Transition-Metal-Catalyzed Cycloisomerizations of Î±,Ï-Dienes

Review pubs.acs.org/CR

Transition-Metal-Catalyzed Cycloisomerizations of α,ω-Dienes Yoshihiko Yamamoto* Department of Basic Medicinal Sciences, Graduate School of Pharmaceutical Sciences, Nagoya University, Chikusa, Nagoya 464-8601, Japan cross-coupling methods in terms of atom economy,3 because metals as well as halides are lost during the coupling processes. In this regard, direct functionalization of ubiquitous carbon− hydrogen bonds has recently received enormous attention, even though specific identification of selective and functional-groupcompatible C−H functionalizing reactions is very challenging.4 Another atom-economical approach to C−C-bond formation is the hydro-carbofunctionalization of π-bonds. One such example is the hydrovinylation of alkenes, which involves the formal insertion of alkenes into the C−H bond of ethylene.5 This fascinating chemistry has been extended to the intramolecular hydrovinylation processes (i.e., cycloisomerizations) of α,ωCONTENTS dienes. The catalytic intramolecular C−C-bond formations are 1. Introduction A particularly important because they provide powerful methods 2. Transition-Metal-Catalyzed Cycloisomerization of for constructing carbo- and heterocyclic molecules, which are α,ω-Dienes C fundamental constituents of natural products, pharmaceutical 2.1. Nickel-Catalyzed Reactions C compounds, and functional materials. In view of their 2.2. Palladium-Catalyzed Reactions E importance, extensive studies on the transition-metal-catalyzed 2.3. Rhodium-Catalyzed Reactions K 2.4. Ruthenium-Catalyzed Reactions L cyclizations of α,ω-bifunctional molecules such as diynes, 2.5. Titanium- and Zirconium-Catalyzed Reacenynes, and dienes have been undertaken.6 Transition-metaltions Q catalyzed cycloisomerizations have been the subject of 2.6. Other Metal Catalysts T particular attention as atom-efficient methods because all of 3. New Aspects of α,ω-Diene Cyclosiomerization U the atoms in the starting materials are preserved during the 3.1. Heterogeneous Catalyst U cyclization process.7 Atom-transfer radical cyclization of 3.2. Recyclable Catalyst Systems using Ionic polyhalo compounds is one typical example that falls into this Liquids V category of environmentally friendly cycloisomerization proc3.3. Microwave-Assisted Cycloisomerization W ess.8 The desired cyclization of trichloroacetamide 1 to yield 4. Cycloisomerizations Involving Conjugated Dilactam 2 can be achieved only via catalytic chlorine-atom enes W transfer within the molecule (Scheme 1a).9 Superfluous 4.1. Cycloisomerization Involving 1,3-Diene W chlorine atoms, however, have to be removed en route to the 4.2. Cycloisomerization Involving 1,2-Diene AA final target molecules. Transition-metal-catalyzed cyclizations of 5. Summary AE enynes are another archetypal example of cycloisomerization.10 Author Information AE For example, the asymmetric cyclization of enyne 3 catalyzed Corresponding Author AE by the chiral rhodium complex furnishes the α-alkylidenelacNotes AE tone 4 in an excellent yield with a high enantiomeric excess Biography AF (ee), as shown in Scheme 1b.11 This reaction is characterized References AF by several intriguing features: (i) substrates are readily accessible and easy to handle; (ii) only a hydrogen atom is transferred during the cyclization, and hence, no extra step is 1. INTRODUCTION required to remove unnecessary atoms or groups; and (iii) the alkene moieties in the obtained products are useful synthetic Transition-metal-catalyzed carbon−carbon-bond formations handles for further transformations. Scheme 1c shows the have been one of the most intensively studied subjects in 1 oxygen-atom-transfer cyclization of sulfoxide 5, catalyzed by a organic synthesis. Among these reactions, transition-metalgold N-heterocyclic carbene complex.12 In this reaction, catalyzed cross-coupling reactions have been successfully 2 benzothiacycloheptanone 6 was obtained in a good yield via applied to the synthesis of a number of complex molecules. a gold carbene complex intermediate. In addition, cycloHowever, transition-metal-catalyzed cross-coupling reactions isomerizations that involve C−C-bond cleavages have also been have a fundamental requirement for prefunctionalized starting materials such as organometallic compounds (organozinc, organoboron, organostananne, etc.) and electrophilic partners Received: February 9, 2012 such as organic halides. This requirement limits the value of the © XXXX American Chemical Society

A

dx.doi.org/10.1021/cr300050n | Chem. Rev. XXXX, XXX, XXX−XXX

Chemical Reviews

Review

cyclic molecular frameworks involving macrocycles;18 however, this innovative approach is nonideal in terms of atom economy, because ethylene molecules are lost during cyclization (Scheme 2b). In contrast, the cycloisomerizations of α,ω-dienes are atom-efficient reactions in which C−C-bond formations are achieved with concomitant transposition of a hydrogen atom, without the need for additional reagents (Scheme 2c).19 Nevertheless, the diene cycloisomerization also suffers from the disadvantage that it is difficult to selectively obtain a single product as an exclusive isomer because the use of certain catalytic systems might result in a mixture of olefinic positional isomers due to facile transposition of alkenes via reversible hydrometalation/β-H elimination under cycloisomerization conditions (Scheme 3). Moreover, the control of cyclization

Scheme 1. Examples of Transition-Metal-Catalyzed Cycloisomerizations

Scheme 3. General Mechanism for Alkene Transposition

developed, as typified by the ruthenium-catalyzed reaction of enyne 7 that involves concomitant CC-bond cleavage and benzylidene-group transfer (skeletal reorganization) to generate the 1,3-diene 8 in a high yield (Scheme 1d).13 More recently, the rhodium-catalyzed cycloisomerization of enynylcyclopropane 9 was reported to involve cyclopropane ring-opening and acetate-group transfer (Scheme 1e).14 The cyclization of α,ω-dienes has been of particular focus because diene cyclizations allow the synthesis of carbo- and heterocycles from readily accessible olefinic substrates. Toward this end, extensive studies have been performed on the cyclization of dienes, promoted by low-valent titanium or zirconium reagents (Scheme 2a).15 This method provides a

regiochemistry is a formidable problem for α,ω-diene cycloisomerizations as compared to α,ω-enyne cycloisomerizations (e.g., Scheme 1b): because of the significant reactivity difference between alkyne and alkene terminals toward transition-metal complexes, the cyclization regiochemistry can be more easily controlled in α,ω-enyne cycloisomerizations. In contrast, the differenciation of two alkene terminals with similar reactivity in α,ω-diene substrates is problematic. To address these issues, considerable efforts have been directed toward the identification of effective catalytic systems and reaction conditions to realize isomer-selective cycloisomerization. This review summarizes the progress of transition-metal-catalyzed cycloisomerization of α,ω-dienes with a particular emphasis on the product selectivity and underlying mechanisms. Because the reaction mechanisms involved and the types of product obtained are strongly related to the transition-metal elements used as catalysts, the next section is outlined based on the classification of the various types of transition-metal promoters. These include nickel-, palladium-, rhodium-, ruthenium-, titanium-, zirconium-, and other metal-based catalysts. The succeeding section summarizes new aspects of α,ω-diene cycloisomerization, which involve heterogeneous catalysts, recyclable catalyst systems, and a microwave-enhanced reaction. The final topic of this review is the cycloisomerization of ene− 1,3-diene and ene−allene substrates, in which one or both of the alkene moieties are incorporated into highly reactive conjugated dienes. The scope of this review is limited to the cycloisomerization of α,ω-dienes involving the transposition of only hydrogen atoms rather than other groups. This is because such cycloisomerizations can be considered to be C−H functionalization reactions as a certain C−H bond can be transformed into a new C−C bond. Thus, intramolecular [m + n]-cycloadditions of α,ω-dienes and congeners, which involve only C−C-bond formations, are beyond the scope of this review, although such reactions can also be regarded as cycloisomerizations.

Scheme 2. Examples of Cyclization of α,ω-Dienes

variety of functionalized cyclic products after trapping the metallacyclopentane intermediates with various electrophiles, but nevertheless, it suffers from the requirement for stoichiometric amounts of transition-metal templates, which is a serious drawback. In view of this limitation, catalytic metallative cyclizations of dienes were developed as an alternative, but stoichiometric reagents such as organomagnesium, organoaluminum, or organosilicon compounds were still required in these methods.16,17 Moreover, functional group compatibility is also problematic for cyclizations using transition metals with high reducing ability and/or highly nucleophilic organometallic reagents. Ring-closing metathesis (RCM) of α,ω-dienes paved the new way to the achievement of B


Chemical Reviews

Review

5).20b The isomer ratios of 18/(19 + 20) were dependent on the combination of the phosphine ligands and the counter-

2. TRANSITION-METAL-CATALYZED CYCLOISOMERIZATION OF α,ω-DIENES 2.1. Nickel-Catalyzed Reactions

Scheme 5. Ni-Catalyzed Cycloisomerization of Diallyl Ether

One of the earliest examples of transition-metal-catalyzed cycloisomerizations of α,ω-dienes was reported by Bogdanović, Wilke, and co-workers.20a In Bogdanović’s review of their investigations on nickel-catalyzed olefin dimerizations,20b several types of cycloisomerizations of linear and cyclic 1,5dienes were outlined, although experimental details were not provided (Scheme 4). They prepared nickel catalysts via

anions of the catalysts. The combined use of triisopropylphosphine and CF3SO3− (TfO−) was optimal for predominant generation of 18 at an 18/(19 + 20) ratio of 94:6. Bogdanović and co-workers extended this method to the asymmetric cycloisomerization of 1,6-heptadienes using Pchiral phosphine ligands L1 and L2 that feature both chiral phosphorus centers and optically active menthyl groups as chiral nonracemic ligands, albeit with low optical yields (Scheme 6).20b Those studies demonstrated that the optical

Scheme 4. Ni-Catalyzed Cycloisomerization of 1,5-Dienes

Scheme 6. Asymmetric Cycloisomerization of 1,6-Dienes

reactions of π-allyl nickel precursors, i.e., [(η3-allyl)NiX]2, with alkylaluminum reagents, i.e., RnAlX3−n, or silver salts, i.e., AgY (Y = BF4, PF6, SbF6, ClO4, etc.), in the presence or absence of phosphine ligands. The resultant nickel complexes were converted to active hydride species, which can be formally denoted as [HNi] and [HNi(PR3)], after reactions with olefin substrates. Alternatively, active nickel hydride species were generated by the treatment of precursors, including Ni(PR3)4 or Ni(cod)2 (cod = 1,5-cyclooctadiene), with acids such as CF3SO3H and HBF4. As shown in Scheme 4a, cycloisomerization of 1,5-cyclooctadiene was promoted by a phosphine-free nickel catalyst to furnish cis-bicyclo[3.3.0]octa2-ene 10 in 96% yield.20 This reaction was considered to proceed via sequential hydrometalation/carbometalation/β-H elimination. Addition of trimethylphosphine produced 1,3cyclooctadiene as a predominant product (93%) via simple alkene transposition as a result of hydrometalation/β-H elimination. The transformation of 1,5-cyclooctadiene was also investigated using other nickel catalyst systems, including Ni(II) 2-ethylhexanoate with alkylaluminum reagents,21a Ni(cod)2 with phosphine ligands,21b and a nickel complex derived from (benzoylmethylene)triphenylphosphorane.21c Triisopropylphosphine and tricyclohexylphosphine were optimal ligands for cycloisomerization of 3-(2-propenyl)cyclooctene 11 to form 12 and 13 (Scheme 4b).20 This cycloisomerization probably starts with reversible hydrometalation of the terminal alkene and the subsequent carbometalation of the cyclooctene and subsequent β-H elimination yielded 12. Further alkene transposition occurred to furnish 13. A similar transposition of the terminal alkene moiety resulted in the formation of cis/ trans-3-(1-propenyl)cyclooctene as side-products (not shown). A related cycloisomerization of cis-1,2-divinylcyclohexane 14 was also reported to be promoted by a tricyclohexylphosphinemodified nickel catalyst to furnish bicyclic products 15 and 16 as major products, although the yields and selectivity were not described (Scheme 4c).20b The nickel hydride catalyst system was also effective for cycloisomerization of 1,6-dienes, e.g., diallyl ether 17 (Scheme

yields were significantly impacted by the configuration at the phosphorus center; reactions using L2 afforded greater optical yields than those employing L1, even though all of the major resultant enantiomers had identical absolute configurations. Since the seminal investigations of Bogdanović, Wilke, and co-workers,20 nickel-catalyzed cycloisomerizations of α,ωdienes have been studied using a variety of nickel catalysts and diene substrates. As an extension of the nickel-catalyzed alkene oligomerizations, Keim and co-workers reported a study on the cycloisomerization of 1,5-hexadiene and 1,6-heptadiene in 1986 (Scheme 7).21c In that instance, the nickel catalyst 22 Scheme 7. Ni-Catalyzed Cycloisomerizations of 1,5-Hexaand 1,7-Octadienes

C


Chemical Reviews

Review

tional nickel precursor ([(η3-allyl)NiBr]2), a counteranion, which is known as BArF (BAr F 4 − ; Ar F = 3,5-di(trifluoromethyl)phenyl), and chiral ligands L3−L5 (Scheme 9).23 Among the chiral nonracemic ligands evaluated in that

derived from Ni(cod)2, PPh3, and (benzoylmethylene)triphenylphosphorane was utilized to perform cyclization of 1,5-hexadiene in toluene at 100 °C. A >99% conversion of the diene was achieved after 60 h to afford exo-methylenecyclopentane as a major product with 74% isomer selectivity. Linear isomers were also formed at 15% selectivity along with trace amounts of methylcyclopentene. Cyclization of 1,7-octadiene with catalyst 23 in hexane at 75 °C reached 71% conversion to give cyclopentenes 24 and 25 as cyclic products at 52% and 15% selectivity, respectively. Six- and seven-membered cyclic products were detected in only negligible traces. These results suggest that 1,7-octadiene failed to undergo cycloisomerization but instead was converted to 1,6-hexadiene via alkene transposition. Then, the resultant 1,6-diene underwent lessregioselective cycloisomerization and subsequent alkene transpositions to furnish 24 and 25. In contrast, catalyst 22 failed to generate cyclization products from 1,7-octadiene. In 1998, Radetich and RajanBabu revisited the nickelcatalyzed cycloisomerization of 1,6-dienes using a cationic nickel catalyst system closely related to those developed by Bogdanović and Wilke, i.e., the combination of [(η3-allyl)NiBr]2, (p-MeOC6H4)3P, and AgOTf (Scheme 8).22 In the

Scheme 9. Ni-Catalyzed Asymmetric Cycloisomerization of Diallylmalonate 26-Et

study, the azaphospholene ligand L3 was superior to phosphoramidites L4 and L5. In the presence of 5 mol % of the catalyst ([(η3-allyl)NiBr]2/NaBArF4/L3), an 81% conversion of diallylmalonate 26-Et was achieved within 17 h of the cycloisomerization to produce exo-methylenecyclopentane 27-Et with 91% isomer selectivity and 79% ee. The ee was lower (39% and 48%, respectively) when L4 and L5 (R = Bu) were utilized. The catalytic activity was enhanced by the use of isolated complexes of the type [(η3-allyl)Ni(cod)]Y (Y = counterions) in conjunction with L3, and the dependence of the catalytic activity and isomer selectivity on Y were investigated using the malonate-derived diene 26-Et as a benchmark substrate (Table 1). The catalytic activity decreased in the order [BArF]− > [SbF6]− > [AsF6]− ≫ [PF6]−, whereas the ee remained almost constant (∼70%). Isomer selectivity remained high with a variation of the counterions (>90%), except in the case of [PF6]−. Decreasing the catalyst loading from 5 mol % to 0.5

