Visible Light-Induced Transition Metal Catalysis

2.1. General Overview

The ability of organocobalt(III) complexes to undergo facile homolytic Co–C (bond dissociation energy (BDE) ≈ 14–42 kcal/mol) bond cleavage, via thermolysis, photolysis, or electrochemical reduction, to produce carbon-centered radicals and persistent Co(II) radicals is well-known.^17,18 These complexes are traditionally obtained from a Co(I) complex and an organic electrophile, and have been used as stoichiometric organic radical precursors. To achieve catalysis, the cobalt intermediate generated after photolysis needs to be reduced to its Co(I) form. This nucleophilic species can then react with the organic electrophile to regenerate organocobalt(III) species. In 1983, Scheffold group disclosed a cobalt-catalyzed reductive acylation of Michael acceptors with acid anhydrides, thus representing the first example of cobalt photocatalysis.¹⁹ In this case, the catalyst turnover was enabled by electrochemical reduction. Several years later, Branchaud and co-worker revealed that elemental zinc could serve as a stoichiometric reductant for the Heck-type reaction of styrene with alkyl bromides.²⁰ However, the first practical, mild, and general catalytic method was developed by Carreira group in 2011. They utilized DIPEA as a base to formally reduce Co(III)H to an anionic Co(I) species via deprotonation, thereby obviating the use of electrolysis or metal reductants for similar reactions.²¹ In addition to acid anhydrides and alkyl halides, other electrophiles have also been successfully employed to produce the corresponding organocobalt(III) intermediate (Figure 2a). Thus, in 2016, Gryko group introduced α-diazo esters,²² followed by the report on epoxides and aziridines from the group of Morandi in the same year.²³ Later, Gryko and co-workers employed 2-S-pyridyl thioesters as a source of acyl radical.²⁴ The same group also showed that electron-deficient bicyclo[1.1.0]butanes (BCBs) could lead to the respective alkyl Co(III) intermediate upon strain-release ring opening.²⁵ Very recently, Komeyama group reported the use of alkyl tosylates as an alternative to alkyl halides.²⁶

The photochemistry of cobalt is not limited to the homolysis of Co–C bond. In 2018, Rovis group described an interesting example of photocycloaddition of diynes and terminal alkynes.²⁷ This transformation involves the photoexcitation of a diyne-ligated Co(II) acetylide complex to its ligand-to-metal charge-transfer (LMCT) state, thereby formally generating a Co(I) center that is capable of oxidative cyclization with the diyne ligand (Figure 2b). Furthermore, the viability of photoinduced SET of cobalt complexes was also disclosed by Wu’s group in 2019.²⁸ In this case, excited Co(II) or Co(III) complex oxidizes H-phosphine oxide to its radical cation, which upon deprotonation produces phosphinoyl radical that subsequently adds to an alkene or alkyne (Figure 2c).

A vast majority of cobalt photocatalysis employs vitamin B₁₂ derivatives and their close analogues such as Co(II) porphyrins and cobaloximes. The structures of these complexes are depicted in Figure 2d.

2.2. Photolysis of Co(III)–Carbon Bond

2.3. Transformations Involving LMCT of Co(II) Acetylides

In 2018, Rovis’ group disclosed a cycloaddition reaction between diynes 15.1 and terminal alkynes 15.2 leading to fused arenes 15.3.²⁷ Diynes with the tether possessing a quaternary carbon reacted smoothly with phenylacetylene to deliver the respective arenes in good to excellent yields (15.4–15.6). Similarly, diynes bearing heteroatoms were suitable substrates (15.7–15.8). This reaction was sensitive to electronic nature of the alkyne partner. Thus, electron-withdrawing groups promoted the reaction rate as well as the yield, while electron-donating groups were detrimental (15.9–15.12). Aliphatic alkynes were less reactive under standard conditions (15.13) (Scheme 15). However, switching the light source to high-energy UV significantly improved the yield. The proposed catalytic cycle begins with the in situ formation of Co(II) acetylide A. The coordination of diyne is crucial, as it leads to a dramatic bathochromic shift of about 100 nm, rendering A photoactive at the employed wavelength. Photoexcited A* features LMCT character. The formally reduced cobalt center is therefore capable of oxidative cyclization with ligated diyne affording Co(III) intermediate B. The aryl radical cation in B is then reduced by DIPEA to produce metallacycle C, which upon migratory insertion with alkyne followed by reductive elimination, furnishes arene 15.3 and Co(I) acetylide D. The latter is then oxidized by DIPEA radical cation or A* to complete the catalytic cycle. The involvement of LMCT state was supported by fluorimetric analysis of reaction mixtures with different alkynes, which revealed a characteristic inverse correlation between emission energy and alkyne electron-richness.⁴² It is noteworthy that cobalt-catalyzed/-mediated [2 + 2 + 2] cycloaddition has long been recognized as a powerful method for construction of benzene or pyridine derivatives; and visible light irradiation has been incorporated occasionally, as exemplified by the early contributions from the groups of Vollhardt^43,44 and Oehme.^45,46 In those cases, however, the role of visible light irradiation is likely to induce ligand dissociation, thereby accessing the active Co(I) catalyst with free coordination sites.

2.4. Fragmentation of P–H Bond of H-Phosphine Oxides

In 2019, Wu’s group employed cobaloxime catalysts for photoinduced P–H bond activation of H-phosphine oxides 16.1.²⁸ With Co(II) catalyst Co8, H-phosphine oxides were coupled with alkenes 16.2 to produce alkenylphosphine oxides 16.3, along with the reduction side products 16.4. Electronically diverse 1,1-diarylalkenes delivered products in good to excellent yields (16.5–16.8). While reactions with styrene derivatives were less efficient, side product formation was substantially suppressed (16.9–16.12). Besides, the trans-isomer was exclusively obtained in all cases. Other diarylphosphine oxides were also competent substrates (16.13–16.14); however, diethylphosphite was not reactive (16.15). This transformation begins with the photoexcitation of Co8 catalyst, which in its excited state is highly oxidizing and thus capable of engaging H-phosphine oxide 16.1 in SET, thus producing phosphinoyl radical A and a Co(I) species. Potentially, the formation of A and Co(III)H species may occur via a direct HAT between 16.1 and Co(II)*. Intermediate A then reacts with alkene 16.2 to produce benzylic radical B, which is then captured by another Co(II) complex to form benzyl Co(III) intermediate C. Finally, β-H elimination of C affords the desired product 16.3. The Co(III)H species, formed along with the product, selectively reduces excess alkene to 16.16 via HAT. The subsequently formed Co(III) complex may undergo radical-polar crossover (RPC) with radical intermediate B to return to its Co(II) form, meanwhile converting B to a carbocation, and eventually product 16.3 upon deprotonation. As Co(III)H does not reduce product, side product 16.4 may stem from protiodemetalation of Co(III) complex C. The intermediacy of phosphinoyl radical was confirmed by EPR studies as well as radical trapping experiment with 2-methyl-2-nitrosopropane (Scheme 16).

In the same report, the authors also explored the reactivity of H-phosphine oxides toward alkynes 16.16. With a different catalytic system, terminal alkynes also provided alkenylphosphine oxides in moderate to good yields, albeit with diminished stereoselectivities in some cases (16.18–16.22). The photoexcited Co(III) cobaloxime (Co7) is an even more potent oxidant. Thus, the catalytic cycle commences with an SET event leading to phosphinoyl radical A, which undergoes addition to terminal alkyne substrate to form vinyl radical D. A subsequent recombination of Co(II) and D generates vinyl Co(III) species E, which upon protiodemetalation furnishes product and regenerates Co(III) catalyst. Reactions with internal alkynes at elevated temperature, on the other hand, led to benzophosphole scaffolds 16.23. Both aryl- and alkyl-substituted alkynes were reactive in this transformation (16.24–16.25). Notably, product 16.26 was obtained as a single regioisomer. Expectedly, H-phosphine oxides bearing para-substituted aryl groups provided a 1:1 mixture of regioisomers (16.27). Similarly to the previous case, phosphinoyl radical A adds to internal alkyne to produce the respective vinyl radical, which in this case undergoes cyclization to afford cyclic intermediate F. Rearomatization of F upon HAT to Co(II) furnishes benzophosphole 16.23.

Later, the same group reported a similar phosphorylation of enamines and enamides 17.1.⁴⁷ A wide range of functionalities at the N-aryl ring of β-cyano enamines are well tolerated (17.4–17.7). Unprotected enamine was also a competent substrate (17.8). Enamines, possessing an ester at the β-position reacted well, but provided the respective products with moderate degrees of diastereoselectivity (17.9–17.11). Enamides were also found to be reactive, providing the desired products in moderate to good yields (17.12–17.14). In addition, examining this phosphorylation method in a more complex setting proven efficient, as well (17.15) (Scheme 17).

3.1. General Overview

Despite enormous popularity of copper catalysis in organic chemistry, the reactivity of copper catalysts under visible light irradiation^48,49 remained underexplored until 2012, when Fu and Peters groups first disclosed a photoinduced Ullmann C–N coupling reaction via an unprecedented radical pathway.⁵⁰ In the same year, Hwang’s group showed that Sonogashira coupling,^51,52 which typically requires a copper/palladium dual catalytic system, under visible light irradiation could be achieved using a sole copper catalyst.⁵³ In both cases, a Cu(I)–nucleophile complex (amide and acetylide, respectively) is involved, which upon photoexcitation serves as a single electron reductant and engages the aryl halide coupling partner in SET events. In 2015, the groups of Dolbier⁵⁴ and Reiser⁵⁵ independently reported excited-state reactivity of cationic Cu(I) complexes, such as Cu(dap)₂Cl, toward fluoroalkyl sulfonyl chlorides. This type of Cu(I) catalyst is commonly referred as a standalone catalyst, as the formation of a Cu(I)–nucleophile complex is not a prerequisite for the photoinduced transformation. These early works laid the foundation for Cu(I) photocatalysis, where a photoexcited Cu(I) species transfers a single electron to organic electrophiles in an inner sphere fashion, thereby engaging them in radical transformations (Figure 3a).

