the reactions that have been carried out. Blagg and co-workers used a platinum dichloride-catalyzed isomerization of eugenol 5 to obtain isoeugenol 18 in excellent yield.634 After multistep modification, isoeugenol 18 was then coupled to racemic taxifolin to afford silybin coniferyl alcohol analogues (structures not shown). In the same paper, the researchers isomerized the allyl group in compound 486 using the same conditions to afford methylstyrene 487 (Scheme 124), after which an ozonolytic cleavage afforded the

formylation products originating from an initial isomerization process in amounts of 26−54%. Finally, it should be noted that a neutral phosphine complexed [Rh(CO)2Cl]2 system, where the phosphine was bis(diphenylphosphino)methane (dppm), has also been investigated in terms of its ability to promote allyl isomerizations; see Scheme 123. Yu and co-workers were able to show Scheme 123

Scheme 124

that when this catalyst system was applied to β,γ-unsaturated ketones 483, the isomerized products obtained, 484, were predominantly the Z-alkenes (E:Z = 1:6 to >1:19) (Scheme 123).631 When applying these reaction conditions to 4-allyl-1,2dimethoxybenzene 17, the isolated isomerized material 485 proved to be mainly the E-isomer (E:Z = 7:1). 5.2.4. Summary Concerning Rhodium-Mediated Isomerizations. In this section, rhodium(III) chloride (hydrate or anhydrous) in alcohol has by far been the most popular ruthenium homogeneous isomerization catalyst, although some work has also been achieved with the Wilkinson’s catalyst, (PPh3)3RhCl. It is perhaps somewhat surprising that more rhodium complexes have not been developed for the allylbenzene isomerization reactions. In terms of supportedrhodium catalysts, Blum and co-workers have performed research in this area for more than 20 years and have developed systems with good activity and minimal leaching. Examples include the isomerization of allylbenzene 3 with ion pairs, RhCl3/Aliquat 336 or [Me3N(CH2)3Si(OMe)3]Cl, entrapped in SiO2 sol−gel matrixes,590 and the application of aqueous microemulsions of these pairs.598 Finally, a summary of the competition between isomerization and hydroformylation processes promoted by various rhodium catalysts has also led to the identification of numerous catalyst systems, which could be fine-tuned to only provide the isomerized phenylpropenoids from allylbenzenes.

corresponding benzaldehyde. It should be noted that as the paper was a communication, little data concerning the actual reaction conditions were detailed. Of interest was that all of the silybin analogues synthesized were tested as potential inhibitors of the Hsp90 protein folding machinery. In 1983, Kurosawa and Asada demonstrated that a small family of η3-allyl ligated platinum complexes 488a−c were able to readily isomerize allylbenzene 3 (Figure 15).635 These

Figure 15.

compounds were used in catalytic amounts (1−3%) in dichloromethane at 25 °C to afford the isomerized allylbenzene 4 as mainly the E-isomer. Of interest was that the isomerization process did not seem to destroy the catalyst because recovered catalysts were shown to still possess the η3-methallyl and triphenylphosphine ligands. The authors proposed that a platinum−hydride species was responsible for the isomerization, but were unable to rule out the possibility that another species in low concentration might also be causing the isomerization. In 2010, Scarso and Strukul demonstrated that platinum(II) complexes such as 489−494, bearing a chelating diphosphine and an alkyl-, or, for improved results, an aryl-moiety could be used for the efficient isomerization of allylbenzenes under mild experimental conditions including air as atmosphere and unpurified commercial solvents.152 The authors pointed out

5.3. Platinum-Catalyzed Isomerizations

5.3.1. Homogeneous Platinum Complexes. Platinum(II) complexes are known to interact with terminal alkenes. However, these complexes have rarely been used to any notable extent as arylallyl isomerization catalysts. It should be noted at this stage that isomerization of allylbenzene substrates has sometimes been observed as a side reaction when platinum complexes have been used. For example, Rakib and co-workers noted that the β-hydrosilylation of substituted allylbenzenes with cyclic siloxane D4H, in the presence of Karstedt’s catalyst (an organoplatinum compound with the formula BB


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that this was the first example of platinum(II) complexes being used as isomerization catalysts (for a recent example where allylbenzene isomerization was observed during platinum(II)mediated intramolecular olefin hydroarylation reactions, see ref 636). The platinum complexes evaluated in this research contained a variety of phosphine backbones, with the most successful platinum complexes (489−494) being shown in Scheme 125. From this study, it was reported that diphosphines

in Scheme 125), could facilitate the isomerization of allylbenzene 3 at 50 °C under 1 bar of hydrogen gas.637 The authors took this result to mean that a platinum−hydride was responsible for this reactivity. 5.3.2. Summary of Platinum-Mediated Methods. As can be seen from the paucity of the examples involving platinum, this transition metal has not seen much application for the isomerization of allylbenzenes. The highlight in this section is definitely the work by Scarso and Strukul, who designed a series of diphosphine platinum(II) complexes, which were able to isomerize a number of allylbenzenes with low catalyst loading (1 mol %), to afford the desired isomerized compounds in high yields and excellent selectivities (>95 E).152

Scheme 125

5.4. Ruthenium-Catalyzed Isomerizations

Application of ruthenium complexes for the purpose of isomerizing allylbenzenes into their respective more thermodynamically stable isomers has also been very popular.638 In addition, the mechanistic aspects pertaining to these organometallic complex-mediated isomerizations have also received interest. The ability of ruthenium complexes to facilitate isomerizations has also been included in general reviews on the reactions promoted by these compounds,639 although their value for the isomerization of allylaryls has not specifically been highlighted. Isomerization strategies have been sporadically described in reviews640 and “highlights”,641 and this section will aim to comprehensively cover what has been achieved, particularly with allylbenzene isomerizations. Initially this section will focus on the use of “regular” homogeneous ruthenium catalysts for the isomerization of allylbenzenes, which will be followed by a number of so-called supported and green approaches. The use of ruthenium hydrides will then be discussed, followed by a significant subsection describing the close relationship between isomerization and metathesis. Finally, a short description of isomerization and C−H activated allylation will conclude the section. 5.4.1. Homogeneous Ruthenium Complexes. The ruthenium trichloride hydrate (RuCl3·xH2O) complex has probably been one of the simplest catalytic systems used, and this Review will provide further examples demonstrating the versatility of this catalyst. In an interesting example involving an anthraquinone skeleton, Castonguay and Brassard reported the isomerization of 2-allyl-1,3-dihydroxyanthraquinone 495 into 1,3-dihydroxy-2-trans-prop-1-enyl-anthraquinone 496 using ruthenium trichloride trihydrate in absolute ethanol at 75 °C (Scheme 126).642 The isomerized product 496 was obtained in

such as 1,4-bis(diphenylphosphino)butane (dppb), with a large bite angle in conjunction with a pentafluorophenyl residue coordinated to Pt, resulted in catalysts being able to isomerize a variety of substituted allylbenzenes under reasonably mild conditions (reaction time of a few hours, 50 °C, CHCl3 as solvent, and catalyst loading 98% isomerization of allylbenzene 3 at 120 °C (ratio E:Z = ∼9:1). In terms of comparison with the polymer-bound variants, it was found that leaching of the transition metal complexes from the sol−gel was less problematic than before, but that subsequent runs of recycled catalyst did result in lower yields, probably due to blockage of the pores, which gave access to the catalytic sites. In addition, the authors of this work mentioned that the sensitivity of the immobilized catalysts to oxygen was significantly reduced, although the system was still found to work best under inert conditions. An interesting use of the isomerization of allylbenzene 3, with potential application in an industrial setting, was described by Joó and co-workers in an essay on “Molecular Catalysis in Liquid Multiphase Systems”.654 In this overview, application of a water-soluble catalyst, RuCl2(tppms)2 [tppms = sodium 3(diphenylphosphino)benzenesulfonate] [i.e., a derivative of RuCl2(PPh3)3], was described for the formation of trans-1propenylbenzene (the cis-isomer was only observed in trace amounts) (Figure 17). Of additional interest was that the rate of isomerization was found to be dependent on the pH of the aqueous phase, and it was found that a buffer keeping the pH in the range 1−8 was required for good selectivity of the transisomer. The researchers mentioned that this dependence on pH could be explained by the isomerization proceeding through a π-allylic insertion mechanism, where β-hydrogen

Figure 17.

abstraction and dehydrochlorination of the intermediate π-allyl system are favored by an increase in basicity. Keeping with the multiphase approaches, Fanun and coworkers employed platinum group catalysts, heterogenized as sol−gel supported ion pairs (for details on how this was done, see ref 598), for the isomerization of allylbenzene 3 in a water− propanol, sucrose laurate (L1695) micellar system.655 For a related sol−gel rhodium complex-mediated isomerization of estragole 7, see ref 656. The authors of this work were particularly interested in reaction conditions that could be used in microreactor systems. These researchers concluded that the catalytic activity of the microemulsion systems employed depended on the nature of the transition metal involved, as well as the type of ligand used (as illustrated in Scheme 133), Scheme 133

and it should be noted that a major driver of using these types of systems was to decrease the amount of organic solvents required. The order of reactivity for these isomerizations under microemulsion phase transfer conditions was found to be RuCl2(PPh3)3 > [(C8H17)3NMe][RhCl4] > [Rh(cod)Cl]2 > IrCl(CO)(PPh3)2 > H2PtCl2 (all as sol−gel encaged catalysts). In another study by Blum and collaborators, they focused on the investigation of the phase and physicochemical behavior of water/n-propanol/cetyltrimethylammonium bromide/allylbenzene micellar systems for the optimization of allylbenzene isomerization.657 5.4.3. Green Approaches with Ruthenium Catalysts. In terms of more environmentally friendly methods, Jasra et al. reported on an in-depth investigation into the solvent-free isomerization of estragole 7 into trans-anethole 20 using a variety of palladium, ruthenium, and rhodium transition metal complexes.658 In this study, it was found that the complex RuCl3(AsPh3)2·MeOH showed the highest conversion (97%) of estragole 7 with good trans selectivity (87%) as compared to other catalyst complexes like PdCl2·3H2O, PdCl2(PPh3)2, RhCl3·3H2O, RhCl(SbPh3)3, and RuCl2(SbPh3)3. In general, the palladium and rhodium transition metal complexes showed a higher conversion in the isomerization reaction, but with much lower selectivity in terms of the ratio of trans:cis isomers. Later, these researchers extended their research to the isomerization of eugenol 5 into isoeugenol 18 and also to the use of a variety of solvent systems to find the optimum BF


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conditions.659 In the second study, RuCl2(PPh3)3 in ethanol was found to work best for estragole 7 (99.7% conversion and 95.4% selectivity). However, RuCl3(AsPh3)2·MeOH in methanol again gave comparable results (94.2% conversion and 98.6% selectivity). Crochet and Cadierno recently disclosed results pertaining to application of ruthenium complexes to the selective isomerization of estragole 7 into the industrially important transanethole 20, this time under “green” conditions (Scheme 134).660 In the first investigation, it was found that the

In a related paper by these researchers, ruthenium complexes 516 and 517, as well as mononuclear derivatives of 516, that is, 518, were used for the same isomerization reaction.662 For a short section on isomerization, in a review on the application of this particular class of ruthenium complexes, see ref 640. In this research, it was concluded that complexes 516 and 518 [where L = P(OMe)3] were particularly efficient and very stereoselective (>99% trans), when methanol was used as solvent. Again, reaction conditions were moderate because the temperature used was 80 °C and the catalyst loading a reasonable 1 mol %, resulting in complete conversion after 5−30 min. In addition, the excellent stereoselectivity was maintained in the more environmentally friendly media such as water−methanol and glycerol−methanol solutions. Cadierno et al. extended the application of the ruthenium(IV) dimer [RuCl(μ-Cl)(η3;η3-C10H16)]2 516 for the specific isomerization of a range of allyl phenyl esters into the corresponding (E)-(1-propenyl)phenyl esters.663 This particular ruthenium complex gave the products with complete Eselectivity and worked well in methanol with high yields of product being obtained. In addition, a broad range of substituents on the aromatic core was tolerated as is illustrated by the successful application of this approach. Of significance in this regard was that the researchers found it necessary to have to acylate the phenol 519 into the corresponding acetate 520 prior to the isomerization step employing the ruthenium(IV) catalyst 516 to obtain the bis-isomerized analogue 521 in moderate yield (Scheme 135). The reason for this rests on the

Scheme 134a

Scheme 135

a

tppms = 3-diphenylphosphinobenzenesulfonate sodium salt, PTA = 1,3,5-triaza-7-phosphatricyclo[3.3.1.13,7]decane, PTA-Bn = 1-benzyl3,5-diaza-1-azonia-7-phosphatricyclo[3.3.1.13,7]decane chloride.

ruthenium complexes 515a−e (0.2−1 mol %) were all very effective in performing the desired isomerization in polar solvents (i.e., water, ethanol, and methanol) at 80 °C, with high selectivity being observed for the trans-isomer of anethole 20 (Scheme 134). It was also found that isomerization reactions were reproducible on a larger multigram scale. In addition, under microwave heating conditions, the reaction time was reduced to less than 5 min, in comparison with traditional heating conditions that required significantly longer times. In a second study, Crochet and co-workers optimized the same set of ruthenium complexes for the isomerization of eugenol 5.661 In this particular study, it was found that the rates were increased by the addition of sodium hydroxide or sulfuric acid to the aqueous reaction media (absence of the transition metal complexes under these conditions resulted in no isomerization being observed). The researchers postulated that both additives resulted in more open coordination sites on the ruthenium metal−sodium hydroxide by promoting the release of the η6coordinated arene ligand and sulfuric acid by increasing Ru−Cl bond dissociation, thus resulting in an isomerization rate enhancement.

finding that were the sequence of the steps used reversed, that is, first isomerization of the allylphenols followed by phenolic acylation, a significant number of by-products were observed during the isomerization step. 5.4.4. Cationic Ruthenium Complexes. The Grotjahn group has in the past been investigating ruthenium catalysts such as cationic complex 522664−666 (Scheme 136) for their propensity to isomerize alkenes over one,667,668 or many positions,669,670 and even engage in productive tandem “internal-to-terminal” isomerization-CM processes.671 In addition, these researchers have been intrigued as to why this complex demonstrates such excellent stereoselectivity for the Eisomers of the isomerized allyl groups.669,670 In terms of application, Erdogan and Grotjahn reported that the ruthenium BG