Scheme 8. Nickel-Catalyzed Cycloisomerization of 1,6Dienes

Table 1. Impact of Counterions on Ni-Catalyzed Asymmetric Cycloisomerization of 1,6-Dienes

presence of this catalyst system, the 1,6-diene 26-Me that features a dimethyl malonate tether underwent cycloisomerization to selectively furnish exo-methylenecyclopentane 27-Me in a high yield. Diallyl ethers 28a−c, which have substituents at the positions α to the ether oxygen, were selectively cyclized to afford 29a−c as major regioisomers. No diastereoselectivity was observed for cyclizations of 28a and 28b. In contrast, the reaction of the tosylamide derivative 30 under similar conditions resulted in the formation of 31 in a low yield. Radetich and RajanBabu also compared the nickel-catalyzed reactions to those employing a relevant palladium catalyst system and found the nickel catalyst to be superior with respect to catalytic activity and isomer selectivity (further expounded in section 2.2). An asymmetric cycloisomerization reaction of 1,6-dienes exhibiting improved enantioselectivity was developed by Leitner and co-workers using the combination of a convenD

Y (cat./mol %)

diene

time/ h

conversion/ %

isomer selectivity/ %

ee/ %

BArF (5) SbF6 (5) AsF6 (5) PF6 (5) BArF (0.5) Al(pftb)4 (0.5) Al(hfpp)4 (0.5) BArF (0.5) Al(pftb)4 (0.5) Al(hfpp)4 (0.5) BArF (5) BArF (5)

26-Et 26-Et 26-Et 26-Et 26-Et 26-Et 26-Et 30 30 30 32 34

0.5 0.5 0.5 0.5 1 1 1 1 1 1 1 1

96 92 88 57 72 79 55 64 11 34 39 >99

91 91 94 84 91 97 91 95 99 99 >99 87

71 72 74 74 72 73 73 33 54 48 91 86


Chemical Reviews

Review

that is different from those of the nickel-catalyzed reactions discussed above. In 1976, Schmitz and co-workers published the pioneering report of palladium-catalyzed cycloisomerization of N,N-diallyl acrylamide 44 (Scheme 12).25a In the presence of ∼6 mol % of PdCl2, 44 was heated for 5 h in boiling isobutanol, giving rise to a variety of cyclized products and a small amount (∼5%) of deallylated product 53. Among these products, 45− 49 were produced via C−C-bond formation between the acrylic olefin and the allyl group, whereas the formation of 50−52 involved the two allyl groups. Most of the substrate was transformed to lactam products (81% combined yield). Thus, the latter products involving the two allyl groups are minor components (∼15% combined yields). These data are indicative of the preference for electron-deficient alkenes in the palladium-catalyzed reaction. This selectivity was further confirmed in the set of experiments that follow (Scheme 13). In an investigation of the palladium-catalyzed cycloisomerization of N-allyl acrylamide 54 and N-acyl diallylamine 57 by the same group,25b it was found that 54 underwent efficient cycloisomerization, with the exception of the N-tert-butyl substrate, to afford lactams 55 and 56. Notably, the more highly substituted isomers 55 were the major products, and this selectivity was higher with bulkier N-alkyl groups. On the other hand, the reaction of 57 furnished the 2-pyrrolines 58 in lower yields. In addition, these inefficient reactions produced the deallylation side-products 59 in ∼10% yield. These results are demonstrative of the facile alkene transposition that occurs during palladium-catalyzed cycloisomerization of 1,6-dienes, with consequent production of mixtures of isomers. Palladium(II) chloride has low solubility in common organic solvents, and hence, it exhibits insufficient catalytic activity for successful cycloisomerization. Grigg and co-workers showed that Pd(OAc)2 effectively catalyzes the cycloisomerization of 1,6-heptadiene derivatives in boiling CHCl3 through which hydrogen chloride was passed prior to the reaction.26 At a 5 mol % catalyst loading, dienes 26-Et, 61, 63, and 65 were transformed to the corresponding cyclopentenes 60-Et, 62, 64, and 66 in 51%−88% yields, respectively (Scheme 14). Similarly, unsymmetrical 1,6-diene 67 was selectively converted to 68 in 68% yield, albeit with a longer reaction time. Use of the ethanol-free solvent enhanced the reaction rates. Thus, the reaction of diene 69-Et, having two terminal methyl groups, was completed within 5 h in ethanol-free chloroform to furnish 70-Et in 82% yield. 1,7-Diene 71-Et was subjected to similar conditions with consequent formation of a 3:2 mixture of 72-Et and the corresponding isomer 73-Et. In contrast, the 1,8-diene 74-Et selectively generated 70-Et in 98% yield as a result of alkene transposition prior to cycloisomerization. On the basis of the fact that deuterium was not incorporated into the product during the Pd(OAc)2-catalyzed reaction in the presence of DCl, a mechanism not involving the palladium hydride species was considered; the reaction is initiated by allylic C−H activation resulting in the formation of the π-allyl alkene complex 75 (Scheme 15).26b Insertion of the remaining alkene into the resultant Pd−H bond is followed by intramolecular attack at the central carbon of the π-allyl group by the alkyl ligand to produce metallacyclobutane 76. Finally, β-H elimination and subsequent reductive elimination affords a 2-cyclopentene. The alternative β-H elimination/ reductive elimination pathway from 76′ also yields a minor product, i.e., exo-methylenecyclopentane. Ensuing studies by Radetich and RajanBabu also illustrated the characteristic isomer selectivity of palladium-catalyzed 1,6-

mol % in the reaction using BArF as the counterion resulted in a lower conversion of 72% at a similar selectivity (91%) and ee (72%). Aluminum-based counterions were also employed for the same reaction. The use of [Al(pftb)4]− (pftb = OC(CF3)3) provided slightly higher conversion and selectivity, whereas the use of [Al(hfpp)4]− (hfpp = OC(CF3)2Ph) resulted in lower conversion. Similar to RajanBabu’s cationic nickel catalyst (Scheme 8), these chiral catalysts showed inferior catalytic activity for the reaction of the tosylamide-derived diene 30 as compared to those of 26-Et. Although isomer selectivity was high, ee was lower, ranging from 33% to 54%. Although the diene 32, which possesses a diol moiety, had a reactivity that was similarly lower than that of 26-Et, the ee was considerably improved to 91%. A higher conversion was obtained without significant erosion of ee (86%) after acetonide protection of the hydroxy groups (34). In striking contrast, diallylmalononitrile failed to cyclize under similar conditions. Leitner’s catalyst was then applied to the cycloisomerization of unsymmetrical 1,6-dienes as outlined in Scheme 10.23d Scheme 10. Ni-Catalyzed Cycloisomerization of Unsymmetrical 1,6-Dienes

Dienes 36a-Et and 38, having terminal phenyl groups, underwent cyclization with >99% conversions to afford the benzylidene-substituted products 37a-Et and 39 with 95% and 71% isomer selectivity, respectively. In contrast, the cyclization of dienes 40-Et and 42, having terminal methyl groups, generated the vinyl-substituted products 41-Et and 43 with diminished isomer selectivity. Combinations comprising nickel(II) phoshine complexes and alkylaluminum reagents were also applied to the cycloisomerization of 1,6-dienes (Scheme 11).24 The use of catalytic Scheme 11. Ni-Catalyzed Cycloisomerization and Deallylation of Diallylmalonate 26-Et

amounts of diethylaluminum chloride was critical for selective cycloisomerization of 26-Et; the deallylation reaction proceeded in the presence of 2 equiv of triethylaluminum. 2.2. Palladium-Catalyzed Reactions

Palladium complexes have been known to catalyze the cycloisomerization of 1,6-dienes in an isomer-selective manner E


Chemical Reviews

Review

Scheme 12. Pd-Catalyzed Cycloisomerization of N,N-Diallyl Acrylamide 44

Scheme 13. Pd-Catalyzed Cycloisomerization of Dienes 54 and 57

Scheme 15. Possible Mechanisms for Pd(OAc)2-Catalyzed Cycloisomerization of 1,6-Dienes

Scheme 14. Pd-Catalyzed Cycloisomerization of α,ω-Dienes

Scheme 16. Palladium-Catalyzed Cycloisomerization of 1,6Dienes

diene cycloisomerization using a catalyst comprising [(η3allyl)PdCl]2, (o-MeC6H4)3P, and AgOTf (Scheme 16).22 The cycloisomerization reaction of diallylmalonate 26-Me generated the exo-methylenecyclopentane 27-Me and dimethylcyclopentene 77-Me in 91% combined yield, with the latter being the major isomer. A similar diene with methyl terminal groups (69Me) underwent cycloisomerization with a longer reaction time to afford cyclopentene isomers 78-Me and 70-Me in 90% combined yield. In contrast, the cycloisomerization of tosylamide 30 afforded five- and six-membered exo-methyleneazacycles 31 and 78. The later compound was the major product under the same conditions, whereas the use of

triphenylphosphine as the ligand led to the predominant formation of 31. Widenhoefer and co-workers discovered the significant impact of hydrosilane additives on the cycloisomerization of 1,6-dienes using a catalyst system closely related to that of RajanBabu.27 In the presence of 1.5 equiv of triethylsilane, diene 26-Et was completely consumed within 20 min at 25 °C to afford cyclopentene 77-Et in 86% yield as a predominant isomer, whereas in the absence of this additive, the relatively sluggish reaction resulted in the formation of a 1.8:3.2:1.0 mixture of 27-Et, 77-Et, and acyclic isomer 79-Et (Scheme 17). This highly selective method yielding thermodynamically favorable cyclopentenes is quite general for various 1,6-diene substrates. This catalyst system was later applied to the unsymmetrical diyne 40-Me (Scheme 18). By employing a lower concentration of the silane ([Et3SiH] = 0.075 M) and a short reaction time, the corresponding cyclopentene 80-Me and ethylidenecyclopentane 81-Me were formed in 85% combined F


Chemical Reviews

Review

Scheme 17. Impact of Et3SiH on Palladium-Catalyzed Cycloisomerization

Scheme 19. Pd-Catalyzed Reactions of 26-Et and 27-Et with DSiEt3

Scheme 18. Pd-Catalyzed Cycloisomerization of Unsymmetrical 1,6- and 1,7-Dienes

that the hydrosilane is the H-donor in this cycloisomerization reaction. The inductive period when using DSiEt3 as a silane indicates the silane was required for the initial formation of a catalytically active palladium hydride, which is stabilized by excess amounts of the silane. By carrying out extensive deuterium-labeling studies, the same group proposed a mechanism for the selective formation of 3-cyclopentene products 77 (Scheme 20).27b At the early Scheme 20. Proposed Mechanism for Formation of 3Cyclopentene 77

yield with a 80-Me/81-Me ratio of 3:1. However, by increasing the Et3SiH concentration and the reaction time, the isomer selectivity was improved. At [Et3SiH] = 0.40 M, 80-Me was predominantly obtained (80-Me/81-Me = 31:1) in 83% yield. These facts imply that silane addition promoted the isomerization of the initially formed alkylidene products to generate the thermodynamically favored cyclopentenes. Similarly, the cycloisomerization of unsymmetrical 1,7-diene 71-Et resulted in the selective formation of a corresponding major isomer 80Et. Kisanga and Widenhoefer monitored the progress of the cycloisomerization of diallylmalonate 26-Et in the presence of [(π-allyl)Pd(PCy3)][BArF] catalyst and Et3SiH at 0 °C using gas chromatography. The formation of the corresponding exomethylenecyclopentane 27-Et was observed without detection of 3-cyclopentene 77-Et until 72% of the starting diene was consumed. When the formation of 27-Et reached a maximum, this compound underwent rapid conversion to 77-Et. In contrast, the corresponding 2-cyclopentene 56-Et failed to undergo isomerization to 3-cyclopentene 77-Et under the same reaction conditions. To elucidate the role of the hydrosilane additive, the palladium-catalyzed reaction of 1,6-diene 26-Et with DSiEt3 was performed at 0 °C.27b As a result, 77-Et-d0−3 was obtained in 92% yield with an average deuterium (D) content of 0.45D/ mol (Scheme 19). Deuterium was exclusively incorporated into the exocyclic methyl groups. The consumption rates of 26-Et with HSiEt3 and DSiEt3 were similar (zero-order rate constants k = 8.1 × 10−5 s−1 and k = 8.8 × 10−5 s−1, respectively), although an induction period was present in the reaction with DSiEt3. Subjection of exo-methylenecyclopentane 27-Et to catalytic conditions with DSiEt3 at room temperature produced significant deuterium incorporation at the terminal methyl group to give 3-cyclopentene 77-Et-d0−3 in 92% yield with an average deuterium content of 0.34D/mol. These results show

stage of the reaction, 1,6-diene 26 was converted to exomethylenecyclopentane 27 via sequential hydrometalation/ carbometalation/β-H elimination. After ca. 70% of the diene was converted, 27 was then consumed by alkene transposition via hydrometalation/β-H elimination to yield thermodynamically more stable tetrasubstituted alkene 77. Widenhoefer’s group also developed another catalyst system by utilizing the 1,10-phenanthroline (phen) ligand (AN = acetonitrile).28 Notably, the new method selectively converted 1,6-dienes 26-Me and 82, without recourse to the use of a hydrosilane additive, to furnish the alternative cyclopentene isomers 60-Me and 83 in 71% and 90% yields, respectively, with high isomeric purity (Scheme 21). This selectivity is Scheme 21. Pd-Catalyzed Cycloisomerization of 1,5- and 1,6-Dienes

G


Chemical Reviews

Review

previously proposed mechanism, the hydropalladation/carbopalladation sequence produces 88, which then undergoes β-H elimination to generate 89. Subsequent reinsertion of the exomethylene moiety into the Pd−H bond followed by β-H elimination produces 90 and 91. Thus, the intermediate alkene complexes 89−91 exist in equilibrium. The direct displacement of the alkene ligands with acetonitrile (AN) furnishes cycloisomerization products 27-Me, 77-Me, and 60-Me; however, these ligand-exchange reactions are necessarily slow due to steric constraints. Alternatively, intramolecular ligand exchange with one of the carbonyl groups in close proximity with the coordinated alkene moiety in 91 proceeds with the formation of the less-hindered carbonyl complex 92, which undergoes facile ligand exchange with acetonitrile to afford the major product 60-Me. In addition, 91 was considered to be in equilibrium with a resting state 93. These proposed processes are consistent with the kinetic and predominant formation of 60-Me. As suggested above, the methoxycarbonyl group plays a significant role in product selectivity. This effect was also investigated with respect to other substituents at the homoallylic position of 1,6-dienes in studies by Widenhoefer and co-workers.28c,d Product selectivity was accounted for in the mechanism shown in Scheme 23 in which trans-carbocyclization possibly leads to cis-91 via cis-94. In contrast, cis-carbocyclization would lead to both 90 and trans-91 via trans-94. The kinetic 60/77 selectivity was confirmed to be 30:1 at 40 °C and 6:1 at 70 °C, which are higher than the trans-selectivity of carbocyclization to form trans-88 preferentially over cis-88 (20:1 at 40 °C and 4:1 at 70 °C). If trans-94 exclusively gives rise to 77 via 90, the trans-selectivity of carbocyclization and kinetic 60/77 selectivity would be identical. Thus, both 60 and 77 are formed from trans-94, and the relevant 60/77 selectivity was calculated to be ∼1:2. In analogous studies by Lloyd-Jones and co-workers regarding the mechanism of cycloisomerization using [(πallyl)Pd(AN)2][OTf] as a precatalyst,29 diallylmalonate 26-Me was allowed to react with 5 mol % of the precatalyst in CHCl3 at 40 °C to produce five-membered ring products. During the early stage of the reaction (up to ∼50% conversion of 26-Me), exo-methylenecyclopentane 27-Me was predominantly formed, whereas the yields of cyclopentenes 60-Me and 77-Me increased in the later stage. This behavior parallels that

similar to that obtained using Grigg’s catalyst. In addition, this system can be applied to the cycloisomerization of 1,5-dienes 84 and 86, albeit at a higher temperature, to obtain similar cyclopentenes 85 and 87 in similar yields, with slightly lower isomeric purity. To elucidate the mechanism, Goj and Widenhoefer monitored the progress of the catalytic cycloisomerization of diallylmalonate 26-Me by GC and found that 60-Me was formed at the very first stage of the reaction, and its relative concentration increased steadily to ∼75% after which it increased slowly to a final value of 88%.28b The relative concentration of 3-cyclopentene 77-Me also increased steadily throughout the reaction to a final value of 7%, while the relative concentration of exo-methylenecyclopentane 27-Me reached a maximum of ∼4% and then decreased slowly to ∼3%. It was also revealed that 2-cyclopentene 60-Me is stable enough to inhibit isomerizations to other isomeric products under the catalytic conditions, whereas the exo-methylenecyclopentane 27-Me underwent partial isomerization to generate 3-cyclopentene 77-Me predominantly. To obtain further insight into the mechanism for the direct and selective formation of 2-cyclopentene 60-Me, the same group also carried out extensive deuterium-labeling studies.28b On the basis of the observations in those studies, a possible mechanism for cycloisomerization of diallylmalonate 26-Me using [(phen)PdMe(AN)][BArF] as the precatalyst was proposed, as shown in Scheme 22. Consistent with the Scheme 22. Possible Mechanism for (phen)Pd+-Catalyzed Cycloisomerization of Diallylmalonate

