Phosphonium Ylide-Catalyzed <i>Z</i> -Stereoselective Synthesis of α-Substituted Monofluoroalkenes from <i>gem</i> -Difluoroalkenes and Triazoles

The defluorinative functionalization of <i>gem</i>-difluoroalkenes provides an effective strategy for the synthesis of monofluoroalkenes. Despite the considerable progress achieved via organo- and transition-metal catalysis, major challenges persist, which means that the development of new catalytic systems remains attractive. Herein, we introduce phosphonium ylide as an organocatalyst to accomplish the synthesis of α-substituted <i>Z</i>-monofluoroalkenes from <i>gem</i>-difluoroalkenes and triazoles. The reaction is promoted with high stereoselectivity and compatible with a broad range of substrates. A wide range of complex natural products and pharmaceuticals are amenable to late-stage modification. Experimental mechanistic studies indicate that the phosphonium cation, formed via capture of the N-protecting group of triazole by the phosphine ylide, may play a key role in governing stereoselectivity.