Mechanistic Insight on the Aerobic Oxidation of Alcohols by Trivalent Copper from DFT Calculations

Density functional theory (DFT) calculations were carried out to elucidate the mechanism of alcohol oxidation by a Cu(III) anilidosalen complex. The study considered singlet, broken-symmetry singlet, and triplet states. During the first step the alcoholate binds to the copper, inducing valence tautomerism, transforming the Cu(III) complex into a Cu(II)-alkoxyradical adduct. Subsequently, an “aniline” ring abstracts a hydrogen from the substrate with a low barrier, yielding a Cu(I) aniline complex and the aldehyde, akin to galactose oxidase. Catalyst re-oxidation is coupled to dioxygen reduction. Initially, dioxygen is reduced by Cu(I) into superoxide, which binds to the metal. Protonation then yields either a Cu(II)-hydrosuperoxo or a Cu(III)-hydroperoxo adduct. Further protonation closes the catalytic cycle by releasing hydrogen peroxide.