Xenon difluoride production

K [14881-07-3], Rb [72151 -96-3], and Cs [72138-72-8]), are prepared by reaction of elemental fluorine, chlorine trifluoride, or xenon difluoride and a mixture of nickel fluoride and alkaH metal fluorides or other metal haHdes (16,17). If the fluorination is carried out using mixed fluorides, a lower temperature can be used, yields are quantitative, and the final products are of high purity. Bis(tetrafluoroammonium) hexafluoronickelate [6310540-8], (NE 2N iF6> prepared from Cs2NiF3 and NE SbE by a metathesis in anhydrous HE, is also known (18). 

Carboxylic acids react with xenon difluoride to produce unstable xenon esters The esters decarboxylate to produce free radical intermediates, which undergo fluonnation or reaction with the solvent system Thus aliphatic acids decarboxylate to produce mainly fluoroalkanes or products from abstraction of hydrogen from the solvent Perfluoro acids decarboxylate in the presence of aromatic substrates to give perfluoroalkyl aromatics Aromatic and vinylic acids do not decarboxylate [91] (equation 51)... 

Heating the xenon difluoride hexane mixture to 80-90"C in a sealed glass tube leads to its inflammation. If the xenon difluoride and hexane (1 5 mol) are heated in a Teflon tube at 105 "C, the products are 1-, 2- and 3-fluorohexanes (28 42 30) with a total yield of 15-20%.13 Under the same conditions cyclohexane has been converted to fluorocyclohexane in 18 % yield. 

Xenon difluoride reacts with alkyl iodides at room temperature to give organic derivatives of polyvalent iodine. When an excess of iodomcthane (7) is treated with xenon difluoride it produces a stable solution of methyliodine difluoride in quantitative yield, the use of hydrogen fluoride as a catalyst allows a substantial amount of product to be obtained, while isopropyl iodide decomposes under the action of the reagent.16-21... 

Fluorination of tris(fluorosulfonyl)methane (10) with xenon difluoride in dichlorodifluoro-methane solution gives fluorotris(fluorosulfonyl)methane in quantitative yield.24 In contrast, no fluorination product has been obtained in the reaction of tris(trifluoromethylsulfonyl)meth-ane with xenon difluoride.25... 

Dienes were treated with xenon difluoride in the presence of a boron trifluoride-diethyl ether complex using mild conditions this formed mainly the products of 1,2-addition.28 Butadiene formed 3,4-difluorobut-1-ene (87%) and the isomer l,4-difluorobut-2-ene (13%), whereas 2,3-dimethylbutadiene quantitatively produced the 1,2-addition product. 

The treatment of tran.v-l-phenylpropene and /rans-stilbene with xenon difluoride in dichloromethane catalyzed by trifluoroacetic acid leads to an approximately equimolar mixture of fluorination and fluorotrifluoroacetoxylation products, the latter process proceeds regio-spccifically.34... 

The hydrogen fluoride catalyzed fluorination of norbornene by xenon difluoride at room temperature leads to a mixture of at least seven components,39 but under milder conditions (— 78 to 26 C, 22 hours) the reaction affords a mixture of two main products 2-e,xo-5-cxo-difluoro-norbornane and 2-c-wfo-5- Yo-difluoronorbornane, ratio 2 1, in a total yield of 51-76%. If the same reaction is carried out in a limited temperature range between — 46 and — 39 C the yield of these products decreases, their ratio becomes equal, and the main product is 2-exo-l-ff //-difluoronorbornane (42 %).40 The structure dependence of the fluorination products of norbornene with xenon difluoride was studied. Solvent, temperature, reaction duration, catalyst (hydrogen fluoride, boron trifluoride, trifluoroacetic acid, pentafluorobenzenethiol) and the routes of product isomerization were analyzed.41-42... 

Alkenes react with xenon difluoride in the presence of some auxiliary reagent to form unsymmetrical addition products. Treatment of indene (8) with xenon difluoridc and methanol in the presence of hydrogen fluoride leads to fluoromethoxylation of the C = C bond, dia-stereomers of 1-alkoxy-2-fluoroindane are formed as the major product, difluoroindane was detected in trace quantities. In the presence of a boron trifluoride-diethyl ether complex the crossover l-fluoro-2-alkoxylated indanes have been obtained as major products.48,49... 

Cyclohexene can be treated with xenon difluoride and /c/V-butylhypochloride to give a complex mixture of products with a 49.4% yield of /ttWi-l-chloro -fluorocyclohcxane.30... 

Alkenes react with xenon difluoride and diphenyldiselcnide to give the corresponding products of 1,2-fluoroselenation. Formation of phcnylselenyl fluoride as the unstable intermediate has been postulated. Cyclohexene (9) has been treated with xenon difluoride and diphcnyl-diselenide in dichloromethane at — 20 C to form tran.v-l-fluoro-2-phenylselenocyclohexane (10) in 73% yield. 2-Phenylselcnocyclohexanol (11) is the major byproduct in 5% yield due to the hydrolytic instability of the fluoroselenide 10. 

