A magnetically recoverable Zn(II) nanocatalyst for efficient synthesis of polysubstituted pyridines in water

A novel magnetically recoverable nanocatalyst, Fe₃O₄@SiO₂-Dop/Oxalyl-ZnCl₂, was designed, synthesized, and successfully applied for the efficient one-pot synthesis of polysubstituted pyridines via a four-component reaction of aromatic aldehydes, malononitrile, ethyl acetoacetate, and aromatic amines. The catalyst was fabricated through a stepwise procedure involving co-precipitation of Fe₃O₄ nanoparticles, silica coating, dopamine functionalization, oxalyl bridging, and coordination of ZnCl₂ to generate Lewis acidic active sites. Comprehensive characterization by FT-IR, XRD, XPS, ICP-OES, TGA, VSM, BET, SEM, TEM, and EDX confirmed the successful construction of a stable core-shell nanostructure with uniformly distributed Zn²⁺ species. Under optimized conditions (5 mol% catalyst, water, reflux), a wide range of polysubstituted pyridines was obtained in excellent yields (90-99%) within short reaction times (25-50 min). The methodology demonstrates broad substrate scope, high functional group tolerance, and operational simplicity. Notably, the catalyst could be easily separated using an external magnet and reused for up to eight consecutive cycles with minimal loss of activity. Compared to previously reported methods, this protocol offers superior efficiency, greener reaction conditions, shorter reaction times, and enhanced recyclability, providing a sustainable and practical approach for the synthesis of valuable pyridine derivatives.