Understanding the influence of the immobilization procedure on the catalytic activity of aminopropylsilicas in C-C forming reactions

Four aminopropylsilicas, prepared following different synthetic methodologies, namely by the grafting method (MCM-41-NH2, KG-60-NH2) and by the sol-gel methodology (HMS-NH2 and APS-NH2) have been tested in classical C-C forming reactions such as the nitroaldol condensation between nitromethane and benzaldehyde to afford nitrostyrene and the Michael addition of nitromethane to 2-cyclohexen-1-one to give 3-nitromethylcyclobexanone. The catalytic activity of these materials is strictly dependent on the dispersion and the accessibility of the active sites which, in turn, depend on the specific method utilized for the catalyst preparation. In the grafting procedure, the aminopropyl groups are linked to the surface of a preformed meso- or macroporous material and consequently the majority of them are easily reachable by the reagents and take part in the reaction; on the contrary, in the materials prepared by sol-gel technique, variable amounts of the propylamine are incorporated within the siliceous framework and only a small portion of the total amine group is usually active. (C) 2003 Elsevier Science B.V. All rights reserved.