Hydrogen Fluoride-Antimony(V) Fluoride1

HF/SbF5
(HF)

[7664-39-3]  · FH  · Hydrogen Fluoride-Antimony(V) Fluoride  · (MW 20.01) (SbF5)

[7783-70-2]  · F5Sb  · Hydrogen Fluoride-Antimony(V) Fluoride  · (MW 216.75) (HSbF6)

[16950-06-4]  · F6HSb  · Hydrogen Fluoride-Antimony(V) Fluoride  · (MW 236.76)

(strong liquid superacid system, used as a catalyst for Friedel-Crafts reactions, isomerization, polymerization, and other acid-related chemistry; efficient for preparation of carbocations and onium ions and their salts1)

Alternate Name: fluoroantimonic acid; hexafluoroantimonic acid.

Physical Data: HF, bp -19.5 °C; SbF5, bp 149.5 °C.

Form Supplied in: HF, gas; SbF5, viscous liquid. Both are commercially available.

Preparative Methods: HF/SbF5 is prepared by mixing required amounts of freshly distilled Hydrogen Fluoride and Antimony(V) Fluoride at low temperature.2 The reaction is exothermic and must be carried out with careful temperature control. The reagent can also be prepared in situ in the reaction flask.

Handling, Storage, and Precautions: HF/SbF5 is extremely corrosive, toxic, and moisture sensitive. It fumes when exposed to air; thus it should be stored under anhydrous conditions in a Teflon bottle and handled with gloves in a well-ventilated fume hood.

General Consideration.

Hydrogen fluoride-antimony pentafluoride (fluoroantimonic acid) is probably the strongest liquid superacid system, possessing the widest acidity range.1 Compared to the widely used Magic Acid system (Fluorosulfuric Acid-Antimony(V) Fluoride), the acidity of fluoroantimonic acid is even higher with the same molar concentration of SbF5. For example, fluoroantimonic acid with 0.5% SbF5 has a H0 of -21, while H0 for HSO3F/SbF5 with the same amount of SbF5 is only -17.2 Hexafluoroantimonic acid (HF:SbF5 = 1:1) has an estimated H0 of -30.1 The X-ray structure of the 1:1 HF:SbF5 complex has recently been established.3

Very strong acidity (and low nucleophilicity of its conjugate anion) make fluoroantimonic acid an efficient medium for the preparation of carbocations and onium ions.1 The same properties have also made the acid an effective catalyst for Friedel-Crafts type reactions, polymerization, isomerization, and other acid-related chemistry.1

Friedel-Crafts and Related Chemistry.

Alkylations of deactivated aromatic compounds such as acetophenone are generally difficult to achieve.4 However, this problem can be overcome with the use of HF/SbF5. It was demonstrated that acetophenone was readily ethylated with ethyl chloride in the presence of HF/SbF5 (eq 1).5 Other primary and secondary alkyl chlorides also react well with the substrate under similar conditions. However, poor results were obtained with tertiary chlorides.

In the presence of HF/SbF5, even alkanes can alkylate aromatics.6 This results from protolysis of alkanes to form carbocations and the subsequent trapping of these cations by arenes. The concept of alkane protolysis was also utilized for the preparation of 4,4-dialkyl-1-tetralones from alkyl phenyl ketones via intramolecular cyclization by using a 5:1 ratio of HF:SbF5 (eq 2).7 The reaction yields are 68-95%.

Upon treatment with HF/SbF5, para-substituted phenols (or their methyl ethers) can be diprotonated,8 first on the oxygen atom and then on the meta carbon.9 The resulting dipositively charged species are exceedingly reactive towards a variety of arenes.9 4-Arylcyclohexenones, the primary products of the reactions, can be further transformed to 3-arylcyclohexenones. The ratio between the two isomers depends on conditions such as reaction time, amount of acid, and the nature of substrates. For example, when p-cresol is reacted with benzene in the presence of HF/SbF5, 4-methyl-4-phenylcyclohexenone and 3-phenyl-4-methylcyclohexenone are obtained in 29% and 33% yields, respectively, after 90 s. With the reaction time increased to 15 min, the yield of 3-phenyl-4-methylcyclohexenone increased to 90% while that of 4-methyl-4-phenylcyclohexenone decreased to 2-3% (eq 3). With the substitution of p-cresol with 4-methylanisole, 6,7-benzo-5-methyltricyclo[3.2.1]octan-2-one is obtained as an additional minor product (eq 4).

