Interconversion biradical conformers

In a related example, Swenton found evidence for only the sr 1,3-shift product with optically active fxJ6>-5,6-diphenylbicyclo[3.1.0]hexene at 130°C. Here log k = 12.6 — 32400/23RT (Scheme 7.33). The lower activation energy is consistent with increased stabilization of a non-planar biradical by a phenyl group and the lack of other products can be reasonably attributed to a higher activation energy for formation of a planar species due to the presence of vicinal phenyl groups which must interact sterically in the transition state for conformational interconversion. 

By analogy to the cleavage reaction of bicyclo[2.2.0]hexane, formation of a boat-like biradical followed by rapid conformational interconversion with a chairlike biradical and then cleavage can provide both CODs. The heat of formation of S and A were reported to be 56 and 53 kcal/mol, respectively/ and if the biradicals are comparable in energy to the transition state producing them, they are roughly 12 kcal/mol less stable, enthalpically, than the bisally lie biradicals which figure so prominently in the other thermal interconversions. Considerations of entropy would make them at least another 5 kcal/mol less stable. 

---