Fluoride acceptors

It will be noted that addition of SbFj to HF considerably enhances its acidity and the. same effect can he achieved hy other fluoride acceptors. such as BFj and TaFj ... 

Consequently, the relative contribution of xenodeborylation to fluorine addition across the C=C double bond and the ratio of the [BF4] to [XCF=CFBF3] anions depends on the electron-withdrawing character of X (acidity of the borane) and the actual ratio of XeF2 (fluoride donor) to XCF=CFBF2 (fluoride acceptor). 

Considering bromine trifluoride as an example, the self-ionization is shown in equation (1). Dissolution of fluoride acceptors, such as antimony pentafluoride, gives compounds formally containing the difluorobromine(III) cation, and correspondingly, dissolution of fluoride donors, such as potassium fluoride, gives compounds containing the tetrafluorobromate(III) anion. 

Finally, the metal-perfluoroalkyl linkage also appears to be less susceptible to facile decomposition by the a- or -elimination pathways that dominate much of the chemistry of hydrocarbon alkyls and lead to metal hydrides. The absence of these reaction pathways, at least for the later transition metals, may reflect the relative strength of the C—F bond versus the M—F bond compared to C—H/M—H analogues (32). However, a-fluoride abstraction reactions can be accomplished with exogenous fluoride acceptors to give fluorinated carbene complexes (see Section III,B,1). One example of an apparent -fluorine elimination reaction is shown in Eq. (2) (33) and presumably is driven by the stronger bond to fluorine formed by early transition... 

Silene 2, 1-fluorosilene 602 and 1,1-difluorosilene 603 have been examined by FT ion cyclotron resonance spectroscopy in order to estimate the jr-bond strength. Bracketing studies with various bases (B) and fluoride acceptors (A) gave values for the proton and fluoride affinity of the silenes284. The TT-bond energy was calculated by thermochemical cycles and found to increase with fluorine substitution. The data are given in Table 16. 

Mixtures of SbFs and fluorosulphonic acid S02F(0H) are among the most powerful proton-donating media known. Here the SbFs acts as a Lewis acid, as usual, but not as a fluoride acceptor ... 

Fig. 6. Orthogonal glycosylation the glycosyl donor is attached to the solid support alternating coupling with a thioglycoside and a glycosyl fluoride acceptor.

Reaction of HF with S03 gives fluorosulphonic acid HS03F, which is even more strongly acidic than sulphuric acid. In mixtures with S03 and powerful fluoride acceptors such as SbF5 it gives superacid media, which are capable of protonating even most organic compounds. 

In accordance with the solvent-system concept fluoride donors like NaF act as bases in this medium (giving Na+ and BrF-4), and fluoride acceptors such as SbF5 act as acids (giving BrF+2 and SbF-4). 

When the reaction is performed in diethyl ether at low temperatures in the presence of hydrogen fluoride acceptors, the fluorination reaction does not take place, instead the corresponding trifluoro(trihydroperfluoroalkoxy)-,i -sulfanes (R = H n = 2,4,6) arc formed. " ... 

Salt/molecule reactions with fluoride acceptors or Lewis acids appear, in all cases studied to date, to form ion pair complexes as the initial reaction product for such Lewis acids as HF, SiFi, BF3 and COF2. There has been no evidence of abstraction as the initial step, although some systems may exist where this step will be preferred. Some spectroscopic effects of ion pairing are observed in the spectra of the product anions, but these still allow for the study of imusual anions in a relatively unperturbed environment. 

The pentafluorides, AsFs and SbFs, are prepared by direct fluorination of the elements or their oxides. Both AsFs and SbFs are used as Lewis acid fluoride acceptors in the preparation of many MFg salts. Anhydrous HF has greatly increased acidity in the presence of SbFs because of the creation of the H2F+ cation by removal of fluoride. The hexafluoroantimonate anion stabilizes many energetic cationic species, for example, [NF4+][SbF6 ]. 

Chlorine trifluoride may serve as a fluoride donor or fluoride acceptor to give cations and anions, depending on the substrate employed (Scheme 9). 

When solid-state and solution studies are carried out on krypton and xenon difluoride adducts incorporating fluoride acceptors in the weak to intermediate range, such as WF40 and MoF40, the results indicate that, in both phases, the adducts contain Ng F M (Ng = Kr or Xe) bridges and ionic contributions to the bonding are minimised 46 49). 

Thus, COFj is a very powerful fluoride acceptor (for comparisons with other fluoride acceptors, see [1212a]), and this tendency dominates many of its reactions with metal fluorides. 

Ultraviolet irradiation of perfluorocarbons in the presence of (C5Me5)2Fe and Li03SCF3 as a fluoride acceptor provides an exceptionally mild system [80] to prepare perfluoroalkenes from perfluoroalkanes and the chemistry can be made catalytic with respect to iron by using zinc to reduce [(C5Me5)2Fe]+(Scheme 6). 

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