Fluoride exchange

Hudis and Wahl have examined the effect of fluoride ion on the exchange rate and have found it necessary to include three fluoride exchange paths... 

Figure 5.53 The transition-state complex for the F + CH3F fluoride-exchange reaction (5.87) (a) the optimized structure and (b) the leading np-ucF donor-acceptor interaction.

Figure 5.54 displays the IRC reaction profile for the fluoride-exchange reaction (5.87). Because the reaction coordinate is the IRC rather than R. c, the barrier profile differs somewhat from that shown in Fig. 3.86. The activation energy is calculated to be 9.54 kcal mol-1 (without zero-point-energy correction), corresponding to an appreciable rate of chemical exchange under ambient thermal conditions.

More conclusive evidence for the intermediacy of benzocyclopropenyl cations was obtained upon chloride/fluoride exchange of 22 with AgF in acetonitrile, which led to the chloro-fluoro derivative 278 and, ultimately, to the difluoro compound 279 in almost quantitative yield. Reaction of 22 with LiAlH AlClj afforded 2,5-diphenylbenzocyclopropene (264), while that with ethyl magnesium bromide lead to the diethyl derivative 280. Phenylmagnesium bromide and methylmagne-... 

FIGURE 4. Fluoride binding estimated by intermolecular fluoride exchange equilibrium constants170... 

These complexes, which are readily synthesized by transmetalation see Transmetalation) from the appropriate Cp metal and MX4 (L = U) or (jj -L)MX3 (L L ), represent a large family of qnite always air-stable componnds. The bromides and iodides are also obtained from chlorine exchange reactions with the appropriate boron trihalide. The flnorides can be obtained by metathesis see Metathesis) of chlorides derivatives with MesSnF. With only one equivalent of tin reagent per mole of dichloride, partial fluorination occms affording mixed hahde metallocenes. The fluorides are also prepared by chloride/fluoride, by chloride/NMe2 followed by NMe2/fluoride exchange, and by oxidative addition see Oxidative Addition). ... 

Tris(morpholine)selenonium fluoride (33), easily prepared by the reaction of JV-trimethylsilylmorpholine (34) with selenium tetrafluoride (35), is a very useful reagent for chloride/fluoride exchange reactions (121. Thus, benzenesulfonyl chloride (36) and diphenylphosphinyl chloride (37) gave upon treatment with 33 the corresponding fluorides 38 and 39 in good yields (Scheme 5). 

Characteristic of germyl fluorides, exchange processes lead to broadened featureless spectra in concentrated solutions. 

Very few non-oxo Movl halides are known. These include Cs[MoF7] and NO[MoF7]-MeCN, prepared by reacting MoF6 with CatF 520 the anions exhibit capped octahedral structures in line with theoretical predictions.521 18F-radiotracer experiments have revealed rapid and complete intermolecular fluoride exchange between [MF7]- and MF6 (M = Mo, W).522... 

A solution of KF or CaFj in a fused ZnClj-KCl eutectic has been also found to be suitable for chloride-fluoride exchange with phosgene [1979]. 

The rate of fluoride exchange is considerably enhanced in the presence of polar solvents e.g. MeCN or bis(2-methoxyethyl)ether, but not by benzene or diethyl ether the rates in MeCN at 25 C were comparable with the gas-solid reactions at 150 C [683]. Similarly, rapid exchange was noted between gaseous CO F j and solid MF 3 (M = La or Tl), HgF j, Hg jF or TIF at 150 C under similar conditions, AgF was largely unreactive [682], It is of interest to note that the pattern of reactivity towards fluoride exchange does not parallel the catalytic effect of the metal fluorides upon the reaction between COF j and F j (see Section 13.7.3). 

Tertiary carbocations may be conveniently prepared in such media from alkyl halides, alcohols, and alkenes. Secondary cations can be observed at low temperatures, but they rearrange readily to more stable tertiary ions. For such cases, special techniques of mixing the reactants, e.g. cocondensation on a cold surface ( molecular beam technique ), have been developed43. Attempts to prepare primary ions in the same manner have not been successful. Methyl and ethyl fluorides exchange halogen but do not generate observable concentrations of cations. All other simple... 

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