Mechanistic Studies on Rhodium-Catalyzed Enantioselective Silylation of Aryl C–H Bonds

Several
classes of enantioselective silylations of C–H bonds
have been reported recently, but little mechanistic data on these
processes are available. We report mechanistic studies on the rhodium-catalyzed,
enantioselective silylation of aryl C–H bonds. A rhodium silyl
dihydride and a rhodium norbornyl complex were prepared and determined
to be interconverting catalyst resting states. Kinetic isotope effects
indicated that the C–H bond cleavage step is not rate-determining,
but the C–H bond cleavage and C–Si bond-forming steps
together influence the enantioselectivity. DFT calculations indicate
that the enantioselectivity originates from unfavorable steric interactions
between the substrate and the ligand in the transition state leading
to the formation of the minor enantiomer.