The Electrocyclic Cyclopropyl‐Allyl Rearrangement. Preliminary communication

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Abstract
MINDO‐2 SCF calculations indicate that ring‐opening of cyclopropyl radical (I) to allyl radical (II) is more favourable via a disrotatory reaction path, the calculated activation energy being ∼30 kcal/mole. (For conrotatory opening the activation energy was found to be ∼44 kcal/mole.) The two critical motions of the nuclei during the transformation are found to be strongly decoupled, i.e. rupture of the CH2βCH2 bond precedes rotation of the CH2 groups. As predicted by qualitative theories both ring‐opening modes are unfavourable since they involve a change in electronic ground‐state symmetry between I and II. The preferred ring‐opening mode is discussed qualitatively in terms of Evans' principle.