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 fiinctionalization, is reported.