Ultrasound-Mediated Synthesis of 2,4,6-Triaryl-Pyridines Using MgAl2O4 Nanostructures

Abstract

Ultrasound-accelerated organic chemical reactions have been increasingly developed by researchers across the globe for the synthesis of organic compounds. Ultrasound irradiation offers an alternative energy source for organic synthesis which are ordinarily accomplished by heating. Ultrasound- mediated reactions proceed by the formation, growth and collapse of acoustic bubbles in the reaction medium. These directly help in shortening the time span of reactions and increasing the yield of products [1]. The pyridine ring system is present in various natural compounds, and many pyridines exhibit a board range of biological activities [2]. Due to their π-stacking ability, some pyridine derivatives are used in supramolecular chemistry [3]. Recently, these heterocyclic compounds have also been evoked considerable attention as these endowed with wide range of pharmaceutical activities such as anticovulsant, antimalarial, anesthetic, vasodilator, and antiepileptic and agrochemicals such as pesticidial, fungicidal and herbicidal [4,5]. Therefore, it has attracted continuous interest to develop procedures for the synthesis of 2,4,6triarylpyridines, Krohnke pyridines. Since Krohnke's original report on the preparation of 2,4,6-triarylpyridines [6], there has been a plethora of research targeting their syntheses [7-12]. Recently, much effort has been devoted to developing more efficient protocols for the synthesis of 2,4,6-triarylpyridines, for example solid-phase synthesis [7], one-pot synthesis under microwave irradiation [8], solvent-free reaction between acetophenones, benzaldehydes, and ammonium acetate in the presence of various catalyst such as I2 [9], heteropolyacid [10], HClO4-SiO2 [11] and ionic liquid [12]. However, many of these processes suffer from drawbacks such as long reaction time, expensive catalyst, undesired side products in reaction with harsh reagents, special care in handling and storing the reagents, cumber some product isolation procedure and environmental pollution. Therefore, a need still exists for further development of versatile reaction conditions in synthesis of 2,4,6-triarylpyridine using an efficient, reusable, inexpensive, eco-friendly, green, great and selectivity catalyst. In recent years, nanostructures have emerged as powerful catalysts in various organic synthesis such as cobalt nanoparticles for synthesis of 1,4Dihydropyridines [13], nanocrystalline MgO for synthesis of 2,4,5-trisubstituted imidazole derivatives [14], silica nanoparticles for preparation highly substituted pyridines [15] and ZnO nano powder for the preparation of 2,4,6-triaryl pyridines