Water catalytic effect on the carbinolamine reaction with amine-catalyzed mesoporous silica nanoparticles

The formation of carbinolamine represents the crucial initial step in the aldol reaction, specifically involving the interaction between p-nitrobenzaldehyde and acetone, facilitated by amine-catalyzed mesoporous silica nanoparticles (amine-MSN). In this process, a nitrogen atom from propylamine, which acts as the catalytic moiety, engages in the formation of a covalent bond with a carbon atom from acetone, leading to the generation of a carbinolamine intermediate. This reaction is significantly influenced by the presence of silanol groups located on the surface of the amine-MSN, which contribute to the catalytic activity. Moreover, the reaction solvent plays a vital role; water serves as an essential solvent that enhances the rate of the aldol reaction. In this work, a comprehensive investigation is conducted to understand the catalytic influence of water explicitly in carbinolamine formation via three distinct pathways: (1) without the assistance of silanol groups, (2) silanol assistance occurs indirectly through hydrogen bonding, and (3) a pathway that showcases direct silanol assistance in the hydrogen atom transfer processes. To analyze the reaction mechanism in the amine-MSN system, this study employs the fragment molecular orbital method, which can accurately treat the complexities associated with large molecular systems like the MSN pore model (Si392O958H348). The concerted mechanism study reveals that the presence of water molecules significantly lowers the energy barrier associated with carbinolamine formation, thereby enhancing the feasibility and efficiency of the reaction. This insight highlights the pivotal role of the solvent and the nature of catalytic sites in optimizing aldol reactions within mesoporous silica frameworks.