Theoretical Studies of the Effects of alpha -methylation and beta- methylation on the Gas - Phase Kinetics of Thermal Decomposition of Allyl formates

Abstract

The gas phase pyrolytic reaction of allyl formate (I), its α-methylated compound (II) and β- methylated compound (III) were studied theoretically with semi-empirical PM3, hatree-fock HF/3-21G and density functional theory, DFT (B3LYP/6-31G*) methods. The decomposition of these compounds proceeds by a concerted [1, 5] hydrogen shift through a six-centered transition state (TS) geometry. The overall result of calculations shows that the reactivity of the thermal decomposition increases consequent to steric releasing effect in the transition state by the methyl group at the α-position, hence it is rate enhancing while reactivity decreases upon β-methylation, decreasing the rate of reaction. Also, it is found that rate enhancement due to C-O bond stretching in the formation of TS is more significant as a rate determinant than the acidic nature of the eliminated formyl hydrogen.

Keywords: Kinetics, hydrogen shift, allyl formate, mechanism, α-,β-methylation