Gold-Catalyzed Oxidative Coupling of Arylsilanes and Arenes: Origin of Selectivity and Improved Precatalyst

The
mechanism of gold-catalyzed coupling of arenes with aryltrimethylsilanes
has been investigated, employing an improved precatalyst (thtAuBr3) to facilitate kinetic analysis. In combination with linear
free-energy relationships, kinetic isotope effects, and stoichiometric
experiments, the data support a mechanism involving an Au­(I)/Au­(III)
redox cycle in which sequential electrophilic aromatic substitution
of the arylsilane and the arene by Au­(III) precedes product-forming
reductive elimination and subsequent cycle-closing reoxidation of
the metal. Despite the fundamental mechanistic similarities between
the two auration events, high selectivity is observed for heterocoupling
(C–Si then C–H auration) over homocoupling of either
the arylsilane or the arene (C–Si then C–Si, or C–H
then C–H auration); this chemoselectivity originates from differences
in the product-determining elementary steps of each electrophilic
substitution. The turnover-limiting step of the reaction involves
associative substitution en route to an arene π-complex. The
ramifications of this insight for implementation of the methodology
are discussed.