Pentadienyl anion thermal cyclization

There are also examples of electrocyclic processes involving anionic species. Since the pentadienyl anion is a six-7c-electron system, thermal cyclization to a cyclopentenyl anion should be disrotatory. Examples of this electrocyclic reaction are rare. NMR studies of pentadienyl anions indicate that they are stable and do not tend to cyclize. Cyclooctadienyllithium provides an example where cyclization of a pentadienyl anion fragment does occur, with the first-order rate constant being 8.7 x 10 min . The stereochemistry of the ring closure is consistent with the expected disrotatory nature of the reaction. 

Electrocyclic closure of both pentadienyl cation and anion have been observed. Cations generated by protonation of dienones close in the predicted conrotatory manner as shown in Equation 12.55.99 The pentadienyl anion, a six-electron system, should close in the disrotatory sense a clear example is the rapid isomerization illustrated in Equation 12.56.100 Photochemical cyclization of pentadienyl cations has been observed Equation 12.57 shows an example in a cyclic system.101 The ready thermal reversion, which should be conrotatory and therefore difficult in the bicyclic system, may possibly occur by a stepwise path.102... 

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. 

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. 

Pentadienyl anion The HOMO of the reactant in ground state is W3 having mirror symmetry, therefore, under thermal conditions cyclization will take place in a disrotatory manner (Scheme 2.14 Figure 2.18). 

What will be the stereochemical course for thermal cyclization of (a) pentadienyl anion (b) pentadienyl cation ... 

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