Gold-Catalyzed Organic Reactions - ACS Publications - American

Chem. Rev. 2007, 107, 3180−3211

3180

Gold-Catalyzed Organic Reactions A. Stephen K. Hashmi* Institut fu¨r Organische Chemie, Universita¨t Stuttgart, 70569 Stuttgart, Germany Received October 17, 2006

Contents 1. Introduction 1.1. Scope, Limitation, and Organization of the Review 2. General Considerations 3. Catalysis Reactions 3.1. Nucleophilic Additions to C−C Multiple Bonds 3.1.1. Simple Nucleophiles 3.1.2. Intramolecular Additions in Conjugated π-Systems and Related Reactions 3.1.3. Ring Enlargement Reactions 3.1.4. Intramolecular Additions with Propargylic/ Allylic Leaving Groups 3.1.5. Enynes as Substrates 3.2. Activation of Carbonyl Groups and Alcohols 3.2.1. Catalytic Asymmetric Aldol Reaction 3.2.2. Condensation with Amines, Alcohols, or Thiols 3.2.3. Three-Component Reactions of Amines, Aldehydes, and Terminal Alkynes 3.3. Carbon Monoxide as Nucleophile 3.4. Hydrogenation Reactions 3.4.1. Alkenes, Dienes, or Alkynes as Substrates 3.4.2. R,β-Unsaturated Carbonyl Groups 3.4.3. Catalytic Asymmetric Hydrogenation 3.4.4. Dehydrogenation Reactions 3.5. Oxidation Reactions 3.5.1. Epoxidation Reactions 3.5.2. Selective Oxidations of Alcohols 3.5.3. Selective Oxidations of Alkanes 4. Conclusion 5. Acknowledgments 6. References

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1. Introduction Gold has accompanied mankind from the very early days; no name of a chemist is associated with the discovery of the element gold. It is probably the only chemical element that literally every adult has heard about. A highly positive normal potential is responsible for a low reactivity and allows gold to occur in nature in elemental form, for example, as nuggets. Some of the oldest and some of the most beautiful ancient art is made of gold, and this stresses the impressive durability of this metal. It has always been precious, which is nicely reflected by the Greek myth of king Midas. * To whom correspondence should be addressed. Tel: 0711-685-4330. Fax: 0711-685-4321. E-mail: [email protected].

A. Stephen K. Hashmi was born in Mu¨nchen, Bavaria, Germany, in 1963. From 1982 to 1991, he studied chemistry at the Ludwigs-MaximiliansUniversita¨t Mu¨nchen, and he obtained both his Diploma and Ph.D. with Prof. G. Szeimies in the field of nickel- and iron-catalyzed cross-coupling and cycloisomerization of strained organic compounds. His postdoctorate with Prof. B. M. Trost at Stanford University from 1991 to 1993 covered transition metal-catalyzed enyne metathesis. After his Habilitation from 1993 to 1998 with Prof. J. Mulzer at the Freie Universita¨t Berlin, the Johann Wolfgang Goethe-Universita¨t Frankfurt, and the Universita¨t Wien, in 1998, he was awarded a Heisenberg fellowship of the DFG for a proposal on gold-catalyzed reactions for organic synthesissstill a major focus of the group. The next stations were at the University of Tasmania at Hobart in 1999 and Universita¨t Marburg from 1999 to 2000. In 2001, he was appointed Professor for Organic Chemistry at Universita¨t Stuttgart; in 2007, he was appointed to a chair in Organic Chemistry at Universita¨t Heidelberg. For his early work in gold catalysis, he in 2001 received a Karl-Ziegler fellowship and in 2002 the ORCHEM price for natural scientists by the German Chemical Society. His work has appeared in about 100 publications, reviews, and book chapters.

Monetary systems based on the rare gold and consequently the desire to possess gold has been the driving force for all kinds of activities such as gold rushes to even the most hostile regions of earth, wars, the conquering of whole continents, and, to come to the more positive effects, for the early development of science and chemistry. Besides the alchemists, famous names like Archimedes and Rutherford are connected with gold. The frequent use of gold in dental medicine, applications in the treatment of arthritis, and recent investigations of the anticancer activity have proven that, unlike, for example, with nickel, no problems of allergic reactions are associated with gold.1 Metallic gold is highly biocompatible, but gold in ionic form is toxic. While the stoichiometric chemistry of gold has intensively and continuously been investigated, this close relationship between gold and chemical applications got lost during the development of catalysis reactions. The periodic table with 81 stable, nonradioactive elements offers only a limited number of building blocks to the chemists exploring catalysis. Therefore, they can hardly afford to skip one of the elements, but they indeed heavily neglected it. Probably a low catalytic

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Gold-Catalyzed Organic Reactions

activity was mistakenly deduced from the inertness of elemental gold that only dissolves in aqua regia or oxidants such as air, the latter only in the presence of strong ligands such as cyanide. The major aim of this review is to show what the few chemists that dared to use gold for the catalysis of organic reactions have already achieved and to stimulate others to occasionally use gold as well. As a key finding of my literature search, I would like to cite Geoffrey C. Bond,2 who coined the following statement in one of the milestone publications of gold catalysis, which probably applies to many of the discoveries in this field: “We are at a loss to understand why these catalytic properties of gold have not been reported before, especially since the preparative methods we have used are in no way remarkable.” A second reason for the neglect of gold as a potential catalyst might be the high value associated with gold, which leads to the assumption that such a catalyst would be unaffordable. Today the label “gold” is still used to express such exclusivity for many things ranging from credit cards to purity grades of chemicals. However, monetary systems do not base on gold any more; in fact, many national banks have started to sell their gold reserves. In combination with other factors, this in the past years has led to constellations where the “queen of metals” was much less expensive than other metals that are used even in large-scale technical catalysis processes like rhodium, palladium, and platinum. In contrast to the latter, thousands of tons of gold are recycled from stoichiometric technical applications, and in addition, thousands of tons are produced in gold mines every year. This, in comparison with rhodium, palladium, and platinum, leads to a higher stability of the price, which is an advantage for possible industrial uses. Furthermore, one should keep in mind that the price of a catalyst is often dominated by the ligand rather than by the metal.3 Overall, a change of paradigm has taken place. While the ancient alchemists investigated the question of how to make gold, now the question is what to make with gold.

1.1. Scope, Limitation, and Organization of the Review This review will cover all gold-catalyzed organic reactions published until October 2006 in both homogeneous and heterogeneous catalysis. Regarding the product, the only restriction is the devotion to organic reactionssthe product must be an organic compound. This excludes complete oxidative degradation of organic molecules to CO2, water, and maybe other inorganic compounds.4 Regarding the catalyst, the major restriction is that gold must clearly be the site of the reaction. Therefore, the myriad of binary, ternary, or higher alloys or mixtures containing gold as well as reactions where the support of the gold might participate are not included in this review. The review is organized by reactivity patterns and by functional groups in the substrates. An organization by the gold catalyst used or the type of bond(s) formed in the product would have been unsuitable, because in most cases many different gold complexes with different oxidation states of gold have been used as precatalysts for the same reaction, the same principles for several different C-element bondforming reactions, and often, several new bonds are formed in one reaction. The chapters are ordered by today’s significance in gold catalysis of organic reactionsssignificance estimated by the number of publications. So far, a number of short reviews

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mainly focusing on gold catalysis in organic synthesis have appeared,5 but many of them are older than 2 years, and the more recent examples were restricted to special aspects. The highly dynamic development in this field is best reflected by the fact that about 50% of the publications covering goldcatalyzed organic reactions have been published in the past 24 months and that in the first 6 months of 2006 more papers than in the whole year of 2005 have appeared in this field.

2. General Considerations Out of the different oxidation states possible for gold, in the presence of organic substrates, gold(0), gold(I), and gold(III) are possible. In aqueous solution, in the absence of stabilizing ligands, gold(I) spontaneously disproportionates to gold(III) and gold(0). From stoichiometric chemistry and theoretical considerations for gold(I), it is known that the fragment R3PAu+ is isolobal to H+ and LHg2+.6 Relativistic effects reach their maximum in the periodic table with gold;6c,7 significant for catalysis is, for example, that in complexes gold(I) can be smaller than silver(I).8 Gold has only one isotope and thus lacks a characteristic isotope pattern in mass spectrometry.9 The nuclear spin of gold is 3/2, but because of a very low sensitivity and a quadropole moment, only a few 79Au spectra in an highly symmetric enviroment have been reported.10 The diamagnetic character of both gold(I) and gold(III) conveniently allows the monitoring of catalysis reactions by NMR. Mo¨ssbauer spectroscopy can deliver information about the oxidation state.11 Ligand exchange processes, which are essential for catalysis, have been investigated. There is only little data on Au(I), which favors an associative mechanism. Au(III) also favors an associative mechanism; the reaction rate was reported to be higher than in the corresponding palladium and platinum but lower than in the nickel complexes.12 Even in stoichiometric organic reactions, the use of gold is rare, as one would expect from the high oxidation potential, and from the analogy to copper in Fehling’s reagent and silver in Tollen’s reagent, gold easily oxidizes a substrate and gets reduced. This has been used in a colormetric determination of vitamin E (R-tocopherol, 1),13 in the stereospecific oxidation of methionine,14 and in the oxidation of C-C double bonds and arenes.15 The most recent report on stoichiometric reactions with gold(III) is the synthesis of fused diporphyrins by oxidative coupling.16 Stoichiometric hydrolysis of thioesters and related compounds has been reported.17 After their troublesome isolation, alkaloids were often characterized as salts with the precious tetrachloroaurate counterion, for example, muscarine (2).18 Gold’s reagent (3),19 a name reagent for stoichiometric use, does not contain the element gold. It is a stoichiometric β-dimethylaminomethylenating agent for ketones or amines (Chart 1).

3. Catalysis Reactions For many reactions, there exist mechanistic proposals, which are supported by labeling experiments, special test substrates, or other experimental evidence; very important is the absence of paramagnetic species, allowing in situ monitoring of the reactions in an NMR instrument. As for many other catalysis reactions, there exists no direct experimental proof for most of the intermediates shown in the mechanistic proposals. However, these assumed intermedi-

3182 Chemical Reviews, 2007, Vol. 107, No. 7 Chart 1

ates help the reader to recognize the often complicated substrate/product relationships and similarities between reactions. The precatalysts are usually well-defined compounds, but in most cases, it is still unknown what the active species really looks like. Therefore, in the intermediates, a generalized gold complex fragment “[Au]” is used to symbolize the unknown active species. For reasons of electroneutrality, occasionally the [Au] carries a negative charge, but one should keep in mind that if the corresponding gold complex fragment is cationic, that part would be uncharged. Most of the reactions, and this was recognized quite early, tolerate both oxygen and acidic protons;20,21 thus, neither air nor humidity need to be excluded, a very important aspect for this application. If related complexes of other transition metals catalyze the same reaction, gold catalysts are often significantly more active.

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organogold intermediate 6 liberates the addition product 7 and the gold catalyst by protodemetallation. 3.1.1.1. Alkynes as Substrates. Alkynes are the most successful and most frequently used reaction partners for gold catalysts.5e 3.1.1.1.1. Hydrochlorination of Alkynes. In 1976, the first experiments were reported by Thomas et al.20 They investigated the reaction of different alkynes (8) with tetrachloroauric acid in aqueous methanol and obtained the corresponding ketones by a Markovnikov addition (9; with internal alkynes, 10 was also obtained) as the major products, less than 5% of the corresponding methyl-vinyl ethers (11) and vinyl chlorides (12). In that key paper for the field, types of products that later became very important in gold catalysis are reported. However, this was not recognized at that time, and the authors did not continue their investigation because they considered the reaction with H[AuCl4] to be a “gold(III) oxidation”. However, for example, the reaction with ethynylbenzene (8d) reached almost six turnovers (“570% yield”, 38% with respect to the phenylacetylene), documenting that gold was a catalyst and not a stoichiometric oxidant (Scheme 2). Scheme 2

3.1. Nucleophilic Additions to C−C Multiple Bonds This is by far the most common reactivity pattern in goldcatalyzed organic reactions. C-C multiple bonds of alkynes, allenes, or olefins coordinate to gold complexes; this very efficiently activates them for the attack of a nucleophile. C-O double bonds in carbonyl groups can be activated as well (see section 3.2). Most popular are alkynes as substrates;5e in the last 12 months, alkenes have received increasing attention; allenes were among the first substrates used, and they are still quite popular.

