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

Abstract

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