Scheme 23. Product Selection Mechanism for (phen)Pd+-Catalyzed Cycloisomerization of Diallylmalonate

H


Chemical Reviews

Review

produced. This is because cis-89 is stabilized by acetonitrile ligation, leading to an enhanced lifetime of this species. As a result, the hydropalladation of the exocyclic alkene moiety of cis-89 occurs to generate the cyclopentylpalladium species endo94. β-H elimination of endo-94 occurs only at the methylene proton to afford 2-cyclopentene 60 exclusively, because the alternative methyne proton is oriented trans to the palladium center and, hence, cannot be abstracted. In contrast, exo-94, which is formed via hydropalladation of 27, preferentially undergoes β-H elimination of the methyne proton to furnish 3cyclopentene 77. Similar selectivity was also achieved using palladium catalysts with nitrogen ligands L6 and L7 (Scheme 25). Heumann and co-workers used the combination of a cationic palladium catalyst with bis(pyrazole) ligand L6 for cycloisomerization of benchmark diene 26-Me in CHCl3 at 45 °C to selectively obtain cyclopentene 60-Me in 92% yield.30a Similarly, the use of a dicationic palladium catalyst system that features ligand L7 converted dienes 95 and 97 to the corresponding cyclopentene derivatives 96 and 98 in high yields with good isomer selectivity using acetonitrile as the solvent. 30b Interestingly, the bicyclo[4.3.0]nonene derivative 100 was quantitatively obtained from 99 using this method. Fairlamb and co-workers also obtained cyclopentenes 60-Me and 96 with high yields and selectivity using the bis(phosphinite) ligand L8 with a bicyclo[3.2.0]heptane backbone (Scheme 25).31 In contrast, cyclohexane-1,3-dione derivative 101 underwent less selective cyclization to afford 102 in a lower yield of 45%. Lloyd-Jones and co-workers revisited the cationic palladium catalyst system developed by Heumann and co-workers and determined that [PdCl(AN)]+ was in an autoionization equilibrium with the paired halide-exchange isomers PdCl2(AN)2 and [Pd(AN)4]2+.32 More importantly, the

observed in the reaction using the catalytic system comprising [(π-allyl)Pd(PCy3)][BArF] and Et3SiH.27 Notably, performing the reaction in a mixed solvent (CDCl3/MeCN = 1:1) at 60 °C for 24 h led exclusively to 2-cyclopentene 60-Me with 47% conversion of 26-Me, which suggests that 60-Me was directly formed from 26-Me at the high concentration of acetonitrile. By performing extensive labeling studies, Lloyd-Jones and coworkers attempted to elucidate the mechanism of the palladium-catalyzed cycloisomerization. As a result, a possible mechanism outlined in Scheme 24 was proposed. A hydroScheme 24. Possible Mechanism for Cycloisomerization Using [(η3-allyl)Pd(AN)2][OTf]

palladation/carbopalladation sequence transforms 26 into the alkylpalladium intermediate trans-88, which then undergoes βH elimination to produce exo-methylenecyclopentane 27 via the alkene-ligated palladium hydride cis-89. In the presence of excess acetonitrile, 2-cyclopentene 60 was predominantly

Scheme 25. Pd-Catalyzed Cycloisomerization of 1,6-Dienes Using Ligand L6−L8

I


Chemical Reviews

Review

obtained, indicating that initial hydropalladation occurred at the less-substituted alkene. The erosion of endo versus exo selectivity can be ascribed to both steric and electronic effects of the phenyl substituent. Evaluation of the electronic impact of aryl substituents furnished evidence that electron-donating substituents such as methyl and methoxy groups favored the formation of the benzylidene products 37b and 37c, whereas the electron-withdrawing CF3 group increased the endo selectivity. These results seem to suggest that an electrondeficient alkene stabilizes the palladium complex due to the favorable back-donation from the palladium center to the alkene, with consequent increase in the endo selectivity. A mixture of 36b and 36e was subjected to similar reaction conditions to observe inversed regioselectivity with respect to the products from 36e; the reaction of 36e provided an 108e/ 37e ratio of 70:30, whereas in the presence of 36b, the observed 108e/37e ratio became 27:73. This result is ascribed to the associative ligand exchange with an electron-rich diene as shown in Scheme 28. In the complexation involving a

commercially available PdCl2(AN)2 is much more reactive than these cationic complexes. They also examined several nitrile ligands and concluded that tBuCN was the most suitable for this reaction. Accordingly, in the presence of 5 mol % of PdCl2(tBuCN)2, diallylmalonate 26-Me underwent clean cycloisomerization in 1,2-dichloroethane (DCE) at 40 °C over the course of 1.5 h to afford 2-cyclopentene 60-Me in 96% yield with 97% selectivity (Scheme 26). Similarly, diester 95 Scheme 26. PdCl2(tBuCN)2-Catalyzed Cycloisomerization of 1,6-Dienes

Scheme 28. Associative Ligand Exchange That Liberate Benzylidene Products 37

and diol 32 were converted to the corresponding products 96 and 103 in 90% and 77% yields, respectively. A carbonate analogue 105 was also obtained in a similar yield of 79% using diene 104 as the substrate. The yield was improved to 96% in the conversion of acetylated diol 106 to 107, although the corresponding benzyl ether 97 failed to undergo cycloisomerization even when reacted at 60 °C for 24 h. Furthermore, diallyl ether and diallylamine were found to be totally ineffective substrates for this protocol. Lloyd-Jones and co-workers further analyzed the mechanism of cycloisomerization of diallylmalonate 26 into 2-cyclopentene 60 using PdCl2(tBuCN)2 as the precatalyst by means of a series of labeling studies.32,33 On the basis of the obtained experimental evidence, a generally proposed mechanism involving hydropalladation, carbopalladation, and β-H elimination seems to be plausible (see Schemes 22−24). However, the selective formation of 2-cyclopentene 60 requires inhibition of the dissociation of the cyclopentylidene ligand from an intermediate such as cis-89 in Scheme 24. To shed light on the ligand dissociation, the cycloisomerization of aryl-substituted dienes 36a−e was investigated (Scheme 27). The parent phenyl-substituted diene 36a underwent cycloisomerization to afford cyclopentene 108a and exo-(Z)-benzylidenecyclopentane 37a in a 108a/37a ratio of 43:57. No other isomer was

palladium(II) center, π → d interaction is dominant, and therefore, the use of an electron-rich diene enhances the dissociation of the benzylidenecyclopentane. It was also discovered that the rate of cycloisomerization of 26 to 60 was significantly increased in the presence of triethylsilane by employing a Pd/Et3SiH ratio of 1:1. It is wellknown that rapid σ-bond metathesis of Si−H with Pd−Cl generates Pd−H species; thus, the net catalytically active species is considered to be HPdCl. It was also elucidated that catalytic turnover was strongly inhibited by a 1,5-diene that was generated from the diallylmalonate via alkene transposition. The overall catalytic cycle is outlined in Scheme 29. Palladium-catalyzed cycloisomerization with dissimilar selectivity that favors thermodynamically less favorable exoalkylidene compounds has been elusive. However, this unique selectivity was achieved using palladium catalysts that feature 1,5-cyclooctadiene (COD) and N-heterocyclic carbene (NHC) Scheme 29. Proposed Catalytic Cycle of Cycloisomerization of 26 Using PdCl2(tBuCN)2

Scheme 27. Cycloisomerization of Aryl-Substituted Dienes 36-Me with PdCl2(tBuCN)2

J


Chemical Reviews

Review

ligands, although the scope of substrates suitable for these methods is limited to relatively few 1,6-dienes (Scheme 30).34,35

Scheme 32. Rh-Catalyzed Cycloisomerization of Diallyl Ether 18

Scheme 30. Pd-Catalyzed Cycloisomerization of 1,6-Dienes Using COD and NHC Ligands

with those of the palladium-catalyzed reaction (Scheme 33).25a By employing a catalyst loading of ca. 0.5 mol %, heating 44 in isobutanol gave rise to various products. The major product was exo-methylenepyrrolidine 50 (56%). In addition, 51 was formed in a low yield via hydrogenation of the acrylic olefin moiety of 50. The combined yield of these two products, which were formed by cycloisomerization involving two allyl moieties, reached 64%. On the other hand, cycloisomerization involving the acrylic moiety also occurred to generate 45, 48, and 110 in a combined yield of 24%. Thus, in contrast to the palladiumcatalyzed cycloisomerization of 44 (Scheme 12), the rhodiumcatalyzed reaction favors electronically neutral alkenes, and exomethylenepyrrolidine derivatives were the major products. This selectivity was further demonstrated by the cyclization of amides 54 and 57 (Scheme 34).25b The reaction of N,Ndiallylamides 57 resulted in the formation of cycloisomerization products 113 and 58 in 75−86% combined yields; exomethylene isomers (113) were the major products, whereas the reaction of N-allyl acrylamides 54 furnished lactams 55 and 111 in moderate 48−55% combined yields. Interestingly, the cyclization regiochemistry of the exo-methylene products was opposite to that of the palladium-catalyzed cycloisomerization of 54. In addition, the deallylation byproduct 112 was also formed in considerable yield (30−38%), which is indicative of the unsuitability of 54 as the cycloisomerization substrate. Grigg and co-workers extended the scope of the rhodiumcatalyzed cycloisomerization to the synthesis of cyclopentane derivatives26b,37 by utilizing Willkinson’s complex as a catalyst to perform the cycloisomerization of 1,6-heptadienes 26-Et, 114, 61, and 63 in refluxing CHCl3 (Scheme 35). The product yield of this reaction was improved by bubbling dry hydrogen chloride through the CHCl3 solvent before use, whereas the reaction was completely suppressed with the use of ethanol-free CHCl3 in the absence of hydrogen chloride. Other rhodium compounds, including RhCl3, [RhCl(CO)2]2, and RhCl(CO)(PPh3)2, showed no catalytic activity. In contrast to the cycloisomerization of the N-allyl amides described above, this carbocyclization selectively afforded exo-methylenecyclopentane derivatives 27-Et, 115, 116, and 117 with ketone or ester substituents at C1 positions in 60%−92% yields. However, diene substrates 67 and 118−120 with at least one internal alkene moiety failed to undergo cyclization. On the other hand, 1,7-diene 71-Et underwent cycloisomerization to deliver a mixture of five isomers E- and Z-121-Et, 122-Et, 80-Et, and 41Et in 62% total yield in the ratio 56:11:13:11:9, which demonstrates the difficulty associated with controlling isomer selectivity. Although no alkene transposition was observed when 115 was subjected to palladium-catalyzed cycloisomerization conditions or when 60-Et was subjected to rhodium-catalyzed conditions (Scheme 36), treatment of 27-Et with Wilkinson’s catalyst in boiling ethanolic hydrogen chloride gave cyclopentene 77-Et (Scheme 37). Furthermore, 77-Et could be obtained directly from diene 26-Et under similar conditions.

Asymmetric cycloisomerization using a palladium catalyst has also been accomplished, albeit with a moderate ee.30a Heumann and co-workers employed a dicationic palladium catalyst with a chiral diimine ligand L9 in the cycloisomerization of the benchmark diene 26-Et (Scheme 31). This system yielded exoScheme 31. Pd-Catalyzed Asymmetric Cycloisomerization of 26-Et Using Chiral Ligands

methylenecyclopentane 27-Et as the major product in 38% yield with moderate 60% ee. The ee of the minor isomer 60-Et was lower (23%). The use of (−)-sparteine L10 in lieu of L9 generated 27-Et and 60-Et in nearly equal yields. The ee of 27Et and that of 60-Et were 60% and 37%, respectively. The scope of this asymmetric reaction has not been reported, and the improvement of ee requires further exploration of superior chiral ligands and palladium catalysts. 2.3. Rhodium-Catalyzed Reactions

Rhodium-catalyzed cycloisomerization of α,ω-dienes was reported for the first time by Malone and co-workers in 1971.36 Their studies revealed that heating the diallyl ether (17) with RhCl 3 ·3H 2 O or [RhCl 2 (C 6 H 11 O)] 2 ·MeOH (109·MeOH) in the presence of a few percent of allyl alcohol furnished exo-methylenetetrahydrofuran 18 (Scheme 32). The turnover number (TON) was reported to be as high as 104, although no information on the isomeric purity of 18 was provided. Subsequent to Malone’s pioneering study, several research groups reported similar rhodium-catalyzed reactions. Schmitz and co-workers investigated the cycloisomerization of N,Ndiallyl acrylamide 44 using RhCl3 and compared the results K


Chemical Reviews

Review

Scheme 33. Rh-Catalyzed Cycloisomerization of N,N-Diallyl Acrylamide 44

Scheme 34. Rh-Catalyzed Cycloisomerization of N-Allyl Amides 54 and 57

Scheme 37. Rh-Catalyzed Isomerization of exoMethylenecyclopentane 27-Et

Z-123-Et/78-Et = 79:9:18 (Scheme 38). Interestingly, the reaction of isomeric 1,8-nonadiene 74-Et under the same Scheme 38. Rh-Catalyzed Cycloisomerization of 1,6- And 1,8-Dienes in EtOH

Scheme 35. Rh-Catalyzed Cycloisomerization of 1,6- and 1,7-Dienes in CHCl3

conditions resulted in a similar combined yield (95%) and an identical isomeric ratio. A considerably lower selectivity was observed for the cycloisomerization of the unsymmetrical 1,6diene 40-Et, which afforded five isomers in a ratio of E-81-Et/ Z-81-Et/80-Et/72-Et/73-Et = 23:3:51:13:10. In contrast, dienes 119 and 120 (Scheme 35) failed to undergo cyclization in refluxing ethanolic hydrogen chloride. Altough the mechanism of rhodium-catalyzed α,ω-diene cycloisomerization has remained unclear, Grigg and co-workers described that both exo-methylenecyclopentanes and 2-cyclopentenes are produced via common intermediates such as 124 and 125 as outlined in Scheme 39.26b Oxidative cyclization of a diene with a low-valent rhodium species generates a rhodacycle intermediate, which undergoes β-H elimination to form 124. Intramolecular reinsertion of the exocyclic alkene moiety into the Rh−H bond produces rhodacyclobutane 125, which finally evolves into exo-methylenecyclopentane or 2-cyclopentene via β-abstraction of Ha or Hb, respectively, and subsequent reductive elimination.

