Abstract
Aerobic oxidation of α-isophorone catalyzed by 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) has achieved comparable performance to that catalyzed by phthalimide-N-oxyl (PINO) with the assistance of Lewis acids (LAs), but the latter efficiency is depressed by LAs. Remarkably changed structural and thermochemical properties of nitroxyl radicals induced by LAs cause the diversely altered reactivity. Persistent and nonconjugated nitroxyl radicals with alkyl groups around N–O parts like TEMPO can be activated, whereas non-persistent ones, with N–O parts conjugated with carbonyl groups like PINO are deactivated by LAs. TEMPO analogues combined with strong but less steric-hindrance LAs exhibit the highest catalytic activity.
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