Gold -catalyzed

Bond, G.C., Louis, C. and Thompson, D.T. (2006) Catalytic Science Series, Vol. 6, Catalysis by Gold (ed. G.J. Hutchings), Imperial College Press, London, UK Hashmi, A.S.K. (2007) Gold-catalyzed organic reactions. Chemical Reviews,... 

Scheme 6/4.37. Gold-catalyzed formation of bicyclohexenes from ene-ynes.

Gold-based catalysis has attracted considerable attention in recent years. A gold-catalyzed aziridination reaction has recently been reported <06JOC5876>. A series of gold catalysts were examined for their ability to catalyze the aziridination of styrene with p-nitrophenylsulfonamide (NsNH2). Styrene and phenyl-substituted styrenes provided the N-nosyl aziridines in good to excellent yields. Cinnamate however provided the aziridine product in only 25% yield. The use of other sulfonamides (e.g. tosyl, trichloroethyl) gave much lower yields of the aziridine product. 

Gold catalyzed reactions are currently enjoying considerable interest, and have also found applications in thiophene ring synthesis. A series of (a-alkoxyalkyl)(o-alkynylphenyl) sulfides 14 was subjected to treatment with catalytic amounts of AuCl, giving the benzo[fe]thiophenes 15 in excellent yields. Some additional, more complex examples were also provided <06AG(E)4473>. 

The first gold catalyzed C-S bond formation was demonstrated in a route to the 2,5-dihydrothiophene 16 via cycloisomerization of the allene 17 which occurred with high chirality transfer (d.r. > 95 5) <06AG(E)1897>. 

An enantiospecific, gold-catalyzed pyrrole annelation reaction was utilized in a total synthesis of rhazinilam 95 <06JACS10352>. Specifically, treatment of allene 81 with gold triflate - triphenylphosphine led to the formation of annelated pyrrole 82, which was subsequently converted into 95. A gold-catalyzed direct coupling of pyrroles with 1,3-dicarbonyls led to the formation (3-(pyrrol-2-yl)enones <06ASC331>. 

An interesting example of a gold-catalyzed cycloisomerization of P-aminoallene 168 to tetrahydropyridine 169 is depicted below <06OL4485>. Patil et al. report a similar gold-catalized hydroamination of allenes to produce 2-vinyl piperidine 170 in good yield <06TL4749>. 

It has been known from the earliest observations in gold chemistry that olefins form very weak 7r-complexes with gold in any of its oxidation states.1-3 Therefore, only a very few stable complexes of this type have been isolated. However, unstable 7r-complexes of olefins with gold compounds are believed to be important intermediates or transient species in gold-catalyzed reactions of unsaturated organic compounds.122-125,131,286-288,307-312... 

Disubstituted furans 62 are obtained from a gold-catalyzed cycloisomerization/dimerization pathway involving terminal allenyl ketones and a,(3-unsaturated ketones <00AG(E)2285>. 

A mechanism for this process has been postulated involving 73-benzylrhodium intermediates (Scheme 9).39 Gold-catalyzed couplings of activated methylene derivatives with olefins have been postulated to proceed via an in situ formed Au(i) intermediate.40 403 Indeed, gold-mediated C-H functionalization processes are becoming very popular (Equation (32)). For other relevant papers on gold chemistry, see Refs 41 and 41a. 

Gold-catalyzed direct C-H functionalizations enable the formation of polyalkylated arenes under mild conditions. In many cases, branched products are obtained. Two mechanisms are thought to operate with electron-rich arenes, an S si2-type mechanism via Au(lll) leads to the linear product. The branched product is obtained via a Friedel-Craft-type alkylation. A silver salt is often added and is believed to generate a more electrophilic Au(m) species. Often regioselectivities are poor and symmetric arenes are employed. Intramolecular variants as well as Michael additions are also known (Equations (72)-(74)).71,71a,71b... 

The Diels-Alder reaction outlined above is a typical example of the utilization of axially chiral allenes, accessible through 1,6-addition or other methods, to generate selectively new stereogenic centers. This transfer of chirality is also possible via in-termolecular Diels-Alder reactions of vinylallenes [57], aldol reactions of allenyl eno-lates [19f] and Ireland-Claisen rearrangements of silyl allenylketene acetals [58]. Furthermore, it has been utilized recently in the diastereoselective oxidation of titanium allenyl enolates (formed by deprotonation of /3-allenecarboxylates of type 65 and transmetalation with titanocene dichloride) with dimethyl dioxirane (DMDO) [25, 59] and in subsequent acid- or gold-catalyzed cycloisomerization reactions of a-hydroxyallenes into 2,5-dihydrofurans (cf. Chapter 15) [25, 59, 60],... 

S. Hofmann, C. Ducati, Neill, R., S. Piscanec, A. Ferrari, J. Geng, R. Dunin-Borkowski, and J. Robertson, Gold catalyzed growth of silicon nanowires by plasma enhanced chemical vapor... 

After the preparation of the manuscript, gold-catalyzed propargylic substitution reactions have been reported by Campagne et al. Georgy, M. Boucard, V. Campagne, J.-M. J. Am. Chem. Soc. 2005, 127, 14180. 

Most gold-catalyzed reactions involve this pattern of reactivity that has been studied since the eighties. The first examples were found with allenes as substrates and subsequently alkenes and alkynes started to be used, the latter being the most popular in the last five years. 

