Evidence for a Reversible Electrophilic Step in Olefin Bromination. The Case of Stilbenes

Article abstract of DOI:10.1021/ja00236a032

Bromonium-bromide ion couple intermediates have been generated by reacting erythro- and threo-2-bromo-1,2-diphenylethanol with gaseous HBr in 1,2-dichloroethane and in chloroform.It has been shown that in both solvents these intermediates mainly collapse to meso-1,2-dibromo-1,2-diphenylethane but also release Br2 to give trans-stilbene.The ratios of trans-stilbene to the meso dibromide obtained in all these reactions ranged between 9:91 and 22:78.The product distributions of the additions of HBr and Br2, both in the absence and in the presence of HBr, have also been determined. cis-Stilbene rapidly added HBr in both solvents, but in 1,2-dichloroethane isomerization to trans-stilbene also occurred to a large extent. trans-Stilbene mainly underwent oligo- or polymerization.The reactions of both olefins with Br2 were found to be not stereospecific.In the presence of HBr, the bromination of trans-stilbene became anti stereospecific, but in the case of cis-stilbene it maintained only a very modest stereoselectivity, and in chloroform HBr addition was predominant.The kinetics of bromination of both trans- and cis-stilbene in 1,2-dichloroethane were shown to follow very cleanly a third-order rate law (second order in Br2).However, the product analysis during the bromination of cis-stilbene showed that significant amounts of the trans olefin were always present at incomplete conversion.It has been shown that the latter olefin is formed by Br2-catalyzed isomerization of the starting cis olefin.All these results can be rationalized by assuming that the formation of bromonium-bromide or bromonium-tribromide ion pair intermediates in the discussed reactions is actually reversible.