Catalysis by basic carbons: Preparation of dihydropyridines
´ ´ E. Perozo-Rondon a, V. Calvino-Casilda a, R.M. Martın-Aranda a, B. Casal b, ´ n-Valle c, M.L. Rojas-Cervantes a,* C.J. Dura
´ ´ ´ ´ Departamento de Quımica Inorganica y Quımica Tecnica, Facultad de Ciencias, UNED, Paseo Senda del Rey no 9, E-28040Madrid, Spain b Instituto de Ciencia de Materiales de Madrid, CSIC, E-28049 Madrid, Spain c ´ ´ Departamento de Quımica Inorganica, Facultad de Ciencias, Universidad de Extremadura, Avda. de Elvas s/n, 06071 Badajoz, Spain Available online 1 December 2005
Abstract The condensation of benzaldehyde and different substituted benzaldehydes, such as 2-nitrobenzaldehyde, 3-nitrobenzaldehyde, and2,4dichlorobenzaldehyde, with ethyl cyanoacetate was carried out using two alkaline carbons (Na-Norit and Cs-Norit) as catalysts in the absence of solvent. The reaction products are precursors in the production of 1,4-dihydropyridine derivatives, which have expanding practical applications as pharmaceuticals in the line of calcium channel blockers. High values of activity and selectivity were obtained.The most active carbon (Cs-Norit), which contains basic sites with pKb = 11.2, is more active than pyridine, and less than piperidine. The selectivity to the desired condensation product when using these activated carbons is, at least, as high as in the case of the homogeneous catalyst. This ‘‘green’’ and ‘‘clean’’ method (alkaline doped carbon catalyst in the absence of solvent) can be extendedto the preparation of other intermediates with medical applications. # 2005 Elsevier B.V. All rights reserved.
PACS: 82.65Jv Keywords: Catalysis; Activated carbon; Dihydropyridines; Fine chemicals
1. Introduction Microporous solids such as activated carbons have shown the utility of those properties in the domain of acid catalysis. However, little is known on their possibilities as basecatalysts. It has been shown  that carbons have basic sites, which are able to catalyse reactions needing weak and medium basic strengths sites. Moreover, their activities as base catalysts, can be modiﬁed by changing the countercation . Different reactions such as isomerization of linear butenes, aldolic condensation, and side chain alkylation in alkylaro´ ´ ´ ´ * Corresponding author at: Dpto.de Quımica Inorganica y Quımica Tecnica, ´ Facultad de Ciencias, Universidad Nacional de Educacion a Distancia (UNED), C/ Senda del Rey 9, E-28040 Madrid, Spain. Tel.: +34 91 398 7352; fax: +34 91 398 6697. E-mail address: firstname.lastname@example.org (M.L. Rojas-Cervantes). 0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.11.017
matics have been studiedon solid bases . There is another type of reaction catalysed by a basic catalyst, i.e. the Knoevenagel condensation, which has been used to measure the strength of the basis sites  and can also be used to form C–C bonds through the reaction of a carbonyl with an activated methylenic group. This type of reaction, which is of interest to prepare unsaturated esters and nitriles, could be alsoused as probe reaction to ‘‘titrate’’ catalysts with different basicity, by changing the pK of the reactant containing the activated methylenic group. Dihydropyridines chemistry is of interest not only from the point of view of fundamental research on heterocyclic compounds , but specially because of expanding practical applications of 1,4-dihydropyridine derivatives as pharmaceuticals in theline of calcium channel blockers [5,6]. In this work, we have employed the Knoevenagel condensation to prepare compounds of the type 1, which are intermediates in the preparation of some dihydropyridines of type 2 (Scheme 1).
´ E. Perozo-Rondon et al. / Applied Surface Science 252 (2006) 6080–6083 Table 2 Some characterization data of the catalysts Catalyst Norit Na-Norit Cs-Norit pH 8.0 8.4...