Effects of primary and secondary surface groups in enantioselective catalysis using nanoporous materials with chiral walls.
Effects of primary and secondary surface groups in enantioselective catalysis using nanoporous materials with chiral walls.
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DOI：
10.1021/ja1017706
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发表时间：
2010-04
影响因子：
15
通讯作者：
A. Kuschel;S. Polarz
中科院分区：
文献类型：
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作者：
A. Kuschel;S. Polarz
介孔材料是固定化各种分子催化剂的重要载体。在固定后催化剂性能提高的情况很少被报道，特别是当它涉及到对映选择性合成时。因此，了解支撑表面的存在如何改变结合催化剂的性质是非常重要的。在本文中，提出了一种新的周期性有序介孔有机硅材料（PMO），其壁完全由手性构建块构成。Al（III）作为路易斯酸中心附着在手性基团上，使材料具有催化活性，例如，用于不对称羰基烯反应。因此，所述材料是研究手性表面影响的有价值的模型体系，也是研究网络中附着在硅醇基上的邻基的模型体系。据报道，与类似的分子参考催化剂相比，表面结合的Al（III）表现出明显更好的性能（更高的ee值）。此外，可以表明，当更多的粗大的二次基团附着在孔壁上时，ee值进一步增加。因此，本报告的主要结论是，在催化剂的空间条件起关键作用的情况下，将催化剂固定在定制的介孔有机二氧化硅材料中有利于催化中心和邻近表面基团之间的协同效应。
Mesoporous materials are valuable supports for the immobilization of various molecular catalysts. Cases in which the performance of the catalyst improves after immobilization have seldom been reported, especially when it comes to enantioselective synthesis. Knowledge of how the presence of the support surface alters the properties of a bound catalyst is therefore very important. In the current article, a new periodically ordered mesoporous organosilica material (PMO) with walls exclusively made of a chiral building block is presented. The attachment of Al(III) as a Lewis acid center to the chiral group furnishes the material with catalytic activity, for instance, for the asymmetric carbonyl ene reaction. Thus, the presented materials are valuable model systems for studying the effect of the chiral surface and also neighboring groups attached to the silanol groups in the network. It is reported that surface-bound Al(III) exhibits significantly better performance (higher ee values) than an analogous molecular reference catalyst. Furthermore, it could be shown that the ee values increase even further when more bulky secondary groups are attached to the pore walls. Therefore, the main conclusion of the current report is that for cases in which steric conditions of a catalyst play a crucial role its immobilization inside a tailor-made mesoporous organosilica material is beneficial with respect to cooperative effects between the catalytic center and neighboring surface groups.