Mechanistic Insights into Light-Driven Acceptorless Dehydrogenation Reactions

Abstract

Dehydrogenation reactions are thermodynamically constrained by their inherent endothermic nature. The concomitant thermodynamic barriers can be overcome via photochemical strategies that harness light to activate intrinsically strong CO, CH, and OH bonds. This review surveys recent progress and challenges in acceptorless light-driven dehydrogenation reactions, focusing on dehydrogenation of linear sp³-hybridized bonds, dehydrogenative coupling reactions, dehydrogenative cyclizations, and dehydrogenation of cyclic hydrocarbons. We identified distinct trends in the catalysts used for light-driven dehydrogenations, including homogeneous unary photocatalysts in which a single molecule absorbs light and catalyzes both oxidation and hydrogen evolution, cooperative homogeneous systems in which two separate catalysts fulfill these roles, as well as heterogeneous systems including nanostructured semiconductors and hybrid materials. In particular, this work uniquely synthesizes mechanistic knowledge across these classes and introduces a unifying classification framework that clarifies how distinct photochemical mechanisms achieve bond activation and hydrogen evolution without external acceptors. First, homogeneous unary photoactive Rh­(I) catalysts promote dehydrogenation of both linear and cyclic sp³-hybridized C–C bonds in hydrocarbon substrates via oxidative C–H addition with subsequent β-hydride elimination. Second, binary homogeneous photocatalytic systems, consisting of a photosensitizer and a transition-metal-based proton reduction catalyst, enable all four types of dehydrogenation reactions via SET. Third, heterogeneous catalysts employed in light-driven dehydrogenation reactions often comprise a semiconductive support material integrated with a transition-metal-based active site, functioning via Mott–Schottky type photoinduced charge separation.

1. Introduction

The inherently endothermic nature of dehydrogenation reactions imposes fundamental thermodynamic constraints on these processes. The hard-to-overcome thermodynamic barriers are typically unaffected by catalysts, thereby necessitating elevated reaction temperatures or pressures. Previous studies have explored photochemical approaches to modulate the thermodynamics of endothermic reactions by enabling the activation of inherently strong CO, C–H, and O–H bonds using light. − Through photoexcitation, the energy of a system is increased, enhancing the thermodynamic favorability of a reaction by opening new pathways within the free energy landscape. In this work, we review the different types of light-driven dehydrogenation reactions reported in the literature, providing an overview of the current state of the art.

Classical thermal dehydrogenation reactions are widely applied in the chemical industry. In 1911, the Russian chemist Nikolay Zelinsky first reported this reaction, demonstrating the conversion of cyclohexane to benzene upon heating to 300 °C in the presence of Pt or Pd catalysts. Later, such thermal dehydrogenation reactions were adopted in the petrochemical industry to produce aromatic compounds from petroleum gas. Additionally, dehydrogenation reactions are employed to convert alkanes into olefins, alcohols into aldehydes and ketones, and formic acid into CO₂ (Scheme ). −

1. Examples of Classical Thermally Activated Dehydrogenation Reactions, Being a) Alkane Dehydrogenation, b) Alcohol Dehydrogenation, and c) Formic Acid Decomposition − .

In addition to high energy demands, the extreme temperatures required for thermal dehydrogenation reactions introduce further disadvantages. Exposure to such high heat can result in low conversion, poor selectivity, and the formation of undesired byproducts that are difficult to separate. − Furthermore, catalysts are prone to deactivation due to coke formation. From a financial standpoint, both reactor and maintenance costs are typically high, as the equipment must withstand temperatures exceeding 500 °C. ,

Owing to unfavorable thermodynamics, dehydrogenation reactions typically require harsh conditions and noble-metal catalysts to promote selective C–H and X–H (X = N, O) bond cleavage while suppressing undesired C–C or X–C (X = N, O) bond scission and associated side reactions. ,, According to Weckhuysen and co-workers, dehydrogenation of C₂–C₄ paraffins generally requires temperatures of 550–750 °C (1 bar) to achieve conversions exceeding 50%. Consistently, industrial heterogeneous dehydrogenation processes operate at temperatures between 500 and 900 °C, resulting in high energy demand and poor selectivity. From a mechanistic standpoint, selective activation of saturated C–H bonds is challenging because competing bonds and functional groups can be more reactive, and elevated temperatures accelerate both desired and undesired pathways.

Oxidants or external acceptors can be used to lower the temperatures required for dehydrogenation reactions. In such catalytic transfer dehydrogenation reactions, a saturated alkane is dehydrogenated by an unsaturated alkene through the transfer of one equivalent of dihydrogen (Scheme a). In pioneering work, Crabtree and co-workers employed cationic iridium­(III) catalysts to dehydrogenate cyclic alkanes and monoalkenes using 3,3-dimethyl-1-butene as external hydrogen acceptor. Jensen and co-workers later developed effective Ir and Rh­(PCP) pincer complexes capable of achieving high turnover in catalytic transfer dehydrogenations at 200 °C, owing to the excellent thermal stability of the pincer ligand. Over time, several iridium complexes bearing PCP pincer analogues have followed, including a POCOP–Ir pincer complex with oxygen atom linkers, a PSCOP-type Ir complex supported by a hybrid phosphinothious/phosphinite pincer ligand with S/O linkers, and a POCNP system featuring an N/O linker combination (Scheme a). ,, Although catalytic transfer dehydrogenations can be used to selectively dehydrogenate alkanes under relatively mild conditions, the requirement for a sacrificial hydrogen acceptor or oxidant results in significant waste and poor atom economy. Alternatively, acceptorless dehydrogenation liberates hydrogen gas through cleavage of C–H and X–H (X = N, O) bonds without the need for an external hydrogen acceptor or oxidant.

2. a) Catalytic Transfer Dehydrogenation, b) Organometallic Acceptorless Dehydrogenation of Alkanes, c) Acceptorless Dehydrogenation via Nature-Inspired Cooperative Base Metal Catalysis ,,− .

Acceptorless organometallic dehydrogenation employs noble metal catalysis to efficiently dehydrogenate alkanes, with dihydrogen as the sole byproduct (Scheme b). This process is conducted at elevated temperatures without external sacrificial electron or hydrogen acceptor, resulting in high energy input but minimal waste generation. Throughout the 1980s, Crabtree and co-workers as well as Goldman and co-workers introduced thermal and photochemical strategies for the selective acceptorless dehydrogenation of alkanes to alkenes with concomitant dihydrogen evolution, relying on Ir and Rh noble metal catalysts for their effectiveness in C–H bond activation. , The addition of light energy reduced the required reaction temperature for the thermal processes from 150 °C to 25–50 °C. ,

Photochemistry is a subdiscipline of chemistry that leverages the absorption of ultraviolet or visible light to drive chemical transformations. Photocatalysis, in turn, is a branch of photochemistry that employs light-activated photocatalysts to lower activation energies and facilitate reactions. , Photochemical dehydrogenation is widespread in nature, as plants and prokaryotic bacteria can harness light energy. For example, protochlorophyllide oxidoreductase (POR) is a photoactive dehydrogenase enzyme involved in light-driven biosynthesis of chlorophyll in bacterial cells (Scheme ). Within the cell, POR efficiently utilizes solar energy to catalyze the biotransformation of Mg-protoporphyrin IX into chlorophyll. Inspired by nature’s ability to perform light-driven reactions, − researchers are increasingly exploring photochemical processes across diverse subfields, including inorganic chemistry, organic chemistry, biochemistry, supramolecular chemistry, and computational chemistry. − Within the field of dehydrogenation reactions, photoinduced enzymatic dehydrogenations observed in nature have inspired West, Huang, and Sorensen to develop binary cooperative organic/base metal catalysis as a strategy for achieving selective acceptorless dehydrogenations under mild conditions, without the need for rare earth elements (Scheme c).

3. Mechanistic Pathway for the Light-Driven Biosynthesis of Chlorophyll Using a Photoactive Protochlorophyllide Oxidoreductase (POR) Enzyme .

In general, the literature distinguishes between two mechanistic classes of homogeneous acceptorless dehydrogenation, with homogeneous referring to the phase of the photocatalyst during the reaction. The first involves organometallic acceptorless dehydrogenation, which proceeds through a consecutive three-step sequence consisting of (i) oxidative addition, (ii) subsequent β-hydride elimination, and (iii) reductive elimination to release dihydrogen gas. The mechanisms accompanying nature-inspired cooperative base metal catalysis proceed via successive hydrogen atom transfer (HAT) steps, where the first catalytic HAT cleaves a strong saturated bond, and a second catalyst initiates a subsequent HAT to liberate dihydrogen gas. Alternatively, light-driven acceptorless dehydrogenations can be performed with photoactive heterogeneous catalysts. Under both homogeneous and heterogeneous conditions, the role of light is to activate the saturated substrates and alter the thermodynamic profile of the reaction, thereby opening energetically favorable reaction paths.

