Spatial arrangement and acid strength effects on acid–base cooperatively catalyzed aldol condensation on aminosilica materials

Abstract

Weak acids are known to enhance the activity of amines in aldol condensation reactions on silica-based catalysts. The effects of acid strength and arrangement of the promoting site with respect to a secondary amine have been investigated in the aldol condensation of 4-nitrobenzaldehyde with acetone. Changing the substituent of this secondary amine from a methyl to an ethyl group decreases the activity. An intramolecular OH function provided by a primary alcohol incorporated on the β-carbon of the amine substituent exhibits a similar cooperativity as an intermolecular OH function provided by neighboring surface silanols. A maximum activity was achieved when the secondary amine with the same primary alcohol-containing substituent was surrounded by surface silanols, indicating the potential advantage of simultaneously activating both reactants by the formation of a hydrogen bond in contrast to the consecutive activation when there is only one promoting site in the vicinity of the amine. Changing the alcohol to stronger acids resulted in a reduced cooperativity with increasing acid strength. After removing the silanols from the surface, the activity of the catalysts which exhibit an intramolecular cooperativity retained about 68–83% of their activities while the activity of the conventional secondary amine was reduced by a factor of four compared to its intermolecularly cooperative counterpart.