Myers−Saito versus C²−C⁶ (“Schmittel”) Cyclizations of Parent and Monocyclic Enyne−Allenes:  Challenges to Chemistry and Computation

The Myers−Saito and the C 2 −C 6 cyclization (“Schmittel” cyclization) of the parent enyne−allene ( Z )-1,2,4-heptatriene-6-yne were investigated with pure density functional theory (DFT) methods and were compared to coupled cluster [CCSD(T)] and Brueckner doubles [BCCD(T)] high-level calculations. Both the Becke−Lee−Yang−Parr (BLYP) and the Becke−Perdew−Wang (BPW91) DFT levels with the 6-31G(d) basis sets are quite suitable to describe the cyclization barriers and are nearly as accurate as much more time-consuming high-level methods. As noted before for the Bergman-cyclization, the hybrid functional B3LYP yields good geometries but is less suitable for energies due to Hartree−Fock mixing. Single-point energy evaluations with the much larger cc-pVTZ basis set do not necessarily improve the results; some even become worse. The computed enthalpy of formation (Δ H f °) of the Myers−Saito product (107 ± 4 kcal mol - 1 ) compares relatively well to the experimental value (103 ± 3 kcal mol - 1 ) where an upward correction within the error bars seems indicated. Using isodesmic equations, the Δ H f ° of the Schmittel product is predicted to be 129 ± 4 kcal mol - 1 . Since BLYP describes the barriers of the parent enyne−allene system quite well, it was utilized to compute the cyclizations of monocyclic enyne−allenes (ring sizes = 7−10 carbons). The Myers−Saito cyclization of the nine-membered ring is associated with the smallest reaction barrier and the highest exothermicity. While ring strain effects are not able to favor the Schmittel products much over the Myers−Saito products, the eight-membered ring closures should give rise to a mixture of both products. The singlet−triplet separations of all cyclization products are very small (1 kcal mol - 1 ); the hydrogen-abstracting ability of such cyclic systems should therefore be rather high.