Spectroscopic Characterization and Electrocyclic Ring Opening of Parent 2-Azetine

Abstract

(N-Boc)-protected 2-azetine serves as a suitable precursor for the generation of parent 2-azetine. High-vacuum flash pyrolysis experiments at 200°C induce the deprotection of the (N-Boc)-protected 2-azetine resulting in the release of carbon dioxide and isobutene allowing the trapping of 2-azetine in a solid argon matrix at 3 K and its spectroscopic characterization by infrared spectroscopy. The assignment is supported by isotopic labeling experiments and excellent agreement with anharmonic vibrational frequencies calculated at the B2PLYP-D3/def2-TZVPP level of theory. At elevated pyrolysis temperatures, 2-azetine undergoes selective ring opening to yield 1-azabutadiene, for which both s-trans-(E) and s-trans-(Z) conformers are observed. Computed reaction pathways and free-energy profiles rationalize the temperature-dependent product distribution and the observed torquoselectivity. These results provide fundamental insight into the structure, stability, and reactivity of 2-azetine and establish matrix isolation as a powerful approach for accessing highly strained nitrogen heterocycles.