The pyrolysis of oxindoles at 850°C

Abstract

Pyrolysis of oxindole at 850ºC/0.7 mm with short contact time gives a complex mixture of products including benzene, benzonitrile, 2-vinylpyridine, o-toluidine, aniline, and o-aminostyrene. The pyrolysis has been investigated by using 1% and ¹³C labelling and by studying the effects of methyl and phenyl substituents. The nitrile carbon of benzonitrile thus formed is derived from C7a of oxindole, and not from C3. Pyrolysis of 7-phenyloxindole forms 1-methylcarbazole, and 4-phenyl-oxindole forms 1-aminofluorene. These results are interpreted in terms of the formation and subsequent rearrangement of arylnitrenes and arylcarbenes. The pyrolysis of oxindole is compared with pyrolyses of o-tolyl azide and tropyl azide. The mass spectrum of oxindole is reinterpreted in the light of the pyrolytic results; the spectrum of [3-¹³Cloxindole shows that loss of H¹³CN is a minor process (25-30%).