Mesoporous niobiosilicate NbMCF modified with alkali metals in the synthesis of chromene derivatives

Article abstract of DOI:10.1016/j.cattod.2016.02.042

We report the synthesis, characterization and activity of new series of catalysts based on Niobium Mesoporous Cellular Foams (NbMCF), modified with alkaline metal elements. These materials exhibit very different acid-base properties attributed to the Nb source for the preparation of the catalysts and the presence and loading of different alkaline metals. The Me/NbMCF materials (where Me are alkaline metals) were tested in the reaction of 2-hydroxybenzaldehyde and ethyl cyanoacetate, under solvent-free conditions, at room temperature, leading to mixtures of the corresponding chromenes 4 and 5, as mixtures of diastereomers erythro/threo, in a 2:1 ratio, respectively, with good to excellent yields. Our experimental results indicate that the metal loading on the catalysts and the acid-base character, and the texture parameters are probably determining factors in the reactivity and the observed diastereoselectivity. The computational study suggests that the presence of alkaline oxides on Me/NbMCF catalyst, exhibiting strong basicity, activates the formation of the nucleophile species, the corresponding enolate, and initiates the reaction. However, the cation size has a deep impact on the stability of the reactant complex so that the effective aldolization could be obstructed by the presence of the bulkier centers.