Interconversion among Dibromocyclopentene Isomers

Trans-3,4, Trans-3,5-, and cis-3, 5-dibromocyclopentenes (1, 2, and 3) underwent facile interconversions in chloroform in the presence of a catalytic amount of tetraethylammonium bromide. The isomerizations involved the nucleophilic attacks by the added bromide ions to these allylic systems through both the S_N2 and S_N2\u27 mechanisms. The S_N2\u27 reactions were associated with syn stereospecificity, and proceeded at rates ca. 1/10 those of the S_N2 reactions. In the absence of added ammonium salt, isomers 1 and 2 isomerized into one another, but isomer 3 remained intact