Fluorosulfuric Acid

HSO3F

[7789-21-1]  · FHO3S  · Fluorosulfuric Acid  · (MW 100.08)

(strong Brønsted superacid1 used as catalyst and reagent for alkylation, isomerization, rearrangement, cyclization, cycloaddition, ring-opening polymerization, and fluorosulfonation; also widely used in generation of stable carbocations)

Alternate Name: fluorosulfonic acid.

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

Solubility: sol nitrobenzene, diethyl ether, acetic acid, ethyl acetate; insol carbon disulfide, carbon tetrachloride, chloroform. It dissolves most organic compounds that are potential proton acceptors.

Form Supplied in: colorless liquid; commercially available.

Handling, Storage, and Precautions: fluorosulfuric acid is highly toxic and corrosive and should always be handled in a fume hood with proper protection. It can be purified by distillation under anhydrous conditions using common glassware. When water is excluded, it may be handled and stored in glass containers, but containers should always be cooled before opening because HF gas pressure may have developed due to hydrolysis. For long-term laboratory storage, however, Teflon bottles are recommended.

Isomerization and Rearrangement.

Many hydrocarbons, especially terpenes, rearrange readily in fluorosulfuric acid, usually at low temperatures, with the intermediacy of carbocations.2-15 Compared to conventional acid systems, the use of HSO3F often alters the normal course of acid-catalyzed isomerization and rearrangement, since the carbocation intermediates formed during the reactions are stabilized in the superacid system. For example, treatment of 1,8-p-menthadiene (limonene) with HSO3F gave a- and b-phellandrenes (eq 1)14 which were not obtained by other weaker acid systems.

Longifolene rearranges to isolongifolene and other tricyclic derivatives in the presence of 50% Sulfuric Acid/Acetic Acid in dioxane, but both longifolene and isolongifolene rearrange to a variety of bicyclic products under the catalysis of fluorosulfuric acid (eq 2).15

Temperature also plays an important role in determining the reaction pathway. Quenching of the reactions with bases or nucleophiles at specific temperatures can control the nature of the products.

Cyclization and Ring Opening.

Fluorosulfuric acid is a promising reagent in catalyzing intramolecular cyclizations at low temperature, giving better yields and higher degrees of structure selectivity compared to conventional acids. Monoterpenoids16-21 and higher terpenoids22-27 cyclize biomimetically by treatment with fluorosulfuric acid at low temperature (eq 3).25

Trienols and dienols cyclize in a similar way, in the presence of an excess of fluorosulfuric acid in 2-nitropropane at -90 °C, to afford 74-87% yields of diastereoisomeric mixtures of odoriferous norlabdane oxides (eq 4).27

Cyclic ethers can be obtained by reacting unsaturated alcohols in fluorosulfuric acid.28-33 Treatment of geraniol or nerol with HSO3F at low temperature gave good yields (78 and 57%) of an iridoid ether (eq 5).21,29b

Benzyl carbinols react with fluorosulfuric acid through various reaction pathways34-36 (rearrangement, dehydration, cyclization, ring expansion), depending on the substrate (eq 6).34

As opposed to cyclization, ring opening of a number of substituted nopinones occurred under fluorosulfuric acid catalysis to give 4-(2-propyl)cyclohex-2-enones (eq 7).37 In industry, fluorosulfuric acid is used as an efficient catalyst to initiate the ring-opening polymerization of tetrahydrofuran.38

Cycloaddition.39-42

Triphenylpropynol reacts in HSO3F with alkenes or dienes to give [4 + 2] or [2 + 2] cycloaddition products (eq 8).42

Formation of Fluorosulfates.

Alkyl fluorosulfates can be prepared by addition of fluorosulfuric acid to alkenes, haloalkenes, and cyclopropanes at low temperatures with good to moderate yields (50-95%) (eq 9).43

Fluorosulfonation.44

Aromatics react with HSO3F in the presence of variable amounts of Antimony(V) Fluoride to give aromatic sulfonyl fluorides and diaryl sulfones (see Fluorosulfuric Acid-Antimony(V) Fluoride).

Generation of Stable Carbocations.

Fluorosulfuric acid is the strongest known simple Brønsted acid, with an H0 value45 of -15.1 (compared to -12 for 100% sulfuric acid, -14.1 for Trifluoromethanesulfonic Acid, and -15 for absolutely anhydrous Hydrogen Fluoride).46 The acidity of HSO3F can be increased by the addition of Lewis acid fluoride (see Fluorosulfuric Acid-Antimony(V) Fluoride). HSO3F has low viscosity, high thermal stability, and a wide liquid range (250 °C from mp to bp). These advantageous properties make HSO3F one of the most frequently used superacids for generating stable carbocations from the corresponding alcohols at low temperature.1,47-50 Generally, neat HSO3F or acid diluted with solvents such as SO2, SO2ClF, freons, and dichloromethane is suitable for the preparation of carbocations stabilized by aryl, cyclopropyl, vinyl, or other p systems. Representative examples are dicyclopropyl carbonium ions (eq 10),47 cyanodiarylmethyl cations (nitrenium ions) (eq 11),48 and the pyramidal (CMe)62+ dication (eq 12).49

An intriguing cyclopropyl cation with a ferrocene substituent has been generated from its trimethylsilyl ether by eliminating trimethylsilyl fluorosulfate (eq 13).51 1,6-Methano[10]annulene undergoes protonation by HSO3F in SO2ClF to form the corresponding arenium ion (eq 14),52 while benzenium ion generation needs a combination of HSO3F and SbF5.53

Related Reagents.

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


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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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