Tertiary : secondary : primary C–H bond relative reactivity in the one-electron oxidation of alkylbenzenes. A tool to distinguish electron transfer from hydrogen atom transfer mechanisms

Abstract

Data of tertiary: secondary: primary C–H bond relative reactivity (TSP selectivity) for a number of electron transfer (ET) and hydrogen atom transfer (HAT) reactions of alkylbenzenes have been critically reviewed and in a few cases supplemented by additional experiments. The resulting picture indicates that there are significant differences in TSP selectivity between ET and HAT reactions. When the HAT mechanism is operating the reactivity order tertiary > secondary > primary C–H bond is always observed. This order never holds in reactions occurring by an ET mechanism where, generally, the secondary C–H bond is the most reactive one and the tertiary centre can be either more or even less reactive than the primary one. Whatever the possible reasons for these differences, it turns out that TSP C–H bond selectivity determinations can afford useful information with respect to the distinction between ET and HAT mechanisms in the oxidations of alkylbenzenes. To check this conclusion a study of TSP selectivity in the oxidation of alkylbenzenes promoted by metalloporphyrins and by microsomal cytochrome P-450 has been carried out, which has allowed us to assign a HAT mechanism to these reactions, in full accord with previous attributions.