Urea-Tethered Porous Organic Polymer (POP) as an Efficient Heterogeneous Catalyst for Hydrogen Bond Donating Organocatalysis and Continuous Flow Reaction

Hydrogen bond donating (HBD) heterogeneous organocatalysis has come to light as a powerful surrogate to Lewis acid activation toward manufacturing biologically important C-C bonds. Notwithstanding the emergence of urea as a functionally diverse moiety to drive homogeneous HBD reactions, its catalytic competency is often muted by self-quenching behavior. Keeping this in perspective, spatial isolation of catalytically active urea functionality inside a porous framework can alleviate this pitfall, rendering a potential solution. The current work reports the fabrication of a porous urea network (IPpop-1) as a superior heterogeneous HBD catalyst toward Friedel-Crafts alkylation of beta-nitrostyrene and indole (yield up to 99%) under mild conditions advocating green chemistry. Experimental evidence that supports the critical step of the catalytic reaction leading to a plausible mechanism was unveiled along with theoretical assistance. Additionally, the versatile bifunctional nature of the catalyst was established from its competence in catalyzing multicomponent Knoevenagel-Michael condensation as well as cyanosilylation reactions efficiently. One-pot cascade catalysis was also achieved under milder reaction conditions with excellent product yields exploiting the dual active sites of IPpop-1. Pertaining to practicality, spherical composite beads were fabricated to perform continuous flow multicomponent Knoevenagel-Michael condensation without compromising the catalytic activity of IPpop-1. Furthermore, regeneration of the spent catalyst (up to 10 cycles) and scalability combined with wide substrate tolerance manifested conceptual feasibility of the polymer catalyst.