Xenon difluoride acids

Xenon difluoride [55], xenon difluoride complexed with dialkyl sulfides [59], and xenon difluoride intercalated with graphite [90] are all effective reagents for the fluonnalion of acids, enolates, or enols (Table 2)... 

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)... 

Xenon difluoride is characteristic in reacting with polarized double bonds (glycals and enol acetates ) in the presence of acid or Lewis acid catalyst,... 

During preparation from perchloric acid and xenon difluoride at —50°C, violent explosions occurred if the reaction mixture was allowed to warm up rapidly [1]. Detonates easily and should be handled with extreme care [2],... 

Bartlett prepared xenon difluoride by the reaction of xenon with silver fluoride in hydrofluoric acid in the presence of boron trifluoride ... 

The action of strong aprotic Lewis acids (antimony(V) fluoride, arsenic(V) fluoride etc.) provokes the ionization of xenon difluoride, leading to the formation of fluoroxenonium salts XcF + MFn or Xe2F3 MFn less strong acceptors of the fluoride ion (hydrogen fluoride, boron trifluoride, etc.) polarize the xenon difluoride molecule. 

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... 

Hydroxy-3-methoxy-L-phenylalanine reacts with xenon difluoride to form after acidic hydrolysis by hydrogen bromide 6-fluoro-3,4-hydroxy-i.-phenylalanine (6-F-DOPA) in 25 % yield.66... 

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... 

Alkyl fluorides have been generated from the reaction of carboxylic acids with one equivalent of xenon difluoride in dichloromethane or chloroform solution. The process has been named fluorodecarboxylation", the fluorine analog of the Hunsdieckerand Kochi reaction.8384 Both primary and tertiary acids react very well, but secondary acids react less readily. A possible scheme involves a free-radical mechanism including an unstable fluoroxenon ester of an appropriate acid.84... 

Table 2. Substituted Fluoroalkanes by Xenon Difluoride Fluorodecarboxylation of Carboxylic Acids...

Perfluoroalkanccarboxylic acids react with xenon difluoride to form corresponding unstable fluoroxenon esters, which decompose to give perfluoroalkyl radicals successfully trapped by various substrates. Treatment of benzene in dichloromethane with a modest excess of tri-fluoroacetic acid and xenon difluoride at room temperature gives trifluoromethylbenzene in 33-42% yield. The better yields are achieved with aromatics bearing electron-withdrawing substituents.8x86... 

Sulfanyl groups are active substrates for perfluorodecarboxylation reactions. 2,3,5,6-Tetra-chloro-4-sulfanylpyridine reacts with xenon difluoride and perfluoroalkanecarboxylic acids in dichloromethane at room temperature to form 2,3,5,6-tetrachloro-4-perfiuoroalkylsulfanyl-pyridines in moderate to high yields.87... 

Imides of perfluorinated succinic and glutaric acids react with xenon difluoride forming the respective yV-fluoroimides 3 in good yield.111... 

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... 

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... 

It is a powerful explosive [1] produced when xenon tetrafluoride or xenon hexafluoride are exposed to moist air and hydrolysed. Some tetrafluoride is usually present in xenon difluoride, so the latter is potentially dangerous. Although safe to handle in small amounts in aqueous solution, great care must be taken to avoid solutions drying out, e.g. around ground stoppers [2]. Full safety precautions have been discussed [2,3,4]. Precautions necessary for use of aqueous solutions of the trioxide as an epoxidation reagent are detailed [5,6]. A safe method of preparation, so far free from explosions, from xenon hexafluoride and difluorophosphoric acid at 0°C has been described [7]. 

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],... 

Electron transfer from perfluoroalkanoic acids to xenon difluoride was also reported to give perfluoroalkyl radicals which were found to add to benzene derivatives [63]. 

Kolbe-type alkylations are, of course, oxidative in nature. There are a few other oxidative processes which lead to perfluoroalkylative addition, namely oxidation of carboxylic acids with xenon difluoride and oxidation of sodium per-fluoroalkylsulfinates [63,287]. 

The dimerization process is observed in the reaction of 3-phenylbicyclo[l. 1. l]pentan-l-oicacid, and free radical intermediates were suggested36,37 however, only a reduction process was found by Della and Headland Michl and coworkers39. Reaction of various bridged carboxylic acids with xenon difluoride gives, beside fluoro-substituted products, also reduced products, while the product distribution depends on the structure of the alkane37,38 (Scheme 10). 

