Cope reaction families:: To be or not to be a biradical

The cyclization reactions of polyunsaturated systems such as enediynes, enyne-allenes, as well as many others, can be structurally related to the Cope reaction and grouped into two branches: 6pi (e.g., Cope, allenyl-Cope) and [2sigma + 4pi] systems (e.g., Bergman, Myers-Saito). After extensive computional (BCCD(T)/cc-pVDZ/UBLYP/6-31G*) examination of a large set of Cope-related reactions, we derive the following simple rule: biradical intermediates are involved along the reaction path if they are stabilized by allylic resonance or aromaticity.