Involving superacids

A review has covered mainly the preparation of aldehydes from carbon monoxide by electrophilic aromatic substimtions involving superacids. In HF-SbFs, some polynuclear aromatic compounds are diformylated. When pyridine-3-carboxaldehyde is compared with benzaldehyde in acid-catalysed condensation reactions with benzene and substituted benzenes to form diaryl-3-pyridylmethanes, it is found to be... 

My research during the Cleveland years continued and extended the study of carbocations in varied superacidic systems as well as exploration of the broader chemistry of superacids, involving varied ionic systems and reagents. I had made the discovery of how to prepare and study long-lived cations of hydrocarbons while working for Dow in 1959-1960. After my return to academic life in Cleveland, a main... 

Dihydrogen (H2) is similarly protonated to by superacids, as was shown by studies using isotopic labeling. The structure of again involves 2e-3c bonding. 

Different types of other coal liquefaction processes have been also developed to convert coals to liqnid hydrocarbon fnels. These include high-temperature solvent extraction processes in which no catalyst is added. The solvent is usually a hydroaromatic hydrogen donor, whereas molecnlar hydrogen is added as a secondary source of hydrogen. Similar but catalytic liquefaction processes use zinc chloride and other catalysts, usually under forceful conditions (375-425°C, 100-200 atm). In our own research, superacidic HF-BFo-induced hydroliquefaction of coals, which involves depolymerization-ionic hydrogenation, was found to be highly effective at relatively modest temperatnres (150-170°C). 

The superacid-catalyzed cracking of hydrocarbons (a significant practical application) involves not only formation of trivalent carbo-cationic sites leading to subsequent /3-cleavage but also direct C-C bond protolysis. 

A fundamental difference exists between conventional acid-catalyzed and superacidic hydrocarbon chemistry. In the former, trivalent car-benium ions are always in equilibrium with olefins, which play the key role, whereas in the latter, hydrocarbon transformation can take place without the involvement of olefins through the intermediacy of five-coordinate carbocations. 

Carboxoninm ions, for example, do not react with alkanes. However, in superacid solution acetaldehyde (or acetone), for example, readily reacts with isobutane involving diprotonated, highly reactive carbocationic species. 

Lower alkanes such as methane and ethane have been polycondensed ia superacid solutions at 50°C, yielding higher Hquid alkanes (73). The proposed mechanism for the oligocondensation of methane requires the involvement of protonated alkanes (pentacoordinated carbonium ions) and oxidative removal of hydrogen by the superacid system. 

These reactions involve the intermediate formation of thiols, followed by condensation to the sulfides. The observation of isomerized products in suitable cases indicates the intermediate formation of carbocations, either by protolysis of alkanes by the superacid or reversible ionisation of the thiol products (149). 

The KF-S reaction presumably involves attack of a fluonnated caibanion on sulfur, whereas the S-Sbp5 reaction may involve electrophilic attack by a cationic sulfur species on the olefin under the strong Lewis acid conditions Electrophilic attack on a fluonnated olefin may also account for formation of a perfluorinated sulfide from reaction of bis(pentafluorophenyl)disulfide with hexafluoropropylene under superacid conditions [IS5] (equation 28)... 

The Pictet-Spengler reaction is an acid-catalyzed intramolecular cyclization of an intermediate imine of 2-arylethylamine, formed by condensation with a carbonyl compound, to give 1,2,3,4-tetrahydroisoquinoline derivatives. This condensation reaction has been studied under acid-catalyzed and superacid-catalyzed conditions, and a linear correlation had been found between the rate of the reaction and the acidity of the reaction medium. Substrates with electron-donating substituents on the aromatic ring cyclize faster than the corresponding unsubstituted compounds, supporting the idea that the cyclization process is involved in the rate-determining step of the reaction. 

It has proved possible to obtain 146 and 147 as stable species in superacid solutions. Another mechanism, involving an intermediate with only one positive charge, has been proposed for certain substrates at low acidities. 

Much of our research has involved the use of dicationic electrophiles in reactions with very weak nucleophiles, such as non-activated arenes and alkanes. By comparison to similar monocationic electrophiles, we have been able to show the extent of electrophilic activation by adjacent cationic centers. For example, carbocations show an increased reactivity with a nearby cationic charge (eqs 3-4).9 When 1,1-diphenyletheneis reacted with superacidic CF3SO3H... 

Acyl-transfer reactions are some of the most important conversions in organic chemistry and biochemistry. Recent work has shown that adjacent cationic groups can also activate amides in acyl-transfer reactions. Friedel-Crafts acylations are known to proceed well with carboxylic acids, acid chlorides (and other halides), and acid anhydrides, but there are virtually no examples of acylations with simple amides.19 During studies related to unsaturated amides, we observed a cyclization reaction that is essentially an intramolecular acyl-transfer reaction involving an amide (eq 15). The indanone product is formed by a cyclization involving the dicationic species (40). To examine this further, the related amides 41 and 42 were studied in superacid promoted conversions (eqs 16-17). It was found that amide 42 leads to the indanone product while 41... 

G. Prakash, G. K. S. Efficient Chemoselective Carboxylation of Aromatics to Arylcarboxylic Acids with Superelectrophilically Activated Carbon Dioxide-Al2CI6/Al System. J. Am. Chem. Soc. 2002, 124, 11379-11391. (d) Klumpp, D. A. Rendy, R. McElrea, A. Superacid Catalyzed Ring-opening Reactions Involving 2-Oxazolines and the Role of Superelectrophilic Intermediates. Tetrahedron Lett. 2004, 45, 7959-7961. 

Zhang, Y. Briski, J. Zhang, Y. Rendy, R. Klumpp, D. A. Superacid-Catalyzed Reactions of Olefinic Pyrazines an Example of Anti-Markovnikov Addition Involving Superelectrophiles. Org. Lett. 2005, 7, 2505-2508. 

The exact mechanism of the reaction between CIO 4 and superacids has as yet not been established, although numerous comments on it were published 19, 21,167, 2S3, 292, 297). Based on our present understanding of superacid chemistry 67, 118, 216) and of the complex formation of FClOs (see Section III, K, 4), a mechanism involving CIOs " as an intermediate is very unlikely. Furthermore, the high yields of FCIO3 (up to 97%) would be surprising in view of the expected instability of ClOs". In our opinion, other mechanisms, such as the one shown, involving protonated perchloric acid 166) are more plausible ... 

Fluorination of cinchona alkaloids has also been investigated. For instance, fluorination of quinine acetate under similar superacidic conditions (HF—SbFs/CHCls) affords a mixture of difluorocompounds in the 10 position that are ephners in 3 (60% yield, 1 1 ratio). This reaction involves a mechanism similar to the one described earlier (protonation, isomerization of carbenium ions, and Cl— F exchange). Curiously, when the reaction is performed on quinine itself, fluorination does not occur and an unprecedented rearrangement takes place (Figure 4.51). ... 

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