Zeolites as base catalysts: Condensation of aldehydes with derivatives of malonic esters

The condensations of benzaldehyde with ethyl cyanoacetate, ethyl malonate, and ethyl acetoacetate were carried out with high activity and selectivity on lithium-, sodium-, potassium- and caesium-exchanged X and Y zeolites. The activity of the zeolites increased with decrease in the framework silicon-to-aluminium ratio of the zeolite and increase in the radius of the counter cation. Under reaction conditions, it was found that most of the basic sites in alkaline X and Y zeolites have pKb⩽10.3, and sites with pKb⩽13 were present only in the CsX sample. This catalyst is more active than pyridine, and less active but more selective than piperidine. It is shown that, on these catalysts, in the condensation reactions studied, the controlling step is not proton abstraction but attack of the carbonyl group by the carbanion.