The correlation of the standard reduction potential of the metal cation for carbon-supported metal chloride catalysts and the efficiency to catalyze the hydrochlorination of ethyne (13) to vinyl chloride (14; Scheme 3) were recognized by Scheme 3

3.1.1. Simple Nucleophiles This is the simplest example of a nucleophilic addition to a C-C multiple bond in an organic substrate 4. First, the gold catalyst interacts with the π-system of the substrate to form the intermediate 5, and then, the nucleophile attacks (Scheme 1). As discussed in specific examples below, there Scheme 1

is much evidence that the nucleophile adds anti to gold to deliver a vinylgold species 6, but for the specific case of norbornenes, a syn addition was reported.22 Then, the

Hutchings21,23 in 1985. Gold(III) is the most active catalyst for that very important industrial process. Until then, mercury salts were used on an industrial scale, but before an application of these gold catalysts in production could be established, the oxidative hydrochlorination of ethene to deliver 14 was developed as an economically preferable route.24 3.1.1.1.2. Hydroamination of Alkynes. Soon after Hutchings’ discovery, Utimoto et al.25 investigated the intramolecular hydroamination reaction of alkynes 15 under mild and neutral conditions, where gold(III) catalysts were superior to palladium(II) catalysts for the 6-exo-dig cyclization. After the initial enamine formation (16) by hydroamination, a subsequent tautomerization led to the thermodynamically more stable imine 17 as product. They used sodium tetrachloroaurate as the catalyst and needed relatively high catalyst loadings of 5 mol % (Scheme 4). Best results were obtained with substrate concentrations of 0.025 M or lower.25b At reflux temperature, the converstions needed 1-2

Gold-Catalyzed Organic Reactions Scheme 4


In an interesting approach by Dyker et al.,29a substrates 25 were assembled by an Ugi four-component reaction. The cyclization of 25 with 3 mol % of AuCl3 in acetonitrile at 80 °C delivered both the products of a 6-endo-dig cyclization to 26 and of the 5-exo-dig cyclization 28, the latter delivering only the shown diastereomer (Scheme 7). Compound 28 Scheme 7

h,25b and at room temperature, they still proceeded, but reaction times were 12 h.25a The 5-exo-dig cyclization of substrates 18-20 was even more effective (Scheme 5).25b T. E. Mu¨ller26a reported that Scheme 5

AuCl3 was less effective for these conversions, but with only 1 mol % of the cationic [AuCl(triphos)](NO3)2 catalyst, for related substrates, a quantitative conversion with an initial TOF of 212 h-1 in refluxing acetonitrile could be achieved.26b The first intermolecular reactions were reported by Tanaka et al.27 for the reaction of alkynes 21 and anilines 22 to imines 23/24 under solvent-free conditions (Scheme 6). With

obviously forms by aromatization of the initial hydroamination product 27. An intramolecular hydroamination of 29 to 30 was observed in reactions for the construction of the complex polycyclic Communesin ring system by Crawley and Funk (Scheme 8).29b Scheme 8

Scheme 6

terminal alkynes, again, a Markovnikov regioselectivty was observed. With catalyst loadings as little as 0.01 mol % of (Ph3P)AuMe and 0.05 mol % of H3PW12O40 as acidic promotor, a TON of 9000 could be achieved for 4-bromoand for 4-cyanoaniline. Aryl, alkyl, and dialkyl acetylenes reacted, and aliphatic amines could not be used as nucleophilic partners. Even phenylhydrazine led to the corresponding phenylhydrazone. Similar reactions of terminal alkyl- and arylacetylenes with anilines using neat substrates and 5 mol % AuCl3 or 5 mol % AuCl3/15 mol % AgOTf at 60-110 °C were reported by Li et al.28

3.1.1.1.3. Hydroalkoxylation, Bis(hydroalkoxylation), and Hydration of Alkynes. During their investigation of the goldcatalyzed hydroamination, Utimoto et al.30a observed that with 2 mol % Na[AuCl4] as the catalyst, alcohols and water can serve as nucleophiles in the gold-catalyzed reactions of alkynes 21 (Scheme 9) as well. Previously, these reactions were known to only be catalyzed by mercury(II) salts under strongly acidic conditions or by palladium or platinum(II) salts.31 The only gold(I) compound tested at that time had two strongly coordinating lingands, K[Au(CN)2], which was catalytically inactive. While for both product types two formulas are depicted, 31 and 32 for ketones or 33 and 34 for ketals, this regioselectivity problem applies only to internal unsymmetric alkynes; for internal symmetric alkynes, 31 ) 32 and 33 ) 34, and for terminal alkynes, a clean Markovnikov selectivity is observed. Another example was reported by Arcadi et al.32

3184 Chemical Reviews, 2007, Vol. 107, No. 7 Scheme 9

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for homogeneous gold-catalyzed reactions, one application of this that we have seen above in Scheme 6. The enol ether intermediate can be obtained in low yield; for example, in substrate 41, methanol attacks at the sterically less hindered position, 42, and a “small amount” of enol ether 43 is obtained (Scheme 12). Propargylic alcohols 44 deliver one Scheme 12

Methyl propargyl ethers 35 allow one to direct the regioselectivity in the hydration of internal unsymmetric alkynes; only the R,β-unsaturated 37 was obtained (Scheme 10).30b Small amounts of the byproduct 36 suggest the latter Scheme 10

to be the intermediate, which forms 37 by elimination. On the other hand, terminal alkynes with a methoxy group in the propargyl position still lead to the normal Markovnikov product; 38 was formed. The intrinsic directing effect of the terminal alkyne was stronger than the one of the propargylic ether. An extension of this principle to the formation of R,βunsaturated esters 40 from alkoxy-alkynes 39 by a AuCl3catalyzed Meyer-Schuster type rearrangement has been reported by Dudley et al. (Scheme 11).33 This is remarkable; Scheme 11

propargylic alcohols did not react with Utimoto’s catalysts. The strength of the procedure is the synthesis of products with bulky substitutents R1 and R2, which would be difficult to prepare by a HWE reaction. Then, in 1998, Teles et al.34 found very high activities (TON up to 105, TOF up to 5400 h-1) for cationic gold(I)phosphane complexes in the addition of alcohols to alkynes! This type of cationic complexes is still the most used type of homogeneous gold catalyst, especially the Ph3PAu+ Xsystem, which was applied by many of the authors cited in this chapter. Teles was also the first to use acidic promotors50

single diastereomer of the cyclic acetals 45. The initial TOFs for the addition of methanol to propyne were strongly dependent on the ligand: Ph3As (430 h-1) < Et3P (550 h-1) < Ph3P (610 h-1) < (4-F-C6H4)3P (640 h-1) < (MeO)3P (1200 h-1) < (PhO)3P (1500-1). Stabilities of the catalysts increase in the opposite direction. The TOF is proportional to the proton concentration. In footnote 16 of that paper,34b even N-heterocyclic carbenes are described (TOF appoximately 1800 h-1), probably the first mention of these as ligands for gold catalysts. Of mechanistic significance is the reaction of diphenylacetylene 46, which initially shows a dominance of the (Z)isomer of 47 (Z:E ) 8:1), but finally, an equilibration to Z:E ) 2:1 is observed (Scheme 13). Scheme 13

This, in combination with a computational investigation of the catalytic cycle, led to the suggestion of a syn oxyauration for the C-O bond-forming step. These calculations also suggested that the alkynes are better ligands for the LAu+ fragment than methanol. Schwarz et al.35 combined a mass spectrometry and a theoretical study. They observed that in the gas phase the reaction does not proceed, suggesting that solvent-assisted hydrogen migration might be crucial for the reactions. With CO as the coligand, which allowed Hayashi and Tanaka et al.36 to even improve the activity of Teles catalysts for the hydration of alkynes, a TOF of 15600 s-1 was accessible. Aqueous methanol was the best solvent; in isopropanol, dioxane, acetonitrile, tetrahydrofuran (THF), dichloromethane (DCM), dimethyl sulfoxide (DMF), or toluene gave lower yields. Sulfuric acid and trifluormethane sulfonic acid were the best acidic promotors. Furthermore, terminal propargylic alcohols, different from Utimoto’s


observation (Scheme 10, 35), delivered not only the Markovnikov product but also 17-20% of the anti-Markovinikov addition followed by elimination. Regarding the functional group tolerance, nitriles, halides, alkoxy groups, and even trisubstituted olefins were tolerated. Wessig and Teubner37 have successfully used this method for ortho,ortho-disubstituted arylalkynes in a total synthesis. In the meantime, the addition of alcohols to alkynes developed to a benchmark test for new catalysts. Raubenheimer et al.38 investigated gold(III) complexes in ionic liquids for the formation of acetophenone from phenylacetylene, but the activity was inferior to the previously described results. Laguna et al.39 used different organometallic gold complexes with the mesityl and the pentafluorophenyl substituent. Under acidic and nonacidic conditions, these showed activity and good yields for catalyst loadings of as little as 0.5 mol %. Stoichiometric experiments delivered evidence for gold(III) as the active species for these organometallic gold catalysts and two intermediates, probably the π-complex of the alkyne and a vinyl gold species. Herrmann et al.40 invesigated N-heterocyclic carbene complexes of gold(I) acetates and B(C6F5)3 as a cocatalyst for the hydration of 3-hexyne. TONs of up to 30 and TOFs of up to 160 were observed in THF; higher activities than in methanol were obtained. Schmidbaur et al.41 used phosphane complexes with carboxylate and sulfonate counterions, found methanol to be better than THF, and obtained TONs up to 650 and TOF of up to 3900 h-1. Catalyst recycling was possible; after five cycles, a total TON of 3400 had been reached. Laguna et al.42 observed a nucleophilic addition to the C-C-triple bond in alkynylphosphane complexes of gold, but there is no evidence for the participation of the gold as catalyst. Most probably, as in the classic Reppe vinylation, alcoholate was the nucleophile. Intramolecular versions of the alcohol addition were also reported. Hashmi et al.43 demonstrated that (Z)-3-ethynylallylalcohols 48 can efficiently cyclize to furans 50 via intermediate 49, which tautomerizes to the thermodynamically more stable heteroaromatic furan (Scheme 14). Liu et Scheme 14


With two intramolecular hydroxy groups as nucleophiles, as in 51, interesting bicyclic ketals 52 were obtained by Geneˆt et al. (Scheme 15).45 AuCl or AuCl3 can be used as catalysts. Scheme 15

Under these reaction conditions, even a styrenelike olefin, which has the same distance to the hydroxy groups as to the alkyne, does not compete. Forsyth et al. have applied related two-fold additions of one hydroxy group and one methyl ether group as the second nucleophile to the synthesis of a trioxadispiroketal containing the A-D rings of the marine toxin azaspiracid.46 Krause and Belting investigated a combination of an intramolecular and an intermolecular addition. The substrate was the homopropargylic alcohol 53, and a second alcohol R4OH was added (Scheme 16).47 Again, acids had to be Scheme 16

added; a series of control experiments suggest a combination of two different catalytic cycles, gold catalysis leading to a dihydrofuran by a fast intramolecular reaction followed by acid catalysis of the slower intermolecular addition of the alcohol R4OH to the enol ether substructure of the intermediate to deliver the ketal 54. Only with tertiary alcohols for R4OH did the reaction fail; other alcohols R4OH can be used neat or in solvents like toluene, DCM, THF, or Et2O but not MeCN. Silylated alkynes (53, R1 ) SiR3) fail to react; one additional carbon atom between the alcohol and the alkyne is tolerated, and internal and terminal alkynes react as well. Another combination of two intramolecular reactions was reported by Barluenga et al.48 The initial cyclization of 55 to an enol ether 56 was followed by a Prins type cyclization, which generated bicyclic compounds 57 containing an eightmembered ring (Scheme 17). Deuteration studies are in Scheme 17

al.44 reported many more examples and extended the methodology to the diastereoselective synthesis of alkylidenedihydrofurans 49, for the case of R4 and R5 different from H, which cannot tautomerize to an heteroaromatic product. Both gold(III) and gold(I) precatalysts prove to be active in different solvents such as MeCN, DCM, and THF. The double-bond geometry of the product is in accordance with an anti-oxyauration. Iodocyclization with I2/base leads to the opposite double-bond geometry.

