Brpnsted-Lewis 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... 

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

The anions from conjugate Brpnsted-Lewis superacids represent the core lithium salts used for commercial lithium batteries (i.e., LiPFg and LiBp4). The acidity order determined from QC calculations is as follows HBF4 (287.7) [Pg.21]

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. 

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. 

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. 

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. 

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... 

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. 

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. 

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. 

Monocationic acyl ions are readily prepared as persistent species in solutions of low nucleophile strength.68 These acyl ions have been thoroughly characterized by IR and NMR spectroscopy, and several acyl ion salts have been characterized by X-ray crystallography. The monocationic acyl ions are often prepared in situ from carboxylic acids, esters, or anhydrides, by the action of a strong Brpnsted acid, or the ions can be prepared from ionization of an appropriate acid halide with a strong Lewis acid. Both methods have been used to prepare acyl-centered dications, some of which can be considered distonic superelectrophiles. As described previously, dicarboxylic acids cleave to the bis-acyl ions in superacid (FSChH-SbFs) provided that the acyl cations are separated by at least three methylene units (eq 54).55 The first bis-acyl dications were reported by Olah and Comisarow, being prepared by the reactions of dicarboxylic acid fluorides with superacidic SbFs (eq 72).69... 

Solid superacids may be made by treating ordinary solid acid catalysts with strong Br0nsted or Lewis acids. For example, if freshly precipitated titanium hydroxide or zirconium hydroxide is treated with sulfuric acid and calcined in air at 500 °C. a very active solid acid catalyst results. The solids consist mainly of the metal dioxides with sulfate ions coordinated to the metal ions on the surface. Likewise, a superacid solid catalyst can be made by treating these metal oxides with antimony pentafluoride. Both catalysts contain both Brpnsted and Lewis acid sites, and they are sufficiently active to catalyze the isomerization of n-butane at room temperature.- ... 

In superacid systems consisting of a composite of a Brpnsted acid and a Lewis acid, the anion interacts chemically with one or more molecules of the latter. Thus, up to 4 equiv of SbF5 has been recommended for conversion of alkyl halides to the corresponding carbocations (38). For those systems, the anion stabilization is accomplished by chemical bonding between the Lewis acid and the anion. The existence of carbocations in such solutions has been demonstrated by various physical methods. 

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. ... 

A superacid is a stronger acid than anhydrous H2SO4. Superacids are made by dissolving a strong Lewis acid (e.g. SbFs) in either of the Brpnsted acids HF or HSO3F. 

Room-temperature ionic liquids have recently attracted a great deal of industrial interest (Carmichael, 2000 Guterman, 1999 see also the brief overview by Earle et al. (2003) and the extensive and detailed review by Olivier-Bourbigou et al. (2010). They are versatile as solvents or nonsolvents for organic substances, and some exhibit strongly temperature-dependent water solubility. They are nonvolatile. Some, notably those containing haloaluminate anions, have widely variable Lewis and Brpnsted acidity (into the superacid range). Others, such as those with tetrafluoroborate or hexafluorophosphate... 

In 1877, Charles Friedel and James Mason Crafts [30a, b] corporately discovered that treatment of amyl chloride with aluminum strips in benzene led to the formation of amylben-zene. This type of transformation was found to be general for alkyl halides and aromatics under the catalysis of Lewis acid. Along with the discovery of the closely related acylation [30c, d], these two men are best remembered by Friedel-Crafts reaction that bears their names. With various modem modifications that appeared in the Uterature, including enan-tioselective variants [31], Friedel-Crafts alkylation and acylation have already become one of the most powerful C—C bond forming reactions in organic chemistry [32]. These methods are recognized to date as of fundamental importance not only in acadania but also in industry [33]. As shown in Scheme 10.18, some heteroaromatics, instead of the aryl component or alcohol, and alkenes instead of halides can be used as suitable substrates. Also, other common Lewis acids like BFj, TiCl, SnCl, ScfOTOj, etc., and Brpnsted acids snch as HF, H SO, and superacids (e.g., HF SbFj, HS03-SbFj) can also used as catalysts. 

---