The photolysis of CuCl₂ salt to produce CuCl and chlorine radical upon irradiation has been recognized for almost 60 years, as described by Kochi in his pioneering work in 1962.⁵⁶ However, the stage for catalysis by visible light-excited Cu(II) complexes⁵⁷ was set only in 2018 by Rehbein, Reiser, and co-workers.⁵⁸ It was shown that the mechanism of photocatalysis by Cu(II) complexes is different to that by Cu(I) counterparts. Thus, the in situ formed Cu(II) azide complex undergoes photoinduced homolytic ligand dissociation, commonly referred as visible light-induced homolysis (VLIH), to release an azide radical. Likewise, alkyl⁵⁹ and chlorine radicals^60,61 can also be accessed from the respective Cu(II) complexes. This represents the general mechanistic scenario for Cu(II) photocatalysis (Figure 3b). Very recently, the detailed spectroscopic studies carried out by Reiser and Castellano groups have provided direct evidence for a homolytic Cu–Cl bond cleavage.⁶²

There is also an example of EnT in copper catalysis. Thus, Liu’s group demonstrated the ability of Cu(I) complexes to activate substrate via EnT, constituting another reactivity mode of excited-state Cu(I) complexes (Figure 3c).⁶³ In this case, a photoexcited Cu(NCS)₂⁻ complex undergoes EnT to vinyl azide substrate, which upon liberation of N₂ forms a reactive 2H-azirine intermediate.

It is worth mentioning that the photochemistry of copper is an immense field of research. Thus, to keep the focus on catalytic properties of visible light-excited copper complexes, the reactions enabled by UV irradiation,^64–72 as well as visible light-induced transformations utilizing copper either as a photoredox catalyst^{73–80,48,81–84} or in stoichiometric fashion,⁸⁵ are not discussed herein.

3.2. Photoinduced SET of Cu(I)–Nucleophile Complexes

3.3. Photoinduced SET of Standalone Cu(I) Complexes

3.4. Photoinduced Homolytic Ligand Dissociation of Cu(II) Complexes

The LMCT character of photoexcited Cu(II) complexes and its subsequent homolytic ligand dissociation have long been recognized.^{56,146–149,85} However, synthetic transformations exploiting this excited-state reactivity had not been reported until 2018, when the groups of Rehbein and Reiser disclosed an oxoazidation reaction of vinyl arenes 56.1 with trimethylsilyl azide and molecular oxygen.⁵⁸ In this transformation, electronically diverse styrene derivatives underwent smooth reaction to deliver the respective ketoazides in good yields (56.3–56.7). Markedly, benzyl chloride functionality did not undergo substitution under the reaction conditions, highlighting notable chemoselectivity of this transformation (56.8). In addition, vinyl heteroarenes (56.9), as well as β-methylstyrene (56.10), were found to be competent substrates. However, no desired product was obtained when unactivated alkenes were employed (56.11). This transformation begins with an off-cycle photoinduced oxidation of Cu(I) catalyst to Cu(II) complex A, which upon ligand exchange with TMSN₃ forms photoactive, dimeric species B. A subsequent photoexcitation leads to the formation of azide radical and Cu(I) complex C. The former then undergoes radical addition to alkene substrate to afford stabilized radical D, which further reacts with molecular oxygen to produce peroxy radical E. Recombination of C and E, followed by release of product 56.2 regenerates complex A and closes the catalytic cycle (Scheme 56).

In the same year, Gong’s group reported an asymmetric alkylation of imine derivatives 57.1 using alkyl trifluoroborates 57.2.⁵⁹ Ligand 57.4 was used in the case of N-sulfonylimines. A wide range of electron-neutral and -deficient benzyl trifluoroborates delivered the respective sulfonamides in excellent yields and enantioselectivity (57.6–57.9). The electron-rich counterparts were equally reactive, however provided products with inferior enantiocontrol (57.10). Similarly, product 57.11 was obtained in almost quantitative yield, but with lower enantioselectivity. Secondary (57.12) and tertiary trifluoroborate (57.13) were also efficient, albeit giving rise to products with diminished stereoselectivity. Substrates possessing other α-carbalkoxy groups were viable reactants, as well (57.14–57.15). However, replacement of α-carbalkoxy groups with methyl group shut down the reaction (57.16). The benzene ring of substrate could also be modified without compromising the reaction efficiency (57.17–57.18). The authors also explored the scope of ketimines in the presence of ligand 57.5. Notably, excellent enantiocontrol was maintained upon varying the substituent at the benzene ring (57.19–57.22). N-Methyl substrate also furnished the corresponding oxindole 57.23 in good yield with excellent enantiocontrol. In contrast to the case using electron-rich amines (cf. Scheme 55), an SET event, either photoinduced or not, is unlikely in this transformation as alkyl trifluoroborates are not capable of reducing Cu(II). Thus, the authors proposed Cu(II) catalyst A and trifluoroborates first undergo transmetalation to produce alkyl Cu(II) complex B. A subsequent visible light excitation leads to the homolytic cleavage of Cu–C bond, thereby producing alkyl radical C and Cu(I) species D. As in the previous case, the former then enters the asymmetric catalytic cycle, while the latter is responsible for its completion (Scheme 57).

In 2020, Wan’s group reported catalytic dichlorination of unactivated alkenes 58.1 with HCl.⁶⁰ Monosubstituted aliphatic alkenes possessing certain functional groups were efficiently dichlorinated in good yields (58.3–58.5). Allylic systems were also applicable, furnishing the corresponding products in moderate yields (58.6–58.7). Furthermore, a wide range of functional groups at aryl substituent, including an iodide, were compatible under these reaction conditions (58.8–58.12). In addition, 1,1-disubstituted alkenes underwent efficient transformation to provide dichlorination products in excellent yields (58.13). The proposed catalytic cycle begins with the photoexcitation of CuCl₂ to its LMCT state, which subsequently undergoes Cu–Cl bond homolysis to produce chlorine radical and CuCl. The former adds to alkene substrate to generate alkyl radical A, which upon reaction with CuCl₂ forges the second C–Cl bond in product 58.2. Meanwhile, CuCl is oxidized back to CuCl₂ in the presence of air and HCl to complete the catalytic cycle. TEMPO trapping experiment indicated the formation of TEMPO–Cl adduct, thus supporting the involvement of chlorine radical. Another piece of evidence comes from an interesting example of HAT from alkene 58.14 to chlorine radical, thereby furnishing trichlorinated product 58.15 (Scheme 58). Of note, the authors also explored dichlorination of styrene derivatives. In those cases, however, a super-stoichiometric amount of CuCl₂ was required.

More recently, Rovis’ group also utilized CuCl₂ as chlorine radical source for HAT in aliphatic C–H alkylation of alkenes 59.1.⁶¹ For activated systems, both primary and secondary C–H alkylations were efficient (59.4–59.6). Cycloalkanes underwent efficient alkylation in excellent yields (59.7). Linear aliphatics were also reactive in this transformation (59.7). However, a mixture of regioisomers was observed in all cases. Notably, an example of selective tertiary alkylation was demonstrated (59.8). In addition, various Michael acceptors with different degrees of substitution proved to be competent substrates for this transformation (59.9–59.15). Furthermore, azodicarboxylate was also found to be an appropriate radical acceptor (59.16) (Scheme 59).

3.5. Photoinduced EnT of Cu(I) Complexes

In 2017, Liu’s group reported a denitrogenative coupling of vinyl azides 60.1 with ammonium thiocyanide involving EnT from the in situ formed Cu(NCS)₂⁻, representing another excited-state reactivity of copper complexes.^63,150 A broad range of (hetero)aryl vinyl azides reacted smoothly under these reaction conditions, affording the respective 2-aminothiazoles in good to excellent yields (60.3–60.9). Likewise, the aliphatic counterparts, bearing diverse functionalities, were found to be competent reaction partners (60.10–60.14). The authors proposed an EnT mechanism for this transformation, which begins with the in situ formation of visible light-absorbing Cu(NCS)₂⁻ complex. Which upon photoexcitation transfers energy to vinyl azide 60.1, leading to the formation of highly strained 2H-azirine A, presumably via the intermediacy of the respective vinyl nitrene.¹⁵¹ Coordination of A to a copper center then forms intermediate B. A subsequent nucleophilic ring opening constructs the C–S bond in C. Finally, intramolecular nucleophilic attack, followed by protonation, produces the cyclized product 60.2. Ir(ppy)₃ photocatalyst, which can also produce 2H-azirines from vinyl azides via EnT,¹⁵² was found to be less effective in this transformation, indicating that the conversion from 2H-azirine to final product may be mediated by copper. Indeed, the reaction of 2H-azirine 60.16 with ammonium thiocyanate was tremendously accelerated by catalytic amount of copper catalyst, thus supporting the second role of copper as a Lewis acid catalyst (Scheme 60).

4.1. General Overview

Iron complexes are well-known for undergoing rapid excited state relaxation via the low-lying metal-centered states.^153–155 This innate nature of iron limits its photochemical reactivities and thus renders it a less popular photosensitizer. Nonetheless, it has been recently shown that certain Fe(III) complexes possess excited-state lifetime up to 2 ns and, thus, are capable of participating in intermolecular quenching events.¹⁵⁶ Therefore, the utilization of Fe-based photoredox catalysts has been reported, and these catalysts were recently reviewed.¹⁵⁷ In the context of visible light-induced iron catalysis, the first example came from Jin’s group in 2019, when they described a decarboxylative alkylation of heteroarenes with alkyl carboxylic acids.¹⁵⁸ This transformation involves the in situ formed Fe(III) carboxylate that upon blue LED irradiation undergoes LMCT to produce carboxyl radical and an Fe(II) intermediate (Figure 4a).

In 2020, Xia, Yang, and co-workers reported an aminoselenation of alkenes with amines and diselenides enabled by another excited-state reactivity of Fe(III) complexes.¹⁵⁹ In this case, amine-coordinated Fe(III) complex undergoes photoinduced SET with diselenide to afford the respective Fe(II) intermediate, selenium radical, and selenium cation (Figure 4b).

