Conformer-Specific Heavy-Atom Tunneling in the Rearrangement of Benzazirines to Ketenimines

5-Methoxy-2H-benzazirine was prepared via irradiation of the corresponding phenyl azide, isolated in an argon matrix at cryogenic temperatures. It undergoes ring expansion to the corresponding ketenimine in the dark at T < 30 K despite a calculated activation barrier of 4.9 kcal mol–1 [B3LYP/6-311++G(d,p)]. Since this rearrangement proceeds with a rate constant in the order of 10–4 s–1, exhibiting only a shallow temperature dependence, the results are interpreted in terms of heavy-atom tunneling. Of the four isomeric benzazirines resulting from the initial photolysis, only one can be observed to rearrange; this conformer specificity is explained by the other potentially observable rearrangements being either too fast or too slow to be detected due to the differences in heights and widths of their respective activation barriers.