Superacids cations

Stabilization ( 900 kJ/mol) with the negative charge that develops on the zeolite framework when the proton is transferred. The major difference between superacids and zeolites is that in superacids cations and anions become solvated by the polar solvent molecules. No such stabilization occurs in zeolites. The chemistry in zeolites, when applied in hydrocarbon catalysis, is much closer to that in a vacuum (dielectric constant s 4) than to that in a polar medium such as a polar superacid (dielectric constant e 80-100). 

Lanthanide halides, nitrates and triflates are not only common reagents in organic synthesis (Fig. 1) but also represent, in dehydrated form, key precursor compounds for the more reactive organometallics (Scheme 2). As a rule, in compounds of strong monobasic acids or even superacids, cation solvation competes with anion complexation, which is revealed by fully or partially separated anions and solvated cations in their solid state structures. The tendency to form outer sphere complexation in coordinating solvents [47] is used as a criterion of the reactivity of inorganic salt precursors in organometallic transformations. 

My work on long-lived (persistent) carbocations dates back to the late 1950s at Dow and resulted in the first direct observation of alkyl cations. Subsequently, a wide spectrum of carbocations as long-lived species was studied using antimony pentafluoride as an extremely strong Lewis acid and later using other highly acidic (superacidic) systems. 

It was not fully realized until my breakthrough using superacids (vide infra) that, to suppress the deprotonation of alkyl cations to olefins and the subsequent formation of complex mixtures by reactions of olefins with alkyl cations, such as alkylation, oligomerization, polymerization, and cyclization, acids much stronger than those known and used in the past were needed. 

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. 

The key initiation step in cationic polymerization of alkenes is the formation of a carbocationic intermediate, which can then interact with excess monomer to start propagation. We studied in some detail the initiation of cationic polymerization under superacidic, stable ion conditions. Carbocations also play a key role, as I found not only in the acid-catalyzed polymerization of alkenes but also in the polycondensation of arenes as well as in the ring opening polymerization of cyclic ethers, sulfides, and nitrogen compounds. Superacidic oxidative condensation of alkanes can even be achieved, including that of methane, as can the co-condensation of alkanes and alkenes. 

The formation of the (X-delocalized norbornyl cation via ionization of 2-norbornyl precusors in low-nucleophilicity, superacidic media, as mentioned, can be considered an analog of an intramolecnlar Friedel-Crafts alkylation in a saturated system. Indeed, deprotonation gives nortricyclane,... 

The superacid-catalyzed cracking of hydrocarbons (a significant practical application) involves not only formation of trivalent carbo-cationic sites leading to subsequent /3-cleavage but also direct C-C bond protolysis. 

The reaction of trivalent carbocations with carbon monoxide giving acyl cations is the key step in the well-known and industrially used Koch-Haaf reaction of preparing branched carboxylic acids from al-kenes or alcohols. For example, in this way, isobutylene or tert-hutyi alcohol is converted into pivalic acid. In contrast, based on the superacidic activation of electrophiles leading the superelectrophiles (see Chapter 12), we found it possible to formylate isoalkanes to aldehydes, which subsequently rearrange to their corresponding branched ketones. 

Volpin called the CH3COCI lAlCk complex an aprotic superacid. The results indicate that the acetyl cation is activated by further 0-complexation with a second molecule of AIX3 (the equivalent of protonation or protosolvation). 

Acetic acid and other carboxylic acids are protonated in superacids to form stable carboxonium ions at low temperatures. Cleavage to related acyl cations is observed (by NMR) upon raising the temperature of the solutions. In excess superacids a diprotonation equilibrium, indicated by theoretical calculations, can play a role in the ionization process. 

The finding that highly deactivated aromatics do not react with N02 salts is in accord with the finding that their greatly diminished TT-donor ability no longer snffices to polarize NOi. Similarly, (j-donor hydrocarbons such as methane (CH4) are not able to affect such polarization. Instead, the linear nitronium ion is activated by the superacid. Despite the fact that is a small, triatomic cation, the 11011-... 

The extent to which rearrangement occurs depends on the structure of the cation and foe nature of the reaction medium. Capture of carbocations by nucleophiles is a process with a very low activation energy, so that only very fast rearrangements can occur in the presence of nucleophiles. Neopentyl systems, for example, often react to give r-pentyl products. This is very likely to occur under solvolytic conditions but can be avoided by adjusting reaction conditions to favor direct substitution, for example, by use of an aptotic dipolar solvent to enhance the reactivity of the nucleophile. In contrast, in nonnucleophilic media, in which fhe carbocations have a longer lifetime, several successive rearrangement steps may occur. This accounts for the fact that the most stable possible ion is usually the one observed in superacid systems. 

Formation of the 3-cyclohexeityl cation from the alcohol in superacid media is followed by more extensive rearrangement to give the methylcyclopentenyl ion, which is tertiary and allylic. ... 

In some cases, NMR studies in superacid media have permitted the observation of successive intermediates in a series of rearrangements. An example is the series of cations originating with the bridgehead ion F, generated by ionization of the corresponding chloride. Rearrangement eventually proceeds to the tertiary ion K. The bridgehead ion is... 

Interpretation of tiie ratio of capture of competing nucleophiles has led to the estimate that bromonium ions have lifetimes on the order of 10 s in methanol. This lifetime is about 100 times longer than fliat for secondary caibocations. There is also direct evidence for the existence of bromonium ions. The bromonium ion related to propene can be observed by NMR spectroscopy when l-bromo-2-fluoropropane is subjected to superacid conditions. The terminal bromine adopts a bridging position in the resulting cation. 

The cyclobutenyl cation is the homoaromatic analog of the very stable cycloprope-nium cation. This ion can be prepared from 3-acetoxycyclobutene with the use of superacid conditions ... 

The KF-S reaction presumably involves attack of a fluonnated caibanion on sulfur, whereas the S-Sbp5 reaction may involve electrophilic attack by a cationic sulfur species on the olefin under the strong Lewis acid conditions Electrophilic attack on a fluonnated olefin may also account for formation of a perfluorinated sulfide from reaction of bis(pentafluorophenyl)disulfide with hexafluoropropylene under superacid conditions [IS5] (equation 28)... 

Strong acids or superacid systems generate stable fluorinated carbocations [40, 42] Treatment of tetrafluorobenzbarrelene with arenesulfonyl chlorides in nitro-methane-lithium perchlorate yields a crystalline salt with a rearranged benzo barrelene skeleton [43] Ionization of polycyclic adducts of difluorocarbene and derivatives of bornadiene with antimony pentafluonde in fluorosulfonyl chloride yields stable cations [44, 45]... 

There is one other substituent which is comparable with the diazonio group in the sense that it is cationic and that it has, in one of its mesomeric structures, a triple bond between the atom attached to the aromatic system and the second atom. It is the acylium group in 7.9. However, no substituent constants are known for this group, obviously because this cation is detectable in measurable concentrations only in superacidic media (see review by Olah et al., 1976). 

Subsequently it was found that the same cations could also be generated from alcohols in superacid-S02 at -60°C and from alkenes by the addition of a proton... 

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