Amino-grafted mesoporous materials based on MCF structure involved in the quinoline synthesis. Mechanistic insights

Abstract

We report here a new series of ordered mesoporous metallosilicates which have been found to be efficient heterogeneous catalysts for the Friedländer condensation. 3-Aminopropyl-trimethoxysilane (APMS) and [3-(2-aminoethylamino)propyl] trimethoxysilane (2APMS) have been supported by grafting on a Mesoporous Cellular Foam (MCF). For comparison, both APMS and 2APMS have also been supported on an Nb-containing Mesoporous Cellular Foam (NbMCF). These hybrid materials have been tested in the reaction between 2-amino-5-chlorobenzaldehyde and ethyl acetoacetate leading to ethyl 6-chloro-2-methylquinoline-3-carboxylate with total selectivity. The condensation was more efficient when using 2APMS/MCF sample which exhibits higher nitrogen content than APMS/MCF. However, this trend was inverted when using the niobiosilicates analogues, most likely as a consequence of the interaction of amine groups with the niobium atoms in the siliceous framework. Experimental and theoretical studies demonstrated that the presence of water in non-activated samples plays an important role in the formation of the corresponding quinoline by stabilizing key transition structures and intermediate species.