Cyclooctadienyl anion, cyclization

Allyl, pentadienyl, and heptatrienyl anions can in principle undergo electrocyclic rearrangements (81). The thermal conversion of a pentadienyl into a cyclopentenyl anion is predicted to be a disrotatory process. The cyclooctadienyl anion cyclizes to the thermodynamically stable isomer of the bicyclo[3.3.0]octenyl ion having cis fused rings (52,82,83). The acyclic pentadienyl anions, however, do not normally cyclize. On the other hand, heptatrienyl anions cyclize readily at — 30°C by a favorable conrotatory thermal process (41,84). This reaction sets a limit upon the synthetic utility of such anions. 

In this case, it is the conrotatory photochemical cyclization that is prevented by strain (it was tried—cyclooctadienyl anion is stable for at least a week at -78 °C in broad daylight) as the product would be a 5,5 traws-fused system. The same strain prevents thermal electrocyclic ring closure of cyclooctadienyl cations. 

There are also examples of electrocyclic processes involving anionic spiecies. Since pentadienyl anion is a six-ir-electron system, its thermal cyclization to a cyclopentenyl anion should be/te tatory. Examples typifying this electrocyclic reaction are rare, and NMR studt of pentadienyl anions indicate that they are rather stable and do not tend to cyclize. Cyclooctadienyllithium is exceptional in this respect, in that it does undergo cyclization to 18, with a first-order rate constant of 8.7 X10 min at 35°C. The stereochemistry of the ring fusion is consistent with the disrotatory nature of the cyclization. Examination of a model of cyclooctadienyl anion reveals that it is quite highly strained, a fact that explains its ready conversion to 18 ... 

Cyclooctadienyl lithium 61 having pentadienyl anion undergoes cyclization by disrotatory motion at 35 °C [33]. 

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