Carbon–Fluorine Bond Activation of Tetrafluoroethylene on Palladium(0) and Nickel(0): Heat or Lewis Acidic Additive Promoted Oxidative Addition

The C–F bond cleavage reaction of tetrafluoroethylene (TFE; CF₂CF₂) with an M(0) complex (M = Pd, Ni) was investigated. The treatment of an M(0) precursor with TFE in the presence of the appropriate monodentate phosphine ligand led to a clean formation of the corresponding η²-TFE adduct (η²-TFE)­M­(PR₃)₂. In the case of the Ni(0) species, in particular, the choice of phosphine ligands is crucial for the preparation of the desired η²-TFE complex: the use of either PCy₃ or P^<i>i</i>Pr₃ resulted in the target adduct, while less sterically hindered phosphines such as PPh₃ and P^<i>n</i>Bu₃ gave the known octafluoronickelacyclopentane as a result of the oxidative cyclization of two TFE molecules. Thermolysis of both palladium and nickel η²-TFE adducts bearing PCy₃ as the ligand resulted in a C–F bond activation reaction and gave the corresponding (trifluorovinyl)metal fluorides, <i>trans</i>-(PCy₃)₂M­(F)­(CFCF₂). The reaction of (η²-TFE)­Pd­(PPh₃)₂ with LiI as an additive allowed cleavage of the C–F bond in THF, even at room temperature, and gave <i>trans</i>-(PPh₃)₂Pd­(I)­(CFCF₂) with a concomitant formation of lithium fluoride. Other metal halides, such as MgBr₂ and AlCl₃, also promoted the C–F bond cleavage of TFE. In addition, the use of either BF₃·Et₂O or B­(C₆F₅)₃ exerted a similar accelerative effect on the C–F bond activation of TFE on either nickel or palladium. The molecular structures of a series of η²-TFE and trifluorovinyl complexes were unambiguously determined by means of X-ray crystallography. The resultant (trifluorovinyl)palladium or -nickel species have shown the potential to utilize a key intermediate in cross-coupling reactions with organometallic reagents to prepare a variety of trifluorovinyl compounds