Carrascosa Eduardo, Meyer Jennifer, Michaelsen Tim, Stei Martin, Wester Roland
Institut für Ionenphysik und Angewandte Physik , Universität Innsbruck , Technikerstraße 25 , 6020 Innsbruck , Austria . Email:
Chem Sci. 2017 Nov 13;9(3):693-701. doi: 10.1039/c7sc04415a. eCollection 2018 Jan 21.
Nucleophilic substitution (S2) and base-induced elimination (E2), two indispensable reactions in organic synthesis, are commonly assumed to proceed under stereospecific conditions. Understanding the way in which the reactants pre-orient in these reactions, that is its stereodynamics, is essential in order to achieve a detailed atomistic picture and control over such processes. Using crossed beam velocity map imaging, we study the effect of steric hindrance in reactions of Cl and CN with increasingly methylated alkyl iodides by monitoring the product ion energy and scattering angle. For both attacking anions the rebound mechanism, indicative of a direct S2 pathway, is found to contribute to the reaction at high relative collision energies despite being increasingly hindered. An additional forward scattering mechanism, ascribed to a direct E2 reaction, also contributes at these energies. Inspection of the product energy distributions confirms the direct and fast character of both mechanisms as opposed to an indirect reaction mechanism which leads to statistical energy redistribution in the reaction complex. This work demonstrates that nonstatistical dynamics and energetics govern S2 and E2 pathways even in sterically hindered exchange reaction systems.
亲核取代反应（S2）和碱诱导消除反应（E2）是有机合成中两个不可或缺的反应，通常认为它们在立体专一性条件下进行。了解反应物在这些反应中的预取向方式，即其立体动力学，对于获得详细的原子层面图像并控制此类过程至关重要。我们使用交叉分子束速度成像技术，通过监测产物离子能量和散射角，研究了Cl和CN与甲基化程度不断增加的烷基碘反应中的空间位阻效应。对于这两种进攻性阴离子，尽管空间位阻不断增加，但在高相对碰撞能量下，表明直接S2途径的反弹机制对反应有贡献。另一种归因于直接E2反应的向前散射机制在这些能量下也有贡献。对产物能量分布的检查证实了这两种机制的直接性和快速性，这与导致反应复合物中统计能量重新分布的间接反应机制相反。这项工作表明，即使在空间位阻较大的交换反应体系中，非统计动力学和能量学也支配着S2和E2途径。
