N,N-dihalophosphoramides-XVI : Ionic addition of diethyl n,n-dibromophosphoroamidate (DBPA) to alkenes and cycloalkenes

Abstract

The addition of DBPA to a variety of phenylethylenes, straight-chain terminal and nonterminal alkenes as well as cycloalkenes in the presence of boron trifluoride etherate has been investigated. It was found that the reaction proceeds smoothly at -20° by adding an olefin to the solution of equimolar amounts of DBPA and boron trifluoride etherate in tetrachloromethane. N-Bromoadducts (mixtures or single isomers depending upon the structure of the olefin) initially formed could be reduced in situ with sodium bisulphite solution to give the corresponding diethyl N-(β-bromoalkyl)phosphoroamidates which in turn afforded β-bromoamine hydrochlorides upon treatment with hydrogen chloride in benzene at room temperature. The regiospecificity typical for Markovnikov addition, as proven by NMR and MS evidence, was observed for unsymmetrical phenylethylenes. The addition of DBPA to ( E)-1-phenylpropene, ( E)-2-butene, and ( Z)-2-butene was also found to proceed stereospecifically affording the corresponding anti-adducts. These results are fully compatible with an ionic addition pathway and can be rationalized by assuming the intermediate formation of an electrophilic complex between DBPA and boron trifluoride. The reaction offers a new approach to aminobromination of alkenes and cycloalkenes and makes possible an easy access to β-bromoamines, the convenient precursors of aziridines.