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Scheme 136

Scheme 137

complex 522 was able to perform a biphasic proton−deuterium exchange at the allylic position of estragole 7, to obtain after isomerization mainly the E isomer 523 (i.e., isomerizations do not occur by way of a metal−hydride species) (Scheme 136). This occurred when using (2−5 mol %) of the catalyst 522 and at moderate temperatures (25−70 °C).672 Later, this research group demonstrated how stereoselective the isomerizations could be, with >99.5% E-isomers being obtained for reactions with low catalyst loadings (0.01−0.1% mol), short times (frequently ca. 10 min), and at ambient temperature.667 Allylbenzene substrates explored by Larsen and Grotjahn included eugenol 5, estragole 7, and the TMS ether 524. Of note was that with eugenol 5 the isomerization was performed on a 4.6 g scale, and that the resultant isomerized E-product was obtained by distillation, without formation of any of the undesired Z-isomer. It should be noted that in the general ruthenium-isomerization field as it currently stands, the Grotjahn catalytic system is cited as the being one of the best in terms of catalyst loading, reaction yields, and E-stereoselectivity. Recently, these same researchers have also extended their arsenal of 2-propenylbenzene isomerization catalysts to include a polystyrene supported version, thus allowing for ease of catalyst removal without a decrease in catalyst selectivity.673 Nolan and co-workers developed another cationic ruthenium complex, which displayed excellent isomerization capabilities.674 Complex 526 was readily synthesized from complex 525 under basic conditions (Scheme 137), which in turn had been synthesized from RuCl2(PPh3)3 as described earlier.675 Complex 525 did in fact promote isomerization of 1-octene, but turned out to have poor substrate scope. However, reaction of 526 with sodium tetrakis-[3,5-bis(trifluoromethyl)phenyl]borate resulted in the production of a cationic ruthenium complex 527 with much better isomerization capabilities. Application of this catalyst to the isomerization of a small library of substituted allylbenzenes gave good conversions (81% to >99%) and good E:Z ratios of >7:1. In addition, the turnover number with allylbenzene 3 was determined to be over 75 000. In some other experiments, involving the ethenolysis of isoeugenol 18 and anethole 20, the precatalyst optimization system 528 was found to be the most effective (for more discussion on the value of ethenolysis and naturally occurring

allylbenzenes, see section 5.1.1, Scheme 105). Finally, the researchers made some preliminary comments regarding the proposed mechanism of the isomerization process by catalyst 527. They proposed that the isomerization either relied on the formation of a ruthenium hydride species, which undergoes the usual hydride addition/elimination sequence, or alternatively proceeds by way of a hydridoruthenium(η3-allyl) intermediate. Of interest with respect to these discussions is that the hydrido complex 529, however, was far less active than the species 527 (see section 5.4.5 for more discussion on ruthenium hydride species). Finally, in this section, concerning competition between hydrogenation and isomerization, Horn and Albrecht observed that when they used ruthenium catalyst 530 shown in Figure 18

Figure 18.

for the transfer hydrogenation of unfunctionalized alkenes, isomerization of allylbenzene 3 was a “competing and much faster process” affording after 10 min a mixture of isomerized (50%) and hydrogenated (6%) products.676 This would suggest that this complex, containing an N-heterocyclic carbene ligand, could be a promising lead as an isomerization catalyst. 5.4.5. Ruthenium Hydrides. An important theme in ruthenium-mediated isomerization reactions is to how ruthenium hydrides affect this process. One of the earliest “modern” investigations into the abilities of ruthenium complexes to facilitate isomerization reactions was performed by Sherman and Olson.677 In this study, it was found that allylbenzene 3 was readily isomerized by the complex [(PPh3)4Ru(π-MeCN)]·MeCN at moderate temperatures BH


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(35−60 °C). As part of the study, proton NMR spectroscopy revealed the presence of two hydrido-η3-1-phenylallyl complexes, one of which was isolated. It was thus postulated that isomerization proceeded by the usual oxidative addition of the Ru(0) species onto an allylic C−H bond, thereby resulting in the η3-1-phenylallyl metal hydride intermediates. Stille and Becker carried out isomerizations of allylbenzenes and anilines, using the ruthenium hydride complex RuClH(PPh3)3678 as a catalyst.679 This catalyst was applied in boiling toluene and with a substrate:catalyst ratio of 200:1. From the experimental results, it was observed that substitution on the aniline nitrogen was essential for isomerization to 532 with the ruthenium hydride. Stille and Becker also showed that 531a and the N-substituted allylbenzene derivative 531b readily underwent isomerization, whereas the free amino derivative 531c resisted isomerization under similar conditions (Scheme 138).

Scheme 139

Scheme 138

Another ruthenium hydride catalyst that has been found to be highly active for the isomerization of arylallyl groups is RuClH(CO)(PPh3)3, and this is arguably one of the most utilized complexes in this regard. Krompiec et al. found this catalyst not only to be effective for the CC bond rearrangement of allylethers, but also that it was able to isomerize allyl-aryl ether groups to give preferentially the (Z)isomers.680 In addition, these researchers investigated and summarized the importance of functional groups (on substrates and solvents) in close proximity to allyl systems in ruthenium hydride-mediated isomerization processes.681 In this particular study, it was noticed that solvents with strong complexing abilities impeded the isomerization of safrole 6 by RuClH(CO)(PPh3)3. This was summarized with the following characteristics being observed: (a) fast isomerization solvents: benzene, toluene, acetone, tetrahydrofuran, diethyl ether, 1,4dioxane; (b) slow isomerization solvents: dimethylformamide, acetonitrile, sulfolane, triethylamine; and (c) no isomerization solvents: dimethyl sulfoxide, allyl-phenyl sulfoxide, imidazole, N-allylimidazole, diethyl sulfide, and allyl ethyl sulfide. It should also be noted that Krompiec and co-workers used the isomerization of allylbenzene 3 as a reference reaction to study the parallel isomerization of other substrates.682 Of importance is the observation that the complex RuClH(CO)(PPh3)3 has seen regular use in synthetic work (as the following examples show). Thus, Evano and co-workers used this ruthenium complex to obtain the substituted 2-(prop-1-en-1yl)phenol 534 from the precursor 533.683 The conjugated phenol 534 was then converted into the natural product abyssenine A 535 over a number of further steps (Scheme 139). In another recent example, Brimble and co-workers used RuClH(CO)(PPh3)3 to isomerize chromone 536, followed by oxidative cleavage to afford naphthaldehyde 537, which was further transformed into the natural products, the chaetoqua-

drins A−C (of which chaetoquadrin A 538 is shown in Scheme 139).684 The complex RuClH(CO)(PPh3)3 has also been applied in medicinal chemistry ventures, an example being the work by Wipf and co-workers in which 6-allyl-7-hydroxy2,3,4,5-tetrahydro-1H-benzofuro[2,3-c]azepin-1-one 539 was isomerized into its 1-propenyl analogue 540 as part of a synthetic strategy toward establishing a library of protein kinase D inhibitors.685 Interestingly enough, these researchers were required to protect the phenol group as the TBS derivative prior to the isomerization. Finally, it should also be noted that when the catalyst RuClH(CO)(PPh3)3 was used on olefins in deuterium oxide, it very effectively mediated a hydrogen− deuterium exchange process. As expected from the previous examples, allylbenzene 3 was isomerized at the same time as the hydrogen−deuterium exchange.686 In 2005, Fogg and co-workers demonstrated that some hydridoruthenium complexes such as RuHCl(CO)(NHC)(PPh3) 541, readily obtained from the parent hydrido complex RuHCl(CO)(PPh3)3, showed high activity for the hydrogenation of unactivated internal olefins and the isomerization of terminal olefins.687 It was observed that activity of the catalyst depended upon the lability of the triphenylphosphine ligand. BI


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For instance, if the ligand was replaced by a less-labile ligand, viz., PCy3, activity of the complex was diminished, and mostly hydrogenated product, propylbenzene 469, was obtained from the autoclave reactors. On the other hand, electron-rich carbenic−ruthenium complexes containing NHC ligands, viz., 541 illustrated in Scheme 140, promoted olefin isomerization in

Scheme 142

Scheme 140

competition with hydrogenation, although it should be stated that the hydrogenation:isomerization ratios were generally still in favor of the hydrogenation process (3:2 to 1:1). Williams and co-workers found that application of a sodium borohydride-Ru(PPh3)4H2−H2O combination for the reduction of alkenes generally gave good results.688 However, when these conditions were tried on estragole 7, only partial reduction to yield 542 was observed, with the majority of the product obtained being the isomerized compound 20 (70%) (Scheme 141). When the reaction time was increased from 2.5

previous section (section 5.4.5) also play an important role in this discussion. 5.4.6.1. Isomerizations by Metathesis Catalysts. Some of the earliest studies on the propensity of ruthenium catalysts, or their decomposition products, to act as isomerization catalysts, were described by Grubbs and co-workers.698,699 In this particular work, the authors demonstrated that at elevated temperatures, the second generation Grubbs catalyst GII decomposed to form the ruthenium hydride complex 543 also illustrated in Scheme 142. To prove that this compound was most likely responsible for the pre- or postmetathetic isomerizations observed in this investigation, it was demonstrated that it readily isomerized the olefinic double bond of allylbenzene 3 on an NMR spectroscopy experimental scale that is further elaborated in Fogg’s work (described below). In a subsequent publication, Grubbs was able to suppress the isomerization reactions during ruthenium-catalyzed metathesis reactions, in particular when using allylbenzene 3 as a test substrate, by the addition of small amounts (10 mol %) of 1,4benzoquinone.700 Use of this additive proved to be efficient for suppressing undesired isomerization in other systems as well. For example, Carreaux and co-workers found that this oxidant was useful when employing cross metathesis for the synthesis of alkenyl boronates, using allylbenzenes and pinacol vinyl boronic ester as substrates (it should be noted though that the Grubbs first generation GI catalyst could also be used without the addition of DDQ).701 In terms of the ruthenium species thought to be responsible for isomerization of olefins during metathesis reactions, Fogg and co-workers recently provided some intriguing insight into the debate.702 Making use of the complexes thus far blamed for the isomerization, viz., 541 and 543, these researchers were able to demonstrate that for the isomerization of estragole 7 these complexes were not “kinetically competent” enough to account for the amount of isomerization observed during homocrossmetathesis with the Grubbs second generation catalyst GII. A structure−activity study was then performed to analyze the effect of the catalyst structure on the extent of isomerization,

Scheme 141

to 22 h, and with an increase in catalyst loading (0.5 to 1.0 mol %), complete reduction to 542 resulted. It would be interesting to discover whether this system could be optimized for isomerization only in preference over reduction because this was unfortunately not investigated by the Williams group (for earlier use of the dihydridotetrakis(triphenylphosphine)ruthenium(II) catalyst, Ru(PPh3)4H2, see ref 689). 5.4.6. Isomerizations and Metathesis. The dramatic increase in the number of reports describing application of novel ruthenium-based catalysts, viz., GI, GII, and 423 illustrated in Scheme 142, coupled with the associated application of these catalysts in metathesis reactions has been a real boon to the field of isomerization catalysis.690,691 The reason for this is that it was rather quickly realized that a significant side-reaction that could occur during the actual metatheses was the isomerization of double bonds.692−697 It should be noted that the hydride catalysts described in the BJ


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and the results are given in Scheme 143. It was fairly clear that, although catalyst 546 caused more isomerization than the Scheme 143

Figure 19.

known isomerization precatalysts, but to no avail. The authors of this work finally postulated that the active species in their isomerization study was the hydrido-carbonyl complex, which formed after heating in methanol. In addition, the researchers mentioned that the first generation Grubbs precatalyst was less effective than the “thermally modified” catalyst and that application of the Hoveyda−Grubbs catalyst 423 under the same conditions only afforded homocoupled products, rather than isomerization. Of additional interest, in relation to the specific substrate 2-allylphenol 158e, was that Kappe and coworkers found that when they used the Grubbs first generation catalyst GI (2 mol %) with the additive Cy3P (10 mol %) in toluene, it resulted in the formation of 2-(prop-1-en-1yl)phenol 159e where the Z-isomer was more abundant (E:Z = 1:4).704 In contrast, using potassium hydroxide in ethanol (microwave heating or flow reactor), or by using the Grubbs first generation catalyst with triphenylphosphine as additive, resulted in a complete reversal of E:Z selectivity. Unfortunately, as the Z-selective conditions could not be performed at higher temperatures, probably due to catalyst decomposition, this process was not attempted in a “high-temperature/pressure capillary flow reactor” which was part of the main theme of the paper. During some early metathesis investigations involving mechanistic studies on ring-opening metathesis polymerization (ROMP) catalysts, Karlen and Ludi found that the catalyst system [Ru(H2O)6]2+ (with counterions bis-p-toluenesulfonate or bis-trifluoromethanesulfonate) facilitated the catalytic CC bond migration of substituted allylbenzenes, instead of giving the expected metathesis results.705 It should be noted that this catalyst was the same one used by McGrath and Grubbs in some in-depth mechanistic studies discussing the hydride addition−elimination mechanism of aqueous alkene isomerizations.706 By using deuterium labeling studies, these researchers were able to conclude that the mechanism of isomerization involved a stereospecific syn 1,2-addition− elimination sequence of a transition metal hydride intermediate, with the transition metal predominantly attacking position 2 of the allyl group.707 The authors also mentioned that this particular mechanism would account for the high selectivity in terms of the trans product being produced during isomerization. The value of this particular study is that the syn 1,2-

complexes 544 and 545, the triphenylphosphine derivatives 547 and 548 were much more active in this regard. This observation provided evidence that the complexes with the weaker donor ligands were more likely to cause isomerization; that is, the more electron-deficient ruthenium metals were more prone to promote isomerization. Clear evidence for this may be found in the impressive isomerization ability of the cationic ruthenium complexes synthesized by Grotjahn and coworkers; see earlier.667,669 Fogg and co-workers thus proposed that the complexes actually responsible for the isomerization during the use of the Grubbs second generation catalyst GII are in fact decomposed ruthenium species that have lost the strongly donating NHC and/or PCy3 ligands. Second, the authors also note that their studies indicate that the isomerization occurs by way of an associative mechanism in terms of the olefin,689 in contrast to the dissociative mechanism proposed in another recent study.692 Fogg and co-workers finally concluded that isomerization of olefins during metathesis reactions could be operating by way of a π-allyl mechanism (rather than via the hydrido species) and that this isomerization would thus be most problematic at high starting material concentrations. Of note in this section is that Hanessian and co-workers developed a very successful isomerization method based on the application of a “thermally modified” Grubbs second generation catalyst GII.703 In this approach, a methanolic solution of the allyl substituted compounds, which included a range of allylbenzenes, was treated with this modified catalyst at 60 °C for 3−12 h (Figure 19). This resulted in the desired propenyl derivatives shown, all in excellent isolated yields, and with a preference for the E-isomers (even indole 552 was readily synthesized). Of interest was that the precursors for the acetophenones 550 and 551 were also treated with catalytic amounts of RhCl3·H2O, RhCl(PPh3)3, and RuH2(PPh3)4, all BK


