Mechanism of the Copper/TEMPO-Catalyzed Aerobic Oxidation of Alcohols

Identifying the mechanism of a catalytic reaction is paramount for designing new and improved catalysts. Several alternative catalytic cycles for the copper/2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO)-catalyzed aerobic oxidation of alcohols to the corresponding aldehydes or ketones were examined using DFT at the SMD(CH₃CN)-RIJCOSX-DSD-PBEB95/def2-TZVP//DF-PBE_D3BJ/def2-SVP level of theory. A catalytic cycle in which TEMPO remains coordinated to copper throughout was identified as the most likely mechanism. There are three components to the catalytic cycle: 1) hydrogen transfer from the alkoxyl ligand to coordinated TEMPO, 2) oxygen activation with formation of a peroxo complex, and 3) alcohol activation with transfer of the OH proton to the peroxo ligand. The oxidation takes place via a six-membered intramolecular hydrogen-transfer transition state. Importantly, this is not the rate-determining step, which instead involves oxygen activation and/or the initial alcohol activation.