Cope Reaction Families: To Be or Not to Be a Biradical

Abstract
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: 6π (e.g., Cope, allenyl-Cope) and [2σ + 4π] 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.