Insight into the Mechanism of Carbonyl Hydrosilylation Catalyzed by Brookhart’s Cationic Iridium(III) Pincer Complex

New experimental findings suggest
partial revision of the currently
accepted mechanism of the carbonyl hydrosilylation catalyzed by the
iridium­(III) pincer complex introduced by Brookhart. Employing silicon-stereogenic
silanes as a stereochemical probe results in racemization rather than
inversion of the configuration at the silicon atom. The degree of
the racemization is, however, affected by the silane/carbonyl compound
ratio, and inversion is seen with excess silane. Independently preparing
the silylcarboxonium ion intermediate and testing its reactivity then
helped to rationalize that effect. The stereochemical analysis together
with these control experiments, rigorous multinuclear NMR analysis,
and quantum-chemical calculations clearly prove that another silane
molecule participates in the hydride transfer. The activating role
of the silane is unexpected but, in fact, vital for the catalytic
cycle to close.