Superacid media

The discovery of a significant number of hypercoordinate carboca-tions ( nonclassical ions), initially based on solvolytic studies and subsequently as observable, stable ions in superacidic media as well as on theoretical calculations, showed that carbon hypercoordination is a general phenomenon in electron-deficient hydrocarbon systems. Some characteristic nonclassical carbocations are the following. 

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,... 

In superacidic media, the carbocationic iatermediates, which were long postulated to exist duting Friedel-Crafts type reactions (9—11) can be observed, and even isolated as salts. The stmctures of these carbocations have been studied ia high acidity—low nucleophilicity solvent systems usiag spectroscopic methods such as nmr, ir, Raman, esr, and x-ray crystallography. 

A review of tile extensive atudies of cariiocaaoQa in superacid media is available in G. A. Olah, G. K. S. 

Scheme S3. Protonation and Ionization of Organic Substrates in Superacid Media...

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... 

Indicate die structure of the ion you expect to be formed as the stable species when each of the following compounds is dissolved in superacid media at —30°C ... 

Ketones and imines with adjacent perfluoroalkyl moieties show sharply reduced abilities to interact with electrophiles. Hexafluoroacetone is not protonated even in superacidic media... 

Superacid media, HF/AsF5 and HF/SbF5, caused perfluorinated 1,2-oxazetidines to ring open by breaking the carbon heteroatom in preference to the N—O bond (89CJC1724). 

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). 

Another approach to a donor adduct of the methylene phosphenium cation is the addition of a phosphonium cation to the phosphaalkyne. The reaction of the protic cation [HPPhal + lCFaSOa] with CjoHuCP produced a white powder which was identified as the P-phosphonio-substituted phosphaalkene [74]. Alternatively to the elimination reaction the phosphaalkynes were protonated. C-protonation of adamantylphosphaacetylene and ferf-butylphosphaacetylene occurred in superacid media under formation of phosphavinyl cations. From these spirocyclic betaines by reaction of l-Ad-C=P (Ad = adamantyl) withB(OTf)3 a phosphavinyl cation could be detected [75]. 

Quantum chemical calculations predict the primary ethylcation, C H/" to have /(-hydrido-bridgcd structure 5 which is 6-8 kcal/mol more stable than the Kekule line-bond structure for the primary cation 4. The ethylcation is not stable enough to be observable directly in superacid media. The NMR chemical shifts were calculated for both isomers 4 and 5 using the GIAO-MP2 method for CCSD optimized structures.23... 

Changing the emphasis to synthetic chemistry in superacid media, in Chapter 8, D. Klumpp examines the chemistry of dicationic electrophiles, demonstrating their enhanced reactivity via heteroatom protonation. In Chapter 9, S. Ito et al. explore the potential utility of stabilized carbocations for designing redox-active chromophores. 

It should be noted that analogous secondary a-methylcyclopropylmethyl cations are observable species in superacidic media at low temperatures.21 Observation of the secondary cyclopropylmethyl carbocations under superacidic conditions indicates the superior charge delocalization into the cyclopropyl ring as compared to the cyclobutyl ring. 

Ionization of the cx,cx-dimethylcyclobutylmethanol in superacidic media similarly gives the ring-expanded carbocation, 1, 2-dimethylcyclopentyl cation... 

Chemical behavior of polycyclic aromatic hydrocarbons (PAHs) and their derivatives in superacidic media continues to attract increased interest (2, 5, 11,... 

In the 1970s, Brouwer and Kifflin reported the reactions of saturated hydrocarbons with aliphatic aldehydes and ketones in superacidic media. Analysis of the products from these reactions suggested that the protonated aldehydes and ketones (carboxonium ions) were reacting at the carbon-hydrogen o-bonds of the alkanes. This was a surprising observation because carboxonium... 

Protonated amides and carboxylic acids have also been shown to activate adjacent electrophilic centers. Although protonated carboxylic acids and amides are not typically considered stable cationic groups, in superacidic media they can be readily generated and many have been observed by spectroscopic studies.16 As an example of electrophilic activation by a protonated carboxylic acid, P-phenylcinnamic acid (35) is diprotonated in super- acid to give the dication (36, observable by NMR) which then reacts with benzene and gives the indanone (37) in good yield (eq 12).17 It is known that the 1,1-diphenylethyl... 

Olah, G. A. Germain, A. Lin, H. C. Forsyth, D. Electrophilic reactions at Single Bonds. XVIII.1 Indication of Protosolvated de facto Substituting Agents in the Reactions of Alkanes with Acetylium and Nitronium Ions in Superacidic Media. J. Am. Chem. Soc. 1975, 97, 2928-2929. 

Zeolites are the main catalyst in the petrochemical industry. The importance of these aluminosilicates is due to their capacity to promote many important reactions. By analogy with superacid media (1), carbocations are believed to be key intermediates in these reactions. However, simple carbocationic species are seldom observed on the zeolite surface as persistent intermediates within the time-scale of spectroscopic techniques. Indeed, only some conjugated cyclic carbocations were observed as long living species, but covalent intermediates, namely alkyl-aluminumsilyl oxonium ions (2) (scheme 1), where the organic moiety is bonded to the zeolite structure, are usually thermodynamically more stable than the free carbocations (3,4). 

Many valuable reviews of the chemistry of these species are given in the new book Dicoordinated Carbocations An introduction by Grob " is followed by reviews of various theoretical studies of vinyl cations, their gas-phase chemistry, their generation by nuclear decay, and their NMR spectroscopic characterization. Vinyl cation production by addition to acetylenes and allenes, by solvolysis, and photolytically are covered, together with the chemistry of the species generated in these various ways. The next chapter deals with the synthetic applications of vinyl cations,and alkynyl and aryl cations are covered in the last chapter. A review of the NMR spectroscopic and quantum-chemical investigation of vinyl cations in superacid media (also of dienyl and 1-cyclopropylvinyl cations) is published separately,as is a review of alkynylcar-... 

Extensive nmr studies of the protonation of trihydroxybenzenes and their methyl ethers (Olah and Mo, 1972) and mono- and dihydroxybenzenes and their methyl ethers (Olah and Mo, 1973) in four different superacid media have recently been published. In the order of decreasing acidity, the media used were ... 

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