Mechanism of thermal eliminations. Part 29. The effect of different silyl groups upon thermal elimination of ketene from ethyl silylacetates

Abstract

A series of ethyl silylacetates, R₃SiCH₂CO₂Et where R₃= Me₃, Me₂Ph, MePh₂, Ph₃, have been prepared and their rates of gas-phase thermal elimination to ketene and the corresponding ethyl silyl ether measured, each over a 50 °C temperature range. Activation energies are in the range 136–144 kJ mol^–1, and log(A/s^–1) values 10.1–10.8. Increase in the electrophilicity of silicon through replacement of one methyl group by phenyl produces a minor increase in reactivity, but further similar replacements eventually produce a rate decrease, which is attributed to steric hindrance. Overall the kinetic data indicate that the strength of the silicon–oxygen bond formed is sufficiently important that alteration in the ester structure is incapable of producing substantial variation in the rate of ketene formation.