Photochemical cyclodehydrogenation of Lewis acid-conjugates of azobenzenes

Abstract

Irradiation of the conjugate acids of azobenzene ( 1a) with Lewis acids like anhydrous AlCl 3, anhydrous SnCl 4 and anhydrous FeCl 3 in 1,2-dichloroethane results in cyclodehydrogenation to benzo[ c]cinnoline ( 2a). The formation of benzidine ( 3a) along with 2a suggests that the reaction is a photochemical disproportionation. The absorption spectra of the conjugate acids in 1,2-dichloroethane reveal that inversion of the n → π* and π → π* singlet state energies occurs on complexation of the azo nitrogen with the Lewis acid. Irradiation of the Lewis acid-conjugates of 2-methylazobenzene ( 1b), 2,2′-dimethylazobenzene ( 1c), 4,4′-dimethylazobenzene ( 1d) and 4-chloroazobenzene ( 1e) also results in cyclodehydrogenation.