The hindered secondary amines, 2,2,6,6-tetramethyl-4-oxopiperidine (II) and 1,9-diaza-2,2,8,8,10,10-hexamethyl-4-oxo-spiro[5.5]undecane (VI), were oxidized with m-chloroperbenzoic acid, and the corresponding hydroxylamine (III) and N-oxyl-hydroxylamine (VIII), respectively, were isolated. The further oxidation of III and VIII with the peracid afforded the corresponding nitroxide radical IV and IX, in good yields respectively. The stoichiometry of the oxidation reaction of the hydroxylamine III to the nitroxide radical IV was elucidated on the basis of the ESR signal intensities of oxidation products at various molar ratios with the peracid and III. An ESR study of the 1-15N-labelled N-oxyl-hydroxylamine (XII) indicated that there was an intramolecular hydrogen transfer between >N–OH and >N–O · groups in the N-oxyl-hydroxylamines VIII and XII.
The reaction of the extremely stable free radical, 2,2,6,6-tetramethyl-4-oxopiperidine-1-oxyl (I), was carried out with benzenethiols (IV). The formation of diphenyl disulfides (VI), benzenesulfinic acids (VII), and 1-(p-substituted benzenesulfinyl)-2,2,6,6-tetramethyl-4-oxopiperidines (X) from the above reaction shows that the radical (I) can act as a hydrogen-abstracting agent, an oxidizing agent, and a radical scavenger. Possible mechanisms for the coupling reaction of the radical (I) with the S-radicals (V) to give the sulfinamides (X) were discussed. A substituent effect was observed in the formation of the disulfides (VI). The N-oxyl radical (I) and another stable free radical, such as 2,6-di-t-butyl-4-methylphenoxyl radical (XXV), were shown to react with the S-radical (XVII) derived from thiamine (XIV) to give new types of thiamine derivatives, S-(2,2,6,6-tetramethyl-4-oxopiperidino) thiamine (XX) and S-(2,6-di-t-butyl-4-methylphenoxy)thiamine (XXVI) respectively.
We have isolated coupled products of such stable free radicals as 2,2,6,6-tetramethyl-4-oxopiperidine-1-oxyl (II), cyclohexane-1-spiro-2′-(4′-oxoimidazolidine-l′-oxyl)-5′-spiro-l″-cyclohexane (IV) and 2,2,6,6-tetramethyl-4-oxopiperidine-1-thiyl derived from bis-[2,2,6,6-tetramethyl-4-oxopiperidyl-(1)]disulfide (VII) with a C-radical derived from α,α′-azo-bis(isobutyronitrile). The reactive site in these stable free radicals was studied from the structure of the coupled products. The homolysis of the disulfide (VII) appeared to be accelerated by an attack of the C-radical, 1-methyl-1-cyanoethyl. The NMR spectra of the products suggested a slow inversion of the nitrogen atom in piperidine derivatives.
ly 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.
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