Oxidation of methoxybenzenes to p-benzoquinones catalyzed by methyltrioxorhenium(VII)

Methoxy-substituted benzenes 1 are oxidized with aqueous hydrogen peroxide catalyzed by methyltrioxorhenium (VII) in acetic acid to yield the p-benzoquinones 3 in moderate yields. An intermediary diperoxo rhenium(VII) complex rather than peracetic acid is the dominating oxidizing species, since oxidation also proceeds in ethanol under peracid-free conditions. Acid played an important role, especially in the oxidation of p-menzoquinones 2 to p-benzoquinones 3. An arene oxide mechanism is postulated for the formation of p-benzoquinones, which would account for the participation of the acid and also overoxidation by cleavage of the arene oxide ring with hydrogen peroxide.