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Quantum Tunneling Reactions in Phenylnitrene Derivatives Generated in Cryogenic Matrices

Quantum Mechanical Tunneling (QMT) is a fascinating phenomenon that allows particles to permeate through potential energy barriers. QMT can manifest itself in ordinary reactions, and one obvious consequence is the increase of reaction rates compared to the expectation of the classical transition state theory (TST). A much more profound consequence of QMT is the control of chemical reaction outcomes, leading to new reactivity paradigms that break with the classic TST principles (e.g., the thermodynamic or kinetic reaction control) [1].

Direct spectroscopic investigations at cryogenic temperatures (e.g. 3-20 K) have been decisive to shed light on the potential relevance of QMT in some organic transformations and to reveal chemical reactivity beyond classical paradigms [2]. In this communication, I will present our contributions to this research endeavors, highlighting pioneering discoveries of QMT reactions in phenylnitrene derivatives [3]. For instance, we discovered that triplet 2-formylphenylnitrene ³2, generated by photolysis of azide 1 in noble-gas matrices at 10 K, rearranges by hydrogen-atom QMT to iminoketene 3. Experimental and theoretical results providing support for the occurrence of different QMT reactions will be discussed.

[1] P. R. Schreiner, J. Am. Chem. Soc., 2017, 139, 15276–15283.

[2] C. M. Nunes, I. Reva and R. Fausto, in Tunnelling in Molecules: Nuclear Quantum Effects from Bio to Physical Chemistry, The Royal Society of Chemistry, 2021; pp 1−60.

[3] (a) C. M. Nunes, S. N. Knezz, I. Reva, R. Fausto, R. J. McMahon, J. Am. Chem. Soc., 2016, 138, 15287. (b) C. M. Nunes, L. P. Viegas, S. A. Wood, J. P. L. Roque, R. J. McMahon, R. Fausto, Angew. Chem. Int. Ed., 2020, 59, 17622. (c) J. P. L. Roque, C. M. Nunes, L. P. Viegas, N. A. M. Pereira, T. M. V. D. Pinho e Melo, P. R. Schreiner, R. Fausto, J. Am. Chem. Soc. 2021, 143, 8266. (d) C. M. Nunes, J. P. L. Roque, S. Doddipatla, S. A. Wood, R. J. McMahon, R. Fausto, J. Am. Chem. Soc. 2022, 144, 20866. (e) J. P. L. Roque, C. M. Nunes, P. R. Schreiner, R. Fausto, Chem. Eur. J., 2024, e202401323.