Gas-phase calculations. Gas-phase geometries were optimized at the ωB97XD 51 /def2-TZVP 52 level of theory. Frequency analyses were performed at the same level to verify the optimized structure to be either a minimum or a transition state, and to obtain the zero-point vibrational, thermal, and entropic corrections, necessary in calculating enthalpies and free energies. For open-shell singlets that appear in the stepwise mechanism, broken-symmetry DFT with spin-projection by the Yamaguchi-Houk procedure was used. On the ωB97XD/def2-TZVP optimized structures, single-point calculations were performed using the strongly contracted N-electron valence state perturbation theory (SC-NEVPT2) and coupled-cluster CCSD(T) methods with the same def2-TZVP basis set. In the SC-NEVPT2 calculations, a (4,4) active space was used for 1,3-cyclohexadiene, which comprises the valence π-type orbitals and the valence π electrons in the molecules. An (8,8) active space was used for the transition states, which consists of the same set of π-type orbitals and π electrons of two molecules of 1,3cyclohexadiene; however, some of the π-type orbitals become more of σ-type orbitals in the bond formation regions in the transition states. The ωB97XD and CCSD(T) calculations were computed using the Gaussian 16 program package. The SC-NEVPT2 calculations were done with the ORCA 4.2.1 program package.