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addition−elimination sequence of the metal hydride species was confirmed without the presence of any chelating functionalities such as carbonyl or hydroxyl groups, as these groups can lead to changes in the mechanism of isomerization.707 Fogg and co-workers investigated the hydrogenation and isomerization abilities of the H2 and the hydridocarbonyl analogues of the Grubbs first generation catalyst GI, complexes 545 and 553, respectively (Figure 20).708 Complex 545 proved

Scheme 144

Figure 20.

to be more reactive than the dihydrogen complex 553, and in terms of the isomerization of allylbenzene 3 versus hydrogenation, this complex was 15× faster (as compared to 30× for complex 553). The superior performance of complex 545, over the dihydrogen complex 553, was postulated to be due to the greater stability of 545. Of additional interest is that Caballero and Sabo-Etienne observed that when they used the related complex RuH2(H2)2(PCy3)2 as a catalyst in the hydroboration of allylbenzene 3 with pinacolborane, any excess alkene was facilely isomerized into trans-ß-methylstyrene.709 It was also noted that no hydrogenation occurred under the reaction conditions used. On an aside involving dihydrido species, Bray and Mawby also demonstrated that ruthenium complexes with the general formula Ru(CO)2H2L2 (where L = PMe2Ph or AsMe2Ph) were able to isomerize allylbenzene 3 (although hydrogenation was observed as a side reaction in this instance).710 Competing isomerization processes have regularly been reported in articles describing the metathetic behavior of new or modified known catalysts. A recent paper by Verpoort and co-workers critically analyzed both the cross-metathesis and the isomerization abilities of the Hoveyda−Grubbs catalyst 423 including a range of novel indenylidene-Schiff base-derived ruthenium complexes synthesized in their laboratories, viz., 554 in Scheme 144.711 In these studies, the ability of the catalysts to promote the cross-metathesis between allylbenzene 3 (1 equiv) and cis-1,4-diacetoxy-2-butene 555 (2 equiv) was investigated, and resulted in the formation of a wide range of crossmetathetic products 556−559, including the isomerized products 4 and 559 (see a related reaction, which used the Hoveyda−Grubbs first generation type catalyst immobilized on mesoporous molecular sieves712). It should also be noted here that Grubbs and co-workers, 713,714 as well as other researchers,715−719 have regularly used allylbenzene 3 as a test substrate for homocoupling (and even heterocoupling720) cross-metathesis reactions. During these experiments, 1propenylbenzene 4 and 1,4-diphenyl-1-butene 559 (due to isomerization after the cross-metathesis of the allylbenzene) are frequently obtained as side-products due to the ability of the ruthenium carbene catalysts or their side products to promote isomerization. In terms of “cross-metathesis”, it should be noted here that eugenol 5 and related phenylpropenoids have frequently been used as substrates for this reaction due to them being considered renewable feedstock.721−723 This reaction, which

is considered reliable (and even being used as an undergraduate laboratory experiment),724 does tend to have the drawback that pre- and postmetathesis alkene migration can be an issue.725 However, Fischmeister and Bruneau were able to demonstrate that the use of the additive 1,4-benzoquinone was able to suppress unwanted isomerization during their cross-metathetic transformation of eugenol 5 into conjugated ester 560 (Scheme 145).725 Other researchers have also recognized the value of using compounds from natural essential oils. For example, dos Santos, Fogg, and co-workers were able to demonstrate that anethole 20, isoeugenol 18, and isosafrole 19, obtained either directly from plant material or readily obtained from their “allyl” precursors 7, 5, and 6, respectively (and generalized as 561 to include other molecules), could readily be converted into their corresponding (E)-cinnamate and (E)-ferulate esters (generalized as 562) by cross-metathesis with various acrylates.726 In an innovative example, making use of an “on purpose” isomerization/cross-metathesis sequence, Carreaux and coworkers were able to further demonstrate the value of making use of tandem processes involving isomerization as one of the key steps.727 In this particular work, ruthenium catalysts such as the modified Hoveyda−Grubbs catalyst 563 were used to generate (E)-(2-arylvinyl)boronates 566 from allylbenzenes 564 and pinacol vinylborate 565 (Scheme 146). Under the optimized reaction conditions, good yields of the (E)-4,4,5,5tetramethyl-2-styryl-1,3,2-dioxaborolanes 566 were obtained. In addition, a wide variety of substitutions on the aromatic partner were tolerated. These researchers demonstrated that the (E)(2-arylvinyl)boronates 566 could readily be converted into their respective (E)-(2-iodovinyl)benzenes and (E)-(2BL


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of ruthenium hydride-bearing Schiff base ligands on the allylbenzene isomerization (Scheme 147).729 Isomerizations

Scheme 145

Scheme 147

were studied in a number of solvents, and the maximum conversion of allylbenzene 3 into 4 was observed when isomerization was carried out in 2-butanol at 80 °C. The researchers pointed out that the inertness of their catalysts, with respect to moisture and air, represented a significant advantage. Scheme 147 illustrates one of Verpoort’s best catalysts, viz., 567, which used a nitro-bearing ligand. Verpoort and co-workers developed another set of O,Nbidentate ruthenium azo complexes as depicted in Figure 21.730

Scheme 146

Figure 21.

The complexes contained either an azo group, viz., 568 or 569, or an imino group in 570. These catalysts were found to be very good at isomerizing allylbenzene 3, and also producing very little of the cis-isomerized products. The group postulated that the absence of the cis isomer could indicate a “lack of cis/ trans isomerization of β-methylstyrene”, which is different from that observed for isomerizations performed with most late transition metal complexes. The Grubbs second generation catalyst GII has found its use in another allyl group isomerization as demonstrated by Nishida and co-workers.562,731 These researchers found that the use of the Grubbs second generation catalyst GII, in the presence of 10 equiv of vinyloxytrimethylsilane, facilitated isomerization of allylbenzenes 1 into 2 (Scheme 148). For allylbenzene itself, the isomerized product was isolated in a poor yield of 34% despite the estimate that the isomerization was quantitative by 1H NMR spectroscopy. On the other hand, 1-allyl-4-methoxybenzene was effectively isomerized (78%). For this interesting application of the second generation ring-

azidovinyl)benzenes (not shown). Finally, the same (E)-(2arylvinyl)boronates 566 could easily be used in Suzuki− Miyaura cross-coupling reactions to afford substituted stilbenes (not shown), or used in a multicomponent Petatis reaction (not shown), thus highlighting the synthetic value of the compounds so readily generated by the isomerization−crossmetathesis reaction sequence. Note should also be taken here of a related tandem process described by Carreaux and coworkers, which involved substituted allylbenzenes and vinylboronic acid pinacol ester in a “cross-metathesis/isomerization/ allylboration sequence” resulting in antihomoallylic alcohols.728 These steps were catalyzed successively by ruthenium carbenes and an iridium catalyst. Verpoort and co-workers continued on this research theme by publishing results based on the ability displayed by a library BM


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catalyst as isomerization facilitator,732 and references cited therein) played an important role in the isomerization of substituted 2-allylphenols 571 to 572, followed by an Oallylation to 573 and RCM with the second generation Grubbs catalyst GII, to afford the 2H-chromenes 574 in mostly good yields.320,733,734 It should be noted that this same catalyst has been used by other researchers on a similar substrate and with the same result, albeit in an unsuccessful attempt to achieve a ruthenium-mediated cyclization.735 Last, with respect to these classes of compounds, it should be noted that when van Otterlo and co-workers applied a double isomerization strategy on substrate 575, followed by RCM with the Grubbs second generation catalyst GII, the 1H-isochromene 576 was produced in a good yield of 83% over two steps.320 In another example from the van Otterlo group, a “double” isomerization strategy on the bis-allyl substrates 577 readily afforded compounds 578, in which, most interestingly, both the O- and the C-allyl groups were isomerized as illustrated in Scheme 150. These compounds were then facilely converted into their respective substituted benzofurans 579 by RCM,565 in generally acceptable yields.320,733,736 A related approach was employed by Wang and co-workers, with the difference being that they generated the substituted benzofurans from bisalkenes, which in turn were generated by potassium butoxide-mediated isomerization and elimination reactions as also shown in Scheme 150.737,738 The Wang group transformed ethers 580 into the conjugated ethers 581 followed by RCM with the Grubbs first generation catalyst GI using an essentially similar approach to afford their benzofurans and applied this

Scheme 148

closing metathesis catalyst, the isomerized products were also obtained with a good E/Z ratio above 8.5. 5.4.6.2. Allylbenzene Isomerizations Applied for Metathesis. In this section are some examples highlighting how allylbenzene isomerization has been used in the preparation of substrates for RCM reactions, as well as an example describing the preparation of novel metathesis catalyst systems. van Otterlo and co-workers used the RuClH(CO)(PPh3)3 catalyst complex described earlier, with much success in their application of isomerization-RCM strategies toward the synthesis of various benzofused scaffolds. This particular approach was successfully used in the synthesis of oxygen- and nitrogencontaining benzofused heterocycles320,321 as exemplified in Scheme 149 in which the ruthenium hydride catalyst RuClH(CO)(PPh3)3 (see reference for earlier use of this Scheme 149

BN


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Scheme 150

Scheme 151

strategy in their further work on substituted coumarins739,740 and 2,5-dihydro[b]oxepines741 (not illustrated). The strategy used by the van Otterlo group was amended to afford nitrogen-containing annulated compounds, albeit with less success. For instance, allylbenzaldehyde 583 was readily isomerized with RuClH(CO)(PPh3)3 to afford 584, which was followed by a reductive amination and N-protection, to produce the bis-alkene compounds 585a (Scheme 151).742 These compounds were then converted into their respective 2,3dihydro-1H-2-benzazepines 586 with the second generation Grubbs catalyst GII. In a similar vein, the bis-allyls 585b were isomerized with the same ruthenium isomerization catalyst RuClH(CO)(PPh3)3, affording 587 in good yields.321,742 When the latter were treated with the second generation metathesis catalyst GII, 1,2-dihydroisoquinolines 588 were obtained. Of interest was that when 585b (R = SO2Bn) was treated with RuClH(CO)(PPh3)3, under high temperature conditions (solventless, 135−140 °C), only compound 589 was obtained, a product from which the N-allyl group had been cleaved under the reaction conditions.743 On an aside, it should be noted here that when Kotha and Shah synthesized 1-benzazepines involving the sequential use of a Suzuki−Miyaura allylation and then an N-allylation reaction followed by RCM, they encountered arylallyl isomerization issues (up to 27%) during

the base-mediated N-allylation reaction (allyl bromide, KOH, TBAI, THF).744 Another example of application of the RuClH(CO)(PPh3)3catalyzed isomerization of allylbenzene derivatives, utilized in the previous examples, can be found in the research published by Lavigne, Grela, and co-workers.745 However, rather than preparing a substrate for metathesis, the researchers used the isomerization to prepare novel metathesis catalysts. In this study, the researchers endeavored to design and evaluate various novel metathesis precatalyst systems based on the Grubbs/Hoveyda olefin metathesis catalyst. To this end, substituted allylbenzenes 590 were isomerized by RuClH(CO)(PPh3)3 to afford a selection of isomerized compounds 591 in variable yields. These styrene derivatives 591 were then reacted with the Grubbs second generation catalyst GII to generate novel polyfunctional metathesis precatalysts 592 and 593 (Scheme 152). In the same paper, the researchers describe a number of other catalysts synthesized from alkylated 2-(prop1-en-1-yl)phenols obtained by the isomerization of 2allylphenol 158e, and the novel metathesis catalysts were then rigorously evaluated. It should also be noted that these researchers previously used rhodium-mediated isomerizations to synthesize another family of novel catalysts in a similar manner (see section 5.2, Scheme 115). The following example involves metathesis and isomerization, albeit that use of the molybdenum catalyst resulted in isomerization (for a section on molybdenum-mediated allylbenzene isomerization, see section 5.11.4). In this work, RCM performed on a series of (η6-arene)chromium complexes also gave interesting results, due to the isomerization of an BO


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Scheme 152

Scheme 153

arylallyl system into the corresponding styrene derivative. When Ogasawara et al. applied the molybdenum catalyst SI to substrate 594, the researchers obtained only the chromium complex with the isomerized double bond, viz., 596, instead of the expected ring-closed product 595 (Scheme 153).746 It was postulated that the isomerization process was competitive due to the fact that the metathesis was being suppressed by the steric bulk of the diphenyl phosphino vinyl functionality. In addition, application of either the first or the second generation Grubbs catalysts GI or GII to RCM of substrate 597 resulted in good yields of the ring-closed product 598, with only small amounts of the ring-contracted product 599 (∼5%) (Scheme 153). Isolation of the latter again provided evidence that the allyl group could be isomerized into the styrene analogue prior to RCM (in later work by the group, (η6-styrene)chromium complexes were utilized to eliminate complications caused by the possible isomerizations of the previously used allyl groups747). 5.4.6.3. Supported Metathesis Catalysts. In this subsection, Bowden and co-workers have demonstrated that the occlusion (adsorption) of Grubbs first and second generation catalysts (GI and GII) into a hydrophobic matrix of polydimethylsiloxane (PDMS) modified the selectivity of the catalysts for isomerization (and metathesis).748,749 Thus, treatment of a range of allyl compounds with the PDMS-occluded catalysts afforded the isomerized products in reasonable to excellent yields. Scheme 154 illustrates an example where eugenol 5 afforded isoeugenol 18 under these conditions, while under “normal” solvated conditions only the homodimerized product 600 was obtained, demonstrating the value of catalyst “tuning” by way of the occlusion method. 5.4.7. Isomerization during C−H Activated Allylations. Ruthenium complexes are regularly used for the generation of carbon−carbon bonds, and examples include those involving the allylation of aryl substrates. In terms of this topic, Oi, Inoue, and co-workers reported on the selective allylation at the orthoaryl-positions of 2-pyridylarenes 601 with allyl acetate in the presence of a ruthenium(II)−phosphine complex to afford mixtures of products 602 and 603 (Scheme 155).750 Of relevance to this particular Review is that these researchers identified postallylation isomerization as a significant challenge,

