Vanadium-catalyzed oxidative bromination promoted by Brønsted acid or Lewis acid

The oxidative bromination of arenes was induced by a vanadium catalyst in the presence of a bromide salt and a Brønsted acid or a Lewis acid under molecular oxygen, which provides an eco-friendly bromination method as compared with a conventional bromination one with bromine. This catalytic reaction could be applied to the bromination of alkenes and alkynes to give the corresponding vic-bromides. Use of aluminum halide as a Lewis acid in place of a Brønsted acid was demonstrated to provide a more practical protocol for the oxidative bromination. From ketones, α-bromination products were obtained. AlBr₃ was found to serve as both a bromide source and a Lewis acid to induce the bromination smoothly. ⁵¹V NMR experiment showed that this catalytic bromination is likely to depend on the redox cycle of a vanadium catalyst under molecular oxygen.