Synthesis of Imidazo[1,2-a]Pyridine Derivatives via a 1,3-Dipolar Cycloaddition Using a Novel Basic Quasi-Homogeneous and Magnetically Separable Catalyst

A multicomponent reaction in the presence of a catalytic amount of novel basic ionic liquid-supported magnetic nanoparticles (BIL@MNPs-SiO2), under solvent-free conditions has been investigated. This catalyst was prepared based on surface modification of magnetic nano-silica with ionic liquid via covalent bonding and characterized extensively by various techniques. The alkalinity of the BIL@MNPs-SiO2 catalyst (pKb value) was obtained at 8.13. 3- chloropropyltrimethoxysilane acted as a linker to connect organic moiety on the surface of magnetic support. The obtained catalyst showed excellent catalytic efficiency in the one-pot reaction between pyridine, 2-bromo-1-phenylethan-1-one, and thiocyanate to afford corresponding fused imidazopyridines through a 1,3-dipolar cycloaddition. Targets’ chemical structures were established by different spectroscopic methods. Under the optimum conditions using 5% by weight of BIL@MNPs-SiO2 to pyridine, a molar ratio of 1.2: 1.2: 1 (mmol), and a temperature of 125 oC, the imidazopyridines were produced in excellent yields (93-98%). According to the TG analysis, the catalyst had good thermal stability since it was stable up to 210 °C. However, the outcome of the hot filtration test demonstrated that the catalyst was stable under the current circumstances. The catalyst functioned as a quasi-homogeneous type because of the union of nano-support characteristics and flexible connectors. Other important advantages of this research are the short reaction time, clean reaction conditions, use of non-hazardous catalyst, the simplicity of the process, use of available raw materials and broad substrate scope, excellent yields towards imidazo [1,2-a] pyridine derivatives as pharmacological compounds. Additionally, the stable catalyst could be easily separated from the reaction mixture by simple magnetic decantation and used in six subsequent chemical cycles without noticeably losing catalytic activity.