Gold(I)-Catalyzed Activation of Alkynes for the Construction of Molecular Complexity

For centuries, gold had been considered a precious, purely decorative inert metal. It was not until 1986 that Ito and Hayashi described the first application of gold(I) in homogeneous catalysis.(1) More than one decade later, the first examples of gold(I) activation of alkynes were reported by Teles(2) and Tanaka,(3) revealing the potential of gold(I) in organic synthesis. Now, gold(I) complexes are the most effective catalysts for the electrophilic activation of alkynes under homogeneous conditions, and a broad range of versatile synthetic tools have been developed for the construction of carbon–carbon or carbon–heteroatom bonds.\nGold(I) complexes selectively activate π-bonds of alkynes in complex molecular settings,(4-10) which has been attributed to relativistic effects.(11-13) In general, no other electrophilic late transition metal shows the breadth of synthetic applications of homogeneous gold(I) catalysts, although in occasions less Lewis acidic Pt(II) or Ag(I) complexes can be used as an alternative,(9, 10, 14, 15) particularly in the context of the activation of alkenes.(16, 17) Highly electrophilic Ga(III)(18-22) and In(III)(23, 24) salts can also be used as catalysts, although often higher catalyst loadings are required.\nIn general, the nucleophilic Markovnikov attack to η2-[AuL]+-activated alkynes 1 forms trans-alkenyl-gold complexes 2 as intermediates (Scheme 1).(4, 5a, 9, 10, 12, 25-29) This activation mode also occurs in gold-catalyzed cycloisomerizations of 1,n-enynes and in hydroarylation reactions, in which the alkene or the arene act as the nucleophile.\nfigure\nScheme 1. Anti-Nucleophilic Attack to η2-[AuL]+-Activated Alkynes\nStructurally, Au(I) predominantly forms linear two-coordinate complexes, although higher coordination numbers are also possible.(30) A significant number of alkyne-gold complexes have been characterized(31, 32) and studied either in solution(32, 33) or theoretically.(34) This selective activation of the alkyne moiety can explain a vast majority of the results experimentally observed for gold(I)-catalyzed cyclization of 1,n-enynes. Nevertheless, complexes of gold(I) with the alkene moiety of the enynes are also formed in equilibrium with the alkyne-gold complexes.(35) Indeed, well-characterized complexes of gold(I) with alkenes have been reported,(36) as well as with allenes(37) and 1,3-dienes.(38)\nDespite the fact that simple gold salts such as NaAuCl4 or AuCl are active enough to catalyze several transformations, gold(I) complexes bearing phosphines or N-heterocyclic carbenes as ligands have found more wide-ranging applications.(39) The active species are often generated in situ by chloride abstraction from [LAuCl] upon treatment with a silver salt bearing a weakly coordinating anion.\nComplexes [LAuY] only exist as neutral species when Y– is a coordinating anion (halides, carboxylates, sulfonates, and triflimide). The corresponding complexes with less coordinating anions, such as SbF6–, PF6–, or BF4–, are in most of the cases not stable. Although, species [AuL]+ (also known as “naked gold complexes”) are often suggested in mechanistic proposals, structural proof for their existence as stable, isolable species is still lacking. Here, for the sake of simplicity in mechanistic schemes throughout this review, LAu+ is used as a surrogate of [LAuL′]+ complexes, where L′ states for a relatively weakly bound ligand such as the substrate (alkyne or alkene), product, or solvent molecule.\nIt is important to remark that when the catalytically active species are generated in situ by chloride abstraction from complexes [LAuCl] in the absence of the alkyne or other unsaturated substrate, much less reactive chloride-bridged dinuclear species [LAuClAuL]Y are readily formed.(40) Formation of these dinuclear complexes could explain, at least partly, the erratic results that have been ascribed as the “silver effects” in reactions in which Ag(I) salts are used in situ to activate neutral gold(I) complexes [LAuY].(41)\nOften, the most convenient catalysts for the activation of alkynes are complexes [LAuL′]X or [LAuX] bearing weakly coordinating neutral (L′)(42) or anionic ligand (X–).(43) These complexes can enter catalytic cycles by ligand exchange with the unsaturated substrate, which proceed by associative mechanisms as observed for Au(I) and other diagonal d10 metal centers.