Carbonium ions dications

Clear, unequivocal experimental evidence has by now been obtained for nonclassical ions such as the norbomyl cation.38 10 The bonding concept required to define nonclassical ions is simply to consider them as penta(or higher)-coordinated carbonium ions involving at least one two-electron three-center (or multicenter) bond, of which CH5+ (themethoniumion-carbonium ion) is the parent, asCH3+ (the methenium ion, methyl cation, carbenium ion) is the parent for trivalent carbenium ions. An example of a hexacoordinate carbonium ion is the pyramidal dication of Hogeveen.41... 

Reaction intermediates such as radicals, carbonium ions, carbanions, cation and anion radicals, dications and dianions can be produced. For example, six reversible redox reactions of 9,9 -bianthryl-10,10 -dicarbonitrile in 0.1 M Bu4NPFg/propio-nitrile solution yield a spectrum of products ranging from the tetra-anion to the dication... 

In most cases this dication is much less stable than the radical anion. It rapidly reacts with a nucleophile present in the solution giving rise to a Il-carbonium ion which is irreversibly reduced to the starting substance ... 

As predicted from the pKr+ values (Table 7.4) the S-di-p-methoxybenzhydryl group [18] is cleaved much more rapidly than other S-benzhydryl thioethers. Konig et al. [55] report complete removal in 2 hr with trifluoroacetic acid (70 ) or in 10 min with added anisole. Photaki et al. [58] report 80% removal in 2 hr (20-25°) with trifluoroacetic acid and 15% phenol. The pKr value of this cation predicts it to be more stable than the S-trityl cation which in turn su ests a more rapid conversion of the conjugate acid of the thioether to the carbonium ion and thiol and a selectivity comparable with trityl in the reaction with nucleophiles. This prediction is not entirely borne out by facts and indeed the S-di-p-methoxybenzhydryl group falls between the benzhydryl and trityl in reactivity. This lowered reactivity is probably due to the presence of significant concentrations of the less reactive dication. 

Several gitonic superelectrophiles have been reported having closely oriented oxonium and carboxonium ion centers, some of which may be considered 1,3-dications. A series of hydroxy-substituted carboxylic acids were studied in FSOsH-SbFs in solution and the oxonium-carbonium dications could be directly observed at low temperature.57 In the case of lactic acid, dication 147 is a persistent ion at — 80°C, but at temperatures above — 60°C, formation of the diprotonated lactide (148) is observed (eq 48). 

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... 

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