Scheme 154

as in many cases up to 30% isomerized products were obtained under the conditions used. In case of 3′-methoxyarylpyridine 604, although the yield was low (30%), no isomerized products were observed with 605 and 606 being the only compounds isolated. The researchers proposed that coordination of the methoxy group to the ruthenium catalyst lowered the catalytic ability of the ruthenium catalyst for the allylation and the olefin isomerization (for a related example involving the C−H allylation of benzoic acids without the concomitant isomerization problems, see the following reference751). With direct reference to the previous research work, Zhang and co-workers made use of a RuCl3-catalyzed allylation, which BP


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these systems the E:Z selectivity for the ruthenium catalysts was very good to excellent, a trait demonstrated by many of the ruthenium catalysts. In terms of other ruthenium catalysts that had good selectivity profiles, and function under “greener” conditions, the work of Crochet, Cardierno, and co-workers should be noted too.660 In terms of exciting catalyst systems, the recent work of the Grotjahn group has produced a series of cationic ruthenium complexes, which are able to isomerize allylbenzenes with very low catalyst loadings (0.01−0.1% mol), resulting in almost exclusively the E-isomers of the isomerized products (the system is also scalable to preparative multigram quantities).664−667 Finally, this section described the close relationship between isomerization of 2-propenylbenzene substrates and ruthenium hydride catalysts (see, for instance, the work by Fogg687,702) and metathesis catalysts (see Grubbs’ contribution699,700) and the impact of these topics on the field of metathesis. From the ample examples provided, it can be clearly seen that the field of metathesis has had an important impact in terms of the discovery and design of new rutheniumcontaining systems that are able to isomerize 2-propenylbenzene systems into their 1-propenyl analogues.

Scheme 155

5.5. Iridium-Catalyzed Isomerizations

Application of iridium complexes as allylaromatic isomerization catalysts has seen sporadic research activity. Of particular interest described in this section is that Ley and co-workers elegantly demonstrated the application of supported iridium catalysts toward total syntheses requiring allylbenzene isomerization, and this will be described in the second part of this section. 5.5.1. Homogeneous Iridium Catalysis. The first work to be described in this section is by Cooper and Caulton, who in 1996 reported the catalytic isomerization of allylbenzene 3 to (E/Z)-β-methylstyrene using Ir(III) hydride halides, hydrido hydroxides, and hydrido alkoxides with the formula Ir(H)2X(PtBu2Ph)2 (where X = F, CI, Br, I, OH, or OCH2CF3).753 In a detailed investigation into the mechanistic aspects of these isomerizations, the authors postulated that significant variations in the rates of the isomerizations could be attributed to differences in the σ- and π-donation from the X ligands, which would then influence the rate of catalysis in a number of ways. The authors of this research postulated that the active isomerization catalyst in these experiments was a 14-valenceelectron iridium(III) species. In addition, from the experimental results obtained pertaining to the isomerization of α,α-d2allylbenzene in the presence of 4-allylanisole, the researchers concluded that the isomerization proceeded by way of a metal hydride insertion-elimination mechanism. In another investigation, Crabtree and co-workers observed significant isomerization of the propene chain of allylbenzene 3 during their investigations into the dehydrogenation of different alkanes with the complexes IrH2(O2CCF3)(PAr3)2. This result turned out to be the exception rather than the rule because in the case of other alkenes, isomerization was observed to be less than 5%.754 The researchers further demonstrated that the complex IrD2(O2CCF3)[P(p-FC6H4)3]2 also catalyzed isomerization of allylbenzene 3. In addition, it was shown that deuterium incorporation occurred at all three of the propene positions. In another investigation by Crabtree and co-workers, it was shown that when the complex [IrH2(Me2CO)2(PPh3)2]BF4 was reacted with allylbenzene, the η6-arene complex [Ir(η6PhCH2Et)L2]BF4 was formed, which proceeded by way of the prop-1-en-1-ylbenzene species.755

occurred only at the ortho position of 2-phenylpyridine 601 (Scheme 156). In this research, allylic compounds (including Scheme 156

allyl chloride, bromide, and acetate) were used, and in contrast to the results by Oi and Inoue described above, the major product turned out to be the isomerized 603, rather than 602.752 5.4.8. Summary Concerning Ruthenium-Mediated Isomerizations. RuCl3·xH2O is arguably the most simple ruthenium system used for the isomerization of fairly robust substrates, although phosphine-containing complexes based on RuCl2(PPh3)3 have also been shown to be important. The water-soluble system designed by Joó and co-workers,654 RuCl2(tppms)2, seems to be an interesting approach with possible larger scale applications, and Jasra et al. have demonstrated that RuCl2(PPh3)3658 (and derivatives thereof659) can be used without solvent or in alcohol solvents (also see the work of Fanun and co-workers in the use of this catalyst in microemulsions655). It should be noted that for most of BQ


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this work tested their catalyst system on a variety of substrates (Scheme 158). Electron-rich allyl-aromatic compounds 1 were

Faller and Crabtree also demonstrated that some air-stable chelated iridium(III) bis-carbene complexes, specifically designed as catalysts for transfer hydrogenation, could facilitate isomerizations.756 These researchers showed that complexes such as 604 could catalyze a double-bond migration when using allylbenzene 3 as substrate (reflux, iPrOH, KOH). After 18 h, βmethylstyrene 4 was obtained as the major product in good yield (>90%) (Scheme 157). Of interest in this case was that no

Scheme 158

Scheme 157

hydrogenation product, that is, propylbenzene, was observed from this reaction. Other iridium complexes encapsulated in “bulky CCC-pincer N-heterocyclic carbene ligands” were also able to facilitate similar isomerizations.757 In addition, these researchers were able to demonstrate by in-depth isotope and crossover experiments that the catalyst structure directly affected the mechanism by which the allylbenzene 3 is isomerized.150 For example, the CCC-pincer complex with the mesityl groups, viz., 605 proceeded by way of a π-allyl mechanism, whereas that of the adamantyl analogue 606 appears to proceed by a yet undetermined isomerization mechanism. Note should also be taken of a recent finding by Brookhart, Krogh-Jespersen, and Goldman who studied the isomerization of 1-alkenes by iridium pincer catalysts by an indepth DFT study and concluded that the “hydride addition pathway” was not active during the isomerization mechanism.758 5.5.2. Supported Iridium Systems. In 2002, Ley and coworkers reported on the development of a polymer-supported iridium isomerization catalyst ●-[(PPh2)2Ir(H)2(THF)2]+PF6−,543,544 based on Felkin’s catalyst,759 and activated it with molecular hydrogen prior to introduction of the substrates. This heterogeneous catalyst system was effective in promoting isomerizations at ambient temperature, and workup was simply affected by filtration and evaporation of the solvent, which afforded the isomerized products. The authors of

isomerized almost exclusively to their respective (E)-products 2, while electron-poor substrates were less susceptible to the catalyzed rearrangement. The researchers also demonstrated that the catalysts were recyclable, although longer reaction times were required in subsequent isomerization runs. The same polymer-bound catalyst was then used by Ley and co-workers during the isomerization of 6-allylbenzo[1,3]dioxol5-ol 48 into 49, a transformation used in the elegant synthesis of the natural product carpanone 607 (Scheme 159).543,760 It is interesting to note that these researchers spent considerable effort in applying other catalyst systems to this isomerization, including polymer-bound catalysts (for example, Wilkinson’s catalyst) and homogeneous systems such as NaOEt−EtOH and KOH−DMSO under microwave heating conditions, without any success. However, when using the iridium system Ir(cod)(PPh2Me)2PF6, under hydrogen activation, the desired isomerized product was obtained in good yields (85−88%) and in excellent trans:cis ratios (127:1). It was on the basis of this result that a heterogenized version of the iridium catalyst was thus synthesized and tested successfully to afford 49 in good yield (97%) and selectivity (92% trans). This compound was then efficiently converted into the natural product carpanone 607 in three steps using only polymer-supported reagents and without the requirement for conventional purification techniBR


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Scheme 159

Scheme 160

ques, an example ably demonstrating the power of this synthetic approach.761 Carpanone 607 is a most interesting compound as it has been proposed that the biosynthetic pathway to this compound involves the dimerization of carpacin 49, because both have been isolated from the same plant material. In 1971, Chapman and co-workers demonstrated that 2-propenylsesamol 49 could be oxidatively dimerized with palladium(II) chloride,762 an approach modified and used by a number of other researchers to obtain these interesting spirohexacyclic structures763−767 (also see the work from Kirchhausen, Shair, and co-workers, who synthesized a 10 000-membered carpanone library based on 1-propenylaromatic derivatives768). Recently, Beifuss and co-workers demonstrated how valuable (E)- and (Z)-1propenylbenzenes are by using laccases to obtain three carpanone diastereoisomers.769 Of interest is that the authors of this work obtained pure samples of (E)- and (Z)-1propenylsesamol 49 by the potassium tert-butoxide in DMSO method described in detail earlier in this Review (see section 4.2). In a related paper requiring isomerization of allylaromatic substrates, the Ley group again focused on using chemistry employing polymer-supported reagents and scavengers to facilitate purification and thus limit chromatographic purifications. In this work, the Ley group used the same hydrogenactivated polymer-bound iridium catalyst described earlier for the isomerization of 4-allyl-2,6-dimethoxyphenol 365 into (E)2,6-dimethoxy-4-(prop-1-enyl)phenol 366 required for the total synthesis of (+)-polysphorin I 369 (Scheme 160).770 See similar syntheses of these compounds in Scheme 87. Mention should also be made here of related work by Riva and co-workers who demonstrated that the immobilized version of Felkin’s iridium catalyst could be successfully used to isomerize 8-allyl-7-hydroxy-4-methyl-2H-chromen-2-one under flow reactor conditions (reaction not shown).771 Blum and co-workers also developed a polymer-bound iridium hydrogen transfer catalyst,772 this time based on the IrCl(CO)(PPh3)2 complex. These researchers found that when they tested their polymer-bound iridium catalyst, ●-[(PPh2)2Ir(CO)Cl]n, with formic acid as donor, the substrate allylbenzene was not hydrogenated efficiently (only 32% propylbenzene obtained). Analysis of the reaction mixture then illustrated that the remaining 68% had rearranged to an equilibrium mixture of (E)- and (Z)-isomers of the β-methylstyrene 4. This result would suggest that it might be possible to optimize this

supported catalyst system to only isomerize allylbenzene substrates. 5.5.3. Summary Concerning Iridium-Mediated Isomerizations. Of interest for the use of iridium allylbenzene catalysis is that there seems to have been more investigation of supported systems than the homogeneous complexes. Of significance has been the development of a polymer-supported iridium isomerization catalyst ●-[(PPh2)2Ir(H)2(THF)2]+PF6− by Ley and co-workers.543,544 This supported complex was then elegantly utilized for the isomerization of available allylaromatics in excellent yields and trans-selectivities, as well as in two total syntheses. 5.6. Iron-Catalyzed Isomerizations

Iron compounds have the advantage of being relatively inexpensive as compared to many of the other transition metals used as catalysts in isomerization reactions and have consequently been rigorously investigated and placed under intensive scrutiny, particularly in recent years.773 5.6.1. Homogeneous Iron Catalysts. Mechanistic studies with respect to the use of iron catalysts for isomerization have also been undertaken, with as an example a fairly recent paper discussing the mechanism by which Fe(CO)5 isomerizes alkenes under photochemical conditions.151 This catalyst was known to isomerize allylaryls, such as allylbenzene 3 and allylpentafluorobenzene, in quantitative yields and with high trans selectivity under photochemical and thermal conditions (see section 6.3).774,775 In addition, the use of a catalytic iron pentacarbonyl (5 mol %)/hydroxide ion combination was also used by Hansen in 1977 to afford the E-isomers of substituted allylarenes (conditions: solventless, 180−190 °C, 20 min).211 The next section will illustrate a number of examples from 1980 on, in which other iron complexes have been applied in the isomerization of allylbenzene compounds. Carrying on with the catalytic theme of iron pentacarbonyl, De Pasquale employed this catalyst in 1980 for the specific isomerization of estragole 7 to anethole 20 in >90% yields on a large scale (45 g of estragole 7).776 The reaction was performed under thermal conditions of 140 °C for 8 h without solvent, and the high purity product was readily obtained by “recycling a distillation precut”. De Pasquale postulated that the reaction mechanism proceeded by way of an initial carbon monoxide dissociation from the iron pentacarbonyl, followed by πcomplexation thereof with the allyl system. A subsequent C−H BS


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oxidative addition then afforded a π-allyl iron hydride from which reductive elimination would lead to the product. Boeckman and co-workers have made use of the De Pasquale conditions to convert the safrole-derived 608 into the substituted prop-1-en-1-ylbenzene isomer 609 (Scheme 161).777 This compound was subsequently employed in the synthesis of the core structure of lycorine (not shown).

