Carbenium ions, superelectrophilic

The reactivity of superelectrophile 46 precludes its direct observation by NMR, but as discussed in Chapter 2, stabilization of the carbenium ion center with para -methoxyphenyl groups allowed the dicationic species (5) to be observed at low temperature. 

In dication 188, the NBO charge at the carbenium ion center is +0.69 and at the acyl carbon is +1.09. The tert-butyl cation has been found to have NBO charge at its carbenium center of +0.67, suggesting a modest superelectrophilic activation of the carbenium ion center in 188, compared with the tert -butyl cation. When charges on the methyl groups are also considered, structure 188 is similar to the protosolvated tert -butyl cation 196. 

There are a number of other aryl-substituted carbodicationic systems that can be properly described as distonic superelectrophiles. For example, dication 45 has been generated from 2,2 -p-phenylenedi-2-propanol in SbFs at —78°C.4 When compared to the dimethyl(phenyl)carbenium ion (cumyl cation) 46, NMR data indicate that the positive charges are dispersed to a considerable extent into the neighboring methyl groups in the dication 45. 

The conversion is thought to involve formation of the carboxonium ion (77) by protonation of the carbonyl oxygen, and subsequent protonation then occurs at the C-H bond. The resulting carboxonium-carbonium dication (78) possesses the maximum possible charge-charge separation for this bicyclic framework. Subsequently, an intermediate carboxonium-carbenium dication (79) is produced, which isomerizes to the tertiary -carbenium ion, and deprotonation provides the product enone (80). Similar distonic superelectrophiles are proposed in other rearrangements of terpenes in superacid.28... 

The electrophilicity index also accounts for the electrophilic activation/deactivation effects promoted by EW and electron-releasing substituents even beyond the case of cycloaddition processes. These effects are assessed as responses at the active site of the molecules. The empirical Hammett-like relationships found between the global and local electrophilicity indexes and the reaction rate coefficients correctly account for the substrate selectivity in Friedel-Crafts reactions, the reactivity of carbenium ions, the hydrolysis of esters, the reactivity at the carbon-carbon double bonds in conjugated Michael additions, the philicity pattern of carbenes and the superelectrophilicity of nitronium, oxonium and carboxonium ions. This last application is a very promising area of application. The enhanced electrophilicity pattern in these series results from... 

An unusual class of gitonic superelectrophiles (having vicinal -dicationic structure) are the protosolvated alkylcarbenium ions (of carbenium-car-bonium dication nature). Several studies have shown that alkylcarbenium... 

It was previously noted that superelectrophilic carboxonium ions may be generated from suitable precursors, including amino-ketones, /V-heteroaromatic ketones and aldehydes, amino-acetals, and other substrates.45 In their superacid-promoted condensation reactions, these compounds often produce ammonium-carbenium superelectrophiles as intermediates in the reactions. As an example, the amino-acetal (229) reacts with arenes in the presence of superacid to give the arylated product (231, eq 72).43... 

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