Peroxometal-mediated environmentally favorable route to brominating agents and protocols for bromination of organics

Abstract

Higher-valent transition metals react with H2O2 to form peroxometallates, thereby activating the coordinated peroxide. Based on the reaction profiles of peroxometal species, environmentally acceptable newer syntheses of tetrabutylammonium tribromide (TBATB), Bu4NBr3, cetyltrimethylammonium tribromide (CTMATB), cetyl(Me)3NBr3, and tetraethyl-ammonium tribromide (TEATB), Et4NBr3, have been developed from the reactions of the corresponding quaternary ammonium bromides with H2O2 and a catalytic amount of vanadium(V) or molybdenum(VI). Other transition metals capable of activating peroxide give similar results. The quaternary ammonium tribromides (QATBs) thus produced, especially TBATB and CTMATB, very efficiently act as clean and selective brominating agents for a variety of organic substrates. Very facile bromination of organic substrates, including aromatics, is also possible by tetrabutylammonium bromide (TBAB) Bu4NBr, either promoted by V2O5-H2O2 or catalyzed by MoO42-H2O2. The scope of the protocols has been underscored, and the relevance to green chemistry has been highlighted.