Transfer of a RCOO or R group from FXeOCOR or Xe(OCOR)2 to aliphatic, aromatic and heteroaromatic molecules has been achieved in recent years. Reactions of difluo-ronitro- and fluorochloronitroacetic acids in the presence of xenon difluoride result in alkylation and acylation of the aromatic ring9, while 1-trifluoroacetoxyadamantane is formed in the reaction of xenon difluoride in the presence of trifluoroacteic acid42. The... 

Acid-catalyzed liquid-phase fluorine addition with xenon difluoride to cis and trans 1-phenylpropene resulted in the formation of vicinal difluoride in high yield63. Photo-initiated reaction in methylene chloride at 0 °C also gave vicinal difluorides in a nonstereospecific manner, and a free-radical mechanism was suggested68. Unfortunately, this radical reaction does not appear to represent a general synthesis since reaction with 1-hexene, cyclohexene and 2,3-dimethyl-2-butene does not give vicinal difluoride products68. 

Liquid-phase fluorination of methyl-substituted benzene derivatives depends strongly on the structure and concentration of the substrate, its molar ratio to xenon difluoride and the catalyst used (Scheme 32). HF-catalyzed fluorination of 1,2,4,5-tetramethylbenzene with an equimolar amount of xenon difluoride gave a small amount of 1,4-difluoro product, while reaction with two equivalents of xenon difluoride also gave the demethylated product l-fluoro-2,4,5-trimethylbenzene, which was also formed by HF-catalyzed fluorination of 1,3,4-trimethylbenzene34. In contrast, trifluoroacetic acid catalyzed fluorination is much more complex, and forms the four products l-(trifluoromethyl)-2,3,5,6-tetrametylbenzene, 2,4,5-trimethylbenzyl trifluoroacetate, l-fluoro-2,3,5,6-tetramethyl-benzene and l-(trifluoromethyl)-2,3,4,5-tetramethylbenzene, the distribution depending on the amount of trifluoroacetic acid used. Similar results were also observed in the fluorination of 1,2,3-trimethylbenzene and 1,3,5-trimethylbenzene34. 

Xenon difluoride reacted with various carboxylic acids, and the type of transformation depends on the structure of the organic molecules35-39. The reaction with primary carboxylic acids involves free-radical intermediates. 6-Hexenoic acid was used as a free-radical clock device in which a A abs of 1.1 x 106 M-1s-1 at 25 °C was determined, while the alkyl radical was also spin-trapped to give an ESR signal37. The primary free radical was trapped by internal cyclization, and (fluoromethyl) cyclopentane in 25% yield was formed, while 6-fluoro-l-hexene could be formed from a radical or ionic intermediate, but 1-fluo-rocycloclohexane was not observed as a product (Scheme 42). 

Phenylpropanoic, 4-phenylbutanoic and 3,3-diphenylpropanoic acids were transformed with xenon difluoride to the corresponding fluorinated products without indication of free-radical rearrangement or cyclization, while phenylacetic acid in the presence of benzene produced diphenylmethane, indicating the involvement of a trivalent species (Scheme 44). Trifluoromethylbenzene was formed in 42% yield by reaction of trifluo-roacetic acid with benzene35-37. 

Nikolenko and coworkers demonstrated that phthalic acid is converted with 2 equiv. of xenon difluoride in acetonitrile to phthalic anhydride, while 2,2 -dicarboxylbiphenyl was converted to benzocoumarin108 (Scheme 45). 

Thiobenzophenone and thiofluorenone are readily transformed with xenon difluoride in to difluoro products90, thiobenzoic acid to benzoyl fluoride90 and dithiobenzoic acid to trifluoromethylbenzene114 (Scheme 47). 

Fluorination with elemental fluorine diluted 1 20 with nitrogen was used to prepare pentafluorophenyl tellurium trifluoride from bis[pentafluorophenyl] ditellurium at — 60° with fluorotrichloromethane as the reaction medium. The yield in this reaction was 80% 4-Methoxyphenyl tellurium trifluoride was obtained by electrochemical oxidation of the diaryl ditellurium in 0.5 M hydrofluoric acid at platinum electrodes2. Bisfpentafluoroethyl] ditellurium and chlorine fluoride (1 6) reacted at — 78° to give pentafluoroethyl tellurium trifluoride, a white solid melting at 95°3. The same compound was obtained with xenon difluoride as the fluorinating agent and melted at 143°4. 

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