Oxidation mechanism of phenols by dicopper-dioxygen (Cu2/O2) complexes

The first systematic studies on the oxidation of neutral phenols (ArOH) by the (mu-eta(2):eta(2)-peroxo)-dicopper(II) complex (A) and the bis(mu-oxo)dicopper(III) complex (B) supported by the 2-(2-pyridyl)ethylamine tridentate and didentate ligands L-Py2 and L-Py1, respectively, have been carried out in order to get insight into the phenolic O-H bond activation mechanism by metal-oxo species. In both cases (A and B), the C-C coupling dimer was obtained as a solely isolable product in similar to50% yield base on the dicopper-dioxygen (Cu-2/O-2) complexes, suggesting that both A and B act as electron-transfer oxidants for the phenol oxidation. The rate-dependence in the oxidation of phenols by the Cu-2/O-2 complexes on the one-electron oxidation potentials of the phenol substrates as well as the kinetic deuterium isotope effects obtained using ArOD have indicated that the reaction involves a proton-coupled electron transfer (PCET) mechanism. The reactivity of phenols for net hydrogen atom transfer reactions to cumylperoxyl radical (C) has also been investigated to demonstrate that the rate-dependence of the reaction on the one-electron oxidation potentials of the phenols is significantly smaller than that of the reaction with the Cu2O2 complexes, indicative of the direct hydrogen atom transfer mechanism (HAT). Thus, the results unambiguously confirmed that the oxidation of phenols by the Cu2O2 complex proceeds via the PCET mechanism rather than the HAT mechanism involved in the cumylperoxyl radical system. The reactivity difference between A and B has also been discussed by taking account of the existed fast equilibrium between A and B.