Speaker
Description
For nearly a century, the role of quantum mechanical tunneling (QMT) in organic chemistry remained barely unrecognized. The interpretation of chemical selectivity has been done solely based on the classical paradigms of thermodynamic and kinetic control. However, it has recently been shown that these paradigms do not cover all aspects of chemical reactivity and QMT can also direct chemical selectivity.1 QMT is very sensitive to the potential energy barrier width of a reaction, which is measured by the mass-weighted displacement of the atoms involved on it. Reactions that occur exclusively through higher, yet narrower, potential energy barriers than other alternative paths operate through tunneling control. There is the perspective that tunneling control will be essential to fully understand chemical reactivity.2
Direct observation of QMT organic reactions has been accomplished in experiments performed at cryogenic temperatures, under which thermal activation is impossible.3 In the present communication, two representative examples of such experiments, related to reactions of aryl nitrenes, will be showcased through a multidisciplinary approach involving organic synthesis, spectroscopy, photochemistry, kinetics and computational chemistry. i) A novel approach where H-tunneling is activated by narrowband IR light irradiation.4 Those results provide the proof-of-concept of an exciting novel strategy to attain control over QMT, opening new avenues to direct chemical transformations. ii) An unprecedented occurrence of two simultaneous conformer-specific reactions that occur through QMT, which selectivity can only be interpreted based on tunneling control.5 Those results highlight the current limitations of the classical paradigms of chemical reactivity and the need to develop a thoroughly understanding of QMT implications in organic reactions.
Acknowledgements
This work was supported by Project PTDC/QUI-QFI/1880/2020, funded by National Funds via the Portuguese Foundation for Science and Technology (FCT). The Coimbra Chemistry Centre − Institute of Molecular Sciences (IMS) is supported by FCT through projects UIDB/00313/2020 and UIDP/00313/2020 cofunded by COMPETE and the IMS special complementary funds provided by FCT. J.P.L.R. acknowledges FCT for a PhD (SFRH/BD/04467/2020) grant. C.M.N. acknowledges FCT for an Auxiliary Researcher grant.
References
1 P. R. Schreiner, J. Am. Chem. Soc., 2017, 139, 15276–15283.
2 P. R. Schreiner, Trends Chem., 2020, 2, 1–10.
3 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.
4 J. P. L. Roque, C. M. Nunes, L. P. Viegas, N. A. M. Pereira, T. M. V. D. Pinho E Melo, P. R. Schreiner and R. Fausto, J. Am. Chem. Soc., 2021, 143, 8266–8271.
5 C. M. Nunes, J. P. L. Roque, S. Doddipatla, S. A. Wood, R. J. McMahon and R. Fausto, J. Am. Chem. Soc., 2022, 144, 20866–20874.
