Distonic superelectrophiles

Knorr cyclization of a range of iV-susbtituted butyramides in triflic acid yields 4-methyl-17/-quinolin-2-one derivatives704 [Eq. (5.257)]. Suggested intermediates of the transformation directly observed by low-temperature multinuclear NMR spectroscopy (HSO3F SbF5 S02C1E, 40°C) are distonic superelectrophiles formed by diprotonation of the two carbonyl oxygen atoms. 

Superelectrophilic intermediates have been categorized into two distinct groups the distonic (distant) and the gitonic (close) superelectrophiles (Table l).22 Distonic superelectrophiles are defined as electrophiles in... 

Charge-charge repulsive effects increase the importance of the resonance form (35b) having dione-type structure (a 1,4-dication and representing a distonic superelectrophile). Despite the importance of the charge separated structure 35b, the system is included here with other 1,2-ethylene dications and gitonic superelectrophiles. 

It might be argued that structures such as 37 and 39 should actually be described as distonic superelectrophiles, because three carbon atoms separate the protosolvated, 2e-3c bonds. However, as it is understood that some of the positive charge resides at the carbon atoms of the -CH4+ groups, the designations as gitonic superelectrophiles can be considered appropriate for 37 and 39. 

The two important resonance structures for the dication are the bis-oxonium structure (73a, a distonic superelectrophile) and the bis-carbenium structure (73b, a gitonic superelectrophile). Although it is understood that various factors should favor structure 73a (including... 

Larger systems (178) produce persistent diacyl ions that may be observed directly by NMR and IR spectroscopy and in some cases are considered distonic superelectrophiles (Chapter 7). Increasing distances between the acyl ion centers lead to structures with electrophilic reactivities similar to monocationic acyl ions. 

While structure 2 is an onium dication, it can not be considered a superelectrophile. Only if the electrophilic site(s) exhibit significantly increased reactivities due to interaction of the onium charge centers can the species be classified as distonic superelectrophiles. 

Several examples of superelectrophiles have already been described in which delocalization of charge may lead to structures that could be considered gitonic or distonic superelectrophiles, depending on which resonance form is being considered predominant. While the bis-oxonium structure (3a) is formally a distonic superelectrophile, the bis-carbenium structure (3c) is considered a gitonic superelectrophile (eq 2). These types of systems have been previously discussed and therefore will not be included in this chapter. In a similar respect, there are examples of equilibration between gitonic (4) and distonic (5) superelectrophilic systems (eq 3).2... 

Although there have been almost no systematic studies of these equilibria, it is expected that for many of these systems the distonic superelectrophile should be preferred in the equilibria, due to favorable separation of positive charge. Besides the number of atoms separating charge centers, another important consideration is the actual distance between charges. If a particular conformational or structural effect leads to charge centers being forced into closer proximity, then this may lead to superelectrophilic activation. Several examples of this effect are described in subsequent sections. 

Water elimination in the superacidic solution is a highly exothermic step, but nevertheless the 2,6-adamantadiyl dication 34 is not formed. This observation suggests that structures like 34 can be distonic superelectrophiles. As in the case of other 1,4- and 1,5-carbodications, the 2,6-adamantadiyl dications are stabilized and persistent when the carbenium centers bear an aryl substituted (vide infra). 

Other adamantane-based dications have also been prepared. Although systems such as diadamanta-4,9-diyl dication (35) and TT-bisadamanta-3,3 -diyl dication (36) have been prepared (both 1,6-dications), experimental data suggests that these systems are more related to the monopositive adamantyl cations rather than distonic superelectrophilic systems.14 However, the adamanta-l,3-dimethyldiyl dication (38) has been prepared in superacid media (eq 16).15... 

There are a number of other aryl-substituted carbodicationic systems that can be properly described as distonic superelectrophiles. For example, dication 45 has been generated from 2,2 -p-phenylenedi-2-propanol in SbFs at —78°C.4 When compared to the dimethyl(phenyl)carbenium ion (cumyl cation) 46, NMR data indicate that the positive charges are dispersed to a considerable extent into the neighboring methyl groups in the dication 45. 

Other distonic superelectrophiles arise from aryl-substituted carbodicationic systems in which the positive charge centers are forced into close... 

Oxidation leads to formation of the new a -bond and aryl-stabilized car-bocationic centers in 62. Interestingly, there is no evidence of proton loss from the dihydro[5]helicene dication 62 and the dication is stable in the presence of the reasonably nucleophilic counter ion, I3-. This again suggests that these stabilized systems are considered only as weakly distonic superelectrophiles. 

There is also the possibility of distonic, superelectrophilic bis-carbo-nium ions. Despite the fact that such species may be important in the superacid-catalyzed cracking reactions of aliphatic hydrocarbons, there have been very few studies of such systems. The structures and energies of small distonic alkonium dications have been studied using ab initio calculations.24 For diprotonated -butane (C4Hi22+) two structures were located as stable minima on the potential energy surface. Structure 63 is formed by a protonation of the two terminal C-H bonds, resulting in a pair of two electron-three center bonds. The other structure (64) arises from protonation of the terminal C-H bond and the most distant C-C bond. 

Another class of distonic superelectrophiles are the carbenium-carbo-nium dications. As discussed, Olah and co-workers found experimental and theoretical evidence for the protosolvated tert-butyl dication ([(CH3) CCH4]2+) and 2-propyl dication ([CH3CHCH4]2"1"), both gitonic superelectrophiles (vide supra).25 However, analogous distonic superelectro-philic systems like 65 and 66 have not yet been studied. 

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. 

There have been a wide variety of carbo-carboxonium dications described in the literature. Some of the related distonic superelectrophiles can be used for remote functionalization of appropriate substrates. For... 

In chemistry involving conversion of camphor, two distonic superelectrophiles are proposed.27 When ketone 76 is reacted with HF-SbFs, the enone 80 is produced (eq 25). 

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... 

In the presence of superelectrophilic trihalomethyl cation, the carboca-tionic center is formed by hydride abstraction generating the distonic superelectrophile (81). Capture of the superelectrophile with fluoride leads to the fluorinated product (82). 

Carbo-carboxonium dications have also been generated by the direct ionization of appropriate functional groups by the action of Brpnsted superacids. For example, unsaturated acids are shown to give the reactive distonic superelectrophiles, which are shown to be moderately reactive.31... 

Protonation of the carboxyl and olefinic groups give the distonic superelectrophiles (86 and 88 eqs 30-31). 

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