A Silver fluoride

The Pandey group has developed a silver fluoride-promoted desily lation of tertiary bis(silyl)amines as an interesting alternative method to access azomethine ylides (Scheme 2.10).18 Notably, this method allows the generation of nonstabilized azomethine ylides under essentially neutral conditions. The starting materials are prepared by a three-step process, sometimes coupled into a single operation. For example, Boc-protected pyrrolidine 36 can be sequentially deprotonated and silylated twice in a one-pot reaction (Scheme 2.10). Removal of the Boc group and alkylation of the free amine leads to bis(silyl)amine 37. When this compound is treated with 2 equiv of silver fluoride in the presence of phenyl vinyl sulfone, rapid formation of products 39 as single stereoisomers results. 

Thionyl tetrafluoride and pentafluorosulfur hypofluorite can be prepared by the fluorination of thionyl fluoride. The product distribution obtained upon fluorination appears to be dependent not only upon the reaction conditions, but on whether a source of fluoride ion is present. Fluorination of thionyl fluoride over a hot platinum gauze or in a Monel pressure reactor produces thionyl tetrafluoride, whereas fluorination in the presence of cesium fluoride or a silver fluoride catalyst jdelds pentafluorosulfur hypofluorite. The role of the fluoride ion in the fluorination has not been conclusively established however, the avail-... 

Variations of these reaction conditions have remained the most frequent method for the synthesis of 6-fluoropurines since these early publications [12,25,102-108]. A development of the conditions was the use of A-methyl pyrrolidine instead of condensing gaseous trimethylamine which led to a simplification of the reaction procedure [35]. Alternatively, a silver fluoride mediated reaction was used to great effect to convert a AJ-norbomyl-6-chloropurine to the corresponding A-norbomyl-6-fluoropurine in good yield (71 %) [109]. 

Many ionic halides dissolve in water to give hydrated ions. The solubility of a given halide depends on several factors, and generalisations are difficult. Ionic fluorides, however, often differ from other halides in solubility. For example, calcium fluoride is insoluble but the other halides of calcium are highly soluble silver fluoride. AgF, is very soluble but the other silver halides are insoluble. 

Uses. Silver fluoride has found many laboratory and special industrial appHcations. It is used as a soft (nHld) fluorinating agent for selective fluorination (7—17), as a cathode material in batteries (qv) (18), and as an antimicrobial agent (19). Silver fluoride is commercially available from Advance Research Chemicals, Inc., Aldrich Chemicals, Cerac Corp., Johnson/Matthey, PCR, Atochem, and other sources in the United States. The U.S. price of silver fluoride in 1993 was 1000— 1400/kg and the total U.S. consumption was less than 200 kg/yr. 

Silver difluoride, commercially available from the same sources as those of AgF, had a 1993 price between 1000— 1400/kg. In spite of the technical success in laboratory experiments, silver fluorides have found limited use on a large scale mainly because of the high cost of the reagents. Demand for silver difluoride is less than 100 kg/yr. 

Silver Fluoride. Silver fluoride, AgF, is prepared by treating a basic silver salt such as silver oxide or silver carbonate, with hydrogen fluoride. Silver fluoride can exist as the anhydrous salt, a dihydrate [72214-21-2] (<42° C), and a tetrahydrate [22424-42-6] (<18° C). The anhydrous salt is colorless, but the dihydrate and tetrahydrate are yellow. Ultraviolet light or electrolysis decomposes silver fluoride to silver subfluoride [1302-01 -8] Ag2p, and fluorine. 

The best known of the hahdes are the trialkyldihalo- and triaryldihaloantimony compounds. The dichloro, dibromo, and diiodo compounds are generally prepared by direct halogenation of the corresponding tertiary stibiaes. The difluoro compounds are obtained by metathasis from the dichloro or dibromo compounds and silver fluoride. The diiodo compounds are the least stable and are difficult to obtain ia a pure state. The tri alkyl- and triaryldichloro- and dibromoantimony compounds are all crystalline soHds which are stable at room temperature that but decompose on heating ... 

Alkyl fluorides have been prepared by reaction between elementary fluorine and the paraffins, by the addition of hydrogen fluoride to olefins, by the reaction of alkyl halides with mercurous fluoride, with mercuric fluoride, with silver fluoride, or with potassium fluoride under pressure. The procedure used is based on that of Hoffmann involving interaction at atmospheric pressure of anhydrous potassium fluoride with an alkyl halide in the presence of ethylene glycol as a solvent for the inorganic fluoride a small amount of olefin accompanies the alkyl fluoride produced and is readily removed by treatment with bromine-potassium bromide solution. Methods for the preparation of alkyl monofluorides have been reviewed. ... 