Reactions of alkynes with xenon difluoride have not been widely investigated. Propyne reacts more slowly with xenon difluoride in the absence of a catalyst than propene, to form a complex product mixture with 33% 2,2-difluoropropene.26 Diphenylethyne treated with xenon difluoride in the presence of hydrogen fluoride forms 1,2-diphenyltctrafluoroethane in 50% yield, comparable yields of 1,2-dialkyltetrafluorocthanes are observed on treatment of 1,2-dialkyl-propenes with xenon difluoride in dichloromethane at room temperature.58... 

Extensive work on the interaction of aromatic compounds with xenon difluoride has been carried out in order to investigate the reaction mechanism and the scope of the fluorination depending on the substituents electronic nature.26-59 62 It has been found that benzene and substituted aromatics react with xenon difluoride at room temperature in the presence of hydrogen fluoride to form the typical products of electrophilic fluorination contaminated with low quantities of difluoro-substituted molecules. 

Trimethylsilyl benzoates react with xenon difluoride in dichloromethane or hexafluorobenzene to give rearranged products, i.e. aryl fluoroformates.74... 

Polyfluoroaromatic compounds react with xenon difluoride in the presence of Lewis acids to give the products of regiospecific 1,4-addition of fluorine, i.e. the corresponding fluorinated... 

To overcome problems associated with the removal of iodobenzene and its derivatives formed upon fluorination of arylalkenes and arylalkynes with (difluoroiodo)arenes, polymer-supported (difluoroiodo)arenes were proposed.139 With these agents, the separation procedures are reduced to filtration of the iodinated polymer. For this purpose popcorn polystyrene is io-dinated and then transformed into the difluoroiodide by treatment with xenon difluoride in the presence of hydrogen fluoride in dichloromelhane at 25 C. The amount of active fluorine bonded to iodine atoms on the polymer support is estimated by iodometric titration. The reactions with phenyl-substituted alkenes result in rearranged gew-difluorides. The procedure provides the same fluorination products as with (difluoroiodo)benzenc (see Section 4.13.) but in much higher yields, e.g. PhCF2CH2Ph (96%), PhCF2CH(Me)Ph (95%). PhCH2CF2H (86%), and l,l-difluoro-2-phenylcyclopentanc (91 %). 

Substitution of hydrogen by fluorine in compounds with acidic a-hydrogens can be achieved with xenon difluoride and some catalysts. Thus, room-temperature fluorination of 1,3-diketones, e g. 20, with xenon difluoride in the presence of the insoluble cross-linked polysty-rene-4-vinylpyridine, either complexed with boron trifluoridc or Nafion-H, as catalyst, gives mono and difluoro products.27-28... 

Investigation of the reaction of xenon difluoride and difluoroiodobenzene with aliphatic 1,3-dienes in the presence of boron trifluoride-diethyl ether complex has shown that under mild conditions this reaction proceeds with kinetic control, to form mainly the products of 1,2-addition of fluorine atoms. The ratio of product is reported in Table 5. 

The reaction of norbornene (5) with xenon difluoride in dichloromethane at various temperatures and in the presence of various catalysts results in the formation of seven products.45 Table 6 shows the significant effect of boron trifluoride-diethyl ether complex as catalyst in the product formation. 

Fhiorination of aromatics. The reagent reacts with toluene to form benzyl fluoride as the major product ( 65% yield). It is also useful for fluorination of phenols- and of alkyl ethers of phenols the or/Ao-isomcr is formed as the major product. Reactions with this reagent thus differ from those with xenon difluoride, which generally favors formation of paro-isomers. 

Xenon Difluoride. XeF2 mw 169.30 colorl linear cryst with a body-centered tetragonal cell structure, possessing a nauseating odor mp 129.03 0.05° d 4.32g/cc. V sol in liq anhydr HF moderately sol in w. Prepn is by reacting two moles of Xe with one mole of fluorine in a Ni or Monel vessel at 400°, quenching the reaction at RT, and isolating the product by vac sublimation. Ref 2 lists nine other techniques for the prepn of the difluoride. The pure compd is stable and can be kept indefinitely in Ni or Monel containers... 

Great differences in product structures and product distributions are obtained by lead(IV) oxide or acetate oxidation of perfluorophenol in different solvents and media. The reaction with the former agent gives a quinoid ether in 22% yield (Table 10).173 The oxidation with lead(IV) acetate has been optimized to such a level as to give perfluoro cyclohexa-2,5-dienone (4) in 65 % yield.174 Treating the phenol with vanadium(V) fluoride or vanadium(III) fluoride as well as xenon difluoride gives a mixture of products,175 therefore, the reactions are only of minor preparative importance. 

