Capture of an elusive nitrile ylide as an intermediate in isoxazole-oxazole photoisomerization.

The unimolecular photochemistry of 3,5-dimethylisoxazole (1) induced by a narrow-band tunable UV laser was studied using low-temperature matrix isolation coupled with infrared spectroscopy. Monomers of 1 were isolated in argon matrices at 15 K and characterized spectroscopically. Irradiation of matrix-isolated 1 at λ = 222 nm (near its absorption maximum) led to the corresponding 2H-azirine 3 and ketenimine 6 as primary photoproducts and also to nitrile ylide 4 and 2,5-dimethyloxazole (5). The photoproducts were identified (i) by comparison with infrared spectra of authentic matrix-isolated samples of 3 and 5 and (ii) using additional irradiations at longer wavelengths (where 1 does not react) which induce selective photoisomerizations of 4 and 6. In particular, irradiation with λ = 340 nm led to the unequivocal identification of the nitrile ylide anti-4, which was transformed into oxazole 5. The details of the 1,5-electrocyclization of the carbonyl nitrile ylide 4 and its structural nature (propargyl-like versus allene-like geometry) were also characterized using theoretical calculations. Thus, the elusive carbonyl nitrile ylide 4 was captured and characterized for the first time as an intermediate in the isoxazole-oxazole photoisomerization.