Differential Tunneling‐Driven and Vibrationally‐Induced Reactivity in Isomeric Benzazirines

• Published in: Chemistry : a European journal Vol. 28; no. 67; pp. e202202306 - n/a
• Main Authors: Nunes, Cláudio M., Doddipatla, Srinivas, Loureiro, Gonçalo F., Roque, José P. L., Pereira, Nelson A. M., Pinho e Melo, Teresa M. V. D., Fausto, Rui
• Format: Journal Article
• Language: English
• Published: Germany Wiley Subscription Services, Inc 01.12.2022; John Wiley and Sons Inc
• Subjects: Chemistry; computational chemistry; heavy-atom tunneling; I.R. radiation; IR spectroscopy; Irradiation; Krypton; Light irradiation; matrix-isolation; Quantum mechanics; Reaction control; reactive intermediates; Reactivity; vibrational excitation chemistry; heavy-atom tunneling; computational chemistry; reactive intermediates; vibrational excitation chemistry; IR spectroscopy; matrix-isolation
• ISSN: 0947-6539; 1521-3765; 1521-3765
• DOI: 10.1002/chem.202202306

Quantum mechanical tunneling of heavy‐atoms and vibrational excitation chemistry are unconventional and scarcely explored types of reactivity. Once fully understood, they might bring new avenues to conduct chemical transformations, providing access to a new world of molecules or ways of exquisite reaction control. In this context, we present here the discovery of two isomeric benzazirines exhibiting differential tunneling‐driven and vibrationally‐induced reactivity, which constitute exceptional results for probing into the nature of these phenomena. The isomeric 6‐fluoro‐ and 2‐fluoro‐4‐hydroxy‐2H‐benzazirines (3‐a and 3′‐s) were generated in cryogenic krypton matrices by visible‐light irradiation of the corresponding triplet nitrene 32‐a, which was produced by UV‐light irradiation of its azide precursor. The 3′‐s was found to be stable under matrix dark conditions, whereas 3‐a spontaneously rearranges (τ1/2 ∼64 h at 10 and 20 K) by heavy‐atom tunneling to 32‐a. Near‐IR‐light irradiation at the first OH stretching overtone frequencies (remote vibrational antenna) of the benzazirines induces the 3′‐s ring‐expansion reaction to a seven‐member cyclic ketenimine, but the 3‐a undergoes 2H‐azirine ring‐opening reaction to triplet nitrene 32‐a. Computations demonstrate that 3‐a and 3′‐s have distinct reaction energy profiles, which explain the different experimental results. The spectroscopic direct measurement of the tunneling of 3‐a to 32‐a constitutes a unique example of an observation of a species reacting only by nitrogen tunneling. Moreover, the vibrationally‐induced sole activation of the most favorable bond‐breaking/bond‐forming pathway available for 3‐a and 3′‐s provides pioneer results regarding the selective nature of such processes. Quantum tunneling and IR vibrational excitation chemistry was discovered to manifest differently in two isomeric benzazirines generated in cryogenic matrices. Computations show that these reactive intermediates have distinct energy profiles regarding two competitive bond‐breaking/bond‐forming pathways. These exceptional results allow probing into the fundamental nature of such unconventional types of chemical reactivity.