3.6.11 Organometallic Complexes of Gold (Update 1, 2011)
V. López-Carrillo and A. M. Echavarren
3.6.11.1 Gold-Catalyzed Cycloisomerizations of Enynes
Gold(I) salts and complexes are the most alkynophilic amongst the electrophilic metals that catalyze cyclization of 1,n-enynes.[1-13] Gold(I) complexes are highly selective Lewis acids with a high affinity for p-bonds linked to relativistic effects, which reach a maximum with gold.[6,14-16] In the reactions of 1,6-enynes 1, the alkyne group is selectively activated by a cationic gold species {[AuL]+} to form an alkyne-gold(I) complex, which reacts intramolecularly with the alkene by formal 5-exo-dig or 6-endo-dig cyclization to form intermediates 2 and 3, respectively (? Scheme 1).[1] A number of alkyne-gold complexes have been characterized[17-21] and studied in solution.[22-25]
? Scheme 1 General Reaction Pathways in the Gold(I)-Catalyzed Cyclization of 1,6-Enynes
Although the vast majority of cyclizations of 1,n-enynes catalyzed by gold(I) can be explained by the selective activation of the alkyne function by gold, complexes of gold(I) with the alkene function of the enyne are actually formed in solution in equilibrium with the alkyne-gold complexes.[26] Indeed, well-characterized complexes of gold(I) with alkenes are known[27-42] and their structures have been studied in solution.[39,40,43,44] The solid-state structure of a cationic allene-gold(I) complex has also been determined.[45]
Formation of C-C bonds can be catalyzed by gold(I) or gold(III) salts or complexes. However, gold(III) may be reduced to gold(I) by easily oxidizable substrates.[46] The most widely used catalysts are cationic complexes [Au(S)(L)]X (L = ligand; S = solvent or substrate molecule) generated in situ by chloride abstraction from complexes [AuCl(L)]. Thus, the precatalyst chloro(triphenylphosphine)gold(I) (or other similar phosphine complex) reacts with 1 equivalent of a silver salt with a noncoordinating anion to generate in situ the cationic catalyst {[Au(PPh3)(S)]X}.[47,48] Similar cationic complexes can be obtained in situ by cleavage of the Au-Me bond in methyl(triphenylphosphine)gold(I) with a protic acid.[47,49-51] More conveniently, a cationic complex {[Au(NCMe)(PPh3)]SbF6} has been prepared as a stable crystalline solid, which allows gold(I)-catalyzed reactions to be performed in the absence of silver salts.[47] A gold-oxo complex {[(Ph3PAu)3O]BF4}[52,53] has also been used as a catalyst in reactions of enynes.[54]
Gold(I) complexes 4-7 bearing bulky, biphenyl-based phosphines, which have been shown to be excellent ligands for palladium-catalyzed reactions,[55,56] lead to active catalysts upon activation with silver(I) salts (? Scheme 2).[57] More convenient as catalysts are cationic complexes 8-11, which are stable crystalline solids that can be handled under ordinary conditions,[58,59] yet are very reactive as catalysts in a variety of transformations.[60-63] Related complexes 12 and 13 with a weakly coordinated bis(trifluoromethylsulfonyl)amide ligand have also been prepared.[64] Gold complexes with highly electrondonating N-heterocyclic carbene ligands[65-67] such as 14-17 are also good precatalysts.[57,68-72] Cationic species 18 and 19,[73] and related complexes,[74,75] as was well as neutral species 20 and 21[76,77] bearing the 1,3-dimesitylimidazol-2-ylidene (IMes) and 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr) N-heterocyclic carbene ligands are active catalysts in many applications. A hydroxygold(I) complex [Au(OH)(IPr)] can also be used as a precatalyst that can be activated with Brønsted acids.[78,79] Open carbenes[80-84] and other related carbenes[20,85-88] also give rise to selective catalysts of moderate electro-philicity. Gold(I) complexes with less donating phosphite or phosphoramidite ligands are the most electrophilic catalysts.[89,90] In particular, readily available complex 22/silver(I) hexafluoroantimonate[91] and its cationic relative 23, bearing tris(2,4-di-tert-butylphenyl)phosphite, are amongst the most reactive gold(I) complexes for the activation of alkynes.[68]
? Scheme 2 Selected Gold(I) Complexes Used as Catalysts or Precatalysts
3.6.11.1.1 Method 1: Cycloisomerization of 1,6-Enynes
3.6.11.1.1.1 Variation 1: Formation of 1,3-Dienes
In contrast to palladium(II), platinum(II),[92-95] and ruthenium(II),[94] gold(I) does not undergo oxidative addition under mild conditions.[6,47,96-98] In the absence of nucleophiles, 1,6-enynes usually undergo various types of skeletal rearrangement reactions by fully intra-molecular processes using a variety of electrophilic metal catalysts.[2-4] The major pathways lead to 1,3-dienes 24 and/or 25, reactions known as single-cleavage and double-cleavage rearrangements (? Scheme 3).[92,93,99-120] These rearrangement reactions proceed under milder conditions using gold(I) catalysts.[47,48,96] For gold(I), the rearrangement is proposed to proceed via intermediates 2 (see ? Scheme 1, Section 3.6.11.1),[97,121] by a mechanism that is consistent with previous work.[99,100,105,114,122-124] Products 26 of a different type of skeletal rearrangement were originally obtained using gold(I) catalysts,[120,125] although this type of compound has since also been obtained using indium(III) chloride[109,110] or iron(III)[96] or ruthenium(II) catalysts.[126] Similar products have also been observed in the reaction of Z-4,6-dien-1-yl-3-ol derivatives with gold or platinum catalysts.[127,128]
? Scheme 3 Gold(I)-Catalyzed Skeletal Rearrangement of 1,6-Enynes
None of the key intermediates involved in the skeletal rearrangement has been spectroscopically characterized,[129] although a gold carbene with an N-heterocyclic carbene ligand has been formed in the gas phase and its reactivity with alkenes has been studied.[130-133] Therefore, the structures of these species are based on density functional theory (DFT) calculations. Some of these intermediates are depicted for convenience as gold carbenes, since backbonding in gold(I) has been shown to be not insignificant.[5,6,134,135] However, according to theoretical calculations, these are highly delocalized structures.[97,121,136,137] In the case of cyclopropyl-containing gold carbenes 2 (see ? Scheme 1, Section 3.6.11.1), these can also be viewed as delocalized cyclopropylmethyl/cyclobutyl/homoallyl carbocations[138] stabilized by gold.[97,121]
Single-cleavage rearrangement reactions of enynes 27 are stereospecific transformations that proceed under mild conditions to give cyclized products 28 (? Scheme 4).[96] Using cationic catalysts 8 or 9 containing bulky phosphine ligands, the rearrangements take place smoothly at temperatures as low as -40 to -60 °C.[121] Similar transformations have been carried out with other gold(I) catalysts.[64,77,139] However, as an exception, enynes such as 29 and 31 with strongly electron-donating groups at the alkene terminus lead to dienes 30 and 32, respectively, with a Z configuration, regardless of the configuration of the starting enynes (? Scheme 5).[140]
? Scheme 4...