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agreement with this mechanistic proposal. However, not only AuCl3 but also PtCl2(COD) catalyzes the reaction, but the latter needed reaction temperatures of 65 °C while the former was active at room temperature. A benzylic ether group is also suitable as a nucleophile. Dube´ and Toste49a were able to convert substrates 58 to indenyl ethers 60 (Scheme 18). Crossover experiments with

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ization of propargyl ketones 65 by initial nucleophilic attack of the carbonyl group followed by aromatization of the intermediate arenium ion 66 to furan 67 (Scheme 21). Scheme 20

Scheme 18

Scheme 21

deuterated substrates indicated an alkyne activation to deliver intermediate 59. Even a chirality transfer was observed, indicating a “memory effect” in the intermediate. 3.1.1.1.4. Hydrocarboxylation of Alkynes. Carboxylic acids are weak nucleophiles. A first intermolecular example using acetic acid was reported by Schmidbaur et al.41 In the reaction of acetic acid with 3-hexyne, besides the enol ester, 3-hexanon was obtainedsThe latter was probably due to the presence of water traces. The intramolecular addition of carboxylic acids to alkynes leads to lactones. Geneˆt et al.49b,c reported the AuCl-catalyzed conversions of substates 61 to 62 (Scheme 19). As in the

Kirsch et al.52 have used R-hydroxy propargyl ketones 68 as substrates for similar reactions, a beautiful route to 3(2H)furanones 69 (Scheme 22). However, in his investigation, Scheme 22

Scheme 19

reaction from Scheme 15, a styrenelike side chain does not compete with the alkyne (again, both in the same distance to the nucleophilic group). AuCl3 is also able to catalyze these reactions. Pale et al.50 demonstrated that with 10 mol % of AuCl and 10 mol % of K2CO3 in MeCN, not only the five- but also the six-membered lactones can be formed. Even a 25% yield of a seven-membered lactone was reported. Bromoalkynes react stereoselectively to yield the (Z)-diastereomers. Shin et al.51 have shown that instead of a free acid the corresponding tert-butyl esters can be used. With carbonates of homopropargylic alcohols (63), where the free acid would not be stable but the tert-butyl carbonates are, AuCl3 and gold(I) catalysts are active for the 6-exo-dig cyclization to cyclic carbonates 64, ultimatively leading to β-hydroxy ketones (Scheme 20). 3.1.1.1.5. Ketones and Imines as Nucleophiles. Here, Hashmi et al.43 initially observed that unlike palladium(II) catalysts, gold(III) catalysts indeed initiated the cycloisomer-

platinum(II) catalysts usually gave better results in toluene at 80 °C. Zhang and Schmalz53 used a related principle for the formation of highly substituted furans from alkynylcyclopropanes 72 (Scheme 23). Not only methanol but also other nucleophiles such as iPrOH, tBuOH, propargylic alcohols, phenols, 2-pyrrolidone, indole, and even acetic acid could be used. Iminelike substrates 76 also react, and one could expect the mechanism to be similar to the principles discussed above, but labeling experiments of Seregin and Gevorgyan54 suggest that the reaction proceeds via vinylidene-carbene complex 77 to deliver indozilines 80 (Scheme 24). It should be noted that Dake and co-workers55 recently demonstrated that similar reactions can efficiently be catalyzed by silver triflate.


Chemical Reviews, 2007, Vol. 107, No. 7 3187 Scheme 25

Scheme 26

Scheme 24

Toste et al.64 Deuterium labeling indicated a reaction of the enol in 88 with the π-coordinated alkyne (Scheme 27). While Scheme 27

3.1.1.1.6. C-C Double Bonds of Arenes and Enols as Nucleophiles. A first example was reported by Sames et al.56 who investigated different transition metal salts for the cyclization of phenyl propargyl ethers 81 to 82 (Scheme 24). However, the yield of the desired product was only 6%; 82% of the starting material was reisolated, and best results were obtained with PtCl4. The intermolecular hydroarylation of alkynes was investigated by Reetz and Sommer.57 For phenylacetylene, a combination of AuCl3 and AgSbF6 was optimal; for ethyl propiolate, gold(I) catalysts gave best results. The observed double bond geometry is in accordance with the mechanism shown in Scheme 1. Recently, even the two-fold intermolecular hydroarylation of alkynes was observed by Hashmi et al.58 He and Shi59 obtained high yields under solvent-free conditions. A related investigation also covered different heterocycles and gold(III) catalysts.60 Intramolecular, gold(I)-catalyzed versions have also been reported by Nevado and Echavarren61 as well as Hashmi et al.62 With indoles in 83 as the intramolecular partner, Ferrer and Echavarren63 obtained up to eight-membered rings (86). These are formed by a 7-endo-dig cyclization to 84 followed by a ring expansion (1,2-shift to 85; Scheme 26) and not by a direct 8-endo-dig cyclization.63 β-Ketoesters 87 can serve as nucleophiles, too. Both the 5-exo-dig and the 5-endo-dig cyclization were reported by

the 5-exo-dig cyclization depends on terminal alkynes, the 5-endo-dig cyclization also tolerates internal alkynes, even iodoalkynes. An enantioselective version of this reaction succeeded only with palladium.64c 3.1.1.2. Allenes as Substrates. Selectivity is quite problematic with allenes; in addition, reactions, chemoselectivity, diastereoselectivity, regioselectivity, and positional selectivity are an issue.65 3.1.1.2.1. Hydroalkoxylation of Allenes. Initially, it was demonstrated43 that nucleophilic attacks of carbonyl oxygen atoms next to allenes are successful (see section 3.1.2.1.). Then, Krause and Hoffmann-Ro¨der66 investigated the cycloisomerization of allenyl carbinols 90 to 2,5-dihydrofurans 91 with good stereocontrol (Scheme 28). While these Scheme 28


reactions also proceeded with acidic catalysts, in the case of acid-labile substrates, only AuCl3 gave good yields of the desired heterocycles. For these reactions, evidence for the in situ reduction of the AuCl3 was obtained,67 and 1,1-disubstituted allenyl carbinols 93 delivered the products of an oxidative dimerization 94 under exclusion of other oxidants and in amounts correlating with the amount of AuCl3 (Scheme 29).

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Scheme 29

as 74 goes along with a high axial to central chirality transfer and a high regioselectivity (Scheme 33).72 Again, the reaction Scheme 33

Hegedus et al.68 investigated related reactions and found gold(I) catalysts to be highly efficient for the cycloisomerization of substrates of type 90. Gockel and Krause69 now reported the synthesis of sixmembered rings by using the higher homologues, β-hydroxyallenes 95 (Scheme 30). Again, the reaction seems to Scheme 30

seems to proceed regioselectively through a vinyl gold intermediate; no allyl gold species is formed. Similar results were obtained in intramolecular reactions.72b Reissig et al.73 conducted the reaction with alkoxyallenes 103, which bear a siloxy group. No dihydropyrrole, only the aromatic pyrrole 104, was obtained (Scheme 34). Scheme 34

proceed via a vinyl gold species; no product from a 5-exocyclization, which would lead to an allyl gold intermediate, was observed. In a remarkable paper, Zhang and Widenhoefer70 described the asymmetric hydroalkoxylation of γ-hydroxy allenes with ee values up to 95% by using a dinuclear gold complex of bis(phosphanyl)biphenyl. 3.1.1.2.2. Hydroacylation of Allenes. In analogy to the reactions shown in Scheme 20, Shin51a used tert-butyl esters 97, which delivered 98 (Scheme 31). Scheme 31

All of these results have to be interpreted with care; the reactions can often also be catalyzed by silver(I), and Ohno and Tanaka et al.74 have just recently shown that the sulfonamides readily cyclize with potassium carbonate in DMF in the absence of any transition metal catalyst. Lee et al. have applied the gold-catalyzed methodology to the synthesis of bicyclic β-lactams.75 3.1.1.2.4. Hydrothiolation of Allenes. Krause et al.76 could even use allenyl thiocarbinols 105 as substrates; these can be cyclized to 2,5-dihydrothiophenes 106 (Scheme 35), and Scheme 35

3.1.1.2.3. Hydroamination of Allenes. In analogy to the allenyl carbinols, different related amines can also be cyclized. This was first shown by Morita and Krause71 for the conversion of 99 to 100 (Scheme 32) The use of free amino groups led to reaction times of days; with sulfonamides, the acetyl, or the Boc protecting group, a fast conversion was possible (but with the latter two the diastereoselectivity was low). In an optimization study, AuCl turned out to be a very good catalyst for this reaction, while cationic gold(I) complexes were ineffective.71b Yamamoto et al. have shown that the intermolecular AuBr3-catalyzed addition of anilines to chiral allenes such

the best results are here obtained with AuCl in CH2Cl2. 3.1.1.2.5. Arenes as Nucleophiles. First results in this field43 indicate that allenyl ketones 107 react with furans 108 (the latter formed in situ) to form R,β-unsaturated ketones 109 (Scheme 36). Yields were low, as 109 can add another furan to form 110. The electron-rich pyrroles are excellent reaction partners for intramolecular reactions, which were applied in a total



synthesis of (-)-rhazinilam by Nelson et al.77 Trisubstituted allene 111 delivers 112 in good diastereoselectivity, and PdCl2(MeCN)2 gives a similar yield but a much lower diastereoselectivity (Scheme 37). Scheme 37

