Diradicals cyclooctatetraene

The most intriguing hydrocarbon of this molecular formula is named bullvalene, which is found in the mixture of products of the reaction given above. G. Schroder (1963, 1964, 1967) synthesized it by a thermal dimerization presumably via diradicals of cyclooctatetraene and the photolytical cleavage of a benzene molecule from this dimer. The carbon-carbon bonds of bullvalene fluctuate extremely fast by thermal Cope rearrangements. 101/3 = 1,209,600 different combinations of the carbon atoms are possible. 

Figure 16-8 shows that the first 3 pairs of electrons are in three bonding molecular orbitals of cyclooctatetraene. Electrons 7 and 8, however, are located in two different nonbonding orbitals. As in cyclobutadiene, a planar cyclooctatetraene is predicted to be a diradical, a particularly unstable electron configuration. 

Six of the eight tt electrons of cyclooctatetraene occnpy three bonding orbitals. The remaining two tt electrons occnpy, one each, the two eqnal-energy nonbonding orbitals. Planar cyclooctatetraene shonld, like sqnare cyclobntadiene, be a diradical. 

Like cyclobutadiene, planar cyclooctatetraene must have one electron in each of the degenerate nonbonding molecular orbitals, making it a diradical. Unlike cydobu-tadiene, however, there is a way for cyclooctatetraene to get out of its energetic misery. Cyclobutadiene must be planar, or very nearly so. By contrast, cyclooctatetraene can distort easily into a tublike form in which most of the angle strain and the diradical nature of the planar form is avoided (Fig. 13.23). 

FIGURE 13.23 In planar cyclooctatetraene, there is both angle strain and diradical reactivity. Both can be avoided by distorting into a tub shape. 

Other theoretical treatments which have been applied to considerations of antiaromaticity include spin-coupled theory, topological methods,and quantum statistical definitions. The relevance of spin-coupled theory has been described The comparison between the (spin-coupled) descriptions of cyclobutadiene, benzene, and cyclooctatetraene clearly indicates that the reason for the lower stability and higher reactivity of antiaromatic systems is due to a simultaneous unfavorable coupling of the spins of all valence orbitals to triplet pairs, which discourages bonding interactions and suggests diradical character. ... 

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