The success of a photochemical reaction can be assessed by its quantum yield (eq ). Quantum yield is defined as the number of molecules undergoing the photochemical process divided by the number of photons absorbed by the reactant. Analyzing quantum yields of photochemical reactions can provide insights into efficiencies, reaction mechanisms, and potential competing pathways. Moreover, quantum yield studies enable reaction optimization and kinetic modeling. , In addition to quantum yield, photochemical reactions can be investigated using steady-state fluorescence spectroscopy, transient absorption spectroscopy, time-resolved spectroscopy, and actinometry. − General analytical techniques such as NMR, GC, HPLC, and MS can also be effectively integrated with these methods for comprehensive analysis. Furthermore, cyclic voltammetry can be used to elucidate photoredox mechanisms by measuring redox potentials.

$$\varphi =\frac{\mathrm{n}\mathrm{u}\mathrm{m}\mathrm{b}\mathrm{e}\mathrm{r}\mathrm{o}\mathrm{f}\mathrm{m}\mathrm{o}\mathrm{l}\mathrm{e}\mathrm{c}\mathrm{u}\mathrm{l}\mathrm{e}\mathrm{s}\mathrm{o}\mathrm{f}\mathrm{p}\mathrm{r}\mathrm{o}\mathrm{d}\mathrm{u}\mathrm{c}\mathrm{t}\mathrm{f}\mathrm{o}\mathrm{r}\mathrm{m}\mathrm{e}\mathrm{d}}{\mathrm{n}\mathrm{u}\mathrm{m}\mathrm{b}\mathrm{e}\mathrm{r}\mathrm{o}\mathrm{f}\mathrm{p}\mathrm{h}\mathrm{o}\mathrm{t}\mathrm{o}\mathrm{n}\mathrm{s}\mathrm{a}\mathrm{b}\mathrm{s}\mathrm{o}\mathrm{r}\mathrm{b}\mathrm{e}\mathrm{d}\mathrm{b}\mathrm{y}\mathrm{r}\mathrm{e}\mathrm{a}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{v}\mathrm{e}\mathrm{m}\mathrm{e}\mathrm{d}\mathrm{i}\mathrm{u}\mathrm{m}}$$ 1

Previous reviews on light-driven endothermic reactions have focused on photochemical CO₂ activation, C–H activation and functionalization, water splitting, and various types of rearrangement reactions. − In 2021, Xu and co-workers published a comprehensive review on photoactive semiconductive materials that enable oxidative organic synthesis accompanied by the release of dihydrogen gas. In 2022, Verma published a valuable review on light-driven dehydrogenation strategies, highlighting the growing interest in photochemical dehydrogenations.

In the present review, we advance beyond these earlier overviews by focusing exclusively on acceptorless light-driven dehydrogenation reactions and by integrating mechanistic understanding across both homogeneous and heterogeneous systems. This review uniquely consolidates the rapidly expanding literature (2010–2025) into a comparative framework covering four major reaction classes with increasing molecular complexity: (i) dehydrogenation of linear sp³-hybridized bonds (Chapter 2), (ii) dehydrogenative coupling reactions (Chapter 3), (iii) dehydrogenative cyclizations (Chapter 4), and (iv) dehydrogenation of homo- and N-heterocyclic hydrocarbons (Chapter 5) (Scheme ).

4. Representative Reactions of Four Different Types of Light-Driven Dehydrogenations: a) Dehydrogenation of Linear sp³-Hybridized Bonds, b) Dehydrogenative Coupling, c) Dehydrogenative Cyclization, d) Dehydrogenation of Cycloalkanes − .

2. Mechanisms of Photocatalytic Dehydrogenation of Linear sp³-Hybridized Bonds

Olefins are key building blocks in the chemical industry for the production of value-added chemicals such as solvents, polymers, fine chemicals, and therapeutics. − They are typically synthesized via catalytic or steam cracking of linear alkanes derived from oil, gas, or naphtha. Short-chain linear olefins, particularly ethylene and propylene, are of special interest, with annual production capacities estimated at 150 and 80 million tons, respectively. These processes account for approximately 300 million tons of annual CO₂ emissions, primarily due to the endothermic nature of the reaction. Given their central role in the chemical industry and the scale of the market, sustainable olefin synthesis has garnered growing academic interest. , Photocatalytic methods have emerged as a promising strategy to reduce the carbon footprint of olefin synthesis, with the first reports dating back to 1988. This interest later expanded to include the photocatalytic dehydrogenation of other linear sp³-hybridized bonds, such as C–O, C–N, and N–N, which are relevant to CO₂ activation and pharmaceutical synthesis.

2.1. Homogeneous Photocatalytic Dehydrogenation of Linear C–C Bonds

In 1988, Nomura and Saito reported the first homogeneous photocatalytic dehydrogenation of a carbon–carbon bond in linear alkanes (Scheme ). Using a trans-Rh^ICl­(CO)­(PR₃)₂ complex (1) and irradiation at wavelengths >340 nm, heptane and octane were dehydrogenated to their corresponding linear unsaturated alkene derivatives. The R-group in the rhodium complex (1) influenced catalytic activity according to the trend PMe₃ > PEt₃ > PPh₃, suggesting that bulky ligands hinder catalytic performance. Moreover, methyl-substituted phosphines appear to enhance catalyst stability, as a maximum turnover frequency (TOF) of 795 h^–1 was achieved using trans-Rh^ICl­(CO)­(PMe₃)₂ for the dehydrogenation of heptane at 92 °C, compared to a TOF of only 136 h^–1 for trans-Rh^ICl­(CO)­(PPh₃)₂ at 82 °C. Unfortunately, detailed information regarding the exact irradiation wavelength, selectivity, product yield, reaction time, and mechanistic pathway was not provided in the original study.

5. Photochemical C–C Bond Dehydrogenation of Linear Alkanes with a Homogeneous Rhodium-Based Catalyst at 92 °C .

A few decades later, in 2014, Beller and co-workers proposed the first plausible reaction mechanism for light-driven rhodium-catalyzed C–C bond dehydrogenation of n-octane (Scheme ). The trans-Rh^ICl­(CO)­(PMe₃)₂ photocatalyst (2) is excited upon irradiation at wavelengths between 320 and 500 nm. The resulting excited-state complex (2*) reacts with n-octane to form the oxidatively added intermediate complex Rh^IIICl­(CO)­(PMe₃)₂(H)­(1-octyl) (3). The rhodium center then undergoes β-hydride elimination to produce the octene product, along with a hydridic Rh^IIICl­(CO)­(PMe₃)₂(H)₂ complex (4). Finally, this hydridic intermediate undergoes reductive elimination to release dihydrogen gas and regenerate the initial trans-Rh^ICl­(CO)­(PMe₃)₂ complex (2). Interestingly, the addition of CO₂ gas or ethyl formate inhibits formation of catalyst-deactivating Rh₂P nanoparticles, thereby preventing catalyst poisoning. Moreover, the inclusion of dinitrogen additives such as 4,4′-bipyridine enhances catalytic activity, increasing the turnover number (TON) from 1459 to 1666 after 14 h, with high product yields. Lastly, side reactions resulting from moisture in air can be suppressed by introducing argon gas.

6. Proposed Mechanism for Photocatalytic Dehydrogenation of a C–C Bond in n-Octane Using a Homogeneous Rhodium-Based Catalyst at 85 °C .

Later, Cartwright and Tunge in 2018 explored whether N-acyl amino acids could undergo decarboxylative elimination coupled with the evolution of dihydrogen gas to form enamides (Scheme ). The radical reaction was carried out at room temperature using a 32 W blue LED and a binary cobalt photocatalytic system. In this context, the photoredox catalyst Mes-2,7-Me₂-Acr-Ph⁺BF₄ ^– (E _1/2 ox = +2.09 V vs SCE) (5), a member of the Fukuzumi acridinium salt family, was selected due to its efficiency in oxidizing the α-amino carboxylate (E _1/2 ox = +0.95 V vs SCE) and compatibility with the reduction potential of Co­(III) to Co­(II) (E _1/2 red = −0.57 V vs SCE), enabling regeneration of the Co^III(dmgH)₂ClPy catalyst (E _1/2 red = −0.68 V vs SCE) (6). The proposed reaction mechanism comprises four steps. First, two Co­(I) equivalents deprotonate the carboxylic group of the amino acid (7), forming an α-amino carboxylate intermediate (8). Second, Co­(III)–H is reduced to Co­(II)–H via two single-electron transfer (SET) events from the excited photoredox catalyst (5) and the α-amino carboxylate intermediate (8). In the third step, the resulting α-amino carboxylate radical intermediate (9) undergoes decarboxylation to release CO₂, and the β-hydrogen of the α-amino radical intermediate (10) is abstracted by Co­(II)–H via a hydrogen atom transfer (HAT) process, forming the final alkene product (11). Upon release of dihydrogen gas, the binary photocatalytic cycle is reinitiated.

7. Proposed Mechanism for Photochemical C–C Bond Dehydrogenation of Functionalized Linear Alkanes with a Binary Catalytic System Consisting of an Acridinium Salt Photosensitizer and a Cobalt Proton Reduction Catalyst .

Notably, Cartwright and Tunge state that the decarboxylative elimination is mechanistically coupled to dihydrogen evolution but they do not address the potential effects of the resulting CO₂ on the photocatalytic activity or whether, as observed by Beller and co-workers, it promotes catalysis. It was demonstrated that the addition of an electron-withdrawing group at the para-position of the aryl ring in R¹ (e.g., CN) increases the yield of the main product to as much as 85%, with an accompanying E:Z selectivity ratio of 81:19 for the aforementioned reaction (Scheme ). Furthermore, this mechanism exemplifies cobalt’s strong propensity to acquire a hydrogen atom via HAT. Moreover, photoredox catalyst (5) enables the conversion of N-acyl amino acids to enamides in yields of up to 95%, highlighting Fukuzumi acridinium salts as a promising class of noble metal-free photosensitizers for light-induced dehydrogenation of functionalized linear alkanes.