decrease in the E/Z ratio was observed for the isomerization of estragole 7 into anethole 20. One of the catalysts tested by Beller and co-workers mentioned in the previous paragraph was the iron complex, Na2Fe(CO)4, and it was concluded that its catalytic application only afforded trace amounts of isomerized compounds under the reaction temperatures tested (80 °C). Of interest was that Reddy and Periasamy had previously used two stoichiometric iron systems using Na2Fe(CO)4 as the key component.780 These authors were able to show that under very mild conditions (25 °C, 12 h), the combinations Na2Fe(CO)4/CuCl or Na2Fe(CO)4/BrCH2CH2Br were able to promote isomerization of safrole 6 and allylbenzene 3 in good yields (75− 87%). Staying in the field of iron carbonyl compounds, the group of Darensbourg reported the isomerization capability of bis(triphenylphosphine)iminium salts of simple mixed-transition metal carbonyl hydrides of the general formula HFeM(CO)8L− (M= Cr, Mo, or W and L = CO or PR3).781 These compounds were found to catalyze the isomerization of allylbenzene 3 under mild conditions, consisting of room temperature and with irradiation from fluorescent lighting. The authors commented that the isomerization ability of the simple heterobimetallic hydrides HFeM(CO)9− was greater than that of either parent homobimetallic hydride [HFe2(CO)8− or HM2(CO)10−] or their fragment components [HFe(CO)4− or M(CO)50]. The use of bis(triphenylphosphine)iminium salts of HFe(CO)4− as an isomerization catalyst was also investigated, and its effectiveness was found to be highly dependent on the various additives. The research group led by Jacobi von Wangelin has in recent years published a number of novel organometallic methods of particular interest in terms of synthetic protocols, which take environmental concerns into account. In terms of isomerization procedures, this group recently reported an interesting protocol, which involved the use of an economical and environmentally benign iron catalyst, Fe(acac)3, in the presence of a reductant.782 The researchers observed the best catalytic activity with respect to the isomerization of allylbenzene 3, by the application of a catalytic amount of Fe(acac)3 (5 mol %) with the simultaneous use of PhMgBr as a reductant. These conditions further proved to be amenable to isomerize a variety of substituted allylbenzenes 1 → 2 (Scheme 163) with good results being obtained in terms of yields (67−98%) and E/Z selectivity generally greater than 15:1. It should be noted though that the researchers found it necessary to add relatively large quantities (50%) of the reductant PhMgBr to improve the reliability of the reactions. Of additional interest is that these researchers developed a one-pot domino reaction involving an iron-catalyzed allylation of an arylmagnesium bromide generated from 608 followed by an iron-catalyzed isomerization of the resultant allylbenzene to afford the desired 1-propenylbenzene 2. These reactions generally occurred in good to excellent yields and thus extend the synthetic availability of the substituted 1-propenylbenzenes by way of a more environmentally benign synthetic strategy. As an aside, the Jacobi von Wangelin research group also identified an isomerization product 611 as the only product detected upon applying their method to an iron-catalyzed biaryl coupling reaction.783 In this particular example, reaction of 1allyl-2-chlorobenzene 609 with phenylmagnesium bromide in the presence of Fe(acac)3 did not result in the expected biaryl

Scheme 161

Crivello and Kong extended this research by using a mixed iron pentacarbonyl (5%)/sodium hydroxide (10%) system to facilitate the isomerization of allylbenzene 3 and 4-methoxyallylbenzene 7 under much more moderate reaction conditions (15:1 v/v ethanol−water mixture at reflux for 30 min).778 In this way, the isomerized products 4 and 20 were obtained in good yields of 86% and 94%, respectively, with high E:Z ratios (96:4). The authors pointed out that the catalyst in this isomerization was most likely the tetracarbonylhydroferrate anion [HFe(CO)4]−, generated under the reaction conditions. In addition, they stressed that their isomerization conditions represent a mild, cheap way to isomerize substrates suitable for academic and industrial applications. It has been known that other iron carbonyl combinations are also able to isomerize terminal into internal double bonds. Thus, Beller and co-workers recently employed the inexpensive and readily available iron carbonyl complex Fe3(CO)12 for the isomerization of terminal olefins.779 These researchers tested the applicability of a number of iron carbonyl complexes and additives and found that the combination of Fe3(CO)12 and base (or potassium chloride) was the most effective. A significant finding in terms of this method was that the isomerization reaction produced the 2-alkene isomers in high yield, with very little further isomerization down the alkyl chain. In the case of the allylbenzene substrates tested 1 → 2 (Scheme 162), the researchers carried out the isomerization reactions Scheme 162

under mild conditions (80−100 °C) to afford the desired isomerized products in good to excellent yields. In addition, in a modification applicable for the isomerization of base sensitive substrates, it was discovered that potassium chloride could be successfully used as an additive in the reaction, in place of KOH. With this change, although the yields were still good, a BT


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researchers also employed supported palladium, nickel, and cobalt complexes in similar hydrogenation/isomerization experiments, with the former in conjunction with oxygenbased coordination groups,787 while the latter two were with phosphorus groups (phosphinate and phosphonate) as the coordinating ligands on the polymeric supports.788 5.6.3. Summary Concerning Iron-Mediated Isomerizations. This area of allylpropenoid isomerization catalysis is particularly of interest due to the lower costs of the transition metal involved. In terms of particularly interesting developments in this area are the iron carbonyl complex Fe3(CO)12, utilized together with KCl, by Beller and co-workers,779 as well as the work performed by the Jacobi von Wangelin group utilizing Fe(acac)3.782 The latter work has been extended to an allylation-isomerization protocol further increasing the value of this work. A final comment is that supported iron catalyst isomerization systems have not seen much recent research, probably due to the inexpensive nature of the transition metal involved.

Scheme 163

5.7. Cobalt-Catalyzed Isomerizations

Another transition metal that has seen application in the field of organometallic-assisted reorganization of alkenes is cobalt, of which a number of complexes have been investigated. 5.7.1. Homogeneous Cobalt Catalysis. Roos and Orchin initially demonstrated that room-temperature isomerization of allylbenzene 3 by HCo(CO)4 [or DCo(CO)4] gave trans-βmethylstyrene 4 (and the corresponding deuterated derivatives thereof).789 After some debate concerning the mechanism of the actual isomerization,790,791 McCormack and Orchin then showed that the HCo(CO)4-catalyzed isomerization of PhCD2CHCH2, carried out in the presence of an excess of p-allyltoluene (which rearranges at a very similar rate to allylbenzene), gave mainly trans-PhCDCHCH3 as product.792 Of importance was that this result lent support to a proposed 1,2-addition−elimination mechanism. Onishi and co-workers investigated photoassisted double bond migration of allylbenzene 3 using the hydridocobalt(I) complexes, CoH[PPh(OEt)2]4793 and CoH[PPh(OMe)2]4.794 The authors proposed that dissociation of a phosphonite ligand occurs during irradiation (see section 6.3), leading to a coordinatively unsaturated species as the active isomerization catalyst. For the isomerizations, pyrex-filtered photoirradiation was provided by a high-pressure mercury lamp. Of interest was that when irradiation was stopped, so too did the isomerization reactions. Because the alkene migrations proceeded again once the mercury light was switched on, the researchers remarked that the isomerizations were “virtually photo-assisted” rather than being “true photocatalytic”.793 This was later supported by identification of a transient coordinatively unsaturated cobalt species, which was responsible for the isomerization catalytic cycle.795 It should be noted that for these photogenerated catalysts, the desired products were obtained in yields of 70− 80%, and as “kinetically controlled E,Z isomeric” mixtures. The same researchers found that electrochemical oxidation of CoH[PPh(OEt)2]4 generated a 17-electron Co(II) species, which facilely catalyzed the isomerization of allylbenzene 3, this time without the requirement of irradiation.796 In addition, under similar experimental conditions, the cobalt complex [CoH{PPh(OEt)2}4]PF6 gave similar results. Satyanarayana and Periasamy demonstrated that a substoichiometric catalyst (50 mol %) generated in situ from CoCl2, PPh3, and NaBH4 (ratio 1:3:1) isomerized allylbenzene

product 610. The only product detected was the isomerized analogue 611. In subsequent research from the Jacobi von Wangelin group,784 they optimized hydrogenation conditions involving iron(0) particles.785 In this work, a catalyst system generated from mixtures of iron chloride salts and ethylmagnesium chloride as the reductant was optimized on the substrate allylbenzene 3. Of relevance to this particular Review was that a number of the conditions applied on this substrate gave appreciable amounts of the cis-isomerized product 90 (24− 52%), along with the desired trans-1-propenylbenzene 4. 5.6.2. Supported Iron Catalysts. From the literature, it appears that supported iron catalysts have seen little application in the isomerization of arylallyl substrates. This is probably not too surprising as the plethora of supported organometallic catalysts based on precious metals is significantly driven by the recyclability of the expensive catalysts, a motivation somewhat missing when iron is used as catalyst. In one rare example, Sukhobok et al. synthesized iron complexes immobilized on a varied series of polymer supports containing phosphorus-based coordinating groups, viz., phosphinite and phosphonate groups, and then demonstrated that these catalysts were useful in the isomerization and hydrogenation of allylbenzene 3.786 Polymer supports that were used included poly-1,2- and poly-1,4butadiene, ethylene-propylene-diene terpolymer synthetic rubber, and respective copolymer combinations. The phosphonate groups were introduced by reaction with PCl5 and subsequent treatment with alcohols, while the phosphinite groups were introduced by radical addition of the polymer to PCl3. Iron chloride was then supported on the polymers as a solution. The researchers subsequently demonstrated that the catalytic activity increased as the capacity of the electrondonating ability of the phosphorus atoms decreased. It should be noted that for these supported catalysts under hydrogen (1 atm), the major reaction was indeed hydrogenation, particularly for the phosphinite-bearing supports. However, because the iron phosphonate-containing polymers were more active, they gave mixtures of products with respect to hydrogenation and isomerization products. It should be noted that these BU


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Weix and Holland have disclosed novel high-spin cobalt(II) complexes with the ability of isomerizing linear alkenes into their cis-2-alkene derivatives.800 Unfortunately, these complexes were less impressive with the allylaryl substrates, leaving room for improvement in this area. Finally, it should also be noted that very little work has been done on supported variants of cobalt isomerization catalysts able to isomerize allylbenzenes.

3 and safrole 6 into their respective (1-propen-1-yl)benzenes 4 and 19 in good yields of 81% and 80% (E isomers only), under very mild temperatures of −10 °C.797 In addition, short reaction times were required, and the reactions were generally complete after 2 h. The researchers tentatively suggested that the active catalytic species had the structure CoHCl(PPh3)2 (also corroborated by other researchers798) and commented that this intermediate could be expected to facilitate isomerization by a hydrocobaltation−dehydrocobaltation synthetic sequence. In terms of comment, it should be appreciated that not many of the isomerization methods discussed in this Review are able to isomerize allylaromatics at temperatures below 0 °C. This particular system should thus be investigated more intensively to determine whether the substrate scope can be extended. Bleeke and Muetterties made an interesting observation that the organometallic complex η3-C3H5Co[P(OMe)3]3 showed potential in the isomerization of allylbenzene, when testing the catalyst in a competitive hydrogenation experiment. 799 Unfortunately, the yields obtained were very low (1.8% cisand 17% trans-1-propenylbenzene obtained), but further investigation might lead to a worthwhile isomerization catalyst. Finally, an important paper highlighting the formation of Zalkenes by way of high-spin cobalt(II) complexes was recently reported by Weix and Holland.800 In this impressive work, the authors demonstrated that the catalyst 612 (Figure 22) was

5.8. Titanium-, Zirconium-, and Hafnium-Catalyzed Isomerizations

Group 4 transition metal complexes have also been investigated as potential isomerization catalysts, and a number of examples will be discussed here. From the literature it appears that most studies dealing with the isomerization potential of these catalysts involve their potential to promote polymerization of small unsaturated monomers,801 and the effect that this isomerization has on the final structure of the resultant polymer; see, for instance, the paper investigating the polymerization of allylbenzene 3 with a variety of zirconium and titanium metallocenes.802 Use of allylbenzene 3 as monomer has enjoyed a significant amount of investigation, sometimes with most interesting results. For example, it has been reported that addition of TiCl4 to this monomer results in very limited polymerization with most of the allylbenzene 3 being isomerized into the conjugated isomer 4.803 Of additional interest is that the majority of examples involving group 4 organometallic complexes require activation by the aluminum complex, methylalumoxane (MAO). In terms of other zirconocene complexes, Negishi and co-workers demonstrated that nBu2ZrCp2 was an effective isomerization catalyst of (2-deuterioallyl)benzene,804 while Taguchi and researchers encountered isomerization of tert-butyl 2allylphenyl[(4-methylphenyl)sulfonyl]carbonate with the zirconium−butene complex prepared from Cp2ZrCl2 and nbutyllithium.805 Rao and Periasamy discovered that a mixture of Cp2TiCl2 (20 mol %), Grignard-grade magnesium, and BrCH2CH2Br facilitated the double bond rearrangement of allylbenzene 3 and safrole 6 into the respective trans products 4 and 19 in yields of 77% and 80% (Scheme 164).806 Of importance here is that

Figure 22.

able to isomerize simple 1-alkenes selectively to give the thermodynamically less favored Z-2-alkenes with ratios as high as E:Z = 1:6. However, initial results for the allylbenzene 3 and 1-allyl-4-methoxybenzene 7 substrates were rather less impressive with the E:Z ratios being ∼2.5−5:1 (yields obtained were also rather disappointing being 20−37%). Experimental results seem to indicate that a bimolecular process was leading to catalyst degradation, and thus by lowering the reaction concentration by a factor of 20 a significant improvement in both yields and selectivities was observed (allylbenzene 3 after 40 h, 4 90%, E:Z = 1:1.4; 1-allyl-4-methoxybenzene 7 after 40 h, 20 73%, E:Z = 1:1.4). To account for the Z-selectivities observed, the authors of this work proposed a steric model based on the square-planar geometry of the cobalt(II) in the transition state during which the β-hydride elimination step occurs. Although results concerning the allylaromatic substrates are still not as good as the simple 1-alkenes, it is hoped that this type of insightful study will provide the impetus for further development of catalysts, which are able to discriminate for the more elusive Z-1-propenylbenzene isomer in the near future (see section 5.11.1 for another example giving predominantly the cis-isomer). 5.7.2. Summary Concerning Cobalt-Mediated Isomerizations. After a flurry of activity in the 1970s and 1980s, the use of cobalt as an isomerization catalyst for allylbenzenes has seen very little research in recent years. Recently, however,

Scheme 164

these conditions have the added advantage of being effective at room temperature and that isomerizations were complete within 2 h. In a subsequent publication, the authors postulated that under the reaction conditions a titanocene−ethylene complex and/or a titanium hydride species could be the agents responsible for the observed isomerization process.807 BV


