Lewis superacids

Brmnsted-Lewis Superacids. Conjugate Friedel-Crafts acids prepared from ptotic and Lewis acids, such as HCl—AlCl and HCl—GaCl ate, indeed, supetacids with an estimated value of —15 to —16 and ate effective catalysts in hydrocarbon transformation (217). 

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. 

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... 

Fig. 23. 75.4-MHz 13C MAS spectra of 2-bromopropane-2-13Creacting on Lewis superacids AlBr3 and SbF5. Spectrum a shows the formation of an adsorption complex (89 ppm for C2 and 29 ppm for the methyl carbons) with AlBr3 at 298 K. Note that 13C label scrambling from C2 to Q is complete in the adsorption complex as indicated by the 2 1 intensity ratio of the 29 ppm to 89 ppm peaks. Spectrum b shows a completely 13C scrambled isopropyl cation (320 and 52 ppm) and a partially 13C scrambled adsorption complex with SbF5 (92 and 26 ppm) at 233 K. (c) Upon raising the temperature, the adsorption complex was completely converted to the isopropyl cation.

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. 

Lewis superacids, such as SbF5, AsF5, PF5, TaF5, NbF5, BF3, tris(pentafluoro-phenyl) borane, boron tris(trifluoromethanesulfonate), and aprotic organic superacids developed by Vol pin and co-workers. 

In Chapter 1, we arbitrarily defined Lewis superacids as those that are stronger than anhydrous aluminum chloride in their reactivity, the most commonly used Friedel-Crafts catalyst. Of course, Lewis acidity is only a relative term concerning specific bases and involved counterions (association, steric hindrance, etc.). The physical properties of some of the Lewis superacids are given in Table 2.3. 

Table 2.3. Physical Properties of Some Lewis Superacids...

Another related boron Lewis superacid, B (CF3)3, is worth mentioning, even though it has not been isolated.59 However, the corresponding anion B(CF )i is known and... 

C6F5SF3 with Lewis superacids.169 Significant deshielding of fluorines in ortho and para positions indicates charge dispersion into the aromatic ring. Cfil 5 derivatives were found to be more stable than their CF3 counterparts. 

In addition to the discussed Br0nsted or Lewis superacidic activation in solution chemistry, there have been reports to suggest that superelec-trophilic species can be formed with solid acids, and even in biochemical systems. For example, Sommer and co-workers have found several examples in which HUSY zeolite has exhibited catalytic activity similar to liquid superacids (eqs 33-34).12 In the same study, the perfluorinated resinsulfonic acid Nafion-H (SAC-13) was found to give products consistent with the formation of the superelectrophile (36, eq 35). 

Carbon-Halogen Vicinal-Dieations Trihalomethyl cations are shown to have enhanced reactivities in superacid solution, while poly-halomethanes in the presence of excess AlBr3 or AICI3 exhibit the properties of aprotic superacids.79 The trihalomethyl cations CX3+ (178, X=C1, Br, I) have been characterized by NMR and IR spectroscopy. The stability of these species is attributed to substantial resonance-stabilization by back-donation from the nonbonded electron pairs of the halogen atoms.22 Trihalomethyl cations are capable of hydride abstraction from alkanes and alkyl groups when the reactions are carried out in the presence of Bronsted or Lewis superacids (eq 46-48).80... 

The Lewis superacids formed by the fluorides are a result of transfer of anions to form complex fluoro anions ... 

Any Lewis acid that is stronger than anhydrous AICI3 has been arbitrarily defined as a Lewis super-acid. " Examples of such Lewis superacids include SbFj, NbFs, BF3, AsFs, TaFs and BiF3. 

Lewis superacid-catalyzed direct alkylation of alkanes is also possible with alkyl cations prepared from alkyl halides and SbFs in sulfuryl chloride fluoride solution. " Typical alkylation reactions are those of propane and butanes by 2-butyl and ZerZ-butyl cations. The ClfU-Sbfs and C2H5F-SbF5 complexes acting as incipient methyl and ethyl cations besides alkylation preferentially cause hydride transfer. Since intermolecular hydride transfer between different carbocations and alkanes are faster than alkylation, a complex mixture of alkylated products is usually formed. A significant amount of 2,3-dimethylbutane was, however, detected when propane was propylated with the 2-propyl cation at low temperature [Eq. (6.36)]. No 2,2-dimethylbutane, the main product of conventional acid-catalyzed alkylation, was detected, which is a clear indication of predominantly nonisomerizing reaction conditions. 

Scheme 12 summarizes a y-radiolysis performed on a mixture of benzene-CF4, and ethylene. Ionization of CF4 yields CFj, which has been called an ionic Lewis superacid.Electrophilic attack on benzene-(i6 gives the conjugate acid of perdeuterated trifluorotoluene, CF3C6DJ, as the first step of Scheme 12. Trifluorotoluene is more basic than ethylene hence, the ion-molecule reaction of CF3C6DJ with ethylene cannot give a Bronsted acid-base reaction. The only thermochemically accessible pathway to ethyl trifluorotoluenes is via ion-neutral complexes containing the ethyl cation...

Alkane Reaction Initiation by Lewis Superacids.- The mechanism of strong Lewis acid action on alkanes is still a... 

Mechanism of Alkane Reactions over Superacids.- Carbenium cations formed upon Brdnsted or Lewis superacid centre action can undergo isomerization. Two types of this reaction are possible. Firstly one in which the state of alkane branching is conserved and secondly one when it is changed. The example of the first... 

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