CASSCF Computational Study of Pseudopericyclic Character in Electrocyclic Rearrangements Involving Heteroatoms

The Complete Active Space Self-Consistent Field (CASSCF) computational method, with the 6-31G* basis set, was used to examine six electrocyclic rearrangements, each involving a 1,2,4,6-heptatetraene skeleton with two variously located oxygen and/or nitrogen heteroatoms, as a way to determine which, if any, are pseudopericyclic as opposed to pericyclic. Primarily through the close examination of the active space orbitals, but also considering transition structure geometries and activation energies, it was concluded that rearrangements 3 → 4, 5 → 6, 7 → 8, and 9 → 10 are pseudopericyclic with two orbital disconnections each, whereas the 13 → 14 and 15 → 16 rearrangements are pericyclic. Our conclusions agreed with those of others in two of the four cases that had been studied previously by density functional theory (3 → 4 and 7 → 8) but ran contrary to the previous conclusions that the 5 → 6 rearrangement is pericyclic and that the 15 → 16 rearrangement is pseudopericyclic. Our results are also compared and contrasted to previous similar ones of ours involving the 3 → 4 electrocyclization (essentially pericyclic), the 11 → 12 [3,3] sigmatropic rearrangement (pseudopericyclic), and similar [3,3] sigmatropic rearrangements (all pericyclic), and detailed rationales for these latest results are provided.