3.1.1.3. Alkenes as Substrates. Again, Thomas et al.78 in 1974 carried out pioneering experiments, which indeed needed stoichiometric amounts of gold. So, gold was, unlike in the experiments by Thomas et al.20 discussed in the introduction to section 3.1.1.1.1., an oxidant and not a catalyst. Still, the products justified the assumption of an initial nucleophilic attack to an olefin π-coordinated to gold(III). 3.1.1.3.1. Hydroarylation of Alkenes. In 2000, Hashmi et al.43 reported the first gold-catalyzed intramolecular addition of a hydroxy group to an activated alkene in 114 to form spirocycle 115 (Scheme 38). In the same publication, the Scheme 38

alkene and a Friedel-Crafts-like electrophilic substitution (119); here, the electron-rich arene would serve as a nucleophile. The same intermediate 120 would be formed in both cases and finally deliver the product 121 by a protodeauration, no β-hydrogen elimination, delivering unsaturated products and reducing the gold catalyst, which is observed. Other electron-rich arenes were also shown to react.81 Organogold compounds of furans related to 118 were subsequently described by Schmidbaur et al.82 He and Shi83 confirmed the concept of a direct auration of electron-rich arenes when they investigated the gold(III)-catalyzed reaction of primary alcohol sulfonate esters with donor-substituted benzenes. A closely related investigation describes the intramolecular reactions of arenes with epoxides.84 Pyrroles are so nucleophilic that the reactions with R,βunsaturated carbonyl compounds are difficult to control;85b,d two-fold addition often dominates in an unselective reaction, and indoles85 or 7-azaindoles85c on the other hand give a highly selective monohydroarylation; benzofurans have also been investigated.60 3.1.1.3.2. Hydroamination of Alkenes. Kobayashi et al.86 discovered that AuCl3 is the most active catalyst for the azaMichael reaction of enones 122 with carbamates 123 (Scheme 40). They screened a large number of Lewis acids; Scheme 40

hydroarylation of R,β-unsaturated alkenes 116 by furans 117 was reported (Scheme 39). Here, two pathways were conceivable as follows: either a direct auration of the furan (118; such aurations have been observed in pioneering stoichiometric reactions of Kharasch79 and Fuchita80) followed by a 1,4-addition to the enone or the activation of the

gold chlorides were among the most reactive catalysts for this reaction. Other catalytically active complexes were PtCl4, IrCl4, and ReCl5. Widenhoefer et al.87 then used unactivated alkenes in intramolecular reactions. A typical example is the reaction


of 125 to 126 (Scheme 41). Five- and six-membered rings

Hashmi Scheme 44

Scheme 41

can be closed that way. With tosyl groups as stronger acceptors on the amine, not only intra- but also intermolecular hydroaminations could be achieved by He et al.88 Deuterium labeling suggested an anti addition of the amine and the gold catalyst. The conversion of 127 to 128 combines both an intermolecular and an intramolcular reaction (Scheme 42). Scheme 42

With the more reactive 1,3-dienes, the intermolecular hydroamination using 5 mol % Ph3PAuOTf in DCE at room temperature proceeds not only with sulfonamides but also with the synthetically much more useful carbamate protecting group Cbz.89 Che et al.90 used microwaves to accelerate the reactions and described the use of carboxamides as nucleophiles. Recent publications show that for the hydroamination of alkenes, quite similar results can be obtained with triflic acid.91 This is not too surprising, as Bergman et al. reported the acid-catalyzed hydroamination earlier.92 3.1.1.3.3. Hydroalkoxylation of Alkenes. Initially, Teles had observed that alkenes do not react with the gold catalysts. He and Yang93 succeeded in the intermolecular addition of weak nucleophiles like phenols and carboxylates to unactivated terminal alkenes. The reaction was catalyzed by 5 mol % of Ph3PAuCl/AgOTf in toluene at 85 °C. The reaction of 129 with phenol delivered a constitutional isomer 131 as the side product (Scheme 43), and control experiments revealed

Another gold-catalyzed addition of alcohols to alkynes was reported by Floreancig et al.94 They formed R,β-unsaturated ketones 138 in situ by the hydration of the alkyne 137 and the elimination reaction discussed in Scheme 10. Then, the hydroxy group closes the ring to deliver 139 (Scheme 45). Scheme 45

Scheme 43

that the gold catalyst was capable of olefin isomerization under the reaction conditions. Gold(III) oxo complexes 132 with norbornene 133 delivered the first and so far only organometallic analogue of an intermediate of a gold-catalyzed reaction, in this specific case the gold-catalyzed addition of water to an alkene (if one considers the µ-oxo atoms in 132 to be equivalents of coordinated water; Scheme 44). Cinellu et al.22 could separate the auraoxetane 135 from the olefin complex 134 and characterize it by an X-ray crystal structure analysis. In a stoichiometric reaction, it ultimatively delivers the epoxide 136.

Li et al.95 extended this principle to 1,3-dienes 140; with phenols 141, an annulation to 144 is possible (Scheme 46). Both diastereomers of the product are formed; it is unknown whether as shown in intermediate 142 the C-C or as shown in intermediate 143 the C-O bond is formed first. 3.1.1.3.4. Enols as Nucleophiles. In a C-C coupling reaction, 1,3-dicarbonyl compounds can be added to activated, styrenelike alkenes, norbornene, and enol ethers.96 A typical example would be the reaction of 145, providing a 1:1 mixture of the diastereomers of 147 (Scheme 47). For the reactions, the alkene has to be added slowly by syringe pump. The authors suggest a C,H activation in the 1,3dicarbonyl compound, but looking at Toste’s experiments (Scheme 27), a reaction of the enol as the nucleophile would be more straightforward. On the other hand, gold complexes of 1,3-dicarbonyl compounds have been studied by Cinellu et al.97



mode of coordination to the alkyne; a normal square-planar tetradentate coodination of the porphyrine would not leave a free coordination site for the substrate. Gevorgyan et al.99 made an important observation with the bromoallenyl ketone 152 (Scheme 49). Which of the two Scheme 49

Scheme 47

3.1.2. Intramolecular Additions in Conjugated π-Systems and Related Reactions In these intramolecular reactions, the nucleophilic group is in conjugation to the double bond, which is activated. In most cases, this ultimatively leads to conjugated (aromatic) products. 3.1.2.1. Carbonyl Groups as Nucleophiles. The cycloisomerization of allenyl ketones 148 was reported by Hashmi et al.43 The carbonyl oxygen atom serves as an intramolecular nucleophile, the Wheland type imtermediate 149, and then delivers the product 151 by aromatization to 150 by proton loss and subsequent protodeauration (Scheme 48). Then, the Scheme 48

different products 155 or 158 was obtained depended on the oxidation state of the gold catalyst. This was explained by an enhanced oxophilic behavior of gold(III) in the sense of intermediates 153 and 155, while gold(I) is more carbophilic (intermediates 156 and 157). Closely related to the reactions of allenyl ketones are Shin’s reactions of tert-butyl allenoates. The initial cyclization step is the same, a nucleophilic attack by a carbonyl group, but then, the tert-butyl group is eliminated and a butenolide is formed.100 Liu et al.101 showed that in reactions of the cross-conjugated dicarbonyl compounds 159 with AuCl3 in DCM alcohol as an external nucleophile that can be incorporated, 162 is obtained (Scheme 50). Scheme 50

furan 151 can react with another molecule of 148, delivering the products from Scheme 36. Che et al.98 have succesfully used gold(III)-porphyrin complexes for the cycloisomerisation of allenyl ketones to furans. The reactions proceed at 60 °C in the presence of triflouroacetic acid, and an impressive TON of 8300 could be reached. The complexes are also active for hydroamination and the hydration of phenylacetylene, which questions the

Larock et al.102 investigated 2-alkynyl-2-alken-1-ones 163. With AuCl3 (Scheme 51), the alkyne seems to coordinate the catalyst (164); then, the carbonyl-oxygen attacks as an intramolecular nucleophile, leading to the furylic cation 165.


Compound 165 then adds methanol (166), delivering 167 as the final product. Other external nucleophiles like electonrich arenes or 1,3-dicarbonyl compounds can also be used. For the transformation from Scheme 51, a recyclable catalytic system in ionic liquids has been reported by Liang et al.103 It is based on Bu4N[AuCl4] in [bmim]BF4; the conversion with 1 mol % of catalyst dropped only from 90% in the first run to 85% in the sixth catalytic run. Yamamoto et al.104 reported a very important reaction principle: ortho-alkynylbenzaldehydes 164 in 1,2-dichloroethane (DCE) (Scheme 52). Nucleophilic attack of the

Hashmi Scheme 53

Yamamoto et al.108 also used enols as interceptors for the pyrylium ions, and Asao and Sato109 demonstrated that even benzyne, which is formed in situ, efficiently undergoes this reaction. The total synthesis of heliophenanthrone by Dyker et al.110 started from a diyne substrate and used an intramolecular alkyne as the cycloaddition partner for the pyrylium ion, a similar strategy that was reported by Yamamoto et al.111 for the total synthesis of (+)-ochromycinone and (+)-rubiginone B2. They also investigated a tethering of the second alkynyl group at the carbonyl group, and the corresponding ketones reacted as expected.112 Oh et al.113 described an alternative pathway; starting from substrate 176 by a 1,3-dipolar cycloaddition, the pyrylium ion intermediate 177 delivered seven-membered cyclic ketone 179 (Scheme 54). The use Scheme 54

Scheme 52

carbonyl oxygen atom delivered the pyrylium intermediates 165, which readily underwent an intermolecular [4 + 2]cycloaddition with alkynes to intermediate 166; then, a ring opening delivers 167. This mechanism was supported by a calculation of Straub,105 who also found that both AuCl and AuCl3 have similar overall barriers for this process. Yamamoto et al. also reported that the AuBr3 has a higher catalytic activity for this conversion.106 Dyker et al.107 extended that methodology to electron-rich alkenes; for example, benzofuran (173) and 174 delivered the annulated 175, which was obtained in the presence of water in excellent regio- and diastereoselectivities (Scheme 53).

of AuBr3 as the most efficient catalyst for the synthesis of products of type 180 and the geminal disubstitution in the malonate subunit seemed to be crucial for this pathway; without the geminal disubstitution, only the usual naphtalines were observed. For both the Yamamoto and the Oh pathways, the second alkyne unit could be replaced by an alkene.114,115 Oh, Han et al.114 used allenes in 181 as intramolecular alkene partners for the pyrylium intermediates (Scheme 55), which turned out to be a very efficient access to polycyclic frameworks of type 182. The pyrylium salts could also be directly protodeaurated; this concept was used by Porco et al.115 in the synthesis of azaphilones. Even the steroid skeleton can be constructed,



and Hildebrandt and Dyker116 used 183 to obtain 184 (Scheme 56). Scheme 56

with AuCl as the catalyst, evidence for the participation of vinylidene species.126 Liu et al.127 cyclized substrates 196; Scheme 59

These reactions proceeding via pyrylium cations are often also catalyzed by copper salts. This has been summarized.117 3.1.2.2. Other Nucleophiles. Asao, Sato, and Yamamoto118 also used nitro arenes instead of formyl arenes, which led to isatogens and anthranils as products. Nucleophiles in ortho-position to an alkynyl group on an arene as in 185 readily cyclized to the corresponding 5-membered heterocycles 188 by an 5-endo-dig cyclization (Scheme 57). For Scheme 57

Scheme 60

X ) O in Hashmi et al.,119 this was shown with AuCl3 in acetonitrile at room temperature; subsequent work of Arcadi et al.120 demonstrated that for X ) NH an efficient cyclization is possible with Na[AuCl4]‚H2O in ethanol under similar conditions. The reaction can be combined with a coupling of R,β-unsaturated ketones.121 Other components can be included in the products; Yamamoto et al.122 investigated the reaction of 2-(alkynyl)phenylisocyanates with alcohols and an intramolecular carbothiolation accompanied by the migration of an electrophilic group. The latest observation in this field was reported by Iwasawa et al.;123 imines of 2-alkynylanilines 189 cyclize to the azomethine ylide intermediate 190, which then in a 1,3dipolar cycloaddition with an alkene (to 191) and subsequent 1,2-migration of a substituent delivers the mitosene skeleton 192 (Scheme 58). Dankwardt124 showed that enolether and enamine substrates of types 192 and 194 (Scheme 59) cyclize in an 6-endo-dig manner to deliver arenes 193 and 195. Not only electron-rich heteroarenes such as 194 but also donorsubstitued phenyl rings react readily.125 Halogens as substituents on the alkyne suffer an interesting 1,2-migration

the product 197 is obtained with PtCl2, but gold catalysts were superior and operated at lower temperatures (Scheme 60).