Meanwhile, the groups of Alcázar and Noël observed an accelerating effect of visible light on iron-catalyzed Kumada coupling.¹⁶⁰ Despite remaining elusive, the origin of acceleration is likely related to the enhanced oxidative addition of aryl chloride to Fe(I) catalyst.

4.2. Transformations Involving LMCT of Fe(III) Carboxylates

In 2019, Jin and co-workers disclosed a decarboxylative Miniscitype alkylation of heteroarenes 61.1 using carboxylic acids 61.2 as alkyl source.¹⁵⁸ Both simple aliphatic and functionalized primary carboxylic acids were viable substrates for this reaction, providing the respective products with moderate to good efficiency (61.4–61.9). Likewise, secondary and tertiary alkylation products were obtained in good to excellent yields (61.11–61.14). A remarkably broad scope of N-heterocycles, such as isoquinolines (61.15–61.16), pyridines (61.17), diazines (61.18–61.21), benzimidazoles (61.22), purines (61.23), and benzothiazoles (61.24), was shown, thus demonstrating the generality of this protocol. Of note, in cases where identical C–H sites are available, a mixture of mono- and bis-alkylated products was obtained (61.25). Besides, benzothiophenes were also found to be competent substrates (61.26). The postulated reaction mechanism begins with the in situ formation of Fe(III) carboxylate complex A. A subsequent photoinduced LMCT of A leads to the formation of carboxyl radical B and a Fe(II) intermediate. The former then extrudes CO₂ to produce nucleophilic alkyl radical C, which undergoes radical addition to protonated heteroarene 61.1 to form radical cation D. Finally, a sequence of deprotonation and oxidation by Fe(III) species affords the alkylation product 61.1. Meanwhile, the Fe(II) intermediate generated in both cycles is oxidized by the terminal oxidant (NaClO₃ or NaBrO₃) to regenerate the Fe(III) catalyst. The radical nature of this transformation was supported by TEMPO trapping experiment (Scheme 61).

Later in the same year, Jin’s group extended this chemistry by reporting a redox-neutral hydroalkylation of Michael acceptors 62.1.¹⁶¹ In this case, addition of the initially formed nucleophilic alkyl radical to 62.1 leads to electrophilic alkyl radical, which is capable of oxidizing Fe(II) intermediate back to its Fe(III) state via RPC pathway, thereby completing the catalytic cycle without the use of an external oxidant. Various aryl- (62.5–62.8) and alkyl-substituted (62.9) malononitriles delivered the corresponding products in good to excellent yields. An example of malonate derivative was also demonstrated (62.10). A broad range of carboxylic acids was found to be competent reaction partners, affording primary (62.11–62.14), secondary (62.15–62.16), and tertiary (62.17–62.18) alkylation products in moderate to excellent yields. Likewise, azodicarboxylates 62.19 proved to be competent radical acceptors, furnishing the respective products with good to excellent efficiencies under similar reaction conditions (62.21–62.25) (Scheme 62).

More recently, Lei and Jin groups reported an intramolecular C–H acyloxylation of 2-arylbenzoic acids 63.1 to access lactones 63.2.¹⁶² It is speculated that in this case, formation of carboxyl radical from Fe(III) carboxylate is followed by a rapid intramolecular cyclization instead of decarboxylation. Substrates bearing diverse functional groups at the aryl substituent underwent smooth transformation to provide benzolactones in good to excellent yields (63.6–63.10). Likewise, variations could be made on the other phenyl ring without loss of efficiency (63.11–63.16). 2-Naphthoic acid derivative was also reactive, albeit with diminished efficiency (63.17) (Scheme 63).

4.3. Difunctionalization of Alkenes

Recently, Yang and Xia’s groups reported aminoselenation of alkenes 64.1 with diselenides 64.2 and amines 64.3.¹⁵⁹ Electronically diverse styrenes reacted efficiently to provide the difunctionalized products in moderate to good yields (64.5–64.9). However, 1,1-diphenylethylene did not provide any product, presumably due to increased steric hindrance (64.10). β-Methylstyrene was also a competent substrate, furnishing product 64.11 with excellent diastereoselectivity. Unactivated alkenes could also be employed, albeit with inferior efficiency (64.12–64.13). Remarkably, a broad range of aniline derivatives was found to be suitable reaction partners for this transformation (64.14–64.19). In general, electron-deficient anilines provided better yields than their electron-rich counterparts. In addition to anilines, azoles also furnished the respective products in good yields (64.20–64.21). In contrast, aliphatic amines were found to be unreactive (64.22). Several other diselenides also provided the desired products but were less effective than diphenyldiselenide (64.23–64.24). Finally, analogous reaction with disulfide was unsuccessful (64.25). The proposed mechanism commences with the in situ formation of Fe(III)–amine complex A. Photoexcited A* undergoes SET with diselenide 64.2 to generate selenium radical C and selenide cation D, which form alkyl radical E and carbocation F, upon addition to alkene 64.1, respectively. Under the oxidizing conditions, both radicals C and E can be oxidized to the corresponding cations. Subsequently, carbocation F either reacts with Fe(II)–amine complex B (path a) or with amine 64.3 (path b) to deliver product 64.3. However, given that electron-poor anilines are more efficient, path b is considered to be less likely. Finally, Fe(III) catalyst is regenerated upon oxidation by the external oxidant. A linear Stern–Volmer relationship confirmed the role of diselenide as a quencher. In addition, the formation of both E and F was supported by trapping experiments with TEMPO (64.27) and H₂O (64.28) (Scheme 64).

4.4. Cross-Coupling Reaction

Iron-catalyzed Kumada coupling is typically limited to electron-deficient aryl halides and primary aliphatic Grignard reagents.^163,164 To employ electron-neutral and -rich aryl chlorides, pressing conditions, such as elevated temperature, as well as prolonged reaction time, are required.^165,166 In 2019, the groups of Alcázar and Noël showed that visible light irradiation enables the use of diverse aryl chlorides 65.1 for Kumada coupling in flow under mild conditions.¹⁶⁰ Thus, differently substituted aryl chlorides underwent smooth transformation with CyMgCl to provide the cross-coupling products in good to excellent yield (65.4–65.7). Notably, substrate conversion was significantly lower in the absence of blue LED irradiation (shown in parentheses), thus confirming the essential role of light in this transformation. Similarly, heteroaryl chlorides provided the respective products in moderate to good yields (65.8–65.11). Of note, the effect of irradiation was marginal, or not noticeable at all, in the latter two cases. Moreover, other primary and secondary alkyl Grignard reagents were found to be suitable coupling partners (65.12–65.14). Kinetic studies, including light on/off experiments indicated crucial effect of visible light irradiation, especially in case of electron-rich aryl chlorides. The proposed mechanism begins with an off-cycle formation of photoactive Fe(I) complex A upon reduction of Fe(acac)₃ by Grignard reagent 65.2. Then a light-accelerated oxidative addition occurs to afford aryl Fe(III) complex B. A subsequent transmetalation with Grignard reagent, followed by the reductive elimination, furnishes the cross-coupling product 65.3 (Scheme 65).

5.1. General Overview

Photophysical properties of organometallic complexes of palladium have been a subject of research for decades.^167–172 For instance, in 1985, Caspar examined several zerovalent palladium–phosphine complexes, which exhibited rather long excited state lifetime up to 5.4 μs.¹⁶⁸ It was also found that the absorption maximum of Pd(PPh₃)₄, one of the common Pd(0) catalysts, lies in the UV region of about 330 nm and tails into the visible light region. Therefore, early examples of Pd(0) photocatalysis operate under UV irradiation.¹⁷³

The first visible light-induced palladium-catalyzed transformation was reported by Gevorgyan’s group in 2016 by disclosing the generation of an unprecedented hybrid aryl palladium-radical species from aryl iodides.¹⁷⁴ In this case, visible light excitation led to the formation of a high-energy Pd(0) species, which was capable of transferring a single electron to aryl iodide, thereby engaging it in hybrid palladium-radical chemistry. To date, the majority of palladium visible light photocatalysis operates via this oxidative quenching pathway, wherein an SET from photoexcited palladium to an organic halide or its equivalent occurs (Figure 5a).^{13,175,14,176,15}

Under this activation mode, visible light irradiation facilitates the “oxidation addition” step of otherwise unreactive substrates under milder conditions in comparison to thermally-^177–180 and UV-induced methods,¹⁷³ thus encompassing a broader spectrum of substrates bearing more diverse functionalities. Furthermore, the formed hybrid aryl palladium-radical species opens up entry to a plethora of one-electron-based transformations, which are much less explored than the well-established two-electron chemistry of Pd. A striking difference between this SET event and that operating under typical photoredox methods³ is its loose relationship with the redox potentials. This can be attributed to a catalyst–substrate interaction, which would not be possible with typical coordinatively saturated photocatalysts. As a result of this inner-sphere SET mechanism, aryl and alkyl halides of reduction potentials beyond the reach for photocatalysts are reactive under the excited-state palladium catalysis.

Although examples remain scarce, visible light irradiation can also play a role in other catalytically relevant elementary steps. A recent example came from Arndtsen’s group, in which they showed that reductive elimination of acyl palladium chloride complex could be promoted by photoinduced homolysis of C–Pd bond followed by an XAT to forge the C–Cl bond (Figure 5b).¹⁸¹ Besides, there were some reports on visible light-accelerated elementary steps of cross-coupling reactions,^182,183 however, in these cases, radical intermediates were not involved, and the origins of acceleration remain unclear.

