Photoreaktionen von 1‐Alkylbenztriazolen

Abstract

The irradiation of benzotriazoles ( cf. Scheme 2 ) with light of 225–325 nm in protic and in aromatic solvents was investigated. In aqueous 0.1 N H 2 SO 4 benzotriazole ( 5 ) and 1‐methyl‐benzotriazole ( 6 ) yielded 2‐amino‐ and 2‐methylaminophenol ( 25 and 26 ), respectively (Scheme 3) . In 2‐propanol 6 , 5‐chloro‐ and 6‐chloro‐1‐methyl‐benzotriazole ( 14 and 15 ) were reduced to N ‐methylaniline, 4‐chloro‐ and 3‐chloro‐ N ‐methyl‐aniline ( 27 , 28 and 29 ), respectively (Scheme 4) . When the benzotriazoles were irradiated in aromatic solvents only C, C coupling products were observed ( cf. Scheme 6 and Tables 1–4 ). It is of importance that 5‐chloro‐1‐methyl‐benztriazole ( 14 ) when decomposed photolytically in benzene solution yielded only 4‐chloro‐2‐phenyl‐ N ‐methyl‐aniline ( 49 ) and its 6‐chloro isomer only 5‐chloro‐2‐phenyl‐ N ‐methyl‐aniline ( 50 ), i.e. the intervention of benzo‐1 H ‐azirine intermediates ( e.g. 53 , Scheme 8 ) can be excluded. The substitution patterns which are observed when 6 is irradiated in toluene, anisole, fluoro‐, chloro‐, bromobenzene and benzonitrile (cf. Table 4) can best be explained by assuming that 6 , after loss of nitrogen, forms a diradical intermediate in the singlet state with highly zwitterionic character. 1‐(1′‐Alkenyl)‐benzotriazoles (cf. Table 7) form on irradiation in cyclohexane solution indoles by intramolecular ring closure of the diradical intermediate and proton shift. After irradiation of 1‐decyl‐benzotriazole ( 8 ) in a glassy matrix at 77K a 7‐line ESR. spectrum characteristic of a triplet radical is observed. This is in agreement with the fact that the lowest lying state of intermediates of type 2 (Scheme 1) should be a triplet state ( cf. [21] [26]).