Photo and earth-abundant metal dual catalysis in organic synthesis

Abstract

This review summarizes the latest research progress on photocatalytic reactions involving typical Sc, Ti, V, Cr, Mn, Fe, Co, Ni, and Cu metal catalysts, mainly focusing on the role of metals and light in the reaction process.

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1. Introduction

As a clean, green, and easily accessible renewable energy source, visible light has attracted significant attention in recent years, and the conversion of various chemicals through photocatalysis is of great significance. In general, visible light-induced photocatalytic reactions are mainly achieved through oxidation quenching cycles, reduction quenching cycles, or energy transfer (EnT) processes between excited-state photocatalysts (PC*) and organic substrates (Scheme 1a). Although visible light-induced reactions have great advantages, it is still difficult to complete some complex reactions through a single photocatalytic system.

Scheme 1 Visible-light-induced catalysis.

Earth-abundant metals mainly refer to first-row transition metal elements, which have an almost unlimited supply. In recent years, the first-row transition metals Sc, Ti, V, Cr, Mn, Fe, Co, Ni, and Cu have attracted widespread attention from organic chemists owing to their low toxicity, low cost, and environmentally friendly characteristics. In general, organic radical intermediates generated through photocatalytic cycling have high reactivity, which makes it highly challenging to achieve specific chemical selectivity, regioselectivity, and, especially, stereoselectivity. In photoinduced 3d-metal catalysis systems, the unique chemical structure of 3d metals promotes relatively rich valence state changes in these elements. Therefore, the generated radicals can easily bind with 3d metals, thus increasing the types of reactions while making the reaction between radicals and substrates more controllable. Compared with photoinduced 3d-metal catalysis systems, traditional precious metals are not only expensive, but also easily undergo the double electron transfer (DET) mechanism; in addition, organic photocatalysts may have a limited light absorption range. Therefore, in the last decade, the combination of photoredox and 3d-metal catalysts have further promoted the rapid development of metal-catalyzed reactions, and the related research results have emerged continuously as well.^1,2 The combination of photoredox and 3d-metals breaks through conversion barriers, which is difficult to achieve with a single catalytic cycle under extremely mild reaction conditions. Mechanistic studies have shown that reaction modes can be roughly divided into two types based on whether an external photocatalyst is needed. In the first case, the reaction still requires the addition of photocatalysts, which engages either a single electron transfer or energy transfer process with earth-abundant metals in order to facilitate the transformation. Another way is for metal complexes to directly absorb visible light to complete the conversion. In this way, no exogenous photosensitizer (PC) is required to promote photocatalytic transformations (Scheme 1b). Herein, we summarize the latest research progress on metal photocatalytic reactions involving the typical metal catalysts Sc, Ti, V, Cr, Mn, Fe, Co, Ni, and Cu, mainly focusing on the role of metals and light in the reaction process. For the convenience of readers, the sections will be elaborated on the formation of bonds. We will classify and discuss the interaction modes between light and 3d metals before the formal discussion.

2. Scandium metallaphotocatalysis

2.1 General overview

The rare-earth 3d metal scandium is widely used as a Lewis acid. Lewis acid catalysis, which can activate heteroatoms, has attracted much attention from chemists and has been well developed in recent decades.³ Combining photocatalysis with Lewis acid catalysis is an elegant strategy that has been developed.^4,5 There are two mainly models for dual photoredox/scandium Lewis acid catalysis (Scheme 2).⁶

Scheme 2 Two main models of visible light-induced scandium Lewis acid catalyzed reactions.

2.2 Photoredox/scandium Lewis acid dual catalysis

In the first model, Lewis acid catalysis can activate the substrate containing heteroatoms and promote the single-electron transfer process with the photocatalytic cycle. In 2008, Yoon's group successfully combined photocatalysis with Lewis acid catalysis using the lithium cation as a Lewis acid.⁷ In recent years, Scheidt's group achieved intermolecular reductive coupling⁸ and the reductive amination⁹ of arylidene malonates via photoredox/Lewis acid cooperative catalysis (Scheme 3a and b), respectively. The single-electron transfer between the scandium ion–arylidene malonate complex and the reduced photocatalyst species is critical for the reactions. In 2020, Shen's group developed the trifluoromethylative difunctionalization of alkenes using photoredox/Lewis acid cooperative catalysis (Scheme 3c). Sc(OTf)₃ was used as a Lewis acid to active the selenium ylide-based trifluoromethylating reagent.¹⁰ The complex of Sc(OTf)₃ and selenium reagent undergoes a single-electron transfer with photoredox process, and then further generates the trifluoromethyl radical.

Scheme 3 Single electron transfer of photocatalysis and scandium catalysis.

In the other mode, Lewis acid catalysis is not directly involved in the photoinduced process. In 2015, Yoon and co-workers developed cooperative Lewis acid-photoredox catalysis of α-amino radical additions (Scheme 4a).¹¹ There is no direct interaction between the scandium catalysis and the photocatalysis. In this process, the Lewis acid determines the enantioselectivity of radical additions, which is independent of the photocatalysis. Recently, Liu and Feng realized the photoinduced Lewis acid-catalyzed enantioselective acylation and alkylation of aldimines via 9-fluorenone electron-shuttle catalysis (Scheme 4b).¹² Depending on the Feng ligands, the enantioselectivity of the reaction could be well controlled by the scandium Lewis acid catalysis. DFT calculations further indicated that the photocatalysis and Lewis acid catalysis were linked through electron-shuttle catalysis without direct interaction.

Scheme 4 Photocatalysis and scandium catalysis without direct interaction.

Scandium has been widely developed as a Lewis acid catalyst, and its combination mode with photocatalysis is worth developing and exploring in the future.

3. Titanium metallaphotocatalysis

3.1 General overview

As a transition metal, titanium has attracted extensive attention from chemists due to its unique reactivity. Over the past few decades, the methodology of combining titanium metals with photocatalysis has been well developed in organic synthesis.^13–15 There are three main patterns in photoinduced titanium catalysis, as shown in Scheme 5.

Scheme 5 Titanium metallaphotocatalysis.

3.2 Titanium dioxide as a photocatalyst

Titanium dioxide and modified titanium dioxide have been used as photocatalysts in heterogeneous catalysis since 1978.¹⁶ The generated holes (h⁺) in the valence band (VB) of TiO₂ can participate in oxidative reactions. Under ultraviolet light, the electrons in the conduction band (CB) are always involved in the reduction reactions (Scheme 5a).¹⁷ Because of this characteristic, TiO₂ is widely used as a photosensitizer in agriculture, environment, biological analysis, energy and other fields.^18–24 There are many reviews on the synthesis, modification and application of TiO₂, and thus they will not be discussed herein.

3.3 Photoinduced single-electron transfer titanium catalysis

It is an elegant strategy to generate a highly active Ti^III complex (such as Cp₂–Ti^IIICl) via single electron transfer in a Ti^IV complex with photocatalysis (Scheme 5b).²⁵ In 2019, Gansäuer's group achieved reductive epoxide ring opening and redox-neutral epoxide radical arylation by combining titanocene catalysis and photoredox catalysis (Scheme 6a).²⁶ Then, Shi and co-workers realized the radical spirocyclization of epoxides using this strategy (Scheme 6b).²⁷

Scheme 6 Single-electron transfer in photocatalysis and titanium catalysis.

Thereafter, a series of reactions such as allene functionalization,²⁸ 1,3-butadiene difunctionalization,²⁹ dehydroxylation of cyclobutanone oximes,^30,31 alkylation of ketones,³² enantioselective pinacol coupling³³ and α-vinyl-β-hydroxy ester formation³⁴ have been reported by Shi, Zhu, Chen, Kanai, Mitsunuma, Calogero and Cozzi et al. via the single-electron transfer of photoredox catalysis and titanium catalysis (Scheme 7).

Scheme 7 Reactions via single-electron transfer in photoredox catalysis and titanium catalysis.

3.5 Ligand-to-metal charge transfer of titanium complex

In 2020, Gansäuer et al. first investigated the photocatalytic process of Cp₂Ti^IVCl₂. Under the radiation of light, the Ti^IV complex is excited by the process of LMCT.³⁵ The excited Ti^IV complex can be quenched by a reducing agent to generate a single-electron catalyst Ti^III complex (Scheme 8a). Recently, Gansäuer and co-workers achieved a regiodivergent epoxide opening reaction using a chiral titanocene complex as a photoredox catalyst for the first time (Scheme 8b).³⁶

Scheme 8 Ligand-to-metal charge transfer in excited Ti^IV complex.

In 2021, Iwasawa's group developed the photocatalytic generation of a Ti^III complex for the dehydoxylative dimerization of benzylic alcohols.³⁷ Under UV-light irradiation, a Ti^III complex and an alkoxy radical are generated from (AcO)_nTi(OiPr)_4−n via the LMCT process (Scheme 9a). Furthermore, in 2022, Kanai and Mitsunuma et al. found that titanium tetrachloride could generate a Ti^III complex and chlorine radicals through the LMCT pathway (Scheme 9b).³⁸ Using the strategy, they realized a chlorine radical-catalyzed C(sp³)–H photoalkylation reaction, which was extended to the direct photoalkylation of aromatic ketones. Then, Walsh and Schelter's group developed a similar transformation using [PPh₄]₂TiCl₆ as the photoredox catalyst (Scheme 9c).³⁹

Scheme 9 Reactions via ligand-to-metal charge transfer in the excited Ti^IV complex.

Combined with photocatalysis, more types of Ti complexes need to be developed to generate active Ti intermediates to realize more interesting chemical transformations.

4. Vanadium metallaphotocatalysis

Among the photoinduced 3d-metal catalysis, vanadium catalysis is relatively rare, but it still has unique properties worthy of attention. In fact, the ligand-to-metal charge transfer (LMCT) process of vanadium complexes under irradiation was observed as early as half a century ago by Bamford and coworkers.^40,41 Through this process, the excited oxovanadium(V) 8-hydroxyquinoline complexes undergo hemolysis reactions to generate alkoxy radicals from alcohols (Scheme 10a). Based on this, Aliwi's group expanded different types of vanadium complexes to realize the transformation.^42–45 Further, Soo applied this property to the degradation of lignin models and achieved promising results.^46–50 This strategy can efficiently break the Csp³–Csp³ bond on the backbone of the polymeric materials under mild conditions without pre-oxidation process (Scheme 10b). In 2020, Wang's group developed oxidative lignin C–C bond cleavage to aldehydes using a commercial vanadium complex, VO(OⁱPr)₃, as the photocatalyst (Scheme 10b).⁵¹ Furthermore, the photocatalytic C–C bond cleavage method can be well applied to the degradation of polymers especially non-biodegradable plastics (Scheme 10c).^50,52,53

Scheme 10 Ligand-to-metal charge transfer process in vanadium complexes.

Recently, Castellano and coworkers delved into the mechanism of this process, particularly the LMCT-activated bond homolysis of vanadium(V) photocatalysts.⁵⁴ In addition, the use photoinduced vanadium catalysis to realize proton-coupled electron transfer (PCET) reactions was studied by Warren and Luca et al. (Scheme 11).^55,56

Scheme 11 Proton-coupled electron transfer process of vanadium complexes.

Compared with other earth-abundant metals, such as Fe and Co, there are only a few modes of the combination of vanadium catalysis and photocatalysis. Thus, there is great potential to develop photoinduced vanadium catalysis to realize more reactions.

5. Chromium metallaphotocatalysis

5.1 General overview

Metal chromium plays an important role in organic synthesis. Both reactions involving equivalent metal chromium and those catalyzed by catalytic amounts of metal chromium have attracted attention from chemists.⁵⁷ In recent years, photoinduced chromium catalytic systems have also been well developed.^58–60 There are three main modes of photochromium synergistic catalytic strategies, as follows: (1) single electron transfer occurs between the chromium catalyst and the photocatalysis; (2) chromium catalysis does not directly interact with photocatalysis; (3) the chromium complex acts as a photosensitizer, and the excited photosensitizer further undergoes single-electron transfer or energy transfer with the substrate; and (4) the excited chromium catalyst undergoes hemolysis via LMCT (Scheme 12).

Scheme 12 Photoredox/chromium dual catalysis.

5.2 Allylation and alkenyl reactions

In 2018, Glorius and co-workers reported the first example of photoredox/chromium dual catalysis to achieve Nozaki–Hiyama–Kishi reaction with good efficiency and diastereoselectivity.⁶¹ In this process, the Cr(III) species undergoes single-electron transfer with the reduced photocatalyst to generate Cr(II) species. Alkyl radicals are captured by the Cr(II) species to form Cr(III) alkyl species, which are further hydrolyzed to give the product (Scheme 13).

Scheme 13 Nozaki–Hiyama–Kishi reaction via photoredox/chromium dual catalysis.

Later, Kanai's group developed the asymmetric allylation of aldehydes using a photoinduced chiral chromium catalysis in 2019.⁶² This strategy allowed direct access to homoallylic alcohols with excellent enantioselectivity and diastereoselectivity (Scheme 14).

Scheme 14 Asymmetric allylation of aldehydes via photoinduced chiral chromium catalysis.

Since then, photoredox/chromium dual catalysis has been well developed for a range of selective allylation and alkenyl reactions. In 2020, Glorius and co-workers reported the chemoselective silyl aminoalkylation of carbonyls for the synthesis of protected 1,2-amino alcohols using photoredox/chromium catalysis (Scheme 15a).⁶³ In this process, the α-aminoalkyl-Cr reagents produced in situ can react as carbanion equivalents. Then, Glorius's group developed a dual photoredox/chromium catalytic strategy to directly access monoprotected homoallylic 1,2-diols (Scheme 15b).⁶⁴ Furthermore, they established the dual photoredox/chromium catalytic strategy with the Hosomi–Sakurai reaction and synthesized a series of valuable chiral homoallylic alcohols in 2022 (Scheme 15c).⁶⁵ They extended photoredox/chromium dual catalysis to the synthesis of α-benzylic alcohols, isochromanones, 1,2-oxy alcohols and 1,2-thio alcohols under mild and robust conditions (Scheme 15d).⁶⁶

Scheme 15 Selective allylation and alkenyl reactions via photoredox/chromium dual catalysis.

5.3 Combination with hydrogen atom transfer catalyst

In 2020, Mitsunuma and Kanai et al. developed a novel photoredox/chromium dual catalysis combined with hydrogen atom transfer catalyst to achieve the allylation of aldehydes using simple alkenes (Scheme 16a).⁶⁷ Further, they realized the linear-selective allylation of aldehydes in 2022 (Scheme 16b).⁶⁸ In 2023, Lu and colleagues reported photoinduced copper/chromium catalysis system for the first time (Scheme 16c).⁶⁹ This strategy achieves direct functionalization of the inert C–H bond to synthesize aryl alkyl alcohols. Recently, Wang's group developed asymmetric C–H addition of N-sulfonyl amines to aldehydes via photoredox, HAT, and chromium triple-catalysis approach (Scheme 16d).⁷⁰ In 2024, Wang and coworkers realized asymmetric Reformatsky reaction for the synthesis of chiral β-hydroxy carbonyl compounds via photoredox/chromium catalysis (Scheme 16e).⁷¹

Scheme 16 Photoredox/chromium dual catalysis combined with hydrogen atom transfer catalysis.

