Heptatrienyl 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 contrast to pentadienyl anions, heptatrienyl anions cyclize readily to cyclohepta-dienyl anions. The transformation of heptatrienyl anion to cycloheptadienyl anion proceeds with a half-life of 13 min at —13°C. The Woodward-Hoffmann rules predict that this would be a conrotatory closure. ... 

Apparently, the pentadienyl anion portion moved along the carbon atom backbone until a heptatrienyl anion was formed, which then underwent the expected cyclization (see Sect. C.I) to a heptadienyl anion, which is of greater stability due to its extra carborn carbon single bond. Finally, while the 3-methylcycloheptadienyI anion was found to be thermally stable, it could be completely converted to the 2-methylcycloheptadienyl anion photochemically. 

Unlike heptatrienyl anions, which cyclize to cycloheptadienyl anions even at — 30°C, nonatetraenyl anion is stable at room temperature and cyclization is not observed even at higher temperatures. In fact, (284) reacts with n-butyl-lithium to give (285) which opens to (286 R = Et) even at — 30°C 7-vinyl-cycloheptatriene reacts similarly to give (286 R = Bu"). ... 

In a number of cases, pentadienyl species are subject to cyclization reactions89-92 (cf., heptatrienyl, Sect. C.I). Thus, cyclooctadienyllithium undergoes a first-order conversion to a bicyclo(3.3.0)octenyl anion with a half-life of 80 min at 35°93. The 1,3,5-triphenyl-... 

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