Mechanism of electrooxidation of substituted phenols in aqueous solutions: Some podophyllotoxin derivatives as models

Etoposide (1) and teniposide (2) proved to be suitable models for oxidation of phenols, because radicals formed by the one‐electron oxidation of their phenolates do not dimerize. Mechanism (1)‐(6) indicates in which pH range the oxidation occurs in a single two‐electron step and in which in two one‐electron transfers. The phenolates are more easily oxidized than phenols. Due to a rapid formation of the phenolate ions at the electrode surface, its oxidation is preferred even at pH 3, about seven pH units below the pKa value. Phenoxenium ions formed in the two‐electron oxidation eliminate CH3O− ions and form ortho‐quinones.