Designing a Triplet 2-Formylphenylnitrene With Fast Tunneling Reactivity.

Quantum mechanical tunneling (QMT) can profoundly alter reaction kinetics and ultimately determine the chemical outcome. Therefore, understanding QMT reactivity and extending its relevance to typical laboratory conditions are important goals. In this study, computational screening identified a 3-trifluoromethyl substituent as the most promising candidate for significantly accelerating the QMT reactivity of triplet 2-formylphenylnitrene. UV-irradiation of the azide precursor 1 in Ar and N2 matrices (14 K) generated the corresponding iminoketene 3 and 2,1-benzisoxazole 4, whereas subsequent visible-light irradiation converted 3 into a benzofused β-lactam 5. The absence of anti and syn-conformers of triplet 3-trifluoromethyl-2-formylphenylnitrene 32 indicates rapid QMT reactions to 3 and 4, respectively, with the former inferred to have a lifetime shorter than tens of ms in an Ar matrix. Preliminary studies using photocatalytic triplet energy-transfer to promote selective generation of 32 from 1 in CH2Cl2 solution (200 K), led to the exclusive observation of 4 by in situ IR spectroscopy. This demonstrates the successful suppression of the intrinsic rapid nitrene dimerization through faster cyclization to 4, which even outcompetes the 1,4-H shift to 3. Overall, this work broadens the understanding of QMT in nitrene chemistry and provides a potential platform for bridging QMT investigations between cryogenic matrices and solution chemistry.