Aromatic hydroxylation of anthracene derivatives by a chromium(iii)-superoxo complex via proton-coupled electron transfer

The chemistry of metal-superoxo intermediates started being unveiled in oxidation reactions by enzymes and their synthetic model compounds. However, aromatic hydroxylation reactions by the metal-superoxo species are yet to be demonstrated. In this study, we report for the first time that the hydroxylation of aromatic compounds such as anthracene and its derivatives by a mononuclear nonheme Cr(iii)-superoxo complex, [(Cl)(TMC)Cr-III(O-2)](+) (1), occurs in the presence of triflic acid (HOTf) via the rate-determining proton-coupled electron transfer (PCET) from anthracene to 1, followed by a fast further oxidation to give anthraquinone. The rate constants of electron transfer from anthracene derivatives to 1 in the presence of HOTf are well analyzed in light of the Marcus theory of electron transfer.