BROMINATION OF ALKENES IN ACETONITRILE - A RATE AND PRODUCT STUDY

The reaction of simple alkenes and aryl alkenes with molecular bromine in damp MeCN occurred with solvent incorporation to give 2-bromo-1-(N-acetylamino)alkanes, 2-methyloxazolines, 2-acetoxyalkylamine hydrobromides, and 2-(N-acetylamino) alcohols. These products arose by the transformation of initially formed 2-bromo-1-(N-acetylamino)alkanes obtained by MeCN attack on bromonium or bromocarbonium ions to give nitrilium tribromide salts. These reacted with water to give 2-bromo-1-(N-acetylamino)alkanes. The kinetic profile of the reaction showed a very fast initial reaction of the alkene and Br2 to yield the nitrilium tribromide, followed by a much slower reaction of Br3- with the alkene. The incorporation of MeCN was Markovnikov and stereospecifically anti. The degree to which incorporation of solvent occurred depended upon the alkene structure and the initial reagent concentrations. A rationalization for the observed chemoselectivity and its dependence on the reaction conditions is offered.