Superacid catalysis

Stereoselectivity in the condensation reaction of 2-arylethylamines with carbonyl compounds to give 1,2,3,4-tetrahydroisoquinoline derivatives was somewhat dependent on whether acid catalysis or superacid catalysis was invoked. Particularly in the cases of 2-alkyl-N-benzylidene-2-phenethylamines, an enhanced stereoselectivity was observed with trifluorosulfonic acid (TFSA) as compared with the weaker acid, trifluoroacetic acid (TFA). Compound 43 was cyclized in the presence of TFA to give modest to good transicis product ratios. The analogous compound 44 was cyclized in the presence of TFSA to give slightly improved transicis product ratios. 

The concentration of protons (and of protonated substrates) is limited at pH values near neutrality. Analogous metal complexes are often stable over a wide pH range including physiological pH, and may exist in the presence of appreciable concentrations of OH- or other nucleophiles. Westheimer2 coined the term superacid catalysis to describe the ability of metal ions to catalyze reactions at pH values where substantial concentrations of the protonated substrates cannot exist. 

It should be also noted that in biological chemistry, following a suggestion by Westheimer,20 it is customary to call catalysis by metal ions bound to enzyme systems as superacid catalysis. Because the role of a metal ion is analogous to a proton, this arbitrary suggestion reflects enhanced activity and is in line with previously discussed Brpnsted and Lewis superacids. 

The halogenation of saturated aliphatic hydrocarbons is usually achieved by free radical processes.523 Ionic halogenation of alkanes has been reported under superacid catalysis. Olah and co-workers524,525 have carried out chlorination and chlorolysis of... 

Davis, A. P., Jaspars, M. Superacid catalysis of the addition of allysilanes to carbonyl compounds. J. Chem. Soc., Chem, Common, 1990, 1176-1178. 

The difference between liquid superacid catalysis and zeolite catalysis relates essentially to the fact that in liquids ionic protonated intermediates are formed, e.g. 

In a biochemical context, superacid catalysis is sometimes used to denote catalysis by metal ions, analogous to catalysis by hydrogen ions. 

The reaction of isobutane with carbon monoxide in the presence of an excess of AICI3 was first reported by Nenitzescu et al. to yield a complex mixture of products, mainly carboxylic acids (Koch-Haaf reaction), but also some methyl isopropyl ketone, a formylation product. Later, Olah et al. louiid " that in the reaction of adamantane (26) with carbon monoxide under superacidic catalysis, formylation (formation of 1-adamantanecarboxaldehyde) effectively competes with Koch-Haaf carboxylation (formation of 1 -adamantanecarboxyUc acid). On the basis of results acquired by the reaction of 1,3,5,7-tetradeu-teroadamantane, formylation was interpreted by a-insertion of the formyl cation into the tertiary C-H bond [Eq. (6.47)]. 

Alkylation of methane by olefins under superacid catalysis was demonstrated both in solution chemistry under stable ion conditions and in heterogeneous gas-phase alkylations over solid catalysts using a flow system. Not only propylene and butylenes but also ethylene could be used as alkylating agents. 

Whereas electrophilic formylation of aromatics with CO was studied under both the Gatterman-Koch condition and with superacid catalysis in some detail, electrophilic formylation of saturated aliphatics remains virtually unrecognized. 

Insertion of protosolvated formyl cation into the C—H a-bond has further been substantiated by carrying out the reaction of 1,3,5,7-tetradeuteroadamantane with CO in superacid media. 3,5,7-Trideutero-l-adamantanecarboxaldehyde-H and 3,5,7-trideutero-l-adamantanecarboxaldehyde-D were obtained in the ratio 94 6. This work provides the first direct evidence for the electrophilic formylation of a saturated hydrocarbon by CO under superacid catalysis. 

Colgupoum, H. M., Lewie, D. F. (1988). Aromatic Poly esterketones Via Superacid Catalysis / in Spec. Polym. 88 Abstracts of the 3-rd International Conference on. New Polymeric Materials. Guildford, 39. 

Furuta, S., Matsuhashi, H. and Arata, K. 2004. Biodiesel fuel production with solid superacid catalysis in fixed bed reactor under atmospheric pressure. Catalysis Commun. 5 721-723. 

Cycloisomerization of methyl-substituted 1,6-diene is also catalyzed by tin(rv) triflate (Equation (8.8)). DFT computations proposed that the mechanism does not involve the direct addition of the tin(IV) cation to a double bond because the catalyst regeneration step would be energetically unfeasible [24]. The active catalyst is a hydrated triflate salt where water molecule plays a decisive role to enable the smooth completion of the catalytic cycle. The diastereoselectivity observed in the cycloisomerization was associated with the transition-state geometries. DFT calculations also showed that protonation and deprotonation occur on a single face of the substrate. These considerations, correlated to the experiments, showed that Brpnsted superacids are not effective in Lewis superacid catalysis. 

PFSA membranes have excellent chemical inertness and mechanical integrity in a corrosive and oxidative environment, and their superior properties allowed for broad application in electrochemical devices and other fields such as superacid catalysis, gas drying or humidification, sensors, and metal-ion recovery. Here, we refer their important applications in electrochemical devices for energy storage and conversion including PEMFC, chlor-alkali production, water electrolysis, vanadium redox flow batteries, lithium-ion batteries (LIBs), and solar cells. 

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