Longicyclic

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... 

Thus, the isodesmic reaction 1 is more exothermic than that of reaction 2 by 108kcal/mol. If longicyclic interactions were of any importance, the isodesmic reaction 2 should be more exothermic.The NMR chemical shifts of the dication were temperature-independent ruling out the contribution of the possible equilibrating tricyclic structures (81 and 82). The donor-acceptor monocation (83) can also be rejected based on the symmetry arguments. 

Longicycles. As was noted before, the longicyclic topology is inherent in compounds possessing at least... 

The longicycles of other types, as, for example, bar-relene (type (2, 2, 2))... 

Goldstein and Hoffmann suggested the new terms bicycloaromaticity and antibicycloaromaticity. The bicycloaromatic compounds are those cyclic conjugated hydrocarbons which contain 4g ji-electrons and stabilized in the shape of laticycles and longicycles. The cyclic conjugated systems which possess 4g -f 2 jc-electrons and are destabilized in the shapes indicated were accordingly proposed to be called antibicycloaromatic. 

Reaction of bullvalene 67 with sodium-potassium alloy in THF or DME led to the bicyclo[3.3.2]decatrienyl dianion 68. The facile formation of 68 in contrast to the unsuccessful reduction of dihydrobullvalene 69 which does not give 70, has been discussed along the lines of longicyclic stabilization of 68 as opposed to the bishomo-antiaromatic nature of the bicyclo[3.3.2]decadienyl dianion 70 56, b). 

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... 

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