Diazo-coupled porous organic polymers as efficient catalysts for metal-free Henry and Knoevenagel reactions

Abstract

Owing to linkage and building block diversity, Porous Organic Polymers (POPs) are versatile materials with high potential in organic catalysis. Herein, nitrogen-rich POPs bearing –OH phenolic groups were synthesized via the diazo-coupling reactions between phloroglucinol and benzidine (PgBd-POP), and between 1,3,5-tris(4-aminophenyl) triazine and 4,4’-[1,1′ -biphenyl]-4,4′ -diylbis(diazene-2,1-diyl))diphenol (TAPT-Bd(PhOH)2-POP), under mild reaction conditions. The mesoporous polymers exhibited a BET surface area of 276 ± 4 m2 g􀀀 1 and 1.94 ± 0.06 m2 g􀀀 1, and chemical stability up to 315 ◦C and 330 ◦C, respectively. Both POPs were used as catalysts in metal-free Henry and Knoevenagel condensation reactions between aromatic aldehydes and nitromethane and ethyl cyanoacetate, respectively, under mild conditions. PgBd-POP showed a remarkable efficiency of up to 99% conversion and >99% selectivity, for the Henry nitroaldol reaction between 4-nitrobenzaldehyde and nitromethane, after only 1 h at room temperature, whereas TAPT-Bd(PhOH)2-POP generally exhibited the highest catalytic activity towards the Knoevenagel reaction, with up to 99% conversion for the condensation of 4-nitrobenzaldehyde with ethyl cyanoacetate, after 1 h at room temperature. These results confirm the potential of this class of porous materials as green catalysts for organic reactions.