Photolysis of 1-aryl -1,2,3-triazoles; rearrangement via 1H-azirines

Abstract

The preparation and photolysis of a seriesof 1-(1-naphthyl)-1,2,3-triazoles, (2)–(5), and of 1-(2-methyl-1 -naphthyl)-1,2,3-triazoles, (6)–(9), with and without electron-withdrawing groups (CO₂R, CONH₂, CN, CHO, COPh) in the triazole ring, are described. In the first series, triazoles (3) with two electron-withdrawing groups, and triazoles (4) with one such group at C-4, mostly give good yields of the expected benz[g] indoles (10) and (11) respectively. Triazoles (5) with the electron-withdrawing group at C-5 also give benz[g]indoles but these are now mixtures of the expected (12) and rearranged indoles (11). Photolysis of the 1-phenyltriazoles (21) and (22) follows the same pattern, those [(22)] with the electron-withdrawing group at C-5 rearranging to give mixtures of indoles (23) and (24). This is explained by a mechanism (Scheme 3) in which the less stable carbene intermediate (13) rearranges to the more stable carbene (15)via the 1H-azirine (14), in competition with its direct cyclisation. This provides the first evidence for antiaromatic 1H-azirines as intermediates in a photochemical reaction. Photolysis of the 2-methylnaphthyl′compounds follows a different path: diester (7b) and dinitrile (7d) give high yields of the deeply coloured 9-methyl-1H-benzo[de]quinoline derivatives (25a) and (25b) respectively, the first stable examples of this ring system to be isolated. Photolysis of the nitrile (8c) gives 9b- methyl-9bH-benz[g] indole-3-carbonitrile (28), a rare example of a stable indole with a ring junction substituent. 1,2-Shifts in the carbenes are only observed with the bis(trimethylsilyl)triazoles (2d) and (6f), in keeping with the known rapid migration of trimethylsilyl groups to carbene centres.