Dicationic carboxonium ions

The proposed dicationic, carboxonium ion intermediates (58-62) have been directly observed by 13C NMR. A more detailed description of this superelectrophilic chemistry is found in chapters 5-7. 

Vicinal 1,2-carbodicationic systems are some of the most important and thoroughly studied superelectrophiles. They include inter alia 1,2-ethylene dications, related carbon-nitrogen superelectrophiles (diprotonated imines and nitriles), protosolvated carboxonium ions, and superelectrophilic tri-halomethyl cations. It is understood that many of these systems involve extensive charge delocalization, and in a sense may not be considered formal 1,2-dicationic systems. For example, diprotonated 2,3-butanedione (34) may be represented formally as a 1,2-ethylene dication (35a, a gitonic superelectrophile), but even in monocationic carboxonium ions, there is a significant amount of double-bond character retained in the carbon-oxygen bond (eq 5).22... 

A series of protonated napthalenediols have also been studied and in some cases the bis-carboxonium dications can be observed by low temperature NMR from HF-SbFj solution.25 In the case of 1,3-napthalenediol (88), the bis-carboxonium ion (89) is formed having the 1,3-dicationic character (Table 1, entry 9). 

Carboxonium-Ammonium and Related Dications A wide variety of species have been generated in which the 1,3-dicationic structure arises from carboxonium ion centers being adjacent (separated by one carbon) to an ammonium or related charge center. These intermediates may be described as reactive dications, yet they have been shown to exhibit electrophilic reactivities comparable to superelectrophiles. 

A series of phosphonium-carboxonium dications have also been studied in superacid catalyzed reactions.313 When the dicationic electrophiles are compared with similar monocationic species, it is clear that the phos-phonium group enhances the electrophilic character of the carboxonium center. For example, protonated acetone is incapable of reacting with benzene in condensation reactions, however, the phosphonium-substituted carboxonium ion (124) reacts in high yield (eq 40). 

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. 

There have been several distonic superelectrophiles described in the literature that are oxonium-centered dications. For example, a series of diols were solvated in FSOsH-SbFs-SC and the dicationic species were persistent at —80°C.77 Upon warming to 25°C, the bis-oxonium ions undergo rearrangement to the more stable carboxonium monocations (eq 77). Barring a concerted mechanism, the transformations are thought to involve the carbo-oxonium dications (i.e., 225) and concomitant hydride shifts. Interestingly, 2,5-hexanediol ionizes in superacid, and with warming the cyclic oxonium ion (229) is formed (eq 78). 

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