Carboxonium- carbenium 1,3 -dications

Carboxonium-Carbenium Dications The protonation of mesityl oxide (41) has been studied by several groups.19 It has been proposed that mesityl oxide can form equilibria in superacid with the 0,0-diprotonated species (42) and (at more elevated temperatures) with the C,0 -diprotonated species (40, eq 12).19a... 

Carboxonium-carbenium dications have also been proposed in the reactions of aryl ethers, phenols, and naphthols, with superacids. When 2-naphthol is reacted with an excess of AICI3 (3 equivalents) and cyclohexane, the product of ionic hydrogenation is observed in 59% yield (eq 15).24 This conversion is thought to occur by double protonation of the 2-naphthol ring to give the dication 48, which is capable of abstracting hydride from cyclohexane. Similar intermediates are formed by the reactions of HF-SbFs with naphthyl ethers (eq 16).25... 

A variety of 1,3-dicationic carboxonium superelectrophiles have been generated from protonation of carboxylic acids and their derivatives. For example, carboxonium-carbenium dications have been proposed in the superacid promoted reaction of cinnamic acid (50) and related... 

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... 

Another type of carboxonium-carbenium dication is obtained from remote functionalization of alkylamides (eq 26).29... 

A number of steroidal systems have also been shown to generate carboxonium-carbenium dications in superacid. These intermediates are known to lead to novel products and their chemistry has been recently reviewed.28d... 

As mentioned, 2-oxazolines may form a ring-opened distonic superelectrophile in reactions in superacid. These carboxonium-carbenium dications are capable of reacting with benzene and moderately deactivated substrates.2 For example, the optically active oxazoline (94) reacts in CF3SO3H to generate the chiral dication (95) and this superelectrophilic species is capable of reacting with o -dichlorobenzene in fair to modest yield and diastereoselectivity (eq 35). 

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. 

Besides the fluorene cyclization, the 1,2-ethylene dications are known to undergo another type of electrocyclization reaction to produce the phenan-threne ring system (see eqs 9 and 11 and Scheme 3).32a Although derived carbenium-carboxonium dications (70) will give the phenanthrene-type products (albeit in low yield eq 15), the bis-carboxonium dication 71 does not. Moreover, the fluorene cyclization occurs readily in 1,1-dihydroxylie systems (eq 10). 

Calculations at the 4-31G level of theory found that the 1,2-bis-carboxonium ion (72) is significantly more stable than the carbenium-carboxonium dication (73). Thus, when compared with the 1,1-dihydroxylie structure 73, the 1,2-dihydroxy lie structure 72 is more effective at stabilizing the 1,2-ethylene dication. This effect leads to the stabilization of dication 71 (eq 16), and not to phenanthrene cyclization. 

Following electrophilic attack on benzene, the carbenium-carboxonium dication 94 is generated, which then gives the product 95. There have been several studies of the chemistry of malonic acid (80) and its esters in superacidic media. It has been shown that diprotonated products (i.e., 81) are formed (Table 1, entry 5).36... 

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