Bronsted acidic ionic liquids: green, efficient and reusable catalyst for synthesis of dihydropyridines

Sustainable chemistry unarguably is one of our main aims of the twenty first century.Chemists are at the forefront of research in this area, injecting lots of attempts andexperiments to considerably decrease energy, matter costs, wastes and risks. As a consequence, various strategies have been envisioned to simplify elimination of wastes and recycling of catalyst. [1] Meanwhile, ionic liquids recently emerged in the literature as a new kind of green and environment friendly solvents. They have also been called to as designer solvents beacuse their properties can be modified by slight changes in the structure, such as the select of the organic cation, inorganic anion and alkyl chain attached to the organic cation. Bronsted acid ionic liquids have their inimitable acidic characters, containing high acid strength and manageable acid density, which have made them well documented. [2] In this methodology, at the beginning, environmentally friendly Bronsted acidic ionic liquids (BILs) have been synthesised by direct reaction of nitrogen heterocyclic compounds with various acids. Then, syntheis of dihydropyrimidines were investigated through one-pot, three-component reaction between substituted aromatic aldehydes, ethyl acetoacetate and urea (thiourea), using these green solvents at different temperature without any catalyst. The best results were afforded when [MIMH]OAc was used at 100 oC for 5-10 min. It is worthwhile to mention that, the products were isolated and no further chromatographic purification wasaccomplished since no impurities were observed by NMR. Increasing the amount of catalyst, however, did not improve the yield significantly. Further investigation on thetemperature effect indicated that high temperature did not reduce the reaction time.