Pyrolysis of benzotriazoles. Relationships between 1- and 2-vinylbenzotriazoles, α- and β-azidostyrenes, N-phenylketenimine and indole. Pitfalls in the use of pyrolysis-mass spectrometry in mechanistic studies

•Flash vacuum pyrolysis (FVP) of vinylbenzotriazoles is reinterpreted.•FVP of N-phenylketenimine PhNCCH2 does not produce indole.•FVP of alpha-azidostyrene does not produce indole but 2-phenyl-1-azirine and phenylacetonitrile.•FVP of beta-azidostyrene does produce indole and 3-phenyl-1-azirine.

Contrary to a previous report based on pyrolysis-mass spectrometry, it is shown that 1-vinylbenzotriazole 4 undergoes flash vacuum pyrolysis (FVP) to N-phenylketenimine (PhNCCH2) 7 and indole 5 in parallel, not consecutive, reactions. N-Phenylketenimine does not rearrange to indole to any significant extent, but on FVP at 950 °C it decomposes in a free radical reaction to yield phenylacetonitrile, acetonitrile, benzene, biphenyl and o-tolunitrile. Phenylacetonitrile itself at 1000 °C yields essentially the same products. 1-(2-Methylpropeny)benzotriazole 22 pyrolyzes to C,C-dimethyl-N-phenylketenimine 23, and at higher temperature to methacrylonitrile 25 and benzene in a free radical reaction. 2-Vinylbenzotriazole 21 rearranges to 1-vinylbenzotriazole, which then affords N-phenylketenimine, phenylacetonitrile and indole at 900 °C. 2-Phenyl-1-azirine 10, phenylacetonitrile and a trace of N-phenylketenimine are formed by FVP of α-styryl azide 8. 2-Phenyl-1-azirine itself pyrolyzes to phenylacetonitrile, benzonitrile and a trace of indole at 900 °C. 3-Phenyl-1-azirine 12, phenylacetonitrile and indole are formed on pyrolysis of β-styryl azide 11, and matrix photolysis of 11 yields 3-phenyl-1-azirine and 1-phenylketenimine (PhCHCNH) 13. The paper emphasizes the dangers of relying solely on mass spectrometry in investigations of mechanisms of pyrolysis reaction. Observed mass spectra may on occasion correspond to very low yields of products that are not characteristic of the main reaction paths. Moreover, there are cases where major FVP reaction products have been nearly impossible to observe by online mass spectrometry.

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