[2.2.2]propellane rearrangements

Both [2.2.2]propellane and the Cope rearrangement follow reaction paths that pass through singlet diradicaloid portions of the potential energy surface. Here we report the results of applying a variety of methods — CASSCF, PUHF, MUMP2, DFT, UDFT, and CI — that have been suggested as methods for obtaining reliable potential energy surfaces for diradicals. All calculations were done with the same 6-31G* basis to facilitate comparison. The results confirm that [2.2.2]propellane exists and has a substantial barrier to opening to the bicyclic diyl or to dimethylenecyclohexane. The open D3h diyl form is lower in energy than propellane, yet occupies only a shallow well, if it exists at all, before opening to the much more stable dimethylenecyclohexane. CASSCF and a judicious use of DFT give similar results. For the Cope problem, by contrast, the DFT results seem more reliable than the CASSCF results.