3.1.3. Ring Enlargement Reactions Hashmi and Sinha128 isomerized alkynyl epoxides such as 198; the latter was easily available by a Sonogashira coupling and an epoxidation to furans 202 with interesting substitution patterns (Scheme 61). The nucleophilic addition of the epoxide-oxygen atom in the alkyne complex 199, leading to intermediate 200, is faster than the nucleophilic addition of the hydroxy group. Again, Wheland type intermediates and a protodeauration are involved. Toste et al.129 reported the ring expansion of ethynyl cyclopropanols and cyclobutanols, a typical example being the conversion of 203 to 206 (Scheme 62). Here, the strained ring in 204 forms a bond to the proximal carbon of the alkyne and not to the distal one. Instead of the proton on the arenium intermediate 200, the proton of the hydroxy group is eliminated. Deng et al.130 succeeded in the insertion of CO2 into oxiranes 207 with polymer-supported nanogold (Scheme 63). TOF values up to 57900 h-1 for the formation of the cyclic carbonates 208 were achieved.


Hashmi Scheme 64

Scheme 62

Scheme 63

3.1.4. Intramolecular Additions with Propargylic/Allylic Leaving Groups This important case is more complex than the previous examples. The reactions can take different pathways. Again, the C-C triple bond is the initial point of interaction between the gold catalyst and the substrate 209; the π-complex 210 then has two possibilities (Scheme 64): (i) attack of the nucleophilic group Y in an 5-exo-dig mode; for X ) NH, the intermediate 211 can directly lead to stable products or 211 rearranges to vinyl carbenoids 212, which then, for example, cyclopropanate an alkene; or (ii) attack of the nucleophilic group Y in an 6-endo-dig mode (213); again, for X ) NH, the intermediate 213 can stabilize by proton loss or a ring opening at X (overall, a stepwise [3,3] rearrangement), leading to allenes 214, which subsequently react with the gold catalyst once more to deliver other products. In both the 5-exo-dig and the 6-endo-dig pathway, a vinylgold species is involved. 3.1.4.1. Propargylic Substrates. 3.1.4.1.1. 5-Exo-Dig Cyclizations Leading to Heterocycles. With an NH group as X, a proton can be eliminated at the stage of the vinylgold species and then release the gold catalyst by protodemetallation. Thus, these reactions after the first step divert to other pathways and do not deliver vinyl carbenoids or allenes.

Independently, in 2004, Uemura et al.123 and Hashmi et al.132 investigated the cycloisomerization of N-propargylcarboxamides, which proceeds by an 5-exo-dig cyclization. Uemura et al. developed a very elegant concept; carboxamides and propargyl alcohols were coupled by 2.5 mol % of a ruthenium catalyst, and then, 20 mol % of AuCl3 was used for the cycloisomerization of the N-propargylcarboxamides in DCE at 80 °C, which delivered the oxazoles under the reaction conditions. Hashmi et al. started from the N-propargylcarboxamides 215 and used only 5 mol % of AuCl3 in either MeCN or CH2Cl2 at 20-50 °C. The yields of the oxazoles 218 were very good; under the mild reaction conditions, the methylene oxazolines 217 could be accumulated up to 95% (Scheme 65). This detection of the Scheme 65

organic intermediate, for the first time accessible by the mild conditions of gold catalysis, allowed a direct study of the diastereoselectivity for the individual steps. The results obtained by isotope labeling indicate that the first step is an anti-oxyauration and the second step is a protodeauration with retention of the relative arrangement of the substituents. This methodology was already used as a step in the synthesis of test substrates for pharmaceutical investigation by Merck.133


Exchanging the positions of oxygen and nitrogen leads to imidates instead of carboxamides. Shin et al.134 and Hashmi et al.135 investigated trichloroacetomidates 219; this intramolecular hydroamination led to the methyleneoxazolines 220, which can tautomerize to the aromatic 221 (Scheme 66). Up Scheme 66


O-propargyl carbamates also cyclize. Then, 4-alkylidene-2oxazolidinones, isomers of 227, were obtained. 3.1.4.1.2. 5-Exo-Dig Cyclizations/Ring Openings Leading to Vinyl Carbenoids. Miki, Ohe, and Uemura were the first to describe this reaction mode for gold catalysis.140 The vinyl carbenoids 229, generated from substrates like 228, were intercepted with styrene, and two diastereomers of the product 230 were formed (Scheme 69). In addition, the Scheme 69

to 3300 turnovers of the catalytic cycle could be achieved, and gold(I) catalysts allowed one to selectively stop the reaction from Scheme 65 at the stage of the nonaromatic product 217. Gagosz and Buzas136 succeeded in the cyclization of propargyl tert-butyl carbonates. Similar to the work of Shin,51 the tert-butyl group instead of a proton is eliminated, and the cyclic carbonates are obtained. An example showing the selectivity is the reaction of the di-alkyne 222, cleanly leading to 225 (Scheme 67). With certain substituents, for example, Scheme 67

isomerization to the allenyl ester 231 was observed. As compared to other transition metal catalysts, the goldcatalyzed reactions were very fast. Recently, an enantioselective version of this reaction using gold(I) complexes of chiral phosphanes was reported by Toste et al.141 One year later, Fu¨rstner and Hannen142 used that approach for the preparation of carene terpenoids by trapping the vinylcarbenoids intramolecularly with an olefin at suitable distance (232 to 235; Scheme 70). Scheme 70

electron-rich groups on the alkyne, a shift of the carbonate group by one carbon atom was observed. Carretero et al.137 as well as Gagosz et al.138 cyclized Bocprotected propargylamines 226 to the corresponding 3-alkylidene-2-oxazolidinones 227 (Scheme 68). The whole system Scheme 68

can also be inverted. Schmalz et al.139 have shown that

The diastereoselectivity was excellent and different from Uemura’s investigation; only “marginal amounts” of the isomeric allenyl acetate were detectable in the crude reaction mixture. A bicyclo[3.1.0]hexane was also accessible by this approach,143 and the face selectivity with chiral substrates for the synthesis of (-)-cubebol was studied.144 Then, Toste at al.145 applied this concept to allylicpropargylic esters. Using a gold(I) catalyst and pivaloates instead of acetates, better results were obtained. A typical example would be the transformation of 234 to 239 (Scheme 71). This reaction, the Rautenstrauch rearrangement, first delivers the enol ester 238, which then hydrolyzes in situ to the ketone 239. In the case of chiral centers at the propargylic-allylic position, a chirality transfer was also possible. A computational study of that chirality transfer indicated that the helicity of the intermediate pentadienyl cation 236, a carbenoid-like species, conserves the chiral information in the intermediate.146 N-heterocyclic carbene complexes of gold(I) have been used in isomerization/intramolecular trapping experiments of Nolan et al.,147 and two-fold rearrangements have been reported by Lee et al.148


Hashmi

Scheme 71

Scheme 73

3.1.4.1.3. 6-Endo-Dig Cyclizations Leading to Heterocycles. Lok et al.149 reported that 2-(N-propargylamino)benzoxazoles such as 240 delivered the dihydropyrimidine 241 (Scheme 72).

Scheme 74

Scheme 72

3.1.4.1.4. 6-Endo-Dig Cyclizations/Ring Openings Leading to Allenic Intermediates. These were described by Zhang and Wang, who reported a tandem 3,3-rearrangement/[2 + 2]cycloaddition150 and a tandem 3,3-rearragnement/Nazarov cyclization.151 Compound 242 by the stepwise, gold-catalyzed 3,3-rearrangement leads to 243; then, in a second catalytic cycle, one double bond of the allene is activated. Electrophilic attack on the indole should form intermediate 245, which then closes to 246 (Scheme 73). If instead of the indole an alkene is present (247), the intermediate allenyl ester 248 and the electrophilic gold(I) catalyst deliver the pentadienyl cation 249, which ultimately yields cyclopentenone 250 (Scheme 74). With propargyl esters 251, the intermediate allenes 252 lead to R-ylidene-β-diketones 255;152 here, gold(III) catalysts give the best results (Scheme 75; the shown catalyst was introduced by Hashmi et al.153). Usually, a good selectivity for the (Z)-diastereomer was observed, and it was assumed that the other geometrical isomer originates from slow subsequent isomerization of 255 by either the gold catalyst or the protons. In a mechanistic investigation, 252 was prepared by another route and then subjected to the gold(III) catalyst, which, as expected, led to 255. Propargylic esters of benzoates and even Boc-protected propargyl alcohols could also be used. With a propargylic TMS group as in 256, the alkenyl enol esters or carbonates 257 were obtained (Scheme 76).154 Here,

the use of dry DCM was important. In wet solvent, 257 was accompanied by significant amounts of the ketone formed by hydrolysis of the enol ester. Toste et al.155 used propargylic esters 260 for goldcatalyzed tandem sequences leading to aromatic ketones 261 (Scheme 77). Here, in most cases, silver(I) catalysts gave superior results, only for the annellation of five-membered rings such as 260 was gold(I) the superior catalyst. As Nolan et al. demonstrated, without the second alkynyl group in benzylic propargylic esters as products, indenes are isolated.156 With allyl groups on the propargyl esters 264, the bicycles 266 were formed (Scheme 78).157 With an additional hydroxy group on the other side of the alkyne, again in propargyl position, 2,5-dihydrofurans with interesting substitution patterns can be synthesized by a combination of the 3,3-rearrangement to the allene and the subsequent cyclization of the allenylcarbinol (compare section 3.1.1.2.1).158 Propargyl vinyl ethers are another class of molecules that can undergo a 3,3-rearrangement to the allene; a high diastereoselectivity and an efficient chirality transfer were



Scheme 78

Scheme 76

Scheme 79

reported.159 Kirsch et al.160 used similar propargyl-Claisen rearrangements and subsequent normal nucleophilic ring closures for the synthesis of highly substituted furans, in combination with amines 269 as intermolecular reaction partners for the synthesis of highly substituted pyrroles 270 (Scheme 79).161 Further combinations of the propargyl Claisen rearrangement with other reactions are a synthesis of dihydropyrans162 and a synthesis of vinylsilanes.163 3.1.4.2. Allylic Substrates. Again, the reactions with alkynes in propargylic substrates are dominant, and only a few reactions with olefins in allylic substrates have been published. Nevado and Echavarren reported one example for an isomerization-Claisen rearrangement by a gold catalyst (271 to 272; Scheme 80), but superior results were obtained with a palladium catalyst (84% yield with PdCl2 instead of 57% with AuCl3).164 Other examples are the aza-Claisen rearrangement of allyl trichloroacetimidates165 and a Claisen rearrangement of aryl allyl ethers followed by an intramo-

lecular hydroarylation of the alkene to yield dihydrobenzofurans.166

3.1.5. Enynes as Substrates 3.1.5.1. Intramolecular Phenol Synthesis. Although the cycloisomerisation of enynes has a long history in transition metal catalysis, gold catalysis could add a new product type for reactions of substrates with enyne substructures. This documents the uniqueness of gold catalysts. The intramolecular reaction of a furan with a terminal alkyne unit in 274, which with 2 mol % of AuCl3 gave the highly substitued phenol 275, was reported by Hashmi et al.167 in 2000 (Scheme 81). Because of the high selectivity


Scheme 81

of the gold catalysis, no side products that could have provided mechanistic insight were detected. Only an intramolecular migration of the oxygen atom was proven by isotope labeling. Then, Echavarren et al.168 reported that platinum(II) also catalyzed this conversion, but side products were obtained. The latter in combination with a computational investigation revealed the intial formation of a cyclopropyl carbenoid intermediate 276 (Scheme 82). With furans

