Knoevenagel and Michael Reactions on Cation-exchanged Zeolites

2 Knoevenagel and Michael Reactions on Cation-exchanged Zeolites 

Besides the general advantages associated with zeolites, the use of exchanged zeolites as heterogeneous basic catalysts has the additional advantage that they can be exposed to the atmosphere, because carbon dioxide and water are not absorbed too strongly and can be removed by thermal treatment. They can be used even in systems in which water or CO2 are involved as reactants or products. 

Despite all these advantages, however, exchanged zeolites have rarely been used as basic catalysts in the production of fine chemicals. This is probably because of their weak basic character and because bulky reactants are involved in many chemical processes. Their basicity is, nevertheless, sometimes sufficient to catalyze Knoevenagel condensations and Michael additions. 

Alkali metal-exchanged zeolites have been used to prepare activated alkenes of interest as prepolymers by Knoevenagel condensation of malononitrile with ketones having different positive charge density on the carbon of the carbonyl group benzophenone, cyclohexanone, and p-aminoacetophenone [85]. The reactivity depends both on ketone structure (the order of reactivity was benzophenone cyclohexanone p-aminoacetophenone) and on the catalyst used. For instance, when malononitrile is condensed with cyclohexanone in the presence of a CsY zeolite conversion was very low. CsX zeolite, however, which has a substantial number of basic sites with pX in the range 9 P a 10.7 and some with 10.7 13.3 could be used to perform 

and Re-exchanged zeolites have also been used as catalysts of the Michael reaction between silyl enol ethers and a, 6-unsaturated carbonyl compounds. This study, performed by Sasidharan et al. [87], focused mainly on the catalytic activity of titanium silicalite molecular sieves (TS-1 and TS-2). They found that TS-1 and TS-2 catalyze 1,4-Michael addition of silyl enol ethers and a,y5-unsatu-rated carbonyl compounds under anhydrous conditions. The zeolites tested as catalysts of this reaction, e. g. ReY, LaY, steamed zeolite Y, and cation-exchanged ZnZSM-5, were less active (or inactive). 

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