Sodium dithionite initiated addition of 1-bromo-1-chloro-2,2,2-trifluoroethane to allylaromatics: Facile synthesis of conjugated dienes substituted with terminal CF3 group

Sodium dithionite effectively promotes the addition of 1-bromo-1-chloro-2,2,2-trifluoroethane to the terminal double bond of allylbenzenes 1. The reactions proceeded in MeCN/H2O to give a 3:1 mole ratio of diastereoisomers of 1-(2-bromo-4-chloro-5,5,5-trifluoropentyl)benzenes 2 as the main products together with small amounts of its reductive debromination products 3. Total yields of 2 and 3 were dependent on the nature of the aromatic ring substituents in 1. Treatment of adducts 2 with DBU in refluxing hexanes resulted in double dehydrohalogenation affording, in good yields, conjugated dienes 4 (1,1,1-trifluoro-5-phenyl-2,4-pentadienes) terminated with the CF3 group at the one end and the phenyl group at the opposite end. These dienes were found to be sufficiently reactive to undergo Diels-Alder condensation with active dienophiles to give trifluoromethylated carbocycles. The reactions of CF3CHClBr with allylheterocycles were less successful and lead to low yields of mixtures of hardly separable compounds or to polymeric resins.