Four-Electron Oxidation of Phenols to <i>p</i>‑Benzoquinone Imines by a (Salen)ruthenium(VI) Nitrido Complex

Proton-coupled electron-transfer reactions of phenols have received considerable attention because of their fundamental interest and their relevance to many biological processes. Here we describe a remarkable four-electron oxidation of phenols by a (salen)­ruthenium­(VI) complex in the presence of pyridine in CH3OH to afford (salen)­ruthenium­(II) p-benzoquinone imine complexes. Mechanistic studies indicate that this reaction occurs in two phases. The first phase is proposed to be a two-electron transfer process that involves electrophilic attack by RuN at the phenol aromatic ring, followed by proton shift to generate a Ru­(IV) p-hydroxyanilido intermediate. In the second phase the intermediate undergoes intramolecular two-electron transfer, followed by rapid deprotonation to give the Ru­(II) p-benzoquinone imine product.