5.4 Three-component reaction

In 2020, Glorius and co-workers extended photo/chromium dual catalysis to a three-component reaction and achieved the difunctionalization of 1,3-diene with good yield and high diastereoselectivity (Scheme 17a).⁷² In this reaction, alkyl radicals are more likely to be captured by 1,3-diene than directly by low-valent chromium species to generate allyl radicals. Then, the allyl radical is captured by the low-valent chromium species to form the chromium allyl species. Later, they achieved radical carbonyl propargylation for the first time by photo/chromium dual catalysis starting with 1,3-enyne (Scheme 17b).⁷³ In 2021, Shi's group achieved the photoredox decarboxylative allylation of aldehydes using NHPI esters as radical precursors and butadiene as substrates. In this process, they reported for the first time that photoexcitation of the Hantzsch ester can directly undergo single-electron transfer with a chromium complex (Scheme 17c).⁷⁴ Unlike the previous 1,2 addition of 1,3-enyne, Wang and colleagues realized asymmetric the 1,4-functionalization of 1,3-enynes. Using the photoredox/chromium dual catalysis, a range of chiral α-allenols can be obtained under mild conditions (Scheme 17d).⁷⁵ Recently, Zhu and coworkers reported the three-component Nozaki–Hiyama–Kishi-type reaction of 1,3-dioxolane, 1,3-butadienes, and aldehydes via photo/HAT/chromium catalysis (Scheme 17e).⁷⁶ In 2024, Li's group⁷⁷ realized the 1,2-difunctionalization of 1,3-enynes through photoexcited Hantzsch ester and chromium catalysis with high activity and selectivity (Scheme 17f).

Scheme 17 Three-component reactions via photoredox/chromium dual catalysis.

5.5 Photoredox/cobalt/chromium catalysis

In 2024, a novel photoredox/cobalt/chromium catalysis approach was developed by Wang's group for the stereoselective homoaldol reaction (Scheme 18a).⁷⁸ Starting from 1,3-dienes, this protocol could be used to access diverse γ-hydroxy carbonyl compounds under mild conditions with good functional group tolerance. In this reaction, Co^III–H is intercepted by the 1,3-diene substrate through the MHAT process to generate a reactive allyl radical, which is subsequently captured by chromium species. Then, Han and Shi et al. reported photoredox/cobalt/chromium triple-catalyzed carbonyl allylation with butadiene, which can be used for the synthesis of valuable homoallylic alcohols (Scheme 18b).⁷⁹

Scheme 18 Reactions via photoredox/cobalt/chromium catalysis.

5.6 Chromium catalysis and photocatalysis without direct interaction

In the other mode, there is no direct interaction between the chromium catalysis and the photocatalysis. In 2022, Shi developed the first photochemical NHK coupling using photoinduced nickel/chromium catalysis (Scheme 19a).⁸⁰ In this process, vinyl nickel species undergoes transmetalation with the chromium species to generate nucleophilic vinyl–chromium species, which is subsequently coupled with carbonyls. The photosensitive Hantzsch esters serve as electron donors to reduce nickel species and as proton donors to hydrolyze the Cr^III–alkoxy species. In 2022, Xiao and colleagues achieved the diacylation of alkenes using a photo/chromium dual system (Scheme 19b).⁸¹ In this reaction, acyl radical and carboxyl radical are formed by the photodecomposition of α-oxocarboxylic acid upon irradiation, which is important for the cross-coupling. Recently, the photo/chromium dual-catalyzed decarboxylative coupling of α-oxo acids was reported by Cao and coworkers (Scheme 19c).⁸²

Scheme 19 Chromium catalysis and the photocatalysis without direct interaction.

5.7 Chromium complexes as photocatalysts

Although conventional photocatalysts tend to be expensive ruthenium and rhodium complexes, inexpensive and readily available chromium complexes can also exhibit photosensitivity. In 2015, Ferreira, Shores, and coworkers developed the first chromium photocatalyst, Cr(Ph₂phen)₃(BF₄)₃ (Ph₂phen = 4,7-diphenyl-1,10-phenanthroline), for catalyzing Diels–Alder cycloadditions (Scheme 20a).⁸³ In the mechanism, a single-electron transfer occurs between the excited chromium complex and the substrate. Moreover, they investigated the role of oxygen in the reaction. Firstly, oxygen quenches the excited chromium complex and produces singlet oxygen. Secondly, singlet oxygen reduces the reduced catalyst to the Cr(III) ground state to form superoxide. Thirdly, the superoxide species reduce the Diels–Alder cycloadduct radical cation to the final product.^84,85 Then, Ferreira further achieved radical cation cyclopropanations using a chromium photocatalyst to synthesize polysubstituted cyclopropanes (Scheme 20b).⁸⁶ In 2018, Ferreira's group realized the (3 + 2) cycloaddition between alkenes and vinyl diazo species via chromium photocatalysis (Scheme 20c).⁸⁷ In 2022, Ferreira obtained a red-shifted chromium photocatalyst, Cr(PMP₂phen)₃(BF₄)₃, by modifying the ligands. Using the novel chromium photocatalyst, dearomative (3 + 2) cycloadditions between indoles and vinyldiazo species were well established (Scheme 20d).⁸⁸

Scheme 20 Cr(Ph₂phen)₃(BF₄)₃ and Cr(PMP₂phen)₃(BF₄)₃ as photocatalysts.

In 2017, Lochbrunner, Opatz, Heinze and colleagues developed the [Cr(ddpd)₂](BF₄)₃ photosensitizer, which could be used for the photocyanation of amines (Scheme 21a).⁸⁹ The excited [Cr(ddpd)₂]³⁺ rapidly decays to the long-lived 2E state and transfers its energy to O₂ to produce singlet oxygen. Then ¹O₂ reacts with amines through a low-energy transition state to form imine cations, which are captured by the cyanide nucleophile to produce the desired product.

Scheme 21 Different chromium photosensitizers [Cr(ddpd)₂]³⁺, [Cr(bpmp)₂]³⁺ and [Cr(tpe)₂]³⁺.

In 2023, Kerzig, Manolikakes, and Heinze et al. reported three different chromium photosensitizers, [Cr(ddpd)₂]³⁺, [Cr(bpmp)₂]³⁺ and [Cr(tpe)₂]³⁺, that could combine sulfur dioxide with organic compound to synthesize sulfones and sulfonamides (Scheme 21b).⁹⁰

In 2022, Wenger and coworkers designed novel ligands for chromium photosensitizers and studied the oxidation properties of the spin-flip excited states of these chromium(III) polypyridine complexes in detail (Scheme 22).⁹¹ They also implemented four photocatalytic reactions including bromination of electron-rich arenes, oxidative cleavage of 1,1,2,2-tetraphenylethylene, deborylative hydroxylation of arylboronic acids and alkenylation of N-phenylpyrrolidines using [Cr(dqp)₂](PF₆)₃ as the photocatalyst.

Scheme 22 [Cr(dqp)₂](PF₆)₃ as a photocatalyst.

Recently, Wang's group achieved the chromium-catalyzed allylic C(sp³)–H addition to aldehydes via photoinduced ligand-to-metal charge transfer (Scheme 23).⁹² In this process, the excited CrBr₃ underwent homolysis through LMCT to generate a bromine radical, which further realized the HAT reaction.

Scheme 23 Photoinduced chromium ligand-to-metal charge transfer catalysis.

6. Manganese metallaphotocatalysis

6.1 General overview

With the development of materials in recent years, photoinduced manganese catalysis has attracted attention from chemists as a promising strategy.^93,94 There are several mechanisms for photoinduced manganese catalysis, as follows: (1) photo-promoted homolysis of Mn₂(CO)₁₀ into a highly reactive monomer, ˙Mn(CO)₅; (2) the manganese complex acts as a photosensitizer, and the excited photosensitizer further undergoes single-electron transfer with the substrate; (3) manganese catalysis does not directly interact with photocatalysis; and (4) single electron transfer occurs between the manganese catalysis and photocatalysis (Scheme 24).

Scheme 24 Photoinduced manganese catalysis.

6.2 Reactive manganese monomer ˙Mn(CO)₅

As early as 1983, Suslick and Schubert discovered the homolysis of a dimeric manganese catalyst, Mn₂(CO)₁₀, into a highly reactive monomer, ˙Mn(CO)₅, under light irradiation.⁹⁵ Since then, a rich series of chemical transformations has been achieved using this reactive monomer of manganese. In 2017, Frenette, Fadeyi and coworkers reported the photoinduced manganese-catalyzed Minisci reaction with unactivated iodoalkanes under mild conditions.⁹⁶ Based on DFT studies, the mechanism for the involvement of light in the generation of the ˙Mn(CO)₅ reactive monomer was proposed, in which the formation of alkyl radicals from manganese radicals with alkyl iodine is the rate-limiting step (Scheme 25).

Scheme 25 Photoinduced manganese-catalyzed Minisci reaction.

In 2018, Nagib's group developed a redox-neutral ketyl radical coupling using ˙Mn(CO)₅ reactive monomer-catalyzed atom transfer.⁹⁷ This strategy could be used for the synthesis of alkenyl iodides with high Z-selectivity and efficiency (Scheme 26).

Scheme 26 Photoinduced manganese-catalyzed redox-neutral ketyl radical coupling.

In 2022, Xie and colleagues achieved the hydrofluorocarbofunctionalization of alkenes by photoinduced manganese catalysis.⁹⁸ Experimental and computational studies showed that the bidentate phosphine ligand used in this reaction not only reduces the energy barrier during atom transfer but also improves the stability and lifetime of the photogenerated metal radicals (Scheme 27).

Scheme 27 Photoinduced manganese-catalyzed hydrofluorocarbofunctionalization of alkenes.

In addition, a range of radical reactions can be achieved via the photoinduced generation of the highly reactive ˙Mn(CO)₅ monomer, including hydrosilylation and hydrogermylation of alkynes,⁹⁹ hydrosilylation of alkenes¹⁰⁰ hydrosulfonylation of alkenes¹⁰¹ and alkynes,¹⁰² fluoroalkylation reactions of 2H-indazoles,¹⁰³ cyclization of unactivated alkyl iodides,¹⁰⁴ and C(sp³)–H functionalization^105–107 (Scheme 28).

Scheme 28 Photoinduced manganese-catalyzed radical reactions.

In 2023, Xie's group achieved the divergent dehydrogenative difluoroalkylation of alkenes by combining photoinduced manganese catalysis with cobaloxime-promoted HAT.¹⁰⁸ In the bimetallic relay catalysis strategy, indium powder plays an important role in bridging the manganese-catalyzed atom transfer and the cobaloxime-catalyzed hydrogen atom transfer (Scheme 29).

Scheme 29 Photoinduced manganese-catalyzed dehydrogenative difluoroalkylation of alkenes.

Meanwhile, Xie and coworkers combined photo-involved manganese activation with gold redox to synthesize a range of Au–Mn complexes under mild conditions.¹⁰⁹ Using the novel Au–Mn complexes as catalysts, the divergent reductive coupling of nitroarenes could be realized (Scheme 30).

Scheme 30 Photoinduced gold-manganese-catalyzed reductive coupling of nitroarenes.

6.3 Manganese complexes as photocatalysts

In addition to facilitating the generation of reactive monomers, photocatalysis has other modes of reaction with manganese catalysis. In 2018, Ackermann's group reported the first example of manganese-catalyzed (het)arene C–H arylation via continuous visible-light photoflow.¹¹⁰ In the proposed mechanism, the manganese catalyst CpMn(CO)₃ undergoes ligand exchange, and then coordination with the substrate to obtain manganese complexes. Subsequently, the excited manganese complex undergoes electron transfer to generate aryl radicals under the irradiation of blue LEDs. This elegant strategy can achieve C–H arylation under mild conditions with good functional group tolerance (Scheme 31).

Scheme 31 Photoinduced manganese-catalyzed (het)arene C–H arylation.

In 2020, Wan and coworkers achieved the hydroxytrifluoromethylation of aliphatic alkenes via photoredox manganese catalysis.¹¹¹ In this process, the excited Mn(III) catalyst undergoes single-electron transfer with CF₃SO₂Na to generate trifluoromethyl radicals, which are further trapped by the alkene (Scheme 32).

Scheme 32 Photoinduced manganese-catalyzed hydroxytrifluoromethylation of aliphatic alkenes.

6.4 Photoinduced single-electron transfer

Recently, Tian, Xiao and colleagues developed the [2 + 2 + 2] cycloaddition of alkynes by photoredox/manganese dual catalysis.¹¹² During the reaction, the high-valent manganese complex can be reduced by efficient single-electron transfer with the excited photocatalyst (Scheme 33).

Scheme 33 Photoinduced manganese-catalyzed [2 + 2 + 2] cycloaddition of alkynes.

6.5 Electro/photo/manganese catalysis

In 2020, Lie and coworkers creatively combined manganese catalysis, photocatalysis and electrocatalysis to realize the oxidative azidation of C(sp³)–H bonds (Scheme 34a).¹¹³ Based on in-depth mechanism studies, they proposed the possible catalytic cycle, in which the Mn(II) complex coordinated by N₃⁻ undergoes single-electron transfer at the anode to generate an Mn(III) complex. The alkyl radical generated by the photocatalytic hydrogen atom transfer process interacts with the Mn(III) complex to obtain azide products. This novel strategy could also be used to install the azido functional group on the tertiary carbon. In 2022, Fu's group utilized the strategy of electro/photo/manganese catalysis to realize the decarboxylative azidation of aliphatic carboxylic acids under mild conditions (Scheme 34b).¹¹⁴

Scheme 34 Electro/photo/manganese catalyzed oxidative azidation of C(sp³)–H bonds and decarboxylative azidation of aliphatic carboxylic acids.

6.6 Novel manganese catalysts

Several novel manganese catalysts have also been developed in combination with photocatalysis. In 2022, Bera and Soo et al. reported a novel Mn(I) complex featuring a hydroxy-functionalized naphthyridine N-oxide ligand (Scheme 35a).¹¹⁵ The Mn(I) complex could be used as a photosensitizer to achieve the conversion of 2-aminobenzyl alcohols to quinolones. In 2022, Kundu and coworkers developed the (NN)Mn(I) complex as a photocatalyst for the dehydrogenative coupling of aldehydes with amines (Scheme 35b).¹¹⁶

Scheme 35 Novel Mn(I) complexes using photo/manganese dual catalysis.

Recently, Lin and colleagues designed an Mn(IV) complex featuring boron-incorporated N-heterocyclic carbine ligands (Scheme 36).¹¹⁷ This strategy can effectively extend the lifetime of the excited Mn catalyst and enable it to undergo a series of transformations to form C–C, C–P, C–B, C–S, C–Se and C–N bonds.

Scheme 36 Mn(IV) complex featuring boron-incorporated N-heterocyclic carbine ligands.

7. Iron metallaphotocatalysis

7.1 General overview

Among the transition metal elements, iron is the most abundant on the Earth's surface. The low cost and low biological toxicity of iron compounds prompted chemists to investigate their potential for use in organic synthesis. Numerous visible light-induced iron-catalyzed organic synthesis methods have been developed, and a variety of iron-catalyzed organic reactions has been applied in industrial production and academic research.^118,119 There are six main models of visible-light induced iron-catalyzed reactions. (1) Iron(III) complexes are excited by visible light, which then undergo an LMCT homolysis process, forming iron(II) complexes and radicals (Scheme 37a). (2) Iron(II) complexes are excited by visible light, which then undergo an MLCT process, resulting in the formation of iron(III) complexes and radicals (Scheme 37b). (3) Iron(n) complexes are excited by visible light and undergo an SET process with the substrate molecules. In this case, the iron complexes act as classical photoredox catalysts (Scheme 37c). (4) There is an interaction between photocatalysis and iron catalysis via an SET process (Scheme 37d). (5) There is no direct interaction between photocatalysis and iron catalysis (Scheme 37e). (6) In some cases, excited-state iron compounds exhibit a heightened propensity for two-electron transfer reactions (Scheme 37f).

Scheme 37 Six main models of visible light-induced iron-catalyzed reactions.

7.2 LMCT homolysis process

Fe(III)–halides, Fe(III)–carboxylates and Fe(III)–alkoxides are the three main types of iron complexes that can undergo the MLCT homolysis process.

7.2.1 Photo-induced formation of halogen radicals from Fe(III)–halides via LMCT hemolysis process. Fe(III)–halides (FeX₃) can mildly generate halogen radicals (X˙) under photoexcitation via the LMCT homolysis process. The X radicals typically react with alkanes via the HAT process or undergo a radical addition process with alkenes (Scheme 38).