Scheme 36. Impossible Interconversions of Cycloisomerization Products

These results imply that the product selectivity is altered by the solvent. 2,7-Nonadiene 69-Et having internal alkene moieties was found to be a totally inefficient substrate for reactions in CHCl3, but it underwent cycloisomerization after being refluxed in ethanolic hydrogen chloride for 24 h to deliver isomer mixtures in 94% combined yield in the ratio E-123-Et/

2.4. Ruthenium-Catalyzed Reactions

Grubbs’ catalyst and its derivatives are extremely efficient catalysts for olefin metathesis. Ring-closing metathesis (RCM) of α,ω-dienes using Grubbs-type catalyts constitutes one of the L


Chemical Reviews

Review

product 126. The effect of varying the allenylidene moiety of the catalyst was also investigated by Fürstner, Dixneuf, and coworkers.39b In contrast to 129a, an inverse trend of chemoselectivity was observed using the closely related catalyst 129b that features electron-deficient p-chlorophenyl terminal groups. RCM was almost exclusively catalyzed using the complex with BF4−, and the use of the corresponding TfO− complex led to the formation of nearly equal amounts of 126 and 31. Ç etinkaya, Bruneau, and co-workers synthesized similar allenylidene complexes 130, which incorporate a benzimidazole ligand in lieu of tricyclohexylphosphine.40 The p-cymene complex 130a selectivity favored cycloisomerization, generating the exo-methylenepyrrolidine derivative 31 as the exclusive product under conditions similar to those outlined above (Table 2). Moreover, the use of a longer reaction time (10 h instead of 4 h) resulted in the exclusive formation of the pyrroline derivative 127, which is ascribed to olefin transposition of 31. In fact, when chlorobenzene was used as the solvent, 98% conversion was achieved after 6 h to afford 31 and 127 in 60% and 38% yields, respectively. This conversion was improved to 100% by utilizing a prolonged reaction time of 10 h, and 127 became the sole product. The corresponding hexamethylbenzene complex 130b showed lower catalytic activity, resulting in a lower conversion even after 20 h and the formation of both cycloisomerization products in almost equal amounts. Ç etinkaya, Dixneuf, and co-workers discovered that related allenylidene complexes 131, which have arene ligands with strongly electron-donating imidazolinylidene side arms, were particularly selective cycloisomerization catalysts.41 Heating the tosylamide-derived diene 30 at 80 °C in chlorobenzene with 2.5 mol % of 131a for 4 h afforded 31 in 94% yield along with small amounts of 126 (Scheme 40). However, this selectivity was limited to this substrate; use of the malonate analogue 26-Et as a substrate exclusively furnished the corresponding RCM product 132 in 87% yield under conditions identical to those

Scheme 39. Possible Mechanisms for RhCl(PPh3)3Catalyzed Cycloisomerization of 1,6-Dienes

most powerful methods for constructing carbo- and heterocyclic frameworks.18 On the other hand, it is well-known that ruthenium hydride species can be generated from Grubbs’ catalyst and that the generated species catalyze olefin transpositions.38 In addition, transition-metal-catalyzed cycloisomerizations usually involve metal hydrides as catalytically active species. Expectedly, cycloisomerization has been recognized as a side-reaction of diene RCM since the end of the 20th century. Dixneuf and co-workers investigated the use of ruthenium allenylidene complexes 129 with the p-cymene and tricyclohexylphosphine ligands as catalysts for RCM of tosylamide 30 (Table 2).39a When weakly coordinating BF4− (129a) was employed as the counteranion, the RCM product 126 was obtained in 31% yield; however, the unexpected cycloisomerization product 31 was also formed as the major product along with the olefin-transposition product 128 (16%). Cycloisomerization was inhibited by addition of a Lewis acid, whereby the use of 10 mol % Et2O·BF3 resulted in the selective formation of 126 in 96% yield, and 31 was obtained in a very low yield (3%). Replacement of BF4− with TfO− highlighted the impact of counteranions on chemoselectivity, where the use of 129a/TfO− led to the quantitative formation of the RCM

Table 2. Ruthenium Allenylidene-Catalyzed Cycloisomerization of N,N-Diallyl Tosylamide 30

cat

solvent, time/h

conversion/%

isomer ratio: 126, 31, 127, 128%

129a/BF4 129a/OTf 129b/BF4 129b/OTf 130a 130a 130a 130a 130b

toluene, 5 toluene, 5 toluene, 5 toluene, 5 toluene, 4 toluene, 10 chlorobenzene, 6 chlorobenzene, 10 chlorobenzene, 20

90 99 97 91 100 100 98 100 79

31, 43, −, 16 99, −, −, − 99, 7, −, − 46, 45, −, trace −, 100, −, − −, −, 100, − −, 60, 38, − −, −, 100, − −, 40, 39, −

M


Chemical Reviews

Review

of 5 mol % of the catalyst, 30 was heated in toluene at 80 °C for 3 h to afford the cycloisomerization product 31 with 93% conversion (Scheme 42). In contrast, the same reaction was carried out with 10 mol % of phenylacetylene to obtain the RCM product 126 with 91% conversion.

Scheme 40. Ruthenium Allenylidene-Catalyzed Cycloisomerization of 1,6-Dienes

Scheme 42. Ruthenium-Catalyzed Cycloisomerization and RCM of N,N-Diallyl Tosylamide

used with 30. In a similar manner, catalyst 131b quantitatively converted 30 to the cycloisomerization product 31. UV irradiation of 131b for 30 min prior to the reaction enabled cycloisomerization of 26-Et under conditions identical to those stated above to generate 27-Et in 87% yield as the sole product. No explanation of this enhancement of cycloisomerization activity upon UV irradiation was provided in that report. Kurosawa and co-workers reported similar results by utilizing 5 mol % of 133, which has an arene ligand equipped with an alcohol side arm, and phenylacetylene at 50 °C for 24 h with consequent RCM of 30 to selectively deliver 126 in 97% yield (Scheme 41).42 The RCM reactivity was ascribed to the

In the RCM of 30 using the Hoveyda−Grubbs’ catalyst, cycloisomerization was also recognized as a side-reaction giving rise to the decomposition product of the alkylidene complex.44 Arisawa, Nishida, and co-workers developed a synthetically useful cycloisomerization protocol by utilizing the secondgeneration (2G) Grubbs’ catalyst 139.45 In the presence of 1 equiv of vinyl silyl ether 138, catalyst 139 at a 5 mol % loading promoted cycloisomerization of 30 in refluxing dichloromethane over the course of 2 h to afford 31 in 86% yield (Scheme 43). Pyrroline 127 was also detected by 1H NMR, and

Scheme 41. Ruthenium-Catalyzed Cycloisomerization and RCM of N,N-Diallyl Tosylamide

Scheme 43. Cycloisomerization of 1,6-Dienes Catalyzed by Grubbs' Catalysts

formation of vinylidene species. In contrast, the same reaction was performed using triethylamine instead of phenylacetylene to obtain the cycloisomerization product 31 in a high yield (94%). The alcohol side arm of 133 was critical for efficient cycloisomerization; in its absence, the cycloisomerization of 30 was much less efficiently catalyzed by the otherwise similar arene ruthenium complex 134 under similar conditions to afford 31 in 26% yield. The authors assumed that the cationic ruthenium complexes were reduced by triethylamine to generate catalytically active hydride species. Thus, they investigated the catalytic activities of the hydride complexes 135 and 136. Although the efficiency of 135 as a cycloisomerization catalyst was poor in the absence of additives, the addition of AgBF4 resulted in the formation of 31 in 24% yield after reaction for 39 h. In contrast, 1 mol % of the hydride complex 136, possessing the alcohol side arm, efficiently catalyzed cycloisomerization of 30 in tetrahydrofuran (THF) at 75 °C over 18 h, even in the absence of additives, to furnish 31 in 61% yield. Moreover, Dixneuf and co-workers reported a similar tuning of the chemoselectivity for cyclization of the N,N-diallyl tosylamide 30 using the NHC complex 137.43 In the presence

its formation was ascribed to olefin transposition of 31 under catalytic conditions. Replacing 138 with vinyl ethyl ether in this protocol resulted in lower catalytic activity. The use of the firstgeneration Grubbs’ catalyst 140 in place of 139 led to the favored formation of the RCM product 126. Further, the Hoveyda−Grubbs’ catalyst 141 exhibited higher olefin-isomerization activity to produce 31 and 127 in 71% and 24% yields, respectively. This method was also effective for conversion of dienes 26-Et and 114 to 27-Et and 115, respectively. Although the active Ru species was not characterized, vinyl ether 138 was considered to be required for the generation of a low-valent ruthenium hydride. Notably, N-allyl and N-homoallyl o-vinylaniline derivatives 142a and 142b underwent selective cycloisomerization to afford 3-methylene-2,3-dihydroindoles 143a and 143b in 80% and 58% yields, respectively (Scheme 44). The corresponding 2,3-dihydrobenzofurans 145a and 145b were obtained in good yields from p-hydrobenzoquinone derivatives 144a and 144b, respectively. This method was applied to the first synthesis of the proposed structure of fistulosin (146), which is a naturally N


Chemical Reviews

Review

Scheme 44. Synthesis of 2,3-Dihydroindoles and 2,3Dihydrobenzofurans

Table 3. [RuCl2(cod)]n-Catalyzed Cycloisomerization of 1,6-Dienes (iPrOH, 90 °C, 24 h)

occurring indole alkaloid.43b However, the 1H NMR data of 146 was not identical to those reported for fistulosin. In 1999, the first diene cycloisomerization using nonmetathesis ruthenium catalysts was reported by Itoh and coworkers.46 They discovered that [RuCl2(cod)]n (147) catalyzed the cycloisomerization of diallylmalonate 26-Me in iPrOH at 90 °C, although this oligomeric complex is scarcely soluble in common organic solvents (Scheme 45). Using 1 mol % of 147,

Table 4. [RuCl2(cod)]n-Catalyzed Cycloisomerization of Unsymmetrical Dienes (iPrOH, 90 °C, 24 h)

Scheme 45. Ru-Catalyzed Cycloisomerization of Diallylmalonate

the exo-methylenecyclopentane 27-Me was obtained in 89% yield with 95% isomeric purity. Decreasing the catalyst loading to 0.1 mol % still furnished 27-Me in a similar yield, albeit with lower isomeric purity. Use of the more soluble analogue, RuCl2(cod)(AN)2, at a 0.1 mol % loading provided an improved yield with similar purity. In contrast, similar complexes that have norbornadiene (nbd) and carbonyl ligands instead of cod were less reactive, and therefore, they had to be used at higher loadings of 5 mol %. The reactions using these catalysts resulted in a moderate isomeric purity (84%−86%), although the yields were high. A 5 mol % loading was also required when Cp*RuCl(cod) (148, Cp* = η5-Me5C5) was used as the catalyst; however, 27-Me was obtained with excellent yield and isomeric purity. The substrate scope of the cycloisomerization using 147 is wide, and a variety of functional groups including esters, ketones, amides, nitriles, and sulfones are tolerated using this catalyst (Table 3). All of the reactions employing 147 selectively afforded exo-methylene isomers in 62−98% yields with isomeric purities from 83% to 100% as determined by GC or high-performance liquid chromatography (HPLC). However, cyclization of diallyl ether, 2,2-diallyl-1,3-dithiane, and diallyldimethylsilane is not promoted by this method. Table 4 summarizes the yields of the exo-methylenecyclopentanes derived from conversion of the unsymmetrical 1,6-dienes,

which were notably high (90−95%). This isomer selectivity is quite unique to the ruthenium catalysts (also see Scheme 43); other catalysts predominantly yielded thermodynamically favored isomers with multiply substituted alkenes (Schemes 10, 14, 18, 30, 35, and 38), and the isomeric purities of the cycloisomerization products obtained from the unsymmetrical O


Chemical Reviews

Review

complete preservation of the deuterium label. In striking contrast, cis-rich 152 was subjected to similar conditions, with the exception that RuClH(PPh3)2 was used as the catalyst instead of 147, giving rise to the trans-rich diene 152 with a trans/cis ratio of 10:1. Because this cis−trans isomerization occurred via hydroruthenation of the alkenylsilane moiety of cis-152 and subsequent β-H elimination of the resultant alkylruthenium species, such hydroruthenation and subsequent insertion of the less-hindered alkene can be excluded as a productive pathway. Furthermore, the reaction of 154 under the same conditions resulted in the diastereoselective formation of 155 with an isotope abundance of 0.7D. The partial loss of the deuterium label is attributed to the hydroruthenation/β-H elimination process. Although the details are unknown, the stereochemistry of the resultant cycloisomerization product 155 can be tentatively proposed as depicted in Scheme 47, if reductive elimination occurs via the ruthenacyclopentane hydride species 156. A catalytic cycle in accord with the presented experimental observations is shown in Scheme 48. The solvent-ligated

1,6-dienes are generally much lower than those of the cyclization products from symmetrical dienes. Remarkably, a 1,6-diene that features a trimethylsilyl terminal group underwent cycloisomerization at a 2.5 mol % catalyst loading to afford the corresponding exo-methylenecyclopentane in 94% yield with 93% purity. In addition, a malonate-derived 1,7-diene underwent exo-methylene-selective cycloisomerization, albeit in a diminished yield. The mechanism of this fascinating isomer selection of ruthenium-catalyzed cycloisomerization is discussed subsequently. The unique regioselectivity of ruthenium catalysts implies that the ruthenium-catalyzed cycloisomerization proceeds with a distinct mechanism. This mechanism was investigated using several control experiments.46b First, the isolable hydride complex 149 was prepared from the oligomeric complex 147 and piperidine (Scheme 46), and cycloisomerization of Scheme 46. Preparation of ClRuH(piperidine)2 (149) from 147

Scheme 48. Plausible Catalytic Cycle of Ru-Catalyzed Cycloisomerization of 1,6-Dienes

diallylmalonate 26-Me was examined using 149 as the catalyst without the alcoholic solvent. The diene 26-Me was heated in toluene at 90 °C in the presence of 10 mol % 149 to afford exomethylenecyclopentane 27-Me in 92% yield with 79% purity. In contrast, phosphine-ligated ruthenium hydride complexes such as RuClH(CO)(PPh3)3, CpRuH(PPh3)2, and Cp*RuH3(PPh3) failed to catalyze the cycloisomerization of 26-Me. Accordingly, the net cycloisomerization catalyst may be a coordinatively unsaturated chlororuthenium hydride species, which is weakly ligated by solvent molecules, and alcohols play the role of a hydride source similar to piperidine. The cycloisomerization of monodeuterated unsymmetrical diene 150 was conducted using 5 mol % of 147 in isopropanol at 90 °C; no deuterium incorporation was observed for the resultant product 151 (Scheme 47). On the other hand, the reaction of the cis-rich diene 152 (cis/trans = 4.3:1) under similar conditions to those outlined above produced 153 with

HRuCl species, which is produced from 147 and isopropanol, undergoes ligand exchange with a 1,6-diene substrate to form 157. This diene complex is transformed into a ruthenacycle hydrido complex 158 via oxidative cyclization. As proposed in Scheme 47, reductive elimination occurs between the moresubstituted alkyl ligand and the hydrido ligand. This regiochemistry can be explained because the C1−Ru bond is plausibly longer and weaker than the Ru−C2 bond due to steric repulsion and, hence, more susceptible to reductive elimination. Subsequent β-H elimination of 159 produces an exo-methylene product and completes the catalytic cycle. Parrain and co-workers investigated the diastereoselective cycloisomerization of diallylated lactones using the ruthenium catalyst 147.47 The saturated lactone 160a was heated in EtOH (in place of iPrOH used in the original protocol) at 80 °C for 12 h to obtain the spirocyclic product 161a in 75% yield with a diastereomer ratio of 67:33 (Scheme 49). Although the use of α-methyllactone 160b as the substrate improved the yield to 92%, 161b was obtained as four diastereomers with lower selectivity. In contrast, unsaturated lactones 162a−c underwent cycloisomerization with improved diastereoselectivity. In particular, spirocyclic dimethyl lactone 163c was obtained with an excellent diastereomer ratio of 95:5. The relative stereochemistry of each diastereomer of 163a was determined

