Group 4 Metallocene Difluoride/Palladium Bimetallic Catalysts for the Reductive Cross-Coupling of Alkynes with Aryl Iodides and Bromides.

A novel protocol has been developed for the selective synthesis of ( E)-alkenes via the reductive cross-coupling of alkynes and aryl halides using a bimetallic catalyst system composed of a group 4 metallocene difluoride (Cp2[M]F2; [M] = Hf or Zr; Cp = cyclopentadienide) and palladium dichloride. This reaction proceeds via a coupling between an aryl halide and an in situ generated alkenyl metallocene intermediate derived from the group 4 metallocene difluoride, a hydrosilane, and an alkyne. For a catalytic reductive coupling, the addition of sodium fluoride (NaF) to the reaction system is required. Moreover, in the presence of NaF, a ligand exchange was observed by NMR spectroscopy in hafnocene diiodide (Cp2HfI2) to afford hafnocene difluoride (Cp2HfF2).