Metal-Enabled Electrophotocatalysis Oxidative Coupling with Hydrogen Evolution

Key words

electrophotocatalysis - oxidative coupling - C–H functionalization - decarboxylation - metal catalysis

1

Introduction

The American Chemical Society (ACS) introduced the concept of ‘Green Chemistry’, which aims to employ chemical principles and methodologies to minimize or eliminate pollutant emissions at their source, thereby achieving environmental protection objectives.[1] With the development of organic synthesis methodologies in recent years, both light and electricity represent cost-effective, environmentally benign, and sustainable energy sources in synthetic chemistry.[2] The integration of photocatalytic and electrocatalytic strategies to achieve single-electron transfer (SET) processes has revealed innovative and distinctive possibilities for activating small organic molecules.[3]

Photocatalytic organic synthesis has emerged as a pivotal branch of synthetic chemistry owing to its environmentally benign nature and operational efficiency. The core of this methodology involves utilizing photonic energy to photoexcite the photocatalyst, thereby driving organic transformations via electron transfer (ET) or energy transfer (EnT) mechanisms.[2a] [4] In 2008, Yoon and co-workers[5] demonstrated that Ru(bpy)₃Cl₂ serves as an efficient photocatalyst for the [2+2] cycloaddition of alkenes. This is a landmark development in photoredox catalysis. MacMillan and Nicewicz[6] further advanced the field by merging photoredox catalysis with organocatalysis. They achieved asymmetric alkylation of aldehydes by using Ru(bpy)₃ ²⁺ as the photocatalyst. These breakthroughs attracted widespread interest in photoredox-mediated reaction development. In its photoexcited state, the redox ability of the photocatalyst is enhanced. Then SET can occur between the excited photocatalyst and the substrate.[7] This process provides mild reaction conditions for organic synthesis. However, conventional photoredox systems remain constrained by the limited excited-state redox potentials of available photocatalysts.[8]

Electrocatalysis organic synthesis has a history spanning nearly two centuries. It started with Faraday’s[9] investigations on acetic acid transformations and Kolbe’s[10] pioneering work on electrocatalytic decarboxylative dimerization. Electrocatalytic organic synthesis has evolved as an environmentally friendly protocol. Replacing stoichiometric oxidants and reductants with electrical energy aligns with sustainable and green chemistry principles. In electrocatalytic systems, redox reactions occur at electrode interfaces, where electrons serve as the ultimate oxidant and reductant.[11] Modern advances in electrocatalytic organic synthesis over the past decade, spearheaded by Baran, Xu, Lei, Ackermann, Lambert, Lin, and Gouin, among others, have achieved remarkable progress across diverse reaction classes.[3d] ^, [12] [13] [14] [15] [16] [17] The precision of electrocatalytic stems from their ability to modulate reaction pathways through controlling electrode potentials thereby enhancing selectivity for the desired transformation. However, the need for additional potentials to generate highly reactive intermediates can lead to uncontrolled side reactions and compromised chemoselectivity. Therefore, the development of electrocatalysis still has challenges.

To overcome the limitations of sole photo- and electrosynthesis, the burgeoning strategy of organic electrophotocatalysis (EPC),[18] [19] [20] integrating photocatalysis with electrocatalysis, has developed. This innovative catalytic paradigm achieves complementary cooperation while preserving the distinctive characteristics of both approaches.[17] A series of efficient catalytic redox, oxidative, and reductive coupling reactions have been developed. Particularly, oxidative coupling exhibits an important role. On the other hand, metal catalysis plays a crucial role in oxidative coupling systems. It can act as a photosensitizer to directly interact with the substrate, and also can react with organic sensitizers to achieve synergistic catalysis.[20] To further amplify the reactivity and broaden the scope of EPC systems, the integration of metallic catalysts into EPC has garnered significant scientific interest. Such triple catalytic systems have remarkably enhanced the potential of EPC in organic synthesis.[4c] [21] To provide researchers with a systematic understanding of these triple systems, this review comprehensively introduces relevant studies, focusing on three mechanistic paradigms: (1) direct interaction of photocatalysts with substrates; (2) indirect interaction of photocatalysts with substrates; and (3) using the substrate intermediate as a photocatalyst. This article specifically addresses metal-involved EPC systems for oxidative coupling reactions with hydrogen evolution, while redox-neutral strategies fall outside the scope of this discussion.