Scheme 47. Ru-Catalyzed Cycloisomerizations of D-Labeled Dienes

P


Chemical Reviews

Review

Scheme 49. Diastereoselective Spiro Cycloisomerization of Diallyllactones

Scheme 51. One-Pot Tandem Double Allylation/ Cycloisomerization of 1,3-Dicarbonyl Compounds

for 20 or 40 h. This one-pot reaction generated the desired exomethylenecyclopentane derivatives in 57−83% yields with 89− 99% isomeric purity. Without adding COD in the second step, a higher catalyst loading (10 mol %) was required to obtain comparable yields and purity. by carrying out nuclear Overhauser effect (NOE) experiments as shown in Scheme 49. Yamamoto and co-workers extended the rutheniumcatalyzed diene cycloisomerization to a one-pot tandem process, which involves double allylation of 1,3-dicarbonyl compounds and subsequent cycloisomerization of the resultant 1,6-dienes.48 Interestingly, both steps can be carried out using a single metal catalyst, provided that the catalytic species generated in the first process can be successfully transformed into that for the second reaction. To realize this requirement, triethylsilane was investigated as a reagent for the generation of a ruthenium hydride catalyst (Scheme 50). Catalyst 148 was utilized for the stated purpose because it is an effective catalyst for both double allylation of 1,3-dicarbonyl compounds and diene cycloisomerization (see Scheme 45). In the presence of 5 mol % of 148 and 1 equiv of Et3SiH, refluxing 26-Me in DCE at 90 °C for 10 h produced the cycloisomerization products 27Me and 60-Me and the silylative cyclization product 164 in the ratio 69.6:23.1:7.3. An extended reflux time of 24 h resulted in an increased yield of 60-Me and 164. Further, considerable amounts of 164 and 3-cyclopentene 77-Me were formed when the p-cymene complex 165 was employed as the catalyst. After double allylation, 148 is expected to be converted to the corresponding π-allyl complex. Thus, the ruthenium(IV) allyl complex 166 was also investigated as a precatalyst, and a similar product ratio was obtained after a reaction time of 24 h. This indicates that the π-allyl complex is transformed into catalytically active hydride species upon treatment with Et3SiH. On the basis of the experiments described above, a one-pot tandem process was developed as outlined in Scheme 51. The first step was carried out using 5 mol % of 148 in DCE at 90 °C for 5 h. Subsequently, 20 mol % each of Et3SiH and 1,5cyclooctadiene (COD) were added, and heating was continued

2.5. Titanium- and Zirconium-Catalyzed Reactions

Titanocenes and zirconocenes have been used as catalysts for oligomerizations/polymerizations and isomerizations of olefins. Diene cycloisomerizations have also been investigated using these catalysts. Titanocene hydride, which was produced upon treatment of titanocene dichloride with LiAlH4 in benzene, catalyzed the cycloisomerization of cis-divinylcyclohexane (cis14) at 70 °C to afford the exo-methylenecyclopentane derivatives cis-15 and trans-15 in 30% yields (Scheme 52).49 Scheme 52. Ti-Catalyzed Cycloisomerization of Divinylcyclohexane

trans-14 was also obtained in 22% yield. In contrast, a similar reaction of trans-14 gave rise to trans-15 and the

Scheme 50. Cycloisomerization of Diallylmalonate Using Ruthenium Catalysts and Et3SiH

Q


Chemical Reviews

Review

methylcyclopentene derivative trans-167 in 42% and 19% yields, respectively; no cis cycloisomerization product was formed. Moreover, the respective reactions of cis- or trans-14 were conducted at 170 °C using 168, both of which generated product mixtures having identical compositions, i.e., trans-167 (58%), cis-167 (18%), and trans-15 (15%). These results indicate that cyclopentene 167 was produced from the exomethylenecyclopentane 15 via olefin transposition and cis- and trans-15 are interconverted via cyclopentene 16, which was isolated from the reaction at 200 °C. Nakamura and co-workers and Lehmkuhl and Tsien independently reported the cycloisomerization of 1,5-hexadiene using a titanium catalyst derived from titanocene dichloride and isopropyl magnesium bromide (Scheme 53).50 During the

In addition to catalysts based on titanocenes, zirconocenebased polymerization catalysts have also been applied to the cyclosiomerization of α,ω-dienes. However, in the case of zirconocene-based polymerization catalysts, the conformational constraint is a requirement for successful cyclization. Christoffers and Bergman reported that treatment of 1,2diallylbenzene 169 with a catalyst comprising 1 mol % of zirconocene dichloride and 4 mol % of methylalumoxane (MAO) at room temperature for 3 days yielded the exomethylenecycloheptane derivative 170 in 70% yield (Scheme 55).51 In contrast, a similar reaction of 1,7-octadiene resulted in Scheme 55. Zirconium-Catalyzed Cycloisomerization of 1,2Diallylbenzene

Scheme 53. Titanium-Catalyzed Cycloisomerization of 1,5Hexadiene

the formation of linear oligomers, indicating the importance of the fused benzene ring in 169 for cycloisomerization; it is not surprising that the benzene ring in 169 reduced the conformational freedom and, as a result, rendered this substrate more cyclizable. Thiele and Erker also showed that conformationally constrained dienes undergo selective cycloisomerization in the presence of Ziegler-type catalysts involving zirconocenes (Table 5).52 cis-Divinylcyclopentane (cis-171) was treated with zirconocene dichloride and MAO (diene/Zr = 90, Al/Zr = 67) at 20 °C for 6 days to obtain the cycloisomerization product cis-

investigation of titanocene-catalyzed isomerization of alkenes, Nakamura and co-workers found that the reaction of 1,5hexadiene with 1 mol % of a titanium catalyst derived from Cp2TiCl2 and 2 equiv of Grignard reagent at 20 °C under neat conditions gave E,E-2,4-hexadiene as the major product.50a In addition to the linear isomerization products, a cycloisomerization product, i.e., methylcyclopentene, was also formed in 27.8% yield. In contrast, Lehmkuhl and Tsien carried out the same reaction in THF with a 4 mol % catalyst loading to obtain a similar product mixture; however, in this instance, exomethylenecyclopentane was formed as the major product via selective cycloisomerization.50b The latter method was successfully applied to the cycloisomerization of divinylcyclohexane (Scheme 54).50b Com-

Table 5. Zirconocene-Catalyzed Cycloisomerization of Divinylcycloalkanes

Scheme 54. Titanium-Catalyzed Cycloisomerization of Divinylcyclohexane

diene

diene/ Zr

Cp2ZrCl2

171

90

67

(tBuCp)2ZrCl2

171

110

67

(tBuCp)2ZrCl2

171

110

67

174

171

200

93

175

171

205

1740

Cp2ZrCl2

14

290

250

174

14

175

220

20 °C, 6d 60 °C, 9d 60 °C, 30 h 60 °C, 76 h 35 °C, 24 h 40 °C, 7d 60 °C

175

14

310

360

45 °C

complex

pared to 1,5-hexadiene, conformationally constrained divinylcyclohexane underwent cycloisomerization with greater efficiency to afford the exo-methylene compound 15 as the major product. Because the reaction conditions were milder than those previously reported (Scheme 52), the stereochemistry of substrate 14 was mostly preserved; the respective reactions of trans-14 and cis-14 furnished trans-15 and cis-15 as the corresponding major products. The former reaction exhibited a higher degree of selectivity. R

Al/Zr

temp/ time

products and yields cis-172 73%, dimers 23% cis-172 41%, cis-173 31%, dimers 12% cis-172 8%, cis-173 64%, dimers 12% cis-172 35%,* cis-173 31%, dimers 4% cis-172 21%, cis-173 24%, dimers 1% cis-15 72%, cis-167 1%, dimers 2% cis-15 36%, cis-167 22%, dimers 11% cis-15 58%, cis-167 1%, dimers 1%


Chemical Reviews

Review

heptadiene and 1,8-nonadiene with consequent formation of linear isomerization products. Similarly, cycloisomerization of 1,6-dienes has been catalyzed by a catalytic system composed of diaryloxytitanium dichloride and a Grignard reagent.54 Okamoto and Livinghouse used cyclohexylmagnesium chloride to activate the titanium reagent. In the presence of 10 mol % of (2,6-Me2C6H3O)2TiCl2 and 25 mol % CyMgCl, a variety of 1,6-dienes underwent cycloisomerization, even at room temperature, to afford exomethylene compounds in 44−90% yields, although the obtained products contained 5−10% of saturated byproduct 177 (Table 6). This method is amenable to ethers and silanes;

172 in 73% yield along with dimeric byproduct (23%). Using (tBuCp)2ZrCl2 with MAO (diene/Zr = 110, Al/Zr = 67), the reaction was performed at 60 °C for 9 h to obtain cis-172 and its isomerization product cis-173 in 41% and 31% yields, respectively. Further extension of the reaction time to 30 h resulted in an increased yield of cis-173 (64%) with concomitant decrease in the yield of cis-172 (8%). The yields of the dimers were constant, independent of the reaction times. Moreover, the indenyl complex 174 exhibited higher catalytic activity as evidenced by its lower loading (diene/Zr = 200), although larger amounts of MAO were used (Al/Zr = 93); the reaction was carried out at 60 °C for 76 h, and cis-172 and cis173 were obtained in 35% and 31% yields, respectively. Notably, the formation of dimers was effectively suppressed (4%). Because catalyst 174 is optically active, slight asymmetric induction was observed for the obtained cycloisomerization products (ee's are 99 20 >99 >99 >99

a

yield/ %

product (selectivity/ %)

96 93 90

31 (5) 214 (87) 31 (20) 31 (>98) 127 (86), 214 (14) 31 (>98)

The cycloisomerization of a benchmark substrate, i.e., diallylmalonate, was also attempted by the same researchers using these recyclable catalyst systems; however, the product selectivity was low. This result is in striking contrast to those observed using the original protocols with conventional solvents, which provided high product selectivities. Asymmetric induction was not observed when chiral ionic liquids were used. 3.3. Microwave-Assisted Cycloisomerization

Microwave (MW)-assisted organic synthesis has received continuous attention because of the dramatic enhancement of reaction rates and improvements of product yields and selectivity that can be achieved under MW irradiation conditions.64 This state-of-the-art technique has also been applied to transition-metal catalysis.65 Fairlamb and co-workers revisited the ruthenium-catalyzed cycloisomerization of 1,6dienes using focused MW dielectric heating instead of conventional heating.66 The cycloisomerization of benchmark substrate 26-Me reached completion within a much shorter reaction time (0.25 vs 24 h) under MW irradiation conditions to furnish 27-Me in 98% yield with 98% purity (Scheme 66).

iPrOH (120 equiv) was added. bReaction was carried out at 40 °C.

[RuCl2(cod)]n (147) and [Pd(AN)4](BF4)2 were found to exhibit high catalytic activity as well as high selectivity in favor of the exo-methylene product 31. Interestingly, the Wilkinson’s catalyst, RhCl(PPh3)3, selectively produced enamide 214, although the product selectivity was slightly lower than the aforementioned ruthenium and palladium catalysts. Evaluation of the recyclability of these ruthenium and palladium catalysts demonstrated that the ruthenium catalyst 147 catalyzed cycloisomerization of 30, remarkably without any loss of activity and selectivity up to 6 cycles (Table 10),

Scheme 66. Ru-Catalyzed Cycloisomerization with MW Irradiation

Table 10. Recyclable Ru Catalyst for Cycloisomerization of N,N-Diallyl Tosylamide

run

conversion (selectivity)/%

isolated yield/%

first second third fourth fifth sixth seventh eighth

>99 (100) >99 (100) >99 (100) >99 (100) >99 (100) >99 (100) 98 (100) 92

97 94 93 96 98 93 96 90

Notably, both the yield (99%) and purity (96%) were greatly improved for the conversion of sulfonamide 30 to pyrrolidine 31 using MW irradiation. A similar improvement in purity was also observed for the MW-assisted cyclosiomerization of unsymmetrical diene 40-Me without erosion of regioselectivity.

although the addition of iPrOH (120 equiv) was required for each cycle. On the other hand, the palladium system required 2.5 mol % of [Pd(AN)4](BF4)2 only, and the reaction could be carried out at a lower temperature (40 °C). However, a significant erosion of both activity and selectivity was observed at the third cycle using this catalyst system (Table 11).

4. CYCLOISOMERIZATIONS INVOLVING CONJUGATED DIENES 4.1. Cycloisomerization Involving 1,3-Diene

1,3-Dienes and 1,2-dienes (allenes) are generally more reactive than simple alkenes. Therefore, the substitution of at least one of two alkene moieties of α,ω-dienes with 1,3- or 1,2-dienes provides a good opportunity to develop a highly efficient diene cycloisomerization. In addition, an additional unsaturated moiety is incorporated into the expected product using this strategy, which can be further functionalized by subsequent transformations, compared to products from α,ω-diene cycloisomerizations. The earliest example of exploitation of this useful cycloisomerization approach was reported by Takacs and co-workers in the late 1980s.68 They prepared a low-valent iron bipyridine (bpy) complex by reducing Fe(acac)3 with Et3Al, which was

Table 11. Recyclable Pd Catalyst for Cycloisomerization of N,N-Diallyl Tosylamide

run

conversion (selectivity)/%

isolated yield/%

first second third fourth

>99 (97) >99 (93) 95 (72) 95 (71)

90 90 68 60 W


Chemical Reviews

Review

corresponding product 225 in 68% yield with >95% isomeric purity. The cycloisomerization of amide-tethered triene 226 also proceeded to produce trans-substituted piperidine 227 in 85% yield. The cycloisomerization reaction of triene 228, possessing an all-carbon tether, also yielded trans-substituted cyclohexane 229 even though a higher temperature of 45 °C was required. In all of these iron-catalyzed triene cycloisomerizations, benzyl ether moieties were critical for selective cyclization. Similar cycloisomerizations of substrates containing unsaturated esters instead of the benzyl ethers were characterized by low yields and product selectivities.70 The iron-catalyzed ene−1,3-diene cycloisomerization was extended to the formation of bicyclic compounds (Scheme 69).71 Cyclization of the pyrrolidine derivative 230 was

then used as the catalyst (Scheme 67). In the presence of 10− 15 mol % of thus-prepared “Fe(bpy)”, triene 215 underwent Scheme 67. Iron-Catalyzed Cycloisomerization of 1,3,8Triene

Scheme 69. Bicycle Formations via Iron-Catalyzed Cycloisomerization

cycloisomerization in benzene at 25 °C within 3−8 h to produce isomers 216. Subsequent acetalization with ethylene glycol under acidic conditions afforded 217a and 217b in 82% combined yield with the 217a/217b ratio of 95:5. Notably, only cis-cyclopentanes were obtained from trans-allylic benzyl ether 215, although the corresponding cis-ether yielded the trans-isomer of 217b as the major product. In the same manner, 218 was converted to 219 as the exclusive product in 66% yield with >93% purity. More importantly, the cyclization of 220 having a methyl substituent at the allylic position resulted in the formation of 221 in 62% yield with excellent diastereoselectivity (150:1). This method was later extended to six-membered-ring formations (Scheme 68).69 Ether-tethered triene 222 was treated with 10 mol % of the iron catalyst, and subsequent acetalization furnished the trans-substituted pyran analogue 223 in 84% yield with excellent diastereoselectivity (>50:1). Moreover, triene 224, possessing an allylic methyl substituent, underwent diastereoselective cycloisomerization to afford the

performed using an in situ-generated iron catalyst with a bisoxazoline ligand to obtain indolizidines 231a and 231b in 65% combined yield with a 231a/231b ratio of over 20:1. Notably, only E enol ethers were formed. In striking contrast, the use of standard Fe(bpy) as the catalyst resulted in erosion of the diastereoselectivity (3:1) and the loss of configurational selectivity of the enol ether moiety. A similar bicyclic-ring formation from tetrahydropyran trans-232 was carried out using the Fe(bpy) catalyst. After acetalization, the desired products 233a and 233b were obtained with good diaseteroselectivity (233a/233b = 91:9), albeit in moderate yield. A higher yield (65%) was obtained when the corresponding cis-232 was used as the substrate at the expense of diastereoselectivity. The diastereoselective iron-catalyzed cycloisomerization was successfully applied to the asymmetric construction of natural product frameworks.72 Iridoid monoterpene natural products were synthesized by preparing tert-butyldimethylsilyl (TBS) ether 234 as the enantiopure common triene substrate, which underwent cycloisomerization to afford a silyl ether mixture 235 (Scheme 70).72a Subsequently, 235 was converted to alcohol 236 and cyclic acetal 237 in 53% and 56% overall yields from 234, respectively. Finally, these intermediates were converted to the natural products (−)-mitsugashiwalactone and (+)-isoiridomyrmecin. Scheme 71 shows the cycloisomerization conditions for the tetrahydroisoquinoline-derived triene 238 using a bisoxazoline catalyst, Fe(L12). The resultant product 239 was then submitted to palladium-catalyzed hydrogenation to ultimately afford 240 (R = Me), which is a homologue of the naturally occurring alkaloid (−)-protoemetinol (240, R = H).72b Furthermore, the iron-catalyzed ene−1,3-diene cycloisomerization was utilized for the construction of the aglycone of the natural iridoid glucoside

