Electrocatalytic Alcohol Oxidation with TEMPO and Bicyclic Nitroxyl Derivatives: Driving Force Trumps Steric Effects

Abstract

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 primary and secondary 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.