In 2021, Huang and co-workers employed a similar cobaloxime-based binary photocatalytic system to homogeneously catalyze the dehydrogenation of functionalized linear alkanes, using 2-ClAQ (12) as photoredox catalyst instead of Fukuzumi acridinium salts (Scheme ). This radical reaction proceeds at room temperature under irradiation with a 20 W LED. The electronegative oxygen atoms in photocatalyst 2-ClAQ (12) stabilize its radical form. In addition, dmgH ₂ (14) was introduced as a stabilizer to enhance the stability of the cobaloxime radical trapper Co^II(OAc)₂·4H₂O (13) in the binary photocatalytic system. The reaction unfolds in four key steps. First, the photocatalyst (12) is excited to a biradical triplet state (15), which abstracts a hydrogen atom from the linear alkane substrate (16) via irreversible HAT, forming an alkyl radical intermediate (17) and a ketyl radical intermediate (18). Next, the reduced Co­(II) species undergoes oxidative binding with the alkyl radical (17) to afford the Co­(III)–alkyl complex (19). Upon photoexcitation, this Co­(III)–alkyl complex promotes β-hydride elimination via a second, reversible HAT step, yielding the olefin product. Finally, Co­(III)–H undergoes SET with the ketyl radical (18) to form Co­(II)–H, which subsequently reacts with protonated photocatalyst 2-ClAQH⁺ (20) to release dihydrogen gas.

8. Proposed Mechanism for Photochemical C–C Bond Dehydrogenation of Linear Alkanes with a Binary Homogeneous Photocatalytic System Consisting of an Acridinium Salt Photosensitizer and a Cobalt-Based Proton Reduction Catalyst .

Overall, the catalytic system developed by Huang and co-workers is effective with a variety of functionalized substrates, including aryl and heteroaryl alkanes, thioethers, ethers, N-aliphatic amides, and carbonyl compounds. The relative reactivity of these functional groups follows the order: thioether > amide > aryl alkane > ether ≈ cycloalkane. When two functional groups are present in the linear alkane, the side bearing the more electron-rich group is dehydrogenated first, as the HAT process is enhanced by electronic effects. Taken together, the studies by Huang and co-workers and Cartwright and Tunge highlight that binary, noble metal-free photocatalytic systems can promote selective and oxidant-free dehydrogenation of functionalized linear alkanes under mild conditions. This was further validated in 2024, when Wendlandt and co-workers reported the synthesis of a range of noncanonical amino acids through terminal selective dehydrogenation of aliphatic amino acids, employing a catalytic system comprising Na₄W₁₀O₃₂ as photocatalyst and Co­(dmgH)­(dmgH₂)­Br₂ for dihydrogen evolution, which afforded yields of up to 86% for leucine derivatives and up to 91% for β-amino acid derivatives under 390 nm LED irradiation in acetonitrile at room temperature over 24 h.

Despite these advances, several challenges remain in the homogeneous light-driven C–C bond dehydrogenation of linear alkanes. For example, Huang and co-workers observed an unfavorable interaction between the excited-state photocatalyst and the cobalt catalyst, which inhibited catalytic performance at elevated cobalt concentrations. In addition, low selectivity toward the E-alkene was observed, despite its higher thermodynamic stability relative to the Z-isomer. Efforts to rationalize this observation remained inconclusive.

2.2. Heterogeneous Photocatalytic Dehydrogenation of Linear C–C Bonds

The first heterogeneously catalyzed, light-driven C–C bond dehydrogenation was reported by Wang and co-workers in 2024. The authors successfully demonstrated the light-driven dehydrogenation of ethane using a platinum-on-zinc oxide (Pt/ZnO) Mott–Schottky-type photocatalyst (Scheme ). Mott–Schottky-type photocatalysts consist of a semiconductor support containing transition-metal active sites, where photon energies equal to or greater than the bandgap of the semiconductor generate electron–hole pairs. The Schottky barrier enhances the rate of proton reduction by suppressing charge recombination, thereby extending the lifetimes of conduction-band electrons and valence-band holes (Figure ). ,

9. Proposed Mechanism for Photochemical C–C Bond Dehydrogenation of Ethane to Ethene over Pt/ZnO Nanoparticles .

1.

a) The common hole–electron transfer mechanism of the heterogeneous C–O bond light-driven endothermic dehydrogenation reactions with the band gap energy of different semiconductors and b) the value of the band gap energy of different functional groups. Reprinted with permission from Verma, p. 4. Copyright 2022 Elsevier. Adapted from Xu and co-workers, p. 13055. Copyright 2021 American Chemical Society.

According to Wang and co-workers, the Pt/ZnO catalyzed dehydrogenation of ethane proceeds via a Mars–van Krevelen mechanism, with the Pt/ZnO photocatalyst achieving a high substrate-to-product conversion rate of 867.8 μmol·h^–1·g^–1 and a product selectivity of 98%. In the proposed mechanism, the Pt/ZnO photocatalyst (21) is photoactivated, generating a hole in the valence band of the oxygen site while the corresponding electron is transferred to the conduction band of the Pt metal in the ethane-coordinated transition state complex (22). This process yields an activated oxygen radical species (O^•–) on the ZnO surface, which abstracts a proton from ethane to form the monodeprotonated complex (23). The same process is repeated on the opposite side of the oxygen species to yield the double-deprotonated complex (24). Subsequently, two negatively charged Pt centers extract two protons from the two protonated oxygen species in 24, resulting in the formation of ethylene and dihydrogen gas as products, thereby completing the photocatalytic cycle.

It was found that weight percentages of the Pt/ZnO photocatalyst exceeding 1.0% lead to decreased ethylene yields (Scheme ). In addition, using Pt as the active catalyst promotes charge separation within the ZnO semiconductor and facilitates desorption of the ethylene product. The Pt/ZnO photocatalyst used by Wang and co-workers exhibits excellent coke-resistance. Mars–van Krevelen-type light-driven dehydrogenation mechanisms have also been observed with Pt/WO_3‑x and Pd/ZnO Mott–Schottky-type photocatalysts, both capable of generating high ethylene yields. , Notably, Pd/ZnO achieves high product selectivity (94.8%) at relatively low temperatures (50 °C), whereas Pt/WO_3‑x requires higher operating temperatures (375 °C) and shows slightly lower selectivity (84.9%).

In early 2025, Zhang and co-workers achieved light-driven propane dehydrogenation using a TiO₂-supported single-atom copper catalyst (Cu/TiO₂), attaining a propylene selectivity of 99% at an irradiation intensity of 500 W cm^–2 at 80 °C. Notably, the reaction temperature could be further reduced to 10 °C in a water-catalyzed propane dehydrogenation process conducted under sunlight. Around the same time, Halas and co-workers also performed dehydrogenation of propane to propylene by using a composite photocatalyst composed of Al/TiO₂ and Pt single-atom clusters in a Mott–Schottky configuration, achieving excellent product selectivity (99%) with low laser power (3500 mW/cm²) at approximately 320 °C. A general trend in heterogeneous Pt-based photocatalysts for linear alkane dehydrogenation is the requirement of elevated reaction temperatures above 300 °C. The use of alternative noble metal active sites, such as partially oxidized palladium, may offer a route to lower-temperature operation. Alternatively, non-noble metal photocatalysts have also proven effective in decreasing reaction temperatures compared to Pt-based photocatalysts, as demonstrated by the copper catalyzed propane dehydrogenation performed by Zhang and co-workers. Altogether, the aforementioned findings demonstrate that unary heterogeneous photocatalytic systems, specifically Pd/ZnO and Cu/TiO₂, can enable heterogeneous C–C bond dehydrogenation of various short-chain linear alkanes, including methane, ethane, and even the more stable propane, under mild or near-ambient conditions with promising selectivities and yields.

2.3. Homogeneous Photocatalytic Dehydrogenation of Linear C–O Bonds

The first homogeneously catalyzed light-driven C–O bond dehydrogenation was reported by Miller and co-workers in 2015, involving the dehydrogenation of formic acid (Scheme ). This reaction proceeds optimally under basic conditions at pH 8. In the proposed mechanism, formic acid is dissolved in water, where it dissociates to form the formate anion. This anion undergoes ligand exchange with the chloride ligand of the precatalyst [Cp*Ir^III(bpy)­(Cl)]⁺ (25) to generate a formate–Ir­(III) complex (26). Owing to the thermodynamic stability of carbon dioxide, this complex undergoes decarboxylation to release CO₂ and generate a hydridic Ir­(III) complex (27). Upon photoexcitation, the hydridic Ir­(III) complex (27) transitions to an excited state (28), in which the hydride ligand becomes even more nucleophilic and prone to protonation by a weak acidhere, water. This reaction forms a hydroxide–Ir­(III) complex (29) and releases dihydrogen gas, completing the catalytic cycle.

10. Proposed Mechanism for Photochemical Dehydrogenation of C–O Bonds in Formic Acid Using a Homogeneous Iridium-Based Photocatalyst .

This unary homogeneous iridium photocatalytic system produces dihydrogen gas with high selectivity (>95% H₂) and a TON exceeding 500 (when R = OMe) (Scheme ). High concentrations of formate were found to inhibit the reaction, while quantum yields improved as light intensity decreased (from 62 mW/cm² to 35 mW/cm² to 13 mW/cm²). The TOF and overall reaction rate were relatively insensitive to pH within the range of 6 to 11. This broad pH tolerance is notable, as most thermal formic acid dehydrogenation catalysts are not stable under basic conditions, even though basicity is advantageous for generating a pure hydrogen stream. Several years later, in 2022, Masaoka and co-workers reported a ternary cobalt photocatalyst capable of dehydrogenating formic acid to high-purity hydrogen gas in the absence of water. However, aside from the reported TOF (229 h^–1), other key mechanistic details and performance parameters, including TON, conversion, yield, and selectivity, have not been disclosed.

