Fluoride ions reactions

Here benzyne formation involves abstraction of a proton (reaction 5) by the base QH5 to form a carbanion which loses fluoride ion (reaction 6) to give benzyne. 

The first application of copper(I)-catalyzed 1,3-dipolar cycloaddition in preparation of [18F]fluoropeptides was reported by Marik and Sutcliffe in 2006 (Figure 14.9) [92]. Three [18F]fluoroalkynes (n = 1, 2, and 3) were prepared in yields ranging from 36% to 80% by nucleophilic substitution of a p-toluenesulfonyl moiety with [18F]fluoride ion. Reaction of these [18F]fluoroalkynes with various peptides (previously derivatized with 3-azidopropionic acid) via the Cu(I)-mediated 1,3-dipolar cycloaddition provided the desired 18F-labeled peptides in 10 minutes at room temperature with yields of 54-99% and great radiochemical purity (81-99%) [82]. 

Fluoride ion reactions are somewhat puzzling. Oxidation of fluoride ion by a cation radical is out of the question and nucleophilic attack would appear to be certain. However, this reaction has failed completely with several cation radical perchlorates, such as those from perylene (Ristagno and Shine, 1971b), and phenothiazine (Shine et al., 1972). It appears that fluoride ion is too weak a nucleophile to participate in such substitution reactions. On the other hand, several cases of anodic fluorination are known oxidation of some aromatics, in solutions containing fluoride ion and at potentials lower than the oxidation potential of fluoride ion, has led to fluorination. This has occurred with naphthalene, which gave... 

Fluoride-ion reactions are the most perplexing of all. A fairly large number of anodic fluorinations have been reported in the literature (41). These are anodic oxidations of aromatics and alkenes at potentials well below that of fluoride ion. Yet, examples of fluorination of isolated cation radicals are, so far, very rare. Reaction of fluoride ion with 3 gave 3 (38%), its 3,10 -dimer (13%) and the well-known green dimer cation (jS). In leading to the dimer, fluoride ion has>behaved as a base. For some years it appeared that fluoride ion was too poor a nucleophile to react as in eq. 31, but we believe now that this is not correct. Mass spectrometry has shown that a monofluoro-N-phenyl-phenoxazine dimer is obtained from S " (8). Most recently mass spectrometry has also shown that a small amount of fluoroperylene is formed from lO " ", in contrast with our earlier report ( ). ... 

The bond dissociation energy of the hydrogen-fluorine bond in HF is so great that the above equilibrium lies to the left and hydrogen fluoride is a weak acid in dilute aqueous solution. In more concentrated solution, however, a second equilibrium reaction becomes important with the fluoride ion forming the complex ion HFJ. The relevant equilibria are ... 

Analytical Procedures. Oxygen difluoride may be determined conveniently by quantitative appHcation of k, nmr, and mass spectroscopy. Purity may also be assessed by vapor pressure measurements. Wet-chemical analyses can be conducted either by digestion with excess NaOH, followed by measurement of the excess base (2) and the fluoride ion (48,49), or by reaction with acidified KI solution, followed by measurement of the Hberated I2 (4). 

Nucleophilic Reactions. The strong electronegativity of fluorine results in the facile reaction of perfluoroepoxides with nucleophiles. These reactions comprise the majority of the reported reactions of this class of compounds. Nucleophilic attack on the epoxide ring takes place at the more highly substituted carbon atom to give ring-opened products. Fluorinated alkoxides are intermediates in these reactions and are in equiUbrium with fluoride ion and a perfluorocarbonyl compound. The process is illustrated by the reaction of methanol and HFPO to form methyl 2,3,3,3-tetrafluoro-2-methoxypropanoate (eq. 4). 

TFEO is by fai the most reactive epoxide of the series. However, ail the reported perfluoroepoxides undergo similar ring-opening reactions. The most important reactions of these epoxides ate those with the fluoride ion or perfluoroalkoxides. The reaction of PIBO and the fluoride ion is an example (27). It also illustrates the general scheme of oligomerization of perfluoroepoxides (eq 5). 

Hexafluoropropylene Oxide HFPO is the most important of the perfluoroepoxides and has been synthesized by almost all of the methods noted. Many attempts have been made to polymerize HFPO (6,8). The most successful has been the reaction of HFPO with fluoride ion at low temperature to give a series of oligomeric acid fluorides which have been end capped to yield stable fluids (eq. 11, where X = H,F). 

Fluorinated ether-containing dicarboxyhc acids have been prepared by direct fluorination of the corresponding hydrocarbon (17), photooxidation of tetrafluoroethylene, or by fluoride ion-cataly2ed reaction of a diacid fluoride such as oxalyl or tetrafluorosuccinyl fluorides with hexafluoropropylene oxide (46,47). Equation 8 shows the reaction of oxalyl fluoride with HEPO. A difunctional ether-containing acid fluoride derived from HEPO contains regular repeat units of perfluoroisopropoxy group and is terminated by two alpha-branched carboxylates. 

Diazotization Routes. Conventional Sandmeyer reaction conditions are not suitable to make fluoroaromatics. Phenols primarily result from high solvation of fluoride ion in aqueous media. 

Chromium Phosphate. Chromium phosphate treatment baths are strongly acidic and comprise sources of hexavalent chromium, phosphate, and fluoride ions. Conversion coating on aluminum precedes by the foUowing reactions (24) ... 

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). 

Fluoride ion attacks the sulfur atom in 2,3-diphenylthiirene 1,1-dioxide to give ck-1,2-diphenylethylenesulfonyl fluoride (23%) and diphenylacetylene (35%). Bromide or iodide ion does not react (80JOC2604). Treatment of S-alkylthiirenium salts with chloride ion gives products of carbon attack, but the possibility of sulfur attack followed by addition of the sulfenyl chloride so produced to the alkyne has not been excluded (79MI50600). In fact the methanesulfenyl chloride formed from l-methyl-2,3-di- -butylthiirenium tetrafluoroborate has been trapped by reaction with 2-butyne. A sulfurane intermediate may be indicated by NMR experiments in liquid sulfur dioxide. 

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