Transition metal-catalyzed coupling reactions are one of the important research directions in organometallic chemistry and play a significant role in the field of synthetic chemistry^[1-2]. Traditional transition metal-catalyzed reactions often require relatively harsh reaction conditions and may be accompanied by the formation of by-products in certain situations, which limits their application in green chemistry. In contrast, light-driven reactions possess characteristics such as mild reaction conditions, low energy consumption, high selectivity, and environmental friendliness, receiving widespread attention in recent years^[3]. In recent years, the combined use of visible light and transition metal catalysts has developed into an efficient and environmentally friendly novel catalytic method that has been widely applied in the field of organic synthesis^[4]. There have been multiple articles summarizing and reviewing the research progress in this field. The MacMillan group^[5] emphasized that light and transition metal catalysis are important means for synthesizing high value-added chemicals, providing innovative ideas for developing new catalytic reactions and promoting the development of organic synthetic chemistry. The Xiao group^[6] summarized a series of studies on multi-component coupling and the construction of C-C and C-X bonds through photo-induced transition metal catalysis under mild conditions, including reaction types such as alkene difunctionalization, alkyne difunctionalization, and spirocyclopropane difunctionalization. Through these reactions, inexpensive raw materials can be converted into complex functional molecules, demonstrating the extensive application potential of this field. Zhang et al.^[7] summarized the decarboxylative coupling reactions of photoredox-catalyzed N-phthalimide esters and their derivatives, pointing out that this type of reaction has been applied to construct various types of C-C and C-X bonds, laying the foundation for related scale-up applications. The Reiser group^[8] described the photo-induced copper-catalyzed difunctionalization of alkenes without the addition of photosensitizers, providing new insights into the application of photo and transition metal cooperative catalysis. The Guérinot group^[9] and the MacMillan group^[10] respectively summarized photo-induced transition metal-catalyzed decarboxylative coupling reactions of carboxylic acids, guiding and promoting the development and application of decarboxylative coupling reactions. The Wei group^[11] outlined the current status and development prospects of copper and light cooperative catalytic radical cyclization reactions, analyzing the impact of ligands, inorganic copper salts, and light sources on the reactions, condensing examples of carbon, nitrogen, and oxygen radicals participating in cyclization reactions, and exploring the potential applications of this method in the field of organic synthesis.

Copper complexes are an important class of transition metal catalysts with many advantages such as low cost, low toxicity, and high catalytic activity, which have received widespread attention in the past decade and have been widely used to catalyze various functional group reactions. Particularly, monovalent copper complexes have strong reducibility and can capture active radicals generated during photoredox processes to form highly active trivalent copper complexes, followed by reductive elimination to achieve cross-coupling of C-C, C-N, C-O and other bonds, achieving the purpose of synthesizing various functionalized organic compounds^[12]. In recent years, significant progress has been made in light-driven copper cooperative catalysis methods, especially in the design and development of novel photoactive substances and copper complexes, which greatly improve catalytic performance and promote the synthesis process of functional natural products, drug molecules, and new materials^[13]. The light-induced copper catalysis method combines the advantages of sustainable light energy, green and pollution-free, high activity, strong stability, low cost, and low toxicity of copper catalysts^[14], providing new opportunities for the synthesis of high value-added chemicals and becoming a hot topic in the international academic community^[15].

Carboxylic acids and their derivatives are important reaction reagents that can be prepared on a large scale from biomass feedstocks. They have the advantages of low cost, abundant variety, and stable chemical properties, making them crucial raw materials for synthesizing high value-added chemicals. In 2017, Su's research group^[16] summarized the transition metal (palladium, silver, copper, etc.) catalyzed decarboxylation C—H bond functionalization reactions of carboxylic acids, pointing out that the decarboxylation C—H functionalization reaction has become a powerful tool for constructing C—C bonds. The decarboxylation process of carboxylic acids, through a series of derivatization reactions, can construct various cross-coupling compounds. This process not only promotes the efficient transformation and utilization of carbon resources but also closely aligns with the national strategic direction of "carbon peak, carbon neutrality," known as the "dual carbon" goals, contributing significantly to achieving green and sustainable development^[17-18].

This article focuses on the latest experimental and theoretical research progress in visible light-induced copper-catalyzed decarboxylative coupling reactions of carboxylic acids and their derivatives, with an emphasis on the decarboxylative coupling reactions synergistically catalyzed by light and copper under the presence of a photosensitizer. As shown in Figure 1, these reactions are mainly used for the construction of C—C, C—N, C—O, and C—S bonds. Using a series of typical reactions as examples, the basic principles and applications of the synergistic catalytic method involving transition metal copper and a photosensitizer are introduced, with the hope of providing insights for future research and applications.