Scheme 38 Two main ways for the production of alkyl radicals using Fe(III)–halides (FeX₃) as catalysts.

In the 1960s and 1970s, Imoto^120–123 reported that light-induced stoichiometric ferric chloride produced chlorine radicals via the LMCT homolysis process, which were used to react with ethylene glycol and toluene for subsequent conversion. Subsequently, an increasing number of research groups have endeavoured to facilitate reactions catalyzed via the LMCT homolysis process using light-induced Fe(III)–halides (FeX₃), resulting in the conversion of a diverse range of compounds, particularly alkyl compounds.^124–160 This strategy of Fe(III)–halides (FeX₃) catalysis via the LMCT process has been effectively used to form various chemical bonds.

7.2.1.1 Formation of new chemical bonds. The first FeCl₃-catalyzed photochemical reaction was achieved by Shulpin in 1990.¹²⁴ In this reaction, the FeCl₃-catalyzed chlorination/oxidation of cyclohexane in air gave chlorocyclohexane, cyclohexanone and cyclohexanol in a total yield of about 20% under visible light irradiation. In 2022, Gong's group employed the FeCl₃/HCl photocatalytic system to achieve the diverse functionalization of a range of aliphatic C–H bonds (Scheme 39).¹²⁵

Scheme 39 Photo-induced FeCl₃ LMCT process to form new chemical bonds.

In 2024, the Hu group successfully employed a photo-iron-catalyzed selective boronization process utilizing sulfone as the oxidant, thereby enabling the selective boronization of alkane terminal C–H bonds in organic compounds.¹²⁷ The mechanistic experiments demonstrated that the HAT process is non-selective with regard to chlorine radicals and alkanes. Furthermore, the deuterium-labeling experiments showed that in the presence of sulfone and boric acid, non-terminal alkyl radicals can undergo another HAT process with high selectivity, thereby enabling the non-terminal alkyl radicals to regenerate alkanes (Scheme 40).

Scheme 40 Photo-induced FeCl₃ LMCT process to form a C–B bond and its mechanism.

Electrophotochemistry offers the benefits of both photochemistry and electrochemistry, facilitating more environmentally friendly and efficient conversions. Numerous interesting works have been carried out by some groups combining iron-catalyzed photoelectrochemical reactions.^{126,128–132,137,161}

In 2023, the Lu group conducted a combined investigation into electrochemistry and photocatalysis, employing iron and nickel as catalysts for the alkenylation and acylation reactions of C(sp³)–H bonds (Scheme 41a).¹²⁸ In the same year, this group achieved alkyl C(sp³)–H arylation and alkylation reactions utilising iron- and nickel-catalyzed photoredox electrocatalytic processes (Scheme 41b).¹²⁹ This method allows the synthesis of either a C(sp³)–C(sp²) bond or the addition of an olefin as a linking group, which in turn allows the construction of both a C(sp³)–C(sp³) bond and a C(sp³)–C(sp²) bond in a single step. This reaction can be achieved with moderate to excellent yields, and a variety of alkyl compounds with C(sp³)–H bonds can be alkylated using this method. Furthermore, a variety of aryl and heteroaryl bromides with electron-withdrawing and electron-donating groups can be involved in this reaction. Notably, functional groups that are typically sensitive to electroreductive conditions, including nitrile, ketone, ester, sulfone, and others, demonstrated remarkable tolerance. Furthermore, this reaction can be employed for the gram-scale synthesis and late-stage modification of natural products and pharmaceutical derivatives. In 2023, the Noël group developed an efficient flow electrophotocatalysis (f-EPC) reactor for electrophotocatalysis, utilising iron as a catalyst to achieve the construction of C(sp³)–N bonds (Scheme 41c).¹³⁰ In 2024, the Lu group successfully combined photo-induced FeCl₃ catalysis with electroredox oxidation, thereby achieving the borylation of alkanes C(sp³)–H bonds (Scheme 41d).¹³¹ The authors proposed the potential mechanism, whereby the Fe(III) species [FeCl₄]⁻ undergo an LMCT homolysis process, resulting in the formation of [FeCl₃]⁻ and Cl radicals in the presence of light. Then, the Cl radicals proceed to react with alkanes via the HAT process, leading to the formation of alkyl radicals. Subsequently, these alkyl radicals can be boronated with B₂cat₂. Subsequently, the Ackermann group reported a similar borylation of alkanes. These two groups presented novel methodologies for the synthesis of alkylboron compounds. However, the capacity to distinguish the C(sp³)–H bonds of specific substrates remains inadequate (Scheme 41e).¹³²

Scheme 41 Electrophotochemical FeCl₃ LMCT process to form a new chemical bond.

7.2.1.2 Formation of two new chemical bonds. In 2020, Zhu reported that FeX₃ (X = Cl or Br) catalyzed the oxidation of olefins to afford α-haloketones. The proposed mechanism posits that the light-induced generation of X radicals from FeX₃ and the subsequent addition of these radicals to olefins result in the formation of β-haloalkyl radicals (Scheme 42a).¹³³ Subsequently, the β-haloalkyl radicals are captured by oxygen, resulting in the formation of α-haloketones in the presence of iron complexes. This method employs the inexpensive and readily available KCl and KBr as halogen sources, which has a broad range of potential applications and is a green and economical method for the synthesis of α-haloketones. In 2023, Zeng and colleagues achieved the C–H functionalization of a series of polymers, including polyethylene and polyethylene glycol, through the use of a photo-induced iron-catalyzed modification (Scheme 42b).¹³⁴

Scheme 42 Photo-induced FeX species LMCT process to form two new chemical bonds.

7.2.1.3 Cleavage of chemical bonds. Photo-induced iron halide catalysis has the capacity to be employed not only for the conversion of small molecules, but also the degradation and recycling of polymers.

In 2021, Hu achieved the oxidation of a wide range of commercial polystyrene plastics to benzoic acid using an FeCl₂-catalyzed oxidation strategy in the presence of light (Scheme 43a).¹³⁵ In 2022, the Stache group achieved the photocatalytic oxidative degradation of polystyrene using 10 wt% of FeCl₃ (Scheme 43b).¹³⁶

Scheme 43 Photo-induced Fe salt LMCT process for the cleavage of chemical bonds.

7.2.2 Photoinduced formation of carboxyl radicals from Fe(III)–carboxylate complexes via LMCT hemolysis. The excitation of Fe(III)–carboxylate complexes by light results in the LMCT homolysis process, whereby carboxyl radicals are formed. These carboxyl radicals can either participate directly in subsequent reactions or become alkyl radicals following decarboxylation, which then engage in further reactions. Carboxylic acids are a simple and readily available class of molecules. This strategy has been employed by numerous research groups to facilitate the utilization of carboxylic acid radicals or alkyl radicals that are formed upon decarboxylation. Consequently, this enables the formation of a diverse range of bonds (Scheme 44).^162–182

Scheme 44 Two main ways Fe(III)–carboxylates produce radicals via the LMCT hemolysis process.

7.2.2.1 Formation of new chemical bonds. In 2019, the Jin Group published a report on the photo-induced iron-catalyzed decarboxylative alkylation of heteroarenes for the first time. The proposed mechanism suggests that Fe(III) compounds coordinate with carboxylic acids to form Fe(III)–carboxylate complexes (Scheme 45a).¹⁶² Subsequently, these complexes are homolytically cleaved to Fe(II) compounds and carboxyl radicals by the LMCT process under light irradiation. Subsequently, the carboxyl radicals undergo decarboxylation to produce the corresponding alkyl radicals, which undergo a Minisci-type reaction with the heteroarenes to give the product. In 2024, Ackerman-Biegasiewicz and colleagues achieved the photocatalytic C(sp³)–C(sp²) coupling of alkylcarboxylic acids and aryliodides using inexpensive Fe and Ni catalysts. Tempo is an effective alkyl radical trapper and a relatively mild oxidising agent (Scheme 45b).¹⁶³ In 2024, Bunescu and colleagues employed Tempo to trap alkyl radicals and utilised its oxidative properties to oxidize Fe(II) species to Fe(III) species (Scheme 45c).¹⁶⁴ This process enabled the photo-induced iron-catalyzed decarboxylative oxidation of alkylcarboxylic acids, which resulted in the formation of a C–O bond. In this study, the researchers conducted comprehensive mechanistic experiments and performed DFT calculations to elucidate the mechanism of the reaction.

Scheme 45 Photo-induced Fe(III)–carboxylate species decarboxylation via LMCT process to form a new chemical bond and its mechanism.

In 2020, the Jin group also reported a photo-induced iron-catalyzed intramolecular aromatic C–H acyloxylation reaction (Scheme 46).¹⁶⁵ In this reaction, probably due to the higher aryl–carboxyl bond energy, carboxyl radicals are directly involved in the reaction rather than undergoing decarboxylation to form aryl radicals. Subsequently, the carboxyl radical was added to another aryl ring, and then oxidized by iron(III) and deprotonated to give the product. Iron(II) was oxidized back to iron(III) by NaBrO₃ to complete the catalytic cycles.

Scheme 46 Photo-induced Fe(III)–carboxylate species LMCT process to form a new chemical bond without decarboxylation and its mechanism.

7.2.2.2 Formation of two new chemical bonds. In 2019, the Jin research group employed the photo-induced iron-catalyzed decarboxylative strategy to generate alkyl radicals, thereby achieving alkylation reactions of olefins and azo compounds (Scheme 47a).¹⁶⁶ This strategy enables the formation of two new bonds. In this protocol, a broad array of Michael acceptors can be alkylated. In 2022, the Xie group successfully synthesised asymmetric tertiary amines using a photo-promoted iron-catalyzed method (Scheme 47b).¹⁶⁷ Nitroaromatics were employed as a nitrogen source, and two different carboxylic acids were used to alkylate them separately. In 2023, West developed a method for the hydrofluoroalkylation of olefins that employs a light-promoted iron-catalyzed combined with a thiophenol-catalyzed hydrogen-atom transfer reaction utilising fluorinated carboxylic acids as the source of fluoroalkyl radicals (Scheme 47c).¹⁶⁸ This method offers the benefits of mild conditions and a broad range of substrates. In contrast to traditional trifluoromethylation reactions utilising trifluoromethylating reagents with oxidising properties, this reaction can also tolerate groups susceptible to oxidation, including heterocycles, sulphides, alcohols and aldehydes. In addition to hydrotrifluoromethylation, this strategy enables the hydrodifluoro-, monofluoro- and perfluoroalkylation of olefins. In 2024, Niu reported the photo-induced iron-catalyzed fluorine–haloalkylation of olefins. The mechanism by which this reaction produces haloalkyl radicals and their addition to olefins to produce new radicals is similar to that reported by West (Scheme 47d).¹⁶⁹ The distinction in this paper is that the newly generated alkyl radical is fluorinated with Selectfluor, resulting in the formation of an N⁺ radical species. This reagent has the capacity to oxidize Fe(II) species directly or indirectly to Fe(III) species. The authors demonstrated that strong Brønsted acids facilitate the LMCT homolysis process between iron and halocarboxylic acids through the utilization of stoichiometric experiments and UV-visible light experiments. Furthermore, they proposed that iron coordination compounds with halocarboxylic acids and acetonitrile are photoactive species, combining UV absorption spectra and HRMS results.

Scheme 47 Photo-induced Fe(III)–carboxylates species LMCT process to form two new chemical bonds.

7.2.3 Photoinduced formation of alkoxy radicals from Fe(III)–alkoxy complexes via LMCT hemolysis. The reaction of alcohols with Fe(III) in the presence of light has been observed to produce alkoxy radicals. These alkoxy radicals have been shown to generate alcohols and alkyl radical species through a HAT process with alkyl C(sp³)–H, or alternatively, to form ketones and alkyl radical species through a β-scission process. The synthesis of complex molecules can be achieved from simple and readily available alcohols using this strategy (Scheme 48).^{142,161,183–189}

Scheme 48 Two main ways by which Fe(III)–alkoxy species produce radicals via LMCT hemolysis process.

In the 1960s, Imoto^120–122 presented a report on the photochemical reaction of glycol in the presence of stoichiometric FeCl₃, which suggested a potential mechanism for homolytic cleavage via LMCT. This mechanism posits that during this process, the Fe(III)–alkoxyl species generates an alkoxyl radical, which subsequently undergoes β-scission to form an aldehyde.

Given that the majority of Fe-mediated alcohol reactions employ FeCl₃ as the initial catalyst or necessitate the incorporation of an external halide anion, the precise underlying mechanism for these processes remains to be fully elucidated. This is particularly pertinent in cases where the LMCT homolysis of Fe halides or Fe(III)–alkoxy species is involved. To validate the involvement of key radical intermediates in these processes, further mechanistic experiments are necessary.

7.2.3.1 Formation of new chemical bonds. In 2021, Hu combined a photo-induced iron-catalyzed LMCT homolysis reaction with a thiophenol-catalyzed hydrogen atom transfer reaction to achieve the cleavage of a C–C single bond in alcohols (Scheme 49a).¹⁴² During the optimisation of the conditions, the authors identified chloride ions as a necessary component and demonstrated the presence of chlorine radicals through corresponding mechanistic experiments. Consequently, the authors proposed that the mechanism of the reaction involves the generation of chlorine radicals from Fe(III)–Cl species by the LMCT homolysis process in the presence of light, followed by HAT of the chlorine radicals with alcohols to produce alkoxy radicals. This alkoxy radical undergoes β-scission to form a ketone and an alkyl radical, which undergoes HAT with the thiophenol to give the product. In 2024, the Mei group achieved the ring-opening arylation of cycloalcohols using a combination of photoelectrochemical and iron/nickel catalytic processes (Scheme 49b).¹⁶¹

Scheme 49 Photo-induced Fe(III)–alkoxy species LMCT process to form new chemical bonds and its mechanism.

7.2.3.2 The formation of two new chemical bonds. In 2021, the Zeng research group reported the photo-induced remote C–H amination of alcohols catalyzed by FeCl₃ as a single catalyst (Scheme 50).¹⁸³ In this work, the authors concluded that the LMCT homolysis of Fe(III) alkoxy species directly generates alkoxy radicals. Also, the alkoxy radicals produce alkyl radicals via the intramolecular 1,5-HAT process, then the alkyl radicals are added to the azodicarboxylate, and finally reduced to the product by Fe(II) species.

Scheme 50 Photo-induced Fe(III)–alkoxy species LMCT process to form two new chemical bonds and its mechanism.

7.2.3.3 Cleavage of chemical bonds. In 2021, the Zeng research group published a report on the oxidation of tert-benzyl alcohol to aromatic acid. This method involves the cleavage of two C–C bonds in the presence of light, utilising FeCl₃ and CeCl₃ in conjunction as catalysts (Scheme 51).¹⁸⁴

Scheme 51 Photo-induced Fe(III)–alkoxy species LMCT process for the cleavage of chemical bonds.

7.2.4 Photoinduced formation of radical species from other types of Fe(III) complexes via LMCT hemolysis. Furthermore, there are other iron complexes that can undergo an LMCT homolysis process in the presence of light to produce radical species, which can be used in subsequent reactions to form various bonds.^190–195

In 2024, West achieved the bifunctionalization of olefins via an iron-catalyzed reaction in the presence of light (Scheme 52).¹⁹⁰ In this reaction, the diazidation of olefins can be achieved by using TMSN₃ as the functionalization reagent, which is capable of constructing two C–N bonds simultaneously. TMSN₃ can coordinate with iron, resulting in the formation of iron azide compounds. These iron azide compounds generate azide radicals under light via an LMCT homolysis process. Alternatively, the use of NaCl in place of TMSN₃ as the functionalization reagent allows the dichlorination or fluorochlorination of olefins.

Scheme 52 Photo-induced Fe(III) species LMCT process to form two new chemical bonds.

