Ferrocene-Stabilized Silicon Cations as Catalysts for Diels–Alder Reactions: Attempted Experimental Quantification of Lewis Acidity and ReactIR Kinetic Analysis
摘要:
The Si-29 NMR chemical shifts of ferrocene-stabilized silicon cations span a wide range depending on the substituents at the silicon atom. These pronounced differences in deshielding of the silicon atom do not translate into significant differences in their catalytic activity in Diets-Alder reactions. It was shown by Lewis pair formation with Lewis base probes (Et3PO and pyridine-d(5)) that there is hardly any difference between these silicon cations after coordination to a Lewis base. This finding not only thwarts experimental quantification of the Lewis acidity of the free Lewis acids but also demonstrates that the reactivity differences are largely due to steric effects for a given counteranion. These observations are further verified by a ReactIR kinetic analysis. The Lewis acidity of silicon cations and their performance as catalysts cannot be correlated with Si-29 NMR chemical shifts as well as resonances of adducts with Lewis base probes, not even for a subset of silicon Lewis acids.
进行系统的实验和理论研究的目的是深入了解二茂铁稳定的硅离子作为硅原子上取代基的函数的独特键合情况,并了解决定29 Si NMR化学位移和亲电性的结构参数这些强路易斯酸。为此,通过硅烷与三苯甲基阳离子的氢化物抽象反应制备了二茂铁稳定的硅阳离子家族的十个新成员,并以多核 1 H和29为特征Si NMR光谱。仔细观察NMR谱图可以发现,其他次要信号不是杂质,而是取代模式与最初形成的阳离子不同的甲硅烷基离子。这些信号的仔细分配提供了实验证明,即受空间影响较小的甲硅烷基离子能够与未反应的硅烷前体交换取代基。密度泛函理论计算提供了对取代基转移的机理性见解,其中迁移基团在涉及二茂铁桥接中间体的协同过程中在两个硅片段之间交换。此外,对29 Si NMR化学位移的量子化学分析揭示了δ(29Si)值和硅阳离子子集的Fe⋅⋅⋅Si距离。电子本地化功能和电子本地化指标分析显示,铁,硅和C'ipso原子之间存在一个