Fluorination catalytic

In the same way as intramolecular displacement leads to particularly stable atomic groupings within a molecule, the disproportionation reactions between several molecules are attributable to the tendency for the more stable compound with the higher fluorine content to form. Reactions of this kind are sometimes used to obtain highly fluorinated compounds from products with lower fluorine contents, for example, the catalytic fluorination of chloroalkanes with hydrogen fluoride or with fluorination agents such as antimony(V) fluoride or antimony(III) fluoride. The chlorine compound formed as the second product of the disproportionation process is reused as the starting material for the preparation of the compound to be dispropor-tionated. 

Neutral electrophilic reagents are usually prepared by reactions involving strong oxidants. For example, the most practical route to relatively stable hypo-halites X0S02F (X=C1, Br, I) [8] is based on the reaction of halogens with peroxide FS0200S02F. This material can be made on a large scale by catalytic fluorination of S03 [34] ... 

An interesting complimentary approach has been devised by Togni and Mezzetti who reported on Ru-F complexes for a catalytic fluorination by exchanging existing halide groups for fluoride [30]. Use of a chiral nonracemic Ru11 catalyst led to initially moderate asym-... 

To summarise, the development of novel enantioselective fluorination methods with the aid of either chiral N-fluoro ammonium salts or transition metal catalysts has established truely practical routes towards chiral fluorinated compounds. Despite the current mechanistic uncertaincies it appears that a door has been opened for exciting and promising further development of asymmetric (catalytic) fluorination reactions in the near future [31, 32]. 

Fluoride Catalysts Their Application to Heterogeneous Catalytic Fluorination and Related Processes... 

In this chapter, recent advances in our understanding of catalytic fluorination under heterogeneous conditions are surveyed from the standpoint of catalyst properties, including developments based on the use of mixed metal fluorides having different structural types, and reaction mechanisms. Much of the newer work has been the result of the need to replace chlorofluorocarbons (CFCs) by alternatives, hydrofluorocarbons (HFCs) or, more controversially, hydrochlorofluorocarbons (HCFCs), following adoption of the Montreal and successor Protocols [2,3]. Where relevant, aspects of catalytic hydrogenolysis, where fluorides have been used as replacement supports in the conventional palladium/carbon catalysts, and isomerization reactions are included. 

Direct evidence for a combination of catalytic fluorination and chlorination [4] was obtained from radiotracer studies in which fluorinated chromia catalysts were labelled with the short-lived (t /2 = 110 min) / + emitting isotope fluorine-18 [11]. Using this isotope it was possible to probe the interactions between HF and various fluorinated chromia catalysts more directly than had been possible hitherto. Three types of surface F-containing species were differentiated, weakly adsorbed HF which was easily removed by an inert gas flow, non-labile F, believed to be bound directly to surface Crin, and catalytically active F which could be incorporated into the organic products [12]. The controversy between dismutation (concerted F-for-Cl and Cl-for-F transfers) and non-concerted halogen exchange processes has been resolved more recently and the evidence is described later in the chapter. What is clear from this early work however, is the importance of aluminium and chromium(III) oxides as catalyst precursors. Fluorination of the surfaces of these oxides is slow (cf [12]) and although there are many references to alu-... 

Heterogeneous catalysis is important in all cases. The catalytic fluorination steps (F-for-Cl replacement) either use existing chlorofluoroethane methodology or new methodology for the catalytic fluorination of the C2 HCFCs. Development of new catalysts was particularly important for the conversion of CF3CH2C1 to... 

In summary, the development of doped catalysts, in both oxide and fluoride families, for catalytic fluorination is at an early stage. Much work remains to be done before a clear picture is obtained, however it is apparent that doping can perturb the crystalline nature of pure phases under some circumstances. When a substantially greater surface area results, a high concentration of acidic surface sites is also observed. There is considerable uncertainty regarding the selectivity and long term stability of doped catalysts and these aspects require further investigation. 

The derivation of a mechanism for a chemical reaction is by its very nature an uncertain process, being dependent critically on the nature and extent of the experimental evidence. Mechanisms that have at their heart a surface process or processes are even more uncertain and when the constraints imposed by the manipulation of HF are also taken into account, it is not surprising that there have been relatively few mechanistic studies made of heterogeneous catalytic fluorination. However a catalytic process cannot be said to be understood fully without a mechanism based on the experimental evidence available and such studies are helpful in the design of the next generation of catalysts. In most cases the work described below involves chromia or y-alumina based catalysts that have been pretreated according to the methods described above. Studies involving C2 and Q compounds are described in turn. 

