Mechanisms and Conformational Control of (4 + 2) and (2 + 2) Cycloadditions of Dienes to Keteniminium Cations

• Published in: Journal of organic chemistry Vol. 85; no. 4; pp. 2597 - 2606
• Main Authors: Ramirez, Melissa, Li, Wenfei, Lam, Yu-hong, Ghosez, Léon, Houk, K. N
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 21-02-2020
• ISSN: 0022-3263; 1520-6904
• DOI: 10.1021/acs.joc.9b03340

Selectivities in (4 + 2) and (2 + 2) cycloadditions of keteniminium cations with 1,3-dienes studied experimentally by Ghosez et al. were explored with ωB97X-D density functional theory. Reactions of keteniminium cations with 1,3-dienes are influenced by the s-cis or s-trans nature of the diene. s-Trans dienes react to give an intermediate enamine that leads to favored formation of (2 + 2) cycloadducts across the keteniminium C–C bond. The first step of the cycloaddition is rate-determining, and reaction occurs by attack on the central carbon of the keteniminium cation and subsequent C–C bond formation. In contrast, s-cis constrained dienes lead to preferential formation of (4 + 2) products by both stepwise and concerted mechanisms involving regioselective addition to the keteniminium C–N bond. Diels–Alder reaction occurs via a concerted mechanism if the diene termini are held in close proximity, as in cyclopentadiene.