Four-Component Fusion Protocol with NiO/ZrO₂ as a Robust Recyclable Catalyst for Novel 1,4-Dihydropyridines

Nickel oxide loaded on zirconia (NiO/ZrO₂) as an expedient catalyst is reported for the synthesis of 18 unsymmetrical 1,4-dihydropyridine derivatives. The Lewis acidic nature of the catalyst proved an excellent choice for the one-pot, four-component fusion reaction with excellent yields of 89-98% and a completion time of 20-45 min. Mechanistic studies show that enamine and imine functionalities are the two possible pathways for the formation of 1,4-dihydropyridines with high selectivity. Crystal structures of two novel compounds (5a, 5c) were reported. The catalyst demonstrated reusability up to six cycles. The reaction at room temperature and ethanol as a solvent make this protocol green and economical.