Studies on Stable Free Radicals. IV. Decomposition of Stable Nitroxide Radicals

Decomposition studies of extremely stable free radicals such as 2,2,6,6-tetramethyl-4-oxopiperidine-1-oxyl (I), 2,2,6,6-tetramethylpiperidine-1-oxyl (VIII) and 2,2,6,6-tetramethyl-4-hydroxypiperidine-1-oxyl (IX) were carried out: although radical I decomposed to give 1-hydroxy-2,2,6,6-tetramethyl-4-oxopiperidine (IV) both in the light and dark, radicals VIII and IX were more stable under these reaction conditions. Therefore it was confirmed that i) a source of hydrogen was the ketonic α-methyene hydrogens, ii) a radical which could give a thermodynamically stable conjugated product by a cycloelimination was easily decomposed and iii) the decomposition of radical I was only accelerated by heating and was independent of the action of light.