Silver(I) Fluoride


[7775-41-9]  · AgF  · Silver(I) Fluoride  · (MW 126.87)

(fluorinating agent1)

Physical Data: mp 435 °C; bp 1150 °C; d 5.852 g cm-3.

Solubility: highly sol water (182 g/100 g H2O), anhydrous hydrogen fluoride (83.2 g/100 g at 11.9 °C); slightly sol methanol (1.5 g/100 mL).

Form Supplied in: golden-yellow powder; commercially available, but very expensive.

Preparative Methods: by the reaction of Ag2CO3 or Silver(I) Oxide in anhydrous Hydrogen Fluoride or aqueous Hydrofluoric Acid

Handling, Storage, and Precautions: a light sensitive, hygroscopic solid. It should be stored in a dark container under an inert atmosphere. It is extremely destructive to tissues of the mucous membranes and upper respiratory tract, eyes, and skin. It emits toxic fumes under fire conditions. It also forms explosive adducts with ammonia; reactions involving liquid or gaseous ammonia should be carried out with extreme caution. Use in a fume hood.

Fluorinations with Silver Fluoride.

Reaction of AgF with alkyl and aryl halides often results in the formation of the corresponding fluorides usually under mild conditions. With geminal dihalides, this conversion has been shown to involve a carbonium ion intermediate (eq 1).2 Chloro quinones react with AgF under high pressure and temperature conditions to give partial conversion to the fluoro quinone (eq 2).3 Cis- and trans-2,3-difluoro-2,3-dihydrobenzofuran can be similarly obtained by the reaction of AgF with trans-2,3-dibromo-2,3-dihydrobenzofuran (eq 3).4 Bromoadamantanes5 and 1-bromobicyclo[3.3.1]nonan-3-one6 undergo fluorination with AgF to yield bridgehead fluorine derivatives (eq 4).

Fluorination of Alkenes by Addition Reactions.

b-Fluoro thioethers can be prepared by the addition of sulfenyl chlorides to alkenes in the presence of AgF. The addition to alkyl-substituted terminal alkenes usually produces anti-Markovnikov adducts with a primary fluorine. Unlike the corresponding chlorides, these materials do not undergo rearrangement to the thermodynamically more stable Markovnikov products (eq 5).7

Silver fluoride adds to electron deficient alkenes. The reaction of 1,1-dichloro-2,2-dicyanoethylene with AgF, for example, does not produce the expected 1,1-difluoro derivative. Instead, this reaction yields the silver salt of trifluoromethylmalononitrile, presumably through the 1,1-difluoro derivative by subsequent addition of AgF to the strongly electrophilic double bond.8 Perfluoroalkylsilver compounds can be prepared by the addition of AgF to perfluoroalkenes.9 The fluorination of tetrabromo-2,3-diaza-1,3-butadiene affords the tetrafluoro derivative.10

Other Fluorinations using AgF.

a-Fluoro-a,b-unsaturated ketones and esters can be formed by the reaction of a-diazo esters with benzeneselenenyl fluoride (prepared by the reaction of Benzeneselenenyl Bromide with AgF).11 Trifluoromethylthiocopper(I) can be readily prepared by the reaction of AgF, carbon disulfide, and copper(I) bromide in acetonitrile. This reagent reacts with iodoaromatics to yield the corresponding trifluoromethylthio derivatives.12 Certain geminal dibromocyclopropanes undergo solvolysis with subsequent fluorination and ring expansion by treatment with AgF in acetonitrile (eq 6).13 Silver fluoride mediates the opening of perfluoro sultones which, in the presence of an appropriate trapping agent, lead to the formation of fluoroalkoxysulfinyl fluorides (eq 7).14

Silver Fluoride as a Desilylating Agent.

A number of unrelated synthetic methods take advantage of AgF as a strong fluoride source. In these methods, fluoride unmasks a reactive intermediate by desilylation, allowing for the desired reaction, in most examples a cyclization or cycloaddition, to take place. For example, t-butyldimethylsilyl carbamates can be treated with AgF, resulting in formation of cyclic carbamates by an intramolecular reaction (eq 8). Only (E)-allyl chlorides provide the cyclic carbamate, since the proposed cyclic transition state is not possible with the corresponding (Z)-allyl chlorides.15 Threo selectivities for the cyclization are poor (3-5:1), but can be dramatically improved by using catalytic allylpalladium(II) chloride. In the absence of a suitable intramolecular trap, decarboxylation takes place. The reaction of AgF with a silyloxycyclopropane, followed by treatment with an allylic chloride, results in the formation of an d,ε-unsaturated ketone (eq 9).16

