Dications Carbodications

Adamantanedimethylium dications 236 Bicyclo[3.3.3]undecane-l,5-diyl dication (manxyl dication) 238 A sulfur-stabilized carbodication 238... 

Trihydroxycyclopropenium ion and homologous ions Hogeveen dication [C6(CH3)6] 256 11 Oxygen-stabilized carbodications 257 References 258... 

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. ... 

The protonation of the methyleneiminium cation gives rise to the superelectrophilic methyleniminium dication, 19, which is a gitonic 1,2-dicationic species (carbenium ammonium species). Such a carbodication has been observed by charge stripping mass spectrometry by Schwarz and coworkers.Olah and coworkers have shown by theoretical calculations that the carbenium ammonium species, 19a, is 27.6 kcal/mol more stable than the three-center two-electron (3c-2e) bonded structure, 19b. ... 

The bicyclo[2.2.2]octane-l,4-diyl dication, 61, and its tricyclopropane derivative, 62, could not be prepared under stable ion conditions even though they were attempted under various reaction conditions. The possible existence of these carbodications was supported by Modified Neglect of Differential Overlap (MNDO) calculations. 

Several 1,4-disubstituted derivatives of the dication, 63, were successfully prepared. The 1,4-diphenyl, 1,4-dimethyl, 1,4-dicyclopropyl-substituted derivatives of carbodication 63 (65, 66, and 67) are exceptionally stable. The 1,4-dimethyl-1,4-cyclohexyl dication, devoid of the adjacent cyclopropyl groups, could not be prepared. The carbocationic center in all these dications are somewhat shielded as compared to their monocations. The C NMR chemical shifts of the carbocationic centers for the dications 65, 66, and 67 are 235.4, 293.4, and 260.8 ppm, respectively. The diprotonated ann -tricyclo(5.1.0.0 )octa-2,6-dione, 68, may be treated as a dicarboxonium ion, instead of dihydroxy dicarbenium ion, since the cabronyl carbon is shielded by only 25.2 ppm, much smaller than that observed for the protonated cyclohexanedione (34 ppm). The para carbons of the phenyl substituents in carbodication 65 are relatively shielded by about 5 ppm from that of the parent l,4-diphenylcyclohexane-l,4-diyl dication showing relatively less delocalization of the charge into the aromatic rings. 

Miller and coworkers have recently prepared a sulfur-stabilized carbodication, 1,3,5,7-tetramethyl-2,4,6,8,9-pentathiabicyclo[3.3.l]nonane-3,7-diylium dication (103). 

The dications display NMR spectra, which are very similar to those of allylic carbocations. The terminal carbons of the dienylic and the allylic systems of the carbodications 107 and 108 show between 192 and 198, characteristic of the typical allylic carbocations. The IGLO-calculated chemical shifts closely match with those of the experimentally observed values for these carbodications. 

Although the acyclic p-hydrido-bridged carbomonocations (e.g., 137) cannot be prepared, Sorensen has shown that the acyclic p-hydrido-bridged carbodications can be readily prepared. The following such carbodications have been prepared from their corresponding diols in FSO3H/SO2CIF 2,6-dimethyl-4-isobutylheptane-2,6-diyl dication (139), 4,6-trimethyl-4-isobutylheptane-2,6-diyl dication (140), and 4,4-diisobutyl-2,6-dimethylheptane-2,6-diyl dication (141). 

The ortho-, meta-, and ura-phenylene bis(l,3-dioxolanium) dications (189-191) have been prepared by the ionization of the corresponding 2-methoxyethyl benzoates in FSO3H or CF3SO3H at 40°C. These carbodications are exceptionally stable to over 100°C. 

Carbodications have also been observed in more rigid systems such as the apical, apical congressane (dimantane) dication 169182 and the polycyclic bridgehead dication 170.182Thebicyclo[2.2.2]octane-l,4-diyl dication 171 was claimed to have been prepared in an earlier study.393 In a subsequent reinvestigation,394 however, dication 171 could not be detected. Instead, only the monocation monodonor-acceptor complex was obtained. 

Multiply Protonated Methane Ions and Their Analogs. In addition to the involvement in a single 2e-3c bond formation, carbon is also capable of simultaneously participating in two 2e-3c bonds in some carbodications. Diproto-nated methane (protiomethonium dication, CH62+) is the parent of such carbodications. 

In a similar respect, ionization of 2,4-dichloro-2,4-dimethylpentane (6) does not give the 1,3-carbodication (7, eq 3).3 Despite the superacidic conditions, deprotonation occurs to give the allylic cation (8). Even substitution by phenyl groups is not enough to stabilize the 1,3-dication. For example 1,1,3,3-tetraphenyl-l,3-propanediol (9) also undergoes the deprotonaton or disproportionation reactions (eq 4).3... 

As in the case of vicinal dications, stabilization of the carbocationic centers has provided several examples of observable 1,3-carbodications. For example, Olah and co-workers were able to generate the cyclopropyl stabilized 1,3-carbodication (17) by ionization of the diol (16) in FSO3H-SbFs at low temperature (eq 6).6... 

There are several other carbodications that may be represented as 1,3-dicationic systems (or nonclassical, highly delocalized systems), including Schleyer s l,3-dehydro-5,7-adamantanediyl dication (28),11 Hogeveen s pyramidal dication (29),12 diallyl dications (i.e., 30),13 dicationic ethers (i.e., 31),14 diprotonated pyrones (32),15 as well as various aromatic dications. 

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