Comments on the mechanism of one-electron oxidation of phenols: a fresh interpretation of oxidative coupling reactions of plant phenols

Abstract

Fieser's critical oxidation potentials can be used to predict the relative ease of 1-electron abstraction from phenols, but the equation for E_crit is pH dependent. For further oxidation to cations (ArO)⁺, E⁺_crit is not pH dependent and so there should be an equipotential pH below which ArO· is metastable with respect to (ArO)⁺ and (ArO)^–. Oxidative C–O–C coupling and aryloxy-polymer formation is ascribed to reactions of ArO· radicals and many C–C coupling reactions to electrophilic attack on phenol molecules by (ArO)⁺ cations.

It is suggested that in plants those biochemical oxidations of phenols which lead to Aryl–O–Aryl structures probably involve the one-electron oxidation of aryloxy-anions to ArO· radicals which then dimerise, but that those oxidations which lead to aryl–aryl coupled products involve the electrophilic attack on phenol molecules by (ArO)⁺ cations. The diversity of the reaction paths which have now been established for biological oxidations of phenols in plants may be due to the involvement of two factors, (a) the oxidation potential of the reagent or active enzyme and (b) the pH of the reacting mixture, in the selection of the preferred oxidation route.