Department of Chemistry, Rikkyo University, Nishi-Ikebukuro, Toshima-ku, 171-8501 Tokyo, Japan.
Chemistry. 2011 Jan 24;17(4):1230-7. doi: 10.1002/chem.201001926. Epub 2010 Dec 7.
The mechanistic dichotomy between concerted E2 and stepwise E1cb of the base-promoted elimination of 2-aryl-3-chloro-2-R-propanols was examined computationally at the HF, M05-2X, and MP2 levels of theory. Optimizations of transition states (TSs) and reaction intermediates, and intrinsic reaction coordinates (IRC) calculations showed that there was a single reaction route for each substrate, and that the mechanism could be changed from E2 to E1cb by making a carbanion intermediate more stable through the introduction of electron-withdrawing substituents. Molecular dynamics simulations revealed that trajectories started at a single TS led directly to two product regions; the carbanion intermediate region in the E1cb mechanism, and the alkene product region in the E2 mechanism, through path bifurcation after the TS. The present system is a new example of bifurcation in reactions of closed-shell molecules. The overall reaction mechanism changes dynamically from E2 to E1cb by a gradual change in the ratio of E2 and E1cb trajectories, rather than a path switch in concurrent pathways.
在 HF、M05-2X 和 MP2 理论水平上，通过计算考察了碱基促进消除 2-芳基-3-氯-2-R-丙醇中协同 E2 和逐步 E1cb 的机理二分法。过渡态 (TS) 和反应中间体的优化以及内禀反应坐标 (IRC) 计算表明，对于每种底物都存在单一的反应途径，并且可以通过引入吸电子取代基使碳负离子中间体更稳定，从而将机理从 E2 改变为 E1cb。分子动力学模拟表明，从单个 TS 开始的轨迹直接通过 TS 后分叉，分别通向 E1cb 机制中的碳负离子中间体区域和 E2 机制中的烯烃产物区域。本体系是闭壳分子反应中分叉的新实例。通过 E2 和 E1cb 轨迹的比例逐渐变化，而不是在并发途径中进行路径切换，整体反应机理从 E2 动态地变为 E1cb。
