Anodic oxidation of Schiff bases. Part I. Oxidation of N-benzylidene-p-anisidines in acetonitrile

Abstract

The anodic oxidations of several N-benzylidene-p-anisidines have been investigated by cyclic voltammetry and controlled potential electrolysis at a glassy-carbon electrode in acetonitrile. All the compounds tested were oxidized irreversibly. N-Benzylidene-p-anisidines showed two anodic waves. The relationship between the peak potentials of their first wave and the σ⁺ values of the substituents on the benzylidene group was linear, whereas that for the second wave was not. On controlled potential electrolysis at the first wave, protonated benzylidene-p-anisidines were identified, and on electrolysis at the potential of the second wave, p-benzoquinone imine and the corresponding benzaldehyde were identified as the main products. When water was added to the solution, the second wave shifted to a lower potential, whereas the potential of the first wave remained unchanged. Hence a combined wave was observed in the presence of sufficient water. On controlled potential electrolysis in the presence of 5% water, p-benzoquinone and the corresponding benzaldehyde were formed as the main products. Analysis of the minor product suggested that the p-anisidine radical cation was formed as an intermediate. A possible reaction mechanism is suggested.