Some Singular Features of Gold Catalysis: Protection of AuI Catalysts by Substoichiometric Agents, and Associated Phenomena

Abstract

This study deals with two striking phenomena: the complete protection against decomposition of hypothetically monocoordinated AuI intermediates [AuL]Y (L = strongly coordinating ligand; Y– = poorly coordinating anion) by addition of small substoichiometric amounts (5 mol% relative to Au) of not strongly coordinating ligands (e.g. AsPh3), and the fact that, in contrast, strongly coordinating ligands cannot provide this substoichiometric protection. The two phenomena are explained considering that: i) the existence of real monocoordinated [AuL]Y is negligible in condensed phases, and the kinetically efficient existing species are dicoordinated [AuL(W)]Y (W = any very weekly coordinating ligand existing in solution, including OH2, the solvent, or the Y– anion); and ii) these [AuL(W)]Y intermediates give rise to decomposition by a disproportionation mechanism, via polynuclear intermediates formed by associative oligomerization with release of some W ligands. It is also shown that very small concentrations of [AuL(W)]Y are still catalytically efficient, and can be stabilized by overstoichiometric adventitious water, so that full decomposition of the catalyst is hardly reached although eventually the stabilized concentration can result kinetically inefficient for the catalysis. These results suggest that, in cases of gold catalysis requiring the use of a significant quantity of gold catalyst, the turnover numbers can be increased, or the concentration of gold catalyst widely reduced, using substoichiometric protection properly tuned to the case.