The working mechanism of the synergistic catalytic system involving photocatalysts and copper complexes encompasses two intercrossing catalytic cycles, known as the photoredox catalytic cycle and the copper complex catalytic cycle (Figure 2). The photoredox catalytic cycle refers to the process where, under visible light excitation, the photosensitizer (Photosensitizer) is excited and oxidized (or reduced) to generate active components (usually radicals). These active components then interact with substrates or transition metal catalysts, undergoing reduction (or oxidation) quenching to regenerate the photosensitizer^[19]. Here, the photosensitizer is also called the photoredox catalyst (Photocatalyst, PC), or simply photocatalyst. Commonly used PCs mainly include metal ruthenium/iridium coordination complexes and organic dyes with conjugated structures (including rose bengal, eosin, acridine, etc.)^[20-21]. These compounds possess unique photochemical and physical properties, including strong visible light absorption, broad spectrum, high redox potential, long excited-state lifetime, and other advantages, making them excellent photoredox catalysts.

The active components generated by the photocatalytic cycle subsequently participate in the copper complex catalytic cycle, achieving efficient conversion from reactants to products^[22]. The copper complex catalytic cycle can be divided into three types according to the valence change of the copper complex, marked as A, B, and C respectively in Figure 2. Pathway A represents a cycle involving only monovalent copper complexes, pathway B involves a copper(I) → copper(II) → copper(I) cycle, and pathway C is a copper(I) → copper(II) → copper(III) → copper(I) cycle. The photocatalytic cycle and the transition metal catalytic cycle intersect and cooperate with each other, jointly completing the catalytic transformation from substrate to target product.

Photocatalysts and copper complexes can promote the decarboxylative coupling of carboxylic acids and their derivatives to efficiently construct C—X (X=C, N, O, S) bonds under synergistic catalysis, obtaining a variety of high value-added chemicals^[9-10,23-24]. The following sections will introduce the photocatalyst and copper co-catalyzed decarboxylative coupling reactions of carboxylic acids and their derivatives according to different types of bond formation.

This paper reviews the experimental and theoretical research progress in the decarboxylation C—X (X = C, N, O, S) bond coupling of carboxylic acids and their derivatives via visible-light-induced photocatalysts and copper complexes synergistic catalysis. The dual catalytic system composed of photocatalysts and copper complexes is an efficient and green novel catalytic method, which can convert inexpensive, abundant, and stable organic small molecule compounds such as carboxylic acids and their derivatives into high value-added chemicals. It has important application prospects in the fields of organic chemistry, biology, and pharmaceutical synthesis. This catalytic method has the advantages of simple operation, mild reaction conditions (generally without the need for additional oxidants), broad functional group compatibility, and product diversity, greatly enriching the content of photochemistry and organic synthetic chemistry.

Despite significant progress in the study of decarboxylative coupling reactions of carboxylic acids and their derivatives via photoredox and copper co-catalysis in recent years, several important challenges remain. First, further understanding of the mechanism of photoredox and copper co-catalyzed reactions is needed. On one hand, the reaction system involves dual catalytic roles of light and copper, making the reaction mechanism complex; on the other hand, the reaction involves high-energy and highly reactive radicals and intermediates, which are short-lived and difficult to capture and characterize. This limitation hinders the in-depth study of the reaction mechanism using experimental methods, making precise control of the reaction (including regioselectivity, chemoselectivity, and enantioselectivity) still highly challenging. Second, the selection and optimization of photocatalysts is another key issue. Although noble metal catalysts (such as ruthenium and iridium) currently in wide use exhibit excellent catalytic performance, their high cost and scarcity limit large-scale applications. Therefore, developing efficient, low-cost organic photocatalysts or transition metal catalysts is a core task for achieving breakthroughs in this field. Third, the scope of substrates needs to be further expanded. The current catalytic systems perform well with specific types of carboxylic acids and their derivatives, but limitations in substrate diversity and applicability still exist, necessitating the development of more efficient catalytic systems. Thus, further experimental and theoretical research into effectively utilizing photocatalysts and copper complexes for co-catalytic decarboxylative coupling is crucial. Clarifying the optical properties of photocatalysts, the catalytic performance of copper catalysts, the activation modes of substrates, and the effects of ligands, additives, and solvents on reaction performance holds significant importance for optimizing and promoting this catalytic method.