Understanding Intramolecular Cooperativity in Acid-Base Silica-Supported Organocatalysts

Carbon-carbon coupling reactions are essential steps in pharmaceutical synthesis as well as biomass valorization toward chemicals and fuels. Often, complex homogenous transition-metal catalysts are required for highly selective carbon-carbon coupling reactions, but they are expensive and not recyclable and thus unfeasible for large scale production. Inspired by enzymatic catalysis, heterogeneous organocatalysts can be designed for controlled selectivity and can utilize multiple functionalities, such as acids and bases, to achieve specific catalytic function. Mesoporous silica provides a weakly acidic solid phase with high surface area to allow for good dispersion of catalytic functional groups. This project studies how the linker length between the amino and alcohol moieties affect the intramolecular cooperativity of basic amino alcohol silanols and the acidic silanols. The functional groups are grafted using a two-step method and HMDS was used between the steps to remove the silanol groups to create a control to demonstrate exclusively intramolecular interactions. Batch liquid-phase kinetics studies utilizing these catalysts in acid-base catalyzed aldol condensation reaction of 4-nitrobenzaldehyde and acetone were performed to determine relative activities and site-specific rates. Experimental data have shown that longer spacer length between the amino and alcohol moieties on the catalytic amines improves conversion and allows for increased intramolecular activity, but the further distance from the promoting acidic silanol sites creates a hindering effect. A future direction will use optimized catalysts from this project to test on more industrial relevant reactions such as upgrading of furfural into a longer chain diesel fuel precursor.