In presence of fluoride ions

On the other hand, we have recently investigated the role played by alkali cations in the synthesis of silicalite-1 in presence of fluoride ions [12]. 

Fluoride ions forms very stable complexes with cerium(IV) and lowers the redox potential considerably. A cerium(IV) sulfate solution is not able to oxidize iodide to iodine when appreciable amounts of fluoride ions are present. The most important fluoro complex is hexafluo-rocerate(IV), [CeFe] ", which can be formed by addition of ammonium fluoride to a solution of cerium(IV). The behavior of cerium(IV) in presence of fluoride ions is markedly different from that of cerium(III). Addition of an ammonium fluoride solution to a solution containing cerium(III) results in the precipitation of gelatinous cerium(III) fluoride, CeF3, which becomes powdery upon standing (Svehla, 1979). The use of cerium(IV) fluoride as a fluori-nating agent is discussed in sections 5.16 and 6.7. 

A classic example of this process is dimerization or oligomenzation of the olefins in the presence of fluoride ion [13], 204, 205], in which the electrophile is... 

Hexafluoro-2-butyne readily undergoes anionic oligomerization in the presence of fluoride ion 24 ], but the intermediate vinylic carbamon can be trapped by highly electrophilic fluormated heterocycles [249] (equation 52)... 

When potassium fluoride is combined with a variety of quaternary ammonium salts its reaction rate is accelerated and the overall yields of a vanety of halogen displacements are improved [57, p 112ff. Variables like catalyst type and moisture content of the alkali metal fluoride need to be optimized. In addition, the maximum yield is a function of two parallel reactions direct fluorination and catalyst decomposition due to its low thermal stability in the presence of fluoride ion [5,8, 59, 60] One example is trimethylsilyl fluoride, which can be prepared from the chloride by using either 18-crown-6 (Procedure 3, p 192) or Aliquot 336 in wet chlorobenzene, as illustrated in equation 35 [61],... 

As a result, the electromotive force (EMF) of the cell is zero In the presence of fluoride ions, cerium(IV) forms a complex with fluoride ions that lowers the cerium(IV)-cerium(IIl) redox potential The inner half-cell is smaller, and so only 5 mL of cerium(IV)-cenum (III) solution is added To the external half-cell, 50 mL of the solution is added, but the EMF of the cell is still zero When 10 mL of the unknown fluonde solution is added to the inner half-cell, 100 mL of distilled water IS added to the external half-cell The solution in the external half-cell is mixed thoroughly by turning on the stirrer, and 0 5 M sodium fluonde solution is added from the microburet until the null point is reached The quantity of known fluonde m the titrant will be 10 times the quantity of the unknown fluoride sample, and so the microburet readings must be corrected prior to actual calculations... 

F H. T. Dean et al. put the correlation between decreased incidence of dental caries and the presence of fluoride ions in drinking water on a quantitative basis... 

Therefore, in order to obtain a 1,4-addition of an allyl residue to an enone, two activation modes can be used reactions take place either under electrophilic conditions with Lewis acid promotion, or in the presence of fluoride ions. This is important as the stereochemical outcome often depends on the activation mode selected. 

The blue fluorescence of dioxetane 25 (X = O and CH2, R = alkyl) in the presence of fluoride ion has been studied <96TL5939>. 

In acidic solution MnOj is usually the end product, although particularly vigorous reductants, e.g. iodide and oxalate ions, convert permanganate to manganous ions. Mn(III) is stable only in acidic solution or in the form of a complex, e.g. with pyrophosphate ion, and it has seldom been reported as the end product of a permanganate oxidation, e.g. for that of Mn(II) in a phosphate buffer and for those of alcohols and ethers in the presence of fluoride ion. ... 

The analogous rhodium complexes exhibit very low activity, even in the presence of fluoride ions [143]. In contrast, (PRjljRhCl (R = Ph, Et) and the corresponding... 

Calza, P., Pelizzetti, E., Mogyorosi, K, Kun, R., and Dekany, I. (2007) Size dependent photocatalytic activity of hydrothermally crystallized titania nanoparticles on poorly adsorbing phenol in absence and presence of fluoride ion. Applied Catalysis B Environmental, 72 (3 1), 314-321. 

Bicyclic trimethylsilyl nitronates undergo stereoselective Henry reactions with benzalde-hyde in the presence of fluoride ion to give cyclic hemiacetals in good yield with high diastereo-selectivity (95% ds) (Eq. 3.68).110... 

The stannylation and silylation of heteroaryl halides has also been studied. The stannylation of 4-iodo pyrimidine was conducted with hexamethyl and hexa-ra-butyl ditin in the presence of fluoride ion and Pd(PPh3)2(OAc)2 as catalyst. The stannylation of 5-bromopyrimidine occurred in the highest yields in the presence of the ligandless catalyst bis(7r-allylpalladium chloride).175 The silylation of bromopyridines, bromopyrimidines, and bromoquinolines also occurred after 96 h at 160 °C with Pd(PPh3)4 as catalyst in HMPT solvent.162... 

