Oxidations Mediated by TEMPO and Related Stable Nitroxide Radicals Anelli Oxidation

Oxidations Mediated by TEMPO and Related Stable Nitroxide Radicals (Anelli Oxidation) 

During the 70 s, Celia et al. treated the hindered secondary amine 52 with / -chloroperbenzoic acid, with the intention of transforming it into the nitroxide 53.1 Unexpectedly, the oxidation of the amine functionality was accompanied by the transformation of the alcohol moiety into a ketone, resulting in the formation of compound 54. 

As peracids react very sluggishly with alcohols, it was apparent that the presence of a nitroxide was playing an important role in the oxidation of the alcohol into a ketone. This seminal serendipitous observation led to the development of the first description of the oxidation of alcohols mediated by catalytic 2,2,6,6-tetramethylpiperidine-l-oxyl (TEMPO) (55), published almost simultaneously by Celia et al and Ganem.3 These authors presented two papers with remarkably similar contents, in which alcohols were oxidized by treatment with MCPBA in CH2CI2 at room temperature in the presence of a catalytic amount of TEMPO (55). In both papers, a plausible mechanism is presented, whereby m-chloroperbenzoic acid oxidizes TEMPO (55) to an oxoammonium salt 56. This oxoammonium salt 56, as detailed in Ganem s paper, can react with the alcohol producing an intermediate 57, which can deliver a carbonyl compound by a Cope-like elimination. 

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