Gold-carbene

Gold carbene complexes preparation, oxidation, and ligand displacement. Journal of Organometallic Chemistry,... 

The cyclic carbene complex shown in equation 3.4 was studied by X-ray diffraction [66], it shows a linear complex (angle C—Au—C 178.6(4)°) and the gold aryl bond distance is 1.993(10) A which is in accordance with such bonds in other known pentafluorophenyl complexes. The gold carbene carbon distance is 1.961(9) A, the dihedral angle between the planes formed by the two organic ligands is 5.35° and the shortest intermolecular Au—Au distance is 3.95 A. 

The majority of gold(I) carbene complexes are pure organometallic compounds and the are out of the scope of this work. Some halide or triphenylphosphine carbene complexes are known and they will be considered here. Nucleophilic addition of alcohols or amines to gold-coordinated isocyanides is one of the best-established methods to obtain gold carbene derivatives. The reaction of H[Au(CN)2] with propene oxide and estirene oxide yields (cyano)carbene complexes (380) avoiding the intermediate step.2257 A cyclic carbene compound is obtained by reaction of a dinuclear isocyanide with amine (Scheme 32).2258... 

A dinuclear gold-carbene complex is obtained when the reaction is carried out with the diamine HNCH3C6H12NHCH3 (Figure 7.13) [11]. This dinuclear compound... 

At the beginning of the new millennium, Hashmi et al. presented a broad research study on both intramolecular and intermolecular nucleophilic addition to alkynes and olefins [18]. One of the areas covered by these authors was the isomerization of co-alkynylfuran to phenols [19]. After that, Echavarren and coworkers identified the involvement of gold-carbene species in this type of process, thus opening a new branch in gold chemistry [20]. And subsequently, Yang and He demonstrated the initial activation of aryl —H bonds in the intermolecular reaction of electron-rich arenes with O-nucleophiles [21, 22]. 

The next study in this field was carried out by Herrmann et al. on the application of N-heterocyclic gold carbenes to the hydration of 3-hexynes in the presence of a Lewis... 

A mechanism proposed for the skeletal rearrangement of enynes involved the presence of gold carbenes [161]. This proposed mechanism was supported by the capture of intermediate gold carbenoids trapped by reactive alkenes in intermolecu-lar cyclopropanation reactions [162]. 

Gold(I)-catalysed cycloisomerizations of enynes (38) afford gold-carbene complexes (39) that have been trapped in an intramolecular fashion yielding highly strained tetracycles (40).38... 

Explosive gold acetylide has been known since the mid-nineteenth century, but systematic studies of this type of compounds were initiated by Coates in 1959. The early work on gold carbonyls dates back to 1925, when Manchot discovered (OC)AuCl, while the related isocyanide complexes were prepared only in 1956 by Sacco. Gold-carbene... 

It is important to note in an overview of this chemistry that the formation of most of the products can be mechanistically explained as proceeding via cyclopropyl gold carbene intermediates. The involvement of these species has been previously proposed in enyne cyclizations with other metals and their structures have been later determined by calculations. ... 

For 1,6-enynes, the initial cyclopropyl gold carbene formed in the 5-exo-dig cyclization (equation 37) evolves to form... 

Additional evidence for the involvement of metal carbenes in these processes was obtained in the reaction of dimeric substrates with a cationic Au(I) catalyst to give disubstituted alkynes (equation 39). These reactions can be explained by isomerization of the initially formed cyclopropyl gold carbene by a [1,3] metallotropic shift, followed by intramolecular trapping of the gold carbene by the alkene. Other examples of [1,3] metallotropic shifts in gold chemistry have been observed recently. ... 

The formation of these products can be rationalized as shown in equation (46). Thus, upon complexation of Au(I) to the aUcyne in the reacting enyne, an endo cyclization occurs to form a cyclopropyl gold carbene, which can be trapped by a nucleophile. However, the lack of stereospecificity observed in some of these reactions, suggests the formation of open six-membered ring cations as intermediates, which can be intercepted by an O-Boc group.3-Hydroxy-1,5-allenynes react similarly with Pt(II) catalysts. ... 

In the presence of external nucleophiles, the situation changes and the behavior of cyclopropyl gold carbenes as delocalized electrophiles becomes manifest. Nucleophihc attack then takes place at either the cyclopropane ring or at the carbene carbon. 

Electron-rich aromatic and heteroaromatic compounds add to 1,6-enynes in the presence of Au(I) catalysts (equations 62 and 63). This reaction leads stereospecifically to adducts related to the products in the aUcoxycychzation by nucleophihc addition on cyclopropyl gold carbene intermediates. Interestingly, in a few cases, addition of the nucleophile takes place at the carbene carbon to give adducts of the type in equation (63). ... 

Other donble bonds on the alkenyl chain can trap the initial gold carbenes leading to cyclopropanes. Thus by using An(I) catalysts, dienynes in eqnations (67) and (68) undergo totally stereoselective cyclizations to yield tetracyclic compounds under milder conditions s n than those required with other metal catalysts. ... 

Carbonyl compounds can also act as the nncleophtles in intermolecular processes with 1,6-enynes. Thus, the gold(I)-catalyzed reaction between enynes and aldehydes or acetone gives stereoselectively tricyclic compounds (equation 71). The transformation is mechanistically intriguing, as it proceeds by a rearrangement of the initially formed cyclopropyl gold carbene (the intermediate in the donble-cleavage mechanism), which is then trapped by the carbonyl compound to form the products. 

A synthesis of cyclopentenones somewhat related to those shown earlier was found from conjugated 1,3-enynes (equation 19) Cyclopentenones are obtained by the hydrolysis of the enol acetates. This transformation involves a 1,3-migration of the acetate to form pentadienyl cation and the formation of the gold carbene after a Nazarov-type cyclization. DFT calculations support this mechanism and provide interesting insight into the mechanism of the final stages of the process. 

Figure 26 Isomers of gold carbene complexes obtained from coordinated isonitriles.

Four main methods have been established for the preparation of gold carbene complexes and a large number of compounds are known. 

Electrophilic Au(I) complexes or their halide AuX analogoues typically cyclize enynes (I, Scheme 58) (475) by a 5-exo-dig pathway to give a variety of cycloisomerization and addition derivatives. The mechanism is proposed to involve formation of a cyclopropyl gold-carbene intermediate... 

Scheme 59. l,8-Dien-3-yne substrate cycloisomerization and ring expansion via a cyclopiopyl gold-carbene intermediate. 

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