Superacid research

Novel organic syntheses that are possible in usual acidic media can be accomplished in superacids, including syntheses of economically important hydrocarbons. The remarkable ability of superacids to bring about hydrocarbon transformations can open up new fields in chemistry. In consideration of the exceptionally high activity of Hquid superacids, research was extended to prepare solid superacids. As for chemical appHcations of liquid superacids, efforts were made to attach them to soHd materials, and the results are found in extensive patent literature [10-13]. 

My research during the Cleveland years continued and extended the study of carbocations in varied superacidic systems as well as exploration of the broader chemistry of superacids, involving varied ionic systems and reagents. I had made the discovery of how to prepare and study long-lived cations of hydrocarbons while working for Dow in 1959-1960. After my return to academic life in Cleveland, a main... 

Over a decade of research, we were able to show that practically all conceivable carbocations could be prepared under what became known as stable ion conditions using various very strong acid systems (see discussion of superacids) and low nucleophilicity solvents (SO2, SO2CIF, SO2F2, etc.). A variety of precursors could be used under appropriate conditions, as shown, for example, in the preparation of the methylcyclopentyl cation. 

Processing heavy oils and bitumens represents a challenge for the current refinery processes, because heavy oils and bitumens poison the metal catalysts used m the refineries. In our research at the Loker Institute, we found the use of superacid catalysts, which are less sensitive to heavy oils, an attractive solution to their processing, particularly hydrocracking. 

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). 

The hydrocarbon research program of the Toker Institute was able in many ways to build on and utilize results of our fundamental work on superacid-catalyzed reactions and their mechanistic aspects (includ-... 

The vastly increased acidity of superacidic systems resulted in the significant new field of superacid chemistry. I began to ask myself whether a similar but more general approach could be used to produce electrophiles of greatly enhanced electron deficiency and thus reactivity. Over the years, there were a number of unexpected results in my own research work, as well as some previously unexplained observations buried in the literature, that seemed worth pursuing. 

However, the more important question of whether AN can be used in C,C-coupling reactions with nucleophiles remains open. It should be noted that Japanese researchers demonstrated in several studies that these transformations can be performed for benzene and certain electron-rich arenes (477). Just the same, this procedure requires severe conditions (the use of superacids at high... 

Much of our research has involved the use of dicationic electrophiles in reactions with very weak nucleophiles, such as non-activated arenes and alkanes. By comparison to similar monocationic electrophiles, we have been able to show the extent of electrophilic activation by adjacent cationic centers. For example, carbocations show an increased reactivity with a nearby cationic charge (eqs 3-4).9 When 1,1-diphenyletheneis reacted with superacidic CF3SO3H... 

Like LiAsFe, LiBF4 is a salt based on an inorganic superacid anion and has moderate ion conductivity in nonaqueous solvents (Table 3). It was out of favor in the early days of lithium battery research because the ether-based electrolytes containing it were found to result in poor lithium cycling efficiencies, which decayed rapidly with cycle number. ° The reactivity of LiBF4 with lithium was suspected as discoloration occurred with time or heating. 

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... 

The difficulties encountered in handling liquid superacids and the need for product separation from the catalyst in batch processes have stimulated research in... 

No NMR evidence ( H and 13C) could be obtained for the vicinal-dication (221). Calculations at the MP2/6-31G level however show dication 221 to be a stable minimum. Its structure has D2symmetry with the two COC units in an almost perpendicular arrangement (dihedral angle between two COC units is 83.7°). Protonated ozone, O3H+, was studied by Olah et al. and by Italian researchers.111 In the superacid catalyzed reactions of ozone, diprotonated ozone is however possibly involved as the de facto intermediate. 

Novel organic syntheses that are not possible in usual acidic media can be accomplished by superacids. The review by K. Arata on Solid Superacids summarizes recent research on synthesis of superacids and their catalytic actions. 

The primary thrust of the material in this chapter will be to indicate to researchers not already familiar with the details of superacid chemistry that, for the synthesis of many inorganic fluorides and other compounds containing fluorine, a favourable reaction medium can frequently be found by careful selection of an appropriate superacid, by deliberate control of the acidity or basicity of that medium and by use of suitable redox and other reactants and precipitants. 

There has been little reported study of preparation of polyatomic cations of metals in superacids. The very limited experience with Bij+ in weakly acidic HF suggests that this may be a fertile field for research. For example, Hg +, which was found to disproportionate in S02 [51], should be stable in protonic superacids or in acidic media such as neat SbFs. Similarly, bismuth cations other than Bij+ should be stable in media of controlled acidity. 

The addition of water causes the breakage of the coordination bonds to yield Bronsted acid sites strengthening Lewis acid sites, as shown in Scheme 17.4, for example. Many research groups report the simultaneous existence of Bronsted and Lewis acid sites or the reversible transformation between Bronsted and Lewis acidity upon hydration or dehydration [61, 106, 152]. Fraenkel suggests that in order to be an effective superacid, sulfated zirconia should contain a critical amount of moisture [155bj. Several workers propose that the strong acidity requires the presence of both Lewis and Br0nsted sites. 

Conversely, the presumed heterolytic C-N bond separation should involve a prior protonation step since this reaction takes place only under strong acid catalysis when it is run at room temperature. In the absence of acid, however, the same process may be thermally induced. There are two potential sites for proton attachment The aziridine nitrogen and the carbonyl oxygen. Both—as protonated species—are suitable to initiate fragmentation of the three-membered ring by way of intermediates VII and VIII, respectively (see Scheme 14.2). While evidence supporting the existence of VIII is available from proton nmr spectral analysis of a V-acrylaziridine in superacid media, other researchers ... 

I continued with my research on sulfuric acid, now quite independently from Ingold. From time to time, he sent a student to me. Ingold chose his students and allocated others to the faculty. I started to branch out, looking at the conductivity of solutions of other substances in sulfuric acid and measuring acidities. That s how I got interested in superacids. 

My research has been mainly in superacids, sulfuric acid, fluorosulfuric acid and hydrogen fluoride, which led me naturally into fluorine chemistry. I didn t do a lot more on VSEPR except for trying to promote it. I was excited by the discovery of the noble-gas compounds, and it was an obvious opportunity to show that it was easy to use VSEPR to predict their structures. At one of the early conferences on noble-gas chemistry, Larry Bartell asked me to predict the structure of XeF. I believe he had already determined the structure by electron diffraction but not yet published it. I told him that it could not be octahedral, which the MO theorists believed they had proved, but was probably a distorted monocapped octahedron, which turned out to be correct. Since then, he has been a great fan of VSEPR and has done much to promote it as a useful theory. 

The isomerization of light paraffin using superacid solid catalysts is a clean way to increase the octane number of hydrocarbons. On this basis, sulfated metal oxides have attracted the attention of many research groups owing to their high activity in acid catalyzed reactions [1]. Sulfated zirconia was found to be a promising catalyst in this field and at the industrial level [2],... 

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