Photolytic and Free-radical Reactions

Free-radical and Photolytic Reactions.—In the course of their systematic study of the free-radical phenylation in the heterocyclic series, Metzger et have examined the behaviour of isothiazole and its 3-, 4-, and 5-methyl homologues from this point of view. The reaction is induced by benzoyl peroxide in the presence of traces of copper at 110 C, and the proportion of the resulting phenylated isomers was estimated by gas chromatography. Analysis of the extensive numerical results shows that the radical activity of isothiazole and its homologues exceeds that of thiazole. The observed free-valency distribution and the radical localization energies are similar for the 3- and 5-positions in isothiazole the observations conflict with the theoretical calculations, and are rationalized by introducing suitable corrections. 

Preliminary observations on the photochemical behaviour of isothiazoles have been briefly reported. The parent base is partially converted into thiazole on irradiation (7% in propylamine, 1% in ether), several unidentified products being formed (15%) side by side. 3-Phenyl- and 3,5-diphenyl-isothiazole are converted into 4-phenyl- (12%) and 2,4-diphenyl-thiazole (48%), respectively, but 5-phenylisothiazole preserves its ring system, merely producing small quantities (2.3%) of the 3-phenyl isomer. The reactions are believed to proceed by way of tricyclic sulphonium cations of the type previously postulated by Kojima and Maeda. It is recalled that the reverse photoisomerization is also on record (see syntheses, above). 

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