Fluoride substitution

The kinetics of fluoride substitution reactions have been studied (133). The relatively low activation enthalpy for the reaction provoked Strehlow to suggest an unusual mechanism (see Ref. (32) for details). 

The XRD pattern of FA is reported in Fig. 4. It is characteristic of the apatite structure. This pattern is very analogous to that of HA. Fluoride substitution for OH is related to a decrease in the a unit-cell dimension and a very slight increase in the c unit-cell parameter. 

Finally, other effects of fluoride addition to HA have been investigated. In particular, the electrical conductivity of HA was found to be modified with fluoride substitution, as an increase in conductivity was observed [82] for OH-F apatites where up to 50% of OH ions were replaced by F. Beyond this proportion, however, a sharp drop in conductivity was pointed out. It seems, however, difficult to... 

Control of the pH and temperature of the precipitating solution is important to provide optimised conditions for stoichiometric, homogeneous, fluorhydroxyapatite formation. Similar conditions and set-up can be used for the synthesis of fluoride-substituted apatite crystals with varying size, crystallinity and morphology depending on the preparation temperature [124] a purge of the synthesis system with nitrogen gas ensures the preparation of carbonate-free fluorhydroxyapatite at ambient temperature [125]. 

Much better yields have been obtained by using phthaloyl chloride to trap the eliminated nitrite ion and tetraphenylphosphonium bromide as a catalyst in the fluoride substitution reactions.140 The less easily accessible 3-fluorobenzonitrile derivatives have been prepared this way. Table 6 illustrates some of the nitro derivatives which have been transformed into fluoro analogs in good yield.140... 

In allyl fluorides, substitution of fluorine can proceed virtually according to an SN2 mechanism. Actually, this substitution is more likely a stepwise reaction with addition of an amine and subsequent elimination of fluoride to form a shifted double bond.11 14... 

Cheng-Lee et al. (2005) demonstrated the multistep synthesis of a radiolabeled imaging probe in a PDMS microreactor, consisting of a complex array of reaction channels, with typical dimensions of 200 pm (wide) 45 pm (deep). Employing a sequence of five steps, comprising of (1) [18F] fluoride concentration (500 /iCi), (2) solvent exchange from H20 to MeCN, (3) [18F]fluoride substitution of the D-mannose triflate 252 (324 ng), to afford the labeled probe 253 (100 °C for 30 s and 120 °C for 50 s), (4) solvent exchange from MeCN to H20, and finally, (5) acid hydrolysis of 254 at 60 °C, the authors demonstrated the synthesis of 2-[18F]-FDG 254 (Scheme 72). 

Antimony(lll) fluoride, " antiinony(lll) fluoride activated with pentavalent antimony salts [anlimony(V) fluoride, antimony(V) chloride, antimony(V) bromide, or by the addition of bromine or chlorine to form pentavalent antimony salts in situ], SbFjX, and antimony(V) fluoride substitute active halogens. In general, antimony fluorides can substitute halogens in polyhalogenated compounds when more than one possible site of fluori-natioii is present, the following order of reactivity is observed (where X = Br or C ) ... 

One of the major problems associated with the hydrolysis of fluorinatcd compounds is the activation of fluoride substitution by per- or polyfliioroalkyl, perfltioroaryl groups, or double bonds linked to the same or adjacent carbon. However, hydrolysis not involving the C-F bond is widely used for the synthesis of 2-(perfluoroalkyl)ethanols, precursors to useful materials such as oil and water repellents, surfactants, and fire-fighting foams. The alcohols... 

Because of the high lattice energy of inorganic fluorides, substitutive fluorination with alkaline fluorides in aprotic solvents is always a two-phase reaction which is very dependent on the origin and the preparation of the solid reagent. 

