An Imidazole-Integrated Porous Organic Polymer as an Expedient Heterogeneous Organocatalyst for One-Pot Synthesis of Pyrans, Chromenes, and Spiropyrans

Multicomponent reactions remain sparsely utilized for the construction of porous organic polymers (POPs). We have synthesized a nitrogen-rich imidazole-based POP (Imd-POP) by subjecting pyrene-4,5,9,10-tetraone, 1,2,4,5-tetrakis(4-formylphenyl)benzene, and ammonium acetate to a multicomponent Debus–Radziszewski reaction in a cost-effective manner. The cascade Knoevenagel condensation–Michael addition–cyclization reaction leads to the formation of a thermally stable and chemically robust POP that incorporates cyclic imidazole moieties integrally. Spectroscopic, microscopic, and other studies reveal that the POP shows permanent porosity and robust stability. It exhibits spherical morphology, as revealed by FESEM, HRTEM, and AFM images. We show that the Imd-POP can be employed as a metal-free heterogeneous bifunctional organocatalyst that serves as a weak base as well as a hydrogen-bond donor/acceptor for one-pot three-component cascade reactions of an array of aldehydes with malononitrile and 4-hydroxycoumarin/cyclohexane-1,3-dione/isatin, leading to chromene-2-carbonitriles, 2-amino-3-cyano-4H-pyrans, and spiropyrans. In contrast to the conventional methods, notable advantages of this green protocol include the formation of products in very good-to-excellent yields without the need for a transition-metal-based catalyst and shorter reaction times. It is shown that the heterogeneous catalyst can be recycled up to 15 runs with no loss of catalytic activity.