Origins of Periselectivity and Regioselectivity in Ambimodal Tripericyclic 8+6/6+4/4+2 Intramolecular Cycloadditions of a Heptafulvenyl-Fulvene

Quantum mechanical calculations and molecular dynamics simulations have elucidated the reaction mechanism for intramolecular cycloadditions of a heptafulvenyl-fulvene tethered by a trimethylene chain. Prior experiments by Liu and Houk reported the formation of only an endo-8+6 cycloadduct at 185 °C. Liu et al. later reported an exo-4+2 Diels–Alder cycloadduct as the major product at 140 °C (Tetrahedron, 1999, 55, 9171). Cycloadditions involve Diels–Alder and an ambimodal intramolecular tripericyclic 8+6/6+4/4+2 cycloaddition. The mechanistic details explain the experimental reports of temperature dependence on the periselectivity of intramolecular cycloadditions. Additional calculations with multireference-based methods CASSCF and NEVPT2 highlight the artifacts of DFT methods and single-reference wavefunction-based CCSD(T) in the description of complete potential energy surface involving various cycloadditions of the heptafulvenyl-fulvene.