FORMATION OF BROMOCARBENIUM BROMIDE ION-PAIRS IN THE ELECTROPHILIC BROMINATION OF HIGHLY REACTIVE OLEFINS IN CHLORINATED APROTIC-SOLVENTS

The kinetics and the products of bromination of several substituted st ilbenes with tetrabutylammonium tribromide (TBAT) have been investigat ed in aprotic solvents at different temperatures, Stilbenes bearing el ectron-withdrawing or moderately electron-donating substituents gave s tereospecifically the anti addition products. The reactions followed a second-order rate law, and an inverse kinetic isotope effect (KIE), K -H/k(D) = 0.85(0.05), was found for the bromination of cis-stilbene. T he reactions of cis- and trans-4,4-dimethoxystilbenes yielded mixtures of meso and d,l dibromides both in chloroform and 1,2-dichloroethane. The rate constants (k(Br3)-) measured for the latter olefins deviated considerably fi om the Hammett correlations, and added bromide had a significant effect on the rates. The reactions of these activated stil benes with molecular Br-2, carried out at low Br-2 concentration, foll owed a mixed second/third-order rate law. The kinetic and product dist ribution data for the reaction, with TBAT, of stilbenes bearing electr on-withdrawing or moderately electron-donating substituents are interp reted on the basis of the known mechanism involving a product- and rat e-determining nucleophilic attack by bromide on the olefin-Br-2 pi-com plex. The data related to the bromination of the more activated methox ystilbenes are rationalized considering that, for these olefins, even in aprotic solvents, the ionization of the initially formed 1:1 rr-com plex to a bromocarbenium bromide ion pair can compete both with the fo rmation of a bromonium-tribromide ion pair and with the nucleophilic a ttack by Br-. For this second-order process (first order in Br-2), the kinetic constants and the activation parameters have been measured in chloroform and 1,2-dichloroethane and the activation parameters have been compared with those related to the third-order Br-2 addition and to the reaction with TBAT.