Recent advances in homogeneous photocatalytic carboxylation incorporated with CO<sub>2</sub>

Abstract

<p indent="0mm">Carbon dioxide (CO₂) has the advantages of low price, non-toxic, abundant and renewable. As CO₂ is an indispensable part of natural photosynthesis, it seems feasible to achieve artificial CO₂ fixation with the aid of light. For light-driven utilization of CO₂, two major categories can be diversified: (1) Photoreduction of CO₂ into other C1 resources, such as carbon monoxide, methanol, formic acid, and methane; (2) photocarboxylation, which is a green and sustainable method of fixing CO₂ into organic molecules. Therefore, taking CO₂ as a C1 synthon to participate in the reaction is an effective means to utilize it. Carrying out carboxylation reaction with CO₂ as raw material to produce acrylic acid, benzoic acid or other high value-added carboxylic acid derivatives has been a very important research significance. The carboxylation reactions using CO₂ usually include: (1) The use of strong nucleophiles such as Grignard reagents; (2) the use of stoichiometric metal reducing agents; (3) high temperature, high pressure and other conditions to activate the thermodynamic and dynamic stable CO₂. In the past decade, the homogeneous photocatalytic reaction has made a great progress. Researchers found photocatalytic organic reactions have many advantages, such as mild reaction conditions, high selectivity, and environmental protection. Compared to conventional thermal reaction system with CO₂ in the carboxylation reaction, the carboxylation reaction in photocatalysis has the following advantages: (1) Avoid the utilization of unstable strong reductant; (2) most of the reaction under mild conditions are performed; (3) during the reaction and post-processing to reduce the environmental impact. This article mainly introduces and summarizes the homogeneous carboxylation reactions involving photocatalytic CO₂ in recent years. At the same time, the study of mechanism is also an important part in the homogeneous photocatalytic reaction, especially in the field of the carboxylation reactions. The current photocatalytic carboxylation reaction can be roughly divided into three categories: (1) The electron transfer process occurs between the substrate and the photocatalyst, and the nucleophilic attack on the CO₂ molecule occurs after the formation of the corresponding anion intermediate to achieve carboxylation; (2) single electron transfer (SET) occurs between the CO₂ molecule and the photocatalyst, forming a CO₂ radical anion and then reaction with the substrates; (3) with assistant of the transition metal to form complex with the substrate and/or CO₂, a series of photocatalytic electron transfer processes and ligand transfer reactions to achieve the carboxylation. In addition, there is a carboxylation reaction realized by a synergistic [4+2] mechanism. Considering the importance of the study on the reaction mechanism in organic catalytic reactions, this article will focus on the employ the different types of intermediates generated in the reaction to achieve the homogeneous photocatalytic carboxylation of various substrates with CO₂, which include four types reaction mechanisms such as anionic intermediates, CO₂ radical anions, transition metal participation, and synergistic processes.