Since the introduction of visible light-induced palladium catalysis in 2016, in just five years, tremendous progress has been made in this field. A historical outline on the development of electrophiles as substrates for the carbon-based hybrid palladium-radical species is presented in Figure 5c. After successfully engaging aryl iodides toward aryl radicals, in 2017, Gevorgyan’s group realized the generation of analogous alkyl species from unactivated alkyl iodides and bromides,¹⁸⁴ which was closely followed by the independent works on bromides from Yu’s group¹⁸⁵ and Shang and Fu’s groups,¹⁸⁶ respectively. Shortly after, Gevorgyan’s group showed that tertiary alkyl iodides, as well as tertiary α-functionalized alkyl bromides, were also competent substrates.¹⁸⁷ Meanwhile, they revealed the employment of vinyl iodides in a separate report.¹⁸⁸ The same group also achieved the use of aryl triflates in 2020, providing a valuable alternative to aryl halides.¹⁸⁹ Very recently, Hong’s group utilized unactivated alkyl chlorides to access alkyl radicals, thus significantly expanding the scope of alkyl halides.¹⁹⁰ RAEs and RAOEs also proved to be viable precursors of alkyl radicals, as shown by Glorius’ group¹⁹¹ and Shang and Fu’s groups.^192,193

As mentioned above, the early reports have shown that aryl, vinyl, and alkyl radicals can be generated from the corresponding (pseudo)halides via visible light palladium photocatalysis. Different types of halides were therefore occasionally studied together in a single report. Accordingly, these references are arranged in Section 5.3 describing alkyl halides.

5.2. Fragmentation of C(sp²)–X Bond

5.3. Fragmentation of C(sp³)–X Bond

5.4. Homolytic Cleavage of N–O Bond

The ability of photoexcited Pd(0) to transfer an electron has also been exploited in the cleavage of N–O bond of RAOEs.^238,239 In 2019, Shang and Fu’s groups employed oxime esters 100.1 and silyl enol ethers 100.2 for synthesis of δ-cyano ketones 100.3.¹⁹³ Various (hetero)cyclobutanone oxime esters underwent smooth cascade transformation in good to excellent yields (100.4–100.10). The scope of silyl enol ethers was also found to be broad without noticeable electronic effects (100.11–100.16). Notably, aryl iodide functionality was well tolerated under these reaction conditions (100.12). Heterocycle-containing substrates were also compatible (100.17–100.18). Of note, in the absence of alkene partner, the corresponding allylic cyanide was obtained. According to the proposed mechanism, the photoexcited Pd(0) undergoes SET with substrate 100.1, leading to hybrid iminyl radical species A, which upon strain-release-driven β-scission produces hybrid alkyl palladium-radical B. As in alkyl Heck reactions, a subsequent radical addition to alkene followed by β-H loss delivers Heck product D, which is readily converted to a ketone product 100.3 under the reaction conditions (Scheme 100).

A more recent example involving oxime esters was disclosed by Xia, Zhou, and co-workers.²⁷⁷ In this work, substrate 101.1 undergoes radical addition to tethered olefin upon generation of iminyl radical. The formed hybrid radical species is then trapped by an external alkene to furnish 1-pyrrolines 101.3. The overall transformation represents an intermolecular Narasaka–Heck reaction.²⁷⁸ Good functional group tolerance was observed across different vinyl (hetero)arenes (101.4–101.9). Besides, the oxime ester substrate could be readily varied to access pyrrolines of diverse structures (101.10–101.15). Interestingly, when electron-deficient alkenes 101.17 were used, the three component coupling products 101.18 were produced instead. Several Michael acceptors were demonstrated as appropriate reaction partners (101.19–101.21) (Scheme 101).

5.5. Miscellaneous

The examples presented so far feature the use of visible light irradiation to induce radical reactivities under mild conditions, which are uncommon for ground state palladium catalysis. The involvement of radical species is supported by mechanistic studies ranging from radical trapping, radical clock, isotope-labeling studies, to EPR experiments. This section briefly covers the reactions in which visible light irradiation was found to be beneficial but did not lead to a switch from classic two-electron mechanism to a radical pathway.

In 2018, Alcázar’s group reported a visible light-accelerated Negishi coupling in flow.¹⁸² Electronically diverse aryl bromides were efficiently coupled with organozinc reagents 102.2 to furnish the cross-coupling products in good to excellent yields (102.4–102.12). Notably, conversion of aryl bromides was significantly lower in the absence of blue LED irradiation (shown in parentheses), thus confirming its crucial role in the transformation. Likewise, aryl chlorides were successfully converted to products in good yields (102.13–102.15). To identify the light-absorbing species, the authors recorded absorption spectra of different combinations of reaction components and observed a bathochromic shift upon addition of organozinc reagent to a mixture of the palladium catalyst and the ligand. Therefore, the mechanism may involve the in situ complexation between Pd(0) and 102.2 leading to a photoactive species A. According to the authors, the photoexcitation of A accelerates the oxidative addition of substrate 102.1 to form Pd(II) complex B and release of the organozinc reagent. The latter then undergoes transmetalation with intermediate B to furnish cross-coupling product 102.3. It should be mentioned that radical scavengers, diphenylethylene and di-tert-butylhydroxytoluene (BHT), did not affect product formation at all, suggesting that SET or other radical processes were not involved in this reaction (Scheme 102).

In 2019, Chu, Sun, and co-workers reported an oxidative reaction cascade between phenol derivatives 103.1 and α-bromoacetophenones 103.2 to access cyclic cores 103.3.²⁷⁹ Both visible light irradiation and elevated temperature were required to achieve good yields of the products (103.4–103.10) (Scheme 103). Separately synthesized ether substrate using the corresponding phenol and bromide was converted to product without loss of efficiency, thus suggesting the in situ etherification followed by the palladium-catalyzed C–C bond formation. However, at this point, the reaction mechanism and the role of light remain elusive.

A more recent example came from Patel’s group, who reported a cascade addition/cyclization of ketodinitriles 104.1 and 104.2 with arylboronic acids 104.3 to construct 3-cyanopyridines 104.4 and 3-cyanopyrroles 104.5, respectively.¹⁸³ This protocol was applicable to a wide range of substrates of different electronic and steric properties (104.6–104.16). A broad scope of boronic acids was also demonstrated (104.17–104.20). Alkylboronic acid, however, was not reactive (104.21). A diverse array of 3-cyanopyrroles, such as 104.22 were successfully synthesized using this method (Scheme 104). The test experiments indicated that the addition of TEMPO led to a drastic decrease in the reaction yield. Nonetheless, based on the fact that no TEMPO adducts were detected, the authors refute a radical reaction pathway. Instead, it was proposed that visible light irradiation accelerates the transmetalation step between palladium species and arylboronic acids. Apparently, more in-depth mechanistic studies are required to establish the exact role of visible light in this transformation.

6.1. General Overview

While nickel catalysts have been frequently used in combination with photocatalysts (cf. Figure 1b),^7,9 the catalytic reactivities of photoexcited nickel complexes are much less studied. In 2018, Doyle group demonstrated that visible light irradiation of Ni(II) aryl halide complexes led to the homolysis of Ni–C bond upon geometrical change, representing the first example of visible light-induced reactivity of nickel complexes.²⁸⁰ Importantly, this offers a new way of generating catalytically active Ni(I) species (Figure 6a).

Later in the same year, Gong and co-workers utilized chiral Ni(II) complex as dual Lewis acid and photoredox catalyst for asymmetric radical conjugate addition.²⁸¹ In this case, a photoinduced SET from an electron-rich amine to the Ni(II)–substrate complex was involved. Almost at the same time, another example of reductive quenching of excited-state Ni(II) complex came from the group of Miyake,²⁸² who coupled aryl halides with amines via a photoinduced generation of reactive aminium radical cation and Ni(I) complex (Figure 6b).

Furthermore, Alcázar’s group observed an accelerating effect of visible light on nickel-catalyzed Negishi coupling reaction.²⁸³ Despite a relatively unexplored area, these initial efforts clearly showcased the versatile reactivities of nickel complexes upon photoexcitation.

6.2. Transformations Involving Photoexcitation of Ni(II) Aryl Halide Complexes

In 2018, Doyle and co-workers studied the photophysics and photochemistry of Ni(II) aryl halide complexes, such as 105.1, using DFT calculations and ultrafast spectroscopy.²⁸⁰ It was found that these complexes displayed long-lived excited states, which were then computationally assigned to be ³MLCT in nature. Later in 2020, they studied a series of these complexes and came to the conclusion that the long-lived species was instead a ³d-d state 105.1b*, which originated from an initially formed short-lived MLCT state 105.1a*.²⁸⁴ This tetrahedral excited state featured an elongated Ni–Ar bond, which is predisposed to homolysis into Ni(I) complex 105.2 and aryl radical 105.3. The former readily dimerized, as observed by ¹H NMR studies, while the formation of the latter, rather than a chlorine radical,²⁸⁵ was unambiguously confirmed by spin trapping experiment with 105.4 (Scheme 105). An important implication of these studies is that visible light irradiation enables a facile generation of catalytically relevant Ni(I) halide from Ni(0) and an aryl halide. As a proof of principle, the authors showed in their 2018 report that C–O coupling reaction between 4′-bromoacetophenone and n-butanol was significantly facilitated by blue light irradiation.

Following Doyle group’s discoveries, Xue’s group evaluated the scope of the C–O coupling of various aryl electrophiles 106.1 with primary or secondary aliphatic alcohols 106.2.²⁸⁶ Interestingly, purple LED was found to be superior, and elevated temperatures were necessary for this transformation. A wide range of (hetero)aryl chlorides were smoothly converted to the respective ethers in moderate to excellent yields (106.5–106.9). However, electron-rich aryl chlorides were unreactive. Various primary and secondary alcohols were competent coupling partners, delivering products in good yields (106.10–106.15). Sterically less hindered primary alcohols reacted selectively in the presence of their secondary counterparts (106.12). Study of the scope of aryl bromides showed good functional group tolerance, featuring an example of intact para-bromo substitution (106.16–106.19). Besides, aryl bromides with a tethered alcohol underwent intramolecular C–O bond formation leading to heterocycles 106.20–106.22 (Scheme 106). Notably, aryl triflates, tosylates, as well as mesylates, were also reactive under these reaction conditions. It is worth mentioning that continuous irradiation was required, presumably to convert catalytically inactive Ni(II) species back to Ni(I). The authors proposed several plausible pathways, including disproportionation,²⁸⁰ homolysis,²⁸⁴ and reduction by base.²⁸²

Using the same nickel precatalyst, Xue and co-workers achieved C–N coupling of nitroarenes 107.1 with aryl halides 107.2.²⁸⁷ Electronically diverse nitroarenes provided the corresponding diarylamines in moderate to good yields (107.4–107.9). As in the previous report, electron-deficient aryl bromides were more reactive coupling partners (107.10–107.14). Aryl chlorides were also successfully coupled with nitroarenes, albeit with diminished yields (107.15–107.16). Notably, drugs such as fenofibrate, could be employed directly to yield aminated analog 107.17 in good yield. In addition, the authors applied this protocol to efficient synthesis of koenoline derivative 107.20 via a sequential C–N/C–C couplings. Preliminary mechanistic studies showed that aniline 107.21 was obtained in good yield upon reacting nitroso compound 107.22 with stoichiometric amount of Ni(II) aryl halide under the standard reaction conditions, thus supporting the intermediacy of 107.22. On the other hand, the involvement of other intermediates, such as aniline (107.23), hydroxylamine (107.24), diazene (107.25), and diazene oxide (107.26), was unlikely (Scheme 107).

