HF-SbFs solutions

Carbocations generated from alkanes using superacids react with carbon monoxide under mild conditions to form carboxyUc acid (188). In this process isomeric carboxyUc acids are produced as a mixture. However, when the reaction is mn with catalytic amounts of bromine (0.3 mmol eq) in HF-SbF solution, regio-selective carboxylation is obtained. / -Propane was converted almost exclusively to isobutyric acid under these conditions. 

Evidence for the involvement of the diprotonated species 40 and 42 includes 13C NMR data, which shows deshielding of the involved carbons as the acidity of the media increases from H0 —8 to —26 (mesityl oxide is estimated to be fully monoprotonated in acids of about H0 —8). Dication 42 has been shown to be capable of reacting even with very weak nucleophiles, for example, abstracting hydride from cyclohexane.19b An analogous species (43) has been proposed in interaction with excess AICI3. Protonated cyclohex-2-enone is converted to 3-methylcyclo-pent-2-enone in HF-SbFs solution at 50°C.20 The reaction mechanism is thought to involve the dicationic intermediate 44 (eq 13). Likewise, aryl-substituted indenones are converted to the dications by reaction in superacid (eq 14).21... 

Ab initio molecular dynamics simulations for the protonation of simple alkanes in HF-SbFs solution showed that the presence of extremely reactive protons at high SbFs concentrations open reaction channels for the C-H protonation with a considerably decreased barrier with respect to dilute solutions and the gas phase. This and the progressive presence of neutral SbFs were suggested to be responsible for the experimentally observed increased reactivity with increasing concentration of SbFs. Furthermore, an electrostatic stabilization of the transition state in solution also contributes to the lowering of the reaction barrier and, as a result, C-H protonation competes with C-C protonation in the case of ethane. 

In HF-SbFs solution 4-aIkyl- and 2,6-dialkylphenols and their methyl ethers (anisoles) are converted to the O-protonated forms, which react with Br2 to afford m-bro-mophenol derivatives selectively (Scheme 14.35) [83, 84]. In HF-SbFs o- and p-bro-mophenols isomerize to m-bromophenol by a 1,2-Br shift in fhe protonated forms [85], When NaBr or KBr is used instead of Br2, the meta selectivity is reduced [86]. ElectrophiUc halogenation of arenes can also be performed with Ar SeCl [87]. 

One recent example of this is the carbonylation of methylcyclopentane in HF-SbFs solutions. An array of fascinating reactions occurs upon protonation of an alkane, and they are illustrated in the following scheme. The relative basicity of a bonds in alkanes is 3° C-H > C-C > 2° C-H >> 1° C-H. Hence, the protonation of methylcyclo-... 

Acid-cataly2ed hydroxylation of naphthalene with 90% hydrogen peroxide gives either 1-naphthol or 2-naphthiol at a 98% yield, depending on the acidity of the system and the solvent used. In anhydrous hydrogen fluoride or 70% HF—30% pyridine solution at — 10 to + 20°C, 1-naphthol is the product formed in > 98% selectivity. In contrast, 2-naphthol is obtained in hydroxylation in super acid (HF—BF, HF—SbF, HF—TaF, FSO H—SbF ) solution at — 60 to — 78°C in > 98% selectivity (57). Of the three commercial methods of manufacture, the pressure hydrolysis of 1-naphthaleneamine with aqueous sulfuric acid at 180°C has been abandoned, at least in the United States. The caustic fusion of sodium 1-naphthalenesulfonate with 50 wt % aqueous sodium hydroxide at ca 290°C followed by the neutralization gives 1-naphthalenol in a ca 90% yield. 

There has been considerable discussion about the extent of hydration of the proton and the hydroxide ion in aqueous solution. There is little doubt that this is variable (as for many other ions) and the hydration number derived depends both on the precise definition adopted for this quantity and on the experimental method used to determine it. H30" has definitely been detected by vibration spectroscopy, and by O nmr spectroscopy on a solution of HF/SbFs/Ha O in SO2 a quartet was observed at —15° which collapsed to a singlet on proton decoupling, 7( 0- H) 106 Hz. In crystalline hydrates there are a growing number of well-characterized hydrates of the series H3O+, H5O2+, H7O3+, H9O4+ and H13O6+, i.e. [H(0H2) ]+ n = 1-4, Thus... 

