CASSCF Calculations Reveal Competitive Chair (Pericyclic) and Boat (Pseudopericyclic) Transition States for the [3,3] Sigmatropic Rearrangement of Allyl Esters.

(10,8)CASPT2/6-31G**//(10,8)CASSCF/6-31G** and CCSD(T)/cc-pVDZ//(10,8)-CASSCF/6-31G** calculations have been performed on the potential surface for the [3,3] sigmatropic allyl ester rearrangements of cis-3-penten-2-yl acetate (16) to trans-3-penten-2-yl acetate (17) and 3-buten-2-yl acetate (21) to trans-2-buten-1-yl acetate (22). The results are compared to DFT (B3LYP/6-31G**) calculations on the known 16 → 17 rearrangement that reported it to be concerted and pseudopericyclic through a boat-shaped transition structure ( Birney, D. M. et al. J. Am. Chem. Soc. 2009 , 131 , 528 - 537 ). The CASSCF calculations, on the other hand, uncovered competitive concerted pathways for both the 16 → 17 and 21 → 22 rearrangements, though it was necessary to apply certain approximations in the former case. While one CASSCF pathway in each case involves a boat-shaped transition structure, similar to the one located through DFT calculations, the other pathway involves a chair-shaped transition structure. Preference for chair or boat is shown to be method dependent. Moreover, examination of the CASSCF active-space orbitals clearly confirms that the boat-shaped transition structures are pseudopericyclic but significantly also established that the chair-shaped transition structures are clearly pericyclic. Conclusions based on these results, and regarding our understanding of pericyclic vs pseudopericyclic reactions, are proffered.

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.