Transesterification over various zeolites under liquid-phase conditions

Abstract

Abstract Transesterification of β-keto esters with a variety of alcohols to the corresponding β-keto esters has been carried out efficiently over aluminosilicates simply by refluxing in toluene under liquid-phase conditions. Aliphatic β-keto esters exhibit the higher reactivity than the aromatic and/or cyclic β-keto esters, whereas other esters such as α-keto esters, α,β-unsaturated esters, normal esters, and α-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 β 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 β-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.