Photocatalytic and catalytic activity of heterogenized W10O324- in the bromide-assisted bromination of arenes and alkenes in the presence of oxygen

Photochemical excitation (lambda > 300 nm) of the decatungstate (nBu(4)N)(4)W10O32, heterogenized with Amberlite IRA-900 and dispersed in a CH3CN/H2O mixture, causes the reductive activation of O-2 to alkyl hydroperoxides. The light-assisted formation of these intermediates represents a new approach for inducing the bromination of activated arenes and cycloalkenes at atmospheric pressure and room temperature. The active species "Br+" is formed as a consequence of a two-electron oxidation of Br- by the photogenerated hydroperoxides. This process is catalyzed by the decatungstate, which, therefore, plays also a catalytic role in addition to the photocatalytic one. Phenol and anisole can be converted to the corresponding mono-brominated derivatives, and a wide range of cycloalkenes to the corresponding bromohydrins and dibromides, with bromohydrins as intermediates for the formation of epoxides. The anionic exchange resin plays a crucial role in fostering the enrichment of bromide anions close to the surface and, consequently, their reaction with the photogenerated hydroperoxides. As a matter of fact, the efficiency of the bromination photocatalytic processes may benefit by the heterogenization of the decatungstate. As to the chemoselectivity, of the photocatalytic process, we have found that the solid matrix plays the important function of increasing the yields of epoxides and bromohydrins from cyclohexene, 1-methyl-l-cyclohexene, and styrene, upon heterogenization of the decatungstate. The solid matrix can also control the chemoselectivity in anisole bromination, by favoring the functionalization of the para position. (c) 2006 Elsevier B.V. All rights reserved.