Gold Complexes for Alkyne Activation

Gold complexes with A -heterocyclic carbenes (NHC) [34—37], such as 11-13 [16, 30, 38 2], cationic 14-16[43, 44] as well as neutral 17 and 18 [45, 46], usually afford more selective catalysts, due to the more donating ability of the NHC in comparison with phosphine ligands. Gold hydroxo complex [Au(IPr)OH] can also be used as catalyst by activation with a Brpnsted acid [47, 48]. Acyclic carbenes [49-53], such as 21, and other related carbenes [54-58] also give rise to selective catalysts of moderate electrophilicity. 

Gold(I) complexes with phosphites as ligands are more active catalysts, due to their more electron-withdrawing character. The complexes 19 [31] and its cationic derivative 20 [42, 59, 60], bearing tris(2,6-di-ferf-butylphenyl)phosphite are among the most electrophilic cationic gold(I) catalysts reported to date. 

In general, gold(III) complexes characterized by a square-planar geometry, are used less than gold(I) complexes. The use of pyridinecarboxylic acid as ligand, as 

Gold(l) complexes exhibit excellent chemoselectivity towards C-C Ti-systems. Although [AuL] does not selectively coordinate alkynes over other rr-systems, alkynes are activated selectively because the nucleophile attack is thermodynamically more favored. In the context of 1,6-enynes, while the [AuL] species indifferently coordinates to both Tu-systems, the addition occurs exclusively to the [AuCalkyne)] complex, which has a lower LUMO than the analogous [Au(alk-ene)] complex [63]. 

The coordination of the [AuL] fragment to the alkyne moiety allows the anti attack of nucleophiles due to the formation of 7c-complex I and, consequently, the formation of fZij-alkenyl-gold complex II as intermediate (Fig. 1.4). 

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