2

Direct Interaction of Photocatalysts with Substrates

2.1

Photocatalyst-Mediated HAT Process

In the field of EPC, the generation of radical cation via HAT mechanisms by excited state photocatalysts has emerged as a critical strategy for achieving efficient and sustainable organic synthesis. Upon visible-light absorption, the photocatalyst undergoes a transition from the ground state to an excited state. The excited state catalyst directly abstracts a hydrogen atom from the substrate (Figure [1]). This method combines light-driven efficient electron transfer with substrate-selective conversion, and has demonstrated unique advantages in recent advances such as C–H bond activation, olefin functionalization, and complex molecular synthesis.[23]

Figure 1 Photocatalyst-mediated HAT process

In 2020, Lei and co-workers[22] reported a synergistic strategy combining manganese catalysis, organic electrosynthesis, and visible-light photocatalysis to achieve oxidative C(sp³)–H azidation (Figure [2]). This approach employed sodium azide as the azidation reagent. The photocatalyst is excited by blue light irradiation, followed by a HAT process to generate an alkyl radical. The alkyl radical then reacts with the N₃–Mn(III) complex to afford the desired azidation product. Concurrently, anodic oxidation regenerates the photocatalyst with a low-potential. A notable advantage of this methodology lies in its ability to activate C(sp³)–H bonds containing large steric hindrance. A series of tertiary and secondary benzylic C(sp³)–H, aliphatic C(sp³)–H, and drug-molecule-based C(sp³)–H bonds in substrates are also well tolerated. In addition, this approach performed the reaction without the necessity of adding excess substrate and successfully avoided the use of stoichiometric chemical oxidants such as iodine(III) reagent or NFSI. It allowed the reaction to occur under mild conditions.

Figure 2 Manganese-catalyzed oxidative azidation of C(sp³)–H bonds under EPC conditions

Figure 3 EPC asymmetric catalysis enables site- and enantioselective cyanation of benzylic C–H bonds

Figure 4 EPC decoupled radical relay enables highly efficient and enantioselective benzylic C–H functionalization

In any C–H functionalization reactions the control of absolute stereochemistry is a desirable but often a challenging goal. Xu and co-workers[23] reported the first EPC asymmetric catalysis strategy for achieving a regio- and enantioselective benzylic C(sp³)–H cyanation reaction (Figure [3]). This approach uses the excited state anthraquinone-2,7-disulfonate (AQDS) catalyst as a HAT reagent. The alkyl radical is trapped by electrochemically generated Cu(II) species, enabling stereoselective C–C bond formation. The catalytic cycle is closed through electrochemical reoxidation of the photocatalyst by using low-potential anodic oxidation instead oxidants. This strategy effectively minimized the over-oxidation of electron-rich substrates and exhibited an exceptional level of functional group tolerance. Notably, substrates containing higher electron density and lower steric hindrance demonstrated preferential reactivity toward cyanation. It enabled the efficient conversion of feedstock chemicals and the late-stage functionalization of complex bioactive molecules and natural products, including those with multiple benzylic sites.

Subsequently, Liu and co-workers[24] advanced the methodology for enantioselective benzylic C–H cyanation by using a similar photoexcited HAT strategy (Figure [4]). By tuning the applied current and the electronics of the anthraquinone (AQ) photocatalyst, the rates of benzylic radical formation and generation of Cu(II) can be matched, which reduces undesired reactivity. They achieved high selectivity across a broad substrate scope including electron-poor and electron-rich benzylic C(sp³)–H bonds. This protocol also exhibited excellent functional group tolerance for the synthesis of complex molecular architectures, such as celecoxib analogue 14, fenazaquin-containing quinazoline 15, and celestolide (16).

2.2

Photocatalyst-Promoted Decarboxylation

Carboxylic acids, presented in pharmaceutical molecules and natural products, have attracted sustained academic interest in their selective functionalization.[25] Alkyl acids usually undergo decarboxylation to generate a carbon-centered radical that can participate in further transformations (Figure [5]). This significantly expands the application scope of carboxylic acids in organic synthesis. The electrochemical decarboxylation was pioneered by Kolbe in the 1840s, and this was regard as the oldest decarboxylative reaction.[26] Despite this historical foundation, the reactivity of carbon radicals generated by decarboxylation remains limited by their propensity for dimerization[27] and over-oxidation to carbocations via electron transfer.[28] Recently, Xu,[29] [30] Fu,[31,32] Wang,[33] and Zhang,[34] among others, have successfully adapted decarboxylative radical cross-coupling using mild EPC conditions. This advancement has enabled direct asymmetric decarboxylative functionalization of carboxylic acids.

