Benzenes with xenon difluoride

The fluorination of aromatic compounds with xenon difluoride has been extensively investigated. The fluorination of benzene with xenon difluoride in the presence of hydrogen fluoride as a catalyst results in the formation of fluorobenzene in 68% yield. Monosub-stituted aromatic systems are reported to give high yields of monofluorinated compounds, the isomer distributions of which arc similar to those observed in electrophilic substitution (Table... 

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. 

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... 

Trifluorodi(phenyl)-A5-stibane is also prepared by treating fluorodi(phenyl)stibane with xenon difluoride (in 98 % yield)10 1 or (difluoroiodo)benzene (in 70% yield).107 The reaction of benzene and substituted benzenes with antimony(V) fluoride has been used to prepare the corresponding diaryltrifluoro-/5-stibane hydrates but in yields as low as 7-20%.108... 

Reaction of pentafluoro-substituted benzene derivatives with xenon difluoride/boron trifluoride results in 1,4- and 1,2-fluorine addition with regiospecificity depending on the substituent.41 For example, the heptafluorocyclohexa-1,4-dienes 1 can be obtained in this way in high yield. 

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. 

Benzene reacts with xenon difluoride in the vapour phase at 200 °C to give a 60% yield of fluorobenzene, and naphthalene and 2-fluoropyridine may also be fluorinated with this reagent. The formation of phenyl benzoate from benzoic acid and benzene in the presence of xenon difluoride (Vol. 2, p. 352) has been discussed in more detail and the reaction has been extended to substituted benzoic acids. ... 

The mechanism for the fluorination of benzene and its derivatives with xenon difluoride in the presence of hydrogen fluoride has been discussed independently in more detail, but similar conclusions about the participation of radical-cations in the early stages of the reaction have been reached. However, the effects of added chlorine and of hydrogen chloride on fluori-nations by this method indicate that free radical-cations are involved only in the formation of biaiyls, the halogenobenzenes themselves being formed directly from a complex of the substrate and fluorinating agent. 

It has been shown that the thermolysis of arenesulphonyl azides with benzene to yield A-arenesuIphonylazepines (44-80%) is aided by the addition of Adogen [30], Sulphonylazepines have also been obtained from the reaction of tetra-n-butylammo-nium salts of sulphonamides with benzene in the presence of xenon difluoride the reaction probably proceeds via the intermediate nitrene [31 ]. 

Diselenides can also be oxidatively fluonnated, as shown in equation 18 [775, 779]. Benzene selenyl fluoride is postulated as an intermediate during the fluo-roselenation of alkenes with diphenyl diselemde and xenon difluoride [73, 120]... 

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. 

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. 

Bardin and coworkers99"101 studied the reactions of xenon difluoride with fluoro-substi-tuted benzene and naphthalene derivatives and in agreement with earlier investigations96"98, mainly addition products were observed (Scheme 34). 

With (difluoroiodo)benzene, prepared from iodobenzene and xenon difluoride, as fluorinat-ing agent eYo-2-.vvH-7-difluoronorbornane becomes the main product. 

Radical-cations may also be involved in the formation of phenyl esters from aromatic carboxylic acids and benzene in the presence of xenon difluoride, and can apparently be detected by e.s.r. spectroscopy during the reaction of iViV-dimethylaniline with perfluoropiperidine. The last reagent gives a low yield of o- and p-fluorophenol in the ratio 3.5 1 in its reaction with sodium phenoxide, and the predominance of ortho substitution is ascribed to a cage mechanism of the type shown in (3). However, towards 2,4,6-tri-t-butylphenoxyl radicals the piperidine acts as a source of fluorine atoms to give the dienone (4). ... 

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