Ni-Unsymmetrical Salen Complex-Catalyzed One-Pot Multicomponent Reactions for Efficient Synthesis of Biologically...

In this report, four new Ni(II)-unsymmetrical salen complexes, [NiL^1-4], were prepared by refluxing Ni(Ac)₂·4H₂O with unsymmetrical salen ligands, H₂L^1-4. All of the synthesized ligands and complexes were characterized by various physicochemical methods. Also, the solid-state structures of [NiL¹], [NiL²], and [NiL⁴] were defined through single-crystal X-ray diffraction methods. The catalytic potential of [NiL^1-4] was investigated by economic and environmentally friendly one-pot-three-component reactions (using reagent: 1,3-dicarbonyls, malononitrile, benzaldehyde, or its derivatives) for the synthesis of biologically active 2-amino-3-cyano-4H-pyran derivatives (total 16 derivatives). After optimization of the reaction conditions, this new synthetic protocol by taking Ni(II)-unsymmetrical salen complexes as catalysts shows excellent conversion with a maximum yield of up to 98% of the effective catalytic products within 1 h of reaction time. In addition, it was observed that the aromatic aldehyde containing an electron-withdrawing group as a ring substituent shows better conversion (up to 98%), and the electron-donating group substituent shows similar or less conversion compared to benzaldehyde under the optimized reaction conditions. From the comparison of results between all these Ni complexes, it was found that the efficiency of the catalytic performance follows the order [NiL¹] > [NiL³] > [NiL²] > [NiL⁴]. A possible reaction pathway was predicted and established through UV-vis spectroscopy. Intermediate II proposed in the reaction pathway was also trapped and characterized through ¹H and ¹³C NMR.