Cyclooctatetraene, dianion planar form

Cyclooctatetraene was reduced electrochemically to cyclooctatetraenyl dianion. In DMF the product is mostly (92%) 1,3,5-cyclooctatriene at —1.2 V. If the potential is lowered the main product is 1,3,6-cyclooctatriene. Previous experiments, in which the anion radical was found to be disproportionated, were explained on the basis of reactions of the cyclooctatetraene dianion with alkali metal ions to form tightly bound complexes, or with water to form cyclooctatrienes. The first electron transfer to cyclooctatetraene is slow and proceeds via a transition state which resembles planar cyclooctatetraene102. 

There is, however, nmr evidence that indicates that the tub form is in rapid equilibrium with a very small amount of the planar form at room temperature. There is about a 15-kcal mole-1 energy difference between the two forms. The dication, C8HS2 , and the dianion of cyclooctatetraene, C8HR2e, which have (4 n + 2)tt electrons, appear to exist in planar conformations (see Exercise 21-16, p. 996). 

Annulene is a 4n -electron system, and thus, the (neutral) planar form would be expected to exhibit antiaromatic behavior. Indeed, Frost s circle analysis predicts an open-shell species (as with [4]annulene see Figs. 2 and 6). Extensive studies of cyclooctatetraene were performed, and it was established that the neutral state avoids the problem of antiaromaticity by adopting a tub conformation. Both the anion and dianion are known to exist in a planar conformation, the latter being aromatic. This fact is also predicted by simple analysis of the [8]annulene dianion by Frost s circle (see Fig. 6). It is predicted that the planar [8]annulene dianion is a closed-shell system. 

The NMR spectrum indicates a planar aromatic structure. It has been demonstrated that the dianion is more stable than the radical anion formed by one-electron reduction, since the radical anion disproportionates to cyclooctatetraene and the dianion ... 

Addition of two electrons to cyclooctatetraene would lead to a 10ji-electron system, the cyclooctatetraenyl dianion, Cyclooctatetraene, which is non-planar, reacts with potassium in tetrahydrofuran to give dipotassium cyclooctatetraenide, which is planar. The C-C bond lengths are all the same (141 pm). At low temperatures, a dication is formed when cyclooctatetraene reacts with SbF.. This also has some aromatic characteristics in keeping with a 6jt-electron system. 

Cyclooctatetraene, as explained on p. 158 of the textbook, is tub-shaped and not planar, because its eight it electrons do not form a 2n+2 number. However, two atoms of potassium can reduce the cyclooctatetraene to a dianion by giving it two electrons, so now it has ten electrons, is aromatic, and is planar. Just one of the many possible delocahzed structures of the product is shown here. 

Cyclooctatetraene could form sandwich or, more likely, halfsandwich complexes in which the 10 v electrons of the planar dianion CsHg - are involved in bonding to a transition metal (Fig. If). 

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