Hydrosilylation of 1-Hexyne Catalyzed by Rhodium and Cobalt–Rhodium Mixed-Metal Complexes. Mechanism of Apparent Trans Addition

Hydrosilylation of 1-hexyne with triethylsilane catalyzed by Rh₄(CO)₁₂, Co₂Rh₂(CO)₁₂, Co₃Rh(CO)₁₂, and RhCl(PPh₃)₃ gives a mixture of cis-1-(triethylsilyl)-1-hexene (1a, major), its trans isomer (2a, minor), and its α-isomer (3a, minor) in excellent yield. Under optimum conditions, the yield of la increases to 96%. The cis/trans ratio depends on the concentration of catalyst as well as the substituents of hydrosilane used. It is found that the lower catalyst concentration, the higher cis/trans ratio. Triethylsilane, dimethylphenylsilane, diethylmethylsilane, and ethyldimethylsilane give thermodynamically unfavorable cis isomers as the major products, whereas chlorodimethylsilane, dichloromethylsilane, and trimethoxysilane do not give cis isomers (1) at all under the usual conditions. A mechanism is proposed to accomodate the observed unique stereoselectivity. The proposed mechanism includes first a silicon shift to the acetylenic bond and the carbene-type zwitterionic rhodium complex as the key intermediate, which undergoes isomerization from a higher energy form (Z complex) to a lower energy form (E complex) followed by reductive elimination to give the cis isomer (1) as the kinetic product.