A Tailored COF for Visible-Light Photosynthesis of 2,3-Dihydrobenzofurans

Abstract

Heterogeneous photocatalysis is considered as an ecofriendly and sustainable approach for addressing energy and environmental persisting issues. Recently, heterogeneous photocatalysts based on covalent organic frameworks (COFs) have gained considerable attention due to their remarkable performance and recyclability in photocatalytic organic transformations, offering a prospective alternative to homogeneous photocatalysts based on precious metal/organic dyes. Herein, we report Hex-Aza-COF-3 as a metal-free, visible-light-activated, and reusable heterogeneous photocatalyst for the synthesis of 2,3-dihydrobenzofurans, as a pharmaceutically relevant structural motif, via the selective oxidative [3+2] cycloaddition of phenols with olefins. Moreover, we demonstrate the synthesis of natural products (±)-conocarpan and (±)-pterocarpin via the [3+2] cycloaddition reaction as an important step using Hex-Aza-COF-3 as a heterogeneous photocatalyst. Interestingly, the presence of phenazine and hexaazatriphenylene as rigid heterocyclic units in Hex-Aza-COF-3 strengthens the covalent linkages, enhances the absorption in the visible region, and narrows the energy band, leading to excellent activity, charge transport, stability, and recyclability in photocatalytic reactions, as evident from theoretical calculations and real-time information on ultrafast spectroscopic measurements.

Bottom–Up Design of Photoactive Chiral Covalent Organic Frameworks for Visible-Light-Driven Asymmetric Catalysis

“…Interestingly, the lifetime of CCOF1 decreased gradually from 9.15 to 4.50 ns when DEBrM was added into the system, suggesting the involvement of electron transfer in the photocatalytic reaction system (Figure 5d). 33 With the above results in hand, the plausible mechanism involving radical species was provided (Figure 5e). First, the aldehyde species access chiral secondary amine units through the 1D nanochannels of CCOFs, which are subsequently activated by the amine groups to form enamine A.…”

Covalent organic frameworks in heterogeneous catalysis: recent advances and future perspective

“…Photoredox catalysis using COFs has traditionally relied on imine-, hydrazone- or azine-linked 2D COFs, which possess inferior chemical stability and π-conjugation. To overcome these drawbacks, significant research efforts have been devoted to developing COFs with novel linkages such as triazine, phenazine, 113 benzoxazole, 114 olefin, 115 and buta-1,3-diene for photoredox catalysis. Among these, sp 2 c-COFs bearing olefin linkages have emerged as promising photocatalysts because of their enhanced π-conjugation and chemical robustness.…”

Optimized Charge Storage in Aza-Based Covalent Organic Frameworks by Tuning Electrolyte Proton Activity

“…COFs are extended organic porous materials which are constructed by the covalent linkage of small organic molecules . The unique π-conjugated skeletons greatly elevate the delocalization of electrons, significantly improving electrical conductivity . Moreover, the highly ordered π–π stacking between layers stabilizes the molecular structure, thereby inhibiting molecular agglomeration and structural collapse. − More importantly, similar to organic small molecules, CO and CN are still the main active functionalities in COFs.…”