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 V₂O₅­H₂O ₂ or catalyzed by MoO₄^2­­H₂ O₂. The scope of the protocols has been underscored, and the relevance to green chemistry has been highlighted.