Natural Bond Orbital Analysis of Pericyclic and Pseudopericyclic 1,5-Electrocyclizations of Conjugated Nitrileimines

• Published in: Bulletin of the Chemical Society of Japan Vol. 77; no. 9; pp. 1671 - 1679
• Main Authors: Fukushima, Kazuaki, Iwahashi, Hideo
• Format: Journal Article
• Language: English
• Published: Tokyo The Chemical Society of Japan 01.09.2004; Chemical Society of Japan
• ISSN: 0009-2673; 1348-0634
• DOI: 10.1246/bcsj.77.1671

Ab initio and DFT studies have revealed that there are three mechanisms in 1,5-electrocyclizations of conjugated nitrileimines. The first one is a pericyclic 1,5-electrocyclization of vinyl or (Z)-imino nitrileimine. Interactions between π-orbitals at C1 and X5 lead to formation of σC1–X5 bonds. The second one is a typical pseudopericyclic 1,5-electrocyclization of formyl nitrileimine. Nucleophilic interaction of lone pair electrons at O5 to π*C1–N2(h) leads to formation of a σC1–O5 bond. The natural bond orbital (NBO) analysis for the reaction has shown that there are two disconnections of orbital interactions at C1 and O5 at the transition state because of the orthogonal array of the forming σC1–O5 bond and the other π-orbitals. The third one is an incomplete pseudopericyclic 1,5-electrocyclization of (E)-imino nitrileimine. Nucleophilic interactions of lone pair electrons at N5 to π*C1–N2(h) and π*C1–N2(v) lead to formation of a σC1–N5 bond. There is a single disconnection of orbital interactions at N5 at the transition state, because the forming σC1–N5 bond and the other π-orbitals are orthogonal at N5 but not at C1, owing to twisting in the nitrileimine moiety.