7.3 MLCT process

In the presence of light, Fe(II) complexes bearing N–O bond-containing compounds, exemplified by dioxazolones or N-alkyloxyamides, can undergo an MLCT process, resulting in the production of Fe(III) nitrogen radical complex species. Subsequently, they may be transformed into nitrogen-containing compounds via further modification (Scheme 53a).^196–198 It is worth mentioning that photo-induced iron-catalyzed reactions involving dioxazolones, N-alkyloxyamides or azides may result in the formation of Fe–nitrene species (Scheme 53b).^199–204 Thus, we discuss the reactions of Fe–nitrene intermediates in this section.

Scheme 53 Reaction models of Fe species via MLCT process.

7.3.1 The formation of new chemical bonds. In 2022, Chen and co-workers reported the photo-induced FeCl₂-catalyzed C(sp²)–N coupling reaction of N-methoxyamides with arylboronic acids (Scheme 54).¹⁹⁶ Based on mechanistic experiments, the authors proposed that light-promoted MLCT processes of Fe(II)–N-methoxyamide complexes form Fe(III)–nitrogen radical species.

Scheme 54 Photo-induced iron species MLCT process to form new chemical bonds.

7.3.2 Formation of two new chemical bonds. In 2021, Bao and colleagues reported the C(sp²)–N coupling reaction of dioxazolones with arylboronic acids, which was catalyzed by FeCl₂ under light irradiation (Scheme 55).¹⁹⁷ The results of UV-Vis experiments indicated that the iron and dioxazolone complexes are photoactive species. The proposed mechanism posits that upon excitation under light, the Fe(II)–dioxazolone complexes undergo a charge transfer process, releasing CO₂ to yield Fe(III)–nitrogen radical species. The latter species interacts with arylboronic acids, thereby facilitating C(sp²)–N coupling in the subsequent transformation.

Scheme 55 Photo-induced iron species MLCT process to form two new chemical bonds and its mechanism.

In 2021, Bao and colleagues achieved the iron-catalyzed decarboxylation of dioxazolones to produce Fe–nitrene species under light, as well as achieving C–H bond amidation, N=S double bond formation and N=P double bond formation reactions (Scheme 56a).¹⁹⁸ The research team posited that an Fe–nitrene intermediate is produced in the reaction process based on their DFT calculations. In 2024, the Bach group employed fluorinated aryl azide in conjunction with an iron catalyst to generate Fe–nitrene species under light conditions, thereby achieving the synthesis of variable 3-arylmethyl-substituted 2-quinolones or 2-pyridones with high regioselectivity and high enantioselective selective C–H bond amination (Scheme 56b).¹⁹⁹

Scheme 56 Photo-induced Fe–nitrene species generated and formation of two new chemical bonds.

7.4 Iron complexes as PC

Some iron complexes have the capacity to undergo single-electron transfer upon photoexcitation, enabling them to engage in an SET process with other molecules. Several research groups successfully synthesised a variety of photosensitive iron complexes and investigated their photochemical properties.^205–214 Some of these complexes have been employed as photocatalysts in diverse reactions.^215–222

7.4.1 Formation of new chemical bonds. In 2015, Cozzi²¹⁵ and colleagues achieved the enantioselective α-alkylation of aldehydes using [Fe(bpy)₃]Br₂ as a photoredox catalyst, thereby extending the findings of MacMillan²²³ (2008), who used [Ru(bpy)₃]Cl₂ as a photoredox catalyst. This marked the inaugural instance of iron compounds being employed as photoredox catalysts in organic synthesis. In 2019, Wärnmark and colleagues synthesised [Fe(phtmeimb)₂]PF₆ with an excited-state lifetime of approximately 2 ns and observed that it exhibited the capacity to undergo an SET process with methylviologen or diphenylamine (Scheme 57a).²⁰⁵ In 2021, Troian-Gautier and colleagues conducted further research into the nature of the [Fe(phtmeimb)₂]PF₆ complex and applied it as a photocatalyst in a benzylic dehalogenation reaction (Scheme 57b).²¹⁶

Scheme 57 Iron complexes act as PC and catalyze the formation of new chemical bonds.

7.5 Combination of photocatalysis with iron catalysis via SET process

The combination of transition metal catalysis and photoredox catalysis allows the synthesis of a diverse range of compounds.²²⁴ The transition metal-catalyzed cycle and the photocatalytic cycle interact through an SET process. Iron, in particular, is capable of undergoing single-electron transfer and can also participate in an SET process with photoredox catalysts.^225–231

7.5.1 Formation of two new chemical bonds. In 2019, Li achieved the dialkylation reaction of olefins through the combination of iron-catalyzed and photocatalyzed processes. This reaction involves the generation of a tert-butoxy radical by DTBP in the presence of a photocatalyst (Scheme 58a).²²⁵ This radical subsequently engages in hydrogen atom transfer (HAT) with the solvent alkane, resulting in the production of an alkyl radical. Subsequently, the alkyl radical is captured by the olefin, resulting in the formation of a new alkyl radical. Then, it reacts with an electron-withdrawing group-substituted alkane, giving the final product. In 2023, the Wu research group successfully completed the 1,2-aryl(alkenyl) heteroatom functionalization of olefins (Scheme 58b).²²⁶ A variety of DBT salts of ortho-, meta-, and para-substituted electron-rich or electron-deficient arene, heteroarenes and styrenes can be converted to DBT salts, and then used in the reaction to achieve the difunctionalization of olefins under mild photoredox/iron-catalyzed conditions. In 2024, the MacMillan group demonstrated the utility of iron catalysis in conjunction with photoredox catalysis for the formation of Fe–carbene intermediates from a diverse array of accessible feedstocks, including carboxylic acids, amino acids, and alcohols (Scheme 58c).²²⁷ This is achieved through the photocatalytic interaction with iron complexes. The Fe–carbene intermediates can be employed in the formation of cyclopropanes with olefins or in the insertion into P–H, S–H and N–H bonds, thereby forming C–P, C–S and C–N bonds, respectively.

Scheme 58 Dual photoredox/iron catalysis for the formation of two new chemical bonds.

7.5.2 Reduction of CO₂. In 2017 and 2018, Robert achieved a photo-iron-catalyzed CO₂ reduction reaction, whereby an SET process exists between the photocatalytic cycle and the iron-catalytic cycle (Scheme 59).^228,229

Scheme 59 Dual photoredox/iron-catalyzed reduction of CO₂.

7.6 No interaction between photocatalysis and iron catalysis

Reactions with iron catalysts and photocatalysts directly without SET or ENT processes also exist.^232–234

In 1989, Saito reported an example of oxidation of alkanes to alcohols or ketones without SET or EnT interaction between photocatalysis and iron catalysis (Scheme 60).²³²

Scheme 60 Photo-iron-catalyzed formation of new chemical bonds.

7.7 Light accelerates iron-catalyzed reactions

When a metal complex is excited by light it can reach an excited state with higher energy. Metal complexes in the excited state have higher energies, and therefore can undergo reactions that are more difficult to occur in the ground state. Gevorgyan^235–239 and Yu^240–242 developed light-excited palladium-catalyzed reactions, which were applied for the transformation of a wide range of compounds. Some two-electron processes (such as oxidative addition, reductive elimination, ligand coordination and dissociation), which are more difficult to occur in the ground state, also occur more easily in the excited state, thus accelerating the reaction. This phenomenon of light promoting the occurrence of two-electron processes in iron complexes has also been found by some research groups in their studies on photo-iron co-catalyzed reactions.^243–245

7.7.1 Formation of new chemical bonds. In 2019, Noël employed flow chemistry to achieve a photo-promoted iron-catalyzed Kumada coupling reaction (Scheme 61).²⁴³ The authors observed the production of a light-absorbing Fe(I) species in the reaction through UV-Vis spectral analysis and postulated that this Fe(I) species can enhance the rate of oxidative addition to aryl chlorides in the presence of light.

Scheme 61 Light-promoted iron-catalyzed formation of new chemical bonds.

7.7.2 Formation of two new chemical bonds. In 2024, Ackermann achieved a photo-promoted iron-catalyzed aryl imine C–H alkenylation reaction (Scheme 62).²⁴⁴ This strategy offers high functional group tolerance, mild reaction conditions and full atom economy. The authors found that light promotes the release of hydrogen reduction to Fe(0) from the precatalyst to facilitate coordination with the substrate imine. Furthermore, light on/off experiments demonstrated that light promoted the dissociation of the Fe–P ligand, thereby releasing a coordination site with the alkynes and accelerating the reaction.

Scheme 62 Light-promoted iron-catalyzed formation of two new chemical bonds.

8. Cobalt metallaphotocatalysis

8.1 General overview

In recent years, dual photoredox and cobalt catalysis has emerged as a new paradigm in organic photocatalysis, which has led to the discovery of unprecedented transformations as well as the improvement of known reactions. This field can be classified into four subcategories, which are based on the interaction modes between visible light and cobalt metal involved. (1) In most cases, the high valence state Co(n) species undergo single-electron transfer (SET) with the reduced-state photocatalyst (PC˙⁻) or excited photocatalyst (PC*) to the low valence state Co(n − 1) (Scheme 63a). (2) The low valence state Co(n) species undergo single electron transfer (SET) with the excited photocatalyst (PC*) to the high valence state Co(n + 1) (Scheme 63b). (3) A cobalt complex as a Lewis acid catalyst activates the substrate containing heteroatoms and promotes the single-electron transfer process in the photocatalytic cycle (Scheme 63c). (4) A cobalt complex can be used as a photocatalyst by harnessing photon energy, and then catalyzes bond breaking/forming events via the traditional or new type of mechanism (Scheme 63d).

Scheme 63 Representative mechanisms in cobalt/photoredox catalysis.

8.2 Reduction quenching

8.2.1 Co(III)–Co(II).
8.2.1.1 Formation of new chemical bonds. Cobalt catalysts facilitate hydrogen atom removal and subsequent H₂ evolution under protonation. After H₂ evolution, the in situ-generated Co(III) species undergoes SET oxidation to the reduced photocatalyst, thereby regenerating both the initial photocatalyst and the Co(II) complex. An early example of merging photoredox and cobalt catalysis via reduction quenching was reported by Wu and colleagues, who combined eosin Y with Co(dmgH)₂Cl₂ for coupling reactions involving various tetrahydroisoquinolines and indoles (Scheme 64a).²⁴⁶ Mechanistically, under visible-light irradiation, eosin Y is excited to its singlet state [eosin Y]*, which undergoes electron transfer with tetrahydroisoquinolines to generate [eosin Y]˙⁻ and an amine radical cation. The amine radical cation deprotonates and is oxidized by Co(dmgH)₂Cl₂, forming an iminium intermediate and Co(I). The iminium intermediate reacts with nucleophiles to afford the cross-coupling product, while Co(I) is protonated to yield Co(III)–H. Upon proton interaction, Co(III)–H releases H₂, regenerating Co(III). Meanwhile, [eosin Y]˙⁻ donates an electron to Co(III), producing Co(II) and restoring the ground-state photocatalyst, thereby completing the catalytic cycle. Through a similar photochemical oxidation of substrates, the dehydrogenative functionalization of amino acids,²⁴⁷ isochromanes,²⁴⁸ (benzo-)thiazoles²⁴⁹ and ortho biaryl-appended 1,3-dicarbonyl compounds^250,251 was also possible. Furthermore, Luo and Wu and colleagues reported the asymmetric dehydrogenative coupling of N-alkyl anilines with ketones, aided by a chiral primary amine catalyst (Scheme 64b).¹⁹⁵ In 2023, Wu and co-workers reported a new method for the synthesis of fluorenones using cobaloxime catalysis without the need for a photoredox catalyst (Scheme 64c).²⁵² The authors proposed that under near UV light irradiation, the aromatic aldehydes enter a highly reactive triplet excited state, which is intercepted by cobaloxime to afford acyl radicals and Co(II) species via single-electron transfer (SET). Subsequent radical cyclization, followed by rearomatization and fluorenone formation is enabled by the Co(II) species. The oxidative functionalization of C–H bonds for the construction of C–heteroatom bonds represents a desirable method for streamlining the multistep synthesis of complex molecules. In 2015, Lei and colleagues reported an external oxidant-free C–H functionalization/C–S bond formation reaction to form benzothiazoles using a combined cobaloxime/photoredox catalytic system (Scheme 64d).²⁵³ In 2016, Wu and Tung and colleagues reported the cross-coupling of benzene with ammonia or water for the synthesis of anilines and phenols via highly oxidizing quinolinium ion QuCN⁺ catalysts and cobaloxime catalysts (Scheme 64e).²⁵⁴

Scheme 64 Cobalt/photoredox-catalyzed dehydrogenation coupling.

In 2018, Wu and co-workers reported another reaction pattern, i.e., the Heck coupling of unactivated aliphatic acids and terminal alkenes (vinyl silanes and vinyl boronates) through the synergistic combination of an organophoto-redox catalyst and a cobaloxime catalyst (Scheme 65a).²⁵⁵ Mechanistically, under alkaline conditions, the deprotonation of acids can lead to the oxidation of the resulting protons by excited photocatalysts, generating alkyl radicals and carbon dioxide. The alkyl radicals undergo radical addition with styrene, forming stable benzyl radicals. Then, these benzyl radicals interact with Co(II), leading to the formation of Co(III) alkyl species. Subsequently, homolysis of the Co–C bond and β-hydride elimination yield the corresponding products, together with Co(III)–H species. Finally, Co(III)–H species and protons react to release H₂. Later, the same group reported a method for the direct alkenylation of alkanes and aldehydes with aryl alkenes (Scheme 65b).²⁵⁶ This strategy relies on direct hydrogen atom transfer (HAT) photocatalysts to produce carbon-centered and carbon-centered radical intermediates, which undergo desaturation catalyzed by cobaloxime to yield the desired olefin and Co(III)–H. In 2019, Xu and co-workers reported a method for the synthesis of allylsilanes from alkenes and tris(trimethylsilyl)silane (TTMSS) by combining photoredox catalysis, hydrogen-atom transfer, and cobalt catalysis (Scheme 65c).²⁵⁷ In 2022, Nagib and co-workers introduced a photo/Co-cocatalyzed radical aza-Heck cyclization method for efficiently converting allyl alcohols into biologically relevant heteroatom-rich compounds (Scheme 65d).²⁵⁸ In the same year, Luo's group reported an asymmetric C–H dehydrogenative allylic alkylation of 2-arylpropenes with β-ketocarbonyls by a related triple catalytic system involving a chiral primary amine, a photoredox catalyst, and a cobaloxime cocatalyst (Scheme 65e).²⁵⁹ The reaction between a chiral primary amine organocatalyst and a β-ketocarbonyl compound produces the corresponding enamine. The enamine undergoes single-electron transfer (SET) oxidation, generating α-iminyl radicals, which readily react with in situ-generated Co(II) species to form the organocobalt intermediate. Subsequent radical addition to 2-arylpropenes, followed by dehydrogenation, leads to the formation of an intermediate via the cobalt intermediate. Finally, hydrolysis of the imine yields the desired product together with regeneration of the organocatalyst.

Scheme 65 Cobalt/photoredox-catalyzed Heck reaction to facilitate the construction of a single bond.

8.2.1.2 Formation of two new chemical bonds. In 2017, the Lei group reported the oxidative [4 + 2] annulation of NH imines and alkenes for the synthesis of multi-substituted 3,4-dihydroisoquinolines utilizing a dual photoredox/cobaloxime catalytic system (Scheme 66).²⁶⁰ Mechanistically, the single-electron oxidation of styrene derivatives by the excited state of the photosensitizer generates the alkene radical cation intermediate, together with the reduced photosensitizer. Following this, nucleophilic attack of imines on the radical cation intermediate leads to benzyl radical intermediates after deprotonation. Radical cyclization and oxidation by Co(II) yield a cation intermediate and Co(I) intermediate. The subsequent elimination of a proton produces the desired 3,4-dihydroisoquinoline product. Meanwhile, the reduced photosensitizer is reoxidized by Co(III), closing the photoredox catalytic cycle. On the cobalt side, Co(II) is reduced to a Co(I) species, which can be protonated to generate a Co(III)–hydride intermediate. Then, this Co(III)–hydride releases H₂ upon interaction with a proton. Subsequently, the same research group²⁶¹ and the Chen group²⁶² sequentially reported several related studies, including the oxidative [4 + 2] annulation reaction of styrene derivatives with electron-rich dienophiles, oxidative cyclization synthesis of tetrahydroquinolines from tertiary anilines and maleimides and selective oxidative dehydrogenative [4 + 2] annulation of imidazo-fused heterocycles with alkenes.²⁶³

Scheme 66 Dual photoredox/cobalt-catalyzed formation of two new chemical bonds and its mechanism.

