Photolysis of a series of α-brominated ortho-xylenes in apolar solvents

Abstract

The α-brominated ortho-xylenes have been subjected to 254 nm irradiation in deaerated benzene, isooctane and benzene–cyclohexene. The product analysis revealed that homolysis of the C–Br bond is followed by a series of hydrogen abstraction and radical recombination reactions resulting in xylenes more and less brominated than the starting compound. The less brominated products are formed with higher quantum yield when cyclohexene is present, due to hydrogen abstraction by the o-benzyl radical formed initially, together with cyclohexene dimers. Additionally, the formation of 2-bromo-2,4,4-trimethylpentane is observed when isooctane is the solvent. The quantum yields observed for the photolysis of 1 and 2 are higher in benzene than in isooctane, suggesting sensitization by benzene. A biradical intermediate of the type o-quinodimethane was expected in the case of (a) photolysis of the o-benzyl radical formed (biphotonic process) or (b) intramolecular hydrogen abstraction. However, the addition of cyclohexene failed to produce the expected Diels–Alder adduct. The synthesis of the novel α,α-dibromo-o-xylene 3 is reported.