Conformational analysis of the cyclopropylacyl, oxiranylacyl, and aziridinylacyl radicals by electron spin resonance spectroscopy

Abstract

A series of ring-substituted cyclopropylacyl, oxiran-2-ylacyl, and aziridin-2-ylacyl radicals have been prepared principally by the reaction of photolytically generated t-butoxyl radicals with the corresponding aldehydes. The e.s.r. spectra show that the cyclopropylacyl σ-radicals exist in s-cis- and s-trans-conformations of approximately equal stability, in which the plane of the acyl group bisects the ring (as it does in the parent aldehyde), and simulation of the spectra through the region of intermediate rates of exchange show that the barrier to rotation is ca. 17.5 kJ mol^–1. The behaviour of the trans-2-ethoxycarbonylcyclopropylacyl and 2,2-dimethylcyclopropylacyl radicals is similar. The oxiranylacyl and trans-3-methyloxiranylacyl radicals exist in the same two conformations with a rather lower barrier, and the N-alkylaziridinylacyl radicals appear to have a lower barrier still.