Transesterification over various zeolites under liquid-phase conditions

Transesterification of beta-keto esters with a variety of alcohols to the corresponding beta-keto esters has been carried out efficiently over aluminosilicates simply by refluxing in toluene under liquid-phase conditions. Aliphatic beta-keto esters exhibit the higher reactivity than the aromatic and/or cyclic beta-keto esters, whereas other esters such as alpha-keto esters, alpha,beta-unsaturated esters, normal esters, and alpha-halo esters fail to undergo the transesterification. The reaction proceeds smoothly with primary alcohols than the tertiary-, cyclic-, and allylic alcohols. The large-pore zeolites such as Y, mordenite, and beta show higher activity than the medium-pore ZSM-5 and the partial removal of framework aluminum from the large-pore zeolites by dealumination increases their activity. The aluminum containing mesoporous MCM-41 shows poor activity. Among the different solvents used toluene leads to the highest activity and the optimum catalyst concentration found was ca. 20 wt.% of the substrate. The activity increases with increasing reaction temperature. The reaction pathway consists of formation of acyl ketene intermediate by the interaction of beta-keto esters with Bronsted acid sites of the catalyst, followed by nucleophilic attack of the alcohol at the electrophilic center and successive elimination of the proton to give the product. (C) 2003 Elsevier B.V. All rights reserved.