Secondary Orbital Effect in the Electrocyclic Ring Closure of 7-Azahepta-1,2,4,6-tetraeneA CASSCF Molecular Orbital Study
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- 作者：James A. Duncan ; David E. G. Calkins ; Mariya Chavarha
- 刊名：Journal of the American Chemical Society
- 出版年：2008
- 出版时间：May 28, 2008
- 年：2008
- 卷：130
- 期：21
- 页码：6740 - 6748
- 全文大小：1419K
- 年卷期：v.130,no.21(May 28, 2008)
- ISSN：1520-5126
文摘
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 TS3→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 TS3→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 π-MO of 3 that is primarily responsible for σ-bond formation in 4, not the terminal allenyl π-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.