Ferrocene-Stabilized Silicon Cations as Catalysts for Diels-Alder Reactions: Attempted Experimental Quantification of Lewis Acidity and ReactIR Kinetic Analysis
Ferrocene-Stabilized Silicon Cations as Catalysts for Diels-Alder Reactions: Attempted Experimental Quantification of Lewis Acidity and ReactIR Kinetic Analysis
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DOI：
10.1021/om401040y
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发表时间：
2014-01-13
期刊：
影响因子：
2.8
通讯作者：
Oestreich, Martin
中科院分区：
文献类型：
--
作者：
Noedling, Alexander R.;Muether, Kristine;Oestreich, Martin
二茂铁稳定的硅阳离子的 Si-29 NMR 化学位移范围很宽，具体取决于硅原子上的取代基。硅原子去屏蔽的这些显着差异并没有转化为它们在迪茨-阿尔德反应中催化活性的显着差异。通过与路易斯碱探针（Et3PO和吡啶-d(5)）形成路易斯对表明，这些硅阳离子与路易斯碱配位后几乎没有任何差异。这一发现不仅阻碍了游离路易斯酸的路易斯酸度的实验定量，而且还表明反应性差异很大程度上是由于给定抗衡阴离子的空间效应造成的。这些观察结果通过 ReactIR 动力学分析得到进一步验证。硅阳离子的路易斯酸度及其作为催化剂的性能不能与 Si-29 NMR 化学位移以及与路易斯碱探针的加合物的共振相关，甚至对于硅路易斯酸的子集也是如此。
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.