Formation of bromocarbenium bromide ion pairs in the electrophilic bromination of highly reactive olefins in chlorinated aprotic solvents

The bromination of cis- and trans-4,4'-dimethoxystibenes with Br2 and tetrabutylammonium tribromide (TBAT) led to the formation of bromocarbenium bromide. Analysis of the activation parameters of 1,2-dichloromethane and chloroform during the formation of bromocarbenium bromide indicated the nucleophilic role of the Br- ion in the TBAT-mediated bromination process. Furthermore, the ionization of the pi-complex to a bromocarbenium-bromide ion pair competed with the formation of a bromocarbenium tribromide ion pair.