Brpnsted superacids

Bromperidol, 4 360t Brompheniramine, 4 359t Bromyrite, 4 304 Bronchitis, effect on heart, 5 107 Bronidox, 4 358t Bronopol, 4 358t Brpnsted acids, 12 190 Brpnsted-Lewis superacids, 12 191-192 Brpnsted superacids, 12 191 Bronze(s), 24 796... 

Neighboring methoxy group assisted asymmetric induction is demonstrated in the TMSOTf-catalyzed reaction of y-alkoxy acetal 18 with allylsilane. A possible cyclic oxocarbenium ion intermediate 19 is proposed (equation 12)55. Siladioxanes 20 react with allylsilane in the presence of a catalytic amount of the Brpnsted superacid TfOH2+ B(OTf)4 to afford the allylated products 21 diastereoselectively (equation 13)56,57. 

Binary Brpnsted superacids such as HF-HSO3F, HF-CF3SO3F, and HB(HS04)4. 

Carbo-carboxonium dications have also been generated by the direct ionization of appropriate functional groups by the action of Brpnsted superacids. For example, unsaturated acids are shown to give the reactive distonic superelectrophiles, which are shown to be moderately reactive.31... 

Ionic liquids can also exhibit superacidity (Brpnsted superacids are acids that are more acidic than pure H2SO4). The dissolution of gaseous HCl in acidic [emim][Cl]/AlCl3 (55mol% of Lewis acid) leads to a superacidic system that has similar properties to those of liquid HF and can be used for the protonation of arenes. The Brpnsted acidity of products dissolved in water-stable ionic liquids can be increased HNTf2 and TfOH display higher chemical activity in [bmim][NTf2] and [bmim][BF4] than in water. ... 

L. Lipping, I. Leito, I. Koppel, I. A. Koppel, J. Phys. Chem. A 2009,113,12972-12978. Gas-phase Brpnsted superacidity of some derivatives of monocarba-closo-borates A computational study. 

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. 

The reader is referred the recent book by Bell and Pines [2] for a more complete overview of the various methods and objectives in NMR studies of solid acids and other heterogeneous catalysis. In the present contribution we illustrate the application of H, and MAS NMR to two archetypal solid acids, Brpnsted sites in zeolites and solid metal halides such as aluminum chloride and bromide powders which exhibit "Lewis superacidity". An important characteristic of the more recent work is the integration of quantum chemical calculations into the design and interpretation of the NMR experiments. 

Lewis acids, 70 413, 47 72 188-190 74 265 79 31, 33. See also Brpnsted-Lewis superacids Super Lewis acids... 

Different catalysts bring about different types of isomerization of hydrocarbons. Acids are the best known and most important catalysts bringing about isomerization through a carbocationic process. Brpnsted and Lewis acids, acidic solids, and superacids are used in different applications. Base-catalyzed isomerizations of hydrocarbons are less frequent, with mainly alkenes undergoing such transformations. Acetylenes and allenes are also interconverted in base-catalyzed reactions. Metals with dehydrogenating-hydrogenating activity usually supported on oxides are also used to bring about isomerizations. Zeolites with shape-selective characteristics... 

In conclusion, extensive research has revealed that the Lewis and Brpnsted acid sites on the promoted sulfated zirconia catalysts are not necessarily stronger acids than the corresponding sites in zeolites, but sulfated zirconia circumvents the energetically unfavorable monomolecular reaction path by following a bimolecular mechanism. The question of superacidity of sulfated zirconia, however, is still debated.312... 

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. 

A quantitative determination of the strength of Lewis acids to establish similar scales (Ho) as discussed in the case of protic (Br0nsted-type) superacids would be most useful. However, to establish such a scale is extremely difficult. Whereas the Brpnsted acid-base interaction invariably involves a proton transfer reaction that allows meaningful comparison, in the Lewis acid-base interaction, involving for example Lewis acids with widely different electronic and steric donating substituents, there is no such common denominator.25,26 Hence despite various attempts, the term strength of Lewis acid has no well-defined meaning. 

Regardless, it is important to keep in mind that superacidity encompasses both Brpnsted and Lewis acid systems and their conjugate acids. The qualitative picture of Lewis acid strengths will be discussed in Section 1.4.7. 

As discussed, superacids, similar to conventional acid systems, include both Brpnsted and Lewis acids and their conjugate systems. Protic (Brpnsted-type) superacids include strong parent acids and the mixtures thereof, whose acidity can be further enhanced by various combinations with Lewis acids (conjugate acids). The following are the most frequently used superacids. 

Solid superacids can be further divided into various groups depending on the nature of the acid sites. The acidity may be a property of the solid as part of its chemical structure (possessing Lewis or Brpnsted sites the acidity of the latter can be further enhanced by complexing with Lewis acids). Solid superacids can also be obtained by deposition on or intercalation of strong acids into an otherwise inert or low-acidity support. 

Hydrogen Fluoride-Trifluoromethanesulfonic Acid. The acidity of this binary Brpnsted acid system has not been measured, but the superacidic properties are mentioned in numerous patents concerning fluorination, olefin alkylation, and hydrocarbon conversion. 

The acidic sites of solid acids may be of either the Brpnsted (proton donor, often OH group) or Lewis type (electron acceptor). Both types have been identified by IR studies of solid surfaces using the pyridine adsorption method. The absorption band at 1460 cm 1 is assigned to pyridine coordinated with the Lewis acid site, and another absorption at 1540 cm 1 is attributed to the pyridinium ion resulting from the protonation of pyridine by the Brpnsted acid sites. Various solids displaying acidic properties, whose acidities can be enhanced to the superacidity range, are listed in Table 2.6. 

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