6.3. Transformations Involving Photoexcitation of Ni(II)–Substrate Coordination Complexes

7.1. General Overview

Iridium photocatalysts are frequently employed in mainstream photocatalysis (cf. Figure 1a,b),^3,6,7,9,285 which is discussed in other review articles in this issue. In this chapter, we describe transformations, in which iridium photocatalysts are engaged not only in the process of harvesting light and subsequently transferring electron or energy, but also in asymmetric transformations. Previously, to achieve asymmetric photochemical transformations, dual-catalytic systems including an additional asymmetric cocatalyst, such as amine,^289–291 N-heterocyclic carbene (NHCs),²⁹² Brønsted or Lewis acid,^293,294 and thiourea,²⁹⁵ were required. The major breakthrough was achieved in 2014, when Meggers group reported an asymmetric photocatalyst capable of harnessing light energy, as well as inducing chirality. Thus, it was shown that chiral-at-metal Ir(III) complexes could catalyze enantioselective α-alkylation of ketones, hence representing the first example of asymmetric iridium photocatalysis.²⁹⁶ The employed bis-cyclometalated complex features two labile acetonitrile ligands, which in situ are readily replaced by substrate to form an iridium–substrate complex, thereby enabling precise control over the stereodetermining step (Figure 7a). As with typical photoredox catalysts, either oxidative or reductive quenching cycles can be operative depending on the reactants and conditions. In 2017, Yoon group utilized chiral hydrogen-bonding Ir(III) triplet sensitizers for asymmetric [2 + 2] photocycloaddition via EnT (Figure 7b).²⁹⁷ In this case, the hydrogen-bonding to a substrate was enabled by a bidentate pyridylpyrazole ligand. Later, the same group reported an interesting intermolecular [2 + 2] reaction, which also occurred via EnT.²⁹⁸ The chiral catalysts developed for the transformations detailed in the following sections are shown in Figure 7c.

7.2. Iridium as Chiral Photoredox Catalyst

7.3. Iridium as Chiral Triplet Sensitizer

In 2017, Yoon and Baik’s groups disclosed another class of chiral Ir(III) photocatalyst capable of EnT to alkene-tethered quinolone derivatives 115.1 upon hydrogen-bonding, thereby accomplishing an enantioselective intramolecular [2 + 2] photocycloaddition.²⁹⁷ Structurally related quinolones provided excellent yields and high enantiomeric excess (115.3–115.7). Placing a chlorine atom at the 8-position led to a drastic decrease of enantioselectivity (115.8). In contrast, product 115.9 with a smaller fluorine atom was obtained with high enantiocontrol. Modification of the alkene tether was also feasible, albeit with diminished enantioselectivities (115.10–115.11). Importantly, substrates in which the nitrogen atom was protected or replaced by an oxygen atom delivered the cycloadducts in good yields, however, with loss of enantiocontrol, thus underlining the crucial role of N–H bond in asymmetric induction. The authors ruled out the possibility of SET as both oxidation and reduction potentials of quinolone 115.14 are out of the range of Λ-Ir4 (1.59 V vs 1.27 V and ←1.7 V vs −0.78 V, respectively). Besides, a marginal bathochromic shift was observed upon mixing substrate and photocatalyst, indicating that substrate coordination was not essential for photoexcitation to occur. On the other hand, the values of triplet energy of 115.14 and Λ-Ir4 were found to be 55.0 and 59.6 kcal/mol, respectively, suggesting an exergonic state change upon triplet energy transfer. A computational analysis of complex between 115.14 and Λ-Ir4 indicated several interactions, including strong hydrogen-bonding of the carbonyl moiety with the N-atom of the pyrazole fragment. In addition, a π–π interaction between substrate and the ligand in proximity was identified, which is in agreement with the orbital analysis revealing a coplanar alignment of the substrate π-space with the ligand π-orbitals to attain an appropriate overlap. Moreover, an unusual NH–π interaction was also identified, which explained the observed negligible enantioinduction in 115.12 and 115.13. Cumulatively, these three substrate–catalyst interactions contributed to the asymmetric induction in this transformation (Scheme 115).

More recently, the same groups reported an enantioselective intermolecular [2 + 2] photocycloaddition of 3-alkoxyquinolones 116.1 and maleimides 116.2 using a similar catalyst Λ-Ir5.²⁹⁸ This reaction was not sensitive to substitution at the aromatic ring providing good to excellent yields and enantioselectivities (116.4–116.8). Methyl substituent at the alkene moiety was also tolerated, albeit with diminished stereoselectivity. N-Substituted maleimides were effective, suggesting that a catalyst–maleimide coordination was not crucial (116.10). Notably, N-protected substrate delivered product 116.11 in 72% e.e., in sharp contrast to the previous report in which the racemates were obtained (Scheme 115), thus indicating a fundamental difference in the interactions involved. Interestingly, Stern–Volmer quenching studies revealed that maleimide was a much more efficient quencher than substrate 116.12. Indeed, reaction of alkene-tethered substrate 115.14 in the presence of maleimide provided the intermolecular [2 + 2] adduct exclusively instead of 115.3. Besides, it was found that the photophysics of Λ-Ir5 was not affected upon substrate coordination. These results support the scenario, in which the exciton of substrate–catalyst complex was localized at iridium center and subsequently transferred to maleimide via intermolecular EnT. To gain further insights into the interactions associated with asymmetric induction, the authors performed computational studies. Thus, DFT calculations of a ternary encounter complex of catalyst, substrate 116.12, and maleimide, suggested that the energy required for EnT to hydrogen-bonded substrate was 3.65 kcal/mol, while that of maleimide was 4.62 kcal/mol. However, the rate of EnT is dependent on the degree of orbital overlap. In this ternary complex, maleimide, instead of substrate, as in the previous case, showed strong π–π interaction with the cyclometalating ligand. Furthermore, two hydrogen-bonding interactions were identified between substrate and maleimide. This binding mode provided a rationale behind the preferential triplet sensitization of maleimide over substrate (Scheme 116).

8.1. General Overview

Similarly to iridium, bis-cyclometalated, chiral-at-metal Rh(III) complexes have been developed for asymmetric Lewis acid catalysis.^{299–302,307} In 2015, Meggers, Gong, and co-workers first explored the excited-state reactivity of this type of catalyst and achieved photoinduced asymmetric α-aminomethylation of ketones.³⁰⁸ As in analogous iridium photocatalysis, a single rhodium catalyst is responsible for both the chiral induction and the redox events (Figure 8a). In addition to α-functionalization, rhodium photocatalysis also enable access to enantioenriched, β-functionalized carbonyl compounds. In 2017, Meggers’ group achieved asymmetric β-C–H functionalization of ketones via radical–radical cross-coupling.³⁰⁹ In the same year, Kang and co-workers employed enones as substrate for enantioselective β-alkylation.³¹⁰

Besides photoredox chemistry, in 2017, Meggers’ group disclosed an example of asymmetric photocycloaddition reaction, further highlighting the versatility of photoinduced Rh(III) catalysis.³¹¹ In this case, photoexcitation of the Rh(III)–enone coordination complex occurs at the alkene moiety, directly leading to a triplet biradical, which subsequently adds intermolecularly to another alkene (Figure 8b). As such, the rhodium catalyst facilitates visible light excitation of the substrate rather than serve as a triplet photosensitizer (cf. Schemes 115 and 116).

Later in 2019, Baslé and co-workers described a photoinduced Rh(I)-catalyzed directed C–H borylation, representing the sole example of Rh(I) photocatalysis.^312,313 In this transformation, visible light irradiation promotes the rate-determining C–H oxidative addition step, thus distinguishing it from the aforementioned examples in which radicals are involved (Figure 8c). The catalysts employed for the transformations described below are depicted in Figure 8d.

8.2. Asymmetric α-Functionalization of Ketones

8.3. Asymmetric Synthesis of β-Functionalized Ketones

8.4. Asymmetric Photocycloaddition

In 2017, Meggers and co-workers disclosed the use of chiral rhodium photocatalyst for [2 + 2] photocycloaddition reaction between enone derivatives 123.1 and olefins 123.2, leading to cyclobutane scaffolds 123.3 bearing three stereogenic centers.³¹¹ High yields and diastereoselectivities, and excellent enantioselectivities were observed regardless of the electronic properties and steric hindrance of the aryl substituent (123.4–123.7). Different auxiliaries were also able to deliver products with comparable efficiencies (123.8–123.10). Remarkably, this method allows for construction of vicinal all-carbon quaternary stereocenters with excellent stereocontrol (123.10–123.12) (Scheme 123). In addition, vinyl ether was found to be a competent alkene substrate (123.13). Of note, the iridium analog could also catalyze this reaction, however, failed to induce any enantioselectivity. The proposed mechanism involves the photoexcitation of the in situ formed complex A to its singlet state, which upon intersystem crossing (ISC) transitions to a triplet state depicted as A* (T₁). This biradical species then reacts with olefin 123.2 in a stereocontrolled fashion, constructing the first chiral center in intermediate B. A subsequent ISC followed by radical recombination forges the second C–C bond to afford cyclobutane C. The desired product is then released upon exchange with the substrate molecule. Computational studies performed on A* (T₁) revealed that spin density was mainly localized at the alkene moiety instead of the metal center, thus refuting the possibility of the triplet sensitization (Schemes 115 and 116). In addition, a redox process was unlikely, as the estimated excited-state potentials of A were insufficient to oxidize or reduce styrene.

