Metal-Catalyzed Hydrosilylation of Alkenes and Alkynes Using Dimethyl(pyridyl)silane

Metal-catalyzed hydrosilylation of alkenes and alkynes using dimethyl(pyridyl)silane is described. The hydrosilylation of alkenes using dimethyl(2-pyridyl)silane (2-PyMe<sub>2</sub>SiH) proceeded well in the presence of a catalytic amount of RhCl(PPh<sub>3</sub>)<sub>3</sub> with virtually complete regioselectivity. By taking advantage of the phase tag property of the 2-PyMe<sub>2</sub>Si group, hydrosilylation products were isolated in greater than 95% purity by simple acid−base extraction. Strategic catalyst recovery was also demonstrated. The hydrosilylation of alkynes using 2-PyMe<sub>2</sub>SiH proceeded with a Pt(CH<sub>2</sub>CHSiMe<sub>2</sub>)<sub>2</sub>O/P(<i>t</i>-Bu)<sub>3</sub> catalyst to give alkenyldimethyl(2-pyridyl)silanes in good yield with high regioselectivity. A reactivity comparison of 2-PyMe<sub>2</sub>SiH with other related hydrosilanes (3-PyMe<sub>2</sub>SiH, 4-PyMe<sub>2</sub>SiH, and PhMe<sub>2</sub>SiH) was also performed. In the rhodium-catalyzed reaction, the reactivity order of hydrosilane was 2-PyMe<sub>2</sub>SiH ≫ 3-PyMe<sub>2</sub>SiH, 4-PyMe<sub>2</sub>SiH, PhMe<sub>2</sub>SiH, indicating a huge rate acceleration with 2-PyMe<sub>2</sub>SiH. In the platinum-catalyzed reaction, the reactivity order of hydrosilane was PhMe<sub>2</sub>SiH, 3-PyMe<sub>2</sub>SiH ≫ 4-PyMe<sub>2</sub>SiH > 2-PyMe<sub>2</sub>SiH, indicating a rate deceleration with 2-PyMe<sub>2</sub>SiH and 4-PyMe<sub>2</sub>SiH. It seems that these reactivity differences stem primarily from the governance of two different mechanisms (Chalk−Harrod and modified Chalk−Harrod mechanisms). From the observed reactivity order, coordination and electronic effects of dimethyl(pyridyl)silanes have been implicated.