Mechanism of the Rhodium-Catalyzed Silylation of Arene C–H Bonds

Mechanistic
studies on the rhodium-catalyzed silylation of arene
C–H bonds are reported. The resting state of the catalyst was
fully characterized by NMR spectroscopy and X-ray diffraction and
was determined to be a phosphine-ligated Rh­(III) silyl dihydride complex
(I). Results from kinetic analysis, stoichiometric reactions
of isolated complexes, deuterium labeling, and kinetic isotope effects
are consistent with a catalytic cycle comprising hydrogenation of
the hydrogen acceptor (cyclohexene) to generate a Rh­(I)–silyl
species, followed by C–H activation of the arene by this Rh­(I)–silyl
species. After oxidative addition of the C–H bond in this mechanism,
reductive elimination of the C–Si bond occurs to generate the
silylarene product. The rate-limiting step (RLS) in the catalytic
cycle is not the oxidative addition of an arene C–H bond; rather,
it appears to be the reductive elimination of cyclohexane during the
hydrogenation process. The influence of the electronic properties
of the arene substituents on the reversibility and relative rates
for individual steps of the mechanism, and on the regioselectivity
of the C–H bond cleavage and functionalization, is reported.