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

Article abstract of DOI:10.1021/jo00155a001

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 chlorodifluoromethane to give the corresponding difluoromethylene olefins in yields which are significantly better than those obtained by the Wittig reaction.The formation of triphenylphosphine oxide is avoided, and all phosphorus-containing moieties can be recovered and recycled.The reaction proceeds by initial dehydrochlorination of chlorodifluoromethane by the ylide to generate difluorocarbene.The intermediate difluorocarbene is then trapped by a second equivalent of the nucleophilic ylide.Mechanistic evidence indicates that either a zwitterionic intermediate or a three-membered cyclic phosphorane can account for the 1,1-difluoro-1-alkene products.The isolation of several 1-hydro-1-fluoro-1-alkene products such as FCH=CHPh, FHC=CPh2, and FHC=CHCH=CHPh after steam distillation of the reaction mixtures, however, can only be accounted for via a three-membered cyclic phosphorane.