Vinylic halogen exchange reactions in 1,2-dihaloperfluorocycloalkenes

Abstract

A variety of 1,2-dihaloperfluorocycloalkenes bearing chlorine, bromine or iodine at the vinylic positions have been synthesized by two different halogen exchange reactions. A novel exchange of vinylic chlorine by hydrogen bromide occurred with a catalyst composed of calcium sulfate and activated charcoal. At 220–280° the yield of 1-bromo-2-chloro- and 1,2-dibromo-perfluorocycloalkenes was 70–95% based on recovered starting material. No addition nor allylic substitution products were found. Metal halide exchange reactions were very slow in low polarity aprotic solvents, but proceeded quickly and in good yields in high polarity aprotic solvents such as dimethylformamide and hexamethylphosphoramide. The observed order of reactivity toward I − was 1,2-dichloroperfluoro-cyclobutene > cyclopentene > > cyclohexene. Displacement of vinylic fluorine halide ion was not observed.