Types of Superacids

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. 

The acidity strength of solid acids is still not well known and is difficult to measure. Claims of superacidity in solids are numerous and will be discussed later in Chapter 2. Among the reviews related to acidity characterization of solids, those of Corma, Farneth and Gorte, and Fripiat and Dumesic are quite significantly representative.30 

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. 

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Similar oxygen stabilized ethylene dications were proposed in several types of superacid-catalyzed condensation reactions involving 1,2-dicarbonyl compounds. For example, 2,3-butanedione condenses in high yield with benzene and the superelectrophile (35) is considered to be the key intermediate, because the monoprotonated species (78) is not sufficiently electrophilic to react with benzene (eq 17).35 Several biologically important a-ketoacids were also found to generate superelectrophiles... 

Although the sulfate superacids are stable enough because of preparatory heat treatment at elevated temperatures, elimination of the sulfate is sometimes observed during reaction as a result of catalyst deactivation, especially in a solid-liquid system. It is hoped to synthesize superacids with the system of metal oxides. We have succeeded in preparing another type of superacid, not containing any sulfate ion but consisting of metal oxides, which can be used at temperatures over 800°C (188-192). 

The type of superacid sites on SO /metal oxides evacuated at 773 K is only a Lewis type according to the IR absorption bands of adsorbed pyridine [31]. Mortcrra ct al. [32] have shown that pyridine adsorbed on Lewis acid sites dominated the spectra of samples evacuated at 673 K and that the addition of water at 300 K significantly increased the amount of Bronsted acidity. Nascimcnto ct al. [8] report that both Bronsted and Lewis acid sites exist on SO4 /Zr02 treated at 723 K and the ratio of Bronsted to Lewis sites changes with the change of sulfur content. Recently. Lunsford ct al. revealed by use of 31P MAS NMR spectra of adsorbed trimcthylphosphinc that three types of Lewis... 

Different types of other coal liquefaction processes have been also developed to convert coals to liqnid hydrocarbon fnels. These include high-temperature solvent extraction processes in which no catalyst is added. The solvent is usually a hydroaromatic hydrogen donor, whereas molecnlar hydrogen is added as a secondary source of hydrogen. Similar but catalytic liquefaction processes use zinc chloride and other catalysts, usually under forceful conditions (375-425°C, 100-200 atm). In our own research, superacidic HF-BFo-induced hydroliquefaction of coals, which involves depolymerization-ionic hydrogenation, was found to be highly effective at relatively modest temperatnres (150-170°C). 

Although H2S is normally a weak acid, it functions as a base in a superacid such as HF/SbF5 in liquid HF. The H3S+ ion is generated, and although solid H3S+SbF5 has been obtained, there is very limited chemistry associated with this type of compound. 

The generation of a-ferrocenyl-P-silyl substituted vinyl cations of type 28 does not require superacidic conditions, they can be generated by protonation of l-ferrocenyl-2-trialkylsilyl alkynes with trifluoroacetic acid at room temperature. The SiR3-groups with larger alkyl substituents increase the lifetime of this type of carbocations. 

Besides the intramolecular acyl-transfer reactions, electrophilic activation is shown to occur with intermolecular Friedel-Craft-type reactions.18 When the simple amides (45a,b) are reacted in the presence of superacid, the monoprotonated species (46a,b) is unreactive towards benzene (eq 18). Although in the case of 45b a trace amount of benzophenone is detected as a product, more than 95% of the starting amides 45a,b are isolated upon workup. In contrast, amides 47 and 48 give the acyl-transfer products in good yields (eqs 19-20). It was proposed that dications 49-50 are formed in the superacidic solution. The results indicate that protonated amino-groups can activate the adjacent (protonated) amide-groups in acyl-transfer reactions. 

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... 

Occasionally rearrangements from more stable to less stable carbocations occur, but only if (1) the energy difference between them is not too large or (2) the carbocation that rearranges has no other possible rapid reactions open to it.9 For example, in superacid medium, in the temperature range 0-40°C, the proton nmr spectrum of isopropyl cation indicates that the two types of protons are exchanging rapidly. The activation energy for the process was found to be 16 kcal mole-1. In addition to other processes, the equilibrium shown in Equation 6.7 apparently occurs.10 In the superacid medium, no Lewis base is available... 

A combination of electrophilic properties of H+ and relatively low nucleo-philicity of the counter anion in superacids makes these materials convenient media for the generation of different types of carbocations as a result of protonation of the carbon-element bond ... 

As discussed previously, several types of reactive dications and superelectrophiles have been directly observed using NMR spectroscopy. These experiments have all used low temperatures (— 100°C to — 30°C) and superacidic conditions to generate the observable reactive dications and superelectrophiles. Some reactive dications and superelectrophiles are stable at low temperatures and can be directly observed by NMR, but at higher temperatures they readily cleave and decompose. The low temperatures also slow down proton exchange reactions and enable the ions to be observed as static species. 

Several types of unsaturated amides have produced gitonic superelectrophiles in superacidic media. For example, reaction of the cinnamamide (60) with benzene in CF3SO3H gives the addition product in quantitative yield (eq 20).27... 

There have been several types of gitonic superelectrophiles having acyl cationic groups as part of a 1,3-dicationic system. Monocationic acyl cations (168) can be prepared as persistent species in superacidic media and salts have even been studied by X-ray crystallography.113 Much of the interest in superelectrophilic species from acyl cations has focused on the protioacyl dications (169, vide supra).61... 

Carbo-acyl dicationic species have been proposed as intermediates in several reports, but these types of distonic superelectrophiles have not yet been sufficiently studied. Work by the Olah group has shown that pro-tonated carboxylic acids cleave to the acyl ions in superacidic media at temperatures above — 10°C.32 In principle, ionization of a second group (such as hydroxyl or olefinic) can generate a carbocationic site adjacent to... 

As described in the previous chapter, a number of diketones have been diprotonated in superacid to generate distonic superelectrophilic species. Several types of 1,4-diketones yield the respective distonic superelectrophiles (eq 49).47... 

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. 

The topic of catalysis with Nafion has recently been reviewed in detail (56). Apart from using Nafion-H primarily as a solid superacid catalyst, a number of reports have described the use of functionalized Nafion derivatives by metal cation exchange to achieve various types of organic reaction. These include a bifunctional catalyst (acid and cation site), a heterogeneous perfluorosulfonate salt (only cation sites), and a trifunctional... 

Benzylation of toluene with benzyl chloride, which is a typical example of Friedel-Crafts alkylation, is known to be catalyzed by Lewis-type superacids such as A1C13 and BF3. This type of catalyst has been mostly used for the Friedel-Crafts reaction, which is one of the most studied of organic reactions. This reaction was performed over several metal oxides and sulfates, and iron sulfates showed an unexpected effectiveness for the reaction (102-104). The catalytic activities of FeS04 and Fe2(S04)3 for the reaction were examined in detail the activities were remarkably dependent on calcination temperature, the maximum activity being observed with calcination at 700°C (105-107). Catalytic actions analogous to the above case were also observed with other Friedel-Crafts reactions, the benzoyl-ation of toluene with benzoyl chloride (108), the isopropylation of toluene with isopropyl halides (109), and the polycondensation of benzyl chloride UIO). 

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