1,2-Migration in β-(acyloxy)ethyl radicals revisited—concerted or stepwise?

Abstract

The acyloxy rearrangement in 2-(acetyloxy)-2-methyl-1-propyl radical (1a) and in 2-(trifluoroacetyloxy)-2-methyl-1-propyl radical (1b) has been investigated with a number of theoretical methods. In both systems the most favorable reaction pathway for 1,2-acyloxy rearrangement leads through a five-membered ring transition state in a concerted fashion. A second pathway through a three-membered ring transition state is only slightly less favorable, while the addition–elimination process through a cyclic 1,3-dioxolan-2-yl radical intermediate has significantly higher barriers. Stationary points corresponding to a contact ion pair could not be found. With respect to the most favorable reaction pathway, the barrier difference between substrates 1a and 1b amounts to 2.5 ± 0.1 kcal mol⁻¹ at a variety of theoretical levels. Solvent effects for the concerted pathways in hydrocarbon solvents, alcohols, and water have been calculated using the PCM scheme and found to be of rather limited magnitude. The barrier difference for reaction of 1a in hydrocarbon solvents (cyclohexane, benzene) and in water was estimated to be around 1 kcal mol⁻¹. Based on kinetic isotope effects calculated for the [3,2]- and [1,2]-acyloxy rearrangement processes a differentiation of pathways is most easily possible through the strongly inverse deuterium isotope effects for d₂-labeling of the radical center in 1 and through the ratio of the ¹⁸O isotope effects on the carboxylate oxygen atoms.