Superacids, intramolecular

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. 

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... 

Besides the intramolecular acyl-transfer reactions, electrophilic activation is shown to occur with intermolecular Friedel-Craft-type reactions.18 When the simple amides (45a,b) are reacted in the presence of superacid, the monoprotonated species (46a,b) is unreactive towards benzene (eq 18). Although in the case of 45b a trace amount of benzophenone is detected as a product, more than 95% of the starting amides 45a,b are isolated upon workup. In contrast, amides 47 and 48 give the acyl-transfer products in good yields (eqs 19-20). It was proposed that dications 49-50 are formed in the superacidic solution. The results indicate that protonated amino-groups can activate the adjacent (protonated) amide-groups in acyl-transfer reactions. 

Saunders and Stofko have observed 1,3- 1,4-, and 1,5-intramolecular shifts from tertiary to tertiary center in superacid and have calculated the activation barriers to be 8.5, 12-13, and 6-7 kcal mole-1, respectively. (M. Saunders and J. J. Stofko, Jr., J. Amer. Chem. Soc., 95, 252 (1973)). 

Electrophilic ring closure of aryl-substituted compounds such as alkenes, halides, alcohols, and carbonyl compounds called cyclialkylation can be induced by conventional Friedel-Crafts catalysts309 and by superacids. Examples are also known in which an intermolecular alkylation step is followed by intramolecular alkylation of the intermediate to furnish a cyclic product. 

Superacid-catalysed intramolecular reactions of some dicationic electrophiles have been investigated.36 The positively charged centres migrate apart and this chemistry gives a new synthetic route to aza-polycyclic compounds. The polycyclic compound (26) can, for example, be formed from reaction of 2-phenyl-3-(l-hydroxy-2-phenyl-ethyl)quinoline (25) with CF3SO3H at 25 °C, loss of water and benzene being involved. Highly diastereoselective polycyclization of homo(polyprenyl)arenes [e.g. (27)... 

Yamato, T., Hideshima, C., Prakash, G. K. S., and Olah, G. A. 1991. Organic reactions catalyzed by solid superacids. 5. Perfluorinated sulfonic acid resin (Nafion-H) catalyzed intramolecular Friedel-Crafts acylation. /. Org. Chem. 56 3955-3957. 

C provides direct support for the dictum that tertiary carbonium ions are more stable than secondary, which are more stable than primary. Primary and secondary alcohols are protonated under these conditions, and the protonated alcohols are the species observed (entry 1), while tert-butyl alcohol yields ter/-butyl cation at rates too fast to measure. The rates of cleavage of protonated primary and secondary alcohols depend on their structure. Protonated sec-butanol cleaves with rearrangement to (CH3)3C and water slowly at -60°C, protonated isobutanol cleaves with rearrangement at —30°C, and protonated n-butanol at 0°C. It is typical of reactions in superacid media that the most stable ion of a particular class is observed because, under conditions in which the ions are long-lived, intramolecular hydride shifts and rearrangement processes occur that lead ultimately to the most thermodynamically... 

The possibility of NMR to follow the intramolecular migration of the selective label in adsorbed hydrocarbons gave the researches a chance to provide an evidence for the formation of alkyl carbenium ions as intermediates in reactions on solid acid catalysts, including zeolites. Carbenium ions in superacids exhibit a unique property to scramble the selective... 

Thus, in the superacidic medium, the direction and character of N-vinylpyrroles protonation change, that is, it results in the formation of stable dications, products of simultaneous protonation of the a-position of the pyrrole ring and p-position of the vinyl group, which are prone (in the case of phenyl substituent) to intramolecular cyclization, and kinetic cations having p-protonated pyrrole undergoing rearrangement via 1,2-hydride shift. 

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