Photodehalogenation and Photodimerization of 2-(4-Halophenyl)benzoxazoles. Dependence of the Mechanism on the Nature of the Halogen Atom

Article abstract of DOI:10.1021/jo00355a017

Irradiation of oxygen-free solutions of 2-(4-halophenyl)benzoxazoles at 300 nm led to dimerization or dehalogenation reactions.The fluoro derivative formed only dimer, while the chloro compound underwent both dimerization and dehalogenation.The bromo and iodo compounds did not form dimers but were dehalogenated.Fluorescence studies showed that the intersystem crossing yield decreased from the fluoro to the iodo compound.While dimerization was found to involve a singlet state, the excited state responsible for photodehalogenation of the bromo and iodo compounds was found to be the triplet on the basis of quenching and sensitization experiments.Triplet excited-state lifetimes were estimated.Photodehalogenation of 2-(4-chlorophenyl)benzoxazole occurs via the singlet state, involving an excimer as demonstrated from the influence of concentration on quantum yield.Photodehalogenation of the chloro and bromo compounds was considerably enhanced in the presence of electron donors such as cis-piperylene and triethylamine.These dehalogenation reactions are believed to proceed via an excited charge-transfer complex.