Lewis superacids catalysts

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). 

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. 

The acidity of these resins, however, increases significantly by treating them with Lewis acid halides. Gates and co-workers151 153 have prepared a superacid catalyst from AICI3 and beads of macroporous, sulfonated polystyrene-divinylbenzene. The... 

Stable nitronium salts, which are readily prepared from nitric acid (or nitrates) with HF and BF3 (and other Lewis acids such as PF5, SbF5, etc.) [Eqs. (5.171) and (5.172)], will nitrate aromatics in organic solvents generally with close to quantitative yield. Because HF and PF5 (or BF3) can be easily recovered and recycled, the method can be considered as a nitric acid nitration using a superacid catalyst [Eq. (5.173)]. 

The first member of this family - vinylidene fluoride - readily adds fluoro-sulfonic acid, even in the absence of the catalyst, and it is nitrofluorinated by a HNO3/HF mixture six times faster than CFH=CF2 [15] however, HF/HS03F superacid does not react at ambient temperature with C1CF=CF2 or CF2=CF2. These two olefins show a similar reactivity in electrophilic reactions. Interaction of polyfluorinated propylenes CF2=CXCF3 (X=H,F) with trifluoro- and tetra-fluoroethylenes proceeds at room temperature in the presence of a Lewis acid catalyst to give the corresponding polyfluoropentenes-2 ... 

This review summarizes the recent works on syntheses of solid superacids and their catalytic action, including Lewis acids and liquid superacids in the solid state, as discussed in Sections Il-IV. Sections VI and VII describe new types of solid superacids we have studied in this decade sulfate-supported metal oxides and tungsten or molybdenum oxide supported on zirconia. Perfluorinated sulfonic acid, based on the acid form of DuPont s Nafion brand ion membrane resin, is also gaining interest as a solid superacid catalyst Nafion-H-catalyzed reactions are reviewed in Section V. 

This work incorporates all acid-catalyzed Friedel-Crafts-like acylation reactions. Thus, classic Lewis and Bronsted acid types are considered together with more innovative and advanced multicomponent superacid catalysts, ranging from rare earth triflates or triflimides and their... 

A Lewis Acid/Ion Exchange Resin Superacids.- Superacid catalysts obtained by the action of AlCl vapours on an ion exchange resin(macroporous, sulphonated poly/styrene-di-vinylbenzene) can be considered as a full analogue to a liquid superacid. This catalyst was obtained and studied by Gates and is characterized by strong proton acidity, following structure ... 

Differently to silicon or germanium, tin triflate was even considered as a Lewis superacid, and investigated in multiple catalytic reactions. Hydroalkoxylation of nonactivated disubstituted or trisubstituted double bonds (Equation (8.7)) was reported to proceed in alcoholic solvents at moderate temperatures in the presence of tin(IV) triflate, without the need of hgands, additives, or cocatalysts [23]. Very important, it is also the control of the selectivity by these catalysts. In this particular reaction, the Markovnikov-type adducts were formed with regiose-lectivities of 84-100%. 

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. 

Solid Superacids. Most large-scale petrochemical and chemical industrial processes ate preferably done, whenever possible, over soHd catalysts. SoHd acid systems have been developed with considerably higher acidity than those of acidic oxides. Graphite-intercalated AlCl is an effective sohd Friedel-Crafts catalyst but loses catalytic activity because of partial hydrolysis and leaching of the Lewis acid halide from the graphite. Aluminum chloride can also be complexed to sulfonate polystyrene resins but again the stabiUty of the catalyst is limited. 

This powerful halide-abstracting ability results in SbFs being the strongest Lewis add with which HS03F forms the strongest superacid , the so-called magic add .70 The Sbv halides are excellent Friedel-Crafts catalysts.269... 

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... 

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