Polymer microgel particles as basic catalysts for Knoevenagel condensation in water

The catalytic activities of basic and thermoresponsive microgel particles for the Knoevenagel condensation reaction in water at ambient temperature were evaluated. The gel particles (GPs) were incorporated with primary-amino, tertiary-amino, imidazole or pyridyl groups. In the Knoevenagel condensation between benzaldehyde and ethyl cyanoacetate, the GPs that had no basic group did not exhibit any catalytic activity. The GPs that had primary-amino, tertiary-amino and imidazole groups exhibited catalytic activity. The catalytic activity of the basic GPs depended on their base strength. The tertiary-aminated microgel particles had quadruple and double the activities of tertiary-aminated silica gel and trimethylamine, respectively. The higher catalytic activity may be attributable to the local enrichment effect of ethyl cyanoacetate. In terms of gel geometry, the activity of the tertiary amines in the microgel particles was higher than that in the bulk gel. The geometry of the microgel particles allowed the rapid uptake of the substrates. Because the base strength of the tertiary amines in the microgel particles decreased with increasing temperature, the catalysts and residual active methylene substrates were separated by dialysis at 80 °C. The recovered microgel particle catalysts were recyclable for an additional Knoevenagel condensation.