Hashmi

Alder reaction were successful. A computational study of a part of the conceivable reaction modes was also conducted.170 For organic synthesis, this reaction was shown to be very successful for the synthesis of benzofurans,119 biaryls,171 dihydroisobenzofurans and isochromanes,172 the synthesis of the natural product jungianol and its diastereomer epijungianol,173 and a simplyfied formal synthesis of jungianol by a domino hydroarylation/cycloisomerization.174 On the other hand, with silyl groups in the 5-position of the furan ring and with gold catalysts, the intramolecular hydroarylation of the alkyne was preferred, leading to interesting anellated furans, while platinum catalysts still delivered the phenols. 62c,175 With N,O-ligands, a ring closure with four atoms in the tether also gave good yields. Tetrahydroisoquinolines were an interesting target.62c,152,153,176 Substituent effects were investigated, while, for example, an acceptor substituent on the furan is not tolerated,168d even mesityl and adamantyl substituents are no problem.177 For such gold-catalyzed phenol syntheses, Corma and Hashmi et al.178 have also used heterogeneous gold catalysts based on nanogold on nanocerium oxide support. 3.1.5.2. Intermolecular Phenol Synthesis. With a binuclear gold(I) catalyst, for the very first time, such reactions proceeding through gold carbenoid intermediates could be initiated in an intermolecular manner.62a The reaction of 280 and 281 delivers 282, and the hydroarylation product 283 is the byproduct (Scheme 83). Neat substrates have to be used, Scheme 83

Scheme 82

as reaction partners of the alkyne, a ring opening then led to 277, and this vinylcarbenoid intermediate reacted with the carbonyl group and delivered the oxepin/arene oxide tautomer 278/279. Ring opening of the arene oxide finally yielded the phenol 275. The intermediacy of the arene oxides was proved.169 The choice of an N,O-ligand for gold allowed the arene oxide to accumulate in the reaction mixture to more than 80%. A full spectroscopic analysis as well as the interception by a Diels-

and long reaction times are necessary, a clear disadvantage. The sequence of the substituents of the 1,2,3,4-tetrasubstituted arene 282 was confirmed by a crystal structure analysis. 3.1.5.3. Alkoxy-Cyclization. Echavarren et al. then thoroughly investigated the behavior of “normal” 1,6-enynes without a furan ring in gold-catalyzed reactions. After initial problems with AuCl3,168,179 in methanol, very promising results were obtained with substrates, which still show the substructure of an enol ether like the furan.180 For example, from 284, in very good yield 285 was obtained (Scheme 84). Apart from the observation of unexpected products in such reactions,181 computational studies180 and enantioselective variants leading to ee values up to 94% for specific substrates,182 an application in the diastereoselective synthesis of R-glucosides183 was reported. Related to these results with 1,6-enynes and alcohols as nucleophiles are results of Geneˆt et al.184 with electron-rich arenes as nucleophiles. 3.1.5.4. Enyne Cycloisomerization and Enyne Metathesis. The gold-catalyzed reactions of enynes have recently attracted significant interest and have been highlighted185 and summarized.186 Here, the use of triarylphosphine gold(I) complexes was the key to success.187 Products of the enyne



Of mechanistic siginificance for the enyne cyclizations is that the intermediate gold carbenoids could be trapped by reactive alkenes in cyclopropanation reactions.201 Closely related are results with allenynes, here Aubert, Fensterbank, and Malacria et al.202 They described a remarkable halide effect on the chemoselectivity of the reaction. Oh et al.203 used allenynes, in situ generated by the rearrangement of propargylic acetates, for the synthesis of naphthalenes. Buzas and Gagosz204 followed similar principles for the in situ formation of 1,3-enallenes and their further conversion to bicyclohexenes.

3.2. Activation of Carbonyl Groups and Alcohols metathesis could be obtained in high selectivity (for example, 288 to 289; Scheme 85). Scheme 85

3.2.1. Catalytic Asymmetric Aldol Reaction A true milestone, not only for gold catalysis but also for asymmetric catalysis in general, was the catalytic asymmetric aldol reaction of aldehydes 295 with isocyanoacetates 296. Ito, Sawamura, and Hayashi used gold(I) complexes of chiral ferrocenes for the synthesis of enantiomerically pure oxazolines 297 (Scheme 88).205 Scheme 88

Other products were methylene cyclohexenes; a second olefin unit offered intramolecularly could be cyclopropanated by the gold carbenoid intermediate [gold carbenoid species generated from ethyl diazoacetate and a gold(I) complex also cyclopropanated olefins].188 With an additional double bond on the alkyne, bicyclic products could be obtained.189 Gagosz et al. investigated alternative pathways that these gold carbenoids can follow190 and introduced a series of important phosphane gold(I) bis-(trifluoromethansulfonyl)imidate catalysts.191 With a nucleophilic group attached to the enyne structure, a double cyclization was reported.192 Shortening the tether to 1,5-enynes delivered bicyclo[3.1.0]hexene derivatives such as 197 (Scheme 86).193 Related Scheme 86

bicycles were observed by Chung et al.194 in the cyclization of some 1,6-enynes. With a hydroxy group in the tether of the 1,5-enynes, aromatic rings could be formed by Grise´ and Barriault.195 In the presence of methanol, Gagosz et al.196 have shown that related 1,5-enynes give functionalized cyclopentenes. Allylstannanes197 and ynamides198 have been used as substrates, and a silyl enol ether substructure in an 1,6-enyne was used for the total synthesis of (+)-lycopladine A,199 with cyclopropyl ethers 292; tricycles 293/294 were accessible as a mixture of diastereomers (Scheme 87).200 Scheme 87

The trans-diastereomer of 297 is the major product, and hydrolysis of this product delivers the interesting syn-βhydroxy-R-amino acids. The postulated transition state 298 would nicely explain the stereoselectivty.206 The amino group in the sidearm of the ligand deprotonates the isocyanoacetate, which is acidified by coordination to gold. An ion pair bonding207 keeps the isocyanoacetate on that coordination site of the gold complex, the carbonyl oxygen atom of the aldehyde then coordinates to the other site, and this directs the facial selectivity of the deprotonated isocanoacetate. Overall, both the carbonyl group and the nucleophile are activated by the gold catalyst. The facial selectivity at the aldehyde is determined by minimal interaction of the group R. The absolute configuration is determined in a cooperative manner by the planar and central chirality of the ferrocene.208 Conformational studies of the ferrocenyl ligands by twodimensional NMR were conducted by Togni and Pregosin et al.209 After the inital aldol addition, the substrate immediately cyclizes to the oxazoline. This reaction has been reviewed several times.207,210 Related reactions are the Knoevenagel-like condensation of benzaldehyde with cyanoacetates and similar C,H-acidic compounds211 and the three-component Mannich reaction of an aldehyde, a ketone, and a carbamate.212


3.2.2. Condensation with Amines, Alcohols, or Thiols

Hashmi Scheme 92

In a key paper, Arcadi et al. described the condensation 1,3-dicarbonyl compounds such as 299, and amines, alcohols, and even thiol 300 are possible under mild conditions with Na[AuCl4] (Scheme 89).213 Similarly, the in situ formation Scheme 89

of enamines of type 304 from chiral amines 302 and 1,3dicarbonyl compounds 303 and their intramolecular hydroamination/isomerization to pyrroles, for example, 303, are shown (Scheme 90).214,215 The Friedla¨nder quinoline synScheme 90

thesis can also be catalyzed by Na[AuCl4];216 the same is true for the Pictet-Spengler reaction.217 Indoles (306) can also add (Scheme 91). The latter can Scheme 91

Hashmi et al.220 could show that this reaction is neither restricted to the more reactive aldehydes; ketones such as acetone react as well. Furthermore, the reaction is not specific to gold as a catalyst! Mercury(II), thallium(III), or even a simple Bronsted acid with a non-nucleophilic counterion, such as 4-toluenesulfonic acid, are equally active. The twofold reaction with a carbonyl group initially forms a benzylic alcohol. The latter then eliminates to the benzyl cation, which as an electrophile reacts with the second electron-rich arene. Kinetic studies showed that this benzylic alcohol cannot be detected under the reaction conditions, which means that the first step, the reaction of the carbonyl compound with the arene, is rate limiting. In light of these results, the goldcatalyzed reactions of benzylic-propargylic alcohols221 or benzylic acetates222 have to be interpreted with care. It is well possible that simply acid, formed from water and the gold(III) salt, is the catalyst. Nair et al. 219c have taken up that thought and reinvestigated their reactions with HCl and different Lewis acids such as Yb(OTf)3, InCl3, Sc(OTf)3, and Zn(OTf)2. They claim higher yields and higher selectivity for the AuCl3 catalyst.

3.2.3. Three-Component Reactions of Amines, Aldehydes, and Terminal Alkynes In the three-component coupling of an aldehyde, an alkyne, and an amine,223 the gold catalyst activates the nucleophile. The reaction can be conducted in water as solvent, and even the benzyl groups in 311 are tolerated; an excellent yield of 314 is obtained (Scheme 93). Scheme 93

either be used directly or be prepared in situ from 2-alkynylanilines and, by the usual addition/elimination steps, lead to products 308.218 Nair et al.219a reported that related R,β-unsaturated aldehydes such as 309 react with indoles (310) or furans to form 1:3 addition products 311 (Scheme 92). Aldehydes also condense in a 1:2 ratio.219b,c

Extensions to acyl iminium ions as electrophiles224 and the addition of electron-rich arenes to N-tosylimines were seen.225 Gold(III) salen complexes have been used as catalysts, and chiral amines as substrates gave excellent diastereoselectivities.226 Other metals such as silver also show


this catalytic activity; this has been reviewed.227 More recent developments are diastereoselective reactions of R-methyl, R-alkoxyaldehydes, and R-hydroxy aldehyde228 as well as an exciting combination of the addition to a carbonyl group in substrates such as 315 and the cyclization of intermediate 317 to isochromenes 318 (Scheme 94).229 Scheme 94

3.3. Carbon Monoxide as Nucleophile Xu et al.230 discovered that terminal olefins 319 with an Au(I) catalyst, sulfuric acid, and one atmosphere of CO deliver tertiary carboxylic acids 323 (Scheme 95). Probably, Scheme 95

the sulfuric acid first protonates the olefin and the secondary carbeniumion 320 isomerizes to the tertiary carbenium ion 321 by a Wagner-Meerwein rearrangement. Then, the carbon monoxide is transformed to this carbenium ion, the resulting acyl cation 322 is attacked by water, and a proton is lost. Overall, this transformation is a low-pressure equivalent of the Koch-Haaf reaction, which normally needs up to 100 atm of CO. The reaction has been conducted with 1-hexene, 1-octene, 1-decene, and cyclohexene. Rhodium and palladium were shown to possess a similar reactivity,231 and the subject has been reviewed.232

3.4. Hydrogenation Reactions Gold-catalyzed hydrogenation is the oldest of the reactions to be discussed in this review. Here, metal catalysis is not


competing with an uncatalyzed background reaction, which, for example, is the case for many oxidation reactions.

