Xenon complexes difluorides

Xenon difluoride [4, 5, 7, 8,10] is a white crystalline material obtained through the combination of fluorine and xenon m the presence of light The reagent is commercially available and possesses a relatively long shelf-life when stored cold (freezer) Xenon difluoride is very effective for small-scale fluormation of alkenes and activated nucleophilic substrates. The reactions are usually conducted between 0 °C and room temperature in chloroform or methylene chloride solutions Hydrogen fluoride catalysis is sometimes helpful Xenon difluoride reacts in a manner that usually involves some complexation between the substrate and reagent followed by the formation of radical and radical cation intermediates... 

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)... 

The starting point for the synthesis of xenon compounds is the preparation of xenon difluoride, XeF2, and xenon tetrafluoride, XeF4, by heating a mixture of the elements to 400°C at 6 atm. At higher pressures, fluorination proceeds as far as xenon hexafluoride, XeFfi. All three fluorides are crystalline solids (Fig. 15.27). In the gas phase, all are molecular compounds. Solid xenon hexafluoride, however, is ionic, with a complex structure consisting of XeF< + cations bridged by F anions. 

Since the discovery of the first noble gas compound, Xe PtF (Bartlett, 1962), a number of compounds of krypton, xenon, and radon have been prepared. Xenon has been shown to have a very rich chemistry, encompassing simple fluorides, XeF2> XeF, and XeF oxides, XeO and XeO oxyf luorides, XeOF2> XeOF, and Xe02 2 perxenates perchlorates fluorosulfates and many adducts with Lewis acids and bases (Bartlett and Sladky, 1973). Krypton compounds are less stable than xenon compounds, hence only about a dozen have been prepared KrF and derivatives of KrF2> such as KrF+SbF, KrF+VF, and KrF+Ta2F11. The chemistry of radon has been studied by radioactive tracer methods, since there are no stable isotopes of this element, and it has been deduced that radon also forms a difluoride and several complex salts. In this paper, some of the methods of preparation and properties of radon compounds are described. For further information concerning the chemistry, the reader is referred to a recent review (Stein, 1983). 

Radon difluoride is quantitatively reduced to elemental radon by water in a reaction which is analogous to the reactions of krypton difluoride and xenon difluoride with water. Complex salts of radon also hydrolyze in this fashion. 

Fluorination of propene by xenon difluoride leads to formation of l,2-difluoropropanc(12%), 1,1-difluoropropane (46 %), 2-fluoropropane (24 %) and 1,1-difluoroethane (9 %). The complex composition of the mixture is explained by the radical character of the reaction. 

The interaction of alkenes with xenon difluoride has been successful in the presence of a catalyst. Dec-l-ene treated with xenon difluoride in the presence of a boron trifluoride-diethyl ether complex in dichloromethane formed 1,1-difluorodecane (26%) and a mixture of 1-fluoro-decene isomers (48%). Under the same conditions 2-fluoro-2,2-dinitroethyl vinyl ether has been converted to T,2 -difluoroethyl-2-fluoro-2,2-dinitrocthyl ether.27... 

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. 

Unsaturated sugars, namely glycals, such as 3,4,6-tri-Oacetyl-l, 5-anhydro-2-dcoxy-D-r//v/-/w7o-hex-1-enetol and the d-/v.yo epimer react with xenon difluoride in the presence of a boron trifluoride-diethyl ether complex to give a mixture of C2 epimeric fluorides.45,46 Tri-O-acctyl-D-galactal treated with xenon difluoride in trichlorofluoromcthane below 5 C forms after hydrolysis (0.5 M HC1) 2-deoxy-2-fluoro-D-galactose in 63% overall yield.47... 

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... 

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... 

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... 

Fluoromalonates can be obtained in moderate yields by fluorination of the corresponding carbanions with xenon difluoride, dimethyl sulfide and boron trifluoride-diethyl ether complex.29... 

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. 

Xenon difluoride reacts with alcohols to form unstable alkoxyxenon fluoride intermediates. Alkoxyxenon fluorides react as positive oxygen electrophiles when boron trifluoride-diethyl ether complex is used as a catalyst. However, these alkoxyxenon fluorides react as apparent fluorine electrophiles with proton catalysts (hydrogen fluoride generated in situ).49... 

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... 

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. 

The fluorination of 1,3-diketones, for example 5.5-dimethylcyclohexane-l,3-dione, with xenon difluoride results in a complex mixture of products however, when the fluorination is carried out with two equivalents of xenon difluoride in the presence of insoluble cross-linked polystyrene-4-vinylpyridine only 2.2-difluoro-5,5-dirncthylcyclohexane-l,3-dione (4) is form-... 

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