Solvent-Induced Reversal of Activities between Two Closely Related Heterogeneous Catalysts in the Aldol Reaction

Abstract

The relative rates of the aldol reaction catalyzed by supported primary and secondary amines can be inverted by 2 orders of magnitude, depending on the use of hexane or water as a solvent. Our analyses suggest that this dramatic shift in the catalytic behavior of the supported amines does not involve differences in reaction mechanism, but is caused by activation of imine to enamine equilibria and stabilization of iminium species. The effects of solvent polarity and acidity were found to be important to the performance of the catalytic reaction. This study highlights the critical role of solvent in multicomponent heterogeneous catalytic processes. ABSTRACT: The relative rates of the aldol reaction catalyzed by supported primary and secondary amines can be inverted by 2 orders of magnitude, depending on the use of hexane or water as a solvent. Our analyses suggest that this dramatic shift in the catalytic behavior of the supported amines does not involve differences in reaction mechanism, but is caused by activation of imine to enamine equilibria and stabilization of iminium species. The effects of solvent polarity and acidity were found to be important to the performance of the catalytic reaction. This study highlights the critical role of solvent in multicomponent heterogeneous catalytic processes.

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

“…However, the discovery rate may be further enhanced by exploiting more rational feedback obtained by means of kinetic modelling in the catalyst design cycle [3][4][5]. base amine sites has a enhancing effect on the catalytic activity of these amines in aldol condensations and other important C C coupling reactions [22][23][24][25][26][27][28][29][30][33][34][35][36][37][38]. Recently it has been demonstrated that an increasing acid strength of the promoting site leads to a decreasing activity which could be explained by a pronounced shift in the equilibrium from the free acid and free base towards the resulting neutralized ion pair [25,26,32].…”

Reaction-dependent heteroatom modification of acid–base catalytic cooperativity in aminosilica materials

“…Additionally, the choice of solvent can impact the activity of the bifunctional catalyst. A recent report demonstrated that primary amines tethered to silica were more active for the aldol condensation of acetone and 4-nitrobenzaldehyde in water than in hexane because water shifted the equilibrium in favor of the aldol product and prevented the amine from forming an imine with 4-nitrobenzaldehyde, which inhibited the reaction [32].…”

Synthesis of high surface area nanomaterials and their application in catalysis

“…Studies of the effect of water in the aldol condensation have been previously reported elsewhere. 38,39 They have demonstrated the role of water in the regeneration of active basic group (amines) at the expense of the inactive iminium form. This kind of observation is unlikely in our case as we don't expect to have amines in our carbons based on our investigations.…”