Super acids, also

The most stable of all alkyl cations is the t-butyl cation. Even the relatively stable f-pentyl and f-hexyl cations fragment at higher temperatures to produce the t-butyl cation, as do all other alkyl cations with four or more carbons so far studied.21 Methane,22 ethane, and propane, treated with super acid, also yield r-butyl cations as the main product (see 2-18). Even paraffin wax and polyethylene give f-butyl cation. Solid salts of t-butyl and f-pentyl cations, e.g., Me3C SbF4, have been prepared from super-acid solutions and are stable below -20°CP... 

The catalysts used in the industrial alkylation processes are strong Hquid acids, either sulfuric acid [7664-93-9] (H2SO or hydrofluoric acid [7664-39-3] (HE). Other strong acids have been shown to be capable of alkylation in the laboratory but have not been used commercially. Aluminum chloride [7446-70-0] (AlCl ) is suitable for the alkylation of isobutane with ethylene (12). Super acids, such as trifluoromethanesulfonic acid [1493-13-6] also produce alkylate (13). SoHd strong acid catalysts, such as Y-type zeoHte or BE -promoted acidic ion-exchange resin, have also been investigated (14—16). 

The aromatic ring has high electron density. As a result of this electron density, toluene behaves as a base, not only in aromatic ring substitution reactions but also in the formation of charge-transfer (tt) complexes and in the formation of complexes with super acids. In this regard, toluene is intermediate in reactivity between benzene and the xylenes, as illustrated in Table 2. 

Allylsilanes react with carbonyl compounds to transfer the allyl group with 1,3-transposition, in the presence of Lewis acids, typically titanium(IV) chloride47. Recently this reaction has been carried out under super-acid catalysis48. Transfer of the allyl group is also induced by tetrabutylammonium fluoride, but in this case reaction takes place regioselectively at the less substituted end of the allyl fragment49. 

Neither methyl nor ethyl fluoride gave the corresponding cations when treated with SbFs. At low temperatures, methyl fluoride gave chiefly the methylated sulfur dioxide salt, (CH3OSO) ShF while ethyl fluoride rapidly formed the rert-butyl and ferf-hexyl cations by addition of the initially formed ethyl cation to ethylene molecules also formed ° At room temperature, methyl fluoride also gave the tert-butyl cation. In accord with the stability order, hydride ion is abstracted from alkanes by super acid most readily from tertiary and least readily from primary positions. 

The major disadvantage of the alkylation process is that acid is consumed in considerable quantities (up to 100 kg of acid per ton of product). Hence, solid acids have been explored extensively as alternatives. In particular, solid super acids such sulfated zirconia SO/ IZr02) show excellent activities for alkylation, but only for a short time, because the catalyst suffers from coke deposition due to oligomerization of alkenes. These catalysts are also extremely sensitive to water. 

However, an atom somewhat heavier than H might well be less susceptible to the perturbations that may disturb the latter as, for example, 13C which also generates an n.m.r. spectrum. Thus the 2-arylpropyl(cumyl) carbocations (56 produced from the corresponding tertiary alcohols in super acid —S02ClF/FS03H/SbF5— solution, cf. p. 181),... 

Protonated amides and carboxylic acids have also been shown to activate adjacent electrophilic centers. Although protonated carboxylic acids and amides are not typically considered stable cationic groups, in superacidic media they can be readily generated and many have been observed by spectroscopic studies.16 As an example of electrophilic activation by a protonated carboxylic acid, P-phenylcinnamic acid (35) is diprotonated in super- acid to give the dication (36, observable by NMR) which then reacts with benzene and gives the indanone (37) in good yield (eq 12).17 It is known that the 1,1-diphenylethyl... 

Kotamino (see also page 29) Street Names Cat valium, green, honey oil, jet, ket, kit kat, purple, special K. special la coke, super acid, super c, vitamin K... 

The tungstophosphoric acid shows high activity, close to H2SO4 used as a benchmark. Regrettably, this acid is soluble in water and hence not usable as a solid catalyst. However, the corresponding cesium salt (Cs2.5) is also super acidic and its mesoporous structure has no limitations on the diffusion of the reactants. Cs2.5 exhibits low activity per weight, hence it is not suitable for industrial applications. 

Subsequently it was found that the same cations could also be generated from alcohols in super acid-S02 at - 60°C 1 and from alkenes by the addition of a proton from super acid or HF-SbF5 in S02 or SO20IF at low temperatures.12 Even alkanes give carbocations in super acid by loss of H. For example,13 isobutane gives the r-butyl cation... 

RIR+), which can be prepared in super-acid solutions (p. 312) and isolated as solid SbFfi salts, are also extremely reactive in nucleophilic substitution.345 Of the above types of compound, the most important in organic synthesis are tosylates, mesylates, oxonium ions, and triflates. The others have been used mostly for mechanistic purposes. 

Lemairc Doussot Guy Chem. Lett. 1988, 1581. See also Hayashi Mukaiyama Chem. Lett. 1987, 1811, 2ll01ah Mo Olah J. Am. Chem. Soc. 1973, 95, 4939. For reviews, see Olah Farooq Prakash, in Hill Activation and Functionalization of Alkanes, Wiley New York, 1989, pp. 27-78 Olah Prakash Sommer, Ref. 44, pp, 270-277 For a review of the thermodynamic behavior of alkanes in super-acid media, see Fabre Devynck Trdmillon Chem. Rev. 1982, 82, 591-614. See also Ref. 46. 

R H) is much faster than alkylation, so that alkylation products are also derived from the new alkanes and carbocations formed in the exchange reaction. Furthermore, the carbo-cations present are subject to rearrangement (Chapter 18), giving rise to new carbocations. Products result from all the hydrocarbons and carbocations present in the system. As expected from their relative stabilities, secondary alkyl cations alkylate alkanes more Teadily than tertiary alkyl cations (the r-butyl cation does not alkylate methane or ethane). Stable primary alkyl cations are not available, but alkylation has been achieved with complexes formed between CH3F or C2H5F and SbFs-212 The mechanism of alkylation can be formulated (similar to that shown in hydrogen exchange with super acids, 2-1) as... 

It is likely that protonated cyclopropane transition states or intermediates are also responsible for certain non-1,2 rearrangements. For example, in super-acid solution, the ions 14 and 16 are in equilibrium. It is not possible for these to interconvert solely by 1,2 alkyl... 

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