Vicinal fluoroalkyl ethers are also obtained from olefins and methyl hypo-fluorite at low temperatures [168] and from the unstable reagents generated from additions of xenon difluoride to methanol [169] or other alcohols [170] Alcohols tend to give complex product mixtures depending on the olefin structure, and best results are often achieved when reactions are run in the presence of boron tri-fluonde Additions of xenon difluoride to trifluoromcthanesulfomc fluorosulfonic, or nitric acids give fluoroxenonium reagents that are stable to about -10 °C and... 

Electrochemical fluorination of pyridine in the presence of a source of fluoride ion gave 2-fluoropyridine in 22% yield (85M11). With xenon difluoride, pyridine formed 2-fluoropyridine (35%), 3-fluoropyridine (20%), and 2,6-difluoropyridine (11%) in a reaction unlikely to be a conventional electrophilic substitution. Xenon hexafluoride has also been used (76JFC179). With cesium fluoroxysulfate at room temperature in ether or chloroform, the major product was 2-fluoropyridine (61 and 47%, respectively). Some 2-chloropyridine was also formed in chloroform solution. In methanol the entire product was 2-methoxypyridine (90TL775). Fluorine, diluted with argon in acetic acid, gave a 42% yield of the 5-fluoro derivative of l-methyl-2-pyridone [82H( 17)429],... 

Conjugated dienes, i.e. 1,3-butadiene and 2,3-dimethyl-1,3-butadiene, added fluorine at 0°C ionically, without rearrangement, with formation of 1,2- and mainly 1,4-difluoro adducts in contrast, xenon difluoride afforded exclusively 1,2-adducts. 1,3-Pentadienes, both cis and trans, gave mostly rearranged products [49] ... 

Trimethylsilyl ethers of several steroidal ketones were a-fluorinated by 4-(difluoroiodo)toluene, in moderate yield because of concomitant elimination, accompanied by the formation of other by-products from nucleophilic substitution to a phenyliodonium intermediate. The analogous reaction with xenon difluoride resulted in much better yields but different stereochemistry [50],... 

Xenon difluoride is able to transfer a fluorine atom to organic molecules without any catalyst, with very reactive organic molecules or at higher temperatures or under photochemical conditions. However, reactions are usually performed in the presence of various types of catalysts, but in some cases a new xenon compound FXeYL is formed (Scheme 1), which can be a source of fluorine atoms, or of a YL or Y group for transfer to an organic molecule. On the other hand, decomposition of the FXeYL molecule results in the desired fluorinated products (FYL, FL or FY). Fluoro-substituted xenon derivatives (FXeYL) can also be further transformed to disubstituted derivatives, which are usually less stable at room temperature, but can also be excellent sources of YL, Y or L groups for transfer to various organic molecules. The next possibility is that xenon difluoride converts an... 

Alkanes do not usually react with xenon difluoride at room temperature, while thermally initiated fluorinations of organic molecules have received much less attention than liquid-phase reactions. Zajc and Zupan16 have shown that several hydrocarbons react with xenon difluoride when heated to 95-120 °C. Reproducible results can be observed only when a teflon jacket is used in the stainless reactor with appropriate preconditioning. Cyclohexane is converted to fluorocyclohexane, while the reaction with -hexane gives three monofluoro-substituted products. Fluorination of adamantane results in the formation of four products, from whose distribution it is evident that the difference in the reactivity between the secondary and the tertiary carbon atom is much larger than the difference between the reactivities of the primary and secondary carbon atoms of hexane. Liquid-phase functionalization of a tertiary carbon atom is observed in reactions in carbon disulfide, where 1-fluoroadamantane is formed17. 

Xenon difluoride reacts also with some halosubstituted alkanes18,19 at room temperature in chloroform, carbon tetrachloride, dichloromethane or bromoform, and depending on the nature of the solvent used chloro or bromo derivatives are isolated with imidazo-(l,2-fr)-pyridazine18. Carbon tetrachloride reacts with xenon difluoride at 180°C, while room-temperature transformations are achieved when various catalysts [antimony trifluoride, tantalum(V) fluoride or silica dioxide] are used, whose structure also influences the product distribution20. Tris(fluorosulfonyl)methane gives a fluoro-substi-tuted product in its reaction with xenon difluoride in difluorodichloromethane21. 

The saturated carbon atom can be functionalized by xenon difluoride in various ketones and diketones where, beside a-mono and a-difluoro ketones, the formation of rearranged products can also be observed however, the course of reaction depends strongly on the catalyst used22-24 (Scheme 5). 

Reactions of xenon difluoride with various carbanions have been extensively studied by Tselinski and coworkers25-28. They found that the solvent plays the most important role in the transformation reactions in dichloromethane give mixtures of up to four products, while acetonitrile is found to be the most convenient solvent (Scheme 6). Excellent results were established with various substrates, while cationic species do not play a very important role. The authors suggested a one-electron transfer oxidation of the carbanion and further formation of various radicals, which is especially evident in reactions in mixtures of acetonitrile and benzene. 

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