Silicon(IV) Fluoride1


[7783-61-1]  · F4Si  · Silicon(IV) Fluoride  · (MW 104.08)

(efficient Lewis acid and fluorination reagent)

Physical Data: mp -90.2 °C; bp -86.0 °C; d (-95 °C) 1.66 g cm-3.

Solubility: sol ether, chloroform, dichloromethane, acetonitrile.

Form Supplied in: colorless gas, commercially supplied in cylinders.

Handling, Storage, and Precautions: very pungent odor similar to that of hydrogen chloride; highly irritating; corrosive to skins and eyes. See also Hydrogen Fluoride.

General Discussion.

Silicon tetrafluoride is an abundant byproduct of fertilizer manufacture. Several attempts have been made to use SiF4 as a fluorinating agent.2 However, tedious experimental procedures and the low nucleophilicity of the fluorine attached to silicon have restricted application of SiF4 as a fluorinating agent. In the presence of Aluminum Chloride, ethyl orthoformate is fluorinated with silicon tetrafluoride (eq 1). Besides ethyl fluoride, the reaction products are ethyl formate and ether.3

The reactions of silicon tetrafluoride with carbon tetrachloride (eq 2) and Phosgene (eq 3) in the vapor phase at elevated temperature give chlorofluorocarbons and carbonyl fluorochloride, respectively.4 The reactions are carried out by passing a mixture of the substrate and silicon tetrafluoride through an electrically heated quartz tube, which is filled with Henger quartz granules.

The ability of silicon tetrafluoride to function as a mild fluorination reagent is demonstrated in the ring-opening fluorination of epoxides (1) and (3) (eqs 4 and 5).5 Aromatic epoxides undergo ring-opening fluorination with silicon tetrafluoride in the presence of Diisopropylethylamine, whereas aliphatic epoxides give fluorohydrins with silicon tetrafluoride only in the presence of Tetra-n-butylammonium Fluoride or water; the involvement of hypervalent fluorosilanes (SiF5- or SiF62-) is postulated.6 Application of this methodology to epoxysilanes affords, after Peterson-type alkenation, fluoroalkenes (5).7 Desilylation of the initially formed fluorohydrin (4) is effected by a catalytic use of Sodium Hydride in the presence of 18-Crown-6, giving the desilylated compound (6) with retention of the configuration at the carbon bearing fluorine.

Alkenes are converted into the corresponding bromofluorides with silicon tetrafluoride-1,3-Dibromo-5,5-dimethylhydantoin (DBH) and water in 1,4-dioxane.8 Stereoselectivity in the bromofluorination of (E)- and (Z)-1-phenylpropenes (7) is noteworthy (eq 6). Both give the anti-bromofluoride (8) selectively, making a contrast to the reagents previously reported for the halofluorination of alkenes.

Silicon tetrafluoride is a useful catalyst for glycosylation.9 The condensation of appropriately protected glycopyranosyl fluorides (9) and trimethylsilyl ethers is conducted under the action of silicon tetrafluoride (eq 7). The pronounced solvent effect is noteworthy: the glycosylation in acetonitrile gives the b-glycosides (11) with moderate to high selectivity, whereas the reaction in ether affords the a-anomers (10) predominantly.

2-Oxazolines (13) are formed from the reaction of epoxides (12) with nitriles promoted by silicon tetrafluoride (eq 8).10 Control experiments show that the ability of silicon tetrafluoride to promote this type of reaction is similar to that of aluminum chloride.

Silicon tetrafluoride is a mild and selective reagent for the cleavage of silyl-protected alcohols.11 The cleavage of various t-butyldimethylsilyl groups by silicon tetrafluoride takes place at ambient temperature in methylene chloride or acetonitrile (eq 9).

1. Shimizu, M.; Yoshioka, H. J. Synth. Org. Chem. Jpn. 1989, 47, 27 (CA 1989, 111, 77 128r); Shimizu, M.; Yoshioka, H. J. Synth. Org. Chem. Jpn. 1990, 48, 1054 (CA 1991, 114, 120 927b); Wilkinson, J. A. CRV 1992, 92, 505.
2. Schmeisser, M.; Jenkner, H. ZN(B) 1952, 7B, 583; Dahmlos, J. U.S. Patent 2 935 531, 1960 (CA 1960, 54, 19 481e); Wasag-Chemie A. G. Br. Patent 837 346, 1960 (CA 1961, 55, 17 498c); Wasag-Chemie A. G. Ger. Patent 1 098 500, 1961, (CA 1962, 56, 3352b).
3. Mason, K. G.; Sperry, J. A.; Stern, E. S. JCS 1963, 2558.
4. Christe, K. O.; Pavlath, A. E. JOC 1964, 29, 3007.
5. Shimizu, M.; Yoshioka, H. TL 1988, 29, 4101.
6. Cotton, F. A.; Wilkinson, G. Advanced Inorganic Chemistry, 2nd ed.; Wiley: New York, 1966: Chapter 19; Muller, R.; Dathe, C.; Moss, D. CB 1965, 98, 241.
7. Shimizu, M.; Yoshioka, H, TL 1989, 30, 967.
8. Shimizu, M.; Nakahara, Y.; Yoshioka, H. CC 1989, 1881.
9. Hashimoto, S.; Hayashi, M.; Noyori, R. TL 1984, 25, 1379.
10. Shimizu, M.; Yoshioka, H. H 1988, 27, 2527.
11. Corey, E. J.; Yi, K. Y. TL 1992, 33, 2289.

Makoto Shimizu

Mie University, Japan

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