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.
Photolysis of the hindered N-Chloroamines, 1-chloro-2,2,6,6-tetramethyl-4-oxopiperidine (IIa), 1-chloro-2,2,6,6-tetramethylpiperidine (IIb), and 1-chloro-2,2,5,5-tetramethyl-4-oxoimidazolidine (IIc), in benzene solution were carried out in an ESR spectrometer cavity. The ESR spectra of the corresponding amino radicals IIIa, IIIb, and IIIc were observed in evacuated solutions. In solutions containing oxygen, it was found that amino radicals IIIb and IIIc, readily reacted with oxygen to give the corresponding stable nitroxide radicals IVb and IVc, from the shapes of spectra and g-values. Amino radical IIIa did not react with oxygen. Although the amino radicals could not be isolated, their formation was confirmed by the isolation of a coupling product with a benzyl radical generated from dibenzylmercury.
New hindered 3,5-dioxopiperazines (VII, IX), hindered piperazines (VIII, X), and the corresponding N-oxyls (XI, XIII, XII, and XIV) were synthesized. The substituent and solvent effects of the nitrogen coupling constants (aN) in the N-oxyls were measured and compared with each other; the introduction of carbonyl groups at the C-3 and C-5 ring positions decreased these values, which increased in a protic polar solvent.
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