Alcoholic functional groups can also undergo light-driven dehydrogenation. In 2019, Chen and co-workers developed an environmentally friendly, low-waste binary photocatalytic system capable of dehydrogenating benzyl alcohols (Scheme ). In this radical-based mechanism, Eosin Y (33) serves as the photocatalyst, while a Ni­(II)–3-mercaptopropionic acid (MPA) complex functions as proton reduction catalyst. The reaction begins with photoexcitation of Eosin Y (33) to its triplet state. This excited triplet state transfers one electron to the Ni­(II) complex via a first SET step, generating Ni­(I) and the Eosin Y radical cation. In a second SET process, an electron from the C–H bond of the benzyl alcohol substrate (34) is transferred to the Eosin Y radical cation, forming a benzyl alcohol radical cation intermediate (35). Deprotonation of this intermediate yields the neutral benzyl alcohol radical (36), while the Ni­(I) species abstracts a proton to form the Ni­(III)–H intermediate. The benzyl alcohol radical (36) then undergoes a second oxidation, releasing an electron to form the benzyl carbocation (37), which, after deprotonation, affords the corresponding ketone product (38). Meanwhile, the Ni­(III)–H bond is homolytically cleaved under light irradiation, producing a hydrogen atom that recombines with the previously released proton and electron to generate dihydrogen gas. The reaction was carried out in water, with the pH adjusted to 8.5 using an aqueous sodium hydroxide (NaOH) solution to create a basic medium. Under these conditions, near-quantitative conversion, product selectivity, and yield (approaching 100%) were achieved.

11. Proposed Mechanism for Photochemical Dehydrogenation of C–O Bonds in Benzyl Alcohol with a Binary Homogeneous Catalytic System, Consisting of an Eosin Y Photosensitizer and a Nickel-Based Proton Reduction Catalyst ,

In 2020, Fuse, Misunuma, and Kanai reported the first acceptorless dehydrogenation of aliphatic secondary alcohols to ketones, employing a ternary hybrid catalytic system composed of a nickel salt catalyst (Ni­(NTf)₂·xH₂O), a mesityl acridinium photosensitizer, and a thiophosphate organocatalyst. Under visible-light irradiation at room temperature, linear aliphatic secondary alcohols and cycloalkanols were efficiently converted to the corresponding ketones in high yields of 78–85% and 70–98%, respectively, with dihydrogen gas as the sole byproduct. Subsequently, in 2022, the Kanai group reported a follow-up acceptorless alcohol dehydrogenation system that achieved comparable yields, based on a self-photosensitizing thiophosphoric acid radical organocatalyst. This catalyst, generated through electron donor–acceptor complex formation between an electronically tuned thiophosphoric acid and protonated N-heteroaromatic compounds, facilitates C–H bond cleavage via hydrogen atom transfer, while a nickel salt (Ni­(NTf)₂·xH₂O) is still needed to promote proton reduction.

A year later, Zhou and co-workers performed homogeneously catalyzed dehydrogenations of alcohol derivatives using a noble metal-free, redox-neutral organophotocatalyst, 4CzIPN, in combination with a thiol to facilitate HAT. Various primary and secondary benzylic alcohol substrates were investigated, with isolated product yields reaching up to 97%. Additionally, the authors demonstrated site-selective oxidation of 1-phenylbutane-1,3-diol, selectively oxidizing the alcohol group adjacent to the aromatic ring to achieve conjugation with the resulting double bond.

2.4. Heterogeneous Photocatalytic Dehydrogenation of Linear C–O Bonds

The first light-driven, heterogeneously catalyzed C–O bond dehydrogenation was reported by Santo and co-workers in 2011. The authors employed a bimetallic Pt_0.5–Au_0.5/TiO₂ Mott–Schottky-type photocatalyst to dehydrogenate ethanol derivatives. Their study focused primarily on catalyst design rather than on the underlying reaction mechanisms. Two years later, redox-selective Ru- and Pd-based Mott–Schottky-type photocatalysts were developed for the dehydrogenation of alcohol derivatives. , For example, rhodium-doped strontium titanate with ruthenium as a cocatalyst (Ru/SrTiO₃:Rh) was shown to dehydrogenate alcohols to aldehydes. Similarly, palladium nanosheets, palladium nanoparticles supported on mesoporous carbon nitride (Pd/CN), and CdS quantum dots coupled with cobalt cocatalysts have been reported to facilitate selective formic acid dehydrogenation. ,,

In 2016, Xu and co-workers were the first to provide detailed mechanistic insights into semiconductor-based heterogeneous C–O bond dehydrogenation of alcohols (Scheme ). They investigated the dehydrogenation of alcohol derivatives using monometallic Ni-modified CdS nanoparticles as the photocatalyst. Incorporation of Ni active sites, via treatment of CdS nanoparticles with NiCl₂·6H₂O, significantly improved conversion and selectivity compared to bare CdS, as the Ni sites inhibited the formation of undesired benzoin and hydrobenzoin byproducts. The proposed mechanism is as follows: first, the Ni/CdS photocatalyst (39) is excited by light and reacts with a secondary alcohol substrate (40) to form an excited alcohol-bound complex (41). Upon photoirradiation, electrons are promoted to the conduction band of Ni, while holes remain localized in the valence band of CdS. Electrons transfer from CdS to the Ni active sites, which participate in proton reduction in the alcohol-bound transition state complex (42). The alcohol is oxidized by the valence-band holes in CdS, forming a single-deprotonated complex (43) that subsequently yields the ketone product (44) and a double-protonated complex (45). Finally, two hydrogen atoms at the Ni site combine to release dihydrogen gas, completing the catalytic cycle and regenerating the Ni/CdS Mott–Schottky-type photocatalyst (39). Interestingly, the reaction proceeds best in absence of water. The system exhibited exceptionally high apparent quantum efficiency (AQE = 48%) and turnover number (TON = 44,000), underscoring the promise of monometallic Ni-based photocatalysts for the efficient dehydrogenation of linear C–O bonds in alcohols.

12. Proposed Mechanism for Photochemical Dehydrogenation of the C–O Bond in Alcohols over a Heterogeneous Cadmium Sulfide Supported Nickel Catalyst .

Over the past decades, light-driven, endothermic dehydrogenation reactions involving C–O bonds have been extensively studied in heterogeneous systems. Numerous publications report high selectivity, conversion rates, and product yields using a variety of photocatalytic systems (Table ). These reactions primarily focus on the dehydrogenation of formic acid and alcohol derivatives, all successfully carried out at temperatures below 50 °C. Examples of catalysts used for formic acid dehydrogenation include: Au₁Pd₂/graphene oxide (GO), AgPd/g-CN, Pt­(P)/CdS/TNTs, Ag₅Pd₅/g-C₃N₄, and AuPd/CNS. Catalysts reported for the dehydrogenation of alcohol derivatives include: Co/CdS, Ag/g-C₃N₄, nickelphosphide (Ni₂P)/CdS, Au–Pt/CdS, Ni _x S _y /ZnS, Ag/CdS, CeO₂/ZnIn₂S₄, and CdS/BiVO₄. Reaction times range from 1 to 125 h, though most reactions are completed within 5 to 12 h. Notably, CdS is frequently employed as a support material in photocatalysts designed for the dehydrogenation of alcohol derivatives. Unfortunately, meaningful comparison of catalytic performance remains challenging, as key parameters such as light intensity and apparent quantum efficiency (AQE) are often omitted in the literature.

1. Various Types of Photochemical C–O Bond Dehydrogenations over Heterogeneous Catalysts.

Photocatalyst	Reactant	Light source	Intensity (mW/cm2)	AQE (%)	T (°C), Atm, t (h)	Ref.
Au1Pd2/GO	Formic acid	420–800 nm of light bulb	500	/	25, /, 2
Au/TiO2	Primary alcohol	300 W Xe lamp, 300–470 nm	/	20	45, N2, 10
AgPd/g-CN	Formic acid	400 nm of light bulb	/	/	30, /, <2
Co/CdS	Benzyl alcohol	300 W Xe lamp, 420 nm	7.3	63	25, N2, 3
Ag/g-C3N4	Methanol	300 W Xe lamp, 350–780 nm	/	/	25, Ar, 4
CoP/RGO/CdS	Formic acid	White LEDs light, λ > 420 nm	/	32	25, /, 10
Ni/Zn0.5Cd0.5S	Benzyl alcohol	300 W Xe lamp, 420 nm	/	53	25, N2, 8
Co–Ni/CdS	Formic acid	750 W Xe lamp, λ > 420 nm	60	16	25, /, 12
Ni2P/CdS	Methanol	300 W Xe lamp, λ > 420 nm	/	/	25, Ar, 3
Pt(P)/CdS/TNTs	Formic acid	150 W light source, λ > 420 nm	/	/	25, N2, 3
Zn3In2S6–W	Aromatic alcohol	300 W Xe arc lamp, λ > 380 nm	/	6	10, /, 12
Ag5Pd5/g-C3N4	Formic acid	300 W light source, λ > 400 nm	/	/	25, /, <1
Au–Pt/CdS	Benzyl alcohol	300 W Xe arc lamp, λ > 420 nm	800	32	25, /, 5
AuPd/CNS	Formic acid	300 W Xe lamp, λ > 420 nm	/	/	25, /, 1
Ni x S y /ZnS	Benzyl alcohol	500 W Xe lamp, λ > 200 nm	/	/	25, /, 3
Ag/CdS	Ethanol	300 W Xe lamp, λ > 400 nm	/	/	25, /, 6
MoP/Zn3In2S6	Formic acid	300 W Xe lamp, λ > 400 nm	/	6	25, /, 10
Ag0.1Pd0.9/2D g-C3N4	Formic acid	300 W Xe lamp, λ > 400 nm	/	28	50, /, 3
CeO2/ZnIn2S4	Aromatic alcohol	300 W Xe lamp, 365 nm	120	/	25, /, 3
Ag/1 AgPd–Ag3PO4/4 g-CN	Formic acid	5 W LEDs, λ > 315 nm	/	/	50, /, <1
FeP/CdS	Formic acid	300 W Xe lamp, λ > 420 nm	/	54	25, /, 125
Ti3C2T x /CdS	Furfuryl alcohol	3 W LEDs lamp, 420 nm	/	/	25, /, 48
CdS/BiVO4	Benzyl alcohol	300 W Xe lamp, λ > 420 nm	/	/	25, N2, 8
UiO-66-(COO)2-Cu	Formic acid	150 W Xe lamp, λ > 390 nm	/	/	25, Ar, 47

^aAQE = apparent quantum efficiency.

