Fluorosulfuric Acid-Antimony(V) Fluoride1

HSO3F-SbF5

[23854-38-8]  · F6HO3SSb  · Fluorosulfuric Acid-Antimony(V) Fluoride  · (MW 316.83)

(a strong conjugate Brønsted-Lewis superacid system1 widely used for the generation of stable carbocations and as catalyst and reagent for alkylation, isomerization, rearrangement, cyclization, oxyfunctionalization, formylation, sulfonation, and fluorosulfonation)

Alternate Name: Magic Acid®.

Physical Data: HSO3F: mp -89.0 °C; bp 162.7 °C; d 1.743 g cm-3. SbF5: bp 149.5 °C; d 2.993 g cm-3.

Solubility: sol SO2ClF, liquid SO2; solubilizes most organic compounds that are potential proton acceptors.

Form Supplied in: colorless liquid; commercially available.

Preparative Methods: HSO3F-SbF5 is prepared by mixing Fluorosulfuric Acid and Antimony(V) Fluoride at rt under dry nitrogen or argon atmosphere. The commercially available Magic Acid is a 50:50 mol % mixture of the two components. Magic Acid diluted in fluorosulfuric acid to various extents is also available. Commercial Magic Acid generally contains some HF in the form of conjugate acid.

Handling, Storage, and Precautions: Magic Acid is highly toxic, moisture sensitive, and corrosive, and should always be handled in a fume hood with proper protection. Glass is attacked by Magic Acid very slowly when moisture is excluded. Therefore, glassware may be used for handling and carrying out reactions involving Magic Acid. Teflon containers are recommended for long-term laboratory storage of Magic Acid.

Introduction.

Of all the superacids, Magic Acid is probably the most widely used medium for the study of stable long-lived carbocations and other reactive cations. The general rule is that the higher the acidity of the medium used, the more stable is the carbocation generated. The acidity of the fluorosulfuric acid-antimony pentafluoride system as a function of SbF5 content has been studied.2,3 The increase in acidity is very sharp at low SbF5 concentration. The H0 value changes from -15.1 for HSO3F to -19.8 for a mixture containing 10% SbF5.2 The acidity continues up to the estimated value of H0 = -26.5 for 90% SbF5 content. The H0 value for the 1:1 molar mixture of fluorosulfuric acid and antimony pentafluoride, known as Magic Acid®, is estimated to be about -23 by a dynamic NMR study.3 The name Magic Acid originated in Olah's laboratory at Case Western Reserve University in the winter of 1966 when a piece of Christmas candle was found to dissolve readily in this acid system, giving the sharp 1H NMR spectrum of the t-butyl cation, a phenomenon considered by the research student involved to be magic.1 A major reason for the wide application of Magic Acid compared with other superacid systems, besides its very high acidity, is probably the large temperature range in which it can be used. In the liquid state, it can be studied at temperatures as low as -160 °C (acid diluted with SO2F2 and SO2ClF) and as high as 150 °C (neat acid).

Magic Acid has been employed as a high acidity medium for isomerization/rearrangement, alkylation, cyclization, carboxylation, formylation, oxyfunctionalization, and related reactions. It also serves as a fluorosulfonating/sulfonating agent for aromatics.

Generation of Stable Carbocations.

Thanks to its high acidity, Magic Acid can be used for the generation of such reactive carbocations as the t-butyl cation and other alkyl cations, while fluorosulfuric acid itself is suitable only for the generation of more stable cations such as aryl- or cyclopropyl-stabilized carbocations.

Carbocations and carbodications have been generated from a variety of precursors in Magic Acid systems.4

Carbocation Generation from Tertiary and Secondary Alcohols.5-11

A wide variety of aliphatic tertiary and secondary alcohols can be ionized to the corresponding alkyl cations by using Magic Acid. Formation of the t-butyl cation (eq 1)5 and a cyclopropyl-stabilized dication (eq 2)6 are representative examples. Primary (and some secondary) alcohols are protonated only at temperatures lower than -60 °C.7 At more elevated temperatures they may cleave to give the corresponding carbocations, which, however, immediately rearrange to the more stable tertiary cations.8,9

Similar to alcohols, aliphatic ethers,12 thiols,13 and sulfides are also protonated on oxygen or sulfur, respectively, at -60 °C in Magic Acid; carbocations are subsequently formed upon raising the temperature. Protonated sulfides, excluding tertiary alkyl, are resistant to cleavage up to +70 °C.13

Carbocation Generation from Alkyl Halides.

