Bicyclic nitroxyl derivatives, such as 2-azaadamantane N-oxyl (AZADO) and 9-azabicyclo[3.3.1]nonane N-oxyl (ABNO), have emerged as highly effective alternatives to TEMPO-based catalysts for selective oxidation reactions (TEMPO = 2,2,6,6-tetramethyl-1-piperidine N-oxyl). Their efficacy is widely attributed to their smaller steric profile; however, electrocatalysis studies described herein show that the catalytic activity of nitroxyls is more strongly affected by the nitroxyl/oxoammonium redox potential than by steric effects. The inexpensive, high-potential TEMPO derivative, 4-acetamido-TEMPO (ACT), exhibits higher electrocatalytic activity than AZADO and ABNO for the oxidation of 1° and 2° alcohols. Mechanistic studies provide insights into the origin of these unexpected reactivity trends. The superior activity of ACT is especially noteworthy at high pH, where bicyclic nitroxyls are inhibited by formation of an oxoammonium-hydroxide adduct.

A new method for Co/NHPI-catalyzed aerobic C–H oxygenation shows excellent tolerance of electronically diverse heterocycles.

Copper/aminoxyl species are proposed as key intermediates in aerobic alcohol oxidation. Several possible electronic structural descriptions of these species are possible, and the present study probes this issue by examining four crystallographically characterized Cu/aminoxyl halide complexes by Cu K-edge, Cu L-edge, and Cl K-edge X-ray absorption spectroscopy. The mixing coefficients between Cu, aminoxyl, and halide orbitals are determined via these techniques with support from density functional theory. The emergent electronic structure picture reveals that Cu coordination confers appreciable oxoammonium character to the aminoxyl ligand. The computational methodology is extended to one of the putative intermediates invoked in catalytic Cu/aminoxyl-driven alcohol oxidation reactions, with similar findings. Collectively, the results have important implications for the mechanism of alcohol oxidation and the underlying basis for cooperativity in this co-catalyst system.