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

Abstract

a b s t r a c tWeak 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 b-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.

Metalloenzyme‐Like Zeolites as Lewis Acid Catalysts for CC Bond Formation

“…Facilitating cooperative interactions among sites in heterogeneous catalysts has proven to be apowerful approach for promoting C À Cc oupling reactions. [7] Analogies can be drawn to enzymatic systems.F or example,c lass II aldolases catalyze the direct aldol reaction of dihydroxyacetone phosphate (DHAP) and aldehydes through cooperative activation of DHAP.T he proposed transition-state model for the active sites of this lyase shows that the Lewis acidic site Zn II coordinates to the carbonyl oxygen atom of the ketone donor, while aglutamate residue functions as aBrønsted base for abstraction of the a-proton. [8] Forhydrophobic pure silica zeolites,a cid-base interactions are critically dependent on physical properties such as the polarizability of the metal atom in the active site and the Brønsted basicity of the oxygen atoms bound to the metal atom.…”

Impact of surface loading on catalytic activity of regular and low micropore SBA‐15 in the Knoevenagel condensation

“…Interestingly, the Knoevenagel condensation is known to be enhanced by cooperative activation of the substrates by amines positioned in close proximity to surface silanols . While work has investigated the design of cooperative catalytic materials for the Knoevenagel condensation, significantly more work has been completed for the aldol reaction and condensation and the nitroaldol reaction and condensation . One design element of interest is the effect of surface loading on catalytic activity.…”

Trace water/amino-modified silica aerogel catalytic system in the one-pot sequential reaction of benzaldehyde dimethyl acetal and nitromethane

“…One-pot cascade reactions, which contain two or more chemical steps and therefore reduce the synthesis steps such as intermediate isolation and purication, save energy consumption, lower operation cost, and decrease the amount of waste, have attracted increasing attention in recent years [1][2][3][4][5][6].…”