N-heterocyclic carbenes and imidazole-2-thiones as ligands for the gold(I)-catalysed hydroamination of phenylacetylene.

Gold(I) complexes of 1-[1-(2,6-dimethylphenylimino)alkyl]-3-(mesityl)imidazol-2-ylidene (C^Imine(R)), 1,3-dimesitylimidazol-2-ylidene (IMes) and of the corresponding thione derivatives (S^Imine(R) and IMesS) were prepared and structurally characterised. The solid-state structure of the C^Imine(R) and S^Imine(R) gold(I) complexes showed monodentate coordination of the ligand and a dangling imine group that could bind reversibly to the metal centre to stabilise otherwise unstable catalytic intermediates. Interestingly, reaction of C^Imine(tBu) with [AuCl(SMe(2))] led to the formation of [(C^Imine(tBu))AuCl], which rearranges upon crystallisation into the unusual complex cation [(C^Imine(tBu))(2)Au](+), with AuCl(2)(-) as the counterion. The activity of the gold complexes in the hydroamination of phenylacetylene with substituted anilines was tested and compared to control catalyst systems. The best catalytic performance was obtained with [(C^Imine(tBu))AuCl], with the exclusive formation of the Markovnikov addition product in excellent yield (>95 %) regardless of the substituents on aniline.