2.5. Photocatalytic Dehydrogenation of Linear Nitrogen-Containing Bonds

A few nitrogen-containing bonds that can be dehydrogenated using light energy have been reported in the literature. For example, in 2016, Su and co-workers successfully performed the heterogeneously catalyzed, light-driven dehydrogenation of the B–N bond in ammonia borane using a Mott–Schottky-type photocatalyst based on graphitic carbon nitride (g-C₃N₄) support. Notably, the reaction was complete in under 1 h and produced dihydrogen with selectivities of up to 100%.

Later, in 2018, Balaraman and co-workers reported the homogeneous light-driven N–N bond dehydrogenation of hydrazobenzene using a binary photocatalytic system, achieving a product yield of 93%. A detailed mechanism was proposed based on experimental data and known redox potentials (Scheme ). The reaction begins with excitation of the photocatalyst [Ru^II(bpy)₃]­Cl₂ (46a) to its excited state. A first SET from the excited Ru­(II) complex to the proton reduction catalyst Co^III(dmgH)₂ClPy (6) produces the oxidized Ru­(III) complex and the reduced Co­(II) complex. Next, the Ru­(III) complex is reduced back to Ru­(II) via a second SET from hydrazobenzene (47), concurrently forming an amine radical cation intermediate (48). This intermediate loses a proton and an electron to generate an amine cation intermediate (49), while the Co­(II) species is further reduced to the double-reduced Co­(I) complex. The Co­(I) species then captures the proton released from intermediate 48, forming a hydridic Co­(III)–H species. Finally, the amine cation (49) releases an additional proton, which combines with the hydride from Co­(III)–H to regenerate the initial Co­(III) catalyst (6), yielding (E)-azobenzene (50) and dihydrogen gas. The relevant redox potentials are E _ox = +0.58 V vs SCE for hydrazobenzene, E _red = +1.29 V vs SCE for [Ru^II(bpy)₃]­Cl₂, E _red = +0.77 V vs SCE for [Ru^II(bpy)₃]­Cl₂, E _red = −0.67 V vs SCE for Co^III(dmgH)₂ClPy, and E _ox = −0.81 V vs SCE for [Ru^II(bpy)₃]­Cl₂. Notably, variations in the position of electron-donating or electron-withdrawing groups on the aromatic ring of the substrate did not significantly affect the yield or reactivity. However, increasing the product concentration was found to decrease the reaction rate, as the photoactive azobenzenes obscure the activity of the photocatalyst by absorbing light within the visible range.

13. Proposed Mechanism for Photochemical Dehydrogenation of the N–N Bond in Hydrazobenzene with a Binary Photocatalytic System, Consisting of a Ruthenium-Based Photocatalyst and a Cobalt-Based Proton Reduction Catalyst .

Light-driven dehydrogenation of C–N bonds in N-heterocycles can also be achieved using both homogeneous and heterogeneous photocatalysts; these will be discussed separately in Chapter 5. Interestingly, dehydrogenation of linear C–N bonds in secondary amines was demonstrated by Chao and Zhao using a supramolecular system. Employing a previously unreported organic thermally activated delayed fluorescence chromophore photocatalyst, the reaction proceeded with product selectivities as high as 99%.

To conclude, in the photochemical dehydrogenation of linear sp³-hybridized bonds such as C–C, C–O, and C–N, unary homogeneous catalysts typically involve Rh­(I) complexes. In binary homogeneous photocatalytic systems, transition metal complexes, such as Ru­(II) or Ir­(III), as well as Eosin derivatives or acridinium salt photosensitizers are commonly paired with Co­(III) or Ni­(II) complexes that serve as proton reduction catalysts. Overall, homogeneous light-driven acceptorless dehydrogenation of C–C, C–O, and C–N bonds generally operates at room temperature in polar solvents, whereas solvent-free homogeneous variants require temperatures above 80 °C. In contrast, heterogeneous catalysts can achieve C–O bond dehydrogenation at room temperature, while C–C bond dehydrogenation has only been achieved at temperatures above 50 °C. Most heterogeneous Pt catalysts used for linear bond dehydrogenations are active at temperatures exceeding 300 °C, suggesting that these Pt-systems might not exploit the full potential of light. In general, heterogeneous photocatalysts designed for the dehydrogenation of linear sp³-hybridized bonds generally consist of a semiconductive support material (e.g., TiO₂, SiO₂, ZnO, or CdS) combined with a transition metal active site (e.g., Pt, Pd, Ni) to facilitate proton reduction. In both homogeneous and heterogeneous systems, dehydrogenation reactions of linear sp³-hybridized bonds tolerate a range of electron-donating and electron-withdrawing substituents. The highest product yields and dihydrogen evolution are typically observed in polar solvents. These reactions are generally carried out under inert atmosphere (Ar or N₂), with irradiation wavelengths between 300 and 420 nm, and at reaction temperatures below 50 °C.

3. Mechanisms of Photochemical Dehydrogenative Coupling Reactions

Cross-coupling reactions are essential Nobel Prize-winning transformations in organic synthesis, enabling the formation of C–C, C–N, and other C–X (carbon–heteroatom) bonds in organic compounds. , Such coupling reactions are widely applied in industry for the synthesis of natural products, polymers, and pharmaceuticals. Traditionally, these transformations rely on homogeneous catalysts, often based on palladium, although heterogeneous systems with metal leaching have also been employed. , Since 2010, light-driven strategies have emerged as a green and sustainable alternative, leading to the development of numerous light-driven dehydrogenative coupling reactions utilizing both homogeneous and heterogeneous photocatalysts. − Due to fundamental differences in reactivity arising from electronic structure, ring strain, and regiochemical constraints, we explicitly distinguish between functional-group couplings and couplings involving ring systems throughout this section.

3.1. Homogeneous Coupling between Functional Groups

The first base- and oxidant-free light-driven homogeneous dehydrogenative cross-coupling reactions between sp³-hybridized functional groups were reported by Wu and co-workers in 2015 (Scheme ). The authors employed a ruthenium-based photoredox catalyst in combination with a cobalt-based proton reduction catalyst to form C–C bonds between glycine esters and β-keto esters. In the proposed mechanism, the photosensitizer [Ru^II(bpy)₃]­(PF₆)₂ (46b) is first excited to its photoactive state. This excited state then accepts one electron from the substrate N-(4-methoxyphenyl)­glycine ethyl ester (51) via a first SET process, reducing the Ru­(II) complex to Ru­(I) and generating an amine radical cation intermediate (53). The Ru­(I) complex subsequently donates an electron in a second SET to reduce the proton reduction catalyst Co^III(dmgH)₂ClPy (6) to Co­(II). Next, the reduced Co­(II) complex is further reduced by an electron from the amine radical cation (53) via a third SET, forming either an imine intermediate (54) or an imine cation intermediate (55). Addition of the ethyl 2-oxocyclopentanecarboxylate substrate (52) results in proton release and generation of the cross-coupled imine product (56). Finally, the double-reduced Co­(I) complex is protonated to form the hydridic Co­(III)–H complex, which reacts with another proton from the medium to release dihydrogen gas. Reaction times of 48 h resulted in complete conversion, with product yields of up to 99%. Notably, the reaction is highly sensitive to moisture, as water decomposes the formed imine intermediates.

14. Proposed Mechanism for Dehydrogenative C–C Bond Cross-Coupling Reactions between Glycine Esters and β-Keto Esters Using a Binary Photocatalytic System, Consisting of a Ruthenium-Based Photocatalyst and a Cobalt-Based Proton Reduction Catalyst .

Subsequently, base- and oxidant-free homogeneous light-driven dehydrogenative cross-coupling reactions were extended to other sp³-hybridized reactants. These include C–C bond formation between arylazo-protected tetrahydroisoquinolines and benzyl/allyl halides, C–C bond coupling involving benzylic C–H bonds, and Si–O bond formation between silanes and alcohols (Scheme ). ,, In all three cases, product yields of up to 96% were achieved.

15. Photochemical Homogeneously Catalyzed Dehydrogenative C–C and Si–O Bond Coupling, Including a) C­(sp³)–C­(sp³) Hetero Coupling of Arylazo-Protected Tetrahydroisoquinolines and Benzyl/Allyl Halides, b) C­(sp³)–C­(sp³) Homo Coupling of Benzylic C–H Bonds, and c) Si­(sp³)–O­(sp³) Hetero Coupling of Silanes and Alcohols ,,

Light-driven dehydrogenative cross-coupling reactions between an sp³-hybridized substrate and an sp²-hybridized substrate facilitate C–C bond formation between a variety of functionalized partners, such as aryl alkenes with alkanes and aldehydes, heteroarenes with a range of unactivated C­(sp³)–H substrates, and benzene with trifluoroacetic acid. − In this context, a notable example was reported by Li and co-workers, who recently achieved a self-promoted light-induced nickel catalyzed acceptorless dehydrogenative C–H acylation of simple alkanes with aromatic aldehydes, without the use of an external photosensitizer, delivering an 85% yield for the coupling of benzaldehyde and cyclohexane (Scheme a). In addition to C–C bonds, dehydrogenative cross-coupling between sp³- and sp²-hybridized substrates offers a synthetic strategy for the formation of C–N, C–O, and C–P bonds (Scheme ). − These reactions have consistently demonstrated promising product yields, conversions, and selectivities. Common photocatalysts employed in such transformations include Eosin B, QuCN⁺ClO₄ ^– and Acr⁺–Mes ClO₄ ^–, often used in combination with Co­(III) derivatives as proton reduction cocatalysts. Alternatively, nickel catalysts with 4,4′-di-tert-butyl-2,2′-bipyridine (dtbbpy) ligands offer a photosensitizer-free approach for couplings involving substrates that can be photoexcited to their triplet states, such as benzaldehydes. ,

