Theoretical analysis of the Cope rearrangement of 1,5-hexadiene

The reaction mechanisms via the transition states of a loose chair, a loose boat, and a Dewar type for the Cope rearrangement of 1,5-hexadiene have been studied by a CiLC-IRC analysis on the basis of a complete active space self-consistent field (CASSCF) molecular orbital (MO) method. The CiLC-IRC analysis showed that the transition state of the loose-chair type is an aromatic electronic state and that of the loose-boat type is nonaromatic electronic. The difference of these mechanisms between the loose-chair type and the loose-boat type comes from the σ-bond distance of bond making and bond breaking. The difference in aromatic nature for the two transition states corresponds to the difference of their activation energies. The diradical character for the reaction pathway through the Dewar-type transition state is also shown.