Photochemical and thermal reactions of heterocycles. Part I. Photolysis, thermolysis, and mass spectral fragmentation of mesoionic 1,2,4-triazol-3-ones

Abstract

Photolysis of the mesoionic 1,2,4-triazol-3-ones (7a–d) gave the corresponding benzimidazoles, azobenzenes, and aryl isocyanates. A mechanism is postulated which involves consecutive valence isomerisations of the triazolone (7) to give the bicyclic intermediate (14) and then the isomeric triazolone (15), followed by fragmentation. An oxidative cyclisation involving the phenyl substituents occured on irradiation of the triphenyl-triazolone (7e). Pyrolysis of the triaryltriazolones (7e and f) gave the corresponding N-(diarylmethylene)anilines (18) by N → C migration of the N(1)-substituent. These results are compared with the high resolution mass spectra of the triazolones (7a–e).