Studies on the mechanism of the enolization reaction of Grignard reagents with ketones

Abstract

The rates of evolution of alkane from reactions of methyl-, ethyl-, isopropyl-, and t-butyl-magnesium bromides with methyl, ethyl, and isopropyl mesityl ketones in diethyl ether were evaluated on the basis of second-order kinetics for the initial stages of the reactions analogous to addition reactions of Grignard reagents to unhindered ketones. Since the rate changes for the four alkylmagnesium bromides for a specific ketone, enolization cannot be the rate-determining step. The relative order for the Grignard reagents for any ketone, Et > Prⁱ > Me Bu^t, is explained in terms of an increasing steric effect in the transition state superimposed on the electronic effect of the Grignard alkyl group. The relative order for the alkyl mesityl ketones, Me > Et Prⁱ, is approximately the statistical order with a steric effect occurring in the isopropyl case. Removal of hydrogen from the α-carbon via a six-membered ring transition state involving the ketone and Grignard reagent is postulated for the rate-determining step, the initial stage of the reaction being co-ordination of the Grignard magnesium to the ketone oxygen in a fast step.