Antimony fluoride: Lewis acidity

Perchloric acid (HCIO4 Ho —13.0), fluorosulfuric acid (HSO3F Ho — 15.1), and trifluoromethanesulfonic acid (CF3SO3H Ho —14.1) are considered to be superacids, as is truly anhydrous hydrogen fluoride. Complexing with Lewis acidic metal fluorides of higher valence, such as antimony, tantalum, or niobium pentafluoride, greatly enhances the acidity of all these acids. 

In a generalized sense, acids are electron pair acceptors. They include both protic (Bronsted) acids and Lewis acids such as AlCb and BF3 that have an electron-deficient central metal atom. Consequently, there is a priori no difference between Bronsted (protic) and Lewis acids. In extending the concept of superacidity to Lewis acid halides, those stronger than anhydrous aluminum chloride (the most commonly used Friedel-Crafts acid) are considered super Lewis acids. These superacidic Lewis acids include such higher-valence fluorides as antimony, arsenic, tantalum, niobium, and bismuth pentafluorides. Superacidity encompasses both very strong Bronsted and Lewis acids and their conjugate acid systems. 

Lewis Acid Complexes. Sulfolane complexes with Lewis acids, such as boron trifluoride or phosphoms pentafluoride (17). For example, at room temperature, sulfolane and boron trifluoride combine in a 1 1 mole ratio with the evolution of heat to give a white, hygroscopic soHd which melts at 37°C. The reaction of sulfolane with methyl fluoride and antimony pentafluoride inhquid sulfur dioxide gives crystalline tetrahydro-l-methoxythiophenium-l-oxidehexafluoroantimonate, the first example of an alkoxysulfoxonium salt (18). 

Grown Ethers. Ethylene oxide forms cycHc oligomers (crown ethers) in the presence of fluorinated Lewis acids such as boron tritiuoride, phosphoms pentafluoride, or antimony pentafluoride. Hydrogen fluoride is the preferred catalyst (47). The presence of BF , PF , or SbF salts of alkah, alkaline earth, or transition metals directs the oligomerization to the cycHc tetramer, 1,4,7,10-tetraoxacyclododecane [294-93-9] (12-crown-4), pentamer, 1,4,7,10,13-pentaoxacyclopentadecane [33100-27-6] (15-crown-6), andhexamer, 1,4,7,10,13,16-hexaoxacyclooctadecane [17455-13-9]... 

The role of Lewis acids in the formation of oxazoles from diazocarbonyl compounds and nitriles has primarily been studied independently by two groups. Doyle et al. first reported the use of aluminium(III) chloride as a catalyst for the decomposition of diazoketones.<78TL2247> In a more detailed study, a range of Lewis acids was screened for catalytic activity, using diazoacetophenone la and acetonitrile as the test reaction.<80JOC3657> Of the catalysts employed, boron trifluoride etherate was found to be the catalyst of choice, due to the low yield of the 1-halogenated side-product 17 (X = Cl or F) compared to 2-methyI-5-phenyloxazole 18. Unfortunately, it was found that in the case of boron trifluoride etherate, the nitrile had to be used in a ten-fold excess, however the use of antimony(V) fluoride allowed the use of the nitrile in only a three fold excess (Table 1). 

Reactions of organoxenonium salts [RXe][EFn+)] with Lewis acids do not result in transformation of the organo group of the cation [RXe]+ (6), but rather give rise to fluoride abstraction from the counteranion [EFn+i]. For example, organoxenonium fluoroborates were converted into [EF6]" salts by using arsenic or antimony pentafluoride or their adducts, EFj L (eqs 9, 10) (22, 30). 

Olah and co-workers in 1963 flrst observed the formation of stable alkylcarbonium ion complexes when t-butyl fluoride was dissolved in excess antimony pentafluoride (serving as both the Lewis acid and the solvent). 

It was indeed found necessary to have excess antimony pentafluoride present in order to obtain stable alkylcarbonium hexafluoroantimonate complexes. Antimony pentafluoride is a liquid Lewis acid fluoride (b.p. 148-150°) of low dielectric constant (e 3), which has been shown by fluorine N.M.R. studies in the pure liquid state and in solution to exist in both cyclic and acyclic polymeric forms involving fluorine bridges. The antimony is in approximately octahedral co-ordination with predominant bridging by coordinating fluorines (Gillespie and Rothenbury, 1963). As fluorine generally does not show bridging properties, the structure of antimony pentafluoride itself indicates the very... 

