Capture of an Elusive Nitrile Ylide as an Intermediate in Isoxazole Oxazole Photoisomerization


Nunes C. M., Reva I., Fausto R.

JOURNAL OF ORGANIC CHEMISTRY, vol.78, no.21, pp.10657-10665, 2013 (SCI-Expanded) identifier identifier identifier

  • Publication Type: Article / Article
  • Volume: 78 Issue: 21
  • Publication Date: 2013
  • Doi Number: 10.1021/jo4015672
  • Journal Name: JOURNAL OF ORGANIC CHEMISTRY
  • Journal Indexes: Science Citation Index Expanded (SCI-EXPANDED), Scopus
  • Page Numbers: pp.10657-10665
  • Istanbul Kültür University Affiliated: Yes

Abstract

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 IS K and characterized spectroscopically. Irradiation of matrix-isolated 1 at lambda = 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 lambda = 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.