CQC, Department of Chemistry, University of Coimbra , 3004-535 Coimbra, Portugal.
Department of Chemistry, Ben-Gurion University of the Negev, Beer-Sheva 841051, Israel.
J Am Chem Soc. 2017 Dec 6;139(48):17649-17659. doi: 10.1021/jacs.7b10495. Epub 2017 Nov 21.
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 s, despite an estimated activation barrier of 7.5 kcal mol. 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.
芳基氮宾的滑动势能面揭示了出人意料且引人入胜的反应。为了探索这一具有挑战性的表面，一种强大的方法是结合使用低温基质环境和可调谐的窄带辐射源。通过这种方式，我们发现了涉及稠环苯并氮杂环丙烷自发环扩张为七元环环状烯酮亚胺的重原子隧穿反应。苯并氮杂环丙烷是由氘代三重态 2-甲酰基苯氮宾在氩基质中光解原位生成的。尽管估计的活化能垒为 7.5 kcal/mol，但该环扩张反应在 10 K 下以约 7.4×10 s 的速率常数进行。此外，尽管绝对温度增加了 2 倍，但速率仅略有增加。考虑隧穿和不考虑隧穿的计算速率常数均证实，在低温条件下，该反应只能通过从基态的隧穿过程发生。还发现，当样品暴露于宽带 IR 辐射时，环扩张反应速率快 1 个数量级以上。
