Oxidative Halogenation Reactions by Using Halide Salt as a Halide Source

Abstract

The bromination reaction of organic compounds is one of the important reactions in organic synthesis, providing bromination products which are key precursors for various transformations. Conventional bromination reaction is carried out using toxic and hazardous elemental bromine. Oxidative bromination reaction by utilization of a bromide ion as a bromide source instead of bromine has attracted a great deal of attention to develop environmentally friendly bromination method. In this account, catalytic oxidative halogenation and aromatization by using halide salt as a halide source are described. An environmentally benign catalytic oxidative bromination reaction was demonstrated by the combination of the redox properties of a commercially available inexpensive vanadium compound and molecular oxygen as a terminal oxidant, wherein vanadium-catalyzed oxidative bromination was promoted by Brønsted acid or Lewis acid. The catalytic chlorination reaction of ketones and alkenes was achieved by using a vanadium catalyst in the presence of Bu₄NI and AlCl₃ under molecular oxygen. The vanadium-catalyzed oxidation system, which consists of a vanadium catalyst, a bromide source, and an acid under molecular oxygen, could be applied to the catalytic oxidative aromatization of 2-cyclohexen-1-ones, affording the corresponding phenol derivatives. Metal-free oxidative bromination reactions was also performed in aqueous media under mild conditions with a combination of tetrabutylammonium bromide as a bromide source, trifluoroacetic acid as an acid, and hydrogen peroxide as an oxidant.