Novel Synthesis Strategies for the Incorporation of Cooperativity in Catalytic Systems.

摘要

Grafting of discrete pairs of functional groups onto a silica surface was demonstrated using an organochlorosilane precursor molecule that can be tethered onto a surface at both ends and that possesses a cleavable silyl ester bond. After grafting to a silica surface, hydrolytic cleavage of the silyl ester bond resulted in the placement of discrete pairs of carboxylic acid and organosilanol groups. This organosilanol moiety was easily transformed into other functional groups through condensation reactions to form pairs such as carboxylic acid/secondary amine, carboxylic acid/pyridine, and carboxylic acid/phosphine. In the case of carboxylic acid/amine pairing, there was evidence of the formation of amide. Herein we present our findings on the application of the carboxylic acid/amine functionalized surface for the aldol and nitroaldol (Henry) condensation reactions. Such reactions have been shown to exhibit cooperativity between nearby acid and base groups through dual activation of the substrates. We show that for both reactions, the activity of the surfaces in which both acid and base are simultaneously grafted in pairs is greater than when groups are grafted sequentially resulting in a random distribution. Furthermore, due to mechanistic considerations, the activity enhancement is much more pronounced in the Henry reaction than the aldol condensation with the paired sample displaying roughly twice the turnover.