Fluoride sources

The movement of fluoride through the atmosphere and into a food chain illustrates an air-water interaction at the local scale (<100 km) (3). Industrial sources of fluoride include phosphate fertilizer, aluminum, and glass manufacturing plants. Domestic livestock in the vicinity of substantial fluoride sources are exposed to fluoride by ingestion of forage crops. Fluoride released into the air by industry is deposited and accumulated in vegetation. Its concentration is sufficient to cause damage to the teeth and bone structure of the animals that consume the crops. 

Kuwajima (75) has provided full details of the regiospecific monoalkylation of carbonyl compounds via their silyl enol ethers, using stoichiometric amounts of fluoride ion. Noyori (76) has given more information on the use of the complex fluoride source (2) (Chapter 18)... 

Fluoride ion can also induce reaction of silyl ketene acetals with electrophilic alkenes. The fluoride source in these reactions is fnT-(dimethylamino)sulfonium diflu-orotrimethylsilicate (TASF). 

The 2-(trimethylsilyl)ethoxymethyl group (SEM) can be removed by various fluoride sources, including TBAF, pyridinium fluoride, and HF.165 This deprotection involves nucleophilic attack at silicon, which triggers (3-elimination. 

The C—Si bond can be activated by NaOH as well as a fluoride source, whereas a series of other inorganic bases such as LiOH, KOH, K2CO3, and Na2CC>3 are less effective. Thus, ( )-hexenylchlorosilane 125 was coupled with 2-chloropyridine, giving rise to 2-( T)-hexenyl-pyridine (126) [103]. 

In 1998, Carreira reported that a catalyst formed from Tol-BINAP, Cu(OTf)2, and 2 equiv of Bu4N+ Ph3SiF2 (TBAT), a soluble fluoride source, was extremely effective in mediating the aldol reaction between a silyldienolate and aromatic or vinyl aldehydes (254). Although initially formulated as a Cu(II) catalyst, subsequent evidence has shown that the active catalyst is a Cu(I) phosphine complex. By using only 2 mol% of the complex, excellent yields and enantioselectivities are observed with a range of aromatic aldehydes (93-95% ee, 86-98% yield), along with some enals (cinnamaldehyde provided the aldol adduct in 83% yield and 85% ee), Eq. 221. 

Sulfonyl chloride 2 is used to protect primary and secondary amines as the corresponding sulfonamide.8 The SES-protected amines are stable compounds that can be readily cleaved by fluoride sources to regenerate the parent amine. 

Aldolization Using Nonracemic Chiral Fluoride Sources. 58... 

Due to ubiquitous exposure to fluoride sources other than drinking water, it is not possible to draw firm conclusions regarding the independent effects of fluoride in drinking water on dental caries and its prevention. It has been estimated that moderate dental fluorosis occurs in 1-2% of the population exposed to fluoride at 1 mg/L in drinking water and in about 10% of the population at 2 mg/L moderate/severe fluorosis occurs in variable percentages ranging up to 33% of the population exposed to fluoride at 2.4-4.1 mg/L [52]. 

Silyl substituted chloromethyl ethers such as 49 serve as a convenient precursor to nonstabilized carbonyl ylides. Treatment with a fluoride source promoted... 

Fluorophosphites are prepared by a two step sequence. The initial step is the reaction of phenolic materials with phosphorus trichloride to prepare a chlorophosphite intermediate. The chlorophosphite is then treated with a fluoride source to convert the chloro- intermediate into the desired fluorophosphite product. Many different fluoride sources have been described in the literature including anhydrous hydrogen fluoride, anhydrous potassium fluoride, and antimony trifluoride. While any of these fluoride sources can be used successfully, we have found that antimony trifluoride (10) works well for small scale, lab preparations. 

The coupling of organosilicon compounds with organic electrophiles was not disclosed until 1988 by Hatanaka and Hiyama, when they demonstrated that through the addition of an appropriate silicophilic nucleophile, those desired pentacoordinate species can be generated in situ and transfer an unsaturated group. Nucleophilic fluoride sources were found to be the additive of choice, typically TASK, TBAF, and, in some cases, KF and GsF. These are the fundamental concepts of what is nowadays called the Hiyama reaction. The use of fluoride... 

Recently, analogues of nucleosides [60], natural products Huperzine-A [61] and Hydroartemisinin [62], and inhibitors of metallo-/ -lactamases have been synthesised [63]. With acylsilane electrophiles, the initial adducts undergo Brook rearrangement which is interrupted by -Si bond fission with loss of fluoride anion (Eq. 16), leading to the formation of extremely useful difluoro-enol silanes [64]. Of the various fluoride sources employed, the tetrabutylam-monium triphenyldifluorostannate described by Gingras appears to be particularly effective. The numerous other methods for trifluoromethylation formed the subject of an exhaustive review [65]. More recently, the Olah group described a chlorodifluoromethyl trimethylsilane which is expected to have a rich chemistry [66]. 

A range of experimental conditions, in terms of supporting electrolyte and fluoride source, have been explored in which reactions of this type have been studied [3]. Typical of such systems are ... 

Tetrabutylammonium(triphenylsilyl)difluorosilicate is a new fluoride source soluble in anhydrous, nonhygroscopic and neutral organic solvents. This reagent converts organic halides and sulfonates into fluorides, however, 4-6 equivalents are required. 1-Iodooctane is converted to 1-fluorooctane (33) in 74% yield, the rest (26%) being the alkene.221 Apparently with tetra-alkylammonium fluorides, the formation of alkenes is generally not completely suppressable. 

Several groups have employed N-halosuccinimdes for the halofluorination of alkenes. Laurent and co-workers have found that triethylamine tris(hydrofluoride) is a suitable fluoride source for the halofluorination of a number of alkenes (Scheme 70).137 Other examples of halofluorinations include NBS and tetrabutylammonium fluoride as the halogen sources for the formal addition of BrF across funtionalized cyclohexenes138 and the halofluorination of alkenes and alkynes by all three N-halosuccinimides and polymer-supported HF.139... 

VF epoxide function is not affected by this reaction. Tetrafluorosilane serves as both a Lewis acid and a fluoride source in the subsequent regioselective epoxide opening to compound 36 SiF4 reacts with water to give silicic acid and hydrogen fluoride, and the latter re-... 

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