Alkyl chlorides, fluorides, and bromides are convenient and frequently used precursors for generation of alkyl cations in HSO3F-SbF5 systems.14 It should be noted, however, that the HSO3F-SbF5 system is less suitable than SbF5 for the generation of alkyl, especially secondary alkyl, cations from the corresponding alkyl halides (see Antimony(V) Fluoride). Ionization of cyclohexyl chloride in Magic Acid is accompanied by isomerization, yielding the 1-methyl-1-cyclopentyl cation (eq 3).8

Carbocation Generation from Unsaturated Hydrocarbons.

Carbocations can be generated by protonation of unsaturated hydrocarbons such as alkenes and cycloalkenes,8,11 cyclopentadienes,15 benzenes and naphthalenes (eq 4),16 pyrenes and cyclophanes,17 unsaturated heterocycles,18 and their derivatives with carbon-heteroatom multiple bonds,1 including carbonyl and nitrile compounds and diazoalkanes (eq 5).19 Compounds with two sites for protonation may undergo diprotonation to give dications in Magic Acid (eq 6).15a

Carbocation Generation from Saturated Hydrocarbons.

Magic Acid, as a strong superacid, can abstract hydride from saturated alkanes, including straight-chain alkanes as well as branched and cyclic alkanes, at -125 to 25 °C to give alkyl cations.6,20 For example, the 2-norbornyl cation is formed through protolytic ionization by dissolving norbornane in Magic Acid (eq 7).20b

Carbocation Generation by Oxidation.

Oxidation of polycyclic arenes such as naphthacene and 1,2-benzanthracene (eq 8)21 gives arene dications (see also Antimony(V) Fluoride).

Isomerization and Rearrangement.

HSO3F-SbF5 is frequently used as a catalyst for the isomerization and rearrangement of terpenoids. The extremely high acidity of HSO3F-SbF5 allows the reaction to be carried out at temperatures as low as -100 °C and with improved selectivity.

In superacid, i.e. HSO3F-SbF5 or HF-SbF5 systems, bicyclic phenols are isomerized to dienones in good yields (eq 9).22 This is an unconventional isomerization process, in which the phenols lose their aromaticity to form nonaromatic dienones. Similar phenol-to-dienone isomerization of estrones occurs with HSO3F-SbF5 as catalyst, accompanied by major dehydrogenation products (eq 10).23

endo-Trimethylenenorbornane isomerizes to its exo isomer in quantitative yield at 0 °C under HSO3F-SbF5 catalysis (eq 11).24 The isomerization will proceed further to adamantane when excess superacid is used (HSO3F-SbF5:substrate ratio = 1:3). For the rearrangement of trimethylenenorbornane to adamantane, HSO3F-SbF5 is, however, less efficient than CF3SO3H-SbF5 or CF3SO3H-B(OSO2CF3)3.25

Alkylation.

Benzene, alkylbenzenes, and halobenzenes undergo alkylation with 2,2,4-trimethylpentane in the presence of HSO3F-SbF5 at temperatures as low as -30 °C with good selectivity (eq 12).26

Cyclization.

HSO3F-SbF5 has been used as a high-acidity catalyst for the cyclization of acyclic isoprenoids at low temperatures. The reaction course and products of the cationic cyclization depend on the acidity of the catalyst and the structural differences in the substrates. Structural changes may also lead to dramatic changes in the reaction course and products. While geranate esters (eq 13)27 and pseudoionones (eq 14)28 are cyclized to monocyclic derivatives, geraniol or nerol (eq 15)29 and geranylacetone (eq 16)30 give bicyclic ethers.

Sulfonation and Fluorosulfonation.

Aromatic compounds react with HSO3F to give arenesulfonyl fluorides.31 When the reactions are carried out in the presence of variable amounts of SbF5, significant amounts of diaryl sulfones are obtained (eq 17).32 In this example, the yields of arenesulfonyl fluorides decreased with increasing amounts of SbF5. In contrast, the yields of diaryl sulfones first increased and then decreased with increasing amounts of SbF5. The highest yield of diphenyl sulfone was obtained when the molar ratio of SbF5:benzene was 1:1.5. Diaryl sulfones were also obtained in high yield from toluene, xylenes, and 1,2,4-trimethylbenzene under similar conditions. Sulfonation of fluoro-, chloro-, and bromobenzenes required higher molar ratios of SbF5:arene (1:3.5) to obtain good yields of the corresponding diaryl sulfones.

Formation of Aromatic Sulfoxides.

