Dications 1,5-distonic

A similar situation is observed in the case of the cation-radical derived from dimethylhexa-phenyl diphosphafnlveninm dication as a resnlt of redaction of the latter by the sodinm salt of the naphthalene anion-radical (Biaso et al. 2006). According to ESR, x-ray experimental data, and results of DFT calculations, the diphosphafnlveninm cation-radical detains the excess electron density within the exocyclic double bond (see Scheme 3.41). Stabilization is gained by conjngation with the two pentavalent phosphorus atoms. Biaso et al. (2006) rationalize the electronic strnctnre throngh two valence-isomeric fluidic forms of the distonic type. 

On the potential energy surface of diprotonated propane (C3H102+), three structures were located as minima (MP2/6-31G level).788 The 1 -//,2-//-diprotonated and 1-/7,3-//-diprotonated forms (458 and 459) are of Cj and C2v symmetry, respectively, whereas the l-//,C-diprotonated structure (460) has C symmetry. Structure 459, a distonic dication with the two charge-bearing centers separated by one carbon, is significantly more stable (by 22.6 kcal mol-1) than structure 458 (a gitonic dication with adjacent charges) and also more stable than structure 460 (by 6.9 kcal mol ). 

Diprotonated ra-butane and isobutane cations (C4H122+) have been computed by Olah et al.788 Two distonic dications [the 1 -//,4-//-di protonated (476) and the terminal C—H and C—C diprotonated (477) forms] were found to have energy minima (MP2/6-31G level), with structure 476 being more stable by only 1.3kcalmol 1. 

For diprotonated isobutane (C4H122+) the structures found as stable minima are analogous to those computed for diprotonated propane (C3H102+). Again, structure 478 (a distonic dication) is 17.7 kcal mol 1 more stable than structure 479 (a gitonic dication), whereas structure 480 is only slightly less stable than 478 (2.6 kcal mol-1).788... 

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. 

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... 

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. 

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. 

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... 

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... 

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... 

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). 

This provides maximum charge-charge separation in the dication. Thus, it is expected that analogous distonic superelectrophiles will likewise tend to undergo protosolvation at the carbon(s) most distant from the acyl cation center. 

Related ammonium-carbenium distonic superelectrophiles (1,4-dications, 103-104) have been shown to possess strong electrophilic reach vies, undergoing arylation with benzene and dichlorobenzene (eqs 40-41).38... 

The formation of distonic superelectrophile 104 provides a high-yield route to the antispasmodic dmg, fenpiprane 105. The 1,4-dication (108), 1,5-dication (106) and 1,6-dication (107) have likewise been shown to react with benzene in high yields.39 The superelectrophilic vinyl-dications (109-110) have also been studied.40,41... 

A number of related distonic superelectrophiles have been generated from A-heteroaromatic compounds (Table 2). Vinyl-dications (111-112) have been produced from the ethynyl pyridines,40 while /V-alkenyl A-heterocycles provide dications (113-115).42a Vinyl-substituted A-heterocycles provide access to distonic superelectrophiles such as dication 116.39 Dications 117 and 118 are generated from their precursors, and both intermediates lead to efficient cyclization reactions with the adjacent... 

Examination of the dication pairs 122/123 and 125/126 may also suggest a further useful definition of the distonic superelectrophiles the distonic superelectrophiles may be distinguished from onium dications that are without superelectrophilic activation (i.e., two isolated cationic centers) by comparison of the energies, reactivities, or structural/electronic criteria, of two closely related species. For example, it is expected that ammonium-carbenium dications 127 and 128 should vary considerably in terms of energy, due to the proximity of the charges. 

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