8.2.1.3 Formation of multiple bonds. In the absence of a nucleophile, photoredox/Co-mediated cascade dehydrogenations offer a route for the synthesis of amines from cyclohexanone. In 2020, Leonori's group used cyclohexanone and amines as raw materials to realize the synthesis of a variety of anilines through the combination of photoredox catalysis and cobalt catalysis (Scheme 67a).²⁶⁴ Under mildly acidic conditions, amine 1 can act as a nucleophile and react with cyclohexanone 2 to generate enamine intermediate A *in situ*. Due to the electron-rich nature of enamines, A undergoes single-electron transfer (SET) with an appropriate photocatalyst, forming nitrogen-centered radical cation intermediate B. Radical cation B enhances the acidity of the β-methylene unit, facilitating deprotonation to produce 5π-electron β-enamine radical C. In the presence of a divalent cobalt co-catalyst, radical C undergoes hydrogen atom transfer (HAT) to form conjugated diene D, while Co(II) is oxidized to a [Co(III)]–H species. The latter releases hydrogen gas upon reaction with a proton source (e.g., DABCO–H⁺), regenerating the [Co(III)] species. Subsequently, [Co(III)] is reduced back to Co(II) by the reduced photocatalyst Ir(II). Given the higher reactivity of conjugated diene D, it can undergo repeated oxidation–dehydrogenation cycles, ultimately yielding aniline 3. Given that aniline 3 also exhibits some reducing ability, the key to this transformation is selecting a photocatalyst with moderate oxidation potential, which can selectively oxidize intermediates A and D without over-oxidizing the final product 3. Later, the same group explored various desaturative synthesis of aromatic compounds via enamine intermediates. In 2022, Leonori's group described a novel strategy for aromatic aldehyde synthesis from the corresponding cyclohexanecarbaldehydes through the synergistic integration of enamine, photoredox and cobalt catalysis (Scheme 67b).²⁶⁵ In 2023, Leonori et al. introduced a new method for the complete aromatization of cyclohexanones to phenols (Scheme 67c).²⁶⁶ More recently, Leonori and co-workers introduced a general platform for the coupling of primary and secondary amines with ketone-containing saturated heterocycles to prepare amine-substituted heteroaromatics including electron-poor pyridines, electron-rich pyrroles, furans, thiophenes and pyrazoles (Scheme 67d).²⁶⁷

Scheme 67 Cobalt/photoredox-catalyzed desaturation of aliphatic compounds for the construction of multiple bonds.

In 2018, Ritter's group reported a method for the generation of olefins from carboxylic acids enabled by the cooperative interplay with a cobalt catalyst (Scheme 68).²⁶⁸ The carboxylate is oxidized by the photoexcited acridinium catalyst to produce an alkyl radical, which is trapped by Co(II) via hydrogen atom transfer (HAT), yielding a Co(III) hydride and the desired alkene. Then, the Co(III) hydride is deprotonated, releasing hydrogen gas and Co(III) species. The Co(III) species can subsequently undergo single-electron reduction by the photocatalyst, completing the catalytic cycles of Co and photocatalyst. In the same year, Tunge's group reported a dual-catalytic strategy for the synthesis of enamides and enecarbamates from amino acids.²⁶⁹ Later, Larionov's group reported the decarboxylative elimination of biomass-derived feedstocks utilizing various acridine/cobaloxime dual-catalytic systems.²⁷⁰ These direct approaches make use of a photoredox catalyst to achieve oxidative decarboxylation in tandem with a cobaloxime catalyst to perform the needed hydrogen atom transfer (HAT). In 2024, Cai's group reported a method for the direct and efficient activation of the carbonyl β-C(sp³)–H bond to form unsaturated ketones via the merger of photoredox and cobalt catalysis.²⁷¹

Scheme 68 Cobalt/photoredox-catalyzed dehydrogenative decarboxyolefination of carboxylic acids to construct multiple bonds.

In 2022, Reuping's group²⁷² and Wu's group²⁷³ reported a photo/cobalt dual-catalyzed method for the synthesis of distally unsaturated ketones through sequential ring-opening C–C bond scission and dehydrogenation of nonstrained tertiary cycloalkanols of variable ring sizes (Scheme 69). The mechanism initiates with SET from aryl substituents of alcohols to the excited state of the photoredox catalyst, generating cation radical species. These intermediates undergo intramolecular PCET in the presence of a base to yield alkoxyl radical species, which can cleave into alkyl radicals and a carbonyl moiety via β-scission of the neighbouring C–C bond. Subsequently, the formed alkyl radical undergoes a desaturation process via cobaloxime catalysis, resulting in the formation of remotely dehydrogenated ketones.

Scheme 69 Cobalt/photoredox-catalyzed synthesis of distally unsaturated ketones via dehydrogenated alcohols to construct multiple bonds.

In 2024, Zuo and co-workers described the first example of the dehydrogenative functionalization of mono-donor cyclopropanes to access diverse allylic N,O-acyl-acetal derivatives through the combination of photoredox and a cobaloxime catalyst (Scheme 70).²⁷⁴ The excited photocatalyst (*PC) is reductively quenched by cyclopropylamide, generating the corresponding radical cation intermediate and reductive species PC. The distonic radical cation is formed after spontaneous β-scission and can be reversibly captured by Co(II), followed by β-H extraction, producing the α,β-unsaturated imine species and Co(III)–H. Then, Co(III)–H can react with a nucleophile, releasing H₂ and regenerating the Co(III) catalyst. The reduced photocatalyst, in turn, can reduce Co(III) back to Co(II), completing the catalytic cycle. Simultaneously, regioselective nucleophilic addition to the imine affords the desired product.

Scheme 70 Cobalt/photoredox-catalyzed dehydrogenative functionalization of cyclopropylamides to construct multiple bonds.

8.2.2 Co(II)–Co(I).
8.2.2.1 The formation of a new chemical bond. In 2016, Lei's group reported the anti-Markovnikov oxygenation of β-alkyl styrenes and their derivatives using water, facilitated by a combination of Fukuzumi's catalyst and cobaloxime (Scheme 71a).²⁷⁵ The mechanism involves the SET oxidation of alkenes into their corresponding radical cations, where the anti-Markovnikov addition of water and deprotonation would furnish a C-radical intermediate, followed by desaturation catalyzed by cobaloxime to yield the desired olefin and [CoIII]–H. As a result, the protonation of cobalt hydride released H₂ and completed the catalytic cycle. Shortly after, the same group explored the dehydrogenative functionalization of alkenes with alcohols and azoles utilizing Acr-Mes-Me⁺ClO₄⁻ as a strongly oxidizing photocatalyst in conjunction with cobaloxime.²⁷⁶ In 2017, the same group reported the phosphonylation of C(sp²)–H bonds (methylarenes, anisoles, polycyclic aromatic hydrocarbons, heteroaromatics, anilines, and olefins derivatives) with no need for excess phosphonylation reagents by merging visible-light photoredox with cobalt catalysis (Scheme 71b).²⁷⁷

Scheme 71 Cobalt/photoredox-catalyzed Heck reaction to facilitate the construction of a single bond.

In 2019, Kojima and Matsunaga group achieved a remarkably selective allylic alkylation reaction with an impressive regioselectivity of >20 : 1 (branched/linear alkene) in most cases (Scheme 72a).²⁷⁸ From a mechanistic perspective, under visible light irradiation, the photoredox catalyst (PC) is excited to its active state (PC*), where it oxidizes an electron donor. Then, the reduced photocatalyst is oxidized by Co(II), returning to its ground state and generating a Co(I) species. The Co(I) complex undergoes oxidative addition with an allylic electrophile, forming a π-allyl cobalt intermediate. Subsequent nucleophilic attack on this intermediate produces the allylated product, regenerating the low-valent cobalt catalyst. Later, using a similar strategy, the Shi and Meng group separately reported the allylation of aldehydes and regio-, diastereo- and enantioselective propargylation of aldehydes (Scheme 72b).^279,280

Scheme 72 Cobalt/photoredox-catalyzed allylation and propargyl reactions for the construction of a single bond.

In 2022, Xiao’ group reported the first visible-light-induced cobalt-catalyzed asymmetric reductive Grignard-type addition for synthesizing chiral benzyl alcohols with high yield and enantioselectivity (Scheme 73a).²⁸¹ In 2023, Zhang and co-workers reported a method for the asymmetric reductive Grignard-type addition of aryl iodides with axially prochiral biaryl dialdehydes, leading to the direct construction of axially chiral secondary alcohols via photo/cobalt catalysis (Scheme 73b).²⁸²

Scheme 73 Cobalt/photoredox-catalyzed asymmetric reductive coupling for the synthesis of alcohol compounds.

8.2.2.2 Formation of two new chemical bonds. Given that cobalt hydride species derived from cobaloximes can be readily generated, it is reasonable to assume that cobalt-hydride-mediated transformations should be possible using cobalt/photoredox catalysis such as hydrofunctionalization of alkenes/alkynes/dienes. In 2021, Matsunaga's group developed the ascorbic-acid-mediated HAT hydrogenation of alkenes in aqueous media by the merging of photo/cobalt catalysis (Scheme 74a).²⁸³ Under visible light irradiation, the photoredox catalyst (PC) is excited to its active state (PC*), which oxidizes ascorbate to afford the reduced PC and the corresponding ascorbic acid radical. The reduction of cobalt(II) with the reduced PC generates cobalt(I), which is converted to cobalt(III) hydride upon protonation. HAT from the cobalt(III) hydride to an unactivated alkene should regenerate the cobalt(II) catalyst and afford an alkyl radical. The second HAT to the resulting alkyl radical from ascorbic acid should realize the hydrogenation of the alkene. Later, they developed a protocol for the photocatalytic deuteration of electron-deficient alkenes using ascorbic acid-d4 as a deuterium source (Scheme 74b).²⁸⁴

Scheme 74 Cobalt/photoredox-catalyzed hydrogenation of alkenes for the construction of two new chemical bonds.

In 2022, Ohmiya and co-workers developed the Markovnikov hydroalkoxylation of unactivated alkenes using alcohols through triple catalysis consisting of photoredox, cobalt, and Brønsted acid catalysts (Scheme 75a).²⁸⁵ Besides, precise control of protons and electrons resulted in a catalytic connection between Co(I) and Co(IV). In the same year, Matsunaga and co-workers reported a strategy for the synthesis of 1,1-diarylalkanes and triarylalkanes from styrenes and cyanopyridine compounds.²⁸⁶ More recently, Liu's group reported a method for synthesis of organosilanes and polycyclic quinazolinones via the hydrolipocyclization or silylation of unactivated alkenes.²⁸⁷ Lin's group reported photo/cobalt catalysis enabling the hydrofluorination of alkenes utilizing Et₃N·3HF as the sole source of both hydrogen and fluorine (Scheme 75b).²⁸⁸ In 2024, the Carreira group developed a general transformation for the cycloisomerization of unactivated olefins utilizing N-, O-, and C-nucleophiles to prepare saturated heterocycles under mild conditions via photo/Co dual catalysis (Scheme 75c).²⁸⁹ In 2024, Xiao's group reported a protocol for the synthesis of a series of chiral functionalized cyclopropanes via the addition of cyclopropenes to imines with mild-to-good yields and high enantioselectivity by leveraging the synergy between photoredox and asymmetric cobalt catalysis (Scheme 75d).²⁹⁰

Scheme 75 Cobalt/photoredox-catalyzed hydrogen functionalization of alkenes to construct two new chemical bonds.

In 2021, Xia and co-workers reported the highly regio- and enantioselective reductive coupling of alkynes and aldehydes toward homoallylic alcohols via photoredox cobalt dual catalysis (Scheme 76a).²⁹¹ Later, they reported a method for the ligand-controlled ene-type or reductive coupling of alkynes and gem-disubstituted alkenes via photoredox cobalt dual catalysis.²⁹² In 2024, Li and co-workers reported a method for the synthesis of axially and centrally dual chiral diaryl ethers in high diastereo- and enantioselectivity by the cobalt-catalyzed photoreductive enantioselective coupling of dialdehyde and alkyne (Scheme 76b).²⁹³ In the same year, Sato and co-workers reported the intramolecular reductive cyclization of alkynals with dual photoredox/cobalt catalysis using H₂O for catalyst turnover to construct substituted cyclic alcohols (Scheme 76c).²⁹⁴

Scheme 76 Cobalt/photoredox-catalyzed reductive coupling of alkynes and aldehydes to construct two new chemical bonds.

In 2017, Rovis and Thullen reported the hydroaminoalkylation of conjugated dienes using a dual-catalyst system (Scheme 77).²⁹⁵ From a mechanistic perspective, the one-electron reduction of the Co(II) complex generates Co(I) in situ. This Co(I) species potentially reacts with an equivalent of carboxylic acid to afford the cobalt hydride, which, upon insertion into conjugated dienes, generates Co(III)–allyl species. This Co(III)–allyl species be reduced by the photocatalyst to generate a Co(II)–ally species. By contrast, amines are oxidized by the excited photocatalyst to furnish nucleophilic α-aminoalkyl radicals. These α-aminoalkyl radicals may then attack the Co(II)–ally species, which undergoes reductive elimination to afford the product and low-valent cobalt complex back into the catalytic cycle. More recently, the Teskey group successfully developed a conceptually distinct single-electron reductive hydropyridylation of dienes via a photo/cobalt-catalyzed strategy.²⁹⁶

Scheme 77 Cobalt/photoredox-catalyzed hydroaminoalkylation of conjugated dienes to construct two new chemical bonds.

In 2023, the research group led by Shi developed a novel strategy for carbonyl allylation with a wide range of amino, oxy, thiol, aryl, and alkyl–allenes to access valuable β-functionalized homoallylic alcohols selective for allenes by the combination of photo Co-MHAT and Ti catalysis (Scheme 78).²⁹⁷

Scheme 78 Cobalt/titanium/photoredox-catalyzed allene functionalization.

8.3 Oxidation quenching

8.3.1 Formation of two new chemical bonds. In 2020, Zhu's group developed a Co/Ru dual catalysis protocol for the Markovnikov-selective intermolecular hydrofunctionalization of styrenes and vinylheteroarenes (Scheme 79a).²⁹⁸ The key step was the excited-state Ru(bpy)₃²⁺ efficient oxidation of the organocobalt(III) complex to organocobalt(IV) complex. In 2021, Sundararaju's group reported a dual-catalytic protocol for the C–H alkynylation of benzamides with bromoalkynes, combining cobalt and photocatalysts (Scheme 79b).²⁹⁹ Mechanistic studies revealed that the excited photocatalyst was quenched by the divalent cobalt species Co(II), generating a catalytically active chiral Co(III) complex. Later, Liu and co-workers developed a photo/cobalt-catalyzed strategy to synthesize a diverse range of indolo[2,3-c]isoquinolin-5-ones via the dearomatization of indoles using readily available N-quinolyl benzamides.³⁰⁰ In 2024, Shi's group and Sundararaju's group independently employed Salox ligands to achieve the asymmetric dearomatization of indoles, delivering high levels of enantioselectivity (Scheme 79c).^301,302

Scheme 79 Cobalt/photoredox-catalyzed hydrogen functionalization of alkenes to construct two new chemical bonds.

