Insertion of Silylenes into Si–H and Si–Cl Bonds. Comparison of Mechanism and Substituent Effects

Abstract

Electronic and steric substituent effects on the insertion reactions of dimethylsilylene (1) and 2,2,5,5-tetrasilylsilacyclopentane-1,1-diyl (2′) into Si–H bonds are compared with those into Si–Cl bonds using DFT calculations at the B3LYP/6-31++G(d,p) level. For both 1 and 2′, the ΔG⧧ value for the insertion into a Si–H bond of SiH4 is close to and only ca. 1.5 kcal mol–1 lower than that into the Si–Cl bond of H3SiCl. Effects of in-plane substituents on the ΔG⧧ values for both Si–H and Si–Cl insertion reactions are mainly electronic and electron-withdrawing substituents lower the ΔG⧧ values. Sensitivity of the Si–H insertion to the substituent effects is similar to that of the Si–Cl insertion. Effects of out-of-plane substituents are largely steric, and their sensitivity for the Si–H insertion is close to that for the Si–Cl insertion. The ΔG⧧ values for the insertion reactions of sterically bulky 2′ are significantly larger than those of 1. The puzzling prior observation that a bulky isolable silylene inserts into the Si–Cl bond of H2SiCl2 and the Si–H bond of Me2SiHCl is explained on the basis of the substituent effects on the insertion reactions.