This topic has been reviewed [2, pp 94, 100-111, 130-134] All of the standard approaches to the synthesis of a compound like methyl 2-fluorostearate from methyl 2-bromostearate result mall yield of the 2-fluoro ester and the unsaturated esters. Although silver fluoride is not a new reagent, its use moist in wet acetonitrile to convert methyl 2-bromostearate to its fluoro ester is a departure from the traditional set of anhydrous conditions (Procedure 6, p 194) [71] In contrast, silver tetrafluoroborate converts a-chloroketones to their respective fluoroketones under anhydrous conditions. The displacement of less activated halogen groups by silver tetrafluoroborate to form their respective fluorides is novel Although silver tetrafluoroborate could not be used to convert an aliphatic terminal dichloromethyl or trichloromethyl group to its corresponding fluoro derivative, it is an effective fluorine source in other situations [72] (Table 8)... 

Rearrangement of fluorine with concomitant ring opening takes place in fluorinated epoxides Hexafluoroacetone can be prepared easily from perfluo-ropropylene oxide by isomerization with a fluorinated catalyst like alumina pre treated with hydrogen fluoride [26, 27, 28] In ring-opening reactions of epoxides, the distribution of products, ketone versus acyl fluoride, depends on the catalyst [29] (equation 7) When cesium, potassium, or silver fluoride are used as catalysts, dimenc products also are formed [29]... 

Alkyl fluorides can be prepared by the Finkelstein reaction. The fluoride anion is a bad leaving group the reverse reaction thus does not take place easily, and the equilibrium lies far to the right. As reagents potassium fluoride, silver fluoride or gaseous hydrogen fluoride may be used. 

A crystalline 5,5 -diene, or bis (vinyl ether), derivative of sucrose has been prepared from 6,6 -dideoxy- 6,6 -diiodo-sucrose hexa-acetate, derived from the 6,6 -ditosylate, by treatment with silver fluoride in pyridine (26). 

The use of the silver fluoride reaction in the preparation of a nucleoside containing a 4, 5 double bond was recently reported (44). Thus treat-... 

O-isopropylidene derivative (57) must exist in pyridine solution in a conformation which favors anhydro-ring formation rather than elimination. Considerable degradation occurred when the 5-iodo derivative (63) was treated with silver fluoride in pyridine (36). The products, which were isolated in small yield, were identified as thymine and l-[2-(5-methylfuryl)]-thymine (65). This same compound (65) was formed in high yield when the 5 -mesylate 64 was treated with potassium tert-hx Xy -ate in dimethyl sulfoxide (16). The formation of 65 from 63 or 64 clearly involves the rearrangement of an intermediate 2, 4 -diene. In a different approach to the problem of introducing terminal unsaturation into pento-furanoid nucleosides, Robins and co-workers (32,37) have employed mild base catalyzed E2 elimination reactions. Thus, treatment of the 5 -tosylate (59) with potassium tert-butylate in tert-butyl alcohol afforded a high yield of the 4 -ene (60) (37). This reaction may proceed via the 2,5 ... 

Acetyl-5-deoxy-l,2-0-isopropylidene-[3-iJ - threo - pent - 4 - enofura-nose (34). (1) From 3-0-acetyl-5-deoxy-5-iodo-1,2-0-isopropylidene- -d-xylofuranose (31). Anhydrous silver fluoride (7.5 grams) was added to a solution of 7.2 grams 26 in dry pyridine (50 ml.), and the mixture shaken at room temperature for 24 hours. The black reaction mixture was diluted with ether (50 ml.), and the supernatant liquid was decanted from the dark, inorganic residue. The residue was further extracted with ether (3 X 50 ml.) and the pyridine-ether solution partially decolorized... 

From 1,2-0-isopropylidene-3,5-di-0-tosyl-/ -d-xylofuranose (21) (29). Treating 29 with silver fluoride in pyridine and isolating as described above for the l-arabino isomer gave a 40% yield of 32 after a reaction time of 48 hours. The product had [ ]D25 — 14.9° and had an infrared spectrum identical with material prepared as above. 

From 5-deoxy-5-iodo-1,2-0-isopropylidene- -d-xylofuranose (30). A solution of 1.14 grams of 30 in pyridine (8.0 ml.) was shaken at room temperature with silver fluoride (2.0 grams). The reaction was slower than with the corresponding 5-tosylate (22) and was complete after 72 hours. The reaction mixture was processed as described above to give a pale yellow sirup which contained, in addition to 28, three minor components. Distillation afforded pure material (0.4 grams, 75%) identical with material prepared as above. 

Methyl 5-deoxy-2,3-0-isopropylidene l3-T>-erythro-pent-4-enofurano-side (43). A solution of methyl 5-deoxy-5-iodo-2,3-0-isopropylidene-/ -D-ribofuranoside (42) (30) (1.0 grams) in pyridine (10 ml.) was shaken for 7 hours at room temperature with anhydrous silver fluoride (1.5 grams). Isolation of the product as described previously afforded the 4-ene (43) as a sirup (414 mg., 70%) which distilled at 40°C. (bath) at 0.1 mm. G.l.c. showed the presence of a major component (95%) together with 5% of another component which was not investigated. [ ]D21 + 26.3° (c, 2.3). Anal Calcd. for C9H1404 C, 58.05 H, 7.6. Found C, 57.9 H, 7.7. 

Silver fluoride is soluble. Therefore, no precipitate forms as Ag ions are added to a solution of F ions. 

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