Figure 5 Photocatalyst-promoted decarboxylation

In 2020, Xu and colleagues[29] reported a decarboxylative alkylation of electron-poor arenes (Figure [6]). This strategy enabled the generation of alkyl radicals from inactive alkanes under oxidant-free conditions. It was also applied in the carbamoylation of heteroarenes. Under photoirradiation, alkyl carboxylate anions undergo oxidation by Ce(IV) via a ligand-to-metal charge-transfer (LMCT) process, leading to decarboxylation and generating an alkyl radical that alkylates quinoline derivatives by a Minisci mechanism. The Ce catalyst is regenerated via anodic oxidation to finish the catalytic cycle. The developed protocol exhibits broad substrate scope, while exhibiting excellent tolerance toward diverse N-heterocycles, including several pharmaceutical molecules. They found that when the RVC anode was replaced with a graphite stick, the yield as well as conversion diminished. This showed the importance of a large electrode surface, which is accessible by light in this type of reaction.

Figure 6 EPC decarboxylative C–H functionalization of heteroarenes

Wang and co-workers[33] extended this decarboxylative strategy by using earth-abundant iron as the metal catalyst to achieve the LMCT-enabled EPC decarboxylative C–H alkylation of quinoxalin-2(1H)-ones (Figure [7]). This strategy uses iron’s LMCT capability to generate alkyl radicals through decarboxylation under light irradiation. The radical engages in C–H functionalization of quinoxalinones. This method also had broad compatibility with diverse carboxylic acids and can maintain excellent functional group tolerance.

Figure 7 EPC decarboxylative C–H alkylation of quinoxalin-2(1H)‑ones

Inspired by related studies, in 2022, Fu and co-workers[31] developed a Ce-Mn bimetallic co-catalytic strategy for the decarboxylative azidation of aliphatic carboxylic acids (Figure [8]). This strategy circumvented the need for chemical oxidants and azide transfer reagents. The reaction involves the participation of the carbon-centered radical through decarboxylation. The azide anions coordinate with Mn(II) species, then undergo electrochemical oxidation to form Mn(III) intermediates species that react with the alkyl radical to give the desired product and regenerate a Mn(II) species. Owing to the mildness of this catalytic system, electron-rich substrates delivered high yields. Notably, the methodology exhibits potential for decarboxylative azidation of drug molecules.

Figure 8 EPC decarboxylative azidation of aliphatic carboxylic acids

Figure 9 EPC asymmetric catalysis enables direct and enantioselective decarboxylative cyanation

Building upon similar strategies, Xu and co-workers[30] achieved enantioselective decarboxylative cyanation through Ce-Cu bimetallic co-catalysis (Figure [9]). This dual catalytic system employed cerium salts to mediate the decarboxylative process and leveraged chiral copper complexes to control stereoselectivity. The proposed mechanism involves several steps. First, Ce(OTf)₃ undergoes anion exchange to form CeCl₆ ^3– in the presence of chloride ions, followed by oxidized and photoinduced LMCT to drive decarboxylation The generated benzyl radical attacks the Cu(II) complex to form an active Cu(III) species. Finally, reductive elimination occurs to deliver the stereocontrolled cyanation product. This method efficiently converted carboxylic pharmaceutical molecules into enantioenriched nitriles under mild conditions, as exemplified by successful transformations of loxoprofen (33), naproxen (34), zaltoprofen (35), and pranoprofen (36).

In 2023, Zhang and co-workers[34] used CeCl₃ and Cu/BOX as co-catalysts to facilitate decarboxylation and cyanation. Both catalysts were regenerated through anodic oxidation. With the addition of ligands, they successfully constructed stereoselective cyanides. Concurrently, Fu and co-workers[32] reported a similar EPC protocol for direct asymmetric decarboxylative cyanation (Figure [10]). They performed DFT calculations on the C–C reductive elimination to elucidate the role of the BOX in enantioselectivity. This environmentally friendly protocol efficiently converted diverse aryl acids into the corresponding nitriles, exhibiting excellent yields, high enantioselectivity, and broad functional group tolerance.

