Heterogeneous Knoevenagel condensation

Base catalysis is one of the less-well developed areas of heterogeneous catalysis. We have developed novel bases derived from amines via the one-step process outlined above. A range of supported amines have been prepared and evaluated in a series of reactions We have also investigated the nature of the amine groups attached to the surface in comparison with those formed by grafting onto pre-formed silica. While many workers have studied the use of basic catalysts for the Knoevenagel condensation of aldehydes, with three articles on the use of MCM derivatives[ 12], little has been done on the more demanding condensation of ketones. 

Further projects dealt with the fabrication of heterogeneous, basic or acidic solid-state catalysts or adsorbents carrying, for instance, amino or sulfonic acid groups. Amino-functionalized silicas were prepared and analyzed for the catalytic activity in Knoevenagel condensation reactions of aldehydes or ketones with ethyl cyanoace-tate ions by Macquarrie et al.154 155 Recently, Zhang et al.156,157 reported on the successful preparation of amino-functionalized silica thin films by means of the EISA approach. 

Lewis base sites in PCPs have the potential to show different catalytic properties from Lewis acid sites.52 165 166 The three-dimensional PCP, [Cd(NC>3)2(4-btapa)2] 6H20 2DMF (4-btapa = 1,3,5-benzenetricarboxylic acid tris[/V-(4-pyridyl)-amide]), has amide groups that act as guest interaction sites on the surfaces of channels with dimensions of 4.7 x 7.3 A2165 A Knoevenagel condensation reaction catalyzed by this PCP demonstrates its selective heterogeneous base catalytic properties, which depend on the size of the reactants. This solid catalyst maintains its crystalline framework after the reaction and is easily recycled. 

Reddy, B.M., Patil, M.K., Rao, K.N., and Reddy, G.K. 2006. An easy-to-use heterogeneous promoted zirconia catalyst for Knoevenagel condensation in liquid phase under solvent-free conditions. Tetrahedron Letters, 47 4409-13. 

Angeletti E, Canepa C, Martinetti G, Venturello P (1989) Amino groups immobilized on silica gel An efficient and reusable heterogeneous catalyst for the knoevenagel condensation. J Chem Soc Perkin Trans 1 105... 

Sebti, S., Saber, A., Rhihil, A. Natural phosphate and trisodium phosphate novel solid catalysts for the Knoevenagel condensation in heterogeneous media. Tetrahedron Lett. 1994, 35, 9399-9400. 

Kantam, M. L., Bharathi, B. Mn(lll) salen catalyst for Knoevenagel condensation - a novel heterogeneous system. Catai. Lett. 1998, 55, 235-237. 

Fig. 15 Heterogeneous Bronsted base catalysis of the Knoevenagel condensation of ethyl cyanoacetate with benzaldehyde using a cross-linked Hu assembly...

In recent years efforts have been focused on the search for new solid catalysts for the Knoevenagel condensation, and numerous types of heterogeneous acidic... 

When the basic catalyst cannot form imine or enamine intermediates, the Knoevenagel condensation occurs by attack of the carbanion on the carbonyl oxygen, which is activated, by hydrogen bonding or protonation, by the acid component of the catalyst [26] (Scheme 3B). Some examples of the use of heterogeneous cat-... 

Under heterogeneous conditions, the Knoevenagel reaction has been used as a well-adopted test reaction to check the activity of the basic sites of different solids, mainly basic zeolites such as alkali-exchanged zeolites [40] or zeolites containing occluded metal oxides [41]. The basic activity of alkali-containing MCM-41 [42] or binary cesium-lanthanum oxide supported on MCM-41 [43] has, moreover, also been evaluated in the Knoevenagel condensation. 

All of these experimental observations are consistent with the heterogeneous nature of 6 as a weak base catalyst for Knoevenagel condensation reactions. 

Gascon et al. [61] reported that IRMOF-3 and Am-functionalized MIL-53, both made from the 2-aminoterephthalic acid Hnker, were active as base catalysts in the Knoevenagel condensation of benzaldehyde and ethyl cyanoacetate in dimethylformamide at 60°C. The IRMOF-3 catalyst showed higher performance than the homogeneous anihne reference catalyst, with a conversion of 90% achieved compared to 55%, respectively. After the removal of the IRMOF-3 catalyst from the liquid reaction product mixture, no further conversion was observed, demonstrating the stability of the heterogeneous catalyst. The Am-MIL-53 catalyst was found to be less active in this reaction probably because of a strong adsorption and a slow diffusion of the reaction substrates and products inside the pores of this particular MOF. 

MCM-41 and silica gel as efficient and reusable heterogeneous catalyst for the Knoevenagel Condensation reactions. Burwell and Leal (1974) first reported selective chemisorption of sulfur dioxide on amine-modified silica gel. Leal et al. (1995) studied carbon dioxide adsorption on amine surface-bonded silica gel, although their CO2 adsorption amount (0.3 mmol/g-sorbent at 1 atm CO2) was low. Further work by Huang et al. (2003) produced high-capacity and selective sorbents for both CO2 and H2S. 

The use of a heterogeneous catalyst not only iaciUtates its reutihzation but also provides certain heat and mass transfer limitations, which are known to modify selectivity and yield. As MSRs offer high spedfic siulaces, the influence of transfer phenomena on the overall reaction can be reduced partially or completely improving the catalytic performance. Some examples reported in the hterature are the Simiki coupling reactions, the Knoevenagel condensation reaction, enzymatic hydrolysis [10], and the esterification reaction [5]. 

J. J., Li, M.J., Qiu, H.Y., and Lai, G.Q. (2006) Synthesis of ionic liquid functionalized SBA-15 mesoporous materials as heterogeneous catalyst toward Knoevenagel condensation under solvent-free conditions. Eur.. Inorg. Chem., 2006, 2947-2949. 

Numerous types of basic heterogeneous catalysts, such as alkahne earth metal oxide, anion exchange resins and alkali metal compounds supported on alumina or zeolite can catalyze various chemical reactions such as isomerization, aldol, Michael, and Knoevenagel condensation, oxidation and transesterification [1], Today considerable attention is devoted to the production of biodiesel (FAMEs) as an alternative for petroleum-derived diesel fuel. Biodiesel is synthesized by direct transesterification of vegetable oil or animal fat with a short-chain alcohol, viz. methanol, ethanol, and isopropanol in presence of an acid, base or enzymatic catalyst [2], Considering the advantages of solid base catalysts, for easy separation and recovery, reduced corrosion and environmental acceptance [1], many studies have been conducted on basic heterogeneous catalysts development for biodiesel production [3-13],... 

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