Theoretical Study of the Oxidation of Phenolates by the [Cu2O2(N,N-di-tert-butylethylenediamine)2]2+Complex

Experiments have shown that the -2:2-peroxodicopper(II) complex [Cu2O2(N,N-di-tert-butylethylenediamine)2]2+ rapidly oxidizes 2,4-di-tert-butylphenolate into a mixture of catechol and quinone and that, at the extreme temperature of 120 degrees C, a bis--oxodicopper(III)phenolate intermediate, labeled complex A, can be observed. These experimental results suggest a new mechanism of action for the dinuclear copper-containing enzyme tyrosinase, involving an early OO bond-cleavage step. However, whether phenolate binding occurs before or after the cleavage of the OO bond has not been possible to answer. In this study, hybrid density functional theory is used to study the synthetic reaction and, based on the calculated free-energy profile, a mechanism is suggested for the entire phenolate-oxidation reaction that agrees with the experimental observations. Most importantly, the calculations show that the very first step in the reaction is the cleavage of the OO bond in the peroxo complex and that, subsequently, the phenolate substrate coordinates to one of the copper ions in the bis--oxodicopper(III) complex to yield the experimentally characterized phenolate intermediate (A). The oxidation of the phenolate substrate into a quinone then occurs in three steps: 1)CO bond formation, 2)coupled internal proton and electron transfer, and 3)electron transfer coupled to proton transfer from an external donor (acidic workup, experimentally). The first of these steps is rate limiting for the decay of complex A, with a calculated free-energy barrier of 10.7kcalmol1 and a deuterium kinetic isotope effect of 0.90, which are in good agreement with the experimental values of 11.2kcalmol1 and 0.83(+/- 0.09). The tert-butyl substituents on both the phenol substrate and the copper ligands need to be included in the calculations to give a correct description of the reaction mechanism.