Scheme 68. Six-Membered Ring Formations via IronCatalyzed Cycloisomerization

X


Chemical Reviews

Review

Scheme 70. Asymmetric Total Synthesis of Iridoid Monoterpenes

Scheme 73. Nickel-Catalyzed Cycloaddition/ Cycloisomerization of 1,3,8,10-Tetraene

with catalytic amounts of Pd(OAc)2, PPh3, and Et3N in refluxing THF (Scheme 74). Alkylidenecyclopentane 246 was Scheme 71. Asymmetric Total Synthesis of Iridoid Monoterpenes

Scheme 74. Palladium-Catalyzed Cyclizations of 1,3,8,10Tetraene

(−)-gibboside by the Takacs group (Scheme 72).72c This group investigated the cyclization of optically active triene 241 using Scheme 72. Asymmetric Total Synthesis of (−)-Gibboside obtained in 65% yield along with a small amount of acetate 247. More importantly, those studies disclosed that the unsymmetrical homologue 248, having a terminal methyl group attached to one of the 1,3-diene moieties, was regioselectively converted to cyclopentane 249 in 95% yield with exclusive trans-selectivity. When the reaction of sulfonamide 250 was performed in the presence of MeOH-d4, the corresponding pyrrolidine derivative 251-d1 was produced with a ∼90% deuterium incorporation at the allylic side-chain. The reaction was considered to proceed via oxidative cyclization, giving rise to the σ,π-bisallyl complex 252, which underwent protonation at the σ-allyl moiety. Subsequent deprotonation of the resultant π-allyl complex 253 ultimately afforded 251-d1. The scope of this selective cycloisomerization is outlined in Table 12. The rhodium-catalyzed ene−1,3-diene cycloisomerization was developed by Mori and co-workers in an investigation of the intramolecular hydroacylation of dienal, where it was serendipitously discovered that an aldehyde substrate with a 1,3,8-triene moiety underwent cycloisomerization to produce alkenylcyclopentene.75 The formyl group was determined to be unnecessary to the success of this cycloisomerization because the treatment of the 2,4,9-decatriene derivative 254 with 10 mol % of [Rh(dppe)]ClO4 (dppe = diphenylphosphinoethane) in DCE at 65 °C for 4 h afforded alkenylcyclopentene 255 in 85% yield as a sole product (Scheme 75). This 1,3-diene product was confirmed to be a thermodynamic product, which was produced via alkene transpositions of kinetically generated isomers including 256. The cationic rhodium complex [Rh(dppe)]ClO4 was an optimal catalyst for this purpose;

Fe(L12) and successfully obtained the precursor of the aglycone. Although the desired 242 was obtained with high diastereomeric purity (>95%), the yields varied over a wide range (40%−80%). The resultant 242 was converted to (−)-gibboside after several steps. During investigation of the nickel-catalyzed [4 + 4]cycloaddition of tetraenes, Wender and Ihle discovered that treating 1,3,8,10-tetraene 243 with catalytic amounts of Ni(cod)2 and PPh3 in toluene at 60 °C generated the desired [4 + 4]-cycloadduct 244 in 70% yield, and that the substrate also underwent cycloisomerization (Scheme 73).73 As sideproducts, the [4 + 2]-cycloadduct 245 and the cycloisomerization product 246 were obtained in 2.6% and 12% yields, respectively. Interestingly, the use of tri(o-biphenyl)phosphine as the ligand instead of PPh3 resulted in predominant formation of 246 in a moderate yield. Takacs and co-workers then extended the scope of the cycloisomerization of 1,3,8,10-tetraenes using palladium catalysts.74 Tetraene 243 underwent cyclization upon treatment Y


Chemical Reviews

Review

Table 12. Pd-Catalyzed Cycloisomerization of Tetraenes (E = CO2Et)

Scheme 76. Rhodium-Catalyzed Cycloisomerization of Trienes

264 was formed via direct cycloisomerization prior to the transposition of the 1,3-diene moiety, which preceded cycloisomerization in the formation of the minor product 265. In contrast, triene substrates that have ether- and tosylamide tethers furnished only intramolecular [4 + 2]-cycloadducts. Sato and co-workers extended this method to tandem intramolecular hydroacylation/cycloisomerization of tetraenals, because both hydroacylation and cycloisomerization proceed with the same cationic rhodium catalyst.76 However, the initial attempt at this tandem reaction using substrate 266 resulted in the failure of the second cycloisomerization, and cycloheptenone 267 with an intact diene substituent was obtained in 66% yield (Scheme 77). Thus, a malonate tether was Scheme 77. Rhodium-Catalyzed Tandem Intramolecular Hydroacylation/Cycloisomerization

Scheme 75. Rhodium-Catalyzed Cycloisomerization of 2,4,9Decatriene

introduced into the substrate 268, because quaternary centers generally facilitate cyclization (Thorpe-Ingold effect).77 The reaction of 268 was conducted in refluxing DCE for 9 h to obtain the desired cyclopentane-fused cycloheptenone 269 in 44% yield. Furthermore, the total synthesis of (±)-epiglobulol was achieved from 269. An alternative type of rhodium-catalyzed cycloisomerization involving ene−1,3-dienes was recently discovered by Li and Yu in which treatment of triene 270 with a cationic rhodium catalyst (formed in situ from 10 mol % each of RhCl(PPh3)3 and AgSbF6) in DCE at 65 °C for 2 h diastereoselectively generated the cis-divinylpyrrolidine 271 in 90% yield (Scheme 78).78a This reaction did not proceed successfully with a 1,7diene substrate. The cycloisomerization of the deuterated substrate 272-d2 yielded 273-d with a significant loss of deuterium labels, indicating that this reaction involves reversible hydrorhodation (275 → 276): carbometalation of Rh−D, subsequent β-H elimination, and C−H reductive elimination can occur, resulting in partial loss of the D label from the oliginal position. Accordingly, the proposed mechanism consists of oxidative addition of the allylic C−H bond of 274 and hydrorhodation of the resultant hydride complex 275 and final reductive elimination. The scope of this cycloisomerization was demonstrated by carrying out the reaction with various

use of the Wilkinson’s catalyst (RhCl(PPh3)3) resulted in the exclusive formation of the intramolecular [4 + 2]-cycloadduct 257a. Likewise, similar cationic complexes possessing bis(diphenylphosphino)propane and bis(diphenylphosphino)butane ligands instead of the dppe ligand afforded the intramolecular [4 + 2]-cycloaddition/alkene transposition product 257b as the major product. This cycloisomerization tolerates the aldehyde and TBS ether functionalities of 258a and 258b to generate the corresponding products 259a and 259b in 77% and 85% yields, respectively (Scheme 76). Similarly, with the use of extended reaction time, 2,4,9-undecatriene 260 having a trans1,2-disubstituted alkene moiety was converted to alkenylcyclopentene 261 in 77% yield. Notably, 261 was also obtained from 1,7,9-undecatriene 262, indicating that the transposition of the unconjugated alkene moiety preceded cycloisomerization. On the other hand, the reaction of 1,7,9-undecatriene 263 produced both six- and five-membered ring products 264 and 265 in 60% and 13% yields, respectively. The major product Z


Chemical Reviews

Review

Scheme 78. Rh-Catalyzed Cycloisomerization of Triene via Allylic C−H Activation

Scheme 79. Rh-Catalyzed Asymmetric Cycloisomerization of Triene

substrates (Table 13). In addition to azacycles, oxa- and carbocyclic compounds were obtained. All products were formed with moderate to excellent diastereoselectivity. The same researchers successfully developed an enantioselective rhodium-catalyzed cycloisomerization of ene−1,3-dienes using chiral phosphoramidite ligand L13 (Scheme 79).78b Triene 270 was treated with a cationic rhodium catalyst derived from 5 mol % each of [RhCl(coe)]2, AgOTf, and L13 in DCE at 70 °C to obtain 271 in 90% yield with >19:1 dr and 90% ee. Similarly, malonate-derived triene 277 was converted to carbocycle 278 in 74% yield with high diastereo- and enantioselectivity. Ether-tethered triene 279 was found to be less reactive under similar conditions: the yield was 64% at 75% conversion and the enantioselectivity was also lower (64% ee) than those obtained for 271 and 278.

Table 13. Rh-Catalyzed Cycloisomerization of Trienes via Allylic C−H Activation

4.2. Cycloisomerization Involving 1,2-Diene

Like 1,3-dienes, 1,2-dienes (allenes) can be involved in cycloisomerization. Since transition-metal-catalyzed cyclizations involving allenes are excellently outlined in a recent review,7d only cycloisomerization of substrates possessing allenes as well as alkenes are summarized herein. The cycloisomerization of allenene was first reported by Trost and Tour in 1988.79 In that study, a combined catalyst, “Ni−Cr”, which is derived from 10 mol % (p-(diphenylphosphino)polystyrene)nickel dichloride and 30 mol % chromous chloride, was utilized for the cyclization of 281-Me in THF/EtOH at room temperature, to obtain exo-methylenecyclopentane 282-Me in 80% yield (Scheme 80). The polymer-supported nickel catalyst was essential for successful cycloisomerization; the use of (Ph3P)2NiCl2 as a nickel component resulted in oligomerization of allenenes. This method was also effective for Scheme 80. Ni−Cr-Catalyzed Cycloisomerization of 1,6Allenenes

AA


Chemical Reviews

Review

diastereoselective cycloisomerization of cyclohexane-tethered allenenes. For instance, 283 underwent smooth cycloisomerization under the previously outlined conditions to furnish the bicyclic product 284 in 86% yield with a 15:1 diastereomer ratio. The first palladium-catalyzed cycloisomerization of allenene was also investigated by Trost and Matsuda in their synthesis of (±)-petiodial.80 Cyanoallene 285 was prepared from farnesyl bromide and subjected to the palladium-catalyzed cycloisomerization conditions to produce cyclopentene 286 selectively in 83% yield (Scheme 81). The previous “Ni−Cr” bimetallic catalyst was not effective for this cycloisomerization.

Scheme 83. Proposed Mechanism of Ru-Catalyzed Cycloisomerization of 1,6-Allenene

Scheme 81. Synthesis of (±)-Petiodial via Pd-Catalyzed Cycloisomerization of 1,5-Allenene

and subsequent β-H elimination. The resultant kinetic product 299 then undergoes isomerization via hydroruthenation/β-H elimination to yield the thermodynamic product 300. Makino and Itoh also developed rhodium-catalyzed cycloisomerizations of 1,6-allenenes.82 Allenenes 303a−c and 305 were treated with catalytic amounts of [RhCl(cod)]2 and P(otolyl)3 in refluxing dioxane to obtain exo-methylenecyclopentanes 304a−c and 306 with good yields and selectivity (Scheme 84). When allenene 303d with a terminal methyl substituent on

The scope of 1,6-allenene cycloisomerization was expanded using a ruthenium hydride catalyst. Kang and co-workers reported the transformation of allenenes 281-Et and 288 upon treatment with RuClH(CO)(PPh3)3 in refluxing toluene into vinylcyclopentenes 287-Et and 289 in 58% and 75% yields, respectively (Scheme 82).81 Similarly, vinylpyrrolines 291 and

Scheme 84. Rhodium-Catalyzed Cycloisomerization of 1,6And 1,7-Allenenes

Scheme 82. Ruthenium-Catalyzed Cycloisomerization of 1,6Allenene

the alkene moiety was subjected to the same conditions, dialkenylcyclopentene 307 was obtained in a high yield with a cis/trans ratio of 2.8:1. Tosylamide-tethered allenene 308 was also converted to the corresponding pyrrolidine 309. In contrast to a majority of diene cycloisomerizations, alkene transposition was inhibited, and therefore, six-membered-ring formation from 310 occurred to afford exo-methylenecyclohexane 311 in a high yield. The oxidative cyclization/β-H elimination/reductive elimination sequence was proposed as a plausible mechanism. Despite the reported use of the palladium-catalyzed cycloisomerization of a cyano-substituted 1,5-allenene in natural product synthesis,80 the scope of the chemistry of this reaction in terms of 1,5-allenene substrates had not been examined prior to the investigation of the cycloisomerization of various substrates and the underlying mechanism by Bäckvall and coworkers.83 In this study, allenene 312a that features a

293 were also obtained in 60−66% yields from the corresponding allenenes 290 and 292. In contrast, cycloisomerization of aryl-substituted ethers 294a,b afforded exomethylenetetrahydrofurans 295a,b with trans-selectivity. In addition, this group obtained similar exo-methylenecyclopentane 297 by carrying out the cycloisomerization of diol 296 for 7 h, although prolonged reaction (12 h) of the same substrate (296) furnished vinylcyclopentene 298 (Scheme 83). These observations imply that the exo-methylene products are kinetic products, which undergo further isomerization to form thermodynamically more stable 1,3-dienes. Thus, a possible mechanism for the stated ruthenium-catalyzed cycloisomerization was proposed as outlined in Scheme 83. The reaction is initiated by hydroruthenation of the internal allenic double bond, which is followed by intramolecular carboruthenation AB


Chemical Reviews

Review

was proposed as shown in Scheme 86, in which hydropalladation of the allene moieties produces vinylpalladium intermediates 322, which undergo intramolecular carbopalladation to generate 323. Finally, β-H elimination results in the formation of 314 and palladium hydride active species. Alternative products 315a,b were formed via olefin transposition of 314a,b. The cycloisomerization of allenenes with consequent generation of six-membered-ring compounds was achieved using the 2G Grubbs’ catalyst 139.84 Mukai and Itoh prepared allenenes 324a and 324b possessing sulfonyl-activated allene moieties and treated them with 20 mol % of 139 in refluxing CH2Cl2 to obtain cyclohexenylsulfones 325a and 325b in high yields (Scheme 87). The use of the first-generation Grubbs’

cyclopentene moiety was subjected to MW heating conditions (5 mol % Pd(dba)2 (dba = dibenzalacetone), 120 °C, 8 min) to furnish 314a and its olefinic positional isomer 315a in 92% combined yield with a 314a/315a ratio of 78:22 (Scheme 85). Scheme 85. Palladium-Catalyzed Cycloisomerization of 1,5Allenenes

Scheme 87. Cycloisomerization of Allenenes Using 2G Grubbs' Catalyst

Similarly, the reaction of the cyclohexene analogue 312b delivered 314b and 315b in 83% combined yield with an improved selectivity. In striking contrast, the use of analogous substrate 312c resulted in no reaction. The cycloalkenyl sidechains are not essential for the palladium-catalyzed cycloisomerization. In fact, allenene 316 having a trans alkene moiety underwent cyclosiomerization under conditions employing a longer reaction time of 40 min to afford 317 and 318 in 74% combined yield with a 317/318 ratio of 73:27. The cycloisomerization of allenene 319 possessing a terminal alkene yielded exo-methylenecyclopentene 320 and Alder−ene product 321 in a moderate yield. To shed light on the mechanism, the cycloisomerization of 312a and 312b was performed in CD3CO2D to obtain the monodeuterated products 314a,b-d and 315a,b-d with varied deuterium contents (Scheme 86). It was also observed that the cycloisomerization did not proceed in aprotic solvents, and decreasing the acetic acid concentration led to a slower reaction rate. On the basis of these facts, a possible reaction mechanism

catalyst and the Hoveyda−Grubbs’ catalyst decreased the conversion efficacy. The analogous allenene 324c, without a malonate moiety, also underwent cycloisomerization to afford the corresponding product 325c, albeit in a lower yield of 77%. In contrast to these carbocyclic formations, similar heterocyclizations of nitrogen- and oxygen-containing substrates were found to be problematic, in which case the addition of 1 equiv of Ti(OiPr)4 improved the yields. Allenenes 326a and 326b having tosylamide tethers were converted to the corresponding nitrogen heterocycles 327a and 327b in 62% yields, respectively, with the recovery of the unreacted substrates. The oxygen-containing analogue 329 was obtained from 328 in a lower yield of 45% using the same method. Furthermore, a remarkable seven-membered-ring formation via cycloisomerization was developed independently by the groups of Itoh and Brummond,82,85 both of whom employed the same rhodium carbonyl complex, [RhCl(CO)2]2, as the catalyst. Makino and Itoh achieved the synthesis of carbocycles 330a,e and azepine 331 from allenenes 303a,e and 308, respectively (Scheme 88). Although the cycloisomerization proceeded under an Ar atmosphere, higher yields were obtained by carrying out the reactions under a CO atmosphere. Brummond and co-workers also reported a similar sevenmembered-ring formation under a N2 atmosphere. The cycloisomerization of allenenes 332a,b and 336 furnished alkylideneazepine derivatives 333a,b and 337 as single stereoisomers in high yields. Allenene 334 with an ether tether underwent cycloisomerization to afford the oxepine analogue 335 in a moderate yield.

