UV-Laser Photochemistry of Isoxazole Isolated in a Low-Temperature Matrix

• Published in: Journal of organic chemistry Vol. 77; no. 19; pp. 8723 - 8732
• Main Authors: Nunes, Cláudio M, Reva, Igor, Pinho e Melo, Teresa M. V. D, Fausto, Rui
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 05.10.2012
• Subjects: Chemistry; Exact sciences and technology; General and physical chemistry; Heterocyclic compounds; Heterocyclic compounds with n hetero atom and also o and/or s, se, te hetero atoms; Heterocyclic compounds with only one n hetero atom and condensed derivatives; Kinetics and mechanisms; Organic chemistry; Photochemistry; Physical chemistry of induced reactions (with radiations, particles and ultrasonics); Preparations and properties; Reactivity and mechanisms; Nitrile; Isoxazole derivatives; Nitrogen heterocycle; Photoisomerization; Isomerization; Theoretical study; Decomposition; Tautomerism; Photochemical reaction; Aldehyde; Ketenes; Infrared spectrometry; Hydrogen cyanides; Ultraviolet radiation; Photolysis; Laser; Irradiation; Reaction mechanism; Spectrophotometry; Moller Plesset partition; Low temperature; Oxygen nitrogen heterocycle; Photochemistry
• ISSN: 0022-3263; 1520-6904
• DOI: 10.1021/jo301699z

The photochemistry of matrix-isolated isoxazole, induced by narrowband tunable UV-light, was investigated by infrared spectroscopy, with the aid of MP2/6-311++G(d,p) calculations. The isoxazole photoreaction starts to occur upon irradiation at λ = 240 nm, with the dominant pathway involving decomposition to ketene and hydrogen cyanide. However, upon irradiation at λ = 221 nm, in addition to this decomposition, isoxazole was also found to isomerize into several products: 2-formyl-2H-azirine, 3-formylketen­imine, 3-hydroxypropenenitrile, imidoylketene, and 3-oxopropanenitrile. The structural and spectroscopic assignment of the different photoisomerization products was achieved by additional irradiation of the λ = 221 nm photolyzed matrix, using UV-light with λ ≥ 240 nm: (i) irradiation in the 330 ≤ λ ≤ 340 nm range induced direct transformation of 2-formyl-2H-azirine into 3‑formylketenimine; (ii) irradiation with 310 ≤ λ ≤ 318 nm light induced the hitherto unobserved transformation of 3‑formylketenimine into 3-hydroxypropenenitrile and imidoylketene; (iii) irradiation with λ = 280 nm light permits direct identification of 3-oxopropanenitrile; (iv) under λ = 240 nm irradiation, tautomerization of 3-hydroxypropenenitrile to 3‑oxopropanenitrile is observed. On the basis of these findings, a detailed mechanistic proposal for isoxazole photochemistry is presented.