Shortly after, the same group described the use of vinyl azides 124.2 to accomplish an asymmetric [2 + 3] photocycloaddition leading to valuable 1-pyrrolines 124.3 containing two consecutive stereocenters.³²³ Examination of the aryl group in substrate revealed that electronic and steric effects had little impact on the reaction outcome (124.4–124.7). Likewise, heteroaromatics could be introduced (124.8–124.9). In addition, quaternary stereocenter could be constructed with high efficiency (124.10–124.11). Various aryl-, vinyl-, and alkyl-substituted vinyl azides were also amenable to this transformation (124.12–124.15). Remarkably, nearly perfect diastereoselectivity and excellent enantiomeric excess were observed for all products. As in the previous report, the iridium counterpart led to the formation of product in racemic form. The catalytic cycle starts with the photoexcitation of the in situ formed complex A to its excited state A*. A subsequent reaction with vinyl azide leads to biradical B along with the establishment of the first stereocenter. Intermediate B then extrudes dinitrogen to produce an iminyl radical in C, which undergoes stereoselective radical recombination to forge the C–N bond. Finally, the product release closes the catalytic cycle. To understand why analogous iridium catalyst did not give any enantioselectivity, the authors carried out similar spin density calculation and found that the iridium center possessed the major spin density, indicative of a different mechanistic scenario involving EnT to free substrate 124.1 followed by cycloaddition, thus resulting in no stereocontrol (Scheme 124).

In 2018, a related work came from Meggers’ group, where racemic cyclopropanes 125.1 were employed in asymmetric [3 + 2] photocycloaddition with alkenes 125.2.³²⁴ A wide variety of Michael acceptors, as well as styrenes were competent alkene partners, delivering the cyclopentane adducts in high yields and good to excellent stereoselectivities (125.4–125.8). Employment of enyne allowed for synthesis of the product 125.9 with all-carbon quaternary stereocenter. Besides, derivatives of natural product, such as aspartame, provided the respective products in excellent yields and stereoselectivities (125.10). Employment of spirocyclic substrate led to product 125.11 with impressive stereocontrol. However, low diastereoselectivities were observed with nonsymmetrical substrates (125.12–125.13). Unlike the previous two photocycloadditions, the proposed mechanism of this reaction involves photoredox processes, wherein a photoexcited complex A undergoes SET with DIPEA to form ketyl radical B, which upon ring opening produces tertiary alkyl radical C. Intermediate C then undergoes stereodetermining cycloaddition step to construct the cyclopentane motif in D. A subsequent single electron oxidation and exchange with the substrate leads to product 125.3 (Scheme 125).

In the same report, the authors also demonstrated analogous cycloaddition with alkynes 126.2. This photocycloaddition reaction also demonstrated a broad scope with respect to alkynes, furnishing cyclopentene scaffolds in high yields with excellent enantiomeric excess (126.4–126.9). Similarly, spiro-cycle 126.10 was obtained with excellent enantioselectivity, while 126.11 was obtained as a mixture of diastereomers. A more complex alkyne was also a competent substrate (126.12) (Scheme 126).

8.5. C(sp²)–H Borylation

The transformations discussed so far involve a photoactive Rh(III) species that initiate the catalytic cycle. In 2019, Baslé and co-workers described the use of NHC–Rh(I) complex Rh6 for directed C–H borylation.³¹² Examination of electronically diverse 2-phenylpyridine derivatives revealed that the electron-donating groups favored the reaction (127.3–127.6). Substitution at other positions was also tolerated (127.7–127.8). However, employment of more electron-deficient pyridyl directing group was less efficient (127.9). Interestingly, a modest level of desymmetrization was observed in 127.10. This protocol was also applicable to C(sp³)–H borylation, as exemplified by 127.11. Stoichiometric experiments between Rh6 and substrate 127.12 indicated the essential role of light in the C–H activation step, as key intermediate aryl Rh(III) hydride 127.13 was formed upon irradiation only. Transmetalation of intermediate 127.13 with B₂Pin₂, followed by reductive elimination of the resultant aryl Rh(III)–BPin complex, furnishes the borylation product 127.2 (Scheme 127).

9.1. General Overview

The advent of gold/photoredox dual catalysis has enabled a range of transformations involving redox processes at the gold center,^325–327 which is uncommon in traditional gold catalysis due to its high oxidation potential.³²⁸ In 2016, Hashmi and co-workers showed that photoexcited gold complexes can also engage in redox processes without an additional photosensitizer, thus representing the first example of visible light-induced gold catalysis.³²⁹ In this report, they described an oxyarylation of alkynes with aryldiazonium salts, which features the in situ formed Au(I)–aryldiazonium charge-transfer complex capable of transferring a single electron to another diazonium upon photoexcitation, thereby producing the key aryl Au(III) intermediate (Figure 9a). Since then, several reactions involving the photocatalytic generation of this intermediate have been achieved.

More recently in 2019, Hashmi’s group disclosed a desulfurizing alkylation of alkenes, which comprises another light-induced reactivity of gold complexes.³³⁰ In this case, dimeric Au(I) complex forms a coordination complex with thiol. Photoexcitation of this complex and LMCT generates a reduced gold intermediate and a thiol radical cation that subsequently desulfurizes to afford the respective alkyl radical (Figure 9b).

It is worth mentioning that an important branch of gold photocatalysis revolves around the use of gold catalyst as a photosensitizer, which is not directly involved in bond forming/breaking events. These works, which are beyond the scope of this review, have been recently summarized by Hashmi and co-workers.³³¹

9.2. Carbofunctionalization of Alkynes and Alkenes

In 2016, Hashmi’s group reported an oxyarylation of internal alkynes 128.1 with aryldiazonium salts 128.2, representing the first example of visible light-induced gold catalysis.³²⁹ Different 1-aryl-1-hexynes were smoothly converted to the respective ketones in good yields (128.4–128.6). Similarly, the alkyl substituent on alkyne could be altered to incorporate various functionalities (128.7–128.10). Both diaryl- (128.11) and dialkyl-substituted (128.12) alkynes were also found to be effective substrates. In addition, diverse aryldiazonium salts delivered the desired products in moderate to good yields (128.13–128.15). However, a trifluoromethyl group was not tolerated, presumably due to decomposition of the corresponding diazonium salt (128.16). The authors also showed that the same reaction conditions could be applied to the oxyarylation of alkene 128.17, thus demonstrating that this process does not require employment of exogeneous photosensitizers, as reported earlier.³²⁵ The role of visible light irradiation was originally attributed to the extrusion of N₂ from Au(III) complex A to produce aryl Au(III) intermediate B. Later, Zhang, Zhu and co-workers performed detailed DFT studies and proposed a different mechanistic scenario involving the formation of photoactive charge-transfer complex C.³³² Upon visible light excitation, C engages another diazonium salt in SET, affording Au(II) complex D and a diazo radical, which gives aryl radical upon extrusion of N₂. The formed aryl radical then recombines with D to form intermediate B (path a), which subsequently forms the key coordination complex E with alkyne substrate. Alternatively, aryl radical can add to Au(I) catalyst to give aryl Au(II) species G (path b). A sequence of SET to 128.2 and coordination to alkyne (i), or vice versa (ii), leads to the same intermediate E, which upon nucleophilic addition of methanol delivers vinyl Au(III) complex F. Finally, reductive elimination followed by hydrolysis furnishes product 128.3 and regenerates the gold catalyst (Scheme 128).

In 2019, Hashmi, Klein, and co-workers described a cyclization reaction featuring light-induced chemodivergent transformations.³³³ Thus, reaction between 2-alkynylphenols 129.1 and aryldiazonium salts 129.2 under photoinduced conditions produced 3-aryl benzofurans 129.3 orazobenzofurans 129.4 under dark. Several examples were demonstrated for each transformation resulting in moderate to good yield of benzofurans (129.5–129.11). Stoichiometric reactions showed that both products 129.13 and 129.14 could be derived from vinyl Au(I) complex 129.12. Thus, both catalytic transformations share a common 5-endo-dig cyclization pathway upon π-coordination of Au(I) catalyst to the alkyne moiety in 129.1 to form vinyl Au(I) intermediate. It then diverges into two paths depending on the reaction conditions. In the upper case, vinyl Au(I) species and diazonium salt form a donor–acceptor complex, which upon photoexcitation leads to C–C bond formation in 129.3. DFT calculations revealed that the triplet excited state of this complex featured an elongated C–N bond of diazonium fragment, thus explaining the extrusion of N₂. On the other hand, vinyl Au(I) can act as a typical organometallic nucleophile and attacks electrophilic diazonium salt, thereby constructing the C–N bond in 129.4 (Scheme 129).

In the same year, Hashmi’s group also reported a PPh₃-mediated desulfurizing protocol to access C–C bond formation product 130.3 from vinyl (hetero)arenes 130.1 and mercaptan derivatives 130.2.³³⁰ Various functionalized styrene derivatives were found to be compatible (130.4–130.9). Di- and trisubstituted alkenes (130.9–130.11), as well as vinyl heteroarenes (130.12), also worked well in this transformation. In addition to ester, mercaptans bearing other electron-withdrawing groups delivered the respective products in good yields (130.13–130.15). Examples of secondary and tertiary alkylation were also demonstrated (130.16–130.17). The proposed catalytic cycle commences with the in situ formation of photoactive coordination complex B from dimeric Au(I) catalyst A and mercaptan 130.2. Photoexcitation of B generates gold complex C and thiol radical cation D via LMCT. The latter is then captured by PPh₃ to produce phosphoranyl radical E, which upon β-scission releases alkyl radical F. A subsequent radical addition to alkene affords benzyl radical G, which then abstracts hydrogen atom from another mercaptan to directly furnish product 130.3 (path a). The concomitantly formed thiyl radical H can react with PPh₃ to propagate the radical chain. In an alternative scenario, G can undergo SET with intermediate C to afford the product via carbanion I, meanwhile closing the gold catalytic cycle (path b). The authors also proposed that trace amount of molecular oxygen may regenerate gold complex A from C. An experimental quantum yield of 19.5% suggested a gold-catalyzed pathway. However, classical radical conditions using AIBN as an initiator provided the product in comparable yield, indicating that both paths a and b could be operative. The intermediacy of alkyl radical was confirmed by radical clock experiment, while the involvement of S-centered radical D or H was supported by the formation of thiol addition product 130.19 in the absence of PPh₃ (Scheme 130). Of note, deuterium-labeling experiments indicated that in addition to mercaptan, solvent may also serve as a hydrogen source.

