Bishomoaromatic dication

The structure of the sandwiched bishomoaromatic dication was confirmed by theoretical calculations at the HF/6-31G level. The stabilization of the carbocation was shown to be mainly due to the bishomoaromatic interactions. The long-postulated longicyclic interactions do not contribute to the stability of the cation, as shown by the energies of the following isodesmic reactions... 

Figure 3.25. Molecular orbital representations of the bonding structure of the parent bishomoaromatic dication C4H82+ (604) and cyclobutadiene dication C4H42+ (605).

Olah and coworkers reported the generation and characterization of the sandwiched bishomoaromatic dication 172 [Eq. (5.26)]. The 11 NMR spectrum of the solution at -80°C indicated that the species has the same symmetry as the starting diol with chemical shifts of some of protons being more shielded. The three absorptions of the NMR spectra were significantly shielded [8 T ... 

The dication 29 could be considered a sandwich bishomoaromatic dication or a 4 jr-electron longicyclic aromatic system or as the alternative homoaromatic species 30. To resolve this problem, Olah and Prakash et al. undertook a theoretical study of these compounds and also, for comparison, monocation 31. Using HF/6-31G and B3LYP/6-31G methods, they obtained optimized structures for 29—31. There is no evidence for longicyclic stabilization in 29, and both... 

Several interesting dications have been prepared and characterized as homoaromatic. Olah et al. (1983) prepared the bishomoaromatic/allylic dication [61], Prakash et al. (1987) studied the related bishomoaromatic... 

The structures of the intriguing dications 156 and 158 were also computed by DFT calculations. The C NMR chemical shifts were also calculated using both GIAO and IGLO methods. Both dications 156 and 158 can also be characterized as 4c/2e a-bishomoaromatic rectangular cyclobutane dications as well as frozen Woodward Hoffmann transition state analogs. 

To further explore the limiting structural and energetic prerequisites for the unique o-bishomoconjugation the structurally related seco-l,16-dodecahedradiene 596 and 1,16-dodecahedradiene 597 were also studied. For 596, only the corresponding bisallylic dication could be observed,1051 1052 whereas for 597 the dication could not be observed at all.1053 The o-bishomoaromatic species 599, however, could be generated via electrochemical oxidation of alkene 598.1054... 

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). 

In-plane delocalized 4C/3e radical cations have been generated by one-electron oxidation of the corresponding dienes (Figure 3). The corresponding dications are cited as a-bishomoaromatic 4C/2e dications, and the lower oxidation potential of the diene shown (0.84 V) compared to the corrsponding mo-noene (1.46 V) is taken as evidence of stabilization of the radical cation by 4C/3e in-plane delocalization. "... 

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