Research Information System
CHEMISTRY-A EUROPEAN JOURNAL, vol.28, no.67, 2022 (SCI-Expanded)
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 (3)2-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 (tau(1/2) similar to 64 h at 10 and 20 K) by heavy-atom tunneling to (3)2-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 (3)2-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 (3)2-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.