A Kinetic View of the Mechanism of the Grignard Reaction with Alkoxysilanes

The kinetics of the reaction of n-butyl- and isopropylmagnesium chloride with alkyltriethoxysilanes (RSi(OEt)(3)) in diethyl ether were determined. The rate constants correlate well with the steric parameters E-s(Si). The resulting steric factors delta fit the range of available susceptibility values for the reactions of organosilicon compounds. The thermodynamic activation parameters for six reactions of ethoxysilanes with alkylmagnesium chlorides in dibutyl ether were determined. The entropy values indicated that two Grignard molecules are involved in the transition state of the reaction. From the correlation analysis of the activation enthalpies for the reactions, it appeared that the inductive effect controls the rate of replacement more considerably than steric requirements in the transition state.