Xenon difluoride aromatics

The results given in both Table 1 and in the reviews [4, 5 7, 8, I0 show that xenon difluoride reacts with a wide variety of aromatic substrates to produce regioselecuvely monofluorinated aromatics An example is the preparation of 6-fluoro-L DOPA [83] (equation 48)... 

Xenon difluoride may be used as the pure reagent or as a graphi te intercalate for the effective fluonnation of polynuclear aromatics [86 87] (equations 49 and 50)... 

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)... 

Safety precautions applicable to direct liquid phase fluorination of aromatic compounds are discussed [1]. Attention is drawn to the hazards attached to the use of many newer fluorinating agents [2], In a study of fluorination reactions of hafnium and zirconium oxides by the fluoroxidisers xenon difluoride, chlorine trifluoride and bromine trifluoride, reactivity decreased in the order given [3],... 

Pyridine is converted into perfluoropiperidine (82) in low yield by reaction with fluorine in the presence of cobalt trifluoride (50JCS1966) quinoline affords (83) under similar conditions (56JCS783). Perfluoropiperidine can be obtained electrochemically. This is useful, as it may be readily aromatized to perfluoropyridine by passing it over iron or nickel at ca. 600 °C (74HC(14-S2)407). Recently, pyridine has been treated with xenon difluoride to yield 2-fluoropyridine (35%), 3-fluoropyridine (20%) and 2,6-difluoropyridine (11%), but it is not likely that this is simply an electrophilic substitution reaction (76MI20500). 

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... 

Difluorotri(phenyl)-/.5-stibane, the most readily accessible among the aromatic anti-mony(V) fluorides, can be synthesized by several alternate methods in addition to those described above, i.e. by substitution of fluorine for chlorine in dichlorotri(phenyl)-/i5-stibane with potassium fluoride109 or hydrogen fluoride,110 and by oxidative fluorination of triphenylstibane with fluorine (in chloroform),111 perfluoro-l-fluoropiperidine,112 (difluoroiodo)ben-zene,107 or xenon difluoride.10 1... 

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. 

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... 

Liquid-phase fluorination of alkyl-substituted anisole derivatives with xenon difluoride depends strongly on the structure of the aromatic molecule. Substitution and addition processes were observed in the case of the methyl derivative, while beside substitution of the hydrogen atom, dealkylation was also established with l-methoxy-4-J-butyl benzene89,90 (Scheme 29). 

The fluorination of 2-bromo-4,5-dimethylphenol with xenon difluoride has again confirmed the strong dependence of the course of reaction on the aromatic structure, and substitution, addition, rearrangement and oxidation processes were in competition91 (Scheme 31). 

The substitution of hydrogen atoms by fluorine in aromatic compounds has been investigated with many classes of reagent, including elemental fluorine. hypofluorites, " fluoroxysulfates. xenon difluoride," and N-F reagents. " ... 

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... 

Xenon difluoride has also been used for the fluorination of enol derivatives [5] and 1,3-dicarbonyl compounds [140], and fluorination of activated aromatic substrates is possible in the presence of a Lewis acid [141]. 

Xenon difluoride is reactive towards aromatic compounds [31], and selective fluoro-desilylation using this reagent has also been reported [32] (Figure 9.10). 

Radical cations derived from Mrfluoro aromatic molecules were first described by Bazhin et al. > The species were made in super acid or oleum solutions and were unambiguously characterized by ESR spectroscopy. Such species have also been proposed as interm tates in the oxidative fluorination of aromatic molecules by high-valent metal fluorides such as cobalt trtfluoride, by bromine trifluoride, and by xenon difluoride. They have also been invoked in the electrochemical fluorination of aromatic molecules in the presence of F. Salts of such radical cation species were unknown prior to the synthesis of C6F6 AsF6 in these laboratories. ... 

One of the first reagents used for electrophilic fluorination ivas xenon difluoride (XeF2) [168], a solid which is easy to handle and which can be used in solvents which are relatively inert toward oxidation, for example acetonitrile and dichloromethane. The reactivity is mostly determined by its strong oxidizing power, rendering its mode of action more oxidative than electrophilic fluorination. With XeF2 not only are typical electrophilic fluorinations of aromatic compounds possible but also the Hunsdiecker-like fluorodecarboxylation of carboxylic acids and fluori-native rearrangements of carbonyl compounds to difluoromethyl ethers [169-171] (Scheme 2.76.). 

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