Superacidic condition

Not only protolytic reactions but also a whole range of varied elee-trophilic reactions can be carried out on alkanes under superacidic conditions. 

On the other hand, under superacidic conditions, alkanes are readily alkylated via front-side CJ-iasertion by carbocationic alkylating agents. The direct alkylation of the tertiary C—H CJ-bond of isobutylene with isobutane has been demonstrated (71). The stericaHy unfavorable reaction of tert-huty fluoroantimonate with isobutane gave a Cg fraction, 2% of which was 2,2,3,3-tetramethylbutane ... 

Friedel-Crafts acylation using nittiles (other than HCN) and HCI is an extension of the Gattermann reaction, and is called the Houben-Hoesch reaction (120—122). These reactions give ketones and are usually appHcable to only activated aromatics, such as phenols and phenoHc ethers. The protonated nittile, ie, the nitrilium ion, acts as the electrophilic species in these reactions. Nonactivated ben2ene can also be acylated with the nittiles under superacidic conditions 95% trifluoromethanesulfonic acid containing 5% SbF (Hg > —18) (119). A dicationic diprotonated nittile intermediate was suggested for these reactions, based on the fact that the reactions do not proceed under less acidic conditions. The significance of dicationic superelectrophiles in Friedel-Crafts reactions has been discussed (123,124). 

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)... 

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. 

Alkylideneallyl cations can be described as resonance hybrids of 1-vinyl-substituted vinyl cations and allenylmethyl cations, and thus contain two reactive sites (the sp- and sp2-hybridized carbons) for nucleophilic addition (Scheme 1) (7,2). Hybridization affects the electronic and steric character of these reaction sites. The electronic property was deduced from the l3C NMR chemical shifts of alkylideneallyl cations measured under superacidic conditions (3) and also from the charge distribution calculated (4). The charge distributions are affected by substituents on the cation the sp2 carbon is more positive than the sp carbon when two methyl groups are introduced at the sp2 carbon. 

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. 

Scheme 1 Analogy between the observable dehydrocyclization of 1 under low temperature superacid conditions, and the possible cyclization of a 2-octyl cation in catalytic dehydrocyclizations at 400-500 C. In the p-H-bridged cations the positive charge is delocalized, so the position of the (+) simply indicates the total charge in these species.

Two chapters in this volume describe the generation of carbocations and the characterization of their structure and reactivity in strikingly different milieu. The study of the reactions in water of persistent carbocations generated from aromatic and heteroaromatic compounds has long provided useful models for the reactions of DNA with reactive electrophiles. The chapter by Laali and Borosky on the formation of stable carbocations and onium ions in water describes correlations between structure-reactivity relationships, obtained from wholly chemical studies on these carbocations, and the carcinogenic potency of these carbocations. The landmark studies to characterize reactive carbocations under stable superacidic conditions led to the award of the 1994 Nobel Prize in Chemistry to George Olah. The chapter by Reddy and Prakash describes the creative extension of this earlier work to the study of extremely unstable carbodications under conditions where they show long lifetimes. The chapter provides a lucid description of modern experimental methods to characterize these unusual reactive intermediates and of ab initio calculations to model the results of experimental work. 

Fluorination of cinchona alkaloids has also been investigated. For instance, fluorination of quinine acetate under similar superacidic conditions (HF—SbFs/CHCls) affords a mixture of difluorocompounds in the 10 position that are ephners in 3 (60% yield, 1 1 ratio). This reaction involves a mechanism similar to the one described earlier (protonation, isomerization of carbenium ions, and Cl— F exchange). Curiously, when the reaction is performed on quinine itself, fluorination does not occur and an unprecedented rearrangement takes place (Figure 4.51). ... 

Benzylic-type cations derived from PAHs have been studied under superacid conditions, where, not surprisingly, they are relatively stable.Lifetimes in water of diastereomeric forms of the benzo[a]pyrene derivative (100) have been deter-... 

The unusual wiefa-bromination of phenols and their methyl ethers [9] under superacid conditions is quite expected because O-protonation inverts polarity of all the nuclear carbon atoms. This reaction is reminiscent of the better known electrophilic substitutions of aniline in acidic media. 

In alkane-alkene alkylation systems it is always the Jt-donor alkene that is alkylated by carbocations formed in the system. In the absence of excess alkenes (i.e., under superacidic conditions), however, the cr-donor alkanes themselves are alkylated. Even methane or ethane, when used in excess, are alkylated by ethylene to give propane and n-butane, respectively ... 

Oxidative carbocationic condensation of methane under superacidic conditions was first achieved in the late 1960s. Olah and coworkers352-355 observed that when methane was introduced into SbF5-containing superacids, C4 and higher alkyl cations were obtained. As the initial reaction [Eq. (3.41)] is endothermic, it became apparent that the superacid oxidatively helped to remove H2 (as HF) ... 

Benzaldehyde reacts readily with benzene under superacid conditions to give tri-phenylmethane in high yields.427,428 The reaction is slow in triflic acid, giving a yield of 90% in 24 h, whereas with the strong superacid CF3SO3H2 B(0S02CF3)4 the product is formed instantaneously in 83% yield.427 Experimental evidence supports the involvement of diprotonated benzaldehyde in the reaction [Eq. (5.76)]. Theoretical calculations show that the 44 0,C(aromatic)-diprotonated cation is the reactive intermediate and not the 45 0,0-diprotonated cation ... 