16. Photochemical Homogeneously Catalyzed Dehydrogenative C–X (X = C, N, O, P) Coupling Involving sp²-Hybridized Substrates, Including a) C­(sp²)–C­(sp³) Hetero Coupling of Benzaldehyde and Cyclohexane, b) C­(sp²)–P­(sp³) Hetero Coupling of Thiazole Derivatives and Diarylphosphine, c) C­(sp²)–O­(sp³) Hetero Coupling of Phenol and Water, d) C­(sp²)–N­(sp³) Hetero Coupling of Alkenes and Azoles, e) Intramolecular Etherification of Arenes via C­(sp²)–O­(sp³) Cross-Coupling, and f) Intermolecular C­(sp²)–O­(sp²) Hetero Coupling of Benzaldehyde Derivatives and Phenol − ,,,

3.2. Heterogeneous Homocoupling of Methane

The first heterogeneously catalyzed light-driven endothermic dehydrogenative coupling of C–C bonds was reported by Chen and co-workers in 2011. Using a modified Zn-based zeolite, they successfully coupled two methane molecules to produce ethane and hydrogen gas (Scheme ). Thermal activation of C–H bonds is particularly challenging due to their high bond dissociation energy of approximately 104 kcal/mol. In this photocatalytic methane coupling reaction, product selectivities of up to 100% were achieved after 24 h of irradiation. However, the mechanism underlying the Zn-catalyzed dehydrogenative cross-coupling of methane remained unexplored at the time.

17. Photochemical Dehydrogenative Homo Coupling of Methane .

^a Δ_C–H denotes the energy required for thermal activation of the C–H bonds.

A few years later, in 2017, Long and co-workers investigated the reaction mechanism of photochemical methane cross-coupling (Scheme ). This radical-based process was carried out using a semiconductor-based unary monometallic photocatalytic system. The initial monometallic Au/ZnO Mott–Schottky-type photocatalyst (57) is first photoexcited and reacts with a methane molecule to form a methyl anion–ZnO complex (58). Subsequently, a photogenerated hole removes an electron from this complex, yielding a methyl radical–ZnO complex (59), which then reacts with another methane molecule to generate an ethane–ZnO complex (60) and a hydrogen atom. The ethane is then released from complex 60, regenerating the initial Au/ZnO photocatalyst (57). Finally, the hydrogen atom combines with an electron and a proton, both originating from the methane substrate, to form dihydrogen gas. Interestingly, the addition of water molecules was found to prevent catalyst deactivation by scavenging excess photogenerated holes. Moreover, the presence of CO₂ suppresses ethene byproduct formation by acting as a competitive electron acceptor, thereby diverting electrons away from undesired radical coupling pathways. Product yield and selectivity in this methane radical coupling reaction were shown to improve with extended irradiation time.

18. Proposed Mechanism for Photochemical Dehydrogenative Homo Coupling of Methane over a Heterogeneous Unary Monometallic Au/ZnO Photocatalyst .

In 2022, Li and co-workers used a Pt-black/TiO₂ Mott–Schottky-type photocatalyst to produce ethane via the dehydrogenative homocoupling of two methane molecules under ambient conditions (Scheme ). In this catalytic system, the Pt metal plays multiple roles: it inhibits overoxidation, promotes charge separation, and lowers the activation barrier, all of which contribute to enhanced photocatalytic efficiency. Interestingly, instead of the expected ethane product, propane was observed as the major product, with propene as byproduct. According to the proposed mechanism, the methane substrate is first photoexcited and adsorbed onto the Pt-black/TiO₂ photocatalyst. On the Pt active site, the excited methane is converted into a platinum–carbene complex (61), releasing one equivalent of dihydrogen. A second photoexcited methane molecule then reacts with the Pt–carbene complex to form a Pt-bound transition state (62) that generates excited ethane. This ethane subsequently reacts with another carbene to yield propane as the major product. Propene was identified as a side product, proposed to form via the reaction of two carbenes through a [2 + 2] cycloaddition-rearrangement process that produces ethylene as an intermediate. Ethylene can then react with a platinum–carbene complex to generate cyclopropane, which undergoes internal rearrangement to yield the more stable side product, 1-propene. Further dehydrogenation studies revealed that propane and butane are more readily converted to alkenes than ethane.

19. Proposed Mechanism for Photochemical Dehydrogenative Homocoupling of Methane to Ethane and Propane Using a Mott–Schottky Type Heterogeneous Pt-Black/TiO₂ Photocatalyst .

The researchers also measured the AQE of the photocatalytic system as a function of light wavelength. A decrease in AQE was observed with increasing wavelength, indicating that the reaction favors UV-light excitation. Moreover, the reaction pathway and product selectivity were found to depend on the degree of agglomeration of the platinum active sites in the Pt-black/TiO₂ photocatalyst. Finally, it was shown that changing the metal precursor (from PtO₄ to PtClO₃) alters the oxidation state of the catalyst from Pt²⁺ to Pt¹⁺. This modification reduces the photoenergy required to excite the photocatalyst and may enhance the stability of the Pt–carbene complex.

3.3. Heterogeneous Coupling between Functional Groups

Between 2020 and 2025, a variety of heterogeneous photocatalysts, such as Pt- or Pd/TiO₂, CdS/SiO₂, PdS/ZnIn₂S₄, MoS₂/CdS, and Ni/CdS, were employed to couple two sp³-hybridized substrates, enabling the formation of C–C, C–N, and S–S bonds (Scheme ). − Among these, the Pd/TiO₂ Mott–Schottky-type photocatalyst achieved high product selectivity of 94% via a radical addition–elimination mechanism, while the Pd/TiO₂ system reached 87% selectivity through a radical–radical coupling mechanism. The CdS/SiO₂ photocatalyst demonstrated near-quantitative selectivity, approaching 100%. Additionally, PdS/ZnIn₂S₄, MoS₂/CdS, and Ni/CdS exhibited excellent catalytic stability in light-driven dehydrogenative cross-coupling reactions, with product selectivities exceeding 99%.

20. Photochemical Heterogeneously Catalyzed Dehydrogenative C–C, C–N, and S–S Bond Coupling of Two sp³-Hybridized Substrates, Including a) Acetonylation of Toluene, b) C–C Homo Coupling of Benzylalcohol, c, d) C–N Homo Coupling of Amines to Imines with Ammonia Release, and e) S–S Homo Coupling of Thiols − .

3.4. Homogeneous Coupling between Ring Systems

In 2013, Wu and co-workers reported the first light-driven dehydrogenative cross-coupling of C–C bonds between five-membered and six-membered ring systems (Scheme ). A graphene-supported RuO₂ nanocomposite (G–RuO₂) was used in combination with Eosin Y to cross-couple N-phenyl-1,2,3,4-tetrahydroisoquinoline and indole. Although G–RuO₂ is a heterogeneous nanosheet material serving as a hydrogen evolution catalyst, the reaction is classified as a homogeneous cross-coupling because substrate activation of N-phenyl-1,2,3,4-tetrahydroisoquinoline (63) by the photosensitizer Eosin Y occurs in a homogeneous solution phase. The proposed mechanism is outlined in Scheme . First, the photosensitizer Eosin Y (33) is excited to its singlet excited state, and subsequently undergoes intersystem crossing to the triplet excited state. The substrate N-phenyl-1,2,3,4-tetrahydroisoquinoline (63) then donates an electron to the triplet excited state of Eosin Y via a first SET process, forming a radical cation intermediate (64). This intermediate releases a proton to the G–RuO₂ nanosheet, generating an oxidized iminium species (65). Upon addition of the indole substrate (66), cross-coupling occurs to form the product (67). The two released protons combine with two electrons, transferred from the G–RuO₂ nanosheet via a double SET process, to form dihydrogen gas. The regenerated Eosin Y (33) re-enters the catalytic cycle. Under optimal conditions, with R¹ = H and a weak electron-withdrawing fluorine substituent (R² = F), a conversion of 91% was achieved, with both the cross-coupled product (67) and dihydrogen gas produced in 96% yield. In contrast, substrates bearing strongly electron-withdrawing groups, such as a nitrile (−CN), rendered the substituted tetrahydroisoquinoline unreactive.

21. Proposed Mechanism for Photochemical Dehydrogenative C–C Bond Cross-Coupling between a Five- and a Six-Membered Ring System Using a Homogeneous Eosin Y/G-RuO₂ Photocatalyst .

Subsequently, a fully homogeneous, noble-metal-free C–C bond cross-coupling between a five-membered and a six-membered ring system was developed by Wu and co-workers in 2014 (Scheme a). In this system, the G–RuO₂ hydrogen evolution catalyst was replaced by a Co­(dmgH)₂Cl₂ complex, resulting in 100% product yield, selectivity, and conversion.

22. Photochemical Homogeneously Catalyzed Dehydrogenative Coupling of Ring Systems, Including a) C­(sp³)–C­(sp²) Hetero Coupling of Tetrahydroisoquinoline Derivatives and Indoles, b) C­(sp²)–C­(sp²) Hetero Coupling of 2-Arylimidazoheterocycles and Azoles, c) C­(sp³)–C­(sp³) Homo Coupling of Tetrahydroquinoline Derivatives, d) C­(sp³)–C­(sp²) Hetero Coupling of Quinoxaline-2­(1H)-ones and Ethers, e) C­(sp²)–N­(sp³) Hetero Coupling of Amines and Cyclohexanones, and f) C­(sp²)–Si­(sp²) Dehydrogenative Hydrolipocyclization and Silylation of Alkenes − .