Treatment of a silylmethylindole with AgF, followed by exposure to an appropriate dipolarophile, results in the formation of a 2,3-dihydro-1H-pyrrolo[1,2-a]indole (eq 10). This cycloaddition works well with a number of related N-[(trimethylsilyl)methyl]-substituted indoles, providing cycloadducts with yields in the range 53-83%. The formation of the azomethine ylide assumes that silver is acting as a very specific Lewis acid, attacking the indole ring to give a silver-bonded carbonium ion.17 a-Cyanoaminosilanes have also been used to generate azomethine ylides for the preparation of pyrrolidines in high yields.18 Reaction of these azomethine ylides with ethyl propiolate yields the fully aromatized pyrroles (eq 11).19

In work by Evans and co-workers, deprotection of a silyl ether with AgF leads to the formation of an intermediate positioned to undergo a rapid Cope rearrangement. Subsequent loss of methanol results in the formation of the deoxylapachol product in 95% yield (eq 12).20 Additionally, certain nitrosoalkenes, which can be active dienes in [4 + 2] cycloadditions, can be prepared from a-chlorosilyloximes by 1,4-elimination in the presence of an appropriate fluoride source, such as AgF.21 Successful cycloadditions are also observed with Cesium Fluoride and Potassium Fluoride (eq 13).

Other Reactions with AgF.

Treatment of vicinal dibromo steroids with aqueous AgF results in the formation of epoxides.22,23 The AgF-initiated methanolysis of 2,2-dibromo-1,3-dimethylcyclopropanecarboxylic acid has also been studied in detail.24 Thiophilic heavy metal salts such as AgF have been used for ring opening of penicillin derivatives (eq 14),25 although other catalysts such as Silver(I) Oxide are better. Treatment of (E)-aryl- and -alkenylpentafluorosilicates with silver fluoride affords symmetrical (E,E)-1,3-dienes in high yields (eq 15).26

Related Reagents.

Iodine-Silver(I) Fluoride; Silver(I) Fluoride-Calcium Fluoride.

1. Meshri, D. T. In Kirk-Othmer Encyclopedia of Chemical Technology, 3rd ed.; Grayson, M.; Eckroth, D., Eds.; Wiley: New York; 1980; Vol. 10, pp 795-797.
2. (a) San Filippo, J., Jr..; Romano, L. J. JOC 1975, 40, 782. (b) Mueller, P.; Etienne, R.; Pfyffer, J.; Pineda, N.; Schipoff, M. HCA 1978, 61, 2482.
3. Feiring, A. E.; Sheppard, W. A. JOC 1975, 40, 2543.
4. Ruzziconi, R.; Sebastiani, G. V. JHC 1980, 17, 1147.
5. Bhandari, K. S.; Pincock, R. E. S 1974, 655.
6. Heumann, A. S 1979, 53.
7. Purrington, S. T.; Correa, I. D. JOC 1986, 51, 1080.
8. Josey, A. D.; Dickinson, C. L.; Dewhirst, K. C.; McKusick, B. C. JOC 1967, 32, 1941.
9. Miller, W. T., Jr.; Burnard, R. J. JACS 1968, 90, 7367.
10. Mitsch, R. A.; Ogden, P. H. JOC 1966, 31, 3833.
11. Usuki, Y.; Iwaoka, M.; Tomoda, S. CC 1992, 1148.
12. Clark, J. H.; Jones, C. W.; Kybett, A. P.; McClinton, M. A.; Jack, M.; Bishop, D.; Blade, R. J. JFC 1990, 48, 249.
13. Loozen, H. J. J.; Robben, W. M. M.; Buck, H. M. RTC 1976, 95, 248.
14. (a) Chen, L.-C., Mohtasham, J.; Gard, G. L. JFC 1990, 49, 331. (b) Chen, L.-C., Mohtasham, J.; Gard, G. L. JFC 1990, 46, 39.
15. Sakaitani, M.; Ohfune, Y. TL 1987, 28, 3987.
16. Ryu, I.; Suzuki, H.; Ogawa, A.; Kambe, N.; Sonoda, N. TL 1988, 29, 6137.
17. Padwa, A.; Gasdaska, J. R. JACS 1986, 108, 1104.
18. Padwa, A.; Chen, Y.-Y. TL 1983, 24, 3447.
19. Padwa, A.; Eisenbarth, P.; Venkatramanan, M. K.; Wong, G. S. K. JOC 1987, 52, 2427.
20. Evans, D. A.; Hoffman, J. M. JACS 1976, 98, 1983.
21. Denmark, S. E.; Dappen, M. S.; Sternberg, J. A. JOC 1984, 49, 4741.
22. Kasal, A.; Trka, A. CCC 1974, 39, 603.
23. Kasal, A. CCC 1976, 41, 140.
24. Hemmingsen, T. H.; Svendsen, J. S.; Sydnes, L. K. ACS 1988, B42, 651.
25. Alpegiani, M.; Bedeschi, A.; Bissolino, P.; Visentin, G.; Zarini, F.; Perrone, E.; Franceschi, G. H 1990, 31, 617.
26. Tamao, K.; Matsumoto, H.; Kakui, T.; Kumada, M. TL 1979, 1137.

Juliette K. Busse & Eric J. Stoner

Abbott Laboratories, North Chicago, IL, USA

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