Potential surface for the quadruply degenerate rearrangement of bicyclo[3.1.0]hex-2-ene.

Previous experimental studies have shown that bicyclo[3.1.0]hex-2-ene and substituted derivatives undergo thermal rearrangements to form three products identical to the starting material save for the position of the labeling substituents. Despite the likely involvement of a symmetric diradical intermediate, cyclohexene-3,5-diyl, these products are formed at unequal rates. DFT (B3LYP), CASSCF, and CASPT2 calculations were performed to map out the details of the potential energy landscape for the reaction. A qualitative hypothesis, based on dynamical considerations, is offered to explain the observed nonstatistical product distribution.