Electrochemical and homogeneous proton-coupled electron transfers: concerted pathways in the one-electron oxidation of a phenol coupled with an intramolecular amine-driven proton transfer.

Proton-coupled electron transfers currently attract considerable attention in view of their likely involvement in many natural processes. Electrochemistry, through techniques such as cyclic voltammetry, is an efficient way of investigating the reaction mechanism of these reactions, and deciding whether proton and electron transfers are concerted or occur in a stepwise manner. The oxidation of an ortho-substituted 4,6-di (tert-butyl)-phenol in which the phenolic hydrogen atom is transferred during the reaction to the nitrogen atom of a nearby amine is taken as illustrative example. A careful analysis of the cyclic voltammetric responses obtained with this compound and its OD derivative allows, after estimation of the various thermodynamic parameters, ruling out the occurrence of the square scheme mechanism involving the proton-electron and electron-proton sequences. Simulation and comparison of the rate constant and H/D kinetic isotope effect with theoretical predictions show that the experimental value of the preexponential factor is ca. 1 order of magnitude larger than the theoretical value. Detailed calculations suggest that an electric field effect is responsible for this discrepancy.