The performance in Zn-MnC>2 alkaline cells of electrolytic manganese dioxide produced in the presence of fluoride ions is compared with commercial EMD materials by I. Makyeyeva et al. The material produced in the presence of fluoride ions was shown to have superior utilization to the commercial EMD. The authors state that the improved... 

In view of this situation we studied a number of samples of Mn02 with different composition and various OH" contents in order to estimate the correlation between the activity of Mn02 and concentration of OH" ions. This compound can be electrochemically deposited on the anode from various aqueous solutions, but electrolytes with sulfate and ammonia sulfate have found widest application [3], It has been determined that the composition and structural parameters of the end-product are governed by the presence of fluoride ion in electrolyte. 

When carried out under standard conditions with Et3SiH/TFA, reduction of acrolein leads to a mixture of allyl alcohol, 1-propanol, and di-n-propyl ether in addition to allyl trifluoroacetate and -propyl trifluoroacetate.434 The 1,2-reduction of cinnamaldehyde with triethoxysilane in the presence of fluoride ion provides the corresponding allyl alcohol in good yields (Eq. 261). 

In an attempt to produce TS-1 at low cost, alternative, cheaper sources of Ti and Si and other bases such as binary mixtures of (tetrabutylammonium and tetraethylammonium hydroxides), (tetrabutylphosphonium and tetraethylpho-sphonium hydroxides), (tetrapropylammonium bromide and ammonia, water, hexanediamine, n-butylamine, diethylamine, ethylenediamine, or triethanolamine) in place of TPAOH have been used (284—294). TS-1 was synthesized in the presence of fluoride ions but the material thus formed contained extraframework Ti species (295-297). 

In addition, Unheim et al. coupled 2-bromothiazole with 2-methylthio-5-stannylpyrimidine (58) to assemble the pyrimidinethiazole 59 [43]. Stannane 58 was prepared by Pd-catalyzed coupling between 2-methylthio-5-bromopyrimidine and hexamethyldistannane in the presence of fluoride ion at ambient temperature. In another case, 2-tributylstannylthiazole was coupled with 2 -deoxyuridine iodide 60 (DMT = dimethoxytrityl) to furnish 5-thiazolyl-2 -deoxyuridine 61 [44]. 

In contrast to the cathodic reduction of organic tellurium compounds, few studies on their anodic oxidation have been performed. No paper has reported on the electrolytic reactions of fluorinated tellurides up to date, which is probably due to the difficulty of the preparation of the partially fluorinated tellurides as starting material. Quite recently, Fuchigami et al. have investigated the anodic behavior of 2,2,2-trifluoroethyl and difluoroethyl phenyl tellurides (8 and 9) [54]. The telluride 8 does not undergo an anodic a-substitution, which is totally different to the eases of the corresponding sulfide and selenide. Even in the presence of fluoride ions, the anodic methoxylation does not take place at all. Instead, a selective difluorination occurs at the tellurium atom effectively to provide the hypervalent tellurium derivative in good yield as shown in Scheme 6.12. 

Table 2. Anodic oxidation of organosilanes in the presence of fluoride ions [21]...

Homoallylic alcohols.1 In the presence of fluoride ion these silanes react with aldehydes to form homoallylic alcohols in high yield (equation I). Crotyltrifluo-rosilanes undergo a similar reaction with aldehydes, but in this case the stereose-... 

Alkynylsilanes will react with bis(tributyltin) oxide in the presence of fluoride ion as catalyst to give the stannylalkynes in excellent yield and hexamethyldisiloxane, which can be removed under reduced pressure (Equation (82)).68 244... 

Allylsilanes in the presence of fluoride ion can be used in place of the Grignard reagents, the reaction of allyltrimethylsilane and bis(tributyltin) oxide giving allyltributyltin in 96% yield.68... 

The reaction of Cjq with silylated nucleophiles [47] requires compounds such as silyl ketene acetals, silylketene thioacetals or silyl enol ethers. It proceeds smoothly and in good yields in the presence of fluoride ions (KF/18-crown-6) (Scheme 3.10). The advantage of the latter synthesis is the realization of the cyclopropanation under nearly neutral conditions, which complements the basic conditions that are mandatory for Bingel reactions. Reaction with similar silyl ketene acetals under photochemical conditions and without the use of F does not lead to methanofullerenes but to dihydrofullerene acetate [48]. 

Solid solutions between carbonated HA and fluorapatite occur naturally in the body, in bone and teeth. In particular, the presence of fluoride ions offers the low solubility and good acid resistance needed for protecting teeth [57]. Note, however, that at very high pH levels, the HA end-member becomes less soluble than fluorapatite [58]. That is why it is often difficult to prepare stoichiometric fluorapatite by precipitation methods involving generally alkaline pH levels. 

Fig. 8. Evolution of synthetic and biological apatites in the presence of fluoride ions. The increase of pH and/or phosphate concentration in solution favours the formation of fluoridated apatite, whereas the increase of fluoride and/or calcium concentration favours CaF2 formation. At physiologic pH and mineral ions concentrations (in all body fluids), the formation of fluoridated apatite is favoured.

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