Condensation of phenols with aromatic compounds has also been successfully applied to the preparation of spiroenones and/or their isomerized ketones via intramolecular cyclization of methoxydiarylethanes or -propanes (eq 5).10 The starting materials are readily available by Friedel-Crafts acylation of phenols and subsequent reduction of the ketones. In the case of 2-methoxy-5-methyl-1,3-diphenylpropane, treatment of the substrate with HF/SbF5 led to exclusive formation of a tetracyclic ketone (a derivative of benzobicyclo[3.2.1]octane) in 90% yield (eq 6).

Electrophilic formylation of arenes with CO in the presence of acids (Gatterman-Koch conditions) is an efficient method for the preparation of aromatic aldehydes. HF/SbF5/SO2ClF is the most active system for this reaction.11 When competitive formylation was performed on benzene and toluene in HF/SbF5/SO2ClF at -95 °C, a kT:kB ratio of 1.6 was obtained. The observed kT:kB ratios for most typical acids range from 155 to 860.11 It has been demonstrated that even diformylation can be achieved on polynuclear aromatics such as naphthalene and biphenyl with the use of fluoroantimonic acid (eqs 7 and 8).12 In these cases, dialdehydes are generally formed only when the SbF5:substrate ratios are larger than 1.

Hydroxylation of aromatics with peroxides and acids generally suffers from secondary oxidation, leading to low selectivity and low reaction yields. With superacids, these secondary reactions can generally be suppressed since O-protonation of the newly formed phenols prevents further reaction.1 The strength of the acids used can play a significant role in the regioselectivity of the reaction. For example, hydroxylation of naphthalene in the weak acid 70% HF-30% pyridine yields 1- and 2-naphthols in a 98.4:1.6 ratio (eq 9); on the other hand, the ratio is changed to 1.8:98.2 when superacidic HF/SbF5 is used (eq 10).13 Benzaldehyde and aromatic ketones were reported to be hydroxylated in HF/SbF5 without formation of products derived from Baeyer-Villiger oxidation.14 When perfluoro-1-indanone was subjected to similar treatment with the H2O2/HF/SbF5 system, 20% perfluorochroman was obtained along with 4% perfluoro-2-(2-carboxyethyl)phenol (eq 11).15 By increasing the ratio of SbF5:substrates, phenols can be further hydroxylated in HF/SbF5. Resorcinols and some other dihydroxyarenes were prepared by this method (eq 12).16,17 Hydroxylation of anilines by H2O2 in HF/SbF5 yields all three possible isomers, with meta derivatives as major products.18

Bromination of para-alkylated or 2,6-dialkylated phenols and their ethers with Br2 in HF/SbF5 leads to formation of meta-brominated products (eq 13).19 The results indicate that the bromination occurs on the O-protonated substrates, since halogenation of phenols under neutral conditions gives ortho- and para-substituted products. Sodium or potassium bromide can also be used in place of Br2 in the reaction.20

Generally, nitration of aromatics is considered to be an irreversible reaction. However, the reversibility of the reaction has been demonstrated in some cases in the presence of HF/SbF5 or superacidic Nafion-H. 9-Nitroanthracene and pentamethylnitrobenzene will transnitrate benzene, toluene, and mesitylene in these acid media.21

HF/SbF5 and other superacids, such as HF/TaF5 and HBr/AlBr3, are able to promote the hydrogenation of benzene and other aromatics to cyclohexanes and compounds derived from them in the presence of proper hydride donors.22 The reaction proceeds through an ionic hydrogenation mechanism. The best hydride donors are isoalkanes (cycloalkanes) with tertiary C-H bonds. The use of molecular hydrogen in ionic hydrogenation reactions is also possible in the presence of tertiary C-H bond-containing hydrocarbons.22a

Upon treating deuteriated methane with HF/SbF5, deuterium-protium exchange was observed.23 Similar exchange was also observed with other alkanes. In the latter cases, protolysis of C-H and C-C bonds to form carbocations accompanies the exchange process.24 It is interesting to compare the isotope exchange of isobutane using D2SO4 and DF/SbF5.25 When D2SO4 is used, all hydrogens on carbon atoms adjacent to the tertiary carbon atom exchange rapidly, whereas the hydrogen on the tertiary carbon is invariably recovered unexchanged (eq 14).26 In contrast, with DF/SbF5 at -78 °C, exchange on the methine proton was observed (eq 15).25a,b More recently, it was reported that deuterium-protium exchange occurs even on the t-butyl cation through the protio-t-butyl dication.25c

