A comparison between nitroxide and hydrazyl free radicals in selective alcohols oxidation

• Properties of nitroxide free radicals compared with hydrazyl free radicals.
• Nitroxide radicals better catalysts in selective aerobic oxidations.
• Distinct oxidation mechanism for nitroxide and hydrazyl.

Literature data are abundant in oxidations catalyzed by nitroxides free radicals, while hydrazyls free radicals are very seldom encountered. In this work we made a comparison between some nitroxides and hydrazyl free radicals towards the ability to selectively oxidize activated alcohols to the corresponding carbonyl derivatives. As nitroxides we used 2,2,6,6-tetramethylpirrolidin-N-oxide (TEMPO) and phtalimide-N-oxyl (PINO) free radicals, while as hydrazyls were used 2,2-diphenyl-1-picrylhydrazyl (DPPH) and 2,2-p-nitrophenyl-1-picrylhydrazyl (DN-DPPH). The differences in the physico-chemical properties between nitroxides and hydrazyls were evaluated by UV–vis, EPR and cyclic voltammetry. For DN-DPPH the X-ray crystal structure was resolved. It has been shown that a nitroxide radical is a far better catalyst in such aerobic oxidations comparatively with a hydrazyl one. The explanation of these results consists firstly in a different mechanism involved in the oxidation procedures and secondly in the variation of the oxidation potentials of nitroxides comparatively with hydrazyls.

The two proposed mechanisms of alcohol oxidation, involving a nitroxide (I) and a hydrazyl (II) free radicalFigure optionsDownload high-quality image (46 K)Download as PowerPoint slide