In 2018, dehydrogenative C–N bond cross-coupling between five- and six-membered ring systems was reported (Scheme b). These reactions used the same Co­(dmgH)₂Cl₂ complex, while comparing the performance of Eosin Y and Acr⁺–Mes ClO₄ ^– as photosensitizers. Remarkably, the maximum product yield was only 2% when using Eosin Y, compared to 64% with Acr⁺–Mes ClO₄ ^–. After optimization of the reaction conditions, an isolated yield of 87% was achieved.

In 2019, C–C bond homo coupling between two tetrahydroquinoline substrates was performed using Ru­(II) complex [Ru^II(bpy)₃]­Cl₂ (46a) and Co­(III) complex Co^III(dmgH)₂ClPy (6) (Scheme c). This system achieved high selectivity for dihydrogen gas evolution, up to 99%. That same year, C–C bonds were formed via cross-coupling of quinoxaline-2­(1H)-ones and cyclic ethers using Eosin Y (33) as the photosensitizer (Scheme d), achieving a maximum product yield of 96%.

In 2020, anilines were synthesized through the coupling of 1,4-oxazinanes with cyclohexanone derivatives, using an Ir-based photosensitizer and a Co-based proton reduction catalyst. This system also reached a maximum product yield of 96% (Scheme e). More recently, in 2024, C–C bond ring-closing reactions were performed using tris­(trimethylsilyl)­silane ((TMS)₃SiH) to generate a five-membered ring (Scheme f). The maximum product yield for this transformation was 81%, which is relatively modest compared to the other cross-coupling reactions described here.

3.5. Heterogeneous Coupling between Ring Systems

In 2012, König and co-workers were the first to report a dehydrogenative cross-coupling of two six-membered rings via heterogeneous photocatalysis. The reaction was oxidant-free and employed a CdS semiconductor photocatalyst. Although a respectable maximum product yield of up to 89% was achieved, the substrate scope was limited to halogenated and phenyl-substituted compounds.

In 2018, Yoshida and co-workers discovered the first light-driven heterogeneously catalyzed cross-coupling between arenes and tetrahydrofuran (THF) (Scheme ). The reaction utilizes a bimetallic dual-photocatalytic system and proceeds via a hole–electron transfer mechanism (Figure ). Upon excitation of the TiO₂ semiconductor photocatalyst, electron–hole pairs are generated. The initial substrate, THF (69), undergoes single-electron oxidation and deprotonation to form an oxidized THF radical intermediate (70). Electrons in the conduction band then reduce the proton released from this intermediate to generate a hydrogen atom. Simultaneously, the arene substrate (68) undergoes radical addition to the oxidized THF radical (70), forming a complex with an Al₂O₃-supported Pd/Au bimetallic catalyst (71). A hydrogen atom is then released via a radical elimination mechanism. The hydrogen atom released from the Pd/Au complex reacts with the hydrogen atom derived from the THF radical intermediate to yield the cross-coupled product (72) and dihydrogen gas. Interestingly, electron-withdrawing R-groups on the arene substrates significantly enhance product selectivity, exceeding 99%. Later that year (2018), the same research group investigated a C–C bond cross-coupling reaction between pyridine and cyclohexane using a Pt/TiO₂ photocatalyst. After 48 h of reaction time, the product selectivity was again reported to exceed 99%.

23. Proposed Mechanism for Photochemical Dehydrogenative Cross-Coupling of Cyclohexene Derivatives and Tetrahydrofuran over a Bimetallic TiO₂/Pd–Au/Al₂O₃ Mott–Schottky Type Photocatalyst .

Acceptorless C­(sp²)–N­(sp²) dehydrogenative cross-coupling between (hetero)­arenes and azoles was reported by Wu and co-workers in 2025. A g-C₃N₄-supported single-atom platinum photocatalyst in combination with a Zn­(OTf)₂ cocatalyst for intermediate radical stabilization enabled selective coupling of thiophenes with a variety of nucleophiles upon 370 nm irradiation in an argon atmosphere at room temperature for 24 h. For example, 3-bromothiophene was successfully coupled with primary functionalized alcohols in moderate to good yields ranging from 50 to 83%. In addition, dehydrogenative coupling of 3-bromothiophene with pyrazole derivatives (Scheme ) afforded comparable yields, ranging from 45% for dimethylpyrazole to 87% for chloro-substituted pyrazole. This work constitutes one of the first examples of light-driven acceptorless dehydrogenative cross-coupling between two sp²-hybridized five-membered rings.

24. Photochemical Dehydrogenative Cross-Coupling between Thiophene and Pyrazole over a Heterogeneous Single-Atom Platinum Catalyst with Zn­(II)­triflate as Cocatalyst .

In summary, photochemical dehydrogenative coupling reactions can be achieved at room temperature using binary homogeneous photocatalysts composed of a photosensitizer, such as a Ru­(II) or Ir­(III) complex, an Eosin derivative, or an acridinium salt, and a Co­(III) complex as proton reduction catalyst. Several heterogeneous photocatalysts capable of facilitating these reactions consist of a semiconductive support material combined with noble metal Au, Pd, or Pt active sites to promote proton reduction. In both homogeneous and heterogeneous media, dehydrogenative coupling reactions tolerate a broad range of electron-donating and electron-withdrawing substituents. These light-driven reactions are typically carried out in polar solvent under inert atmosphere (Ar or N₂), with irradiation wavelengths ranging from 350 to 450 nm. Light-driven dehydrogenative methane coupling has been achieved under solvent-free conditions at moderate temperatures (25–43 °C) using heterogeneous noble metal catalysis, offering a promising future route to energy-efficient and fully electric methane valorization with concomitant dihydrogen production.

Over the past 15 years, advances in light-driven dehydrogenative coupling reactions have enabled bond formation between a wide variety of functional groups and ring systems under ambient conditions in polar solvents, most often acetonitrile. The wide substrate scope explored to date naturally prompts critical reflection on the potential practical applications of light-driven dehydrogenative couplings in the fine chemicals and pharmaceutical industries. Although acetonitrile is commonly used as solvent in industrial pharmaceutical processes, research efforts have been directed toward its replacement with more environmentally benign alternatives or the implementation of solvent recovery strategies. Future research could therefore explore whether these strategies can be translated to light-driven dehydrogenative coupling reactions, thereby decreasing dependence on harmful solvents in line with green and circular chemistry principles.

4. Mechanisms of Photochemical Dehydrogenative Cyclization Reactions

According to the IUPAC definition, a cyclization reaction is “the formation of a ring compound from a chain by formation of a new bond”. , Cyclization has gained prominence in organic synthesis due to chemists’ long-standing interest in mimicking the cyclic structures commonly found in nature. The sustained interest in the selective synthesis of homo- and N-heterocycles is largely driven by their ubiquity in natural products and chiral pharmaceuticals. , The selectivity of cyclization reactions can be finely tuned through the use of appropriate catalystsoften transition-metal-basedwhich enable stereo- and regioselective control. , For example, Pd-based catalysts facilitate asymmetric cyclization reactions to produce enantiopure cyclic compounds. In 2016, the first report on photochemical intramolecular cyclization via selective C–C bond formation was published. Shortly thereafter, light-driven strategies were also developed for cyclization reactions involving the formation of C–N, C–O, and C–S bonds under homogeneous conditions.

4.1. Homogeneous Photochemical Dehydrogenative Cyclization of C–C Bonds

The first homogeneous intramolecular light-driven dehydrogenative C–C bond cyclization forming five- and six-membered rings was reported by Wu and co-workers in 2016 (Scheme ). In this reaction, indoles are synthesized from enamines using an Ir^III(ppy)₃ complex (74) as photosensitizer and a Co^III(dmgH)₂(4-CO₂Mepy)Cl complex (75) as proton reduction cocatalyst, with dihydrogen gas as a coproduct. To support the feasibility of this light-driven endothermic dehydrogenative cyclization, the redox potentials of the catalytic intermediates were determined: (E ^IV/III = +0.73 V vs SCE for Ir^III(ppy)₃, E ^III/II = −2.3 V vs SCE for Ir^III(ppy)₃, E _ox = −1.83 V vs SCE for *Ir^III(ppy)₃, E _red = +0.26 V vs SCE for *Ir^III(ppy)₃, and E ^III/II = −0.51 V vs SCE for Co^III(dmgH)₂(4-CO₂Mepy)­Cl, E ^73a·+/73a = +0.71 V vs SCE for the amine substrate). These values confirm that electron transfer from the excited Ir complex (E _red = +0.26 V vs SCE for *Ir^III(ppy)₃) to the Co^III complex (E ^III/II = −0.51 V vs SCE for Co^III(dmgH)₂(4-CO₂Mepy)­Cl) is thermodynamically feasible.

25. Proposed Mechanism for Photochemical Intramolecular Dehydrogenative Cyclization of Enamines into Indoles Using a Binary Homogeneous Iridium and Cobalt-Based Catalytic System ,

In the proposed radical mechanism (Scheme ), Ir^III(ppy)₃ (74) is first photoexcited and undergoes SET to reduce the Co^III(dmgH)₂(4-CO₂Mepy)Cl complex (75) to Co­(II). Simultaneously, the resulting Ir­(IV) species oxidizes the amine substrate (73) via a second SET to generate the radical cation intermediate (76) and regenerate the ground-state Ir^III(ppy)₃ complex (74). Upon light activation, the N–H proton of the radical cation (76) dissociates into solution, forming a neutral radical amine intermediate (77), which resonates with a radical enamine structure (78). This intermediate undergoes intramolecular radical cyclization to yield a radical indole intermediate (79). The reduced Co­(II) complex then accepts an electron from 79, forming a double-reduced Co­(I) species. This Co­(I) complex is protonated at the nitrogen atom to generate a hydridic Co­(III)–H species. Subsequently, the Co­(III)–H abstracts a proton from the tertiary carbon of the cationic indole intermediate (80), forming an aromatic ring via homocyclization. This intermediate then tautomerizes to yield the final aromatic indole product (81). The release of dihydrogen gas completes the cycle and initiates a new catalytic turnover. Notably, hydrogen bonding plays a key role in controlling selectivity by favoring the Z-conformation over the E-conformation of the enamine substrate.