9.3. Cross-Coupling Reactions

In 2017, Hashmi, Xie, and co-workers disclosed the first visible light-induced gold-catalyzed cross-coupling reaction of boronic acids 131.1 and diazonium salts 131.2,³³⁴ complementing the reported methods which require an additional photosensitizer.^321,331 Using this method, electron-neutral and -deficient arylboronic acids delivered the coupling products in moderate to good yields (131.4–131.6). However, the electron-rich counterparts were much less efficient (131.7). Heteroaryl boronic acids were also effective substrates for this transformation (131.8). Likewise, diazonium salts bearing an electron-withdrawing group reacted more efficiently (131.9–131.12). It is worth mentioning that aryl pinacolboronates were also viable cross-coupling partners (131.13). As established earlier, this transformation also involves the photoinduced generation of aryl Au(III) species A from Au(I) catalyst and aryldiazonium 131.2. In this case, the counterion BF₄⁻ also serves as an activator of boronic acid (B), thereby facilitating the subsequent transmetalation step to produce intermediate C, which upon reductive elimination furnishes product 131.3 and regenerates the gold catalyst (Scheme 131).

In 2018, the same group achieved a chemoselective Hiyama coupling using bifunctionalized arenes 132.1 to access highly valuable biarylboronates 132.3.³³⁵ As in the previous case, diazonium salts with electron-withdrawing groups were effective coupling partners, delivering products with BMIDA-, BPin-, or BNep functionality in moderate to good yields (132.4–132.9). Remarkably, an iodo substituent was also tolerated, highlighting the chemoselectivity of this protocol (132.10–132.11). This transformation was also applicable to substrates with different substitution pattern (132.12–132.13), as well as those bearing additional functional groups (132.14–132.16). Besides, other arylsilanes were also viable substrates (132.17–132.18). Notably, iodo- (132.19) and triflyl (132.20) groups on arylsilane also remained intact under the reaction conditions, thus constituting another class of bifunctionalized substrates for this reaction (Scheme 132). The authors also demonstrated a series of iterative modular functionalization of the obtained products, thus further highlighting the synthetic utility of this method.

Shortly after, Hashmi’s group demonstrated the viability of several other B- and Si-based reagents 133.1 for cross-coupling reactions.³³⁶ Thus, MIDA boronates, potassium trifluoroboronates, trimethoxysilanes, and bis(catecolato)silicates, were coupled with aryldiazonium salts in moderate to high yields (133.4–133.5). In addition, the authors showed that alkynylation was also feasible using alkynyltrimethylsilanes 133.6, as exemplified by successful synthesis of products 133.9 and 133.10. Interestingly, bistrimethylsilylacetylene afforded the double arylation product 133.11 in good yield (Scheme 133).

10.1. General Overview

In recent years, tetrabutylammonium decatungstate (TBADT), a tungsten-based polyoxometalate complex, has gained immense popularity in organic synthesis (Figure 10).^337,338 Photoexcited TBADT is capable of abstracting hydrogen atoms from C–H sites of BDE up to 105 kcal/mol,³³⁹ thus rendering TBADT a superior catalyst for the activation of strong C–H bonds.

Notably, photoexcited TBADT features highly electrophilic oxygens with partial radical character³⁴⁰ and is therefore primed for HAT at electron-rich C–H sites. As such, polarity-matched hydridic C–H bonds undergo faster HAT than the protic ones. Moreover, due to the larger size of TBADT, HAT occurs preferentially at less hindered sites. Taken together, one can strategically exploit the polar and steric factors for highly site-selective C–H functionalization reactions.

Given the absorption maximum at 324 nm, most of the TBADT-catalyzed reactions are performed under UV-irradiation.^341,342,340 Nonetheless, this broad absorption band tails into the violet region, thus indicating that visible light irradiation can also be utilized.^343–347 In 2009, Fagnoni’s group first demonstrated the viability of sunlight as a light source.³⁴⁸ Since then, several transformations using purple LED irradiation have been reported.

10.2. C–H Alkylation

In 2009, Fagnoni’s group reported a solar light-driven C–H alkylation of Michael acceptors 134.1 with unactivated- (134.4) and activated (134.5) aliphatics as well as aldehydes (134.6). Several examples of Michael acceptors were also demonstrated (134.7–134.9).³⁴⁸

Several years later, the groups of Fagnoni and Ryu reported a regioselective β-alkylation of cyclopentanone under very similar conditions.³⁴⁹ As in the previous case, various Michael acceptors were found to be suitable substrates for this reaction (134.10–134.13). Notably, 3-methylcyclopentanone underwent selective transformation at the methine carbon (134.14) (Scheme 134). Interestingly, a much lower β-selectivity was observed with other cyclic and acyclic ketones. Both transformations share the same mechanism, which involves HAT from the more hydridic and weaker C–H sites of 134.2 to photoexcited decatungstate catalyst to form alkyl radical A. A subsequent radical addition to electron-deficient alkene 134.1 affords electrophilic radical C. Finally, back HAT from reduced decatungstate B affords alkylation product 134.3 and regenerates the catalyst.

In 2020, P.-S. Wang group incorporated chiral phosphoric acid (CPA) catalyst into analogous alkylation reaction to access α-enantioenriched ketones 135.3 from enones 135.1.³⁵⁰ With cyclohexane as the alkyl source, differently substituted 1-tetralones delivered the respective products in moderate to good yields with high enantiocontrol (135.5–135.8). Enones with fused heterocycle (135.9), as well as those of different ring sizes (135.10–135.11), were also viable substrates. Acyclic enone proved to be more challenging, as exemplified by the low yield and enantioselectivity of product 135.12. In addition to cycloalkanes, hydrocarbons containing benzylic- (135.13), allylic- (135.14), as well as aldehydic (135.15) C–H sites, afforded the alkylation products with moderate to high efficiencies (Scheme 135). The authors proposed the proton transfer from CPA to enol intermediate A as enantiodetermining step. Of note, this method requires a substantial excess (5–20 equiv) of hydrocarbons to achieve efficient hydroalkylation.

Very recently, Q. Wang and co-workers reported synthesis of unnatural α-amino acids via C–H alkylation of aldehydes with dehydroalanine methyl ester derivatives.³⁵¹ Sterically and electronically diverse benzaldehydes reacted efficiently to provide the corresponding products in good yields (136.4–136.8). Notably, the allylic ether moiety of 136.8 remained intact in this transformation. Likewise, heteroaryl aldehydes were found to be effective substrate (136.9). This protocol could also be applied to nonaromatic aldehydes (136.10–136.11), as well as unactivated hydrocarbons bearing a tertiary C–H site (136.12), albeit with lower efficiency. Moreover, switching the substrate to chiral Karady–Beckwith alkene provided product 136.13 with excellent diastereoselectivity.

Shortly after, P.-S. Wang’s group independently reported a closely related asymmetric version of this transformation.³⁵² Simple cyclic hydrocarbons furnished the products with excellent enantioselectivity (136.15–136.16). Likewise, a broad range of toluene derivatives was found to be suitable substrates for this reaction (136.17–136.21). Other substrates possessing weak C–H bonds also underwent smooth transformation with good enantioselectivity (136.22–136.24) (Scheme 136). Importantly, the N–H bond in substrate is crucial to enantioinduction, as protecting the nitrogen with a methyl group led to the formation of racemic products. Of note, the authors also applied this protocol toward α-aryl carboxylates from the corresponding α-aryl acrylates.

In addition to Michael acceptors, vinyl (hetero)arenes have also been employed for C–H alkylation. In 2019, C. Wang and co-workers reported a hydroacylation reaction of α-trifluoromethylated vinyl (hetero)arenes.³⁵³ Styrene derivatives containing a range of functionalities were compatible (137.4–137.8). Vinyl heteroarenes also provided the respective products uneventfully (137.9). Besides aliphatic aldehydes, the aryl counterparts were also found to be suitable C–H sources (137.10–137.12). N-Phenylformamide also delivered the respective product, albeit with diminished efficiency (137.13). It is noteworthy that β-F elimination, a common side reaction in previous methods,^354–356 was not observed in this protocol.

More recently, Prieto and Taillefer employed disulfide as a cocatalyst for C–H alkylation of styrenes.³⁵⁷ Various styrene derivatives (137.14–137.17), as well as 2-vinylpyridine (137.18), reacted smoothly with formamide to deliver the corresponding amides in moderate to good yields. N-Alkyl (137.19) and -aryl (137.20) formamides were also found to be suitable reaction partners. However, tertiary amide did not yield the desired product (137.21). Other C–H sources, including aldehydes (137.22–137.23) and 1,3-benzodioxole (137.24), also worked well in this transformation. The authors also demonstrated an example of alkylation with PhSiH₃ (137.25). This protocol features the use of disulfide as a polarity reversal catalyst to circumvent the redox potential mismatch between benzylic radical A and reduced decatungstate B. Thus, photocatalytically generated thiol radical, which has much lower reduction potential than A, undergoes efficient single electron reduction by B followed by protonation to form thiol, which in turn, transfers a hydrogen atom to A to furnish product 137.20 (Scheme 137).