(44) Thus, large negative activation entropies characteristic of associative mechanisms have been determined for the rate determining ligand exchange reactions of substituted alkyne(45, 46) and alkenes(36o) on commonly used Au(I) catalysts. Although nitriles are frequently used as weakly coordinating neutral ligands, 1,2,3-triazole(46, 47) or other related ligands(48) have also been employed.\nThe properties of gold(I) complexes can be easily tuned sterically or electronically depending on the ligand, consequently modulating their reactivity in the activation of alkynes, alkenes, and allenes.(27, 29f, 49) Thus, complexes containing more donating N-heterocyclic carbenes (3) are less electrophilic than those with phosphine ligands (4, 5) (Figure 1).(28) Complexes with less donating phosphite ligands (6) and related species are the most electrophilic catalysts.\nfigure\nFigure 1. Increase in electrophilicity with decreased donating ligand ability in gold(I) complexes.\n\nGold(I) complexes bearing weak-coordinated ligands such as Me2S, thiodiglycol, or tetrahydrothiophene (tht) have been widely used for the preparation of soluble gold(I) complexes, commonly starting from a gold(III) source.(50) Complex [Au(tmbn)2]SbF6 (tmbn = 2,4,6-trimethoxybenzonitrile), in which gold(I) is supported by two nitrile ligands, can be used for the in situ preparation of a variety of chiral and achiral cationic complexes [LAu(tmbn)]SbF6, including complexes immobilized on a polymeric support.(42a) Other immobilized gold(I) complexes have also been prepared.(51) The use of gold complexes bearing chiral ligands has led to the development of efficient asymmetric gold-catalyzed transformations.(52) Less common precatalysts used in gold(I)-catalyzed transformations are gold hydroxo complex IPrAuOH, which is activated in the presence of Brønsted acids,(53) open carbenes,(39c, 54) and other related complexes,(55) which give rise to selective catalysts of moderate electrophilicity. Cyclopropenylylidene-stabilized phospenium cations, which behave similarly to classical triaryl- and trialkylphosphines, have also been used as ligands in gold-catalyzed reactions.(56)\nThe effect of the counteranion has been studied in detail for several gold(I)-catalyzed transformations.(57, 58) Thus, for the intermolecular reaction of phenylacetylene with 2-methylstyrene catalyzed by [t-BuXPhosAu(NCMe)]Y, it was found that yields increase depending on the counteranion in the order Y = OTf– < NTf2– < BF4– < SbF6– < BARF (BARF = 3,5,bis(trifluoromethyl)phenylborate). By using the bulky and noncoordinating anion BARF, yields are increased by 10–30% compared to those obtained when Y = SbF6–, probably due to a decrease in the formation of the unproductive σ,π-(alkyne)digold(I) complexes from the initial alkyne.(57)\n1.2Scope and Organization of the Review\nHomogeneous gold(I)-catalysis has experienced an outbreak in the past decade leading to the discovery of a remarkable amount of new synthetically useful transformations. Thus, in recent years many groups have used gold catalysis in key steps of total synthesis taking advantage of the unique catalytic ability of gold to build molecular complexity under mild reaction conditions.\nSeveral reviews have been published on gold(I)-catalyzed reactions of alkynes, enynes, and related substrates,(5, 7, 25-28, 59) as well as on gold(I)-catalyzed reactions of allenes(60) and cascade gold-catalyzed reactions.(61) Moreover, specific reviews focused on gold-catalyzed carbon-heteroatom bond formation(62) and on the use of gold catalysis in total synthesis(63) have also been published. In this review, we will cover reactions of alkynes activated by gold(I) complexes, including recent applications of these transformations in the synthesis of natural products. According to the aim of this thematic issue, the main focus is on the application of gold(I)-catalyzed reactions of alkynes in organic synthesis, although reactions are organized mechanistically. Reactions of gold(I)-activated alkenes and allenes, as well as gold(III)-activated alkynes, will not be covered.\nThe discussion has been primarily organized based on the different reactions catalyzed by gold(I) complexes that alkynes can undergo. When possible, inter- and intramolecular processes, as well as the applications in total synthesis, are treated in specific subsections.