On the photophysical behaviour of 4-halo-5-phenyl-oxazoles and isoxazoles: A correction and observations on the photoinduced isomerisation and degradation of methyl 4-halo-5-phenyl-isoxazole-3-carboxylates

Photophysical data in cyclohexane solutions, including measurements of energies of lowest excited singlet and triplet states, fluorescence quantum yields, lifetimes, phosphorescence and triplet-singlet difference absorption spectra, reported in a previous Letter as due to methyl 4-halo-5-phenylisoxazole-3-carboxylates [S. M. Fonseca, H.D. Burrows, C.M. Nunes, T.M.V.D. Pinho e Melo, A.M.d'A. Rocha Gonsalves, Chem. Phys. Lett. 414 (2005) 98] are reassigned to the isomeric oxazoles. These are compared with data on the isoxazoles synthesised by an unambiguous route. In addition, results are presented on the photoinduced isomerization of 4-halo-5-phenylisoxazoles to oxazoles in cyclohexane, which is shown to proceed via a nonconcerted pathway, possibly involving thermal reaction of an azirine intermediate. (C) 2009 Elsevier B. V. All rights reserved.