Fluoride-ion transfer reactions

Fluoride ion transfer reactions have not been established for FBr03 and may be unlikely, (see p. 879). 

The molecular symmetry is Cj (like F3CIO Fig. 17.26d) and there is some evidence for weak intermolecular association via Fax-Br - Fax bonding. Fluoride ion transfer reactions have been estabUshed and yield compounds such as [F2BrO]+[AsF6] , [F2BrO]+[BF4]" and... 

Chloride and fluoride ion-transfer reactions in halomethanes and haloethanes have also been studied by Dawson, Henderson, O Malley, and Jennings... 

Numerous interhalogen compounds, as well as cations and anions, are known. Several engage in halide/fluoride ion transfer reactions. 

This is a fluoride-ion transfer reaction and the ions Brp2 and Brp4 are the characteristic solvent cation and anion respectively. Thus SbP 5 is regarded as an acid of this system, since it ionizes in the following manner ... 

Polymerization of methacrylates is also possible via what is known as group-transfer polymerization. Although only limited commercial use has been made of this technique, it does provide a route to block copolymers that is not available from ordinary free-radical polymerizations. In a prototypical group-transfer polymerization the fluoride-ion-catalyzed reaction of a methacrylate (or acrylate) in the presence of a silyl ketene acetal gives a high molecular weight polymer (45—50). 

The whole sequence of reactions represents a tour de force in the elegant manipulation of extremely reactive compounds. F3CIO2 is a violent oxidizing reagent but forms stable adducts by fluoride ion transfer to Lewis acids such as BF3, AsF5 and PtFe. The structures of F3CIO2 and [F2C102] have C2v symmetry as expected (Fig. 17.26e and i). 

An alternative view of the interaction of an alkali metal cation with a fluoride-containing anion is one of Lewis acid/base competition. The reactions discussed in the preceding section involved the reaction of an alkali fluoride salt with a Lewis acid with subsequent fluoride ion transfer to the Lewis acid. However, the alkali metal cation is a Lewis acid as well, and the degree of perturbation of the anion by the cation may be dependent on the differences in fluoride ion affinity of the Lewis acid and the alkali metal cation. The fluoride ion affinities for a variety of Lewis acids are well known from ICR (53,54,64) studies, while the fluoride ion affinities for alkali metal cations are the heterolytic bond dissociation energies of the gas phase alkali fluoride molecules... 

Burton and co-workers, as part of a series of ground-breaking studies on fluorinated organometallic systems [14], have established that trifluoromethyl derivatives may be obtained by reaction of halofluoromethanes with copper and other metals. The process involves electron transfer from the metal, with subsequent loss of halogen to form difluorocarbene which, in turn, generates very active fluoride ion by reaction with the solvent. The full process is indicated in Figure 10.60. 

Hydrogen ion transfer reactions. In these reactions, also known as Bronsted-Lowry acid-base reactions, a hydrogen ion is transferred from one species to another. Fluorite (Cap2) is etched by acids because a hydrogen ion is transferred from the acid to the fluoride ion and forms the molecule HF. 

Ion cyclotron resonance spectroscopy has been used to observe fluorine atom and fluoride ion transfer from neutral NF3 to PFJ and PFJ, respectively, in PF3-NF3 mixtures according to the reactions PFJ + NF3- Pp4 + NF2 and PFJ + NF3 PF3 + Np2 [3]. Product distributions in the combustion system PF3-NF3-He calculated for various reactant ratios at 1500 and 2000 K are listed in [4]. 

This variant of the Peterson reaction can be applied in forming the nitrophenyl-substituted methylenecyclopropa[f>] naphthalene 158 by employing KF with a catalytic amount of TBAF as the fluoride ion transfer reagents in acetonitrile (Scheme 2.101) [284]. This method is applicable to nitro compounds that are sensitive to strong bases. 

In contrast to the difiuorosulfiir hnides, reaction of TASF with die corre onding S-trifiuoromethyl monofluorosulfiir hnides leads to reasonably stable anions with pseudo pentacoordinated sulfiir (41). It is not unlikely that in both sterns the sulfiir centers are primarily attacked by F under formation of RC(0)NSF3 and RC(0)NSF2CF3 , re ectively. There are no profound theoretical investigations on the different stabilities of these two types of anions. Nevertheless, for deconq>osation to occur fluoride ion transfer fi om sulfiir to carbon is neccessary. If this occurs only fi om an equatorial position, then the difference in stability could be eiqilained. 

The range of nueleophiles whieh have been observed to partieipate in nueleophilie aromatie substitution is similar to that for S[, 2 reactions and includes alkoxides, phenoxides, sulftdes, fluoride ion, and amines. Substitutions by earbanions are somewhat less common. This may be because there are frequently complications resulting from eleetron-transfer proeesses with nitroaromatics. Solvent effects on nucleophilic aromatic substitutions are similar to those discussed for S 2 reactions. Dipolar... 

Electronegatively substituted acetylenes, such as dimethyl acetylenedicar-boxylate, do not react under normal conditions but will add the elements of hydrogen fluoride by reaction with fluoride ion (e g, CsF or tetraalkylammonium dihydrogen trifluoride) and a proton source under phase-transfer conditions [49, 50] (equation 8)... 

Dichloro monomers can also be polymerized with bisphenols in the presence of fluorides as promoting agents.78 The fluoride ions promote the displacement of the chloride sites to form more reactive fluoride sites, which react with phenolate anion to form high-molecular-weight polymers. Adding 5-10 mol % phase transfer catalysts such as A-alkyl-4-(dialkylamino)pyridium chlorides significantly increased the nucleophilicity and solubility of phenoxide anion and thus shortened the reaction time to one fifth of the uncatalyzed reaction to achieve the same molecular weight.79... 

There are only a few examples of low-temperature conditions reported to lead to a species behaving as an o-QM. All of these, except for our O-acyl transfer methods that will be discussed later, use a fluoride ion to trigger the formation of the o-QM in an almost instantaneous manner. In these examples, a high concentration of the intended nucleophile is necessary to prevent any side reactions with the o-QM, because given the low-temperature conditions its formation is usually irreversible. 

B. Potassium allyl- and crotyltrifluoroborates undergo addition to aldehydes in biphasic media as well as water to provide homoallylic alcohol in high yields (>94%) and excellent diastereoselectivity (dr >98 2). The presence of a phase-transfer catalyst (e.g., B114NI) significantly accelerates the rate of reaction, whereas adding fluoride ion retards the reaction (Eq. 8.70).165 The method was applied to the asymmetric total synthesis of the antiobesity agent tetrahydrolipstatin (orlistat).166... 

---