Photocatalysis Meets Copper Catalysis: A New Opportunity for Asymmetric Multicomponent Radical Cross-Coupling Reactions

Metal-Bound Heteroatom Radicals: Advancing Site-Selective C-H Functionalization

Selective transformation of C-H bonds represents a frontier research area in synthetic chemistry. While the high reactivity of radicals provides an alternative and efficient pathway for C-H bond functionalization, controlling their selectivity─particularly in processes such as site-selective hydrogen atom abstraction (HAA)─remains a long-standing and unresolved challenge in radical chemistry, largely due to the lack of effective regulation strategies. This review deliberately avoids a comprehensive discussion of the field's current state or landmark discoveries in C-H functionalization. Instead, by focusing on recent advances in metal-catalyzed, highly site-selective C-H bond transformations, this Perspective elucidates how metal-bound radicals enable precise hydrogen abstraction for targeted functionalization. This emerging paradigm offers innovative strategies for regulating radical behavior, potentially unlocking novel radical-mediated selective transformations─including but not limited to the precise functionalization of C-H bonds.

Cu(I)-catalysed chemo-, regio-, and stereoselective radical 1,2-carboalkynylation with two different terminal alkynes

Transition-metal-catalysed asymmetric multicomponent reactions with two similar substrates often suffer from the lack of strategies to control the chemo-, regio-, and stereoselectivity of these substrates due to the close similarity in the chemical structures and properties of each reagent. Here, we describe a Cu(I)-catalysed asymmetric radical 1,2-carboalkynylation of two different terminal alkynes and alkyl halides with high chemo-, regio-, and stereoselectivity by using sterically bulky chiral tridentate anionic N,N,P-ligands and modulating alkynes with different electronic properties to circumvent above-mentioned challenges. This method features good substrate scope, high functional group tolerance of two different terminal alkynes, and diverse alkyl halides, providing universal access to a series of useful axially chiral 1,3-enyne building blocks.

Photoinduced Copper-Catalyzed Regio- and Diastereoselective Multicomponent [3 + 2 + 1] Radical Cyclization To Access Tetrahydropyridines

The use of simple raw materials to construct complex piperidine scaffolds via multicomponent reactions is highly desirable from the perspectives of atom and step-economy. In this Letter, we present a photoinduced copper-catalyzed three- or four-component [3 + 2 + 1] radical cyclization, utilizing inexpensive and readily available feedstock amines, alkynes, and aldehydes, to synthesize multisubstituted bicyclic or spirocyclic tetrahydropyridines. This method is notable for its mild conditions, atom-economic approach, excellent regio- and diastereoselectivity, and the simultaneous activation of two α-amino C(sp3)-H bonds, resulting in the formation of three C-C bonds and one C-N bond in a single step. Mechanistic studies suggest that the α-aminoalkyl radical is the key intermediate in this reaction, which undergoes sequential radical addition, 1,5-HAT, and 6-exo-trig-type radical cyclization.

Radical functionalization of allenes

Allenes exhibit comparatively lower stability compared to alkenes and alkynes, which confers heightened reactivity to these compounds. Recently, the radical functionalization of allenes has progressed considerably, leading to a renaissance in the synthesis of functional natural products, drugs and their analogues, but summary work addressing this aspect has not been reported. This review systematically summarizes recent advancements in the field of radical functionalization of allenes reported within the past five years. It encompasses the difunctionalization and trifunctionalization of the three carbon atoms in allenes, as well as the functionalization of C-Y bonds (Y = H, Br). The representative studies are categorized based on the type of radicals generated, including C-, N-, O-, S-, and Se-centered radicals. For individual more complex reactions, the mechanisms are explored and briefly discussed.

Direct Asymmetric α-Alkylation of β-Ketocarbonyl Compounds with Simple Olefins by Photoredox-Nickel-Hydrogen Atom Transfer Triple Catalysis

Although the asymmetric α-alkylation of carbonyl compounds with activated olefins has already been established, extending this methodology to less activated or nonactivated olefins remains a significant challenge due to the polarity mismatch in these ionic processes. An alternative approach involves the activation of the parent carbonyl compounds into electrophilic α-carbonyl radicals, which could potentially overcome this limitation. However, the lack of efficient catalytic systems has impeded the wide adoption of this strategy, particularly in realm of the catalytic asymmetric reactions. Here, we present a cooperative triple catalytic system that integrates photoredox, chiral Lewis acid, and hydrogen atom transfer (HAT) catalysts to achieve a direct asymmetric α-alkylation of β-ketocarbonyl compounds using simple olefins as alkylating agents. By combining a multifunctional chiral nickel Lewis acid with an iridium photoredox catalyst and a thiophenol catalyst under visible light, we have developed a highly efficient process that is temporally synchronized to facilitate a novel mechanism of electron and hydrogen transfer. This triple catalytic approach enables the intermolecular coupling of β-ketocarbonyl compounds with both less and non-activated olefins. This redox-neutral protocol provides an atom- and step-economic route to enantioselectively synthesize high-value molecules featuring an all-carbon quaternary stereocenter from feedstock chemicals, while only consuming photons.

