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 mo des were identified using density functional (DFT) and coupled-cluster meth ods. The geometries obtained with several DFT functionals are quite similar ; for consistency's sake, we employed BLYP/6-31G* geometries; Brueckner dou ble 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 f ulvene 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 s tabilization. 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 dimeth ylenecyclobutene biradical 12 is 69 kcal mol(-1) endothermic and is hardly of preparative interest. A new cyclization of 3 should lead to the seven-me mbered ring biradical 13, which is located 33 kcal mol(-1) above 3 and 24 k cal 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 con ditions. All other possible modes of cyclization of 4 did not lead to stabl e products. Benzannelation has a significant effect on the endothermicities of the Bergman and Myers-Saito cyclizations, which are 8-9 kcal mol(-1) ab ove the parent reactions due to reduced aromatization energy in the naphtha lene derivatives. The endothermicities of the other cyclization pathways ar e largely unaffected by benzannelation.