Self-protonation mechanism in the electroreduction of hydroxyimines

The electrochemical reduction mechanism of N-(p-hydroxybenzylideneaniline), N-benzylidene-p-hydroxyaniline and of the corresponding methoxy derivatives has been investigated in DMF, by cyclic voltammetry, controlled-potential electrolysis and coulometry. All the imines examined show two cathodic voltammetric peaks which correspond, for the methoxy derivatives, to two successive one-electron reduction steps while for the hydroxy substituted compounds they are attributable to the partial two-electron reduction of the substrate in the form of undissociated molecule and of its conjugate base. The latter stems from an intermolecular proton transfer from the substrate to its basic reduction intermediates, featuring a self-protonation mechanism. Kinetic analysis of the voltammetric results has allowed the electrode reduction mechanism to be fully characterised and the rate constant of the proton transfer rate-determining step to be evaluated.