An Eco-Friendly Improved Protocol for the Synthesis of Bis(3-indolyl)methanes Using Poly(4-vinylpyridinium)hydrogen Sulfate as Efficient, Heterogeneous, and Recyclable Solid Acid Catalyst
Highly efficient and eco-friendly protocol for the synthesis of bis(3-indolyl)methanes by the electrophilic substitution reaction of indole with aldehydes catalyzed by poly(4-vinylpyridinium)hydrogen sulfate was described. Excellent yields, shorter reaction times, simple work-up procedure, avoiding hazardous organic solvents, and reusability of the catalyst are the most obvious advantages of this method. 1. Introduction Indole derivatives have emerged as important class of nitrogen containing heterocycles and are known to possess broad spectrum of biological and pharmacological activities [1, 2]. In particular, bis(indolyl)methanes (BIMs) which are isolated from terrestrial and marine natural sources such as parasitic bacteria, tunicates, and sponge are found as possible antibacterial, anticarcinogenic, genotoxic, and DNA-damaging agents [3]. BIMs are active cruciferous substances for promoting estrogen metabolism [4] and have the ability to prevent cancer by modulating certain cancer-causing estrogen metabolites [5]. Owing to their diverse biological properties, many methods have been developed for their synthesis using various catalytic systems such as amberlyst-15 [6], iodine [7], boric acid [8], fluoroboric acid [9], sulfamic acid [10], NbCl5 [11], silica sulfuric acid [12], cellulose sulfuric acid [13], zeolite [14], ceric ammonium nitrate [15], polyvinylsulfonic acid [16], dodecylsulfonic acid [17], dodecylbenzenesulfonic acid [18], HClO4-SiO2 [19], ZrOCl2 8H2O [20], Dy(OTf)3 [21], protic solvent [22], and ionic liquids [23]. However, most of these reported methods suffer from one or several drawbacks such as low yields, prolonged reaction times, use of hazardous, expensive, moisture-sensitive, and large quantity of reagents, involving harsh reaction conditions, tedious workup procedure, and difficulty in recovery, and reusability of the catalysts. Therefore, still there is a need to develop an efficient, eco-friendly, and versatile method for the synthesis of bis(indolyl)methanes. In continuation of our research towards the synthesis of biologically important molecules using novel methodologies [24], we report herein a simple, highly efficient, and eco-friendly method for the synthesis of bis(3-indolyl)methanes using poly(4-vinylpyridinium)hydrogen sulfate [P(4-VPH)HSO4] [25] as heterogeneous and reusable solid acid catalyst. 2. Results and Discussion The electrophilic substitution reaction of indole with aryl aldehydes catalyzed by P(4-VPH)HSO4 is shown in Scheme 1. The reaction smoothly proceeds at room temperature under grinding technique to
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