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tigation to the use of a Cp2TiCl2−magnesium system (see Rao and Periasamy conditions above), which also readily gave the Eisomers of allylbenzene 3 and safrole 6 at room temperature with yields above 92% and E:Z ratios >96:4 (conditions: Cp2TiCl2:Mg:substrate = 1:5:10).814 Titanocene and zirconocene have been known to display interesting properties such as the ability to methylenate carbonyl functional groups and even promote ring-closing by way of alkene-carbonyl coupling reactions and RCM. For example, when Bennasar and co-workers attempted RCM on the substrate 615, only complex mixtures of products were obtained, which included the isomerized compounds 616 and 617 in variable yields of 20−30%.815,816 In addition, when Taguchi and co-workers attempted an alkene-coupling reaction on substrate 618 with in situ prepared Cp2ZrCl2, the isomerized product 619 was obtained instead of the expected cyclized product (Scheme 166).389

Earlier, Brubaker and co-workers demonstrated that polymer-bound versions of TiCp2Cl2 and TiCpCl3 that had been reduced with n-Buli were effective catalytic systems in terms of being able to rearrange allylbenzene 3 into cis- and trans-1-propenylbenzenes 90 and 4, respectively, and in conversions greater than 80%.808 The researchers showed that in terms of efficacy, the polymer-bound reduced TiCpCl3 was more than twice as effective as the polymer-bound reduced TiCp2Cl2. In addition, it was observed that the polymer beads with the smaller loading of titanocene dichloride were more active, a fact attributed to the better “site isolation” on the beads with lower loading.809 A limitation of the polymersupported titanium catalysts was that recycling leads to a significant decrease in isomerization potency (from ∼90% to ∼15%). Of interest is that Brubaker and co-workers also prepared polymer-supported zirconocene and hafnocene complexes, which were capable of facilitating allylbenzene 3 isomerization after reduction with butyllithium.810 On the topic of supported catalysts, it should also be noted here that in 1980, Schwartz and Ward demonstrated that silica-supported zirconium hydrides were able to isomerize allylbenzene 3.811 Whereas Brubaker and co-workers used n-Buli as a reductant, Lee and Lee were able to demonstrate that a TiCp2Cl2−LiAlH4 system was also very effective as an isomerization catalyst system.812 A significant advantage of this combination was that the researchers were able to isomerize allylbenzene 3, allylanisole 7 and safrole 6 of general formula 1 at the low temperature of 0 °C. In addition, good selectivities for the respective trans-isomers 2 were observed (Scheme 165).

Scheme 166

Scheme 165

Another set of titanium and zirconium complexes that have been found to have catalytic isomerization prowess are the complexes TiCl2[N(PPh2)2]2 and Zr(NPhPPh2)4.817 HeyHawkins and Eisen found that application of these compounds resulted in trans-methylstyrene 4 being formed selectively from allylbenzene 3. In the case of the titanium complex, the isomerized product 4 was obtained in quantitative yield after only 15 min, while the zirconium complex took somewhat longer (77% yield after 16 h at 80 °C). It should be noted that both catalysts had to be activated by MAO, prior to the addition of allylbenzene 3. In addition, the researchers confirmed that both complexes were efficient in polymerization reactions resulting in high-molecular-weight elastomeric polypropylene. Averbuj and Eisen demonstrated the isomerization of allylbenzene 3 with the octahedral early transition cationic complexes cis-[p-Me-C6H4C(NSiMe3)2]2Zr(Me)2 620 and C3tris[(NSiMe3)(N′-myrtanyl)benzamidinate]ZrCl 621 in addition to cis-[p-Me-C6H4C(NSiMe3)2]2Ti(Me)2 622 (structures not shown). These complexes were all activated by the presence of MAO and gave excellent conversions when applied as isomerization catalysts. Substrates tested in NMR spectroscopic scale experiments included allylbenzene 3, and all three complexes afforded the isomerized product with varying selectivities of E-β-methylstyrene 4:Z-β-methylstyrene 90 as follows: 620 68:32, 621 90:10, 622 100:0.818 The authors of this work proposed that the isomerizations occurred by several insertions of the zirconium or titanium hydrides, followed by β-

On the basis of research demonstrating that TiCp2Cl2 catalytic systems were effective at promoting the isomerization of allylbenzenes, Quian and co-workers investigated the effect of cyclopentadienyl ring substituents on the stereoselectivity of the isomerization process.813 To this end, these researchers synthesized a range of (C6H4CR1R2Ar)2TiCl2 complexes and combined them with isopropylmagnesium chloride to isomerize a range of terminal olefins, including allylbenzene 3, under mild conditions (RT and diethyl ether as solvent). For the complexes made with the cyclopentadienyls 613 and 614 (Figure 23), β-methylstyrene 4 was obtained in good yields of 96% and 99%, with E:Z ratios of 97:3 and 99:1, respectively. Later these researchers extended their isomerization inves-

Figure 23. BW


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hydrogen eliminations, to afford the most stable internal alkene (as per Saytzeff’s rule). In related work, the Eisen research group reported on the isomerization of allylbenzene 3 with the complexes [{NSiMe3)C(Ph)} 2 CH] 2 TiCl 2 623 and [{NSiMe 3 )C(Ph)} 2 CH][(NSiMe3)C(Ph)NC(Ph)CH(SiMe3)]ZrCl2 624 (structures not shown), once again in the presence of MAO at room temperature.819 The former catalyst gave the trans-isomer 4 almost exclusively, while the latter afforded the trans-βmethylstyrene 4 (93%), along with a small amount of 1phenyl-1-butene (4%). The fact that no cis-β-methylstyrene 90 was obtained was inferred by the authors to mean that there was no isomerization between the cis and trans isomers, in contrast to that observed for late transition metal catalysts where the isomerized products are in equilibrium. The authors of this work also postulated that the mechanisms for the isomerizations by the two transition metal complexes were different, in that 624 provided product by way of an olefin insertion and subsequent β-hydride elimination, while complex 623 was proposed to facilitate isomerization by way of an allylic C−H activation pathway. Finally, in the section describing contributions to this area by Eisen and co-workers, another titanium catalyst was studied by this group in an effort to understand the mechanism in terms of how the complex was able to polymerize propylene. In this particular study, the catalyst was confirmed to be the bis(dimethylmalonate)-bis(diethylamido) titanium complex,820 which again was activated by MAO and demonstrated its ability to promote isomerization of allylbenzene 3 to afford 95% of the β-methylstyrene 4 in 20 h at room temperature. It should be noted that this experiment was performed on an NMR spectroscopy experimental scale. 5.8.1. Summary Concerning Titanium, Zirconium, and Hafnium Catalysts. Not much recent research has been performed on allylbenzene isomerizations by the group 4 metals, and most of the successful work is based on the complex Cp2TiCl2 and derivatives thereof. Only Brubaker and co-workers808 have investigated supported versions of these catalysts, so this area of research is still very open to research.

Scheme 167

yield (94% isoeugenol 18, 99% after 15 min). It was noted that at a NaCN/NiSO4 ratio of ∼1 only a small amount of hydrocyanation occurred to afford the side-product 626 in 0.1% (Scheme 168). However, at larger NaCN/NiSO4 ratios, viz., Scheme 168

5.9. Nickel-Catalyzed Reactions

This section on nickel-mediated isomerizations begins with the descriptions of two water-soluble catalyst systems. The first involves a two-phase system reported by Monflier and coworkers in 1998821 in which the optimized nickel catalytic system was comprised of bis(cycloocta-1,5-diene)nickel(0) [Ni(cod)2], the water-soluble tetra-sulfonated phosphine dppbts 625, and a Brønsted acid (HX), dissolved in either water or a water−methanol mixture (Scheme 167). The best results were obtained when acids possessing coordinating anions (i.e., HCl or HI) were used, although these acids also led to a lower E:Z ratio. For weakly water-soluble substrates such as allylbenzene 3, the reaction rate was greatly improved by the addition of methanol to the solvent mixture. In another example of a water-soluble system, Vittori and coworkers reported that the cyanide-stabilized nickel(0)-tpptscyanide complex in basified water was suitable for the isomerization of allylbenzene 3 and eugenol 5 at room temperature (tppts = triphenylphosphine-3,3′,3″-trisulfonic acid trisodium salt 627).822 The reaction was performed in a water−ethanol mixture containing nickel(II) sulfate, tppts, sodium cyanide, and sodium borohydride, and the reaction was over in less than 6 min to give the isomerized products in high

8:1, 4-phenylbutyronitrile 626 becomes the major product and can be obtained with selectivities of over 70%. An added advantage of this method was that it involved a simple catalyst separation protocol that facilitated isolation of the desired isomerized products. Very recently, Norrby, Skrydstrup, and co-workers reported the development of a versatile nickel-catalyzed Heck reaction involving electron-rich aryl triflates and butyl vinyl ether.823 In a specific example mentioned in this paper, when these researchers used eugenol triflate 628, having an allyl group on the benzene ring, the isomerization of the allylic double bond occurred under the reaction conditions shown, giving the cis- 630 and trans-adducts 629, along with the nonisomerized ketone product 631. The authors of this work postulated that the isomerization was being promoted by a “long-lived nickel hydride complex” formed under the reaction conditions (Scheme 169). In another striking example making use of the bis(cycloocta1,5-diene)nickel(0) catalyst Ni(cod)2, Ong and co-workers demonstrated how a tandem isomerization, followed by a C−H activation, could result in the regioselective “hydroheteroarylation” of allylarenes.824 In this innovative work, it was BX


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Scheme 169a

a

to afford 633. Of interest was that the researchers were able to demonstrate that addition of the trimethyl aluminum sufficiently slowed the nickel-promoted isomerizations, so that the linear products 634 were now preferably formed. It should also be noted that a number of heterocycles with activatable C−H bonds, 635, could be used in the hydroheteroarylation of allylarenes, which also included benzoxazole 636, 1-methylindole 637, and 1-methylimidazole 638 (the reader is also referred to related work by Yamakawa and Yoshikai who recently performed similar reactions on the C2position of indoles with a cobalt-based catalyst825). In 2010, Hu and co-workers reported the isomerization of allylbenzene 3 using the nickel pincer complex [(MeNN2)Nipropyl].826 This particular catalyst isomerized allylbenzene 3 on an NMR spectroscopy scale into 1-propenylbenzene 4 at 70 °C, with the trans isomer being isolated as the major product. The researchers experimentally confirmed that the [(MeNN2)Nipropyl] complex initially generates a catalytically active hydride species by β-H elimination, after which allylbenzene 3 inserts into the Ni−H bond to form both the terminal and the internal nickel alkyl species, which are in equilibrium via isomerization. β-H elimination from the latter species is then thought to give the product, 1-propenylbenzene 4 (Scheme 171). Finally, an

DPPF = 1,1′-bis(diphenyphosphino)ferrocene.

demonstrated how reaction of the general allylbenzenes 1 with 1-methyl-1H-benzo[d]imidazole 632 under Ni(cod)2-mediated conditions resulted either in branched 633 or linear 634 products (Scheme 170). It should be noted that addition of Scheme 170

Scheme 171

interesting aside is that [(MeNN2)Ni-propyl] was also able to isomerize cis-stilbene into the thermodynamically more stable trans-stilbene under reaction conditions similar to those used before (10% catalyst, 70 °C, 20 h). The cationic nickel hydride-promoted isomerization of olefinic bonds, where the nickel species was obtained by electrogeneration, has been demonstrated by the Bontempelli group.827 These researchers carried out the isomerization of allylbenzene 3 using the cationic nickel hydrides [NiHL4]+ [where L = 1/2 dppe, PPh3, or P(OEt)3] [dppe = 1,2bis(diphenylphosphine)ethane]. Of particular interest is the fact that these nickel species were formed in situ by electrogeneration of the corresponding Ni(0) complexes in the presence of hydrogen ions. The researchers were able to demonstrate that, although the more stable cationic nickel hydrides with dppe exhibited low reactivity, the PPh 3 containing hydrides exhibited high reactivity, despite the fact that they decayed rapidly. Of interest is that because there is continuous electrogeneration of these unstable hydrides, isomerization reactions were able to be performed effectively and with good yields of the isomerized products being obtained (>92%). These researchers have earlier described olefinic isomerizations promoted by the electrogenerated cationic nickel(I) complex [Ni(PPh3)4]+.828 In this case, the corre-

catalytic amounts of trimethyl aluminum was required for the linear products to be formed. The authors were able to show that under the AlMe3-free conditions, the isomerization of the allyl group to the trans-β-methylstyrene proceeded “within a few minutes” and postulated that the branched product would then be obtained by way of oxidative addition of the heterocyclic C−H bond, followed by migratory insertion of the 1-propenylbenzene and ultimately a reductive elimination BY


