A Density Functional Molecular Orbital Study of the C2−C7 and C2−C6 Cyclization Pathways of 1,2,4-Heptatrien-6-ynes. The Role of Benzannulation

• Published in: Journal of the American Chemical Society 2000-09, Vol.122 (38), p.9265-9270
• Main Authors: Wenthold, Paul G, Lipton, Mark A
• Format: Article
• Language: English
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja002050+

The electronic factors influencing the competition between C2−C7 (Myers−Saito) and C2−C6 (Schmittel) cyclizations of 1,2,4-heptatrien-6-yne and 1-ethynyl-2-propadienylbenzene are investigated using Becke3LYP/6-31G* calculations. The relative energies of the biradical products formed upon cyclization are calculated using a simple bond additivity estimate and by direct calculation. Both approaches predict that benzannulation stabilizes the bis-dehydromethylfulvene biradical that forms upon C2−C6 cyclization with respect to the biradical that forms upon C2−C7 cyclization by ca. 10 kcal/mol. Transition state calculations predict that benzannulation decreases the barrier for C2−C6 cyclization, relative to that for C2−C7 cyclization, by 2.3 kcal/mol. The results suggest that benzannulation plays an important role in promoting C2−C6 cyclization.