The possibility of catalytic fluorination and chlorination occurring in C2 HCFCs has been demonstrated for CHC1FCF3 using the precursor CoCl2 supported on alumina which is subsequently treated with HF [97]. Mechanisms based on... 

Although the procedure described utilizes metal vessels for ease of treating the cesium fluoride and to avoid the formation of large amounts of silicon tetrafluoride, glass may be used. This procedure has been used for preparation of up to 0.015 mole of CF2(0F)2 at a time with the aid of liquid nitrogen to condense excess fluorine into the vessel. The method is analogous to that used in the preparation of pentafluorosulfur hypofluorite by the static, catalytic fluorination of thionyl fluoride in the presence of cesium fluoride. ... 

Cryogenic preconcentration of sample (catalytic fluorination after GC to form SFb) GC/ECD 6.25 ng/m3 NR Johnson and Bates 1993... 

Catalytic fluorination involves mixing the reacting compound and fluorine diluted with nitrogen in the presence of the catalyst.58 The catalyst may be copper gauze, silver-coated copper gauze, or cesium fluoride. An example is C<,H6 + 9F2 -c 205 >.C0F12 + 6HF... 

Carbonyl compounds can also be catalytically fluorinated to give fluoro-hypochlorites RR CF(OCl) by using C1F in presence of CsF.60 Similarly the CsF-catalysed interaction of C02 and F2 gives difluorobis(fluorooxy)-methane, CF2(OF)2.61... 

Peroxydisulfuryl fluoride is usually synthesized by catalytic fluorination of sulfur trioxide.However, pure peroxydisulfuryl fluoride can be obtained in a relatively simple way by the reaction between fluorosulfuric acid and dioxygenyl hexafluoroarsenate. The method lends itself to preparation of smaller quantities (a few grams) of peroxydisulfuryl fluoride. The reaction can be carried out in an openable nickel or Monel pressure vessel, into which dioxygenyl hexafluoroarsenate is transferred from a photochemical reactor (see the preceding synthesis of dioxygenyl hexafluoroarsenate) or, more simply, in the same Pyrex-glass flask in which dioxygenyl hexafluoroarsenate has previously been synthesized. 

Chromium(VI) tetrafluoride oxide CrF40 was first synthesized via fluor-ination of either metallic chromium or CrOj, and the route has been modified to use CrOj at lower temperature. Very recently, fluorination of Cr02F2 with KrF2 in anhydrous HF was reported. However, this route requires the use of a rather exotic fluorinating agent. The procedure described below involves the catalytic fluorination of Cr02F2 in the presence of CsF. 

Peroxydisulfuryl difluoride can be prepared by reaction of fluorine with an excess of sulfur trioxide at about 250° and by the combination of fluorine fluorosulfate with sulfur trioxide. The catalytic fluorination of sulfur trioxide vapor by fluorine in the presence of a heated catalyst of copper ribbon coated with silver(II) fluoride provides a good method for the preparation of a product which can be purified easily. 

Scheme 13 Catalytic fluorination with a chiral ruthenium catalyst employing a nucleophilic fluorine source...

Although the adsorption behaviour of CFCs, particularly CCI3F and CCI2F2, on clean metal surfaces has been studied extensively, information relating to surfaces that are relevant to catalytic fluorination is sparse. 

Specific heterogeneous catalytic reactions will now be discussed in turn starting with those processes, catalytic fluorination and catalytic fluorina-tion followed by hydrogenolysis of a chlorofluoroethane, that are relevant to the production of CF3CH2F (HFC-134a). These three sections are followed by discussions of catalytic reactions involving CpCFCs and Cp HCFCs and of room temperature catalytic fluorination. 

A.2 The heterogeneous catalytic fluorinations HCFC-133a and HCFC-123a HFC-125... 

Figure 8.4 Halogen exchange and intramolecular isomerisation model proposed for the heterogeneous catalytic fluorination reaction C2Cl6(CFC-110) - C2CIF5 (CFC-115). (Reproduced with permission from ref. 30.)...

Catalytic fluorination of CCI4 and dismutation of Cj HFCs under heterogeneous conditions... 

Although there has been some recent interest in the catalytic fluorination of CCI4, notably over chromium(III) fluoride derived from thermal decomposition of (NH4)3CrF6 [42], most attention has been paid to dismutations within the HCFC—CHCl3 F series. Reactions on several metal oxides and halides have been studied by Kemnitz and coworkers and evidence for dismutation behaviour adduced from thermodynamic considerations and... 

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