Bifunctional Mesoporous Silica for Cooperative Catalysis Prepared Through Sequential Grafting of An Organic Base and An Organic Acid

Coupling reactions, such as aldol, nitroaldol, and Knoevenagel reactions that are typically acid or base-catalyzed, can be accelerated through the addition of a second component that acts cooperatively.  Depending on the nature of the interaction, the two components can also quench one another, inhibiting the catalytic activity.  Mutually reactive and cooperative functional groups can be separated through grafting onto mesoporous silica, permitting cooperativity and enhancing the rate of reaction relative to the single component system.  Here we demonstrate the sequential grafting, spectroscopic characterization, and catalytic activity of a cooperative catalyst with an organic acid and an organic base attached to the surface of a mesoporous silica.  The acid component was protected using a thermally cleavable protecting group, demonstrating for the first time that two mutually reactive functional groups can be sequentially grafted onto mesoporous silica.  The bifunctional catalyst conclusively demonstrates the importance of the strength of the acid component for the aldol condensation.  This concept is shown to be broadly applicable to similar C-C bond forming reactions.  The observed trend of acid strength has provided direction for future catalyst design.