Can Fulvenes Form from Enediynes? A Systematic High-Level Computational Study on Parent and Benzannelated Enediyne and Enyne−Allene Cyclizations

Abstract

Apart from the well-known Bergman, Myers−Saito, and Schmittel ring closure reactions of parent enediyne (4) and enyne−allene (3), novel cyclization modes were identified using density functional (DFT) and coupled-cluster methods. The geometries obtained with several DFT functionals are quite similar; for consistency's sake, we employed BLYP/6-31G* geometries; Brueckner double energy single points [BCCD(T)/cc-pVDZ] on these geometries were used to determine the relative energies. The C1−C5 cyclization of 4 leading to fulvene biradical 8 is 40 kcal mol-1 endothermic, and the product lies 31 kcal mol-1 above 1,4-didehydrobenzene 7 because of the lack of aromatic stabilization. The heat of formation (ΔfH°) of 8 is predicted to be 172.0 ± 1.0 kcal mol-1. Yet another ring closure of 4 leading to dimethylenecyclobutene biradical 12 is 69 kcal mol-1 endothermic and is hardly of preparative interest. A new cyclization of 3 should lead to the seven-membered ring biradical 13, which is located 33 kcal mol-1 above 3 and 24 kcal mol-1 above the Schmittel product 6. As the transition structure for both cyclizations differ by 11 kcal mol-1, 13 may form under suitable conditions. All other possible modes of cyclization of 4 did not lead to stable products. Benzannelation has a significant effect on the endothermicities of the Bergman and Myers−Saito cyclizations, which are 8−9 kcal mol-1 above the parent reactions due to reduced aromatization energy in the naphthalene derivatives. The endothermicities of the other cyclization pathways are largely unaffected by benzannelation.