· Antimony(V) Fluoride
· (MW 216.74)
(one of the strongest Lewis acids,1 used as catalyst for Friedel-Crafts reactions, isomerization, and other acid related chemistry;1 an efficient acid system for preparation of carbocations and onium ions as well as their salts;1 a fluorinating agent, and a strong oxidant)
Alternate Name: antimony pentafluoride.
Physical Data: bp 149.5 °C.
Solubility: SbF5 reacts with most organic solvents, forming solids with ether, acetone, carbon disulfide, and petroleum ether. SbF5 is soluble in SO2 and SO2ClF.
Form Supplied in: viscous liquid; commercially available.
Handling, Storage, and Precautions: SbF5 is extremely corrosive, toxic, and moisture sensitive. It can be purified by distillation. SbF5 fumes when exposed to atmosphere. It should be stored under anhydrous conditions in a Teflon bottle and handled using proper gloves in a well ventilated hood.
Antimony pentafluoride is one of the strongest Lewis acids reported and is capable of forming stable conjugate superacid systems with HF and FSO3H (see Hydrogen Fluoride-Antimony(V) Fluoride and Fluorosulfuric Acid-Antimony(V) Fluoride).1 Its complex with Trifluoromethanesulfonic Acid is, however, less stable and cannot be stored for extended periods of time. Nevertheless, CF3SO3H/SbF5 is also a useful acid system when prepared in situ.2 The most important properties of SbF5 include its high acidity, strong oxidative ability, and great tendency to form stable anions.3 The chemistry of SbF5 is mainly characterized by these properties. The major applications of SbF5 in organic synthesis include oxidation, fluorination, and as a catalyst for Friedel-Crafts type reactions and other acid related
chemistry, and as a medium for preparation of carbocations and onium ions.1
Friedel-Crafts and Related Chemistry.
The use of SbF5 as a Friedel-Crafts catalyst has significantly expanded the scope of these reactions.3 This is made possible through the strong Lewis acidity and oxidative ability of SbF5. Influenced by the strong acidity, compounds of otherwise very weak nucleophilicity such as perfluorocarbons can react readily with aromatics. Furthermore, the strong oxidative ability of SbF5 enables unconventional substrates such as methane to be oxidized to form positively charged species, which in turn can be applied as electrophilic reagents. On the other hand, these factors also considerably restrict the use of SbF5 in organic synthesis since the high reactivity makes it a less selective Lewis acid for many reactions.
Alkylation of arenes proceeds readily under SbF5 catalysis.3 In addition to alkyl halides, alkyl esters and haloesters have also been applied to alkylate arenes under the reaction conditions.4 Perfluoro or perchlorofluoro compounds have been similarly used as alkylating agents in the presence of SbF5.5 For example, perfluorotoluene reacts with pentafluorobenzene to form perfluorodiphenylmethane in 68% yield after the reaction mixture is quenched with HF. When H2O is used in the quenching step, perfluorobenzophenone is obtained in 93% yield (eq 1).5a
Acylation of pentafluorobenzene to form ketones either with acid halides or with anhydrides has been achieved with an excess of SbF5.6 In the case of dicarboxylic acid dichlorides or anhydrides as acylating agents, diketones are obtained as the reaction products (eq 2). The reaction of the anhydrides of dicarboxylic acids with aromatic compounds does not stop at the keto acid stage, as is the case when other Lewis acids are used. Using phosgene in place of acid chlorides results in the formation of pentafluorobenzoic acid in good yield.3
Perfluorobenzenium salt can be conveniently prepared from the reaction of 1,4-perfluorocyclohexadiene with SbF5.7a-c In the presence of SbF5, this salt is able to react with three equivalents of pentafluorobenzene to yield perfluoro-1,3,5-triphenylbenzene (eq 3).7a-c When 2,2´-di-H-octafluorobiphenyl is used as the substrate, perfluorotriphenylene is obtained in 50% yield (eq 4). Perfluoronaphthlenenium ion also reacts with polyfluorinated arenes in a similar fashion.7a-c The above reaction offers a facile approach for the preparation of these perfluorinated polynuclear aromatic compounds. Oxidation of perchlorobenzene with SbF5 in the presence of pentafluorobenzene leads to the formation of coupling products.7d
Friedel-Crafts sulfonylation of aromatics with alkane- and arenesulfonyl halides and anhydrides has been studied.8 Good yields of sulfones are generally obtained (eq 5). In the case of pentafluorobenzenesulfonyl fluoride with pentafluorobenzene, decafluorodiphenyl sulfone is formed along with decafluorodiphenyl.8c A convenient approach for synthesizing symmetrical aryl sulfones is to react aromatics with Fluorosulfuric Acid in the presence of SbF5 (eq 6).9 Certain phenylacetylenes react with SO2 and benzene in the presence of SbF5 to form benzothiophene S-oxides10 (eq 7). In some cases, 1,1-diphenylvinylsulfinic acids were also obtained as side products of the reaction. Sulfinyl fluoride reacts with arenes similarly under the catalysis of SbF5 to give sulfoxides (eq 8).8c
Oxidation of elemental sulfur and selenium with SbF5 leads to the formation of doubly charged polyatomic cations.11 These cations are able to react with polyfluorinated arenes to form diaryl sulfides or selenides (eq 9).12a-c Under similar treatment, polyfluorodiaryl disulfides or diselenides also react with aromatics to form diaryl sulfides and selenides, respectively (eq 10).12a,12b
In the presence of SbF5, inorganic halides such as NaCl and NaBr can serve as electrophilic halogenating agents.5b,13 Even deactivated arenes such as 5H-nonafluoroindan can be brominated under the reaction conditions (eq 11).13b
Functionalization of alkanes has been achieved under SbF5 catalysis.3 Halogenation of alkanes is one of the widely studied reactions in this field.14 A valuable synthetic procedure is the reaction of alkanes with methylene chloride (or bromide) in the presence of SbF5 (eq 12).14d In these reactions, halonium ions are initially formed, which in turn abstract hydride from hydrocarbons. Quenching of the resulting carbocations with halides leads to the desired haloalkanes.