3.4.1. Alkenes, Dienes, or Alkynes as Substrates In 1906, Bone and Wheeler233 published the probably first example of a gold-catalyzed organic reaction. They studied the uptake of hydrogen by the gold in the presence of O2 at 600 °C.234 The only directly observable parameter was the pressure in the apparatus; thus, the possibilities for the interpretation of the results were quite limited. Still, it was found that the rate of reaction is mainly dependent on the pressure of hydrogen and that oxygen had a slight retarding effect, providing possible evidence for the interaction of hydrogen with the catalyst being rate limiting. Later work confirmed that finding.235 In 1950, Couper and Eley236 demonstrated that gold surfaces could convert para-hydrogen to ortho-hydrogen. Several studies also showed that both gold foil237,238 and supported gold catalysts239-242 were active for exchange reactions at temperatures above 200 °C, which demonstrated that gold surfaces can activate hydrogen. The first results on alkene hydrogenation come from the disproportionation of cyclohexene (224), which simultaneously served as a hydrogen donor and as the substrate for hydrogenation. In 1963, Erkelens, Kemball, and Galwey243 reported that gold films at 469-615 K catalyze this reaction and deliver both the hydrogenation product cyclohexane (225) and the dehydrogenation product benzene (226); the same reactivity was observed for gold powder at 476-558 K (Scheme 96).244 Scheme 96

As expected for thermodynamic reasons, the portion of benzene increases with increasing temperature and decreasing hydrogen pressure. The energy of activation for the hydrogenation was significantly higher than the one for dehydrogenation; thus, the dehydrogenation product 226 dominated, and only a small amount of 225 was isolated. In 1966, Yolles, Wood, and Wise investigated the gasphase hydrogenation of 1-butene and cyclohexene at 110 °C with highly dispersed supported gold.245 The reaction rates were first-order in alkene pressure and second-order in chemisorbed hydrogen (the latter being supplied through a palladium silver alloy membrane under the gold layer). In 1971, the more reactive alkynes were investigated;246 2-butyne at temperatures between 335 and 355 °C showed an induction period in which (Z)-2-butene was formed; after that, (E)-2-butene and finally n-butane were also obtained. 1-Butene was observed only in traces. At higher temperatures, C-C bond cleavage to methane, ethane, ethylene, propylene, and propyne was also detected. The skeletal isomerization of neopentane at 700-750 K on gold powder described by Boudart and Ptak is closely related, since in an isomerization, C-C bond cleavage also takes place.247 In the same context, one could discuss the formation of pentane from butane on Pt/Au alloys, which increases with Au content of the alloy (up to 31% with 52% Au at 408 °C).248 The milestone paper by Bond, Sermon, Webb, Buchanan, and Wells followed in 1973.2 They were the first to


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Table 1. Alkenes and Alkynes Investigated in Heterogeneous Gold-Catalyzed Hydrogenation Reactions no. of carbon atoms C2 C3 C4 C5 C6 C7 C8 C10

substrate

refs

ethene ethyne propene propyne 1-butene 1,3-butadiene 2-butyne 1-pentene cyclohexene 2-hexyne benzene toluene styrene phenylacetylene naphthalene

250, 251 2, 252, 253 242, 249, 254 255, 256 245b, 245c 2, 257-259 2 241 243, 244, 245a, 245c, 260 261 262 263 264 265 263, 266

demonstrate that with gold on silica, γ-alumina, or boehmite at only 373-490 K, an efficient hydrogenation is possible with as little as 0.01 wt % gold on the SiO2 support, for example, in the hydrogenation of 227 to 228 (Scheme 97). Scheme 97

investigated in the heterogeneous gold-catalyzed hydrogenation to date. In 1966, the hydrogenation of benzene, a reaction with a remarkably high activation energy of 209 kJ/mol, was investigated.262 The electrocatalytic reduction of ethylene on gold was investigated kinetically by Bryne and Kuhn; the kinetic isotope effect for hydrogen vs deuterium evolvement on gold was 4, and the kinetic isotope effect for ethane hydrogenation vs deuteration was 1.5.267 In spite of all of these efforts, it is impossible to correlate the results. Even if one ignores all of the speculations and assumptions, different supports, different catalyst precursors, different substrates, different temperatures, and different pressures were used. No clear mechanistic picture of these relatively simple hydrogenation exists today268,269sa major difference to the homogeneous gold-catalyzed hydrogenations (see section 3.4.2). There, after a much shorter time, more mechanistic insight was achieved! The chemoselectivity, especially in the hydrogenation of alkynes and R,βunsaturated carbonyl compounds (see next section), justifies future investigation.

3.4.2. R,β-Unsaturated Carbonyl Groups The supported gold catalysts can show very high selectivity for the hydrogenation of R,β-unsaturated aldehydes 234 (Scheme 99), and much of this work has recently been Scheme 99

With 1,3-butadiene 229 and 2-butyne 230 as substrates, a chemoselective monohydrogenation was observed, no diastereoselectivity in the case of 1,3-butadiene (E/Z ) 1:1) but a good diastereoselectivity (E/Z ) 1:8) in the case of 2-butyne (Scheme 98). Scheme 98

Au/γ-Al2O3 catalysts containing less than 5% Au241 were inactive for the hydrogenation of 1-pentene, but very different results were obtained using Au/SiO2 catalysts. With a massive excess of hydrogen, low concentrations of Au supported on SiO2 were active for 1-pentene hydrogenation, and the maximum activity was observed with 0.04 wt % Au. This observation indicates that hydrogenation reactions could be sensitive to both the Au particle size and the nature of the support. The hydrogenation of ethene with 5% Au/SiO2 is first-order in hydrogen, and the order of reaction for ethene is 0.4.241 Interesting results were obtained with deuterium and propene on Au/SiO2;249 D2 reacts much slower than H2, suggesting that the breaking of a H-H bond is the ratedetermining step. Guzman and Gates investigated the hydrogenation of ethene with mononuclear gold complexes on MgO powder at atmospheric pressure and 353 K. EXAFS and XANES data provided evidence for AuIII as the active species. IR spectroscopy showed ethyl gold species as the reactive intemediate. The reaction order in hydrogen was 0.5.250 Table 1 shows the few alkene and alkyne substrates that have been

reviewed by Claus.268 Au/ZrO2 and Au/ZnO catalysts were highly selective for the formation of crotyl alcohol (235; R2 ) H and R1 and R3 ) Me) from the hydrogenation of crotonaldehyde, and selectivities up to 81% at conversions of 5-10% could be observed.270,271 These supported gold catalysts preferentially hydrogenated the CdO bond rather than the CdC double bond. They also showed that the addition of low amounts of sulfur from thiophene promoted this selective hydrogenation. This is one of the few examples of promotion for a gold catalyst, and it can be anticipated that this will become a useful research approach for the design of improved catalysts. Similar to Cu272-274 and Ag275 catalysts, sulfur shows a promotional effect. Only relatively small particles (ca. 2 nm) showed the effect; for larger particle sizes, the promotional effect decreased. However, the selectivity for crotyl alcohol increased with the Au particle size; calcination of a 5 wt % Au/ZnO at temperatures between 250 and 400 °C increased the selectivity for crotyl alcohol. The hydrogenation of acrolein (234; R1, R2, and R3 ) H) is also possible using Au/SiO2, Au/ZrO2, Au/TiO2, Au/ZnO, and Au-In/ZnO.276 The supported gold catalysts have been


applied to more complex substrates, including citral,277 benzal acetone,278 and pent-3-en-2-one.279 Corma and Serna280 achieved the regioselective selective reduction of a nitro group, even in the presence of other reducible functions, using gold nanoparticles supported on TiO2 or Fe2O3. The chemoselective hydrogenation of functionalized nitroarenes with H2 under mild reaction conditions was demonstrated, providing a previously unknown route for the synthesis of the industrially important cyclohexanone oxime from 1-nitro-1-cyclohexene. Homogeneous gold-catalyzed hydrogenation was first mentioned by Muller,281 who in 1974 stated that HAuCl4/ Sb(SC6F5)3 hydrogenated ethene to ethane in ethanol at 0 °C at 1 atm of hydrogen, but details were not provided. Almost 30 years later, in 2003, Arcadi and co-workers282 developed a gold-catalyzed synthesis of pyridines; the final step of this reaction is the dehydrogenation of the initially formed dihydropyridine intermediate 241 to the pyridine 242 (Scheme 100). This dehydrogenation is connected with Scheme 100

hydrogenation by the principle of microscopic reversibility. The fate of the hydrogen liberated was not investigateds Two equivalents of alkyne 240 was used in these reactions, and thus, it is conceivable that a transfer hydrogenation directly delivers the hydrogen to 240. On the basis of computational studies, Corma et al.283 showed that for nonaysmmetric hydrogenation using semisalen type ligands, a heterolytic mode of hydrogen activation and not a homolytic mode in the sense of, for example, an oxidative addition, is the probable pathway. The oxidation state of the gold does not change in this pathway. However, none of the intermediate species suggested have yet been observed. Related reactions are the diborylation of styrenes, where a rhodium catalyst mainly delivered unsaturated, monoborylated products by β-hydrogen elimination, but a gold catalyst formed in situ from [(PEt3)AuCl], and 1,2-dicyclohexylphosphanylethane was claimed to only deliver the desired diborylated product.284 No experimental details of Scheme 101


this reaction have been published. The same authors285 described the gold-catalyzed hydroboration of imines, in which the phosphane gold(I)-chloride complex catalyzed reaction was more than 40 times faster than the uncatalyzed reaction. Hexaalkyldistannanes are useful reagents for organic synthesis,286 but they are quite expensive. The formation of hexabutyldistannane from the relatively cheap tributylstannane by the intermolecular dehydrogenative dimerization forming a tin-tin-bond is possible.287 Hosomi et al.288 in 2000 reported the successful hydrosilylation of ketones and imines with PhMe2SiH and 3 mol % of the [AuCl(PPh3)]/PBu3 catalyst. After workup, the primary alcohols or the secondary amines could be isolated. This methodology parallels the hydroboration of imines mentioned above,285 and it shows a remarkable chemoselectivity since it allows a differentiation between aldehydes and ketones, the latter not being reduced. Ito and Sawamura et al.289 recently discovered the dehydrogenative silylation of alcohols with HSiEt3 and the goldxantphos complex 244. The chemoselectivity is nicely demonstrated by substrate 243, and the product 245 still contains the R,β-unsaturated enone subunit, which is not reduced (Scheme 101). Other functional groups such as alkenes, alkynes, alkyl halides, tertiary alcohols, aldehydes, ketones, or carbamates are tolerated. For the reaction mechanism for these reactions, in analogy to Corma’s proposal,283 a gold monohydride species was suggested. Relatively few papers have appeared on homogeneous gold-catalyzed hydrogenation, but it is interesting to see the mechanistic insight that has already been achieved. There is significant potential for methodical and synthetic work.

3.4.3. Catalytic Asymmetric Hydrogenation The enantioselective hydrogenation of alkenes and imines by gold(I)-Me-Duphos complexes gave very good results.290 In most cases, the gold catalyst reaches the activity of the platinum and iridium catalysts, and ee values clearly exceed the values obtained with the other two classical hydrogenation catalysis metals. Still, this is only a singular publication, and the absolute configuration of the products obtained seems to be unknown; it also remains unclear whether with all three metals the same enantiomer is prefered.

3.4.4. Dehydrogenation Reactions Additional information might be gained from dehydrogenation, which is connected to hydrogenation by the principle


of microscopic reversibility (and the first step of dehydrogenation is intrinsically connected to isomerization). Gold in the form of foil, wire, powder, and sponge as well as Au/SiO2 and Au/Al2O3 is capable of dehydrogenating formic acid to hydrogen and CO2 at temperatures between 373 and 523 K.291 Activation energies are typically around 60 kJ/mol. The isomerization of 1-butene to 2-butene is catalyzed by gold at 573-673 K.275b With a gold film, a dehydrogenative cyclization of n-pentane to cyclopentadiene and cyclopentene at 650-700 K was described.292 Another study covered the hydrogen transfer reaction between cyclohexane and benzene over different supported gold catalysts at 200-250 K.293 These experiments are in close relationship with the formation of cyclohexane and benezene in the hydrogenation of cyclohexene described above.243,244 For 1-butyne, the formation of butene over metallic gold films (no hydrogen added, a transfer hydrogenation might be conceivable) was mentioned in a publication.294 Chemically related is the reaction of silanes, for which also a H/D exchange295 and an addition to 1-octene296 were observed. Still, all of these results do not help to gain better insight in the mechanism of hydrogenation.

3.5. Oxidation Reactions In paticular, supported gold catalysts have been found to be effective for the epoxidation of alkenes and the oxidation of alcohols. There are some studies suggesting that C-H activation may be feasible with supported gold catalysts.

