Nanosphere of Semicrystalline Polyaniline Powder: An Effective, Versatile, and Reusable Catalyst for Hantzsch Reaction

Polyaniline salt containing 1-hydroxyethane-1,1-diphosphonic acid with nanosphere morphology in semicrystalline powder form was successfully synthesized by interfacial polymerization pathway and demonstrated as polymer based acid catalyzed in the synthesis of 1,4-dihydropyridines and polyhydroquinoline derivatives. This catalyst promotes the reaction under solvent free condition in excellent yield in 10？min with recyclability. The advantages of this methodology are the easy synthesis of polymer containing acid group catalyst in powder form having easily handlable, efficient, versatile, and reusable nature. 1. Introduction Multicomponent reactions (MCRs) are important for generating high levels of diversity because they allow more than two building blocks to be combined in a practical, time saving, one-pot operation to give rise to complex structures by simultaneous formation of two or more bonds. MCRs are extremely convergent, producing a remarkably high increase of molecular complexity in just one step [1, 2]. 1,4-Dihydropyridines are well known as calcium channel modulators and have emerged as one of the most important classes of drugs for the treatment of cardiovascular diseases. Cardiovascular agents such as nifedipine, nicardipine, amlodipine, and other related derivatives are dihydropyridyl compounds, effective in treatment of hypertension [3, 4]. 1,4-Dihydropyridine derivatives possess a variety of biological activities such as vasodilator, bronchodilator, antiatherosclerotic, antitumor, geroprotective, hepatoprotective, and antidiabetic activity [5, 6]. Thus, the synthesis of this heterocyclic nucleus is of much importance. In 1882, Hantzsch reported first synthesis of symmetrically substituted 1,4-dihydropyridines by the one-pot, four-component condensation of two molecules of ethyl acetoacetate, aromatic aldehyde, and ammonia [7]. The standard Hantzsch procedure does not need the intervention of any additive or reagent and the reaction was originally conducted either in acetic acid or at reflux in alcohol for rather long periods, resulting in lower modest yields of condensation products. Replacement of ammonia by ammonium acetate allowed the efficient synthesis of Hantzsch compounds in aqueous medium as well as under solvent free conditions [8, 9]. Realizing the importance of polyhydroquinoline derivatives in the synthesis of various drug sources, numerous methods have been reported such as conventional heating [10, 11], various catalysts such as L-proline [12], ceric ammonium nitrate (CAN) [13], silica perchloric acid (HClO4–SiO2) [14],