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

Abstract

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 β-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.