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