Some Singular Features of Gold Catalysis: Protection of Gold(I) Catalysts by Substoichiometric Agents and Associated Phenomena

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 weakly 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
be 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.