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-PyMe₂SiH) proceeded well in the presence of a catalytic amount of RhCl(PPh₃)₃ with virtually complete regioselectivity. By taking advantage of the phase tag property of the 2-PyMe₂Si 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-PyMe₂SiH proceeded with a Pt(CH₂= CHSiMe₂)₂O/P(t-Bu)₃ catalyst to give alkenyldimethyl(2-pyridyl)silanes in good yield with high regioselectivity. A reactivity comparison of 2-PyMe₂SiH with other related hydrosilanes (3-PyMe₂SiH, 4-PyMe₂SiH, and PhMe₂SiH) was also performed. In the rhodium-catalyzed reaction, the reactivity order of hydrosilane was 2-PyMe₂SiH ≫ 3-PyMe₂SiH, 4-PyMe₂SiH, PhMe₂SiH, indicating a huge rate acceleration with 2-PyMe₂SiH. In the platinum-catalyzed reaction, the reactivity order of hydrosilane was PhMe₂SiH, 3-PyMe₂SiH ≫ 4-PyMe₂SiH > 2-PyMe₂SiH, indicating a rate deceleration with 2-PyMe₂SiH and 4-PyMe₂SiH. It seems that these reactivity differences stem primarily from the governance of two different mechanisms (Chalk-Harrod and modified ChalkHarrod mechanisms). From the observed reactivity order, coordination and electronic effects of dimethyl(pyridyl)silanes have been implicated.