Hirashima, T; Manabe, O; Hiyama, H
An initial step of photodecomposition of azobenzenes (H, H; H, NEt2; H, NMe2; H, OMe) was studied; Azobenzenes were irradiated with low or high pressure mercury lamps in methanol, benzene, and cyclohexane in the presence of β–naphthol under nitrogen atmosphere. The generation of diazonium ion or diazo radical from azo species was suggested from the formation of l–phenylazo–2–naphthol, which was identified with authentic material by mass spectrometry. The product may be formed by the coupling of the diazonium ion or diazo radical with β–naphthol. As the generation of two kinds of diazonium ion or diazo radical from 4,4′–asym–substituted azobenzene was expected, accordingly two kinds of azo dyes were anticipated to be obtained, but only one azo dye which has the diazo component derived from relatively the more electronegative substituent in aryl group of the original azobenzene was detected. The substituent effects in the photodecomposition of azobenzenes were found to be similar to those in the fragmentation of the azobenzenes (H, NEt2; H, NMe2; H, OMe; H, NHCOMe; H, NH2; Cl,NEt2; Me, NEt2; NH2, NEt2; MeO, NEt2; 2 Br(o,p), NEt2) to diazonium ions in mass spectra. A Hammett's relation was found between the ratio of the intensity of fragments of the two diazo species in mass spectra of the azobenzenes and the difference of σ (= Δσ). The more electronegatively substituted diazonium ion or diazo radical was found to be produced more preferentially, and the result accounts well for the formation of 1–arylazo–2–naphthol. A photo–redox reaction was observed in the irradiation of azobenzene in cyclohexane with low pressure mercury lamp and dicyclohexyl and hydrazobenzene were obtained. In the case of N, N–dialkylaminoazobenzenes, photodealkylation of N–alkyl group proceeded and the diazonium ion was hardly detected. 1973, The Chemical Society of Japan. All rights reserved.