Zanetti D, Nassif N, Antonelli AR (2001) Surgical repair of bone defects of the ear eanal wall with flexible hydroxylapatite sheets A pilot study. OtolNeurotol 22 745-753 Zaremba CM, Morse DE, Maim S, Hansma PK, Stucky GD (1998) Aragonite-hydroxylapatite conversion in gastropod (abalone) naere. Chem Mater 10 3813-3824 Zerahn B, Kofoed H, Borgwardt A (2000) Increased bone mineral density adjacent to hydroxy-apatite-eoated ankle arthroplasty. Foot Ankle Inti 21 285-289 Zhang Y, Fu T, Xu K, An H (2001) Wet synthesis and eharacterization of fluoride-substituted hydroxylapatite. J Biomed Eng 18 173-176... 

An early observation in the development of fluorosilane cross-coupling noted the affect of heteroatom substitution at the silicon on the facility of coupling. In the cross-coupling reactions between (l-octenyl)silyl fluorides and 1-iodonaphthalene, it was determined that fluoride substitution on silicon was essential for the reaction... 

Alkylation of j8-diketones using polymers as supports for the intermediate enolate anion has also been reported (Gelbard and Colonna, 1977). Reaction of several cyclohexyl j3-diketones with Amberlite IRA-9(K) formed a resin-linked j8-diketonate that could be readily alkylated. Similarly, it was shown that alkylation of phenoxide anions can be readily effected when the anions are supported via an ionic bond with the resin (Gelbard and Colonna, 1977). Alkylations of /3-diketones were also shown to occur if a fluoride-substituted, strongly basic resin (Amberlyst A26, A27, or Dowex MSA-1) is used (Miller et at., 1978). In this case, the presence of the fluoride ion was necessary before reaction would proceed. Considering the report on the use of Amberlite IRA-900— a very similar resin— the necessity for a fluoride ion is puzzling. In the same publication, the 0-alkylation of phenols, the sulfenylation of /3-diketones, and the Michael addition of a thiol to an ot,/8-unsaturated ketone were also investigated. 

Wang J, Wang B (2002) The process comprises substituting 2,4,5-trifluoro-3-nitrobenzoyl fluoride with CI2 at 190-195 °C for 16-18 h to obtain 3-chloro-2,4,5-trifluorobenzoyl fluoride substituting. CN Patent 1357548,10 Jan 2002... 

Observation of the hydration process indicated the following. In the first few minutes of the dissolution of free lime, the hydration of anhydrite and hemihydrate and the formation of calcium aluminate hydrate and monosulfate hydrate occur. Ettringite is formed within one hour, monosulfate hydrate within 2-6 hours and C-S-H gel within 1-16 hours, with the maximum heat ofhydration of C3S at about 10 hours. Measurements of the non-evaporable water indicate that the amount of combined water is 60-80% of the theoretically determined total amount of combined water at complete hydration. (The total amount of combined water was estimated to be 36%.) The amount of ettringite in the paste is estimated to be about 18-25% for the period one hourto seven days. The monosulfate content increases from about 10% at six hours to about 15-25%inone day. The alite in Reg Set cement paste is approximately 65-70% hydrated in one day and 8 0-95% hydrated in seven days. It is suggested that fluoride is tied up as A1(0H)2F. The possibility of fluoride-substituted ettringite and the formation of halo-aluminate hydrates of the form ( A CaXj wHjO is conceivable. 

Sn2 fluorination reactions are conducted in the presence of quaternary ammonium fluorides (substitution of other halogens) or in the presence of DAST or Deoxo-Huor (substitution of hydroxyl group) to provide the corresponding fluoro derivatives with inversion of configuration. Although widely used, this approach is not an asymmetric synthesis in the strict sense. In contrast to asymmetric electrophilic... 

Reactions with azide- and phthalimide nucleophiles afforded a potential entry into a-aminonitriles, which in turn are precursors for a-amino acids. a-Fluoronitriles have been prepared by fluoride substitution and, from unactivated cyanohydrins, using dieth-ylaminosulfur trifluoride (DAST) [92]. For aliphatic substrates these reactions produced a-fluoronitriles with a high e.e. in moderate yields. With aromatic substrates considerable racemization occurred, even if the a-sulfonyloxynitiile was allowed to react in situ with the appropriate nucleophile [93]. 

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