Photochemical cyclopropyl allyl anion

Thus with the cyclopropyl anions 331- and 340 - it has been established that the disrotatory mode, as predicted by Woodward and Hoffmann , is the preferred one. It is however not clear whether a photochemical cyclopropyl-allyl anion or a thermal cyclopropyl radical ring-opening (the latter caused by photochemical electron ejection) takes place. It has also been realized that systems with X = vinyl or Br (342- and 343-, respectively) do not open photochemically . 

Photochemical cyclopropyl-allyl anion transformations have been observed by Newcomb and Ford 20c) and by Fox 22) for example, the photochemical disrotation of 10 to give 12. 

It was only after Woodward and Hoffmann had predicted a conrotatory mode for the thermal cyclopropyl-allyl anion transformation in 1965 9) that new interest developed in this reaction 10). But it was first shown by Huisgen and coworkers U) by means of the iso-rc-electronic uncharged aziridine 1 which gives the azomethine ylids 2 and 3, respectively, that the stereochemistry of the thermal and the photochemical reactions agrees with the prediction. 

Tolbert [1-3] has summarized the various photochemical pathways open to allyl anions in general, including electron photoejection. Other pathways include ring closure to a cyclopropyl anion, E-Z isomerization, protonation and a-bond cleavage. With the availablity of a greater number of photochemical pathways for allyl anions in general, it is not surprising that electron photoejection is not an important pathway for aryl-stabilized allyl anions. One of the most studied... 

The thermal cyclopropyl anion to allyl anion rearrangement is predicted to be conrotatory and the photochemical rearrangement to be disrotatory. ... 

By these simple rules Woodward and Hoffmann predicted a disrota-tory course for the opening of the cyclopropyl cation in its ground state to the corresponding allyl cation, while the thermal opening of cyclopropyl radical and anion to allyl radical and anion is conrotatory. A glance at Fig. 2 clearly shows the reason. Reverse predictions can be made for photochemically induced reactions. 

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