Comparative study for3,3´-[(4-X-phenyl)-methanediyl]bis(1H-indoles) synthesis catalyzed by Ce(III) cations
Abstract
Results of comparative study for 3,3´-[(4-X-phenyl)methanediyl]bis(1H-indoles) synthesis starting from 4-X-benzaldehyde (X = H, OCH3, N(CH3)2, NO2, OH) and 1H-indole in dichloromethane solution at room temperature and catalyzed by cerium trichloride heptahydrate or by new resin supported Ce(III) catalyst are described. It was found that Ce(III) cations supported on weakly acidic macroporous low-cost industrial resin have at least the same or better catalytic activity as CeCl3.7H2O (or other mentioned catalytic systems) advantageously its easy separation for a recycling.
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Introduction
Solid support metal catalysis is a way for a combination of both positive properties connected with heterogeneous and homogeneous metal catalysis, i.e. a simple separation of catalyst by sedimentation, filtration or by similar processes (heterogeneous catalysis) and on other hand an utilization of auxiliary ligands for an optimal catalytic action of a metal in the case of the homogeneous metal catalysis. [1, 2] Auxiliary ligands are in the optimal cases a part of supporting matter. Solid supported catalysts are, thanks to these properties, suitable as catalysts for the synthesis carried out according to the principles of Green Chemistry. [3]
Recently, we found and described a possibility for support of some metal cations by an application of weakly acidic macroporous low-cost industrial acrylate resins. [4] Also cerium(III) cations can be immobilized/supported in this manner. Cerium(III) in many different forms (i.e. chloride, nitrate, supported on silica gel and other), as a Lewis acid, attracted the attention especially due to its high reactivity, low toxicity, ease of handling, stability, air tolerance and finally low cost. [5, 6]
The catalytic efficiency of Ce(III) cations in synthesis of 3,3´-[(4-X-phenyl)methanediyl]bis(1H-indole) derivatives was studied in this paper. Alkylated indole derivatives are frequently present in many pharmaceutical and natural products and also in wide range of functional synthetics. [7, 8] Furthermore, 3,3´-[(4-X-phenyl)-methanediyl]bis- -(1H-indole) derivatives show a broad range of biological activities such as anti-fungal, anti-inflammatory, anti-pyretic, cardiovascular, anti-convulsant, COX2 inhibitory and selective β-glucuronidase activity. [9, 10, 11] This is one of the reasons, why is in recent years paid more attention to the methods of their syntheses.
Derivatives of 3,3´-[(4-X-phenyl)methanediyl]bis(1H-indoles) are usually prepared from carbonyl compounds and indole in the presence of Lewis acids or protic acids. [9, 12, 13, 14] The catalysts used for their synthesis may be even zirconium or palladium complexes [15] or CeCl3.7H2O [16].
Conclusion
In this comparative study we proved, that catalytic system of Ce(III) supported on weakly acidic macroporous resin of polyacrylate type has in the regioselective C3 alkylation of indoles at least the same efficiency as CeCl3.7H2O and some other catalysts chosen from literature (containing for example palladium, copper or scandium). Furthermore, this catalytic system has all advantages of supported catalysts and as solid support was used low-cost stable industrial resin.