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

Abstract

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 PiPr3 resulted in the target adduct, while less sterically hindered phosphines such as PPh3 and PnBu3 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, trans-(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 trans-(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.