Chemistry: Tyrosinase reactivity in a model complex: An alternative hydroxylation mechanism

The binuclear copper enzyme tyrosinase activates O₂ to form a μ-η²:η²-peroxodicopper(II) complex, which oxidizes phenols to catechols. Here, a synthetic μ-η²: η²-peroxodicopper(II) complex, with an absorption spectrum similar to that of the enzymatic active oxidant, is reported to rapidly hydroxylate phenolates at -80°C. Upon phenolate addition at extreme temperature in solution (-120°C), a reactive intermediate consistent with a bis-μ-oxodicopper(III)-phenolate complex, with the O-O bond fully cleaved, is observed experimentally. The subsequent hydroxylation step has the hallmarks of an electrophilic aromatic substitution mechanism, similar to tyrosinase. Overall, the evidence for sequential O-O bond cleavage and C-O bond formation in this synthetic complex suggests an alternative intimate mechanism to the concerted or late stage O-O bond scission generally accepted for the phenol hydroxylation reaction performed by tyrosinase.