Reactive intermediates. Part XXV. Investigation of the pyrolysis of 1,4- and 1,5-diphenyl-1,2,3-triazoles by use of 13C-labelled compounds

Abstract

1,4-Diphenyl-[4-¹³C]1,2,3-triazole (1) and 1,5-diphenyl-[5-¹³C]1,2,3-triazole (2) have been prepared by the addition of azidobenzene to phenyl[1-¹³C]acetylene. Pyrolysis of the triazoles in the vapour phase gave the same four compounds in each case: 1-, 2-, and 3-phenylindole, and N-(phenylvinylidene)aniline, which was isolated as its hydrolysis product, phenylacetanilide. It has also been established that 1-, 2-, and 3-phenylindoles can be interconverted by vapour phase pyrolysis. ¹³C N.m.r. spectra of these indoles, and of phenylacetanilide, are reported. An analysis by fast Fourier transform ¹³C n.m.r. of the positions of ¹³C enrichment in the products derived from the triazoles has shown a substantial degree of scrambling of the ¹³C label in the products derived from 1,5-diphenyl-[5-¹³C]1,2,3-triazole, which provides strong evidence for the intermediacy of a 1H-azirine in the reactions. The products derived from 1,4-diphenyl-[4-¹³C]1,2,3-triazole show little or no scrambling of the label.