The stereoselective synthesis of (Z)-HFC=CFZnI and stereospecific preparation of (E)-1,2-difluorostyrenes from (Z)-HFC=CFZnI via an unusual Pd(PPh3)4-Cu(I)Br co-catalysis approach or (Z)-HFC=CFSnBu3

(Z)-HFC=CFZnI was stereoselectively synthesized from activated zinc dust and (Z)-HFC=CFI that was synthesized from chlorotrifluoroethene in a sequential manner. Compared to (E)-HFC=CFZnI, (Z)-HFC=CFZnI 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)-alpha,beta-difluorostyrenes were stereospecifically prepared in good to excellent yields under mild conditions from aryl iodides and (Z)-HFC=CFZnI 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)-HFC=CFSnBu3 and aryl iodides with an electron-withdrawing group underwent Stille-Liebiskind coupling to afford (E)-alpha,beta-difluorostyrenes. (C) 2010 Elsevier B.V. All rights reserved.