Diverse catalytic efficiency of nitroxyl radicals tuned by Lewis acids in the oxidation of hydrocarbons

• Lewis acids contrarily affect the catalytic efficiency of nitroxyl radicals.
• TEMPO analogues are activated, but PINO analogues are inactivated by Lewis acids.
• TEMPO with strong but less hindrance Lewis acids shows the highest catalytic activity.
• The remarkably altered features of organocatalysts account for their activity.

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.

Promotion or Depression? Organocatalyst nitroxyl radicals like TEMPO show improved activity, whereas ones similar to PINO display decreased performance by combination with Lewis acids.Figure optionsDownload as PowerPoint slide