Xenon(II) Fluoride1

XeF2

[13709-36-9]  · F2Xe  · Xenon(II) Fluoride  · (MW 169.29)

(relatively mild fluorine source, capable of electrophilic fluorination of carbon-hydrogen bonds; radical fluorinating agent; fluorodecarboxylation of carboxylic acids)

Physical Data: 129 °C (triple point); d 4.32 g cm-3.

Solubility: sol H2O (25 g/L at 0 °C), with slow decomposition; sol CCl4, CH2Cl2, HF, BrF3, MeCN.

Form Supplied in: white solid; commercially available.

Preparative Methods: prepared from xenon and fluorine.2

Handling, Storage, and Precautions: strong oxidizing agent; hydrolyzes in humid air. Like all other potential sources of elemental fluorine with the ability to fluorinate any substrate, it must be treated with great respect and caution, and should only be used with the proper equipment and precautions for handling reactive fluorinating agents. Contamination by XeF4 will generate explosive XeO3 upon hydrolysis.

Fluorodecarboxylation.

Xenon difluoride has the unique ability to convert aliphatic carboxylic acids to fluoroalkanes (fluorodecarboxylation) without fluorinating other portions of the molecule (eq 1).3-5 This reaction is the fluorine analog of the Hunsdiecker and Kochi reactions. The substrates may contain aryl, aryloxy, and ketonic groups (Table 1).3 Compounds containing hydroxy groups give complex mixtures. Secondary acids like cyclohexanecarboxylic acid, as well as amino acids, cholic acid, cinnamic acid, trans-phenylcyclopropanecarboxylic acid, and benzoic acid resist decarboxylation. Optically active a-trifluoromethyl-a-methoxyphenylacetic acid is fluorodecarboxylated to racemic a-fluoro-a-trifluoromethyl-a-methoxytoluene (1).4 When this reaction is carried out in the presence of Bu4N+ 18F-, the 18F- is incorporated into (1) in 65% radiochemical yield.4,5 A limitation is illustrated by the reaction of CF3CO2H and excess XeF2 in benzene to give (trifluoromethyl)benzene.5 A general procedure for fluorodecarboxylation is to combine equimolar amounts of the acid and XeF2 in CH2Cl2 in a polyethylene bottle, slowly bubble HF into the solution, and store overnight at rt. The next day the solution is washed with dilute NaHCO3 and purified. Although Bromine Trifluoride also promotes fluorodecarboxylation,6 it can easily fluorinate other parts of the molecule.

Electrophilic Fluorination of Carbon-Hydrogen Bonds.

The reaction of XeF2 and hydrocarbons must be approached with caution. Usually a small amount of HF is used as a catalyst. The results of the direct fluorination of substituted benzenes with XeF2 are shown in Table 2.7,11

At 25 °C, XeF2 fluorinates methyl phenyl sulfide; in a separate experiment, fluorination of fluoromethyl phenyl sulfide gives a 58% yield of the difluorinated product (eq 2).8

Addition of Fluorine to Alkenes.

XeF2 can be a convenient way of adding fluorine to an alkene. When ethylene is fluorinated, the reaction proceeds at 0-25 °C in CHCl3 to produce a mixture of CH2F-CH2F (45%), CHF2-CH3 (35%), and CHF2-CH2F (20%).9 The differences in the reactivities of XeF2 and XeF4 have been described.10 Fluorination of butadienes with XeF2 or difluoroiodobenzene shows that XeF2 is more selective and favors 1,2-addition (eq 3). When 2,3-dimethylbutadiene is fluorinated with XeF2, only 1,2-difluoro-2,3-dimethyl-3-butene is formed.10 Examples of the fluorination of a variety of unsaturated cyclic alkenes can be found in German and Zemskov.1

Fluorination of Nitrogen-Containing Aromatic Compounds.

A CH2Cl2 solution of pyridine reacts with XeF2 to give 2-fluoropyridine (35% yield), 3-fluoropyridine (20%), and 2,6-difluoropyridine (11%). Fluorination of benzylamine in this way gives a 40% yield of the o-fluoro isomer and 2% of the p-fluoro isomer. Reaction of XeF2 with 8-hydroxyquinoline leads to a 35% yield of 5-fluoro-8-hydroxyquinoline as the only identifiable product. The product distributions from the reactions of XeF2 with aniline and benzylamine are given in Table 2.11

The use of XeF2 in synthesis is limited by its cost. Over the years, its cost has decreased and its availability has increased. As this trend continues, many more applications will be discovered for this novel and apparently versatile reagent.


1. German, L.; Zemskov, S. New Fluorinating Agents in Organic Synthesis; Springer: Berlin, 1989.
2. Williamson, S. M. Inorg. Synth. 1968, 11, 147.
3. Patrick, T. B.; Johri, K. K.; White, D. H. JOC 1983, 48, 4158.
4. Patrick, T. B.; Johri, K. K.; White, D. H.; Bertrand, W. S.; Mokhar, R.; Kilbourn, M. R.; Welch, M. J. CJC 1986, 64, 138.
5. Patrick, T. B.; Khazaeli, S.; Nadji, S.; Hering-Smith, K.; Reif, D. JOC 1993, 58, 705.
6. Halpern, D. F.; Robin, M. L. U.S. Patent 4 996 371, 1991.
7. Shaw, M. J.; Hyman, H. H.; Filler, R. JACS 1970, 92, 6498.
8. Zupan, M. J. JFC 1976, 8, 305.
9. Shieh, T. C.; Feit, E. D.; Chernick, C. L.; Yang, N. C. JOC 1970, 35, 4020.
10. Shellhamer, D. F.; Conner, R. J.; Richardson, R. E.; Heasley, V. L. JOC 1984, 49, 5015.
11. Anand, S. P.; Filler, R. JFC 1976, 7, 179.

Donald F. Halpern

Murray Hill, NJ, USA



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