In 2019, Dilman’s group employed N-tosyl imines 138.1 as radical acceptor to access alkylation products 138.3.³⁵⁸ As established in the previous reports, simple aliphatics (138.4), ethers (138.5–138.6), amides (138.7), and aldehydes (138.8) were all found to be viable C–H sources. Aldimines bearing other aryl- (138.9) and alkyl (138.10) substituents also underwent smooth transformation. On the contrary, ketimines afforded much lower yield, presumably due to the steric hindrance (138.11). In addition to a mechanism that resembles the one described before (cf. Scheme 134), the authors proposed an alternative pathway, where electrophilic N-centered radical, formed upon radical addition, undergoes HAT with the C–H source to furnish product 138.3 and propagate the radical chain (Scheme 138).

10.3. C–H Alkynylation and Deuteration

Very recently, Capaldo and co-worker reported a C–H alkynylation protocol using methanesulfonyl alkynes 139.1 proceeding via radical addition/elimination pathway.³⁵⁹ The scope with respect to the C–H sources was similar to the examples presented so far (139.4–139.7). Various functional groups could be introduced at the phenyl ring of substrate (139.8–139.11). Besides, an example of aliphatic substrate was also demonstrated (139.12). The employment of sulfonyl substituent as a radical leaving group was crucial, as the chloro analogue 139.13 led to vinylation product 139.14 (Scheme 139).

As detailed in Scheme 95, thiol can be incorporated in decatungstate catalysis to facilitate back HAT to the organic radical intermediate. In 2020, Q. Wang’s group reported the deuteration of aldehydes 140.1 via a direct hydrogen isotope exchange (HIE) with D₂O, using thiol 140.3 as a DAT catalyst.³⁶⁰ A broad range of functionalized (hetero)aryl aldehydes were efficiently deuterated (140.4–140.13). Notably, products 140.10 and 140.11, possessing allylic and benzylic C–H bonds, respectively, were obtained chemoselectively. Aliphatic aldehydes also underwent deuteration with high level of deuterium incorporation (140.14–140.16). In some cases, additional deuteration occurred at the α-C–H site to the carbonyl group. Furthermore, more complex substrates, such as ibuprofen derivative (140.16), can also be selectively deuteriated.

In the same year, Wu and co-workers reported the same transformation under similar conditions.³⁶¹ Likewise, (hetero)aryl- (140.4, 140.17–140.21) and aliphatic (140.22–140.23) aldehydes underwent efficient isotope exchange to provide the deuterated products. The authors also extended this chemistry toward deuteration of aliphatic C(sp³)–H bonds. Thus, both unactivated and activated C–H sites were successfully deuterated (140.26–140.28). As in the previous cases, further deuteration at other positions may occur (140.29–140.31) (Scheme 140). In addition, a range of pharmaceutical compounds, such as edaravone (140.32), mexiletine (140.33) and sesamol derivative (140.34), were amenable to this transformation, further highlighting the generality of this protocol.

11.1. Manganese

Although there are reports on manganese-catalyzed organic reactions under visible light irradiation,³⁶² a majority of them feature an off-cycle, photoassisted generation of active manganese species, which then catalyzes the reaction in its ground state. A notable example is the photolysis of Mn₂(CO)₁₀ to its reactive monomeric form.^363–368 As such, there are only a handful of photocatalytic reactions, where a photoexcited manganese complex undergoes redox events with a substrate molecule via an inner sphere mechanism.

In 2018, Ackermann’s group disclosed the first example of visible light-induced manganese catalysis, in which a Mn(I) catalyst for C–H arylation of (hetero)arenes 141.2 with aryldiazonium salts 141.1 was employed.³⁶⁹ Electron-deficient diazonium salts reacted efficiently with various arenes to form the biaryl products in moderate to good yields (141.4–141.18). Likewise, heteroarenes, including furans (141.9–141.13), pyrroles (141.14), as well as thiophenes (141.15–141.17), were found to be viable substrates for this transformation. Of note, products 141.16 and 141.17 were obtained with excellent regioselectivity. The proposed mechanism begins with an off-cycle ligand exchange between CpMn(CO)₃ and arene 141.2 to form complex A, which undergoes a second exchange with diazonium salt 141.1 to produce visible-light absorbing species B. Then photoexcitation of B triggers an SET from Mn(I) to diazonium salt, leading to the formation of aryl radical C and Mn(II) complex D. A subsequent radical addition of the former to another arene molecule affords intermediate E, which is then oxidized by Mn(II) intermediate D to carbocation F, meanwhile regenerating the Mn(I) catalyst (path a). Alternatively, radical E can be oxidized by diazonium salt (path b). Finally, deprotonation of intermediate F furnishes the reaction product 141.3. In accord with the proposed mechanism, the competition experiment showed that electron-deficient diazonium salt 141.18 reacted preferentially over its electron-rich counterpart 141.19. Besides, a parallel KIE value of 1.0 indicated that the C–H bond cleavage is not the rate-determining step. In addition, the formation of aryl radical was supported by radical trapping experiment with 1,1-diphenylethylene (141.21) (Scheme 141).

In 2020, Wan and co-workers showed that photoexcited Mn(III) complex can be reduced by Langlois’ reagent (NaSO₂CF₃), representing an example of reductive quenching cycle in manganese photocatalysis.³⁷⁰ In this work, unactivated alkenes 142.1 underwent hydroxytrifluoromethylation with NaSO₂CF₃ under aerobic conditions. Monosubstituted alkenes with pendant ester functionality were found to be suitable substrates for this transformation (142.3–142.7). Likewise, imides (142.8) and thioesters (142.9) delivered the respective products in good yields. 1,1-Disubstituted alkenes also underwent smooth reaction, furnishing tertiary alcohol with high efficiency (142.10). The catalytic cycle commences with an SET event from NaSO₂CF₃ to photoexcited Mn(III) catalyst, producing trifluoromethyl radical and a Mn(II) complex. The former then adds to unactivated alkene 142.1 to afford alkyl radical A, which is subsequently trapped by molecular oxygen to generate peroxy radical B. Finally, intermediate B reacts with the Mn(II) complex to deliver product 142.2, presumably via the respective Mn(III) peroxo intermediate, and complete the cycle (Scheme 142). Radical trapping experiment revealed the formation of TEMPO–CF₃ adduct, thus supporting the radical nature of this reaction.

11.2. Ruthenium

In contrast to its rich chemistry as a conventional photosensitizer,^1,6 ruthenium had not been utilized as a transition metal catalyst in the realm of photocatalysis until recently. In these scarce reports, however, visible light-photoexcited ruthenium catalysis has exhibited interesting reactivity, which is distinct from that in its ground state.

In 2019, the groups of Ackermann and Greaney simultaneously reported a photoinduced Ru(II)-catalyzed meta-selective alkylation of arenes 143.1.^371–373 In the protocol developed by Ackermann and co-workers, a range of tertiary alkyl bromides underwent efficient reaction with 2-phenylpyridine to provide the respective alkylation products in good to excellent yields (143.4–143.7). Substrates with substituted phenyl ring were also found to be effective (143.8–143.9). Remarkably, this protocol enables a selective meta-alkylation regardless of the steric environment (143.10). Secondary alkyl bromides were generally less efficient, leading to the corresponding products in moderate yields (143.11–143.13). α-Bromo esters also proved to be viable reaction partners (143.14). The authors also showed that in addition to pyridine, other directing groups, such as pyrazole (143.15) and oxazoline (143.16), also delivered the desired products, albeit with lower efficiency.

Greaney’s method relies on use of alkyl iodides in this transformation. Thus, alkylation with tertiary (143.17–143.18) and secondary alkyl iodides (143.19–143.22) was achieved with moderate to good efficiencies. This protocol exhibits high functional group tolerance, as exemplified by the efficient synthesis of products 143.23–143.26. As in the previous case, the authors also demonstrated the viability of other directing groups for this transformation (143.27). It is noteworthy that primary alkyl iodides were also reactive under the same conditions, but delivered ortho-alkylation products instead.

The proposed mechanism involves the in situ formation of photoactive cyclometalated Ru(II) complex A from ruthenium catalyst and substrate 143.1. Upon photoexcitation, complex A undergoes SET with alkyl halide 143.2 to produce alkyl radical B and Ru(III) species C. A subsequent addition of nucleophilic radical B at the position para-to-Ru(III) leads to complex D, which is then oxidized upon intramolecular SET. Finally, deprotonation followed by protiodemetalation releases alkylation product 143.3 and regenerates Ru(II) catalyst. Mechanistic studies performed by the two groups confirmed the radical nature of this reaction. In addition, fluorescence quenching studies established the role of alkyl halides as the quencher of the photoexcited complex A (Scheme 143).

11.3. Platinum

While an array of platinum photosensitizers has been developed for various applications, such as photodynamic therapies^374–377 and synthetic chemistry,^378,379 organic transformations wherein the platinum catalyst is directly involved in bond breaking/forming events remain scarce.

In 2004, Tung, Wu, and co-workers disclosed a visible light-induced Pt(II)-catalyzed hydrogen production from Hantzsch ester derivatives 144.1.³⁸⁰ Remarkably, platinum catalyst 144.3 efficiently catalyzed the dehydrogenation with quantitative formation of H₂ at catalyst loading as low as 0.1 mol %. On the other hand, Hantzsch esters with secondary alkyl or benzyl substituent at the 4-position underwent dealkylation, delivering pyridines 144.4 and alkanes 144.5. The proposed mechanism begins with the photoinduced excitation of platinum catalyst 144.3 to its MLCT state, which undergoes intermolecular HAT at the 1-position of substrate 144.1 to produce Pt(III) hydride and N-centered radical A in a solvent cage. A subsequent atom transfer affords pyridine derivative 144.2 or 144.5, and Pt(II)–H₂ or Pt(II)–RH complex B. The latter then regenerates Pt(II) catalyst upon hydrogen or alkane elimination. Deuterium-labeling studies revealed a lower photoluminescence quenching constant for Hantzsch ester deuterated at the 1-position compared to the one at 4-position, thus indicating the first HAT occurs at the nitrogen site (Scheme 144).

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