Cationic Rhodium Complex-Catalyzed Highly Selective Dehydrogenative Silylation of Styrene

Cationic rhodium complex-catalyzed dehydrogenative silylation of styrene with hydrosilanes was studied. Dehydrogenative silylation was competitive with hydrosilylation. The selectivity was strongly dependent on the reaction temperature and the molar ratio of styrene to hydrosilane. The selectivity of dehydrogenative silylation was 92% when using 3 equiv of styrene to 1 equiv of triethylsilane ( 1a ) in refluxing diethyl ketone. The nature of the hydrosilane also affected the reaction. As the steric bulk of hydrosilane increased, the selectivity of dehydrogenative silylation increased. Dehydrogenative silylation product ( 2d ) was obtained exclusively in 81% yield when using triisopropylsilane ( 1d ). The amount of ligand had a large influence on the selectivity of the reaction. As the amount of PPh 3 used increased, the selectivity of dehydrogenative silylation increased. These results were reasonably explained according to the mechanism involving the insertion of styrene into the rhodium−silicon bond which was formed by the oxidative addition of hydrosilane to rhodium complex.