Effects of Primary and Secondary Surface Groups in Enantioselective Catalysis Using Nanoporous Materials with Chiral Walls

Abstract

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 AlIII 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 AlIII 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.