James A. Duncan

CASSCF Molecular Orbital Calculations Reveal a Purely Pseudopericyclic Mechanism for a [3,3] Sigmatropic Rearrangement

A comparative CASSCF/6-31G*-level computational study of the concerted [3,3] sigmatropic rearrangements of cis-1-iminyl-2-ketenylcyclopropane (15), cis-1-iminyl-2-propadienylcyclopropane (17), and cis-1-iminyl-2-keteniminylcyclopropane (19) to give products 16, 18, and 20, respectively, was conducted. Analysis of the active space MOs of TS(15-->16), TS(17-->18), and TS(19-->20) suggests that the 17 --> 18 and 19 --> 20 rearrangements are classically pericyclic, whereas the 15 --> 16 rearrangement is pseudopericyclic with two orbital disconnections-one involving the nitrogen lone-pair orbital and the other the carbonyl carbon of the ketene moiety. The novel TS(15-->16) was also found to have a highly planar, tight, geometry, whereas TS(17-->18) and TS(19-->20) were both shown to have the boat-shaped geometry expected for classically pericyclic [3,3] sigmatropic rearrangements. Results of calculations on the [3,3] sigmatropic rearrangements involving additional transition structures, TS(21-->22), TS(23-->24), TS(25-->26), TS(27-->28), TS(29-->30), and TS(31-->32), demonstrate the relative uniqueness of the pseudopericyclic one, TS(15-->16).

Secondary Orbital Effect in the Electrocyclic Ring Closure of 7-Azahepta-1 ,2,4,6-tetraene-A CASSCF Molecular Orbital Study

Results of (10,9)CASSCF/6-31G* and B3LYP/6-31G* level calculations on the potential surface for the electrocyclic ring closure of E-7-azahepta-1,2,4,6-tetraene 3 to 1-aza-6-methylidenecyclohexa-2,4-diene ( 4) are reported, as well as parallel calculations on the electrocyclizations of hepta-1,2,4,6-tetraene 5, hexa-1,3,5-triene 7, Z and E-1-aza-1,3,5-hexatrienes 9 and 10, and Z-7-azahepta-1,2,4,6-tetraene 12 for purposes of careful comparison. The 3 --> 4 rearrangement has been studied computationally with density functional theory (DFT) by others, leading to disagreement over whether it is pseudopericyclic (de Lera, A. R.; Alvarez, R.; Lecea, B.; Torrado, A.; Cossío, F. P. Angew. Chem., Int. Ed. 2001, 40, 557-561; de Lera, A. R.; Cossío, F. P. Angew. Chem., Int. Ed. 2002, 41, 1150-1152) or pericyclic (Rodríguez-Otero, J.; Cabaleiro-Lago, E. Angew. Chem., Int. Ed. 2002, 41, 1147-1150). In accordance with disrotatory motion, the normal mode vectors for TS 3-->4 calculated at the (10,9)CASSCF/6-31G* level show a greater magnitude of rotation of the N1-H group relative to the N1-C2 bond being formed than in TS 3-->4 calculated at the B3LYP/6-31G* level. Furthermore, comparison of orbital correlation diagrams constructed entirely from localized complete active space (CAS) molecular orbitals (MOs) for the electrocyclizations of 3, 5, 7, 9, and 10 suggest that it is the highest occupied delocalized pi-MO of 3 that is primarily responsible for sigma-bond formation in 4, not the terminal allenyl pi-bond MO. However, there does appear to be a special secondary orbital effect role for the nitrogen lone-pair and hence the process is likely neither purely pericyclic nor pseudopericyclic.

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.

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.