In the presence of SbF5, alkyl chlorides are ionized to form carbocations, which can be trapped with CO.15 Quenching the reaction intermediates with water or alcohols yields the corresponding carboxylic acids or esters. For instance, halogenated trishomobarrelene reacts with CO in SbF5/SO2ClF to give, after treatment with alcohol and water, a mixture of acid and ether (eq 13).15a
Studies have been carried out on the alkylation of alkenes in the presence of SbF5, especially fluoroalkenes.16 For example, treatment of 1,1,1-trifluoroethane with SbF5 in the presence of tetrafluoroethylene yields 90% of 1,1,1,2,2,3,3-heptafluorobutane.16c Perfluoroallyl or -benzoyl compounds with varying structures have also been utilized as alkyating agents (eqs 14 and 15).16b,e-h At elevated temperatures, intramolecular alkylation was observed in certain cases with perfluorodienes, forming perfluorocyclopentenes or perfluorocyclobutenes (eq 16).17
Acylation of alkenes can also be similarly effected with SbF5.18 Reaction of acetyl fluoride with trifluoroethylene produces 1,1,1,2-tetrafluorohexane-3,5-dione in 40% yield.18a When antimony pentafluoride is used in excess (more than 6 molar equivalents), 1,1,1,2-tetrafluorobutan-3-one is formed as a side product. Benzoyl fluoride reacts with difluoroethylene to form the expected ketone in 39% yield.18a a,b-Unsaturated carboxylic acid fluorides have also been used in reactions with perfluoroalkenes.18b In these cases, a,b-unsaturated ketones are obtained (eq 17). Enol acetates of perfluoroisopropyl methyl ketone and perfluoro-t-butyl methyl ketone react with acetyl fluoride to provide the corresponding b-diketones (eq 18).18c
Oxygenation of dienes with molecular oxygen to form Diels-Alder-type adducts can be effected by Lewis acids and some salts of stable carbenium ions.19 In the case of ergosteryl acetate, SbF5 is by far the most active catalyst (eq 19).19a
Isomerization and Rearrangement.
Isomerization of perfluoroalkenes can be realized with SbF5 catalysis.20 The terminal carbon-carbon bonds of these alkenes are usually moved to the 2-position under the influence of this catalyst (eq 20). A further inward shift generally occurs only if H or Cl atoms are present in the 4-position of the alkenes. As a rule, the isomerization leads to the predominant formation of the trans isomers. Terminal fluorodienes also isomerize exothermally into dienes containing internal double bonds in the presence of SbF5. With a catalytic amount of SbF5, perfluoro-1,4-cyclohexadiene disproportionates to hexafluorobenzene and perfluorocyclohexene. The disproportionation proceeds intramolecularly in the case of perfluoro-1,4,5,8-tetrahydronaphthalene. The starting material is completely converted to perfluorotetralin (eq 21).20f
Like most Lewis acids, SbF5 promotes the rearrangement of epoxides to carbonyl compounds,21 and SbF5 is an efficient catalyst for this reaction. However, the migratory aptitude of substitutent groups in the reaction under SbF5 catalysis is much less selective compared to that promoted by weak Lewis acids such as Methylaluminum Bis(4-bromo-2,6-di-t-butylphenoxide).21a Nevertheless, SbF5 is well suited for the rearrangement of perfluoroepoxides.21b-e Excellent yields of ketones are obtained from the reaction. When diepoxides are used, diketones are obtained as the reaction products (eq 22).21c
Fluorination and Transformation of Fluorinated Compounds.