The carbenium ions formed in the protolysis of alkanes with HF/SbF5 can be trapped in situ with CO.27 Hydrolysis of the resulting acyl cations yields the corresponding carboxylic acids (eq 16) (Koch reaction).28 With excess alkane present, ketones can be obtained (eq 17). It is noteworthy that in the carbonylation of propane, addition of catalytic amounts of bromides or halomethanes enhances the selectivity in favor of secondary C-H bond activation. Most (99%) of the products are derived from the isopropyl cation; only negligible amounts of methane and ethane are formed.29

Superacid-catalyzed oxygenation of alkanes has been well reviewed.30 Both peroxy compounds and ozone can serve as oxidants. For example, in the oxygenation of neopentane with O3 in HF/SbF5/SO2ClF, both methylated and ethylated acetones were obtained.30

1,2-Dichloroethane reacts with tetrafluoroethylene in HF/SbF5 to form 1,1,1,2,2-pentafluoro-3-chlorobutane in 80% yield (eq 18).31 The product is formed by addition of a-chloroethyl cation, rearranged from the initially formed b-chloroethyl cation, to tetrafluoroethylene and subsequent quenching with fluoride. Small amounts of 1,1,2,2-tetrafluoro-1,3-dichlorobutane and 1,1,2,2-tetrafluoro-1,4-dichlorobutane were also obtained. When 1,2-dibromoethane is used, 1,1,1,2,2-pentafluoro-4-bromobutane was obtained in 50% yield (eq 19).31 No products arising from rearrangement were observed in this case.

An interesting synthetic method has been developed for carboxylation of bicyclic enones in HF/SbF5.32 It was demonstrated that the diprotonated a,b-unsaturated ketones react with CO to form acylium ions. Quenching of these acylium ions with methanol led to the corresponding carboxylic esters in good yields (eq 20).

Generally, alcohols with short carbon chains (C1-C4) are not carbonylated under the usual Koch conditions. However, it was found that these alcohols readily react with CO in fluoroantimonic acid to give the corresponding carboxylic acids in high yields.33a Even diols reacted under similar conditions to give dicarboxylic acids.33b Some cyclization products were also obtained. g-Butyrolactones react with CO at atmospheric pressure in HF/SbF5 containing an excess of SbF5 to give dicarboxylic acids in good yields (eq 21).33c Methyl alkyl ketones with alkyl groups having five or more carbons can undergo Koch reaction to form the corresponding oxo carboxylic acids (eq 22).34

Oxidation of 3-keto steroids with O3 has also been carried out in HF/SbF5 (eq 23).35 When oxygenations of ethers were performed under similar conditions, oxo alkyl ethers were obtained, with the oxo functionality generally three carbons away from the ether linkage (eq 24).36

Superacid-catalyzed ionic hydrogenation is not limited to aromatic compounds; it has been successfully applied in natural product chemistry (eq 25).37-39 Protonated enones or dienones can be conveniently reduced in the presence of isoalkanes or with molecular hydrogen.

Isomerization and Rearrangements.

The high acidity of HF/SbF5 makes fluoroantimonic acid an efficient isomerization catalyst for hydrocarbons. Isomerizations of fluorotoluenes, polyhalobenzenes, and aromatic sulfones have been carried out.40,41 Ortho- or para-bromo phenols were reported to isomerize to meta isomers in HF/SbF5 through intramolecular 1,2-Br shifts. In contrast, Trifluoromethanesulfonic Acid-catalyzed isomerizations proceed through an intramolecular mechanism.42 It was also reported that rearrangements of 4-alkylated and 2,6-dialkylated phenolic ethers can occur in HF/SbF5. The alkyl groups originally attached to oxygen are rearranged to the meta positions of the compounds (eq 26).43 This dealkylating ability of HF/SbF5 has been successfully employed in the synthesis of 11-deoxyanthracyclines (eq 27).44 Rearrangement of s-butylbenzene to isobutylbenzene under AlCl3 catalysis necessitates higher temperatures; however, the isomerization proceeds under milder conditions when HF/SbF5 is used.45