Alcohols instead of olefins can also be used in these reactions. Although secondary and tertiary alcohols react preferentially in the presence of H2S04,112 SbCl5—S02,113 and acids with Cu(I) or Ag(I) ions,105 107 108 114 even methanol was shown under superacidic conditions to give acetic acid.115... 

The preparation of acetic acid represents a special case. Olah and coworkers as well as Hogeveen and coworkers have demonstrated that CO can react with methane under superacidic conditions, giving the acetyl cation and by subsequent quenching acetic acid or its derivatives (see Section 7.2.3). Monosubstituted methanes, such as methyl alcohol (or dimethyl ether), can be carbonylated to acetic acid.115 Similarly, methyl halides undergo acid-catalyzed carbonylation.115,116 Whereas the acid-catalyzed reactions can be considered as analogs of the Koch reaction, an efficient Rh-catalyzed carbonylation of methyl alcohol in the presence of iodine (thus in situ forming methyl iodide) was developed by Monsanto and became the dominant industrial process (see Section 7.2.4). 

Even methane, the least reactive alkane, was shown to undergo carboxylation under superacidic conditions.115,176 The formation of carboxylated products (acetic acid and methyl acetate) from methane was first observed by Hogeveen and coworkers by trapping methyl cation formed in SbF5 (60°C, 50 atm CO pressure) followed by quenching with H20 or MeOH. The intermediate methylcarboxonium ion (CH3CO+) and CH3CH2CO+ formed in a similar reaction of ethane were identified by NMR spectroscopy.176,177... 

The phenyl cation (134) firstpostulated by Waters335 is a highly reactive species oflow stability and plays a fundamental role in organic chemistry—for example, in the chemistry of diazonium ions. According to gas-phase studies and calculations, its stability is between that of the ethyl cation and the vinyl cation.336 Since it is an extremely electrophilic and short-lived species, it could not be isolated or observed directly in the condensed phase. For example, solvolytic and dediazoniation studies under superacidic conditions by Faali et al.337,338 failed to find evidence of the intermediacy of the phenyl cation. Hyperconjugative stabilization via orf/zo-Me3Si or... 

Lammertsma412 has also obtained dication 198 by the protonation of hexahydro-pyrene in Magic Acid solution. This is the first example of a p,p -diprotonated naphthalene derivative. Koptyug and co-workers270,413 have studied a,a -diproto-nated hexa- and octamethylnaphthalenes 199 under superacidic conditions. 

Cyclooctatetraene dications 225407 4m and benzocyclobutadiene dications 226471 472 are well studied. The parent cyclooctatetraene dication is still elusive, despite repeated attempts to prepare it under a variety of superacid conditions. 

The generation of the stable cyclopropenylcarbinyl cation 268 under superacid conditions is an exceptional example562 [Eq. (3.63)]. It appears that in this case the bulky terf-butyl group prevents the usually fast rearrangement of cyclopropenylcarbinyl cations to the corresponding homocyclopropenylium ions. 

Cubylcarboxonium ions have been also studied by Prakash, Olah, and co-workers.579,580 The parent cation 281 prepared under superacid conditions was stable at low temperature but decomposed to cubylacylium cation 282 as a result of further protonation and dehydration [Eq. (3.71)]. In addition to cation 281, di- and tetra-carboxonium ions and the corresponding protonated methyl esters were also observed as long-lived species stable under superacidic conditions. Experimental evidence and theoretical data indicated that the strained cubyl system effectively stabilizes the carbocationic centers through C—C bond hyperconjugation (283). On the basis of 13C data, three conformers of protonated dimethyl cubane-l,4-dicarboxylate (284-286) could be identified. 

A great number of trialkyl(aryl) selenonium and telluronium ions are known, and their synthesis does not require the use of strong electrophilic alkylating or arylating agents.268,269 The synthesis and transformations of triorganotellurium ions have been treated in recent reviews.270,271 However, acidic selenonium and telluronium ions can be obtained only under superacidic conditions. 

Levisalles and co-workers466 and Wentrup and Dahn467 studied enoldiazonium ions generated under superacidic conditions. More recently Laali and coworkers468 have shown that the stability of enoldiazonium ions can be greatly enhanced by an a-silyl functionality. Equation (4.137) shows the characteristic diazonium ion products. 

Other Substituted Diazonium Ions. A series of aminodiazonium ions have been prepared under superacidic conditions [Eq. (4.141)]. Schmidt495 described the preparation and IR spectra of protonated hydrazoic acid 212 and methylazide as their hexachloroantimonate salts. Olah and co-workers496 have carried out a comprehensive study on aminodiazonium ions (protonated azides) by H, 13C, and 15N NMR spectroscopy. Even the electrophilic aminating ability of aromatics of 212 has been explored.496 The tetrachloroaluminate salt of 212 has also been prepared496... 

Generation of the hydroxydiazonium ion HON2+ as long-lived species under superacidic conditions was not successful.507 According to an early theoretical study, the ion has a structure similar to that of methoxydiazonium ion 218. Recent high-level calculations442 [MP2(FU)/6-31G level] have found the O-protonated structure of Cs symmetry to be the global minimum but it is only 2.5 kcal mol-1 more stable than A-protonated nitrous oxide. Of the dications formed by a second protonation, the 0,A-diprotonated nitrous oxide is more stable than the O, (9-diprotonated nitrous oxide by 7.5 kcal mol... 

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