Amino-grafted SBA-15 material as dual acid–base catalyst for the synthesis of coumarin derivatives

• Amino-grafted SBA-15 is an efficient catalyst for the synthesis of coumarins.
• The reaction occurs by Knoevenagel condensation followed by non catalytic lactonization.
• MAP/SBA-15 exhibiting basic properties can be considered as dual acid–base catalyst.
• Substitution in 3 affects to the electrophility and acidity of CHO and OH groups, respectively.

We report herein an experimental and theoretical study concerning the preparation of the coumarins (2) from 2-hydroxybenzaldehydes (3) and ethyl acetoacetate (4) promoted by amino-grafted SBA-15 materials. The reaction takes places by Knoevenagel condensation between reagents, in the absence of any solvent and under mild conditions, followed by a non catalytic lactonization step.Amino-grafted SBA-15 loaded with secondary amine groups, MAP/SBA-15, was found to be the most efficient and totally recyclable catalyst as compared with its analog containing tertiary amine groups, DEAP/SBA-15 (where MAP and DEA are methyl aminopropyl and diethyl aminopropyl groups, respectively). Our theoretical analysis confirms that the steric congestion and the absence of NH protons as the catalytic active sites are determining factors for the increasing of the activation barrier and, therefore, for the lower catalytic activity and reactivity to the formation of the coumarin.The computational study herein reported demonstrates that MAP/SBA-15, a traditional basic mesoporous silica having NH protons, shows a dual acid–base catalytic behavior when used in the synthesis of coumarins via Knoevenagel condensation.Otherwise, the substitution in 2-hydroxybenzaldehydes (3) at 5-position clearly influences the acceptor and acidity capacities of the CHO and the OH groups as experimentally and theoretically demonstrated.

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