Protolytic ionization

Acid-catalyzed isomerization reactions of alkanes as well as alkylation and condensation reactions are initiated by protolytic ionization. Available evidence indicates nonlinear but not necessarily triangular... 

The reverse reaction of the protolytic ionization of hydrocarbons to carbocations, that is, the reaction of trivalent carbocations with molecular hydrogen giving their parent hydrocarbons, involves the same five-coordinate carbonium ions. 

In the acid-catalyzed isomerization of straight-chain alkanes to higher-octane branched ones, after initial protolytic ionization, alkyl and hydrogen shifts in the formed carbocations lead to the most branched and therefore thermodynamically preferred, generally tertiary, carbocations. Intermolecular hydrogen transfer from excess alkane then produces the isomeric isoalkane with the formed new carboca-tion reentering the reaction cycle. 

Superacids were shown to have the ability to effect the protolytic ionization of a bonds to form carbocations even in the presence of benzene.190 The formed car-bocations then alkylate benzene to form alkylbenzenes. The alkylation reaction of benzene with Ci—C5 alkanes (methane, ethane, propane, butane, isobutane, isopentane) are accompanied by the usual acid-catalyzed side reactions (isomerization, disproportionation). Oxidative removal of hydrogen by SbF5 is the driving force of the reaction ... 

Consequently, the reverse reaction of protolytic ionization of hydrocarbons to carbenium ions—that is, the reduction of carbenium ion by molecular hydrogen — can be considered as alkylation of H2 by the electrophilic carbenium ion through a pentacoordinate carbonium ion. Indeed, Hogeveen and Bickel have experimentally proved this point by reacting stable alkyl cations in superacids with molecular hydrogen [Eq. (5.7)]. 

The same phenomenon was observed61 with propane (Figure 5.6) and isopentane (Figure 5.7) above 20 mol% of SbF5, reversible protonation and protolytic ionization decrease rapidly whereas the conversion of the alkane with concomitant reduction of SbF5 increases. H-D exchange data observed in small alkanes are collected in Table 5.1. 

The first evidence for the protonation of alkanes under highly acidic (superacid) conditions was independently reported by Olah and Lukas, as well as by Hogeveen and coworkers. " Protolytic reactions of hydrocarbons in superacid media were interpreted by Olah as an indication of the general electrophilic reactivity of covalent C-H and C-C single bonds of alkanes and cycloalkanes. The reactivity is due to the donor ability of the a-bond electron pairs via a three-center, two-electron (3c-2e) bond formation and follows the trend tertiary C-H > C-C > secondary C-H primary C-H. The transition state for protolytic ionization of hydrocarbons was presumed to be linear. It was later suggested that such 3c-2e interactions in carbocations generally tend to be nonlinear (even in stericaUy crowded cases) in nature (see also Chapter 1) [Eqs. (6.3) and (6.4)]. That is, such interactions are similar to transition states proposed for frontside Se2 reactions. 

H-D exchange in HF-SbFs via hypercoordinate isotopic methonium ions [Eq. (6.5)] without any detectable side reactions (see Chapter 5, Section 5.4.1.1). Exchange involving protolytic ionization via CH + HD is improbable in the case of methane, because of the unfavorable, highly energetic nature of the primary methyl cation. However, in higher homologous alkanes protolytic ionization takes place with ease. 

The direct reduction of SbFs in the absence of hydrocarbon by molecular hydrogen necessitates, however, more forcing conditions (50 atm, high temperature), which suggests that the protolytic ionization of alkanes proceeds probably via solvation of the pentacoordinate carbocation by SbFs and concurrent ionization-reduction [Eq. (6.13)]. 

Olah and coworkers also prepared the parent dodecahedryl cation 29 under superacid conditions [Eq. (6.20)]. Upon standing in the superacid medium at -50°C, it slowly and irreversibly transformed into 1,16-dodecahedryl dication 31. The formation of dication 31 can be rationalized by protolytic ionization of the C-H bond at C(16) farthest from the cation center presumably involving hypercarbon intermediate 30. 

The superacid-catalyzed alkylation of benzene with alkanes was also achieved. Alkyl cation formation for the required electrophilic attack again involves protolytic ionization of alkanes via pentacoordinate carbocations [Eq. (6.37)]. Sperenza and coworkers,on the other hand, have shown that phenyl cations generated in the gas phase readily insert into the C-H bonds of simple alkanes to provide the corresponding alkylated aromatics [Eq. (6.38)]. 

J. Sommer, J. Bukala, M. Hachoumy, R. lost, Reversible protonation of isobutane in liquid superacids in competition with protolytic ionization, J. Am. Chem. Soc., 1997, 119, 3274-3279. 

Protolytic ionization of methylcyclopentane gives an equilibrium mixture of the tertiary 1-methyl-1-cyclopentyl cation (29) (more stable by about 40 kJ/mol) and the secondary cyclohe l cation (32) (Scheme 5). At low temperature, irreversible reaction of 29 with CO leads to ion 30, which, after reaction writh ethanol, gives the 31 ester. Product composition, in this case, reflects the difference in stability of the intermediate carbocations. Since the carbonylation step is reversible at higher temperature, and carbocation 32 has a much higher affinity for CO, the concentration of 33 in solution continuously increases to yield, after quenching with ethanol, the 34 ester. This is an example of a kinetically controlled product formation through a thermodynamically unfavorable intermediate. 

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