8.4 Photocatalyst–transition metal catalysis without interactions (Lewis)

8.4.1 Formation of two new chemical bonds. The notable proficiency of cobalt as a Lewis acid catalyst, especially compared to heavier traditional transition metals, highlights its potential for cost-effective and versatile catalysis. In 2019, Xiao and co-workers first developed novel chiral Co^II complexes as efficient catalysts for the visible-light-induced Giese reaction, enabling the coupling of both alkyl and acyl radicals (Scheme 80a).³⁰³ This method offers synthetically valuable chiral ketones and 1,4-dicarbonyl compounds with generally high yields and excellent enantioselectivity. More recently, Xiao disclosed the asymmetric radical coupling of α,β-unsaturated 2-acyl imidazoles and α-silylamines to give β,β-disubstituted γ-amino acid derivatives with acyclic quaternary carbon stereocenters by merging photocatalysis and cobalt catalysis (Scheme 80b).³⁰⁴ Gong's group reported the chemo and stereoselective reductive cross-coupling between common aldehydes with a broad array of carbonyl and iminyl compounds by photoredox-mediated cobalt catalysis (Scheme 80c).³⁰⁵ The Hong group developed a novel platform for generating persistent ketyl radicals, facilitated by a cobalt Lewis acid catalyst and versatile acyl triazoles (Scheme 80d).³⁰⁶ Besides, they further demonstrated two representative radical cross-coupling reactions with other radical precursors such as DHPs and trifluoroborates. In these reactions, Co as a Lewis acid coordinates with the substrate such as imidazole, pyridine, and N-acyl hydrazone, enabling substrate activation for single-electron transfer (SET) to generate persistent radical and subsequent radical–radical cross-coupling to obtain the product. Initially, R2-DHP was oxidized by the excited photocatalyst, and then alkyl radicals were generated. Meanwhile, α,β-unsaturated ketones were activated by the chiral Co catalyst. Then, stereoselective conjugated addition occurred to deliver Co, and its isomer was generated via keto–enol tautomerism. Finally, a single reduction by the reduced state of the photocatalyst, followed by protonation and ligand exchange, afforded chiral ketone products.

Scheme 80 Photo-induced cobalt-catalyzed formation of two new chemical bonds.

8.5 Cobalt complexes as photocatalysts

8.5.1 Formation of new chemical bonds. Although various cobalt salts and complexes in combination with exogenous photocatalysts have been utilized to facilitate diverse dehydrogenative transformations and C–C or C–heteroatom bond formations, Co(III) complexes, such as vitamin B12 and its derivatives, can also function as photocatalysts. This occurs through the homolytic cleavage of Co–C bonds ((BDE) ≈ 14–42 kcal mol⁻¹) to produce carbon-centered radicals and persistent Co(II) radicals under irradiation mediated by ligand-to-metal charge transfer (LMCT). In 2011, the Carreira group reported the novel and mild cobalt-catalyzed, intramolecular Heck-type coupling of alkyl iodides with olefins utilizing DIPEA as a base to formally reduce Co(III)H to an anionic Co(I) species via deprotonation (Scheme 81).³⁰⁷ Co(I) cobaloximes are well-known to undergo SN₂ reactions with alkyl halides, forming Co(III)–alkyl intermediates. Upon photoinduced LMCT, these intermediates generate alkyl radicals that undergo cyclization, and subsequently recombine with Co(II) to form Co(III) species. This LMCT process also facilitates hydrogen atom transfer (HAT) at the αC–H bond, yielding the elimination product. Deprotonation of the resulting Co(III)–H complex regenerates the catalyst. Later, using the same strategy, the Carreira group developed a method for the synthesis of allylic trifluoromethanes from styrene derivatives and 2,2,2-trifluoroethyl iodide.³⁰⁸ In 2016, the Morandi group demonstrated the use of epoxides and aziridines as electrophiles and intramolecular coupling with alkenes catalyzed by a cobalt catalyst.³⁰⁹ More recently, El-Sepelgy and co-workers reported the intramolecular endo-selective Heck reaction of iodo- and bromomethylsilyl ethers of phenols and alkenols using the Co-1 catalyst.³¹⁰

Scheme 81 Cobalt/photo-catalyzed coupling of alkyl iodides with alkenes via LMCT.

The viability of photoinduced SET of cobalt complexes was also disclosed by Wu's group in 2019 (Scheme 82).³¹¹ In this case, the excited Co(II) complex oxidizes H-phosphine to its radical cation, which upon deprotonation produces a phosphinoyl radical, subsequently adding to an alkene and affording radical intermediate II. This intermediate II combines with another Co(II) catalyst to form metallic intermediate III. After removal of the Co(III)–H intermediate via β-H elimination, the product is obtained. Later, the same group reported a similar phosphorylation of enamines and enamides.³¹²

Scheme 82 Photoinduced cobalt catalysis for the synthesis of alkenylphosphine oxides.

In 2020, Dong and co-workers developed the cobaloxime-catalyzed dehydrogenative cyclization of o-teraryls under photochemical conditions (Scheme 83).³¹³ The reaction starts with photoexcited conrotatory 6π electrocyclization to generate transient trans-dihydro-triphenylene intermediate (I). Then, the photoexcited Co(II) catalyst would be able to oxidize I to give a highly delocalized radical cation intermediate (II), and the resulting anionic Co(I) abstracts a proton from II to obtain Co(III)–H and radical intermediate III. Then, Co(II) abstracts the hydrogen from the weakened C–H bond of radical intermediate III to produce the product and Co(III)–H. Finally, the Co(III)–H intermediate would undergo homolytic cleavage of the Co–H bond for H₂ evolution. Later, Yang's group reported the cobaloxime-catalyzed acceptorless dehydrogenative cyclization of ortho biaryl-appended 1,3-dicarbonyl compounds under photochemical conditions.³¹⁴

Scheme 83 Photoinduced cobalt catalyzed dehydrogenative cyclization of o-Teraryls.

8.5.2 Formation of multiple bonds. In 2019, Rovis’ group reported a cycloaddition reaction between diynes and terminal alkynes, producing fused arenes (Scheme 84).³¹⁵ The catalytic cycle begins with the in situ formation of Co(II) acetylide (A). Coordination of the diyne induces a bathochromic shift of ∼100 nm, making A photoactive at the applied wavelength. Upon photoexcitation, A* exhibits LMCT character, enabling oxidative cyclization with the coordinated diyne to generate a Co(III) intermediate (B). The aryl radical cation in B is reduced by DIPEA, forming a metallacycle (C). The subsequent migratory insertion of the alkyne, followed by reductive elimination yields the arene and regenerates Co(I) acetylide (D). This cycle is completed by oxidizing D with the DIPEA radical cation or the excited-state A*.

Scheme 84 Photoinduced cobalt catalyzed [2 + 2 + 2] cycloaddition of alkynes to construct multiple bonds.

9. Nickel metallaphotocatalysis

9.1 General overview

The interaction modes between visible light and nickel metal can be classified into four models based on the presence or absence of a photocatalyst in the reaction (Scheme 85), as follows: (1) generally, the high valence state Ni(n) species undergo single-electron transfer (SET) with the reduced-state photocatalyst (PC˙⁻), releasing the ground-state photocatalyst (PC) and the low valence state Ni(n − 1) to complete the photocatalytic cycle and nickel catalytic cycle. (2) The high valence Ni(n) species in the nickel catalytic system can be directly obtained by single-electron oxidation of low valence Ni(n − 1) species with the excited-state photocatalysts (PC*) through photocatalytic cycling. (3) In addition to the single-electron transfer process, Ni(n) can also undergo energy transfer processes with photocatalysts to form high-energy excited organic Ni(n) intermediates to complete coupling reactions. (4) In addition to the above-mentioned common modes, in certain specific reactions, the photon-absorbing nickel complex is activated by visible light to reach an unstable excited state in the absence of photocatalysts, which then undergo photolytic cleavage to generate low-valence Ni species and a halogen radical.

Scheme 85 Photoredox/nickel dual catalysis.

9.2 Formation of new chemical bonds

In this research field, MacMillan, Molander, Doyle and other research groups^316,317 have reported many pioneering works, avoiding the requirement of stoichiometric amounts of organometallic reagents or metal reducing agents in traditional cross-coupling reactions. In this section, they will be classified according to the different free radical precursors.

9.2.1 Reactions involving alkyltrifluoroborates as radical precursors. Compared with boric acid or borate, alkyltrifluoroborates are a class of reagents with the advantages of easy preparation and laboratory stability, making them excellent C(sp³)-type nucleophiles in cross coupling reactions. The cross-coupling reaction between alkyltrifluoroborates and bromobenzene was reported by Molander's team in 2014³¹⁸ (Scheme 86a). Benzyltrifluoroborate (E^red_1/2 = +1.10 V vs. SCE) underwent oxidative cleavage under irradiation with a 26 W compact fluorescent lamp (CFL) at room temperature for 24 h to produce a C-centered radical, which then combined with an Ni catalyst through an energetically barrierless, single-electron metalation process to achieve the construction of alkyl–aryl linkages. Subsequently, to demonstrate the broad potential of single-electron conversion in this dual catalytic cross-coupling reaction, Molander and colleagues also extended their photoredox-nickel catalytic strategy to the coupling reaction of secondary alkyltrifluoroborates with a relatively higher reduction potential (E^red_1/2 = +1.50 V vs. SCE). In 2015, this group firstly disclosed the cross-coupling reaction of secondary alkyltrifluoroborates with an array of aryl bromides enabled by the synergistic combination of photoredox and Ni catalysis (Scheme 86b).³¹⁹ In 2016, a cross-coupling reaction of secondary β-trifluoroboratoketones and -esters with aryl bromides was realized by the same group (Scheme 86c).³²⁰

Scheme 86 Reactions involving alkyltrifluoroborates as radical precursors.

After determining the mechanism of alkyltrifluoroborates and arylbromides in nickel/photoredox dual catalytic through a density functional theory (DFT) study, Kozlowski and Molander³²¹ further expanded the types of electrophiles. In 2017, Molander's group used N-acylpyrrolidine-2,5-diones as electrophilic reagents to synthesize aliphatic ketones³²² (Scheme 86d). In the same year, they merged photoredox and nickel catalysis to realize a cross-coupling reaction between acyl chloride and potassium trifluoroalkanoate³²³ (Scheme 86e). In addition, this team used dimethyl dicarbonate (DMDC) as an activator for the in situ activation of carboxylic acids to achieve the construction of acyl-sp³ bonds through photoredox/nickel catalysis³²⁴ (Scheme 86f).

Trifluoroborates with heteroatoms introduced at the α-position are more prone to single-electron transfer to break C–B bonds. As early as 2015, Molander³²⁵ et al. first disclosed that enantiopure α-amino radicals formed from chiral N-trifluoroboratomethyl amino acids were efficient for cross-coupling reaction with aryl bromide through photoredox/nickel dual catalysis (Scheme 86g). Subsequently, they sought to apply this chemistry broadly to 1° alkoxymethyltrifluoroborates and acyclic 2° alkoxyalkyltrifluoroborate systems³²⁶ (Scheme 86h) and synthesized the important flavanone core starting from functionalized 2-trifluoroboratochromanones³²⁷ (Scheme 86i). In 2023, an efficient coupling reaction between allyl trifluoroborates and aryl halides was developed by Liu's group³²⁸ to achieve the regioselective preparation of diverse substituted allylic benzenes by adding allyl to nickel species to produce a π-allyl nickel(III) intermediate (Scheme 86j).

9.2.2 Reactions involving 1,4-dihydropyridines as radical precursors. 1,4-Dihydropyridine derivatives (DHPs) prepared from aldehydes in one-step can undergo oxidative fragmentation to form a wide range of C-center radicals, such as alkyl, acyl, and carbamoyl groups, which subsequently participate in cross-coupling reactions.³²⁹ In 2016, Molander's³³⁰ team used 1,4-dihydropyridine as a Csp³-centered alkyl radical precursor to achieve a coupling reaction with the electrophilic reagent (hetero) aryl bromide (Scheme 87a). In the same year, Nishibayashi³³¹ et al. generated alkyl radicals through the C–C bond cleavage of 4-alkyl-1,4-dihydropyridines to achieve a coupling reaction with aryl iodides via the combination of nickel and photoredox catalysts. Unlike Molander's work, this reaction required an equivalent amount of base as additive to deprotonate DHP (Scheme 87b). In 2018, Molander's group³³² firstly used dihydropyridyl saccharide motifs as free radical precursors to access arylated saccharides under mild conditions, providing a new pathway for the synthesis of non-classical reverse aryl C-glycosides (Scheme 87c). In 2021, Liang³³³ firstly achieved the regio-selective alkylation of alkyl 1,4-dihydropyridine derivatives with propargylic carbonates without alkyl organometallic reagents, efficiently obtaining a series of alkylated allene products under mild conditions (Scheme 87d). It is worth mentioning that in 2024, Chi's group³³⁴ used amino acid-derived redox-active dihydropyridines as carbamoyl radical precursors to establish a direct method for the synthesis of amide-linked C-glycosyl amino acids and peptides under photoredox and nickel catalysis. 20 natural amino acids, peptides, and their derivatives could efficiently undergo glycosylation to obtain C-glycosyl products in moderate to excellent yields with excellent stereoselectivity (Scheme 87e).

Scheme 87 Reactions involving 1,4-dihydropyridines as radical precursors.

The above-mentioned works all required the addition of photocatalysts to induce C–C bond cleavage of DHPs. Considering the specificity of the substrate, Melchiorre³³⁵ discovered that 4-alkyl-1,4-dihydropyridine could become a strong reducing agent under the excitation of violet-light-emitting diode, which could activate the reagents through SET, while undergoing homolytic cleavage to directly generate alkyl radicals and participate in the nickel catalytic cycle (Scheme 87f).

9.2.3 Reactions involving carboxylic acid and alcohols as radical precursors. Carboxylic acid is an ideal source of radicals, which can be obtained by removing one molecule of carbon dioxide through a photoredox process to obtain C-centered radicals. Recently, many research groups have carried out cross coupling reactions of carboxylic acids and halogenated hydrocarbons based on photoredox/Ni dual catalytic systems. In 2014, the pioneering work on the coupling of carboxylic acids and aryl halides through the synergistic combination of photoredox and nickel catalysis was reported by MacMillan's group.³³⁶ Initially, the direct decarboxylative C(sp³)–C(sp²) cross coupling was achieved by using Ir[dF(CF₃)ppy]₂(dtbbpy)PF₆ as the photocatalyst and inexpensive amino acids and α-oxy- or benzylic-substituted carboxylic acids as the source of alkyl radicals (Scheme 88a). In 2015, this group demonstrated that vinyl halides are also suitable substrates for participating in coupling reactions³³⁷ (Scheme 88b). Subsequently, in 2016, this research group³³⁸ demonstrated that Ir[dF(CF₃)ppy]₂(dtbbpy)PF₆ was an effective photocatalyst for the decarboxylative coupling of carboxylic acids with alkyl halides under blue light to furnish a new method for the formation of C(sp³)–C(sp³) bonds (Scheme 88c). In 2016, Fu's and MacMillan's group³³⁹ successfully applied photoredox and nickel catalysis to the field of asymmetric decarboxylative C(sp³)–C(sp²) cross-coupling reactions, obtaining valuable enantioenriched benzylic amines under mild reaction conditions (Scheme 88d). In the following years, based on a dual nickel- and photoredox-catalyzed modular approach, Davidson's group³⁴⁰ and Flanagan's group³⁴¹ successively reported the reaction of α-heterocyclic carboxylic acid with aryl bromides in conjunction with the chiral pyridine–oxazoline (PyOx) ligand to obtain enantiomerically enriched N-benzylic heterocycles and the C(sp²)–C(sp³) decarboxylation arylation reaction of α-amino acids and DNA-tagged aryl halides. The discovery of these reactions further promotes the vigorous development of this field.

Scheme 88 Reactions involving carboxylic acid as radical precursors.

