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 P i Pr3 resulted in the target adduct, while less sterically hindered phosphines such as PPh3 and P n 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, 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.

Regioselective CF Bond Activation of Hexafluoropropylene on Palladium(0): Formation of a Cationic η2‐Perfluoroallylpalladium Complex

“…[3] Recently, we also demonstrated that the C À F bond cleavage of TFE using a group 10 metal was accelerated by the addition of not only metal halides such as lithium iodide but also boron Lewis acids, such as BF 3 and B(C 6 F 5 ) 3 . [4] Our next concern was to develop a novel strategy for the regioselective CÀF bond activation of perfluoroolefins. Hexafluoropropylene (HFP; 1) is the simplest perfluoroalkene, second only to TFE, and 1 contains four different types of fluorine atoms whereas TFE consists of chemically equivalent ones (Scheme 1).…”

“…[16] 3 was too unstable to be isolated. [4] The perfluoroallyl ligand in 3 a showed reactivities toward nucleophiles. When 3 a was treated with an equimolar amount of either PCy 3 PCy 3 , 5 a; XR 3 = DABCO) was generated in quantitative yield (Scheme 5).…”

Palladium-Mediated Organofluorine Chemistry

“…224 Thus, LiI has been used as additive to ease R-F activation by forming a [PdRIL 2 ] and LiF which has a larger lattice energy than LiI. 245,246 Also, a the addition of a hard Lewis acid which can form an adduct with fluoride has the same effect. 246 Oxidative addition of arylfluorides is preceded by arene coordination and this has been shown experimentally for Ni(0) compounds which coordinate octafluoronaphthalene, 305 and fluorobenzenes.…”

“…245,246 Also, a the addition of a hard Lewis acid which can form an adduct with fluoride has the same effect. 246 Oxidative addition of arylfluorides is preceded by arene coordination and this has been shown experimentally for Ni(0) compounds which coordinate octafluoronaphthalene, 305 and fluorobenzenes. 306,307 where a new P-F bond has been formed and a cyclohexyl group is transferred from the phosphorous to the metal (Scheme 102).…”

Palladium-Catalyzed Cross-Coupling Reactions of Perfluoro Organic Compounds

“…The first palladium-catalyzed cross-coupling reaction of 1 with aryl zinc compounds in the presence of LiI is also discussed [41,57]. We then discuss the development of the active Pd(0)/PR 3 species that enabled the oxidative addition of the C-F bond of 1 using no additives.…”