On the Photochemistry of 1,2‐Benzisoxazole: Capture of Elusive Spiro‐2 H ‐azirine and Ketenimine Intermediates

The photochemistry of 1,2‐benzisoxazole ( 1 ) was studied using low‐temperature matrix isolation coupled with infrared spectroscopy and quantum chemistry calculations. We identified, for the first time, spiro‐2 H ‐azirine 2 and ketenimine 3 as intermediates in the photoisomerization of 1 to 2‐cyanophenol ( 4 ). These results constitute indirect evidence for the existence of vinylnitrene intermediates in the photochemistry of 1,2‐benzisoxazoles. The potential energy surface (PES) resulting from the N–O bond cleavage of 1 was compared with the respective PES of the parent isoxazole. Calculations at the CBS‐QB3 level show that no stabilization is gained for the triplet vinylnitrene upon introduction of a benzene ring fused with isoxazole. However, the energies of 2 and 3 are higher by 13–15 kcal/mol comparing with the 2 H ‐azirine and ketenimine analogs resulting from isoxazole, which explains why they had not been observed before. Our general mechanistic proposal also predicts well the photoisomerizations of 2 and 3 to 4 .