Chemical Reviews

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3 isomerization.833 In this work, they found that monometallic gold and palladium catalysts displayed good activity, and that bimetallic gold−nickel nanocomposite catalysts displayed interesting synergistic effects in terms of activity. In a related study, Vasil’kov and co-workers used XPS to study the synergistic effect observed during allylbenzene 3 isomerization by gold and nickel supported on silica oxide.834 These catalysts were prepared by the vapor deposition method. Furthermore, nanosized gold particles were also immobilized on γ-Al2O3 by anion adsorption, and it was found that these nanosized gold particles exhibited excellent catalytic activity in the isomerization reaction of allylbenzene 3.835 The authors of this work were able to conclude that as the size of gold nanoparticles decreased from 40 to 2 nm, the activity increased. In point of fact, it was experimentally determined that particles with a size greater than 40 nm displayed virtually no activity. A theoretical approach to validate this has been published.836 Of interest too was that the ratio between the trans and cis isomers of the βmethylstyrene generated also depended on the average particle size and decreased by a factor of ∼2 as the average particle size decreased from 13 to 2 nm. A subsequent computational paper confirmed that the catalytic activity of the gold nanoclusters described was dependent on the oxidation state of the gold involved in the catalysis.837 In addition, this study seemed to indicate that a gold−hydride complex was responsible for the isomerization process. A subsequent quantum chemical study by the same authors, this time focusing on the influence of the charge state of the gold atoms involved, confirmed that this particular parameter was indeed important in the plausible isomerization mechanisms. 838 It should be noted that Tkachenko and co-workers had earlier investigated the synergistic effects of gold and nickel catalysts supported on aluminum oxide (prepared by both deposition-precipitation and impregnation approaches) on the ability to isomerize allylbenzene 3. This was done by using a variety of techniques, including DRIFT, SPS, and XAS.839 The researchers proved that isomerization on Au/Al2O3 proceeded with a low rate, while monometallic Ni/Al2O3 proved inactive. It was found that in the active bimetallic Au−Ni/Al2O3 catalysts there was a prevalence of Ni2+ cations, and that Au0 nanoclusters coexisted with Au3+ cations. The authors thus postulated that the coexistence of cationic species increased the catalytic activity in the allylbenzene isomerization reaction. In an interesting paper in this section, comparing the intramolecular hydroaminations of alkenes and alkynes by gold and silver catalysts, Ujaque and co-workers noted that only the gold catalysts facilitated the isomerization of allylaromatic compounds in the presence of triflic acid. For example, when allylbenzene 3 or 4-methoxyallylbenzene 7 were treated with TsNH2 and catalytic [(PhO)3P]AuCl/AgOTf, hydroamination products were obtained, with structures which indicated that the double bond had been isomerized (see section 4.3.1 for more on isomerizations during hydroaminations). In fact, this isomerization occurred under TfOH-catalyzed conditions, but not under conditions using only the AgOTf as catalyst. The authors were unable to explain the difference in the abilities of the gold and silver catalyst and at the time were of the opinion that a great deal more research would be needed to determine the reasons for this apparent discrepancy in reactivity.840 It should be realized that allylbenzene isomerization has been observed during the use of homogeneous catalysis by gold complexes for more complex processes. These include the (PPh3)AuCl/AgOTf isomerization-hydroamination reactions

sponding triethylphosphite nickel(I) complex showed no catalytic activity, which was postulated to be due to the phosphite ligand being a poor leaving group. It should be highlighted that electrochemical methods can demonstrate advantages over standard chemical procedures, particularly in an industrial sense, making this work of interest for the practical isomerization of allylbenzenes. Next, an example describing use of a supported nickel isomerization system will be discussed. In this case, Potapov made use of gel-immobilized Ni(acac)2 nickel complexes for the specific hydrogenation and isomerization of allylbenzene 3.829 This research involved an examination of the catalytic properties of the nickel complexes when they were immobilized in the bulk of three-dimensionally crossed-linked butyl acrylateacrylonitrile-acrylic acid-methylacrylamide polymers. Of additional importance was that diethyl aluminum was required as cocatalyst for the reactions to proceed. Second, it was found that the hydrogenation and isomerization rate of allylbenzene 3 was significantly influenced by the Al:Ni ratio, from both cocatalyst and catalyst, respectively. In this research, it was noted that both the rates of hydrogenation and isomerization increased with an increase in the reaction temperature, with a maximum conversion being observed at 40 °C. Finally, it should be stated that Prasad and Pillai demonstrated that Raney nickel was able to hydrogenate and isomerize various allylic compounds.830 The results were disappointing in that Raney nickel was not a very good isomerization catalyst for allylbenzene 3 (yields never higher than 20%). However, full hydrogenation to produce propylbenzene proved to be quantitative if sufficient time was given for the reaction to proceed to completion. 5.9.1. Summary Concerning Nickel Isomerizations. Nickel is another relatively inexpensive transition metal, and it would be expected that complexes of this transition metal have been extensively studied for the isomerization of allylbenzenes. Rather surprisingly, this does not seem to be the case, although recently Ong and co-workers elegantly made use of Ni(cod)2 tandem isomerization−C−H activation protocol.824 In terms of supported systems, apart from some older work, there appears to be little concerning nickel-promoted allylbenzene isomerization in the literature. However, the reports on water-soluble catalysts systems, by Monflier821 and Vittori,822 could be of interest in terms of larger scale homogeneous catalysis. 5.10. Gold-Catalyzed Isomerizations

In recent years, application of gold and gold complexes as catalyst has experienced a significant and growing interest.831 Some examples of the use of supported gold catalysts as well as one example of a homogeneous application are given below. Gold nanoparticles immobilized on alumina oxide supports have been found to be highly active in their ability to facilitate aromatic side chain allyl isomerization. Nikolaev and coworkers demonstrated the activity of gold nanoparticles immobilized on alumina in this reaction.832 The catalytic activity in terms of allylbenzene 3 isomerization was measured using 0.1 g of catalyst with a sample metal content of 10−5− 10−6 mol, 7.5 × 10−4 mol of allylbenzene 3, and the isomerization was performed at 170 °C. In concluding this work, the authors pointed out that the measurement of the heat of adsorption of allylbenzene 3 on the catalysts could serve as an indication of the efficiency of the catalysts for olefin isomerization. These researchers also studied a series of monoand bimetallic nanocomposite catalysts in terms of allylbenzene BZ


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described by Che and co-workers,841 and the (PPh3)AuOTfmediated addition of phenols and carboxylic acids to olefins described by Yang and He.842 Finally, isomerization of the allylbenzene scaffold has been observed during a microwave heating-mediated intermolecular hydroarylation between alkenes and indoles in the presence of (PPh3)AuCl/AgOTf, as described by Wong and Che843 (see also a cobalt-mediated version by Yamakawa and Yoshikai825). 5.10.1. Summary Concerning Gold Isomerizations. Apart from some gold complex-mediated allylbenzene isomerizations occurring as side reactions during more involved homogeneous processes, most of the research of interest has focused on heterogeneous systems. These include work by Nikolaev,832 Vasil’kov,834 Tkachenko,839 and co-workers on the use of gold nanoparticles immobilized on alumina oxide supports and the factors that affect their ability to facilitate the allylbenzene isomerization.

summary, under both hydrogen and inert atmospheres, the rhenium complexes catalyzed the isomerization of α-olefins including allylbenzene 3, giving predominantly cis-β-olefins, which in itself is a rather unusual result (see section 5.7.1, Figure 22 for another example giving predominantly the cisisomers). 5.11.2. Niobium- and Tantalum-Catalyzed Isomerizations. Brubaker and co-workers were successful in synthesizing a number of polymer-supported niobium and tantalum catalysts 641 and 644, which demonstrated a variable ability to isomerize allylbenzene 3 into (1-propen-1-yl)benzene 4.847 The following supported catalysts were made as described in Scheme 172: ●-CpNbCl4, ●-CpNbCl3, ●-CpNbCl2, Scheme 172

5.11. Miscellaneous Transition Metal Catalysts

A number of isolated reports of transition metal-mediated isomerizations facilitated by “unusual” transition metals will be listed here. As a general rule, if a certain transition metal has less than five references in terms of its catalytic use as an isomerization entity, it is listed in this section. 5.11.1. Rhenium-Catalyzed Isomerizations. Isomerization of allylbenzene ligands by cationic rhenium complexes has been observed.844 Other researchers have used rhenium complexes specifically for the purpose of isomerization. For example, Berke and co-workers described how they observed formation of the isomerization products of allylbenzene 3, viz., 4 (by NMR spectroscopy), when applying a catalytic hydrogenation system comprised of rhenium hydride and various boron Lewis acid cocatalysts.845 These workers proposed that the catalytic system was comprised of the rhenium hydrides [Re(Br)(H)(NO)(PR3)2] (where R = iPr or Cy) and borane (BH3) from the Me2NH·BH3 (DMAB) cocatalyst, the former being generated from the complexes [ReBr2(NO)(PR3)2(η-H2)]. The group suggested that isomerization of allylbenzene 3 was competing with the hydrogenation because, in this case, the particular allyl complex(es) formed under the reaction conditions were blocking the catalytic sites. In addition, the TOF (h−1) for the hydrogenation of allylbenzene 3 was also very slow when compared to other alkenes hydrogenated in the study. Gvinter et al. synthesized various rhenium complexes from oxopentachloro(oxopentabromo)rhenic acid with cyclic sulfurcontaining ligands, viz., thiophane and 2-propylthiophane, and examined their activity toward the hydrogenation and isomerization of olefins and dienes.846 They showed that the catalytic activity of the complexes was a function of the ligand environment, the nature of the substrate, the reaction conditions, and the solvent used in the experiments. They studied the reactions in methanol and toluene and demonstrated that the rate of conversion in methanol was higher than in toluene. Among the isomerized products, quite unusually, the cis-isomer formed in higher amounts than the trans-isomer (ratios of cis:trans from 25:14 to 25:7). In addition, the ratio of trans- and cis-1-propenylbenzene was significantly different in toluene (20:1) than in methanol. In addition, a blanket of argon seemed to only afford the cis-isomer for both solvents. It was also found that at a temperature of below 70 °C, the primary reaction occurring was isomerization, but that at 100 °C the competing hydrogenation reaction began to prevail. In

●-Cp 2 Nb(BH 4 ), ●-CpTaCl 4 , ●-(PhCH 2 )NbCl 4 , and ●-(PhCH2)TaCl4. Some of these supported organometallic complexes, when treated with n-BuLi to form their reduced equivalents, were found to be efficient isomerization catalysts, as shown by the two examples in Scheme 172. The researchers found that reduction of the supported organometallic complexes was essential to achieve good isomerization results and postulated that the actual catalysts were the polymersupported hydride derivatives. 5.11.3. Yttrium-Catalyzed Isomerizations. Rare earth metal complexes have also been used in allylbenzene 3 isomerization reactions, albeit fairly seldom. Qian and coworkers used a (C5H5)3-yttrium/sodium hydride system to effectively isomerize allylbenzene 3 in good yield (99%) and with good E:Z selectivity of 9:1 into 4.848 The reaction was performed at 45 °C, and the reagents were used in the ratio 0.05:7.8:1 for (C5H5)3Y:NaH:allylbenzene. The authors proposed that it was likely that an organolanthanide hydride CA


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arylallyl compounds 1.860 β-Methylstyrenes 2 were successfully obtained including those derived from safrole 6, estragole 7, allylbenzene 3, and 1-allyl-3,4-dimethoxybenzene 17 (Scheme 173 gives the generalized summary). Yields were generally

monomer, generated under the reaction conditions used, facilitated the isomerization reaction (see section 4.3.4 for the few examples utilizing sodium hydride for the isomerization of allylbenzenes). 5.11.4. Chromium-, Molybdenum-, and TungstenCatalyzed Isomerizations. Although molybdenum has not seen extensive investigation for isomerization reactions, Le-thi and Farona found that arenetricarbonylmolybdenums were able to isomerize allylbenzene 3.849 These researchers found that treatment of allylbenzene 3 with toluenetricarbonylmolybdenum (1 mol %) at 105 °C and for 24 h afforded the isomerized product 4 in 73% yield (E:Z = 54:46). In addition, tungsten and chromium arenetricarbonyl complexes were used, and found to be able to affect the isomerization transformation, albeit with less efficacy. 3-Phenyl-3,3-d2-1-propene (C6H5CD2CHCH2) was also isomerized and it was found that C6H5CDCHCH2D was the main product, in accordance with that expected for an allylic shift process postulated by Frankel for these types of arenetricarbonyl metal complexes.850 In another example concerning reactions of transition metal hexacarbonyl complexes, Kuchen and co-workers found that when (2-allylphenyl)phosphine was used as a ligand, this compound was isomerized to a mixture of [2-(prop-1-en-1yl)phenyl]phosphine isomers.851

Scheme 173

above 70% with the trans-isomers predominating (trans:cis ratio 7.6−12.3:1). Of interest was that the isomerizations occurred under the electrolytic conditions, even when no triphenylmethane was added, presumably by another anionic species, but that the results were less favorable in terms of yield (in particular, due to the formation of the corresponding propylbenzene byproducts that formed due to a competing hydrogenation process became more prevalent). Finally, Sawaki and co-workers discovered that “electroreduction” of electron deficient allylbenzenes 1 in dry DMF containing tetrabutylammonium perchlorate as electrolyte gave first the respective 1-propenylbenzenes 2 (0.5 F/mol of charge), and after a further 2−2.5 F/mol of charge gave a mixture of DL- (major) and meso- (minor) 2,3-dimethyl-1,4diphenylbutanes 645 (Scheme 174).861 The authors demon-

6. MISCELLANEOUS METHODS FOR ISOMERIZING ALLYLBENZENES 6.1. Electrochemical Methods for Isomerizing Allylbenzenes

Scheme 174

As a result of their applicability in the area of polymerization chemistry, allylbenzene 3 and its derivatives have been investigated as monomers for the synthesis of polymers with novel characteristics.852,853 Isomerization of these substrates to give the substituted styryl derivatives has been observed, along with the subsequent changes in polymer structure and properties. A number of examples describing these prepolymerization isomerizations are presented here (for another recent example involving a chlorotris(triphenylphosphine)triethylsilane isomerization system, based on previous work by Mirza-Aghayan et al.854 and giving a 1-propenylbenzoxazine that was incorporated into a polymer, see ref 855). The Oktem research group reported that during the electrochemical polymerization of 4-allyl-1,2-dimethoxybenzene 17, an initial isomerization of this substrate into 1,2dimethoxy-4-(1-propenyl)benzene 485 was followed by the actual polymerization process. In these experiments, performed at room temperature, a constant potential electrolysis of the reactant was carried out in an acetonitrile−tetrabutylammonium−tetrafluoroborate solvent−electrolyte couple.856 Of interest is that the isomerization-polymerization of substituted allylbenzenes is known to proceed by way of this pathway and has been researched extensively.857 Examples in the literature include the polymerization of estragole 7 (electrochemical),858 2-allylphenol 158e (electrochemical),859 and allylbenzene 3 (radiation and electrochemical),611 although it should be noted that homopolymerization of allylbenzene 3 without prior isomerization was successful when a metallocene catalyst was used.802 In an interesting electrochemical isomerization procedure, Nakajima and co-workers demonstrated that the triphenylmethyl anion, generated by way of a one-electron reduction under electrolysis, was able to isomerize a number of topical

strated that when acetonitrile, pyridine, dimethoxyethane, and wet DMF were used as solvent, interestingly, the reaction afforded only the isomerized allylbenzenes 2 (43−86%). 6.2. Allylbenzene Isomerization by Way of Flash Vacuum Pyrolysis

An interesting example involving isomerization of the double bond in allylbenzene 3 was observed from the work of Aitken et al., who performed high temperature (600 °C) flash vacuum pyrolysis (FVP) on a number of organic substrates.862 In the investigation, pyrolysis of benzylic and other aryl/alkyl and aliphatic halides was studied by FVP over magnesium. It was determined that the halopropylbenzenes 646 and 647 underwent elimination of HX, accompanied by an apparent isomerization of the initially formed allylbenzene 3, to give a mixture of all three isomers, viz., 3, and the E- and Z-isomers 4 and 90, respectively, of β-methylstyrene (Scheme 175). It should be noted that pyrolysis of 647 had previously been CB


