Carboxonium-ammonium dications

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. 

Among other distonic superelectrophiles described in the literature, there are carbo-onium dications. These include carbo-carboxonium dications, carbo-ammonium dications, and related ions. Despite the separation of charge in these superelectrophiles, some have been shown to have very high electrophilic reactivities. I. ike the carbodications described previously, the discussion here is limited to those systems that have been shown to have electrophilic reactivities greater than the related monocationic onium ions, as well as structural criteria supporting their designation as a distonic superelectrophilic species. 

A number of studies have reported the formation of carboxonium-centered distonic dications. These systems include diprotonated diketones, diproto-nated dicarboxylic acids, their derivatives, and others. A wide variety of mixed dications have also been reported such as ammonium-carboxonium dications, oxonium-carboxonium dications, acyl-carboxonium dications, and other species. Many of these carboxonium-centered dications exhibit properties indicative of distonic superelectrophiles. 

Reaction of acetal 104 with benzene in the presence of CF3SO3H leads to product 107 in high yield. This conversion involves formation of the ammonium-carboxonium dication (105), a reactive dication possessing some 1,3-dicationic character. Reaction with benzene and subsequent loss of methanol generates another reactive dication (106), which then gives the product. The superelectrophilic character of the ammonium-carboxonium dications is indicated by their reactions with moderately deactivated arenes, such as o-dichlorobenzene. 

Initial ionization gives an ammonium-carboxonium dication, which then produces the ammonium-carbenium dication (230). A variety of dicationic electrophiles like 230 have been proposed. 

Among the reported distonic superelectrophiles, a significant number of ammonium-carboxonium dications and related species have been studied. It has been shown that these electrophiles show enhanced reactivities compared with monocationic carboxonium ions. For example, 4-piperidone (172) is diprotonated in superacidic CF3SO3H to give the distonic superelectrophile (173), which condenses with benzene in high yield (eq 59).56 In contrast, cyclohexanone forms the monocationic carboxonium (174) ion, but ion 174 is not sufficiently electrophilic to react with benzene (eq 60). 

The observed electrophilic reactivity is indicative of superelectrophilic activation in the dication 173. Other ammonium-carboxonium dications have also been reported in the literature, some of which have been shown to react with benzene or other weak nucleophiles (Table 4).1 42b 57-60 Besides ammonium-carboxonium dications (175-179), a variety of N-heteroaromatic systems (180-185) have been reported. Several of the dicationic species have been directly observed by low-temperature NMR, including 176, 178-180, 183, and 185. Both acidic (175, 180-185) and non-acidic carboxonium (176-177) dicationic systems have been shown to possess superelectrophilic reactivity. The quinonemethide-type dication (178) arises from the important biomolecule adrenaline upon reaction in superacid (entry 4). The failure of dication 178 to react with aromatic compounds (like benzene) suggests only a modest amount of superelectrophilic activation. An interesting study was done with aminobutyric acid... 

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