Taming Inert B-H Bond with Low Energy Light: A Near-Infrared Light-Induced Approach to Facile Carborane Cluster-Amino Acid Coupling

The selective functionalization of inert B-H bonds in carborane clusters has been a formidable challenge. Recent advances have witnessed such reactions through photoredox methods utilizing ultraviolet or visible light irradiation. However, high-energy light sources often suffer from poor energy efficiency, a limited substrate scope, undesired side reactions, and low scalability. Here, we present the first successful B-H bond functionalization under low-energy near-infrared (NIR) light using a carborane-based electron donor-acceptor complex. Both photophysical investigations and theoretical modeling reveal a facile single-electron transfer from the carborane cage to the electron-deficient photocatalyst, generating a carborane cage radical under NIR light irradiation. The follow-up radical pathway enables the direct coupling of carboranes with amino acids or oligopeptides, yielding a diverse array of carborane-functionalized amino acids or oligopeptides. Beyond expanding the known chemical space of boron cluster derivatives, we further demonstrate that carborane-based amino acids with imaging and targeting capabilities could serve as promising multifunctional boron carriers for boron neutron capture therapy. Thus, the selective B-H bond functionalization of the carboranes via NIR light not only provides a straightforward and practical strategy in boron cluster synthetic chemistry but also lays the foundation for the development of next-generation boron-containing biomolecules and advanced functional materials.

Enantioselective Desymmetrization of Biaryls via Cooperative Photoredox/Brønsted Acid Catalysis and Its Application to the Total Synthesis of Ancistrobrevolines

Photoredox catalysis has emerged as a powerful tool for forming and breaking chemical bonds, further taking hold with its integration with asymmetric catalysis. While the dual-catalytic approach has led to successful examples of the control of stereogenic centers, the control of stereogenic axes has remained underexplored. In this study, an acylimine intermediate was generated through photoredox catalysis, and a symmetric substrate, 2-arylresorcinol, was desymmetrized with the aid of chiral phosphoric acid catalysis. Using this approach, a stereogenic center and stereogenic axis were successfully controlled to provide a natural-product-driven compound. The origins of enantioselectivity and diastereoselectivity were investigated through a density functional theory study of four possible enantiodetermining transition states. Consequently, the first total syntheses of the ring-contracted naphthylisoquinoline alkaloid ancistrobrevolines A and B were accomplished concisely. This approach provides not only a novel methodology and strategy to synthesize naphthylisoquinoline alkaloids but also a direction to advance catalytic research and total synthesis studies.

Photobiocatalytic Reaction: From Single Component to Triple Components

Despite remarkable advances in photoenzymatic methodologies, the development of photoenzymatic multicomponent reactions remains a significant challenge. In this Highlight, we showcase a recent breakthrough by Prof. Xiaoqiang Huang and co-workers, who achieved enantioselective triple-component radical sorting via visible light-induced biocatalysis under mild conditions. This innovative approach opens new avenues for the synthesis of complex molecules with high precision and efficiency, marking a remarkable advance in the field of enzymatic photocatalysis.

Photocatalytic Strecker-Type Reaction for the Synthesis of Primary α-Aminonitriles

A practical photoinduced direct N-O cleavage of oxime ethers via a single electron transfer (SET) process was developed, enabling controlled generation of N-H imines via iminyl radical intermediates. By employing this strategy, an efficient Strecker-type reaction was established to construct a variety of primary α-aminonitriles using TMSCN as a cyanide source. This protocol showed exceptional tolerance to various functional groups, delivering the corresponding products in good yields. Mechanistic investigations indicate the involvement of iminyl radicals and a radical/polar crossover sequence.

Photosensitized Imino-Thiocyanation of Alkenes

A metal-free photosensitized 1,2-imino-thiocyanation of olefins has been established by using the easily accessible bifunctional reagent S-cyano-N-(diphenylmethylene) thiohydroxylamine. A wide range of olefins were successfully transformed into the corresponding β-iminothiocyanates in moderate to high yields. This protocol stands out for its metal-free nature, broad substrate compatibility, and high atom and step economy, providing an effective strategy for assembling β-amino thiocyanate-containing scaffolds.