Effects of amine structure and base strength on acid-base cooperative aldol condensation

tAminated silica materials are known to efficiently catalyse aldol condensations, especially when silanolgroups are neighbouring the amine function. The effect of the amine structure and base strength hasbeen analysed experimentally and by kinetic modelling using commercially available precursors to graftprimary, secondary and tertiary amines on the silica surface. While primary amines are arranged in aclustered manner on the catalyst surface, secondary amines are arranged randomly which results in ahigher percentage of promoted amines in the low silanol-to-amine ratio range. An enamine compoundformed by the reaction between the amine active site and acetone has been identified as the key interme-diate to explain the experimental observations. In the case of a primary amine this enamine intermediatecan form an inhibiting imine with which it is in equilibrium. As a secondary amine has only one hydrogenatom bonded to the nitrogen atom, the inhibiting imine cannot be formed, resulting in a comparativelyhigher concentration of reactive enamines on the catalyst surface. In case of a tertiary amine the forma-tion of the reactive intermediate is impossible due to the absence of any hydrogen atom bonded to thenitrogen atom. The activation entropies of all reaction steps occurring on the amine sites, as obtained byregression, could be correlated to the deprotonation entropies of the amine sites. As the deprotonationenthalpy does not account for steric effects, no such correlation could be found between the activationenergies of these reaction steps and the deprotonation enthalpies of the amine sites