Visible Light‐Driven Carboxylation of Olefins by Using 2D Metal‐Free Covalent Organic Framework as Intrinsic Photocatalyst: A Sustainable Approach for CO₂ Utilization

Abstract

Photocatalytic metal-free carboxylation of olefins offers an environment-friendly approach for resolving the increasing energy issue as well as mitigating the dilemma caused by the greenhouse effect. Carboxylation of styrene and its derivatives by photocatalytic CO 2 reduction reaction shows great potential for sustainable utilization of greenhouse gas CO 2 into valuable chemicals. Herein, we have constructed a highly crystalline and thermally stable 2D porous covalent organic framework (COF) having very low band-gap energy, 1.8 eV and it shows efficient photocatalytic activity towards the carboxylation of aryl-alkenes in good yields in presence of p-terphenyl as a co-catalyst at ambient temperature under visible light irradiation and 1 atmospheric CO 2 pressure which facilitates to overcome the current transition metal catalyzed approaches. In addition, TR-OT COF is a proficient and highly selective catalyst with outstanding recyclability. This suggests that this COF material is a novel photocatalyst for CO 2 reduction towards α, β-carboxylation of styrene and its derivatives under 20 W light-emitting diode (LED) light.

Recent Advances in the Use of Covalent Organic Frameworks as Heterogenous Photocatalysts in Organic Synthesis

“…The presence of p ‐terphenyl as co‐catalyst and triethanolamine (TEA) as a sacrificial electron donor was necessary ( Scheme ). [ 193 ] The α‐to‐β product ratios are not correlated with the substituents on styrene. For example, the alpha position is more prone to reaction in styrenes including methoxy, nitro, chloro, and cyano groups.…”

“…Finally, the carboxylated product is generated via the protonation of the anion, and the TR‐OT‐COF is regenerated by the sacrificial electron donor TEA. [ 193 ]…”

“…Finally, the carboxylated product is generated via the protonation of the anion, and the TR-OT-COF is regenerated by the sacrificial electron donor TEA. [193] Figure 32. Suggested mechanism for photocatalytic C-3 formylation of indoles.…”

Recent advances in metal-free covalent organic frameworks for photocatalytic applications in energy and environmental fields

“…Islam et al 104 reported the construction of a highly crystalline and stable 2D-COF with a very low band gap for the carboxylation of unsaturated olefins with CO 2 . The COFs were synthesised by Schiff base condensation using trimethyl resorcinol and o -toluidine as building blocks.…”

Visible Light-Driven C(sp3)-H Carboxylation of Diverse Amines with CO2 into α-Amino Acids Using an Eco-Friendly and Reusable Covalent Organic Framework

“…On the contrary, the band gap of organic semiconductors is tuned straightforwardly via the involvement of a wide range of monomeric units. − Recently, organic porous materials like conjugated microporous polymers (CMPs), , carbon nitrides, − hyper-cross-linked polymers (HCPs), and covalent triazine-based frameworks (CTFs) have been investigated for light-driven reduction of CO 2 . The aforementioned organic materials are generally amorphous in nature; in contrast, COFs can be capable of combining porosity with their crystallinity. − Recently, we demonstrated a metal-free crystalline covalent organic framework as a reusable heterogeneous photocatalyst for visible-light-promoted photocatalytic carboxylation of olefins with CO 2 into carboxylic acid . COFs were also selected as photocatalytic systems for water splitting, , and for electrochemical reduction of CO 2 . , These highly ordered crystalline porous polymers also have the potential to be a great asset for direct photoreduction of CO 2 into value-added chemicals: for instance, an azine-functionalized COF, N 3 -COF, was described to promote gas phase selective photoreduction of CO 2 into methanol .…”