Cu/TEMPO Alcohol Oxidation Follows an Open-Shell HAT Mechanism.

The mechanism of Cu/aminoxyl-catalyzed aerobic alcohol oxidation has been widely debated. Two pathways have been proposed: hydride transfer to an oxoammonium-like intermediate or hydrogen atom transfer (HAT) to the aminoxyl radical followed by single-electron transfer (SET). Using ab initio (CASSCF/MR-DDCI3) methods, we show that the reactive Cu intermediate is dominated by an open-shell singlet Cu(II)-aminoxyl configuration (|J| ≈ 3400 cm-1). Intrinsic bond orbital analysis reveals a stepwise HAT/SET mechanism, with the two electrons of the C-H bond traveling to different acceptors in succession. These findings reconcile all experimental observations, and provide a unified mechanistic basis for the observed redox cooperativity and selectivity across Cu/aminoxyl catalyst systems.