The stereoselective synthesis of (Z)-HFCCFZnI and stereospecific preparation of (E)-1,2-difluorostyrenes from (Z)-HFCCFZnI via an unusual Pd(PPh3)4–Cu(I)Br co-catalysis approach or (Z)-HFCCFSnBu3

(Z)-HFCCFZnI was stereoselectively synthesized from activated zinc dust and (Z)-HFCCFI that was synthesized from chlorotrifluoroethene in a sequential manner. Compared to (E)-HFCCFZnI, (Z)-HFCCFZnI was more challenging to prepare in terms of sluggish metallation and formation of by-products, and underwent slower and incomplete Negishi coupling with aryl iodides. In a modification of Negishi coupling, (E)-α,β-difluorostyrenes were stereospecifically prepared in good to excellent yields under mild conditions from aryl iodides and (Z)-HFCCFZnI with the co-catalysis of Pd(PPh3)4/Cu(I)Br. Experimental investigation and mechanistic rationalization suggested that Cu(I)Br would be a scavenger of free ligands for the facilitation of Pd(PPh3)2 formation, and a supplier of ligand for the metathesis process. Alternatively, (Z)-HFCCFSnBu3 and aryl iodides with an electron-withdrawing group underwent Stille–Liebiskind coupling to afford (E)-α,β-difluorostyrenes.

Graphical abstractFigure optionsDownload full-size imageDownload as PowerPoint slideResearch highlights▶ The first stereospecific preparation of (E)-1,2-difluorostyrenes. ▶ The first stereospecific preparation of (Z)-HFCCFZnI. ▶ The use of (Z)-HFCCFSnBu3 for the preparation of (E)-1,2-difluorostyrenes. ▶ An unusual co-catalysis of Cu(I)Br/Pd(PPh3)4 in the Negishi coupling of vinylzinc reagents.