Figure 10 Cu-catalyzed enantioselective decarboxylative cyanation via the synergistic merger of photocatalysis and electrochemistry

2.3

Photocatalyst-Mediated SET Process

Electron-rich aromatic compounds, which typically exhibit low oxidation potentials, can undergo oxidation via EPC processes to generate radical cation intermediates. These intermediates are regard as electrophilic centers that attract nucleophilic reagents to engage directly with the aromatic core, thereby initiating C–H functionalization (Figure [11]). This newly formed radical has a strong tendency to re-aromatize to build an aromatic ring, which enabling direct C–H functionalization of aromatic skeleton through this coupled oxidation mechanism.

Figure 11 Photocatalyst-mediated SET process

In 2023, Xu and Lai[35] reported an EPC strategy for enantioselective heteroaryl cyanophoric difunctionalization of alkenes (Figure [12]). This strategy used an acridinium salt and a chiral copper complex as co-catalysts. Photoexcitation of the acridinium salt generates its excited state, which oxidizes the heteroarene to form a radical cation intermediate. The radical cation regioselectively attacks the terminal position of the arylalkene, generating a benzylic radical cation. The benzylic radical cation then engages with the chiral copper catalyst to enable the efficiently conversion of the arylalkene into an enantioenriched nitrile. The catalytic cycle is closed through electrochemical reoxidation of both the acridinium salt and copper catalyst. This process was a rare example of heteroaromatic radical cation-mediated asymmetric catalysis, which combined the advantages of photoredox catalysis and asymmetric electrocatalysis. this method facilitates the formation of two C–C bonds while circumventing the need for external chemical oxidants, thus provide a novel strategy for the synthesis of chiral organic molecules.

Figure 12 EPC asymmetric catalysis enables enantioselective heteroarylcyanation of alkenes via C–H functionalization

3

Indirect Interaction of Photocatalysts with Substrates

Metal-chloride complexes, such as Fe(III) and Ce(IV), exhibit photochemical instability. Under specific wavelength irradiation, a LMCT process occurs.[36] This process releases a highly reactive chlorine radical species that promotes indirect HAT with the substrate. Then, the carbon radical can construct complex compounds through addition reactions, coupling reactions, and others (Figure [13]). This strategy offers alterative reaction pathways distinct from direct interaction of photocatalysts with substrates. Compared with traditional photocatalysis, photoexcitation instantaneously produces active chlorine radicals, and significantly shortens the excitation time. This strategy has been successfully applied to EPC organic synthesis.

Figure 13 Indirect interaction of photocatalysts with substrates

Organosilanes are key components in medicinal chemistry and molecular materials, and are also widely used as multifunctional intermediates in organic synthesis. In 2023, Ackermann and co-workers[37] reported an EPC iron-catalyzed silylarylation of alkenes (Figure [14]). Mechanistic investigations revealed that the Fe(III) complex undergoes photoinduced generation of a chlorine radical that acts as a HAT reagent to activate the silane Si–H bond. The silane-centered radical undergoes addition to acrylamide to give a silicon-substituted indole, while anodic electro-oxidation ensures regeneration of the iron catalyst. Notably, the photoinduced LMCT of Fe(III) complexes and the HAT process enabled the radical-polarity-matched Si–H and Ge–H activation, bypassing the comparable redox potential of Si/Ge–H and C–H bonds. This strategy offered a new opportunity to selectively synthesize variety of Si-incorporated oxindoles with excellent chemo- and regioselectivity.

Figure 14 EPC Si–H and Ge–H activation by iron catalysis

In 2023, Nöel and co-workers[38] extended halogen radical-mediated HAT processes to achieve the C–H amination of tetrahydrofuran (Figure [15]). They reported accelerated electrophotocatalytic C(sp³)–H heteroarylation achieved using iron(III) chloride as a catalyst in an efficient continuous-flow reactor setup. This reaction produces carbon-centered α-oxyalkyl radicals through a similar LMCT and HAT process. This radical is electrochemically oxidized to a stable electrophilic carbon cation, which is trapped by a nucleophilic reagent to form the desired C–N bond. This new flow reactor concept simultaneously accommodated photons and electrons in the microchannel allowing for the handling of transient species. The electrophotochemical heteroarylation occurred at room temperature, demanded no external oxidants, and ensured short reaction times, enhancing productivity.

