Theoretical studies on Myers-Saito and Schmittel cyclization mechanisms of hepta-1,2,4-triene-6-yne

The mechanisms of the Myers-Saito cyclization and the Schmittel cyclization of hepta-1,2,4-triene-6-yne are studied by ab initio multireference MO methods (CASSCF and MRMP2 methods). For the Myers-Saito cyclization, two transition states with C(s) and C₁ symmetries are located. The transition state with C1 symmetry is only 1.5 kcal/mol lower in energy than that with C(s) symmetry at the MRMP2 calculation level. The obtained activation energy at the transition state with C₁ symmetry and the reaction energy are 16.6 and 16.2 kcal/mol exothermic, respectively. For the Schmittel cyclization, two transition states with C(s) and C₁ symmetry are also obtained. The transition state with C₁ symmetry is 7.9 kcal/mol lower in energy than that with C(s) symmetry. The transition state with C₁ symmetry for Schmittel cyclization is 6.7 kcal/mol higher in energy than that for the Myers-Saito cyclization. The reaction mechanisms are analyzed by a CiLC-IRC method. The interactions of orbitals for the Myers-Saito and Schmittel cyclizations can be distinguished.