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 reactivity of bromine trifluoride is significantly enhanced by Lewis acids, such as tin(IV) chloride, antimony(V) chloride, titanium(IV) chloride, which are exchanged to the corresponding fluorides and give complexes with bromine trifluoride. Thus, the reaction of 2,2,2-tri-fluoroethyl or 2,2,3,3-tetrafluoropropyl 2,3-dibromopropanoate with bromine trifluoride in the presence of 1 mol% tin(IV) chloride affords the corresponding 2,3-difluoropropanoates in 85-87% yield.110... 

Arenediazonium hexafluoroantimonates were described as early as 1935,275 but their fluorodediazoniation was only studied much later.276 They deliver fluoroaromatics in better yields than other diazonium salts only when electron-withdrawing groups (N02, C02H) are present on the ring. Otherwise, antimony(V) fluoride formed in the reaction acts as a strong Lewis acid and catalyzes the formation of byproducts. The phenomenon is more pronounced with diazonium hexafluoroarsenates which deliver fluoro compounds in acceptable yields only from nitroanilines.276 With unsubstituted or electron-releasing substituted anilines, difluo-rinated arenearsonic acids are often the major product. 

A series of functionalized alkenes has been subjected to selective epoxidation reactions. Oxidation of perfluorinated vinyl polyethers by bubbling oxygen through the liquid that contains a catalytic amount of a Lewis acid, e.g. antimony(V) fluoride, results in the formation of acid fluorides together with a smaller amount of C = C bond cleavage.76 Perfluorinated pentaaIkyI-2.3-dihydrofuran 38, an example of an unsaturated cyclic ether that is quite stable owing to perfluoroalkylation. is epoxidized by hypochlorite to the product 39 at 45 C.62... 

A wide range of Lewis acids have been used in the extensive studies of the conversion of perfluoro(2-melhyloxirane)28 into hexafluoroacetone. including antimony(V) fluoride,31-32 aluminum trichloride,30 aluminum chlorofluoride (from A1C1, and CFClj).23-33 alumina pre-... 

Dehydrofluorinations using acidic reagents have been described using Lewis acids such as antimony(V) fluoride, e.g. formation of 1 and2,14or ion-exchange resins, e.g. formation of 3.99... 

Covalent fluorides of group 3 and group 5 elements (boron, tin, phosphorus, antimony, etc) are widely used in organic synthesis as strong Lewis acids Boron trifluoride etherate is one of the most common reagents used to catalyze many organic reactions. A representative example is its recent application as a catalyst in the cycloadditions of 2-aza-l,3-dienes with different dienophiles [14] Boron trifluoride etherate and other fluormated Lewis acids are effective activators of the... 

It is quite stable thermally we expect the octahedral Mn(IV) complex (d3) to enjoy a considerable degree of crystal field stabilisation. Christe reasoned that the unstable MnF4 could be displaced by treating K2[MnF6] with a stronger Lewis acid than MnF4. Antimony(V) fluoride immediately sprang to mind (Section 9.2), and fluorine gas was obtained by the reaction ... 

Particularly good results are obtained when anhydrous hydrogen fluoride is used in combination with a Lewis acid, such as antimony(V) fluoride or boron trifliioride-diethyl ether complex, for the ring opening of epoxides bearing electron-withdrawing substituents. Per-fluoro(2.2-dimethyloxirane) is opened to pcrfluoro(2-methylpropan)-2-ol (1) in 56% yield with hydrogen fluoride and catalytic amounts of antimony(V) fluoride at 100 C. ... 

While the first two allow the isolation of the reaction products, the third does not. When a Lewis acid is mixed with an acyl halide, a donor-acceptor complex RCOX. .. MtX , or full ionisation by halide ion transfer, to give RCO MtX +i, or both, take place. The extent of ionisation depends mostly on the nature and strength of the Lewis acid used Thus, for example, acetyl halides react with stannic chloride and titanium tetrachloride to give mostly the coordination complex while with antimony pentachloride, pen-tafluoride and boron fluoride they give the conesponding acylium salts. Many of these... 

In 1986, Carl O. Christe discovered that the strong Lewis acid SbFj displaces the weaker Lewis acid MnF from the hexafluoromanganate(TV) ion, [MnFg]. MnF is thermodynamically unstable, and it decomposes into MnFj and F2. Christe heated a mixture of potassium hexafluoromanganate(IV) and antimony(V) fluoride in a passivated Teflon-stainless steel container at 150°C for 1 hour. He obtained elemental F2 in better than 40% yield. The overall reaction may be represented as... 

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