Remarkable Substituent Effects on the Activation Energy of Silylene Insertion into Si-Cl Bond. A Theoretical Study

Abstract

A systematic DFT calculations using the 6-31++G(d,p) basis set for dimethylsilylene (:SiMe₂) insertion into Si-Cl bonds of various chlorosilanes (YR¹R²SiCl) have revealed that the insertion proceeds concertedly via three-membered cyclic transition states (TS) and the activation free energies are affected remarkably by the substituents on chlorosilanes. Effects of in-plane substituents Y located in the three-membered ring plane at TS are mostly electronic. Electron-withdrawing Y accelerates the insertion via enhanced nucleophilic interaction between silylene lone-pair orbital with antibonding sigma(Si-Y) orbital at the TS. The activation free energies correlate with sigmaI constants as a scale of inductive effect of Y. The effects of out-of-plane substituents (R¹, R²) are mainly steric; bulky substituents increase the activation free energy. The good linear correlation of the activation free energies with Taft's steric substituent constants(E_S) was observed. Interestingly, there is no significant difference in the out-of-plane substituent effects between electron-withdrawing Cl and electron donating Me.