By treatment with HSO3F-SbF5 (1:1) and Sulfur Dioxide, alkylbenzenes, halobenzenes, and alkylhalobenzenes were converted to their corresponding diaryl sulfoxides along with small amounts of diaryl sulfides as minor products (eq 18).33 In the absence of SO2, aryl sulfone formation is the dominant process, although sulfoxide is also formed. Unsymmetrical (mixed) sulfoxides can be prepared by addition of one molar equivalent of an arene to the solution of the second arene and Magic Acid-SO2 in Freon at low temperatures.

Formylation and Carboxylation.

Formylation of aromatic compounds such as benzene, toluene, xylenes, mesitylene, indan, tetralin, and halobenzenes is achieved in HSO3F-SbF5 under atmospheric CO pressure at 0 °C (eq 19).34 However, in the cases of alkylbenzenes, both formylation and sulfonation took place under these reaction conditions to give alkylbenzaldehydes and formylalkylbenzenesulfonyl fluorides, as well as small amounts of alkylbenzenesulfonyl fluorides and bis(alkylphenyl) sulfones. With benzene and halobenzenes, because of their lower reactivity only aldehydes were produced.

Saturated hydrocarbons, including branched and unbranched chain alkanes as well as cycloalkanes, react with carbon monoxide in the presence of Copper(I) Oxide in HSO3F-SbF5 to afford tertiary and secondary carboxylic acids in high yield (eq 20).35 The reaction proceeds at 0 °C under 1 atm CO. In some cases the reaction involves cleavage of C-C bonds and isomerization of the intermediate carbocations.

Oxyfunctionalization of Hydrocarbons.

When treated with Ozone36 or Hydrogen Peroxide (98%)37,38 under Magic Acid catalysis, alkanes, including methane, ethane, butanes, and higher alkanes as well as haloalkanes,39 undergo electrophilic oxygenation followed by carbon-to-oxygen alkyl group migration giving, via alkoxycarbenium ions, ketones and alcohols (eq 21). Aliphatic alcohols, ketones, and aldehydes react with ozone in Magic Acid solution to give bifunctional oxygenated derivatives such as diketones, hydroxyl ketones, and glycols (eq 22).40 The relative reactivity of s-bonds in alkanes with protonated ozone was found to be R3C-H > R2(H)C-H > R(H2)C-H > C-C.36

Aromatic compounds such as benzene, alkylbenzenes, and halobenzenes can be directly oxygenated with hydrogen peroxide in Magic Acid or other superacids, giving phenols (eq 23).41 The phenols formed are protonated by the superacids and thus are deactivated against further electrophilic attack or oxidation. When naphthalene was treated with hydrogen peroxide in Magic Acid at -78 °C, 2-naphthol was obtained (92% regioselectivity) along with small amounts of dihydroxynaphthalenes (eq 24).42 Unlike phenol derivatives, naphthols can be further hydroxylated with hydrogen peroxide in superacid systems to dihydroxynaphthalenes, since the unprotonated ring of the protonated naphthols can still be attacked by the electrophilic hydroxylating agent.

Miscellaneous Reactions.

In the presence of HSO3F-SbF5, Peroxydisulfuryl Difluoride (S2O6F2) reacts smoothly with 1,1,2-trichlorotrifluoroethane at rt to give 1,2-dichlorotrifluoroethyl fluorosulfate (eq 25).43 S2O6F2 alone does not react with 1,1,2-trichlorotrifluoroethane even at 150 °C, while in the presence of HSO3F the reaction occurs only at temperatures higher than 150 °C.

Polyfluoroarenes, such as m-H2C6F4, m-O2NC6F4H, m- and p-BrC6F4H, (p-HC6F4)2, C6F5H, 1,3,5-F3C6H3, and m-FSO2C6F4H, have been thallated by Thallium(III) Trifluoroacetate in HSO3F-SbF5. The thallated products can be converted to polyfluoroiodoarenes by treatment with aqueous Sodium Iodide (eq 26).44

Although HSO3F-SbF5 is known to be a strong oxidizing system, it has been reported that octafluoroazoxybenzene was reduced by HSO3F-SbF5 at 20 °C to octafluoroazobenzene in quantitative yield (eq 27).45 The mechanism of the process is still unclear.

Isoalkanes are brominated by Br2 in HSO3F-SbF5 to yield mono-, di-, and tribromoalkanes (eq 28).46 Cleavage of C-C bonds occurs when isooctane is reacted under similar conditions, leading to butyl bromides.

Fluorinative dediazoniation of an arenediazonium salt occurred at 0 °C in Magic Acid; (eq 29)47 however, fluorosulfonation accompanied the dediazoniation.

Related Reagents.

Antimony(V) Fluoride; Fluorosulfuric Acid; Hydrogen Fluoride; Hydrogen 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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