3.5.1. Epoxidation Reactions Most research has focused on the oxidation of propene 246 to propene oxide 247, which is a major research target, as this is a commodity chemical used in the manufacture of polyurethane and polyols (Scheme 102). Although the epoxidation of ethene with dioxygen is a commercial process operating with selectivities in excess of 90% using a supported Ag catalyst,297 the oxidation of propene with its allylic hydrogens has proved to be much more problematic, and typically, selectivities lower than 10% are observed with many catalysts. However, very recently, Lambert et al.298 have shown that supported catalysts can give selectivities of about 50% at 0.25% conversion, but the selectivity declines rapidly with increasing conversion. Scheme 102

Haruta et al. were the first to demonstrate the potential of supported gold catalysts for the epoxidation of propene with dioxygen in the presence of H2 as a sacrificial reductant. H2 permits the activation of O2 at relatively low temperatures, permitting the selective oxidation of propene to occur.299 The need for the requirement of sacrificial H2 for the epoxidation of alkenes using supported gold catalysts has been shown to be not essential in a recent study by Hughes et al.300 In this study, it was shown that using catalytic amounts of peroxides could initiate the oxidation of alkenes with O2. A range of substrates were used to show the effect, and very high selectivities of the epoxides were observed with cyclohexene, styrene, cis-stilbene, and cyclooctene. In

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addition, the selectivity was shown to be highly dependent on the solvent, and the best results were achieved in substituted benzenes. However, these catalysts were also effective in the absence of solvents. The yields of epoxides obtained under these solvent-free conditions were similar to those obtained for supported gold catalysts using sacrificial H2.301

3.5.2. Selective Oxidations of Alcohols The oxidation of alcohols and polyols represents a demanding and very important target. Supported platinum and palladium nanoparticles are generally acknowledged as effective catalysts for the oxidation of polyols, for example, in carbohydrate chemistry for the oxidation of glucose to gluconic acid and the oxidation of glycerol to glyceric acid. However, these catalysts, although well-established, have disadvantages, namely, they exhibit poor selectivity with complex substrates. Rossi and Prati with co-workers302 have shown that supported gold nanoparticles can be very effective catalysts for the oxidation of alcohols, including diols. In these studies, the presence of base was found to be essential for the observation of activity, as it is considered to be essential for the first hydrogen abstraction, and this is a significant difference from Pd and Pt catalysts that are effective in acidic as well as basic conditions. The catalysts were Au/carbon, and they were observed to be effective for a range of substrates. In addition, the catalysts were shown to be effective with gas-phase reactants, and in this case, no base addition is required.303 These researchers have extended their studies to the oxidation of sugars, and a similarly high catalytic efficiency for the oxidation of glucose and sorbitol has been observed.304 In recent studies, they have reported the synergistic effect of the addition of Pd or Pt to the Au/ carbon catalysts for the selective oxidation of D-sorbitol to gluconic and gulonic acids.305 These studies using Au/carbon catalysts were extended by Carrettin et al.306 to show that Au supported on graphite can oxidize glycerol to glycerate with 100% selectivity using dioxygen as the oxidant under relatively mild conditions with yields approaching 60%. It was observed that the selectivity to glyceric acid and the glycerol conversion were very dependent upon the glycerol/NaOH ratio. In general, with high concentrations of NaOH, exceptionally high selectivities to glyceric acid can be observed. However, decreasing the concentration of glycerol and increasing the mass of the catalyst and the concentration of oxygen lead to the formation of tartronic acid via consecutive oxidation of glyceric acid. Interestingly, this product is stable with these catalysts. It is apparent that, with careful control of the reaction conditions, 100% selectivity to glyceric acid can be obtained with 1 wt % Au/C. Under comparable conditions, supported Pd/C and Pt/C always gave other C3 and C2 products in addition to glyceric acid and, in particular, also gave some C1 byproducts. Subsequent studies by Porta and Prati302n have shown further aspects of this complex oxidation. In many catalytic studies, the support-catalyst interaction is a crucial factor controlling reactivity. Interestingly, Rossi and co-workers307 have shown that “naked” gold colloidal particles can be very effective catalysts for the oxidation of glucose 248 to gluconic acid 250. They demonstrated that the initial rates for these nonsupported particles were identical to the rates observed with Au/carbon catalysts operated under the same conditions, hence confirming that the support is of limited importance in the origin of the catalyst activity in


these oxidation reactions. The support-catalyst interaction is, however, essential for the observation of a stable catalyst system. Subsequently, Mertens et al.308 have also shown that colloidal gold can catalyze the oxidation of 1,2-diols. Tsunoyama et al.309 have shown that benzyl alcohol can be oxidized using oxygen in aqueous media with gold nanoclusters stabilized on polymers. One of the most significant advances in the field of alcohol oxidation has been the observations of Corma and coworkers310 showing that an Au/CeO2 catalyst is active for the selective oxidation of alcohols to aldehydes and ketones and the oxidation of aldehydes to acids. In these studies, the catalysts are active at relatively mild conditions, without the addition of a solvent, using O2 as oxidant without the requirement for the addition of NaOH to achieve high activity. The results were shown to be comparable to, or higher than, the highest activities that had been previously observed with supported Pd catalysts.311 The catalytic activity was ascribed to the Au/CeO2 catalyst stabilizing a reactive peroxy intermediate from O2. Subsequently, Hutchings et al.312 showed that alloying Pd with the Au in supported Au/ TiO2 catalysts enhanced the activity for alcohols under solvent-free conditions by a factor of over 25. With these reactive catalysts, primary alcohols and diols can be readily oxidized, further extending the scope of selective oxidation reactions that are possible with gold catalysts. More recently, attention has started to be given to the mechanism of oxidation of alcohols using gold catalysts, mainly for homogeneous systems. The best molecular model of oxidation with oxygen using supported gold catalysts was developed by Rossi et al.,304b and because the reactions proceed through an aldehyde-hydrate, it is conceivable that not only the oxidation from an aldehyde to a carboxylic acid but also the oxidation of an alcohol proceeds in a comparable manner. On the basis of a thorough kinetic analysis304b for the oxidation of glucose to gluconic acid,307 which, for example, shows an activation energy of 47 ( 1.7 kJ/mol (quite similar to the value of 49.6 ( 4.4 kJ/mol observed for the Hyderase enzyme) and the reaction being first-order in oxygen, the authors suggest a fast adsorption of the glucose on the gold; the rate-determining step would then be the oxidation of the glucose by oxygen dissolved in the aqueous phase. Hydrogen peroxide could be detected, but it decomposed quickly under the reaction conditions. These glucose oxidations can reach an initial TOF of 50120 h-1. In a subsequent investigation, Rossi et al.313 could show that these oxidations do not proceed by a radical pathway but by a two-electron mechanism, delivering gluconate and hydrogen peroxide. The hydrogen peroxide decomposes due to the basic medium before it reaches a concentration at which it would efficiently compete with oxygen as the oxidant. Scheme 103 shows the mechanistic picture for that oxidation reaction. In aqueous medium, the two cyclic diastereomers of glucose (β-D-glucose 248) and the acyclic aldehyde form are in equilibrium. Nucleophilic addition of hydroxide will form 249, which is adsorbed on the catalyst (250). Then, oxygen reacts with 250 to form the peroxy complex 251, and then, the hydrogen atom of the coordinated aldehyde-hydrate is eliminated as a proton, and the product gluconic acid (252) desorbs from the catalyst (Scheme 103). Hydrogen peroxide is also released. While the gold colloids with a mean particle diameter of 3.6 nm (Rossi’s “naked” gold particles)307 are located on the


Scheme 104

Scheme 105

borderline between heterogeneous and homogeneous catalysis, Shi et al.314 recently described the selective oxidation of alcohols to aldehydes or ketones using soluble complexes of gold. This reaction also works with air, for example, with benzyl alcohol (253) and the catalyst 254 (which is formed in situ from AuCl and an anionic ligand, so the structure of the precatalyst is not proven), and a 100% conversion was detected by gas chromatography, which also indicated a 99% selectivity toward the product benzaldehyde (255) (Scheme 104). These reactions allow an oxidation by molecular oxygen, sometimes even by air, and thus are beautiful examples for green chemistry.

3.5.3. Selective Oxidations of Alkanes The activation of C-H bonds in alkanes is of immense commercial significance. As supported nanocrystaline gold catalysts have been shown to be effective for selective oxidation of alkenes and alcohols, it is not surprising that attention is now being applied to the oxidation of C-H bonds. In this respect, attention has so far been focused on cyclohexene activation under mild conditions. This is one of the most important alkane activation processes currently operated industrially in the oxidation of cyclohexane to cyclohexanol and cyclohexanone, which in review has been acknowledged to be a reaction that continues to be a significant challenge.315 The aerobic oxidation of cyclohexane (256) is a step in the production of nylon-6 and nylon-6,6,


and the worldwide production exceeds one million tons per year. The use of gold catalysis for this application has been initiated by Zhao et al.,316 who have shown that gold can activate cyclohexane at 150 °C; selectivities of ca. 90% can be achieved for the Au/ZSM-5 catalyst and >90% for a Au/ MCM-41 catalyst although an initial induction period was apparent with these catalysts. These gold catalysts can be resused, but the activity gradually declines and the selectivity suffers a shift from cyclohexanone (259) to cyclohexanol (258). These studies show that supported gold catalysts are indeed active for the activation of C-H bonds but that the temperature used for these studies was 140-160 °C and that high selectivities can be achieved (Scheme 105). A recent study by Xu et al.317 has investigated the potential oxidation of cyclohexane at temperatures well below 100 °C, using oxygen as an oxidant, with a gold catalyst, since, at this temperature, higher selectivities might be expected. In this study, Au/carbon catalysts were contrasted with supported Pt and Pd catalysts. The Au/carbon catalysts were identical to those that were found to be highly effective for the epoxidation of alkenes.318 In this study, a reaction inhibitor was also investigated (1,4-difluorobenzene). The selectivity to cyclohexanone and cyclohexanol observed was very high at low conversion, but this declined rapidly with enhanced conversion at longer reaction times. However, the gold catalysts were found to give identical performance to the Pt and Pd catalysts, and in general, the selectivity observed was just a function of cyclohexene conversion. It is clear that considerable ingenuity will be required in future research concerning the preparation and design of gold catalysts if C-H bond activation is to be successfully achieved. This subject has been approached with homogeneous catalysts as well. Schwarz et al. reported that, unlike PtAu+,319 Au2+ cannot activate methane in the gas phase, but Au(CH2)+ in collisions with ammonia delivers CH2N2+ and AuH.320 In solution, complexes of gold were reported to catalyze the oxidation of alkanes to alkylhydroperoxides by H2O2 in acetonitrile at 75 °C.321 Alcohols and carbonyl compounds were formed in minor amounts only. It was even claimed that gold helps bacteria to oxidize methane.322 Periana et al.323 reported that gold oxidizes methane to methanol in concentrated sulfuric acid with selenic acid as the oxidizing reagent at 180 °C with a maximum TON of 30. This reaction has been discussed in depth by de Vos.324

4. Conclusion While neglected by organic chemists for a long time, gold catalysis of organic reactions now has become a highly active field. While most of the activity still focuses on alkynes as substrates and nucleophilic reagents, olefinic substrates and other reaction modes such as hydrogenation and oxidation are catching up. Stereoselectivity, which has been investigated by the pioneers of the field, is back on the agenda. C,H activation might be one of the important future fields. For retrosynthesis, the product/starting material relationship will become increasingly complicated, as many of the transformations form several new bonds and now more and more tandem sequences are reported. Looking at the speed at which the field is expanding today, I doubt that it will be possible to update a review on “goldcatalyzed organic reactions”; future reviews, if they should still be readable, will probably only be able to cover the new

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developments in certain subfields. Clearly, we are still on the increasing side of the golden wave.

5. Acknowledgements I thank the Deutsche Forschungsgemeinschaft for continuous support in the field of gold catalysis since 1999 by a Heisenberg fellowship and individual projects (Ha 1932/ 5-1, Ha 1932/6-1, Ha 1932/6-2, Ha 1932/9-1, and Ha 1932/ 10-1), the Fonds der Chemischen Industrie, and the European Union (AURICAT EU-RTN, HPRN-CT-2002-00174).

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