Carbocations dicationic

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. 

Much of our research has involved the use of dicationic electrophiles in reactions with very weak nucleophiles, such as non-activated arenes and alkanes. By comparison to similar monocationic electrophiles, we have been able to show the extent of electrophilic activation by adjacent cationic centers. For example, carbocations show an increased reactivity with a nearby cationic charge (eqs 3-4).9 When 1,1-diphenyletheneis reacted with superacidic CF3SO3H... 

The success of carbocation chemistry lies in the stabilization of carbocations in a medium of low nucleophilicity. Superelectrophiles, in turn, are reactive intermediates generated by further protonation (protosolvation). This second protonation increases electron deficiency, induces destabilization, and, consequently, results in a profound increase in reactivity. In particular, charge-charge repulsive interactions6 play a crucial role in the enhanced reactivity of dicationic and tricationic superelectrophilic intermediates. As various examples of acidity dependence studies show, without an appropriate acidity level, transformations may occur at much lower rate or even do not take place at all. In addition to numerous examples of superacid catalyzed reactions, various organic transformations, in which the involvement of superelectrophilic intermediates is invoked or superelectrophiles are de facto observed in the condensed state, are also included in this chapter. 

In a study involving the superacid-catalyzed reaction of amino-alcohols, a chiral, dicationic electrophile was observed by low temperature 13C NMR.31 Ionization of benzylic alcohols in superacids can generate stable carbocations, such as the trityl cation. Because of the resonance stabilization of the carbocationic centers, they are fairly weak electrophiles, incapable of reacting with benzene (eq 31). However, it was shown that adjacent ammonium groups can increase the electrophilic reactivities of the diphenylethyl cations (eq 32). 

Olah, G. A., Diume, K., Kelly, D. P., and Mo, Y. K. (1972). Stable Carbocations. CXXIX. Mechanism of the benzidine and Wallaeh rearrangements based on direct observation of a dicationic reaction intermediate and related model compounds. J. Am. Ghent. Snc. 94, 7438-7447. 

Phenylethyl-substituted pyridinecarboxaldehydes (69) were shown to generate dicationic electrophiles such as the dicationic carboxonium ion (70)." ° These undergo cyclization to the dicationic carbocation (71), which may be trapped by arenes or water. The chemistry provides a useful route to triarylmethanes or 10,ll-dihydro-5//-benzo[4,5]cyclohepta[l,2-( ]pyridin-5-ones. 

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]. 

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