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Two examples involving allylbenzenes, one using a homogeneous and the other employing a heterogeneous approach, will next be described. Isomerization of allylaromatic compounds has been achieved by use of inorganic solid acids,869 in particular the zeolites.390 For example, Gates and co-workers demonstrated that eugenol 5 could be readily isomerized by HY zeolite at 200 °C and at atmospheric pressure.870 The authors pointed out that eugenol 5, a ligninderived product,871,872 is a valuable resource, which, through isomerization and hydrogenation, can be converted into a range of valuable secondary products. The researchers also used a platinum-based catalyst supported on γ-Al2O3, with and without a hydrogen atmosphere, to obtain a range of products from eugenol 5, based on various hydrogenation and isomerization processes. In related work, Holloday and coworkers made use of eugenol 5 as a lignan model.873 These researchers were able to demonstrate that reaction of eugenol 5 with various Lewis and Brønsted acid catalysis in ionic media gave varying, but small, amounts of isoeugenol 18, among the other products obtained. Finally, Ipaktschi and Brü c k demonstrated that during Claisen reactions of O-allyl aromatics, occurring on Y zeolites under microwave irradiation, 1propenylaromatics were also obtained in yields below 10%, as a result of post-Claisen-isomerization under the conditions used.874 In terms of Lewis acids, Chattopadhyay and co-workers used an excess of boron trifluoride etherate (5−10 equiv) in chlorobenzene for an extended period of 24 h to directly afford a range of substituted 2-(prop-1-en-1-yl)anilines 649 from the respective N-allylanilines 648 in good yields for the combined aza-Claisen isomerization reaction.875 In addition, the method could be tailored to selectively produce the 2allylanilines 650 if so desired, which could then be converted into compounds 649, thus highlighting the synthetic power of the method (Scheme 177). It should be mentioned here that this control is admirable as others have reported problems with isomerizations when using related substrates.876 It should also be noted here that aluminum trichloride in carbon disulfide, in the presence of acetyl chloride, has previously been used to promote a Claisen-allylaryl isomerization-acetylation process.877

Scheme 175

effected at a lower temperature of 390−450 °C, with similar results.863 When the high temperature FVP was performed on the individual compounds 3, 4, and 90, in the case of allylbenzene 3, it gave the E-β-methylstyrene 4 in a good yield of 86% (with 14% starting material recovered). 6.3. Photochemical Isomerization of Allylbenzenes

Application of photochemistry for the isomerization of allylbenzenes has been rather sporadic, even though the method has been used to achieve olefin isomerization.864 It should be noted that certain transition metal catalyst systems, specifically those containing iron151,774,775 and cobalt,793,794 have been used with photoirradiation. However, these examples have been described in the sections under the transition metals themselves. Isomerization of allylbenzenes under photochemical oxidative conditions was observed by Mukerjee and co-workers in 1985.865 These researchers found that when dillapiole 11 was irradiated with a low-pressure mercury lamp, isodillapiole 23 was obtained in low yield (8%) illustrated in Scheme 176. Scheme 176

Scheme 177 When dillapiole 11 was irradiated with a tungsten light source in the presence of oxygen and rose bengal adsorbed on silica gel, 23 was again obtained, albeit in a slightly lower yield (6%). It may be that with some optimization photochemistry might provide a better yielding approach for the isomerization of allylbenzenes, but the initial results are not that promising.

7. ACIDS AS ISOMERIZATION CATALYSTS Acid catalysis has seen far less utilization in the isomerization of simple allylarenes. Of interest is that substituted allylbenzenes containing the (2-methylallyl)benzene motif are susceptible to isomerization under acidic conditions. For example, 1,3difluoro-2-(2-methylallyl)benzene was readily isomerized with p-toluenesulfonic acid (benzene, heat, 10 h),866 while related, more complex systems were readily isomerized with hydrogen chloride in methanol,867 trifluoroacetic acid in methoxybenzene (0 °C),868 and sulfuric acid (0 °C, 2 min).131 CC


Chemical Reviews

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8. BIOCHEMICAL ISOMERIZATION OF ALLYLBENZENES Perhaps one of the innovative future methodologies that may emerge would be isomerization of allylaromatic compounds via a natural biocatalytic process. Isomerization processes in a number of biocatalytic pathways have indeed been noted by several researchers. As an example, Itoh and co-workers demonstrated that the plant cultured cells of Caragana chamlugu are able to biotransform estragole 7 into a variety of products including 4-methoxycinnamaldehyde 651, 4methoxycinnamyl alcohol 652, and 4-methoxybenzaldehyde 653 (Scheme 178).878 This work stems from other studies,

propenylbenzene 2, has been much applied over the last 30 years or so. This isomerization step has found use in industrial settings (particularly for the fragrance, flavor, and pharmaceutical sectors), as well as in academic research, especially in the synthesis of important natural products. It is thus quite easy to appreciate the reasons why synthetic organic, inorganic, and physical chemists are still showing such a keen interest in this deceptively simple, but still intriguing, transformation. From the conclusions associated with each subsection of this Review, it should be noted that there is no “one size fits all” (catalytic) system. However, it can also be seen that the abundant research in this area has resulted in some “preferred” candidates among the base- and transition metal-mediated methods. In terms of where the isomerization of 2-propenylarenes into 1-propenylarenes could see more development, these challenges could include the following: (a) The development of more Z-selective catalysts−a challenge, as the vast majority of allylbenzene isomerization reactions afford the thermodynamically more stable E-isomer. (b) The development of more isomerization procedures scalable to industrial processes. Current methods involving more-than-stoichiometric amounts of transition metal hydroxides are obviously not environmentally friendly, and new processes involving heterogeneous systems should get more attention. Processes that occur at room temperature would also be preferable, as heating or cooling has significant energy consequences for any industrial process. (c) The incorporation of these allyl isomerizations into synthetic processes, which are more atom and energy “economical”. These could include tandem, one-pot, and multicomponent reaction strategies in which the intermediate 1-propenylarene could be further synthetically elaborated to afford compounds with additional complexity. Examples of these could include recent demonstrations of transition metalmediated allylbenzene tandem isomerization-hydroheteroarylation catalysis of indoles825 and (benz)imidazoles824 and the allylbenzene “cross-metathesis/isomerization/allylboration sequence” resulting in antihomoallylic alcohols by Carreaux and co-workers.728 (d) The application of biosynthetic strategies making use of enzymes or even whole cells for the production of specialty or large-scale 1-propenylaromatics is definitely attainable in the near future. Advances in biotechnology involving the fields of plant physiology and microbiology should pave the way for the “green” production of allylbenzene starting materials, as well as their isomerized 1-propenylbenzene versions. This should allow for the more sustainable production of complex organic molecules depending on these starting materials.

Scheme 178

which have demonstrated that certain soil bacteria are able to utilize eugenol 5 and isoeugenol 18 as food sources.879−881 The biotransformations processes illustrated in Scheme 178 were proposed to occur via a radical pathway. It is thus possible to envisage that future processes will utilize biomass engineered to produce valuable phenylpropenes; these will include bioprocesses882−885 in which large amounts of allylpropenoid compounds could be isomerized in vivo into their isomerized derivatives (for example, the large-scale conversion of estragole 7 into anethole 20886). Scientific investigation concerning biotransformation of naturally occurring, and thus potentially renewable, feedstock has become an active area of research. Of particular relevance to this Review are recent publications on the value of eugenol 5, isoeugenol 18, and related compounds in this regard.887,888 In addition, the list of microorganisms able to biotransform these substrates is increasing, due to new discoveries being made.889−892 Our deeper knowledge and understanding of the workings concerning the genes found in these organisms responsible for these valuable natural reactions will undoubtedly drastically increase over time.893−897 With this as current background, we are quite convinced that a new area of innovative research that will impact the application of isomerization transformations will emanate from research into microorganism-mediated biotransformations.

AUTHOR INFORMATION Corresponding Author

9. CONCLUSIONS This Review illustrates in a comprehensive way that isomerization of a generalized 2-propenylbenzene 1 into the thermodynamically more favored internal isomeric alkene, 1-

*Fax: +27 21 808 3360. E-mail: [email protected]. Notes

The authors declare no competing financial interest. CD


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Biographies

Gareth Arnott obtained his Ph.D. from the University of Cape Town, Mohammad Hassam obtained his Master’s degree in Chemistry from

South Africa, in 2003, working on distally functionalized resorcinar-

Lucknow University (2003) and carried out his Ph.D. under the

enes. From 2004 to 2006 he worked with Prof. Jonathan Clayden at

supervision of Dr. Chandan Singh at the Central Drug Research

the University of Manchester on dearomatizing cyclizations onto

Institute, Lucknow (2009), specializing in synthetic organic and

pyridine systems. In 2007, he was appointed in the Department of

medicinal chemistry. After obtaining his Ph.D., he joined GVK

Chemistry and Polymer Science at the University of Stellenbosch,

Biosciences Pvt Ltd., Hyderabad, as a senior research associate for a

where he began his independent research career. His research interests

period of 15 months. He then joined the van Otterlo group at

lie in synthetic methods and interesting aspects of chirality, notably the

Stellenbosch University, South Africa (2010−2012) as a post doctoral

inherent chirality of calixarenes and resorcinarenes.

fellow. Currently, he is a Sinica fellow at the Institute of Chemistry, Academia Sinica, in Taiwan. His research interests include new methodologies for the synthetic transformation and synthesis of novel molecules of therapeutic importance.

Ivan Green received his Ph.D. degree in Organic Chemistry in 1973 from the University of Cape Town. He was made a full Professor in 1986 and Senior Professor in 1990 at the University of the Western Cape where he lectured for 39 years until his retirement on 31st July Abu Taher received his B.Sc. (2003) from The University of Burdwan

2011. To date he has authored and coauthored over 150 scientific

with a National Merit Scholarship (Govt. of India) and M.Sc. (2005)

papers, given 40 podium lectures at international conferences, and

from the same University. In 2010, he received his Ph.D. degree from

supervised some 30 M.Sc. and 18 Ph.D. students in South Africa and 6

University of Kalyani. He was a postdoctoral fellow at the University of

Ph.D. students internationally. He is a regular referee for eight

Stellenbosch for two years (2011−2012), followed by a second

International Scientific Journals. After retirement he moved to the

postdoctoral stint at the University of Johannesburg. His research

University of Stellenbosch where he is an Honorary Research

interests mainly deal with (i) the synthesis of potential bioactive

Associate, mentoring M.Sc. and Ph.D. students in the synthesis of

heterocycles, (ii) the design and synthesis of kinase inhibitors, and (iii)

small libraries of compounds as potential anticancer and HIV/Aids

the design and synthesis of metal nanocomposites and their

treatment regimens, as well as in the isolation of alkaloid-based

applications in catalysis and electrocatalytic sensors.

scaffolds for anticancer evaluation. CE


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Willem van Otterlo was born in Amsterdam, The Netherlands. In 1999 he graduated with a Ph.D. (Chemistry) under the mentorship of Professors Charles de Koning and Joseph Michael (University of the Witwatersrand, Johannesburg, South Africa). After a postdoctoral research stint in Canada (Professor Stephen Hanessian, University of Montreal, Quebec, Canada), he returned in 2001 to his alma mater to take up a lecturing position, progressing to the level of associate professor in 2008. Since 2010 he holds the chair of Organic Chemistry in the Department of Chemistry and Polymer Science at Stellenbosch University. He has been a recipient of a number of awards including the University of the Witwatersrand’s Friedel-Sellschop Award (2004), the South African Chemical Institutes Raikes Medal (2004), The ViceChancellors Distinguished Individual Teachers Award (2007), and an Alexander von Humboldt Foundation-Georg Forster Research Fellowship for Experienced Researchers (2008−2009). Much of his current research time focuses on the use of novel synthetic methods to generate new organic molecules that target diseases such as HIV, malaria, and various cancers.

ACKNOWLEDGMENTS We gratefully acknowledge the Faculty of Science at Stellenbosch University for research support. In addition, the South African National Research Foundation (NRF Pretoria) is also thanked for research finances. H.M. and A.T. (Innovation fellowship) thank the NRF for postdoctoral funding. W.v.O. also thanks the School of Chemistry, University of the Witwatersrand, for an enjoyable collegial research environment during his time of employment. DEDICATION This work is dedicated to Stephen Hanessian (University of Montreal, Quebec, Canada) in celebration of his 80th birthday and in acknowledgment of his contribution to organic chemistry, and to the Henning family for the solitude at Penkelly Farm, The Crags, where most of this Review was compiled. REFERENCES (1) Dewick, P. M. Medicinal Natural Products-A Biosynthetic Approach, 3rd ed.; Wiley: Chippenham, 2009; pp 156−159. (2) Petersen, M.; Hans, J.; Matern, U. Biosynthesis of Phenylpropanoids and Related Compounds. In Annual Plant Reviews, 2nd ed.; Wink, M., Ed.; Wiley-Blackwell: Oxford, 2010; Vol. 40, pp 182−257. (3) Chapuis, C.; Jacoby, D. Catalysis in the Preparation of Fragrances and Flavours. Appl. Catal., A 2001, 221, 93−117. (4) Boulogne, I.; Petit, P.; Ozier-Lafontaine, H.; Desfontaines, L.; Loranger-Merciris, G. Insecticidal and Antifungal Chemicals Produced by Plants: A Review. Environ. Chem. Lett. 2012, 10, 325−347. CF


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NOTE ADDED IN PROOF During the preparation and review of this manuscript a number of papers related to the topic of this review were published. For completeness, they are listed as follows (under the different approaches that were utilized for the isomerization reaction): (a) Bases: Potassium tert-Butoxide by Ray898 and Brimble,899 and isomerization occurring under Wittig alkenylation conditions (nBuLi, PPh3MeBr) by Dong;900 (b) Metal complexes: (i) palladium(II) complexes by Yang901 and Schaetzer,902 (ii) rhodium(III) by Kim,903 (iii) ruthenium complexes by Chattopadhyay,904 Grubbs905 and Cadierno,906 (iv) iron(III) complex by Jacobi von Wangelin,907 and (v) a cobalt(II) complex by Hilt.908

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Isomerization of Allylbenzenes - Chemical Reviews (ACS Publications)

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