Ethylene dication

Tetrakis(dimethylamino)ethylene dication 224 Acetyl dication 224... 

The elusive ethylene dication (C2H4) has been observed in the gas phase, and its potential surface has been well characterized by ab initio theory at MP3/6-31G /6-31G level. The perpendicular conformer (D2d) has been shown to be lower in energy than the planar conformation (D2h) by an unusually large value, 28.1 kcal/mol. The other possible isomeric carbodications also have very low-activation barriers for the conversion into the perpendicular conformer. ... 

A range of substituted ethylene dications have also been calculated by ab initio... 

A variety of dications have recently been prepared and characterized by X-ray diffraction. The tetrakis(dimethylamino)ethylene dication 402 has been generated by... 

The novel four-center two-electron delocalized o-bishomoaromatic species 593, 594,599,601a, and 603 are representatives of a new class of 2jt-aromatic pericyclic systems. These may be considered as the transition state of the Woodward-Hoffmann allowed cycloaddition of ethylene to ethylene dication or dimerization of two ethylene radical cations985 (Figure 3.25, 604). Delocalization takes place among the orbitals in the plane of the conjugated system, which is in sharp contrast to cyclobutadiene dication 605 having a conventional p-type delocalized electron structure (Figure 3.25). 

There have been many studies related to tetraaryl-1,2-ethylene dications, formally gitonic superelectrophiles, which show extensive charge delocalization into the aryl rings (Scheme 1). Low temperature 13C NMR studies have shown that the para carbons are deshielded, from < 13C... 

Figure 1. Ethylene dication (1) and calculated NBO charges for the tort-butyl cation (2) and the proto-tort-butyl cation (3).

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... 

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. 

Carbon-Carbon Vicinal-Dications Ethylene dications are a major group of carbon-centered superelectrophilic systems. The parent ethylene dication (CH2CH22+, 1) has been studied experimentally in the gas-phase, as well as in several theoretical studies.1 Two electron... 

Table 2. Generation of aryl-substituted 1,2-ethylene dications.

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. 

Other oxygen stabilized ethylene dications have been prepared and studied using stable ion conditions. For example, Olah and White were able to observe as early as 1967 diprotonated oxalic acid (74) in FSOsH-SbFs solutions.33 This species and related dications were also studied subsequently by theoretical methods. Diprotonated a-keto acids and esters have likewise been generated in superacidic media, such as 75-77 34... 

Similar oxygen stabilized ethylene dications were proposed in several types of superacid-catalyzed condensation reactions involving 1,2-dicarbonyl compounds. For example, 2,3-butanedione condenses in high yield with benzene and the superelectrophile (35) is considered to be the key intermediate, because the monoprotonated species (78) is not sufficiently electrophilic to react with benzene (eq 17).35 Several biologically important a-ketoacids were also found to generate superelectrophiles... 

Another ethylene dication with significant nitrogen stabilization is the tetrakis(dimethylamino)ethylene dication (88). By reaction of the sub-stituted-ethylene with CI2 or Br2 (eq 23),... 

Triflic acid-catalyzed Friedel-Crafts acylation reactions of aromatics with methyl benzoate give benzophenone products in good to excellent yields (eq 33). To explain the high level of electrophilic reactivity of this system, protosolvated species are proposed as possible intermediates (eq 32). In the triflic acid-catalyzed cyclization of some ethylene dications, protonation of the ester group is thought to be a key activation step. Reaction of a-(methoxycarbonyl)diphenylmethanol with TfOH gives the fluorene product in 94% yield (eq 34). ... 

Of course, no it-bond can be formed without electrons in the it-orbitals and, as expected, the planar conformation of ethylene dication has a relatively long 1.587A bond. In a sharp contrast to the neutral ethylene, the ethylene dication is stabilized by 28kcal/mol in the perpendicular geometry, formed upon 90° rotation around the C-C bond. In the more stable twisted geometry, the C-C bond is relatively short (1.398 A,... 

Kolomeitsev and Roschenthaler [40] produced tetrakis(dimethylamino)ethylene dication stabilized perfluoroaUcyl tliiolates via reduction of bis(perfluoroalkyl)disulfides... 

Scheme 11 Tetrakis(dimethylamino)ethylene dication trifluoromethyl thiolate as a stable reagent for substitution of aromatic halides...

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