As a consequence, salts of H3O2" cannot be prepared from aqueous solutions but they have been obtained as white solids from the strongly acid solvent systems anhydrous HF/SbFs and HF/AsFj, e.g. ( 5)... 

Brouwer and Mackor (1964) found that concentrated and stable solutions of a series of tertiary alkyl cations can be prepared in HF-SbFs and their proton magnetic resonance spectra were recorded. The t-butyl, t-pentyl and t-hexyl cations were observed in this solvent system. The spectra were identical with those obtained previously in SbFs and FSOsH-SbFs solvent systems. 

The dicationic species have also been obtained from /3-ketoacids, fi-ketoesters, and /-i-ketoamides in superacid solutions (Table 1, entries 2-4). Diprotonated acetoacetic acid (75) can be observed by low-temperature NMR under stable ion conditions.34 Likewise, diprotonated methylacetoacetate (77) can be observed by NMR at temperatures lower than — 80°C in FS03H-SbF5-SC>2 solution.35 With ethyl acetoac-etate in HF-SbFs, the equilibrium constant for the dication-monocation equilibrium has been estimated to be at least 107, indicating virtually complete conversion to the superelectrophile.35 The /3-ketoamide (78) is found to give the condensation products 95 in good yield from CF3SO3H and the superelectrophile 79 is proposed as the key intermediate in the condensation reaction (eq 25 ).27... 

The stability of cr-complexes 2 and their easy formation are substantially governed by the nature of the counter anion. Thus, in HF solution, thiophene and alkylthiophenes form cations stable at temperatures below —40 °C, in HF—BF3 the same cations are stable up to —20 °C and 2,5-dimethyl-2H-thiophenium ion, generated in HF—SbFs is sufficiently stable even at -1-60 °C (66RTC1072). The high stability of 2H-thiophenium ions formed by protonation in the presence of AICI3 in dichloroalkanes as solvents (75ZOR424) is undoubtedly caused by the AICI4 counterions as well as by favorable solvation conditions. The stability of thiophenium ions is mainly caused by their peculiar structure. 

In very strongly acidic nonaqueous solutions (such as HF/SbFs) H2S acts as a base (proton acceptor) and the white crystalline... 

The structures of hydrocarbon ions can be derived directly from a measured NMR spectrum. The measurement of such a spectrum is possible only in the liquid phase, and under conditions such that the ions have long lifetimes (>10 sec) and are formed in high concentrations. Concentrated and stable solutions of carbonium ions can be prepared by dissolving hydrocarbons (or appropriate halides, alcohols, ethers) in strong acids such as HF-SbFs or FS03-SbF5. During the last five years, numerous NMR spectra of carbonium ions formed in such solutions have... 

An interesting example of thermodynamic vs kinetic control was observed in the temperature dependence of carboxylation of methylcyclopentane in the presence of HF-SbFs (90). Carboxylations were carried out by bubbling CO through the reaction mixture either at —40°C or 0°C followed by quenching the resulting solution with ethanol. A reaction at -40°C yields quantitatively ethyl... 

In the presence of Brpnsted acids, thiophene is prone to protonation with the formation of thiophenium ions - C-protonated thiophenes (Scheme 1, E=H). Thiophenium ions were generated in pioneering investigatiOTis by treatment of thiophene with the excess of HF, HF-BF3 or HF-SbFs and later also with H2SO4, HSO3F and its mixtures with Lewis acids [4—7]. C-2-protonated thiophenes are stable in solution, even at room temperature, and were investigated by NMR spectroscopy (Scheme 2) [8]. 

The highest acidity level measured according to the Ho acidity scale is around -27 for 90% SbFs in HSO3F. However, based on rate measurements in superacidic reactions the fluoroantimonic acid system is predicted to be even stronger [Fig 2.3], Similar to BF3 in HF, SbFs also ionises in dilute solutions to produce the H2F cation. The lower Hammett acidity of the HF/BF3 system compared to SbFs and TaFs, can be attributed to the lower concentration of the H2F " cation produced in the HF/BF3 system. The HF/BF3... 

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