In addition to carboxylic acids, in situ-formed mixed anhydrides, α-oxo acid and alkyl alcohols activated in situ with oxalyl chloride can also participate in decarboxylation coupling reactions. For example, in 2015, MacMillan's group³⁴² demonstrated that the mixed anhydrides formed in situ from carboxylic acids and acyl chlorides could efficiently synthesize ketones after undergoing metal insertion–decarboxylation–recombination (Scheme 89a). In the same year, this research group achieved the direct decarboxylation arylation reaction of α-oxo acid and aryl halide under mild reaction conditions.³⁴³ Meanwhile, the drug molecule fenofibrate can be rapidly constructed through this strategy (Scheme 89b). In 2016, MacMillan's group³⁴⁴ demonstrated that alcohols activated by simple oxalyl chloride can generate C-centered radicals to complete cross-coupling reactions with a broad range of aryl halides under mild conditions (Scheme 89c).

Scheme 89 Reactions involving mixed anhydrides, α-oxo acid and alkyl alcohols as radical precursors.

9.2.4 Reactions involving hydrocarbons as radical precursors. As the most common compounds, hydrocarbons can activate the C–H bond under visible light to obtain alkyl radicals, which greatly enriches the application scope of the dual catalytic model. Thus far, the activation of C–H bonds to obtain alkyl radicals can be achieved through hydrogen atom transfer (HAT), where the dual catalytic cross coupling initiated by the hydrogen atom transfer process involves two specific ways, one is to directly use HAT catalysts (such as halogens, TBADT), and the other is to use an amine reagent to conduct targeted H atom abstraction. In 2016, Doyle³⁴⁵ reported the direct C(sp³)–H cross-coupling reaction between aryl chlorinated hydrocarbons and ethers. It is worth noting that there are three models of interaction between visible light and nickel in this double catalytic system. Firstly, the Ni(II) aryl chloride intermediate generated by the oxidation addition of Ni(0) to aryl chloride undergoes single-electron oxidation to obtain Ni(III) species. Subsequently, the Ni(III) species undergo homolysis under the irradiation of visible light to form chlorine radicals, which act as a HAT reagent to capture a hydrogen atom from the ether to obtain alkyl radicals, and subsequent reductive elimination gives the desired product. Finally, a single electron-transfer (SET) reduction of the Ni(I) intermediate and Ir(II) species would regenerate both the Ni(0) and Ir(III) catalysts (Scheme 90a). In the same year, Molander's group³⁴⁶ also achieved a photochemical nickel-catalyzed C–H arylation reaction. Unlike Doyle's work, the photocatalyst facilitates nickel excitation and the generation of a bromine radical through an energy-transfer pathway (Scheme 90b). Using a similar energy-transfer strategy, Li et al.³⁴⁷ used inexpensive and readily available thioxanthen-9-one as the photocatalyst and NiBr₂·3H₂O as the nickel catalyst to achieve the efficient synthesis of diarylamines. According to the mechanism, firstly aryl halides undergo oxidative addition with Ni(0) to obtain an aryl–Ni(II)–bromide complex, which undergoes rapid ligand exchange to afford a new aryl–Ni(II)–amido. At the same time, the photosensitizer thioxanthen-9-one absorbs energy to reach the excited state. The energy of the triplet photosensitizer thioxanthen-9-one is transferred to the aryl–Ni(II)–amino intermediate, which upon subsequent reductive elimination generates the target coupling compounds and Ni(0). The following year, Doyle's group³⁴⁸ used aryl chloride and 1,3-dioxane as starting materials to achieve the redox-neutral formylation of aryl chlorides under mild conditions (Scheme 90c). In 2023, Xu's group³⁴⁹ used TBADT as a hydrogen-atom-transfer (HAT) photocatalyst to generate acyl radicals by capturing the H atom on aldehydes, achieving a coupling reaction with α-bromophosphonates. It is worth mentioning that the mechanism exploration found that there is no direct interaction between photocatalysis and nickel metal catalysis.

Scheme 90 Reactions involving hydrocarbons as radical precursors.

In 2017, MacMillan's group³⁵⁰ activated aldehydic sp² H–C(O) bonds through a polarity-matched hydrogen extraction strategy using electrophilic quinoline radical cations, achieving the efficient construction of ketone compounds under mild conditions (Scheme 91a). The following year, the same group³⁵¹ achieved the selective functionalization of alcohol α-hydroxy C–H bonds through the combination of photoredox/Ni catalysis, hydrogen atom transfer catalysis and Lewis acid activation modes (Scheme 91b). In 2022, Studer's group³⁵² described a strategy of Ni and photoredox catalysis to achieve the β-C–H arylation of aldehydes and ketones with (hetero)aryl bromides (Scheme 91c).

Scheme 91 Reactions involving hydrocarbons as radical precursors.

9.3 Formation of two new chemical bonds

With the development of photoredox and transition-metal dual catalysis, a series of elegant photoredox and nickel dual-catalyzed cascade reactions have received widespread attention in recent years, making it possible to obtain a range of valuable organic compounds from simple and abundant commercially available raw materials through efficient and mild methods.

9.3.1 1,2-Dicarbofunctionalization. In 2018, Chu and colleagues³⁵³ first disclosed that tertiary alcohol derivatives are efficient alkylation reagents for intermolecular three-group cascade reactions with terminal alkynes and aryl bromides through photoredox and nickel dual-catalyzed cross-coupling reaction (Scheme 92a). In this reaction, when Ir[dF(CF₃)ppy]₂(dtbbpy)(PF₆) was used as the photocatalyst and NiCl₂·glyme was used as the nickel catalyst, excellent efficiency and syn-stereoselectivity products could be obtained by the irradiation of visible light in a DMSO solvent system at 36 °C for 18 h. The mechanism was envisioned to proceed through four consecutive events, as follows: (1) the tertiary alkyl oxalate prepared through a one-pot method from tertiary alcohol was excited by the photocatalyst through single-electron oxidation, resulting in the generation of a tertiary alkyl radical upon the loss of two molecules of CO₂. (2) The resulting alkyl radical undergoes regioselective addition to C≡C bonds to produce a linearized alkenyl radical. (3) The generated alkenyl radical participates in the nickel catalytic cycle, resulting in an E-alkene product with lower Gibbs energy. (4) The E-alkene product acts as a photosensitizer, which undergoes a photoinduced energy transfer pathway, resulting in E → Z isomerization to obtain the rare Z-alkene products with syn-stereoselectivity. Afterwards, the same group³¹⁷ achieved the selective, intermolecular 1,2-alkylarylation of olefins under mild and redox-neutral conditions via photoredox/nickel dual catalysis (Scheme 92b).

Scheme 92 1,2-Dicarbofunctionalization reactions.

A novel three-component carbofunctionalization of alkenes via photoredox and nickel cooperative catalysis was developed by Nevado et al. in 2019³⁵⁴ (Scheme 92c), in which secondary and tertiary alkyl silicates were used as alkyl radical precursors and various aromatic halides were employed as electrophilic reagents to realize the 1,2-alkylarylation of olefins under mild conditions. Based on the construction of anti-Markovnikov-type hydroalkylation through terminal alkynes and carboxylic acids, the same group reported a photoredox/nickel dual-catalyzed protocol to obtain trisubstituted olefin by using the inexpensive NiBr₂ as the nickel catalyst, dtbbpy as the ligand, and Cs₂CO₃ as the base under the irradiation of a 34 W blue LED for 12 h at room temperature. Based on the two-component decarboxylative cross-coupling reactions, Aggarwal's group³⁵⁵ achieved the first three-component metallaphotoredox-catalyzed decarboxylative conjunctive cross-coupling reaction between carboxylic acid, vinyl boronic esters, and aryl iodide in 2020 (Scheme 92d). In addition, Molander and coworkers developed an Ni/photoredox dual catalysis three-component coupling reaction of vinyl boronate with alkyl trifluoroborates and aryl bromides (Scheme 92e).³⁵⁶

In 2020, Martin et al. used two different electrophilic partners to achieve the 1,2-dicarbofunctionalization of vinyl boronates through the combination of photoredox and nickel catalysis, which opened a new strategic way in the field of cross electrophilic coupling (Scheme 92f).³⁵⁷ Unlike other alkyl reagents, this process was initiated through a reductive-quenching photocatalytic cycle, in which, as the sacrificial electron donor, TMEDA reacted with the excited state of 4CzIPN to generate the reduced photocatalyst (4CzIPN˙⁻), and the fluorescence quenching experiment also confirmed this conclusion. A transient carbon-centered radical was generated through single-electron transfer (SET) between 4CzIPN˙⁻ and tertiary alkyl bromides.

Primary alkyl radicals generated from traditional free radical precursors are not compatible in three-component coupling reactions due to the overwhelmingly competitive two-component cross-coupling. Thus, in 2020, Yuan's group disclosed that primary alkyl radicals derived from α-silyl amines were efficient reagents for the three-component conjunctive cross-coupling of alkenes through photoredox/nickel dual catalysis (Scheme 92g).³⁵⁸ Based on previous work, in 2024, the same research group firstly accomplished a three-component cascade process by adding Heck-type aryl electrophilic reagents to olefins as the starting step for triggering the cascade process to afford regio-reversed 1,2-arylalkylation products.³⁵⁹

9.3.2 1,2-Sulfonylarylation. Heteroatomic radicals exhibiting electrophilic properties are prone to undergo radical addition reactions with electron-rich olefins. In 2019, Nevado³⁵⁴ disclosed the visible light-driven dual photocatalytic oxidation–reduction/Ni-catalyzed 1,2-sulfonylarylation reaction of electron-deficient olefins with aryl halides and sodium sulfite salts (Scheme 93a). In the same year, Rueping³⁶⁰ et al. reported a straightforward and efficient three-component cross-coupling reaction for the construction of multi-substituted vinyl sulfoxide compounds (Scheme 93b). It is worth noting that the reaction could construct anti-addition or syn-addition alkenes stereoscopically by regulating photocatalysts with different triplet energies.

Scheme 93 1,2-Sulfonylarylation reactions.

9.3.3 1,2-Silicoarylation and 1,2-aminoarylation. Silicon-containing compounds are widely present in natural products and functional material molecules. However, despite the advances in the difunctionalization of alkenes, carbosilylation encounters problems such as harsh reaction conditions and poor functional group tolerance. In 2019, Hu³⁶¹ et al. attempted to use TMS₃SiH with a weak Si–H bond as the radical precursor to achieve a three-component 1,2-arylsilylation with electron-deficient terminal alkenes and aryl halides (Scheme 94a). In 2021, Studer³⁶² et al. used 2,2,2-trifluoroethoxy carbonyl-protected α-amino-oxy acids as N-radical precursors to achieve a three-component 1,2-aminoarylation reaction with electron-rich alkenes and aryl bromide (Scheme 94b).

Scheme 94 1,2-Silicoarylation and 1,2-aminoarylation reactions.

10. Copper metallaphotocatalysis

10.1 General overview

According to the nature of the excited copper species involved and whether an external photocatalyst is required during the reaction, the field of visible-light-induced copper catalysis can be divided into four different categories (Scheme 95), as follows: (1) interaction between photocatalysis and copper catalysis through SET process with the addition of a photocatalyst. (2) Copper(I) complexes act as photocatalysts after coordination with ligands, and then complete the catalytic cycle through single-electron transfer. (3) New Cu(I) complexes formed in situ by nucleophilic reagents or alkynes reacting with Cu(I) salts serve as photoexcited species to absorb visible light and reach the excited state. (4) The Cu(II)L_x complexes are excited by visible light to form [Cu(II)L_x]*, which then undergoes a ligand-to-metal charge transfer (LMCT) homolysis process to form Cu(I) complexes.

Scheme 95 Photoredox/copper catalysis.

10.2 Formation of new chemical bonds

With the rapid development of copper photocatalytic technology, numerous research groups have made outstanding contributions. The section is divided into the construction of C–C bonds,^363–378 C–N bonds,^379–385 C–O bonds^386–391 and C–S bonds.

In 2019, Chen and colleagues³⁶⁴ disclosed a method for achieving the enantioselective radical ring opening cyanation of oxime esters through dual photoredox and copper synergistic catalysis, through which a wide range of optically active alkyl dinitriles was produced in high yield and excellent enantioselectivity in the presence of box type ligand L₁ as a chiral ligand (Scheme 96a). The mechanism was envisioned to proceed through two cycles. The photocatalyst Ph-PTZ absorbs light energy to reach the excited state, followed by the single-electron reduction of oxime ester 1 to obtain iminyl radical 2 together with the formation of the carboxyl anion (RCO₂⁻). The carboxyl anion (RCO₂⁻) can promote the formation of the cyanide anion from TMSCN, which undergoes ligand change with the copper(I) salt to form L₁Cu(I)CN. Then, the oxidized photocatalyst oxidizes the L₁Cu(I)CN complex to the active substance L₁Cu(II)(CN)₂ via an SET process after trapping another molecule of cyanide anion from TMSCN. At the same time, cyanoalky radical 3 generated by selective β-C–C bond cleavage of imine radical 2 is captured by L₁Cu(II)(CN)₂, and chiral Cu(III) complex II is generated through intermediate I. Finally, product 4 is obtained through the reduction and elimination with regeneration of the L₁Cu(I)CN complex to complete the copper catalytic cycle. Inspired by Fu and MacMillian's work on asymmetric decarboxylation aromatization catalyzed by light and nickel, a variety of achiral carboxylic acids was successfully converted into enantiomeric-enriched alkyl nitriles compounds under irradiation from 12 W blue LEDs, with Ir(ppy)₃ and CuBr/L₁ in a 4 : 6 mixed DMF/p-xylene solvent system by Liu's team in 2017³⁶³ (Scheme 96b). Then, in 2019, Xiao et al.³⁶⁵ developed the first catalytic asymmetric propargylic radical cyanation of propargyl esters through an interwoven photocatalytic cycle and a copper catalytic cycle with high reaction efficiency and enantioselectivity (Scheme 96c). In 2020, Yu's group³⁶⁷ successfully achieved an enantioselective HLF-type remote C(sp³)–H cyanation reaction (Scheme 96d), and in 2022, a photoredox/Cu dual-catalyzed enantioselective remote cyanation through 1,4-heteroaryl migration was achieved by Zhu's team³⁷² (Scheme 96e). In addition, numerous groups carried out outstanding work through similar reaction mechanisms, which further enriches the types of reactions catalyzed by visible light and copper metal.^366,376,378

Scheme 96 Construction of C–C bonds with the participation of photocatalysts.

In 2022, Han's group³⁷⁴ implemented a cross-dehydrogenative coupling (CDC) strategy between aldehydes and electron-deficient olefins using Cu(dap)₂Cl as a copper catalyst under the irradiation of a 525 nm green light (Scheme 97a). Mechanistic studies showed that copper(I) complexes, upon coordination with ligands, can act as photocatalysts under the irradiation of green light, mediating the cleavage of O–O bonds in tert-butylperoxy 2-ethylhexyl carbonate (TBEC) to generate a tertbutoxy radical (tBuO˙) and releasing CO₂. The nucleophilic acyl radicals generated by the hydrogen atom transfer (HAT) process between tertbutoxy intermediates and aldehydes can undergo 1,4-addition with olefins to obtain electrophilic radicals, which undergo a halide transfer with Cu(II)–Cl intermediates to form β-chloroketones. The β-chloroketones undergo elimination in the presence of a base to form the cross-dehydrogenative coupling products. In addition, Xu's group³⁷⁰ achieved the visible light-induced Cu-catalyzed asymmetric C(sp³)–H alkylation reaction using glycine derivatives and NPhth esters as reaction substrates (Scheme 97b). It is worth mentioning that mechanistic studies have shown that there are two models of interaction between visible light and copper in this catalytic cycle, as follows: (1) the chiral copper intermediate formed in situ by the coordination of quinoline-8-glycinate with Cu(I)L* can function as photocatalysts upon exposure to visible light. (2) Monitoring the steady-state absorption of the reaction mixture revealed that LMCT requires the involvement of visible light for the photoreduction of Cu(III) to Cu(II).

