Dicationic Carbonyl Compound

Dicationic Carbonyl Compound In general, stable carbocations do not react with non activated benzene. In 1991, Shudo et al demonstrated that multiply positively charged (dicationic and tricationic) species have sufficient electrophilicity toward benzene. In the presence of super Bronsted acids (TfOH, TfOH-SbFs), Friedel-Crafts type reaction of benzene was carried out with cinnamaldehyde, its derivatives, and nitromethane (Scheme 2.13) [34]. 

In all cases, superelectorophilic dicationic intermediates3 5 were suggested to be involved in the activation of carbonyl compounds based on the observation that protonated /V-heterocycles significantly enhance the reactivity of adjacent carbo-cationic centers. For example, cyclohexanone and acetophenone are unreactive toward benzene in triflic acid, whereas 4-piperidones252 and acetylpyridines254 react readily. Likewise, 3-pyridinecarboxaldehyde is able to alkylate deactivated... 

As a further example, a,j8-unsaturated carbonyl compounds possess two sites which can undergo reduction the carbonyl group and the adjacent C—C-double bond. While an abundant hterature is available on the selective reduction of the carbonyl group, often achieved with relative ease, the selective hydrogenation of the double bond suffers from the lack of simple methods using practical chemicals. Selective ionic hydrogenation of ,j8-unsaturated carbonyl compounds with cyclohexane was previously known to proceed only in superacidic conditions due to the necessity of dicationic, superelec-trophilic activation of the enones. H-form zeolites with acidity well below superacidity are however also able to induce the reduction of o j8-unsaturated carbonyl compounds with cyclohexane, in analogy, or sometimes even better than the parent , superacid mediated reactions. ... 

The resistance to cleavage is an indication of the superelectrophilic character of dication 150c. Several aromatic compounds have likewise been shown to produce dicationic species upon the protonation of carboxyl and carbonyl functional groups. Other bis-carboxonium dications have been described involving protonation of carbonyl, amide, and other groups.50 These distonic superelectrophiles (152-153) have been shown to be useful in condensation reactions (eqs 52-53). 

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