Scheme 86. Possible Mechanism for Pd-Catalyzed Cycloisomerization of 1,5-Allenenes

AC


Chemical Reviews

Review

yield. A similar mechanism involving allylic C−H bond activation was postulated on the basis of the transformation of deuterium-labeled 340-d2 resulting in the selective formation of 341-d2 with >99% deuterium contents. Furthermore, the concomitant cycloropane ring-opening and cyclization of 342 that has a terminal methyl substituent on the allyl moiety produced the bis(isopropylidene)azocine derivative 343 in a moderate yield. Seemingly similar cycloisomerization proceeds with 1,6- and 1,7-bisallene substrates, as independently reported by Lu and Ma and Mukai and co-workers (Scheme 91).87,88 Lu and Ma

Scheme 88. Rh-Catalyzed Cycloisomerization of Allenenes via Allylic C−H Activation

Scheme 91. Rhodium-Catalyzed Bisallene Cycloisomerization

This type of cycloisomerization was proposed to proceed via allylic C−H activation, intramolecular carborhodation of the allene moiety, and reductive elimination as outlined in Scheme 89. This mechanism is corroborated by the conversion of 332ad2 into 333a-d2 with the concomitant deuterium transfer.85 Scheme 89. Proposed Mechanism of Seven-Membered-Ring Formation

treated 1,6-bisallene 344 with 2 mol % [RhCl(CO)2]2 in DMF at 110 °C to obtain bis(isopropylidene)cycloheptene 345 in 71% yield, whereas Mukai and co-workers employed [RhCl(CO)(dppp)]2 (dppp = bis(diphenylphosphino)propane) as the catalyst for cycloisomerization of the sulfonyl-activated 1,7bisallene 346 to obtain bis(methylene)cyclooctene 347 in an excellent yield along with a small amount of the cyclocarbonylation product 348. Investigation of the transformation mechanism using the deuterium-labeled substrate 349-d2 gave rise to the corresponding cyclooctene 350-d2 in a good yield without loss of the deuterium labels. The pattern of deuterium transposition was identical with that of the cycloisomerization of allenenes (Schemes 89 and 90). However, Mukai and coworkers also obtained cyclobutane-fused products in varied yields from the reaction of several substrates. For instance, 1,6bisallene 351 was subjected to the aforementioned cycloisomerization conditions to furnish the cycloheptene derivative 352 as a major product, and cyclobutane-fused product 353 was also formed in 13% yield (Scheme 92). Similarly, use of the

Vinylidenecyclopropanes with unsaturated side-chains were also found to undergo similar cycloisomerization to achieve eight-membered-ring formation (Scheme 90).86 Lu and Shi treated 338 with the same rhodium carbonyl complex [RhCl(CO)2]2 as the catalyst under a CO atmosphere to form the cyclopropane-fused azocine derivative 339 in 77% Scheme 90. Eight-Membered-Ring Formation via RhCatalyzed Cycloisomerization

Scheme 92. Prausible Pathways for Exocyclic Dienes and Cyclobutanes

AD


Chemical Reviews

Review

tosylamide derivative 354 as the substrate gave rise to cyclobutane 356 as the major product. This intramolecular [2 + 2]-cycloaddition was not observed in the absence of the rhodium catalyst under the same conditions. On the basis of these results, both products were considered to stem from common rhodacycle intermediates such as 357 with concomitant hydrogen shifts and reductive elimination (Scheme 92). Asymmetric cycloisomerization of 1,6-allenene was achieved using a chiral nonracemic gold catalyst that features the 3,5xylylbinap ligand L14.89 Gagné and co-workers optimized the reaction conditions of the asymmetric cyclization of allenene 358. As a consequence, the use of the combination of 5 mol % 3,5-xylylbinap(AuCl)2 complex with 15 mol % silver salts in MeNO2 solvent was found to be optimal (Scheme 93). The

Table 14. Gold-Catalyzed Asymmetric Cycloisomerization of 1,6-Allenenes

Scheme 93. Gold-Catalyzed Asymmetric Cycloisomerization of 1,6-Allenene

highest ee was obtained using AgOTf as the silver salt, whereas AgOTs gave the best isomer selectivity. This carbophilic Lewisacid-catalyzed cycloisomerization was proposed to involve electrophilic attack of the gold allene complex on the disubstituted alkene and subsequent deprotonation/protodeauration as shown in Scheme 93. The substrate scope of this fascinating asymmetric cycloisomerization is summarized in Table 14. Moderate ee's were obtained for the examined substrates with the exception of the bis(sulphone) derivative. The ratios of olefin positional isomers varied depending on the substitution pattern of the substrates. The 3,5-xylylbinap(AuCl)2 complex was also isolated, and its crystal structure shows that there is a π−π stacking interaction between the two P-bound aryl groups. The conformational rigidity of this precatalyst due to the π−π stacking interaction was ascribed to the observed asymmetric inductions. However, the use of the isolated precatalyst resulted in slower reaction and a diminished ee of 21%.

asymmetric cycloisomerizations. There is, nevertheless, room for improvement of the substrate scope, the isomeric selectivity, and the enantiomeric selectivity. In addition, by combining the newly developed catalytic systems with state-of-the-art techniques such as a microwave treatment and the use of unique solvent systems, further progress in diene cycloisomerizations is being made possible. For example, recyclable catalyst systems for the 1,6-diene cycloisomerization have been established using ionic liquids and scCO2 as solvents. Further investigations into the utilization of heterogeneous catalysts and carbophilic Lewis acid catalysts as well as less examined substrates would result in the development of unprecedented modes of cycloisomerization and would increase the synthetic value of α,ω-diene cyclization.

5. SUMMARY The transition-metal-catalyzed cycloisomerizations of α,ωdienes have been continuously investigated because such cycloisomerization reactions provide atom-economical routes to various carbo- and heterocyclic compounds. Nevertheless, early catalytic systems generally lacked the requisite isomeric selectivity and the general substrate scope. Considerable efforts devoted to the identification of selective and versatile catalyst systems have led to recent developments of late-transitionmetal-catalyzed cycloisomerizations of 1,6-dienes with wide substrate scope and high isomeric selectivity, including

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. AE


Chemical Reviews

Review

Biography

(9) Motoyama, Y.; Hanada, S.; Niibayashi, S.; Shimamoto, K.; Takaoka, N.; Nagashima, H. Tetrahedron 2005, 61, 10216. (10) (a) Zhang, L.; Sun, J.; Kozmin, S. A. Adv. Synth. Catal. 2006, 348, 2271. (b) Michelet, V.; Toullec, P. Y.; Genêt, J.-P. Angew. Chem., Int. Ed. 2008, 4268. (11) Lei, A.; He, M.; Zhang, X. J. Am. Chem. Soc. 2002, 124, 8198. (12) Shapiro, N. D.; Toste, F. D. J. Am. Chem. Soc. 2007, 129, 4160. (13) Chatani, N.; Morimoto, T.; Muto, T.; Murai, S. J. Am. Chem. Soc. 1994, 116, 6049. (14) Li, X.; Zhang, M.; Shu, D.; Robichaux, P. J.; Huang, S.; Tang, W. Angew. Chem., Int. Ed. 2011, 50, 10421. (15) For selected examples, see: (a) Nugent, W. A.; Taber, D. F. J. Am. Chem. Soc. 1989, 111, 6435. (b) Rousset, C. J.; Swanson, D. R.; Lamaty, F.; Negishi, E. Tetrahedron Lett. 1989, 30, 5105. (c) Takahashi, T.; Kotora, M.; Kasai, K. J. Chem. Soc., Chem. Commun. 1994, 2693. (d) Urabe, H.; Hata, T.; Sato, F. Tetrahedron Lett. 1995, 36, 4261. (e) Probert, G. D.; Harding, R.; Whitby, R. J.; Coote, S. J. Synlett 1997, 1371. Also, see: (f) Negishi, E. In Comprehensive Organic Synthesis; Trost, B., Fleming, I., Paquette, L., Eds.; Pergamon: Oxford, U.K., 1991; Vol. 5, pp 1163−1184. (g) Titanium and Zirconium in Organic Synthesis; Marek, I., Ed.; Wiley-VCH: Weinheim, Germany, 2002. (h) Negishi, E. Dalton Trans. 2005, 827. (16) For selected examples using organomagnesium or organoaluminum reagents, see: (a) Knight, K. S.; Waymouth, R. M. J. Am. Chem. Soc. 1991, 113, 6268. (b) Wischmeyer, U.; Knight, K. S.; Waymouth, R. M. Tetrahedron Lett. 1992, 33, 7735. (c) Knight, K. S.; Wang, D.; Waymouth, R. M.; Ziller, J. J. Am. Chem. Soc. 1994, 116, 1845. (d) Negishi, E.; Jensen, M. D.; Kondakov, D. Y.; Wang, S. J. Am. Chem. Soc. 1994, 116, 8404. (e) Shaughnessy, K. H.; Waymouth, R. M. J. Am. Chem. Soc. 1995, 117, 5873. (f) Dzhemilev, U. M. Tetrahedron 1995, 51, 4333. (g) Yamaura, Y.; Hyakutake, M.; Mori, M. J. Am. Chem. Soc. 1997, 119, 7615. (h) Shaughnessy, K. H.; Waymouth, R. M. Organometallics 1998, 17, 5728. (17) For selected examples using organosilanes, see: (a) Onozawa, S.; Sakakura, T.; Tanaka, M. Tetrahedron Lett. 1994, 35, 8177. (b) Molander, G.; Nichols, P. J. J. Am. Chem. Soc. 1995, 117, 4415. (c) Fu, P.-F.; Brard, L.; Li, Y.; Marks, T. J. J. Am. Chem. Soc. 1995, 117, 7157. (d) Molander, G. A.; Dowdy, E. D.; Schumann, H. J. Org. Chem. 1998, 63, 3386. (e) Molander, G. A.; Schmitt, M. H. J. Org. Chem. 2000, 65, 3767. (f) Widenhoefer, R. A. Acc. Chem. Res. 2002, 35, 905. A related reductive cyclization was also reported, see: (g) Molander, G. A.; Hoberg, J. O. J. Am. Chem. Soc. 1992, 114, 3123. (18) (a) Armstrong, S. K. J. Chem. Soc., Perkin Trans. 1 1998, 371. (b) Deiters, A.; Martin, S. F. Chem. Rev. 2004, 104, 2199. (c) McReynolds, M. D.; Dougherty, J. M.; Hanson, P. R. Chem. Rev. 2004, 104, 2239. (d) Gaich, T.; Mulzer, J. Curr. Top. Med. Chem. 2005, 5, 1473. (e) Brown, R. C. D.; Satcharoen, V. Hetrocycles 2006, 70, 705. (f) Michaaut, A.; Rodriguez, J. Angew. Chem., Int. Ed. 2006, 45, 5740. (g) Gradillas, A.; Pérez-Castells, J. Angew. Chem., Int. Ed. 2006, 45, 6086. (h) Majumdar, K. C.; Rahaman, H.; Roy, B. Curr. Org. Chem. 2007, 11, 1339. (i) Monfette, S.; Fogg, D. E. Chem. Rev. 2009, 109, 3783. (19) For an excellent review with respect to mechanistic aspects of cycloisomerization of 1,6-dienes and 1,6-enynes, see ref 7b. For a short review in Japanese: The Chemical Times, Kanto Chemical Co., Inc. 2004, No. 4, 2. (20) (a) Bogdanović, B.; Henc, B.; Karmann, H.-G.; Nüssel, H.-G.; Walter, D.; Wilke, G. Ind. Eng. Chem. 1970, 62 (12), 34. (b) Bogdanović, B. Adv. Organomet. Chem. 1979, 17, 105. (21) (a) Maly, N. A.; Menapace, H.; Farona, M. F. J. Catal. 1973, 29, 182. (b) Miura, Y.; Kiji, J.; Furukawa, J. J. Mol. Catal. 1975/76, 1, 447. (c) Behr, A.; Freudenberg, U.; Keim, W. J. Mol. Catal. 1986, 35, 9. (22) Radetich, B.; RajanBabu, T. V. J. Am. Chem. Soc. 1998, 120, 8007. (23) (a) Böing, C.; Franciò, G.; Leitner, W. Chem. Commun. 2005, 1456. (b) Böing, C.; Franciò, G.; Leitner, W. Adv. Synth. Catal. 2005, 347, 1537. (c) Diez-Holz, C. J.; Böing, C.; Franciò, G.; Hölscher, M.; Leitner, W. Eur. J. Org. Chem. 2007, 2995. (d) Böing, C.; Hahne, J.; Franciò, G.; Leitner, W. Adv. Synth. Catal. 2008, 350, 1073.

Yoshihiko Yamamoto was born in Nagoya in 1968. He obtained his B.S. (1991), M.S. (1993), and Ph.D. (1996) degrees from Nagoya University, where he was appointed as an Assistant Professor in 1996 and promoted to an Associate Professor in 2003. In 2006, he moved to Tokyo Institute of Technology. After returning to Nagoya University in 2009, he was promoted to a Full Professor in 2012. He was awarded the Incentive Award in Synthetic Organic Chemistry, Japan (2003); the Japan Combinatorial Chemistry Focus Group Award in Synthetic Organic Chemistry, Japan (2004); and the Tokyo Tech Award for Challenging Research (2006). His research interests are focused on the development of organometallic reagents and catalysis and their application to the synthesis of biologically important molecules.