Scheme 97 Construction of C–C bonds without photocatalysts.

In 2021, Shi's team³⁶⁹ revealed that alkyl thianthrenium salts can act as suitable alkylation reagents for Sonogashira-type coupling reactions with terminal alkynes in the presence of copper catalysts under irradiation with blue LEDs (Scheme 98a). Subsequently, Zhang's group³⁷³ demonstrated that the binding of the bisoxazoline diphenylamine ligand to 1,1′-bi-2-naphthol (BINOL) significantly increased the reduction potential of copper complexes. Based on this, asymmetric C(sp³)–H alkylation of 2-iodobenzamide derivatives with alkynes was achieved successfully under photo-irradiation through intramolecular 1,5-hydrogen atom transfer (HAT) (Scheme 98b). In the same year, Zhu and colleagues³⁷⁵ synthesized a series of heteroaryl-tethered chiral alkynes compounds with good regioselectivity and enantioselectivity using racemic NHPI esters and terminal alkyne as reaction substrates through the 1,4-heteroaryl migration strategy (Scheme 98c). The mechanism studies showed that in the presence of a base, the complex formed between the terminal alkyne and Cu(I) act as a photoactive substance that absorbs visible light to reach a photoexcited state, which can undergo single-electron transfer (SET) with the NHPI ester to generate alkyl radicals and Cu(II) complexes. Subsequent 1,4-heteroaryl migration of the alkyl radicals generates a more stable benzylic radical, which then combines with the Cu(II) complex to form the chiral alkyne product, while regenerating Cu(I) species to complete the copper catalytic cycle. Through a similar reaction mechanism, Wang et al.³⁷⁵ also proposed a photoinduced copper-catalyzed cross coupling reaction of aliphatic fluorides with terminal alkynes, obtaining valuable propargyl monofluorides under mild conditions (Scheme 98d).

Scheme 98 C–C bond construction involving complexes formed in situ by nucleophilic reagents or alkynes reacting with Cu(I) salts.

In addition to terminal alkynes, azoles can also be used as carbon nucleophiles to participate in C–C coupling reactions. For example, Zhang et al.³⁶⁸ demonstrated that a copper(I)/carbazole-based bisoxazoline (CbzBox) complex can serve as a photo and chiral catalyst to achieve the asymmetric alkylation of azoles under mild conditions (Scheme 98e).

To solve the problem of uncontrolled stereoselectivity and frequent side reactions in traditional asymmetric S_N2 reactions involving amides and alkyl halides, Fu's team³⁸² used a photoinduced, copper-catalyzed method to achieve the C–N coupling of amines with racemic tertiary alkyl chloride electrophilic reagents in 2022 (Scheme 99a). In 2023, Xie and coworkers disclosed a divergent radical C–N coupling of trifluoromethylated arenes and carbazoles, through which a range of α,α-difluoromethylamines was achieved through photoexcited copper catalysis. It is worth mentioning that different ligands can be screened to regulate the reaction products in the reaction (Scheme 99b). In 2023, Bolm's team³⁸⁴ used in situ-generated Cu(I)–NH sulfoximine complexes as photoactive species to achieve coupling reactions with alkyl diacyl peroxides under blue LED irradiation, demonstrating the important application of Cu(I) complexes in C–N coupling reactions (Scheme 99c). In 2023, Ritter's group³⁸⁵ firstly used arenes with different electronic structures as aryl donors to achieve the N-arylation of NH-sulfoximines via photoinduced sulfoximine-to-copper charge transfer. Mechanism studies have shown that this approach successfully extends the LMCT concept to other copper-philic substrates. In addition, the C–N coupling reaction catalyzed by photocatalysts and copper catalysts has also been flourishing in recent years.^379–381

Scheme 99 Construction of C–N bonds under copper photocatalysis.

In 2020, Nguyen used a combination of visible light and copper catalysis to effectively construct diastereoselective C(sp³)–O bonds through the cross-coupling reaction between glycosyl bromides and aliphatic alcohols (Scheme 100a). During the reaction process, aliphatic oxygen acted as a nucleophile to coordinate with the Cu(I) center, forming the corresponding copper(I)–oxygen complex, which was then excited by visible light to generate the photoexcited Cu(I)–alcohol intermediates and engage in electron transfer with α-glycosyl bromide to participate in subsequent cycles. This strategy highlighted the potential of copper-mediated photochemistry in complex molecular synthesis, enabling the stereoselective construction of α-1,2-cis glycosides under mild conditions. In 2023, Raha Roy et al.³⁹¹ use CuCl₂ as a copper catalyst and KBrO₃ as a terminal oxidant to realize the intramolecular dehydrogenative coupling of 2-aryl benzoic acid. As shown in Scheme 100b, the reaction underwent a photoinduced LMCT process of Cu(II) species. Firstly, 2-aryl benzoic acids complexed with CuCl₂ to form Cu(II)–carboxylate species, which acted as photoexcited species to absorb visible light and generate carboxyl free radicals through the ligand-to-metal charge transfer (LMCT) process, and Cu(II) was reduced to Cu(I) at the same time. Then the formed carboxyl radical underwent intramolecular nucleophilic addition to form a cyclic intermediate, which was converted to the desired benzocoumarin derivatives after subsequent oxidation with Cu(II), followed by deprotonation. Following the idea of visible light-induced Cu(II)–O bond homolysis, the Stahl group designed a Cu/2,2′-biquinoline catalyst to make it possible to restrict benzylic C–H esterification with limiting C–H substrate (Scheme 100c). The mechanism studies showed that blue-light irradiation promotes ligand-to-metal charge transfer (LMCT) in the (2,2′-biq)Cu(II)-OBz complex, reducing Cu(II) to Cu(I), which activates the peroxide to generate an alkoxyl radical hydrogen-atom-transfer species. In addition, Collins’ team³⁹⁰ pioneered the use of the dynamic kinetic resolution (DKR) method to achieve the dynamic kinetic decomposition of secondary alcohols through tandem photo- and biocatalysis (Scheme 100d).

Scheme 100 Construction of C–O bond under copper photocatalysis.

In 2022, a novel dual metal/photocatalysis system incorporating two diphosphine ligands with different properties and a single copper catalyst was presented by Carretero team for the anti-borohydride of alkynes³⁹² (Scheme 101a). Very recently, Noble et al. demonstrated that Cu(II)Cl₂ can be excited by visible light to form [Cu(II)Cl₂]*, which then generates chlorine radicals through the LMCT process to promote the C(sp³)–H boronization of non-activated alkanes (Scheme 101b). Based on the important biological significance of organosulfones, Yang et al.³⁹³ achieved the sulfonylation of aryl halides and sulfinates using a single ligand Cu(I) formed in situ as a photocatalyst under visible light assistance (Scheme 101c). To broaden the reaction types of C–X bonds, in 2023, Duan's group³⁹⁴ reported a simple strategy for the photoinduced copper-catalyzed synthesis of phosphines using N-(acyloxy)phthalimides as the alkyl radical precursor and secondary phosphine borane as the coupling partners. In addition, this group demonstrated that N-fluorocarboxamides are also suitable radical precursors for the construction of C(sp³)–P bonds (Scheme 101d).

Scheme 101 Construction of C–B, C–S and C–P bonds under copper photocatalysis.

10.3 Formation of two new chemical bonds

Based on the two-component reaction, numerous groups have attempted to add styrenes,^395–402 terminal alkynes, 1,3-dienes, or 1,3-enynes to the reaction system to achieve a three-stage cascade reaction by merging photoredox catalysis with copper catalysis. Normally, this type of reaction requires a combination of photo- and copper-catalyzed cycling, where the radicals induced by photoredox first add to the styrenes (terminal alkynes, 1,3-dienes, or 1,3-enynes) to generate new radical intermediates, which further couple with nucleophilic complexes to obtain difunctionalization products. For example, in 2018, Pan et al.³⁹⁵ achieved the asymmetric cyanoalkylation of olefins by using primary, secondary, and tertiary alkyl-substituted NHP esters as alkylation reagents (Scheme 102a). Chen's group³⁹⁶ demonstrated that oxime esters can undergo C–C bond β-cleavage to form acyl and cyanoalkyl radicals through photoredox and copper catalysis, achieving highly enantioselective three-component olefin dicarbofunctionalization reactions (Scheme 102b). Similarly, He's group³⁹⁷ achieved coupling reactions of aryl alkenes, aroyl chlorides (or aromatic and aliphatic sulfonyl chlorides), and TMSCN under mild conditions. In 2023, Xu's team⁴⁰³ achieved the 1,2-difunctionalization reaction of terminal alkynes through the copper metallaphotocatalysis strategy.

Scheme 102 Dual catalytic reactions involving olefins.

In addition, by utilizing the unique chemical properties of copper, Wu's group achieved the visible-light-induced, copper-catalyzed three-component difluoroalkyl thiocyanidation of olefins using fluorinated halogenated hydrocarbons as radical precursor reagents without additional photocatalysts (Scheme 102c).

Due to their unique structural characteristics and properties, allenes are important in a wide range of natural products and pharmaceuticals. In recent years, the 1,4-difunctionalization of 1,3-enynes by merging photoredox catalysis and copper catalysis has been successfully developed, which has become a powerful tool for the construction of multi-substituted allenes under mild reaction conditions. In 2021, Lu's group⁴⁰⁴ disclosed the first example of the radical 1,4-carbocyanation of 1,3-enynes under copper/photoredox dual catalysis (Scheme 103a). Deng et al.⁴⁰⁵ developed a visible-light-induced copper photoredox dual catalyst for the 1,4-perfluoroalkylcyanation of 1,3-enynes, achieving the efficient synthesis of structurally diverse perfluoroalkylated allenes through a radical relay involving the resonance of the propargyl radical with the allenyl radical (Scheme 103b). In addition, Wang et al.⁴⁰⁶ also synthesized multiple functionalized allenes with broad functional group tolerance using cyclic butanol and pentanol derivatives as radical precursors (Scheme 103c).

Scheme 103 Dual catalytic reactions involving 1,3-enynes.

1,3-Dienes can be extensively difunctionalized in the alkene moieties through the 1,2-, 1,4-, or 3,4-selective pathway to rapidly construct various functionalized compounds. In 2022, Chen's group⁴⁰⁷ used commercially available alkyl bromide reagents as radical precursors and carboxylic acids as oxygen-based nucleophiles to achieve the selective 1,4-difluoroalkylesterification of 1-aryl-1,3-dienes via photoredox and copper catalysis, providing a new and convenient pathway for the synthesis of difluoroalkylated allylic esters (Scheme 104a). In addition, this research group successfully extended the reaction system to an intramolecular two-component system, using 1-(1,3-butadienyl) benzoic acid and methyl bromodifluoroacetate as substrates to achieve the efficient synthesis of 3-substituted benzobutyrolactones. In the same year, this group achieved the 1,2-aminooxygenation reaction of 1,3-dienes using N-aminopyridine salts and nucleophilic alcohols (Scheme 104b).⁴⁰⁸ In addition, the free radical precursor used in this strategy was also extended to N-Boc amidopyridinium salts by Zhu et al., which further enhanced the synthetic utility of dual photoredox and copper-catalyzed three-component reactions in organic synthesis (Scheme 104c).⁴⁰⁹

Scheme 104 Dual catalytic reactions involving 1,3-dienes.

11. Conclusion and perspective

In summary, photoinduced 3d-metal catalysis has been well developed over the past few decades. Especially in recent years, this elegant method has attracted significant attention, and initially applied in organic synthesis, the pharmaceutical industry and environment, biological analysis and other fields. Based on the interaction modes of photocatalysis and 3d-metal catalysis, herein, we summarized the photoinduced 3d-metal catalytic reactions of Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu. There are four main modes of photo-metallic dual catalytic strategies, as follows: (1) single-electron transfer occurs between the metal catalyst and the photocatalysis; (2) the excited metal catalyst undergoes an LMCT or MLCT process; (3) the 3d-metal complex acts as a photosensitizer; and (4) the metal catalysis does not directly interact with the photocatalysis. Using clean light energy, 3d-metal catalysis can realize a wide variety of chemical reactions to build carbon–carbon bonds, carbon–heteroatomic bonds, or even multiple chemical bonds under mild conditions.

The amount of catalyst used in the photoinduced 3d-metal catalytic system is still relatively high. In some cases, the amount of the expensive Ru and Ir photosensitizer is 3 mol%, while the amount of 3d-metal catalyst can be as high as 10 mol%. In addition, although the related reactions can be applied to practical applications, there are still problems such as relatively low catalytic efficiency and difficulty in meeting the requirements of stable light source. Therefore, in recent years, scientists have begun to adopt flow reaction devices to shorten the reaction time and improve the reaction efficiency, making it more suitable for industrial preparative synthesis.

Various modes of interaction and various bonding methods determine that photoinduced 3d-metal catalysis still has great potential for development (Table 1). In the future, different interaction modes of transition metal catalysis and photocatalysis are worth exploring. Compared with metal catalysis or photocatalysis, dual catalysis systems have more reaction modes to achieve more transformations. Then, the asymmetric reactions catalyzed by photo/transition metals dual catalysis are also worth attention, and more chiral ligands and chiral systems should be developed. Meanwhile, the mechanism of the interaction between photocatalysis and transition metal catalysis deserves further investigation and can provide inspiration for the development of more novel reactions. In addition, replacing the expensive Ru and Ir photosensitizers with earth-abundant metal complexes is also a future development. It is believed that photoinduced 3d-metal catalysis will be continuously developed to realize more interesting and useful chemical transformations in the next decade.

Table 1 Reaction types of photoinduced 3d-metal catalysis

Entry	3d-metal	C–C bond formation	C–X bond formation	Multiple bonds formation	Other reaction types
1	Sc			Radical addition, etc.
2	Ti	Homocoupling, etc.		Radical cyclization, hydrofunctionalization, difunctionalization, etc.	Epoxide opening, etc.
3	V			PCET reaction, etc.	C–C bond cleavage, etc.
4	Cr	Cyanation of amines, etc.	Bromination, deborylative hydroxylation, etc. (X = Br, OH etc.)	Allylation of aldehydes, hydrofunctionalization, difunctionalization, decarboxylative coupling, etc.
5	Mn	Minisci reaction, cross-coupling, etc.	Cross-coupling, dehydrogenative coupling, C–H functionalization, decarboxylative azidation, etc. (X = P, B, S, Se, N, N3, etc.)	Radical coupling, radical addition, difunctionalization, cycloaddition, etc.	Reductive coupling, etc.
6	Fe	Minisci reaction, cross-coupling, C–H alkylation, etc.	C–H functionalization, decarboxylative functionalization, C–X cross-coupling, etc. (X = B, N, O, Cl etc.)	Hydrofunctionalization, difunctionalization, etc.	C–C bond cleavage, reduction of CO2, N–X bond formation (X = S, P), etc.
7	Co	Heck-type reaction, cross-coupling, etc.	Heck-type reaction, C–X cross-coupling, etc. (X = P, S, O, N, etc.)	Giese-type reaction, hydrofunctionalization, dehydrogenation, difunctionalization, cycloaddition, etc.	Isomerization of alkenes, substitution reaction, etc.
8	Ni	Cross-coupling, C–H functionalization, intermolecular cascade reaction etc.	C–X cross-coupling, etc. (X = B, N, O, S, P etc.), decarboxylative functionalization	Difunctionalization, etc.
9	Cu	Cross-coupling, Cu(II)–Cl bond homolysis	C–X cross-coupling, etc. (X = N, S, O etc.), C–H bond functionalization	Difunctionalization of olefins

---

Data availability

All data included in this study are available from the corresponding author upon request.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We gratefully acknowledge the financial supports from the Zhejiang Provincial Natural Science Foundation of China (LDQ24B020001), NSFC (22271249), National Key R&D Program of China (2021YFA1500200 and 2021YFF0701600), and the Fundamental Research Funds for the Central Universities (226-2022-00224 and 226-2024-00003).

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