Dynamic Intermediates in the Radical Cation Diels–Alder Cycloaddition: Lifetime and Suprafacial Stereoselectivity

Cation radical Diels–Alder
cycloadditions proceed via an
acyclic intermediate that exists on a flat region of the potential
energy surface. Competition between cyclization and C–C bond
rotation results in varying levels of suprafacial stereoselectivity.
Quasi-classical trajectories were used to explore reaction dynamics
on this surface. Even though there is no discernible energy barrier
toward cyclization, a dynamically stepwise process is found, for which
the acyclic intermediate is found to reside for several hundreds of
femtoseconds. In a small number of cases, exceptionally long lifetimes
(>1000 fs) are found, leading to a loss of alkene stereochemistry.

阳离子自由基Diels-Alder环加成反应经由存在于势能面（potential energy surface）平坦区域的非环中间体进行。环化与C-C键旋转之间的竞争会导致同面立体选择性（suprafacial stereoselectivity）水平出现差异。本研究采用准经典轨迹（quasi-classical trajectories）对该势能面上的反应动力学展开探究。尽管环化过程不存在可观测的能垒，但研究发现了动态分步过程：在此过程中，非环中间体可存续数百飞秒（femtoseconds）。在少数案例中，观测到了极长的中间体寿命（>1000飞秒），进而导致烯烃立体化学特征的丧失。