Nitroxide radicals during photolysis

Photochemical dissociation of Ph CN (368) into Ph C and radicals is found to be a minor process. Formation of nitroxide radicals in photolysis of aromatic nitro and nitroso compounds (235-238) is probably also a minor but concerted process. Nitroanions (218) have been observed during photolysis of basic aqueous or alcoholic solutions... 

The tert-butoxy radical is liberated by photolysis of XXXVIa (Scheme 9). Its formation was confirmed by reactions which produce more stable radicals detectable by the ESR method135. The heptane solution of XXXVIa was irradiated by light 360—480 nm in the presence of 2,6-di-tert-butyl-4-methylphenol or 2,4,6-tri-tert-butylphenol. These sterically hindered phenols were transformed by the liberated tert-butoxyl into phenoxyls. Their identification by the ESR method is the indirect evidence of tert-BuO formation. The direct evidence was achieved by means of nitro-sobenzene CVI (R=H) and nitrosodurene CVI (R=Me) as spin-traps. The nitroxide radical CVII (R=H or Me) is formed in both cases and was identified by ESR spectrum135. This spin-adduct is unstable. The intensity of its spectrum gradually decreases during irradiation and the spectrum of another radical appears which is less intense but invariable in time. Judging from its hyperfine structure, it may be formulated as CVIII (R=H or Me). More detailed data on the character of substituents RJ-R2 is not yet known. 

The chromophore must be attached close to the nitroxide moiety to ensure an efficient intramolecular energy transfer from the chromophore to the NO-C bond which should also undergo homolysis in the macroalkoxyamine. However, direct attachment of the aromatic chromophore to the alkoxyamine nitrogen, as in 44, leads to competitive cleavage of the N-OC bond because the resulting aminyl radical is resonance stabilized (see Section 6.3). On the other hand, alkoxyamine 45 undergoes a selective NO-C photolysis. Linear increase of Mn with conversion was observed during NMPP of -butyl acrylate with 45, as expected for controlled polymerization (Scheme 4.18). ... 

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