Figure 15 Accelerated EPC C(sp³)–H heteroarylation enabled by an efficient continuous-flow reactor

In 2024, Lu and co-workers[36] achieved C(sp³)–H boronation using a FeCl₃-induced HAT strategy, which extending the substrate range to aliphatic alkanes (Figure [16]). Mechanistic studies showed that B₂cat₂ is reduced to produce radical anions at the cathode. The boron radical anion couples with reactive alkyl radicals to deliver the desired C(sp³)–H borylation products. In addition, the alkyl carbon radical can also react with B₂cat₂, affording the borylated product with H₂ evolution on the cathode, and the generated boryl radical generated is quenched by oxidation or reduction during the reaction. The strategy demonstrated remarkable steric site selectivity, enabling selective borylation of terminal alkanes. It was also compatible with diverse methylsilane substrates, where borylation preferentially occurred at α-silyl C(sp³)–H bonds. Ackermann and co-workers reported analogous strategies to achieve EPC borylation of germanium- and stannane-containing substrates.[37]

Figure 16 EPC driven iron-catalyzed C(sp³)–H borylation of alkanes.

Cerium with +3 and +4 oxidation states is also widely used in EPC.[39] Zeng and co-workers reported a strategy for the construction of nitrogen-containing polycyclic compounds using EPC by applied cerium catalysts (Figure [17]). This method generated a chlorine radical by LMCT as a HAT reagent, and activated aliphatic C(sp³)–H as the radical donor. Then, this radical underwent radical addition/cyclization cascaded with alkene. By using this method, a variety of alkylated benzimidazo-fused isoquinolinones and other N-containing polycycles were synthesized with high efficiencies under external oxidant-free conditions. Compared with previous reports for the construction of these polycycles from carboxylic acids, alkylboronic acids,[40] NHPI esters,[41] or Katritzky salts,[42] this electrophotocatalytic strategy features high step- and atom-economy.

Figure 17 Electrophotoredox/cerium-catalyzed unactivated alkanes activation for the sustainable synthesis of alkylated benzimidazo-fused isoquinolinones

4

Substrate Intermediate Used as a Photocatalyst

In early studies, the Yoon group[43] discovered that chiral Lewis acid catalyzed accelerate visible-light photoinduced, electron-transfer, [2+2] cycloadditions of unsaturated carbonyls. Building upon this precedent, Meggers and co-workers[44] reported combines photoelectrochemistry with asymmetric catalysis to achieve enantioselective dehydrogenative [2+2] photocycloaddition between alkyl ketones and alkenes under rhodium (Rh) catalysis (Figure [18]). This strategy combined EPC with Rh-catalyzed asymmetric synthesis. This strategy enabled the construction of up to four consecutive stereocenters by simultaneously activating two C(sp³)–H bonds and two carbon centers. The use of a robust chiral Lewis acid, which catalyzed the dehydrogenation of ketones and the photocycloaddition, resulted in the asymmetric induction. This EPC asymmetric method had broad utility in the construction of complex molecules. It was successfully applied to the synthesis of the chiral natural product melicoptine C.

Figure 18 EPC asymmetric dehydrogenative [2+2] cycloaddition between C–C single and double bonds via the activation of two C(sp³)–H bonds

5

Summary

In this review, we highlight recent advancements in metal-enabled EPC reactions for oxidative cross-coupling reactions with hydrogen evolution. Furthermore, the integration of metallic species into EPC platforms demonstrates their remarkable potential. The synergy of light and electrical energy not only ensures mild reaction conditions but also circumvents the need for stoichiometric oxidants. These methodologies have exhibited exceptional performance in C–H activation, decarboxylation, and cross-coupling reactions. While the synergistic EPC approach is still in its infancy, it holds immense promise for advancing organic synthesis through sustainable and atom-economical pathways.

Current research has demonstrated the high reactivity and selectivity of EPC, yet realizing its full potential in organic synthesis faces significant challenges. First, existing EPC strategies rely on photocatalysts as electron-transfer mediators. But the scarcity of available photocatalyst types coupled with their high cost restricts practical scalability. Second, current EPC methodologies predominantly depend on custom-built reaction systems, limiting their application to large-scale industry syntheses. Third, the mechanistic understanding of EPC reactions remains contentious. For SET-driven processes, evaluating the lifetimes of substrate and catalyst excited states is critical. However, the transient nature of short-lived intermediates complicates experimental characterization, leaving the dynamics of excited organic radical ions largely unexplored. Future progress should focus on the development of robust mechanistic tools and the design of tailored reactor systems capable of enabling complex, previously inaccessible transformations. These advancements will expand EPC applications, offering vast opportunities for sustainable and precise organic synthesis.

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