CONCENTRATION-DEPENDENCE OF THE STERIC COURSE OF THE BROMINE ADDITION TO ARYLALKENES - THE CASE OF STILBENES

The ratios of meso- to d,l-1,2-dibromo-l,2-diphenylethane obtained in the bromination of cis- and trans-stilbene in 1,2-dichloroethane have been determined by HPLC as a function of the reagent concentrations. With stoichiometric reagents both reactions were nonstereoselective down to 10-3M concentrations, but became stereospecific and stereoconvergent to give the meso dibromide at [Br2] ≼2 Χ10-3 M and 100-fold lower olefin concentration. Kinetic measurements have shown that under all conditions the reaction was occurring through the same rate-determining step. A rationalization, involving tight or solvent-separated ion-pair intermediates, is proposed. © 1990, American Chemical Society. All rights reserved.