American Chemical Society
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Photochemistry of 2‑Formylphenylnitrene: A Doorway to Heavy-Atom Tunneling of a Benzazirine to a Cyclic Ketenimine

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journal contribution
posted on 2017-11-07, 00:00 authored by Cláudio M. Nunes, Igor Reva, Sebastian Kozuch, Robert J. McMahon, Rui Fausto
The slippery potential energy surface of aryl nitrenes has revealed unexpected and fascinating reactions. To explore such a challenging surface, one powerful approach is to use a combination of a cryogenic matrix environment and a tunable narrowband radiation source. In this way, we discovered the heavy-atom tunneling reaction involving spontaneous ring expansion of a fused-ring benzazirine into a seven-membered ring cyclic ketenimine. The benzazirine was generated in situ by the photochemistry of protium and deuterated triplet 2‑formylphenylnitrene isolated in an argon matrix. The ring-expansion reaction takes place at 10 K with a rate constant of ∼7.4 × 10–7 s–1, despite an estimated activation barrier of 7.5 kcal mol–1. Moreover, it shows only a marginal increase in the rate upon increase of the absolute temperature by a factor of 2. Computed rate constants with and without tunneling confirm that the reaction can only occur by a tunneling process from the ground state at cryogenic conditions. It was also found that the ring-expansion reaction rate is more than 1 order of magnitude faster when the sample is exposed to broadband IR radiation.