Reactive intermediates. Part XXIV. 1H-Azirine intermediates in the pyrolysis of 1H-1,2,3-triazoles

Nitrogen has been extruded from several 1,2,3-triazoles by flash vacuum pyrolysis and the fate of the resulting iminocarbenes has been determined. 1-Alkyl-4,5-diphenyl-1,2,3-triazoles (1) gave nitriles (Scheme 1) and isoquinolines and hydroxyisoquinolines (Scheme 2), the former by Wolff rearrangement and the latter by 1,4-hydrogen transfer in the iminocarbene. 1,4-Dimethyl-5-phenyl-1,2,3-triazole (3) and 1,5-dimethyl-4-phenyl-1,2,3-triazole (4) both gave 3-methylisoquinoline; a mechanism involving a common, 1H-azirine intermediate is proposed (Scheme 4). From both 4- and 5-phenyl-1-(1-phenylvinyl)-1,2,3-triazole [(5a) and (6a)] mixtures of 2,4- and 2,5-diphenylpyrrole were isolated; similarly the corresponding 1-phenyltriazoles gave mixtures of 2- and 3-phenylindole (Scheme 5). 4,5-Diphenyl-v-triazole also gave 2-phenylindole. The formation of these products is rationalised in terms of 1H-azirine intermediates in the pyrolyses.