Reactive intermediates. Part XXIII. Pyrolysis of 1-phthalimido-1,2,3-triazoles: formation and thermal reactions of 2H-azirines

Abstract

Pyrolysis in the vapour phase of 4,5-disubstituted 1-phthalimido-1,2,3-triazoles (1) gives 2H-azirines (2) as the primary isolable products. The azirines undergo further thermal reactions under the conditions of the pyrolysis: with a methyl or an ethyl 2-substituent they are cleaved to nitriles and phthalimido-carbenes, but with a phenyl 2-substituent they rearrange to indoles. 4-Methyl-5-phenyl-1 -phthalimido-1,2,3-triazole (1c) and 5-methyl-4-phenyl-1-phthalimido-1,2,3-triazole (1d) give identical mixtures of azirines and their pyrolysis products, indicating that the products are formed through a common intermediate, considered to be the antiaromatic 2-methyl-3-phenyl-1-phthalimido-1H-azirine.