REFERENCES (1) (a) Transition Metals for Organic Synthesis; Beller, M., Bolm, C., Eds.; Wiley-VCH: Weinheim, Germany, 2004. (b) Hegedus, L. S.; Soderberg, B. C. G. Transition Metals in the Synthesis of Complex Organic Molecules, 3rd ed.; University Science Books: CA, 2009. (2) (a) Cross-Coupling Reactions: A Practical Guide; Topics in Current Chemistry 219; Miyaura, N., Ed.; Springer-Verlag: Berlin, 2002. (b) Metal-Catalyzed Cross-Coupling Reactions, 2nd ed.; de Meijere, A., Diederich, F., Eds.; Wiley-VCH: Weinheim, Germany, 2004. (3) (a) Trost, B. M. Science 1991, 254, 1471. (b) Trost, B. M. Angew. Chem., Int. Ed. 1995, 34, 259. (c) Trost, B. M. Acc. Chem. Res. 2002, 35, 695. (4) (a) Activation of Unreactive Bonds and Organic Synthesis; Topics in Organometallic Chemistry 3; Murai, S., Ed.; Springer-Verlag: Berlin, 1999. (b) Handbook of C−H Transformations: Applications in Organic Synthesis; Dyker, G., Ed.; Wiley-VCH: Weinheim, Germany, 2005. (5) (a) RajanBabu, T. V.; Nomura, N.; Jin, J.; Radetich, B.; Park, H.; Nandi, M. Chem.Eur. J. 1999, 5, 1963 and cited references. (b) RajanBabu, T. V. Chem. Rev. 2003, 103, 2845. (6) (a) Lautens, M.; Klute, W.; Tam, W. Chem. Rev. 1996, 96, 49. (b) Ojima, I.; Tzamarioudaki, M.; Li, Z.; Donovan, R. J. Chem. Rev. 1996, 96, 635. (c) Aubert, C.; Buisine, O.; Malacria, M. Chem. Rev. 2002, 102, 813. (d) Zhang, Z.; Zhu, G.; Tong, X.; Wang, F.; Xie, X.; Wang, J.; Jiang, L. Curr. Org. Chem. 2006, 18, 1457. (7) (a) Trost, B. M.; Krische, M. J. Synlett 1998, 1. (b) Lloyd-Jones, G. C. Org. Biomol. Chem. 2003, 1, 215. (c) Jiménez-Núñez, E.; Echavarren, A. M. Chem. Rev. 2008, 108, 3326. (d) Aubert, C.; Fensterbank, L.; Garcia, P.; Malacria, M.; Simonneau, A. Chem. Rev. 2011, 111, 1954. (8) (a) Clark, A. J. Chem. Soc. Rev. 2002, 31, 1. (b) Eckenhoff, W. T.; Pintauer, T. Catal. Rev. 2010, 52, 1. (c) Pintauer, T. Eur. J. Inorg. Chem. 2010, 2449. (d) Muñoz-Molina, J. M.; Belderrain, T. R.; Pérez, P. J. Eur. J. Inorg. Chem. 2011, 3155. AF


Chemical Reviews

Review

(24) (a) Nečas, D.; Turský, M.; Tišlerova, I.; Kotora, M. New J. Chem. 2006, 30, 671. (b) Nečas, D.; Ramella, D.; Rudovaká, I.; Kotora, M. J. Mol. Catal. A: Chem. 2007, 274, 78. (25) (a) Schmitz, E.; Urban, R.; Heuck, U.; Zimmermann, G.; Gründemann, E. J. Prakt. Chem. 1976, 318, 185. (b) Schmitz, E.; Heuck, U.; Habisch, D. J. Prakt. Chem. 1976, 318, 471. (26) (a) Grigg, R.; Mitchell, T. R. B.; Ramasubbu, A. J. Chem. Soc., Chem. Commun. 1979, 669. (b) Grigg, R.; Malone, J. F.; Mitchell, T. R. B.; Ramasubbu, A.; Scott, R. M. J. Chem. Soc., Perkin Trans. 1 1984, 1745. (27) (a) Widenhoefer, R. A.; Perch, N. S. Org. Lett. 1999, 1, 1103. (b) Kisanga, P.; Widenhoefer, R. A. J. Am. Chem. Soc. 2000, 122, 10017. (28) (a) Kisanga, P.; Goj, L. A.; Widenhoefer, R. A J. Org. Chem. 2001, 66, 635. (b) Goj, L. A.; Widenhoefer, R. A J. Am. Chem. Soc. 2001, 123, 11133. (c) Goj, L. A.; Cisneros, G. A.; Yang, W.; Widenhoefer, R. A J. Organomet. Chem. 2003, 687, 498. (d) Goj, L. A.; Cisneros, G. A.; Yang, W.; Widenhoefer, R. A J. Organomet. Chem. 2004, 689, 2845. (29) (a) Bray, K. L.; Charmant, J. P. H.; Fairlamb, I. J. S.; LloydJones, G. C. Chem.Eur. J. 2001, 7, 4205. (b) Bray, K. L.; LloydJones, G. C. Eur. J. Org. Chem. 2001, 1635. (30) (a) Heumann, A.; Moukhliss, M. Synlett 1998, 1211. (b) Heumann, A.; Giordano, L.; Tenaglia, A. Tetrahedron Lett. 2003, 44, 1515. (31) (a) Fairlamb, I. J. S.; Grant, S.; Whitwood, A. C.; Whitthall, J.; Batsanov, A. S.; Collings, J. C. J. Organomet. Chem. 2005, 690, 4462. (b) Fairlamb, I. J. S.; Grant, S.; Tommasi, S.; Lynam, J. M.; Bandini, M.; Dong, H.; Lin, Z.; Whitwood, A. C. Adv. Synth. Catal. 2006, 348, 2515. (32) (a) Bray, K. L.; Fairlamb, I. J. S.; Lloyd-Jones, G. C. Chem. Commun. 2001, 187. (b) Bray, K. L.; Fairlamb, I. J. S.; Kaiser, J.-P.; Lloyd-Jones, G. C.; Slatford, P. A. Top. Catal. 2002, 19, 49. (33) Bray, K. L.; Lloyd-Jones, G. C.; Muñoz, M. P.; Slatford, P. A.; Tan, E. H. P.; Tyler-Mahon, A. R.; Worthington, P. A. Chem.Eur. J. 2006, 12, 8650. (34) Oh, C. H.; Kim, J. D.; Han, J. W. Chem. Lett. 2001, 1290. (35) Song, Y.-J.; Jung, I. G.; Lee, H.; Lee, Y. T.; Chung, Y. K.; Jang, H.-Y. Tetrahedron Lett. 2007, 48, 6142. (36) Bright, A.; Malone, J. F.; Nicholson, J. K.; Powell, J.; Shaw, B. L. J. Chem. Soc., Chem. Commun. 1971, 712. (37) Grigg, R.; Mitchell, T. R. B.; Ramasubbu, A. J. Chem. Soc., Chem. Commun. 1980, 27. (38) Schmidt, B. Eur. J. Org. Chem. 2004, 1865. (39) (a) Picquet, M.; Touchard, D.; Bruneau, C.; Dixneuf, P. H. New J. Chem. 1999, 141. (b) Fürstner, A.; Liebl, M.; Lehmann, C. W.; Picquet, M.; Kunz, R.; Bruneau, C.; Touchard, D.; Dixneuf, P. H. Chem.Eur. J. 2000, 6, 1847. (c) Bassetti, M.; Centola, F.; Sémeril, D.; Bruneau, C.; Dixneuf, P. H. Organometallics 2003, 22, 4459. (40) Ö zdemir, I.; Ç etinkaya, E.; Ç etinkaya, B.; Ç içek, M.; Sémeril, D.; Bruneau, C.; Dixneuf, P. H. Eur. J. Inorg. Chem. 2004, 418. (41) (a) Ç etinkaya, B.; Demir, S.; Ö zdemir, I.; Toupet, L.; Sémeril, D.; Bruneau, C.; Dixneuf, P. H. New J. Chem. 2001, 25, 519. (b) Ç etinkaya, B.; Demir, S.; Ö zdemir, I.; Toupet, L.; Sémeril, D.; Bruneau, C.; Dixneuf, P. H. Chem.Eur. J. 2003, 9, 2323. (42) Miyaki, Y.; Onishi, T.; Ogoshi, S.; Kurosawa, H. J. Organomet. Chem. 2000, 616, 135. (43) (a) Sémeri, D.; Bruneau, C.; Dixneuf, P. H. Helv. Chim. Acta 2001, 84, 3335. (b) Lo, C.; Cariou, R.; Fischmeister, C.; Dixneuf, P. H. Adv. Synth. Catal. 2007, 349, 546. (44) Connon, S. J.; Blechert, S. Bioorg. Med. Chem. Lett. 2002, 12, 1873. (45) (a) Terada, Y.; Arisawa, M.; Nishida, A. Angew. Chem., Int. Ed. 2004, 43, 4063. (b) Terada, Y.; Arisawa, M.; Nishida, A. J. Org. Chem. 2006, 71, 1269. (c) Arisawa, M.; Terada, Y.; Takahashi, K.; Nakagawa, M.; Nishida, A. J. Org. Chem. 2006, 71, 4255. (46) (a) Yamamoto, Y.; Ohkoshi, N.; Kameda, M.; Itoh, K. J. Org. Chem. 1999, 64, 2178. (b) Yamamoto, Y.; Nakagai, Y.; Ohkoshi, N.; Itoh, K. J. Am. Chem. Soc. 2001, 123, 6372.

(47) Michaut, M.; Santelli, M.; Parrain, J.-L. Tetrahedron Lett. 2003, 44, 2157. (48) Yamamoto, Y.; Nakagai, Y.; Itoh, K. Chem.Eur. J. 2004, 10, 231. (49) Mach, K.; Sedmera, P.; Petrusová, L.; Antropiusová, H.; Hanuš, V.; Tureček, F. Tetrahedron Lett. 1982, 23, 1105. (50) (a) Akita, M.; Yasuda, H.; Nagasuna, K.; Nakamura, A. Bull. Chem. Soc. Jpn. 1983, 56, 554. (b) Lehmkuhl, H.; Tsien, Y.-L. Chem. Ber. 1983, 116, 2437. (51) Christoffers, J.; Bergman, R. G. J. Am. Chem. Soc. 1996, 118, 4715. (52) Thiele, S.; Erker, G. Chem. Ber./Rec. 1997, 130, 201. (53) (a) Waratuke, S. A.; Johnson, E. S.; Thorn, M. G.; Fanwick, P. E.; Rothwell, I. P. Chem. Commun. 1996, 2617. (b) Thorn, M. G.; Hill, J. E.; Waratuke, S. A.; Johnson, E. S.; Fanwick, P. E.; Rothwell, I. P. J. Am. Chem. Soc. 1997, 119, 8630. (54) Okamoto, S.; Livinghouse, T. J. Am. Chem. Soc. 2000, 122, 1223. (55) (a) Okamoto, S.; Livinghouse, T. Organometallics 2000, 19, 1449. A similar result was reported: (b) Sturla, S. J.; Buchwald, S. L. Organometallics 2002, 21, 739. (56) (a) McLain, S. J.; Schrock, R. R. J. Am. Chem. Soc. 1978, 100, 1315. (b) Smith, G.; McLain, S. J.; Schrock, R. R. J. Organomet. Chem. 1980, 202, 269. (57) For the formation of exo-methylenecyclopentanes via decomposition of isolated fused metallacyclopentanes, see: McDermott, J. X.; Wilson, M. E.; Whitesides, G. M. J. Am. Chem. Soc. 1976, 98, 6529. Also see ref 15c. (58) (a) Bunel, E.; Burger, B. J.; Bercaw, J. E. J. Am. Chem. Soc. 1988, 110, 976. (b) Piers, W. E.; Shapiro, P. J.; Bunel, E. E.; Bercaw, J. E. Synlett 1990, 74. (59) (a) Kerber, W. D.; Koh, J. H.; Gagné, M. R. Org. Lett. 2004, 6, 3013. (b) Feducia, J. A.; Campbell, A. N.; Doherty, M. Q.; Gagné, M. R. J. Am. Chem. Soc. 2006, 128, 13290. (60) Grau, F.; Heumann, A.; Duñach, E. Angew. Chem., Int. Ed. 2008, 45, 7285. (61) Zhao, C.; Toste, F. D.; Bergman, R. G. J. Am. Chem. Soc. 2011, 133, 10787. (62) Alvaro, M.; Das, D.; Garcia, H.; Leyva, A. Tetrahedron 2004, 60, 8131. (63) Corma, A.; García, H.; Leyva, A. J. Organomet. Chem. 2005, 690, 3529. (64) Miao, X.; Feder-Kubis, J.; Fischmeister, C.; Pernak, J.; Dixneuf, P. H. Tetrahedron 2008, 64, 3687. (65) (a) Lidström, P.; Tierney, J.; Wathey, B.; Westman, J. Tetrahedron 2001, 57, 9225. (b) Perreux, L.; Loupy, A. Tetrahedron 2001, 57, 9199. (c) De la Hoz, A.; Díaz-Oriz, Á .; Moreno, A. Chem. Soc. Rev. 2005, 34, 164. (d) Caddick, S.; Fitzmaurice, R. Tetrahedron 2009, 65, 3325. (e) Santagada, V.; Frecentese, F.; Perissutti, E.; Fiorino, F.; Severino, B.; Caliendo, G. Mini-Rev. Med. Chem. 2009, 9, 340. (f) Martínez-Palou, R. Mol. Divers. 2010, 14, 3. (66) (a) Coquerel, Y.; Rodriguez, J. Eur. J. Org. Chem. 2008, 1125. (b) Appukkuttan, P.; Van der Eycken, E. Eur. J. Org. Chem. 2008, 1133. (c) Mehta, V. P.; Van der Eycken, E. V. Chem. Soc. Rev. 2011, 40, 4925. (67) Fairlamb, I. J. S.; McGlacken, G. P.; Weissberger, F. Chem. Commun. 2006, 988. (68) (a) Takacs, J. M.; Anderson, L. G. J. Am. Chem. Soc. 1987, 109, 2200. (b) Takacs, J. M.; Myoung, Y.-C.; Anderson, L. G. J. Org. Chem. 1994, 59, 6928. (69) (a) Takacs, J. M.; Anderson, L. G.; Creswell, M. W.; Takacs, B. E. Tetrahedron Lett. 1987, 28, 5627. (b) Takacs, B. E.; Takacs, J. M. Tetrahedron Lett. 1990, 31, 2865. (70) Takacs, J. M.; Newsome, P. W.; Kuehn, C.; Takusagawa, F. Tetrahedron 1990, 46, 5507. (71) (a) Takacs, J. M.; Weidner, J. J.; Takacs, B. E. Tetrahedron Lett. 1993, 34, 6219. (b) Takacs, J. M.; Weidner, J. J.; Newsome, P. W.; Takacs, B. E.; Chidambaram, R.; Shoemaker, R. J. Org. Chem. 1995, 60, 3473. AG


Chemical Reviews

Review

(72) (a) Takacs, J. M.; Myoung, Y. C. Tetrahedron Lett. 1992, 33, 317. (b) Takacs, J. M.; Boito, S. C. Tetrahedron Lett. 1995, 36, 2941. (c) Takacs, J. M.; Vayalakkada, S.; Mehrman, S. J.; Kingsbury, C. L. Tetrahedron Lett. 2002, 43, 8417. (73) Wender, P. A.; Ihle, N. C. J. Am. Chem. Soc. 1986, 108, 4678. (74) (a) Takacs, J. M.; Zhu, J.; Chandramouli, S. J. Am. Chem. Soc. 1992, 114, 773. (b) Takacs, J. M.; Clement, F.; Zhu, J.; Chandramouli, S. V.; Gong, X. J. Am. Chem. Soc. 1997, 119, 5804. (75) (a) Sato, Y.; Oonishi, Y.; Mori, M. Organometallics 2003, 22, 30. (b) Oonishi, Y.; Saito, A.; Mori, M.; Sato, Y. Synthesis 2009, 969. (76) Oonishi, Y.; Taniuchi, A.; Mori, M.; Sato, Y. Tetrahedron Lett. 2006, 47, 5617. (77) For reviews on Thorpe-Ingold effect, see: (a) Jung, M. E. Synlett 1999, 843. (b) Jung, M. E.; Piizzi, G. Chem. Rev. 2005, 105, 1735. (78) (a) Li, Q.; Yu, Z.-X. J. Am. Chem. Soc. 2010, 132, 4542. (b) Li, Q.; Yu, Z.-X. Angew. Chem., Int. Ed. 2011, 50, 2144. (79) Trost, B. M.; Tour, J. M. J. Am. Chem. Soc. 1988, 110, 5231. (80) Trost, B. M.; Matsuda, K. J. Am. Chem. Soc. 1988, 110, 5233. (81) Kang, S.-K.; Ko, B.-S.; Lee, D.-M. Tetrahedron Lett. 2002, 43, 6693. (82) (a) Makino, T.; Itoh, K. Tetrahedron Lett. 2003, 44, 6335. (b) Makino, T.; Itoh, K. J. Org. Chem. 2004, 69, 395. (83) Närhi, K.; Franzén, J.; Bäckvall, J.-E. Chem.Eur. J. 2005, 11, 6937. (84) Mukai, C.; Itoh, R. Tetrahedron Lett. 2006, 47, 3971. (85) Brummond, K. M.; Chen, H.; Mitasev, B.; Casarez, A. D. Org. Lett. 2004, 6, 2161. (86) Lu, B.-L.; Shi, M. Angew. Chem., Int. Ed. 2011, 50, 12027. (87) Lu, P.; Ma, S. Org. Lett. 2007, 9, 2095. (88) Kawamura, T.; Inagaki, F.; Narita, S.; Takahashi, Y.; Hirata, S.; Kitagaki, S.; Mukai, C. Chem.Eur. J. 2010, 16, 5173. (89) Tarselli, M. A.; Chianese, A. R.; Lee, S. J.; Gagné, M. R. Angew. Chem., Int. Ed. 2007, 46, 6670.

AH


Transition-Metal-Catalyzed Cycloisomerizations of Î±,Ï-Dienes

Recommend Documents

Transition-Metal-Catalyzed Cycloisomerizations of Î±,Ï-Dienes