Can fulvenes form from enediynes? A systematic high-level computational study on parent and benzannelated enediyne and enyne-allene cyclizations

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 C-1-C-5 cyclization of 4 leading to fulvene biradical 8 is 40 kcal mol(-1) endothermic, and the product ties 31 kcal mol(-1) above 1,4-didehydrobenzene 7 because of the lack of aromatic stabilization. The heat of formation (Delta H-f(o)) 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.