Stereoselective Palladium-Catalyzed C-F Bond Alkynylation of Tetrasubstituted gem-Difluoroalkenes.

A stereoselective Pd(PPh3 )4 -catalyzed C-F bond alkynylation of tetrasubstituted gem-difluoroalkenes with terminal alkynes has been developed. This method gives access to a great variety of conjugated monofluoroenynes bearing a tetrasubstituted alkene moiety with well-defined stereochemistry. Chelation-assisted oxidative addition of Pd to the C-F bond is proposed to account for the high level of stereocontrol. An X-ray crystal structure of a key monofluorovinyl PdII intermediate has been obtained for the first time as evidence for the proposed mechanism.