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.

中文翻译：

---

（4 + 2）和（2 + 2）二烯与Keteniminium阳离子的环加成反应的机理和构象控制。

Ghosez等人通过实验研究了酮亚胺阳离子与1,3-二烯在（4 + 2）和（2 + 2）环加成反应中的选择性。用ωB97X-D密度泛函理论进行了探索。烯酮铵阳离子与1,3-二烯的反应受二烯的s-顺式或s-反式性质影响。s-反式二烯反应生成中间体烯胺，该烯胺导致在酮亚胺CC键上形成（2 + 2）环加合物。环加成反应的第一步是确定速率，反应是通过攻击酮亚胺阳离子的中心碳并随后形成CC键而发生的。相反，s-顺式约束的二烯通过逐步的和协同的机制而导致优先形成（4 + 2）产物，所述机制涉及区域选择性地添加至酮亚胺CN键。