Radical-Anionic Cyclizations of Enediynes:  Remarkable Effects of Benzannelation and Remote Substituents on Cyclorearomatization Reactions

The reasons for large changes in the energetics of C1−C5 and C1−C6 (Bergman) cyclizations of enediynes upon one-electron reduction were studied by DFT and Coupled Cluster computations. Although both of these radical-anionic cyclizations are significantly accelerated relative to their thermal counterparts, the acceleration is especially large for benzannelated enediynes, whose reductive cyclizations are predicted to proceed readily under ambient conditions. Unlike their thermal analogues, the radical-anionic reactions can be efficiently controlled by remote substitution, and the effect of substituent electronegativity is opposite of the effect on the thermal cycloaromatization reactions. For both radical-anionic cyclizations, large effects of benzannelation and increased sensitivity to the properties of remote substituents result from crossing of out-of-plane and in-plane MOs in the vicinity of transition states. This crossing leads to restoration of the aromaticity decreased upon one-electron reduction of benzannelated enediynes. Increased interactions between nonbonding orbitals as well as formation of new aromatic rings (five membered for the C1−C5 cyclization and six membered for C1−C6 cyclizations) are the other sources of increased exothermicity for both radical-anionic cyclizations. The tradeoff between reduction potentials and cyclization efficiency as well as the possibilities of switching of enediyne cyclization modes (exo or C1−C5 vs endo or C1−C6)) under kinetic or thermodynamic control conditions are also outlined.