SbF5 is a strong fluorinating agent. However, its use is largely limited to the preparation of perfluoro- or polyfluoroorganic compounds.22 SbF5 has also been used for transformation of perfluoro compounds.22
When hexachlorobenzene is subjected to SbF5 treatment, 44% of 1,2-dichloro-3,3,4,4,5,5,6,6-octafluorocyclohex-1-ene is obtained as the major product of the reaction (eq 23).23 Using SbCl5 as reaction solvent leads to better reaction control, resulting in an increase in the product yield.23c In the case of polyfluoronaphthalenes, polyfluorinated tetralins are obtained.23d Treatment of hexafluoro-2-trichloromethyl-2-propanol with SbF5 gives perfluoro-t-butyl alcohol in 92% yield (eq 24).24
Hydrolysis of CF3 groups to CO2H can be induced by SbF5.25a In this reaction, the CF3-bearing compounds are first treated with SbF5 and the resulting reaction mixtures are subsequently quenched with water to form the desired products. For example, perfluorotoluene was converted to pentafluorobenzoic acid in 86% yield by this procedure. When perfluoroxylenes and perfluoromesitylene go through the same treatment, the corresponding di- or triacids are obtained in high yields. It is also possible to partially hydrolyze perfluoroxylenes and perfluoromesitylene through a stepwise procedure and to isolate intermediate products. The hydrolysis procedure is also applicable to other halogen-containing compounds.25b
Under SbF5 catalysis, Trifluoromethanesulfonic Anhydride is readily decomposed to trifluoromethyl triflate in high yield (eq 25).26 This method is the most convenient procedure reported for preparation of the triflate.
Preparation of Carbocations, Onium Ions, and Their Salts.
SbF5 is a preferred medium for the preparation of carbocations and onium ions.1 In fact, the first observation of stable carbocations was achieved in this medium.1,27 By dissolving t-butyl fluoride in an excess of SbF5, the t-butyl cation was obtained (eq 26). Subsequently, many alkyl cations have been obtained in SbF5 (either neat or diluted with SO2, SO2ClF, or SO2F2). The 2-norbornyl cation, one of the most controversial ions in the history of physical organic chemistry,28 was prepared from exo-2-fluoronorbornane in SbF5/SO2 (or SO2ClF) solution (eq 27).1 Bridgehead cations such as 1-adamantyl, 1-trishomobarrelyl, and 1-trishomobullvalyl cations have also been similarly prepared with the use of SbF5.15a,29
Carbodications have also been studied.1 One convenient way of preparing alkyl dications is to ionize dihalides with SbF5 in SO2ClF (eq 28). In these systems, separation of the two cation centers by at least two methylene groups is necessary for the ions to be observable.1 Aromatic dications are usually prepared by oxidizing the corresponding aromatic compounds with SbF5 (eq 29).1,30 In the case of pagodane containing a planar cyclobutane ring, oxidation leads to the formation of cyclobutane dication which was characterized as a frozen two-electron Woodward-Hoffmann transition state model (eq 30).31
SbF5 has also been found useful in the preparation of homoaromatic cations.1,32 For example, the simplest 2p monohomoaromatic cations, the homocyclopropenyl cations, can be prepared from corresponding 3-halocyclobutenes in SbF5 (eq 31).32a
Perfluorocarbocations are generally prepared with the aid of SbF5.33 Perfluorobenzyl cations and perfluorinated allyl cations are among the most well studied perfluorinated carbocations. It was reported that in the perfluoroallyl cations containing a pentafluorophenyl group at the 1- or 2-position, the phenyl groups were partially removed from the plane of the allyl triad (eq 32).33b
Use of SbF5 as ionizing agent offers a convenient route to acyclic and cyclic halonium ions from alkyl halides.1 Three-membered ring cyclic halonium ions are important intermediates in the electrophilic halogenations of carbon-carbon double bonds.1 More recently, a stable 1,4-bridged bicyclic bromonium ion, 7-bromoniabicyclo[2.2.1]heptane, has been prepared through the use of SbF5 involving an unprecedented transannular participation in a six-membered ring (eq 33).34
Reaction of a,b-unsaturated carbonyl compounds with diazo compounds generally gives low yields of cyclopropyl compounds. The rapid formation of 1-pyrazolines and their subsequent rearrangement products 2-pyrazolines is the reason for the inefficiency of cyclopropanation of these substrates. However, in the presense of SbF5, the cyclopropanation of a,b-unsaturated carbonyl compounds with diazocarbonyl compounds proceeds very well to produce the desired products in good yields (eq 34).35
Fluorosulfuric Acid-Antimony(V) Fluoride;
Hydrogen Fluoride-Antimony(V) Fluoride;
Methyl Fluoride-Antimony(V) Fluoride.
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George A. Olah, G. K. Surya Prakash, Qi Wang & Xing-ya Li
University of Southern California, Los Angeles, CA, USA
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