Rearrangement of phenols to dienones occurs readily in superacids. Some simple bicyclic phenols and their ethers were investigated as model compounds using HF/SbF5.46 The method is also applicable to natural products.47 For example, treatment of estrone derivatives in HF/SbF5 followed by aqueous bicarbonate workup led to estra-4,9-diene-3,17-dione (eq 28). The opposite rearrangement of phenols to dienones, i.e. dienones to phenols, can also be achieved in an HF/SbF5 medium (eq 29).48

n-Alkanes can be readily isomerized to branched alkanes in superacidic media; HF/SbF5 has been used for this process.49 With a catalytic amount of HF/SbF5, endo-trimethylenenorbornane was converted to exo-trimethylenenorbornane at rt in 98% yield.50 With increasing amounts of acid, some adamantane was formed in the reaction mixture. Raising the reaction temperature to 100 °C increases the yield of adamantane to 47% (eq 30).51

HF/SbF5 can induce isomerization of pregnane-3,20-diones to mixtures of isomers containing the 13a-isomer (eq 31).52 The reaction is proposed to occur through the cleavage of the C(13)-C(17) bond. Many estrane derivatives can be prepared through HF/SbF5-catalyzed isomerization.53

Isomerization or rearrangement catalyzed by superacids often differs from conventional acid catalysis; the high acidity leads to stabilized reaction intermediates. A pertinent example is the rearrangement of camphor. In concentrated H2SO4, it was mainly converted to 3,4-dimethylacetophenone (eq 32);54 in HF/SbF5, however, a mixture of three aliphatic ketones was obtained (eq 33).55

Carbocations and Onium Ions.

The corrosive and toxic nature of HF makes HF/SbF5 a less frequently used acid system for the preparation of carbocations compared to HSO3F/SbF5. It is preferred, however, in the generation of arenium ions, since high acidity is required for their formation (eq 34).56 Interestingly, upon treatment with HF/SbF5, azoxybenzene is deoxygenated to form a dication believed to be responsible for the Wallach rearrangement product (eq 35).57 HF/SbF5 is able to dehydrate aliphatic ketones to form allyl cations (eq 36).58 It was found that the rate of dehydration increases with the acidity of the medium.

HF/SbF5 is frequently used for the preparation of onium ions. The early work has been reviewed.1 More recently, HF/SbF5 has been employed to protonate (CN)2, XCN (X = halogen), CF3CN, and MenS(CN)2 - n.59

Since SbF6- (and Sb2F11-) is a stable anion, hexafluoroantimonic acid has been widely used in the preparation of carbocations and onium salts.60 Representative examples include the t-butyl cation, and tricyclopropylcarbenium, 1-adamantyl, alkyloxocarbenium, haloacetylium, methoxycarbenium, dimethylbromonium, tetramethylenebromonium, nitronium, and nitrosonium salts (eqs 37 and 38). Many of these salts have found valuable applications in organic synthesis. The X-ray structure of the isolated t-butyl hexafluoroantimonate salt has recently been established.61

Polymerization.

HF/SbF5 is also an efficient cationic polymerization catalyst.1 Here, only its application in the preparation of macrocyclic ethers is included. The importance of these crown ethers has been well documented; however, their preparation is usually tedious and expensive. It was reported that HF/SbF5 and other acids such as HF/BF3 readily oligomerize ethylene oxide to mixtures of cyclic ethers62 which can be subsequently separated (eq 39). The key to cyclic ether formation is the presence of anhydrous HF in the conjugate acid systems; chain polymers would otherwise be obtained.

Related Reagents.

Antimony(V) Fluoride; Fluorosulfuric Acid; Fluorosulfuric Acid-Antimony(V) Fluoride; Hydrofluoric Acid; Hydrogen Fluoride; Nafion-H.


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49. For example, see: (a) Bassir, M.; Torck, B.; Hellin, M. NJC 1987, 11, 437. (b) Bassir, M.; Torck, B.; Hellin, M. BSF 1987, 760, 554. (c) Fabre, P. L.; Devynck, J.; Tremillon, B. BSF(1) 1982, 5. (d) Bonifay, R.; Torck, B.; Hellin, M. BSF(1) 1977, 808, 1057.
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53. (a) Jacquesy, J. C.; Ung, H. L. T 1976, 32, 1375. (b) Jacquesy, J. C.; Jacquesy, R.; Narbonne, C. BSF 1976, 1240. (c) Jacquesy, R.; Narbonne, C. BSF(2) 1978, 163.
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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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