Ylide-Carbene Chemistry. Synthesis of 1,1-Difluoro-1-alkenes

摘要

The reaction between nonstabilized alkylidenetriphenylphosphoranes and chlorodifluoromethane has been found to be a useful alternative to the Wittig reaction for the synthesis of many difluoromethylene olefins. Both primary and secondary ylides which do not contain strongly electron-withdrawing substituents within the alkylidene portion of the ylide react with chlorodifluormethane to give the corresponding difluoromethylene olefins in yields which are significantly better than those obtained by the Wittig reaction. The formation of triphenyl-phosphene oxide is avoided, and all phosphorus-containing moieties can be recovered and recycled. The reaction proceeds by initial dehydrochlorination of chloridifluoromethane by the ylide to generate difluorocargene. The intermediate difluorcorbene is then trapped by a second equivalent of lthe nucleophilic ylide. Mechanistic evidence indicates that either a zwitterionic intermediate or a three-membered cyclic phosphorane can account for the 1,1-difluoro-1-alkalene products such as FCH=CHPh, FHC-CPh2, and FHC=CHCH-CHPh after steam distillation of the reaction mixtures, however can only be accounted via a three-membered cyclic phosphorane.