Hydrothiolation of Allenes The first example of a gold-catalyzed carbon-sulfur bond formation was published by Kraus et al. who synthesized 2,5-dihydrothiophenes by allenyl thiocarbinols [38]. The best results were obtained with AuCI in CH2C12, providing an impressive diastereoselectivity. In the same study, other coin-metal precatalysts were tested but only gold afforded the cydization product. 

During the studies of Lee et al. with several organometallic reagents for the development of new [5-lactam antibiotics a gold-catalyzed synthesis of bicydic 15-lactams 28 was described. In this case, the suggested reaction pathway also involved a vinyl gold intermediate [41]. 

Initial studies by Yamamoto et al. developed a highly efficient gold-catalyzed intramolecular hydroamination of allenes under very mild conditions [42]. 

Reactions by Arenes The work of Hashmi et al. to form a, 3 -unsaturated ketones by reacting allenyl ketones and furans presented the first results in these types of gold-catalyzed processes [28] (Scheme 8.3). 

From the encouraging results obtained in the reactions of a series of gold(III) oxo complexes with olefins [56], Cinellu et al. tried to achieve the supposed oxametalla-cyclic intermediate, which had never been isolated before [25]. In the reaction of 8 and norbornene 56, if the - atoms were considered to be equivalents of coordinated water, and it was therefore possible to talk about the gold-catalyzed addition of water to an alkene. The metallaoxetane 58 was separated from the gold-alkene complex 57 and characterized by X-ray crystal structure analysis. The subsequent stoichiometric reaction yielded epoxide 59 (Scheme 8.5). 

Research in this area was developed further by Li et at. using 1,3-dienes 64 for the gold-catalyzed annulation of phenols and naphtols [58, 59]. These generated various dihydrobenzofuran derivatives. The best yields were achieved when the catalytic system included enough AuCl3 and silver salt to remove halogen atoms and deliver cationic gold. 

Reactions by Arenes In 2000, the first evidence of a gold-catalyzed hydroarylation of a, 3-unsaturated alkenes was reported by Hashmi et al. in their research with furans. In this publication, they proposed two possible mechanisms for the process ... 

Alkynes are by far the most popular and studied functional group in gold catalyzed reactions since Thomas et al. chose them for their research in 1976 [53, 84]. 

A nucleophilic addition to the triple bond in alkynylphosphine derivatives was observed by Laguna and Bardaji and although there was no evidence for a gold catalyzed cycle, reaction conditions were extremely milder than in the classic Reppe vinylation [94] (Scheme 8.12). 

Alcohol addition was also studied by Hashmi et al. in intramolecular processes [28]. Through gold catalyzed cyclizations of (Z)-3-ethynylallyl alcohols 157, these authors were able to obtain furans 159. Reaction occurred via intermediate 158, which tautomerized the heteroaromatic furan, which is thermodynamically more stable (Scheme 8.15). 

A novel gold catalyzed example of three-component addition was recently reported by Shi et al. (Equation 8.44) [106]. Terminal aryl alkynes, alcohols and 2-(arylmethy-lene) cyclopropylcarbinols provided an intermolecular tandem hydroalkoxylation/ Prins-type reaction to form 3-oxabicyclo[3.1.0]hexanes from simple materials and under mild conditions, catalyzed by the system AuClPPh3/AgOTf. The proposed mechanism for this reaction is shown in Scheme 8.19. 

Hydroamination of Alkynes The discovery of palladium-catalyzed intramolecular addition of amines to acetylene coupled with the spectacular contribution of Hutchings opened the door for the synthesis of several nitrogen heterocycles. The first study in this field was performed by Utimoto et al., who researched gold catalyzed intramolecular 6-exo-dig hydroamination. Tautomerization of the initial enamines allowed them to obtain imines, which were thermodynamically more stable [111] (Scheme 8.20). 

Floreancig and Hood recently incorporated gold-catalyzed heterocycle formation in the total synthesis of (+)-Andrachcinidine, a natural component extracted from the plant Andrachne aspera, which has medicinal properties. Gold catalysis was required in the last step of the synthesis, as shown in Scheme 8.22 [119]. 

A related work by Nakamura et al. was recently reported to show the gold-catalyzed process of aminosulfonylation, the formal addition of a nitrogen-sulfur bond to an alkyne moiety, and environmentally benign synthesis of a wide variety of 3- and 6-sulfonylindoles, present in many biologically active compounds [120]. 

For the synthesis of furanones, Kirsch et al. studied the gold-catalyzed hetero-cyclization/1,2-migration cascade reaction of a-hydroxy propargyl ketones [139]. 

After compiling many results obtained in similar studies of different substrates (alkenes, dienes, alkynes and so on), the results cannot be correlated to draw definitive conclusions due to the wide variety of parameters that can influence the reaction (substrates, catalyst precursors, supports, pressure, temperature and so on) [9, 208-214]. This is maybe the main reason why there are no clear mechanistic explanations for this simple reaction, unlike homogeneous gold-catalyzed processes. 

Bailie et al. were the first to mention alcohol formation from aldehydes by supported gold-catalyzed selective hydrogenation. The reaction of the formation of crotyl alcohol from crotonaldehyde showed high selectivity (up to 81%) at conversions of 5-10%, with preferential hydrogenation of C=0 rather than the C=C bond [216]. The addition of thiophene promoted this selective hydrogenation. This promotional effect was also observed in similar situations for Cu and Ag, but it was not very common for gold. 

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