The above-mentioned reaction tolerates a variety of electron-donating and withdrawing substituents, with maximum product yields reaching 95% for electron-donating methyl, methoxy, or phenyl substituents on the six membered ring of the indole precursor (Scheme ). Adversely, a strong electron-withdrawing trifluoromethyl group reduced the yield to 70%. In addition to electronic effects, steric factors also appear to influence the outcome of the cyclization. Meta-substitution across from the phenyl group (73a) affords regioisomers and lower product yields compared to meta-substitution close to the ring substituent (73b). Moreover, the use of polar solvent mixtures (e.g., DMF + iPrOH) was found to enhance product yields, likely by promoting charge separation processes.

Between 2017 and 2021, Zhang and co-workers conducted a series of studies on light-driven dehydrogenative intramolecular C–C bond cyclization reactions between five- and six-membered rings, resulting in the formation of aromatic carbon homocycles (Table ). Several consistent trends were observed. Polar solvents such as dichloromethane (DCM) and ethanol–water mixtures were commonly used, with higher ethanol concentrations correlating with increased product yields. Notably, these reactions did not require transition-metal-based photocatalysts to achieve cyclization. Activation of C–H bonds in six-membered aromatic systems (e.g., phenyl, pyridine, and bipyridine rings) was typically promoted by electron-donating groups. Although substituent effects on five-membered ringssuch as furanwere explored in a limited number of cases, only one study (Zhang and co-workers) explicitly addressed this point, reporting that electron-donating substituents on the furan ring can enhance product yields. All reactions were carried out under ambient conditions and an inert argon atmosphere. Finally, each intramolecular C–C bond cyclization was accompanied by the evolution of dihydrogen gas, proceeding via tautomerization followed by syn-elimination.

2. Various Types of Homogeneous Photochemical Intramolecular Dehydrogenative C–C Bond Cyclization Reactions between a Five-Membered Aromatic Ring (Often Furan) and a Six-Membered Aromatic Ring under Ambient Conditions .

Substrate scope	Light source	Solvent	MPY (%)	FGP	T (°C), Atm, t (h)	Ref.
2,3-Di(hetero)arylchromen-4-one	500 W Hg lamp, 365 nm	EtOH:H2O (7:1, v/v)	86	EDG	25, Ar, 3
4-Phenyl-3-heteroarylcoumarins	500 W Hg lamp, 365 nm	EtOH:H2O (9:1, v/v)	76	EDG	25, Ar, 2
o-Phenylfuranylpyridines	500 W Hg lamp, 365 nm	DCM	85	EDG	25, Ar, 1–4
3-Phenyl-4-(2-heteroaryl)pyrazoles	500 W Hg lamp, 365 nm	EtOH or EtOH–H2O (2:1, v/v)	83	EWG	25, Ar, 6–9
3-Styryl indoles	30 W UV lamp, 350 nm	MeCN	98	None	25, Ar, 6

^aMPY = maximum product yield by different functional groups, FGP = functional group preference of six-membered rings, EDG = electron-donating groups, EWG = electron-withdrawing groups.

Between 2020 and 2024, several research groups investigated intramolecular photoinduced dehydrogenative cyclization reactions between six-membered ring systems to synthesize polycyclic aromatic hydrocarbons (Table ). In some cases, these reactions involve photoinduced Scholl-type cyclizations, characterized by intramolecular C–C bond formation between two six-membered rings (Scheme ). In general, these dehydrogenative cyclizations are carried out in polar solvents such as acetonitrile, tert-butanol, trifluoroacetic acid, or dichloromethane/dimethyl sulfoxide mixtures. The reactions are typically photoactivated using light with wavelengths ranging from 250 to 500 nm. Electron-donating substituents are generally favored to enhance product yields. Most reactions are conducted under ambient conditions and an inert atmosphere (Ar or N₂). In the majority of cases, transition-metal-based photocatalysts, such as cobaloxime and iridium complexes, are employed to facilitate these cyclizations and promote the formation of aromatic polycycles. Interestingly, phosphindole ring cyclization requires the addition of a Lewis acid, such as bismuth­(III) trifluoromethanesulfonate, along with sodium bicarbonate. This specific reaction exhibits excellent performance, with product yields exceeding 99%. It is notable that none of the references discussed in this paragraph report the AQE or the light intensity used in the reactions.

3. Various Types of Homogeneous Photochemical Intramolecular Dehydrogenative C–C Bond Cyclization Reactions between Two Six-Membered Aromatic Rings under Ambient Conditions .

Photocatalyst	Substrate scope	Light source	Solvent	MPY (%)	FGP	T (°C), Atm, t (h)	Ref.
Co(dmg(BF2))2(OH2)2	o-Teraryls	254 nm of UV lamp	MeCN	96	EDG	25, /, 18
/	(E)-N,1-Diphenylformimines	2000 W UHP, 500 nm	tBuOH	85	EDG	45, /, 2
/	(E)-2-Phenyl-3-styrylpyridines	64 W UV lamp, 254 nm	CF3COOH, EtOH	90	EDG	25, Ar, 3–8
Fac-Ir(ppy)3, Co(dmgH)2pyCl	o-Biaryl-1,3-dicarbonyls	3 W LEDs, 440 nm	DCM	95	EDG	25, Ar, 8
[Ir(dF(CF3)ppy)2(dtbbpy)]PF6	3-Vinyl-2-aryl-indoles	30 W LEDs, 460 nm	DCM/DMSO (10:1)	98	EDG	25, Ar, 12–18
Bi(Otf)3, NaHCO3	2,3-Diarylbenzophopholes	40 W LEDs, 456 nm	MeCN	>99	None	50, N2, 22

^aMPY = maximum product yield by different functional groups, FGP = functional group preference, EDG = electron-donating groups, EWG = electron-withdrawing groups.

26. Example of a Scholl-Type Dehydrogenative Cyclization Reaction .

4.2. Homogeneous Photochemical Dehydrogenative Cyclization of C–N Bonds

The first homogeneously catalyzed dehydrogenative cyclization between an intermolecular imine and alkyne to form a C–N bond was reported by Li and co-workers in 2018 (Scheme ). In this reaction, the Acr⁺–Mes ClO₄ ^– complex (84) serves as a metal-free photosensitizer, while Co^III(dmgH)₂(4-CO₂Mepy)Cl (85) functions as a proton reduction catalyst to generate dihydrogen gas. The redox potentials of all catalytic species were measured to evaluate the feasibility of electron transfer steps: E _ox = +1.62 V vs SCE for bis-4-methylphenyl methanimine, E _ox = +1.86 V vs SCE for 1,2-diphenylacetylene, E _red = +2.06 V vs SCE for the photosensitizer, E _ox = −0.55 V vs SCE for photosensitizer, E _red = −0.77 V vs SCE for Co^III(dmgH)₂(4-CO₂Mepy)­Cl, E _red = −1.04 V vs SCE for Co^III(dmgH)₂(4-CO₂Mepy)­Cl, and E _oxi = −0.87 V vs SCE for oxygen. These values confirm that bis-4-methylphenyl methanimine can engage in a SET process with the excited state of the photosensitizer, but not with its ground state or with either the Co­(III) or Co­(II) species. However, due to its high quenching rate, the excited photosensitizer can undergo SET to reduce the Co­(III) complex.

27. Proposed Mechanism for Photochemical Intermolecular Dehydrogenative Cyclization of an Imine and an Alkyne Using a Binary Homogeneous Photocatalytic System Consisting of an Acridinium Salt Photosensitizer and a Cobalt-Based Proton Reduction Catalyst .

In the proposed radical mechanism (Scheme ), the reaction begins with excitation of bis-4-methylphenyl methanimine (82), triggering homolytic cleavage of the N–H bond and generating an iminyl radical intermediate (86) and atomic hydrogen. Meanwhile, the Acr⁺–MesClO₄ ^– complex (84) is excited to form Acr^•–Mes^•+ClO₄ ^–. In the first SET step, the acridinyl radical donates an electron to reduce the Co­(III) complex to Co­(II) (oxidative quenching), while the Mes radical cation accepts an electron from the hydrogen atom released by substrate 82, generating the first proton in solution (reductive quenching). Next, the 1,2-diphenylacetylene substrate (83) is added and reacts with the iminyl radical (86) via radical addition to form a radical alkene intermediate (87). This intermediate undergoes radical cyclization with the phenyl ring, yielding an arene radical intermediate (88). This species promotes homolytic C–H bond cleavage to produce the cyclized product (89), a second proton, and an electron. This electron returns to the photosensitizer, reducing Co­(II) to Co­(I) via a second SET process. The resulting Co­(I) species binds the first proton (from the N–H bond cleavage in 82) to form a hydridic Co­(III)–H complex, which subsequently abstracts the second proton (from the C–H bond cleavage in 88) to release dihydrogen gas. The overall quantum yield of the reaction was determined to be 0.19.

Interestingly, this intermolecular cyclization reaction is regioselective. Product 90a contains identical functional groups on both substituents (R), whereas product 90b features distinct functional groups on each side (Scheme ). A systematic study revealed that strong electron-withdrawing groups decrease the product yield, while weak electron-withdrawing and electron-donating groups are preferred, enabling product yields of up to 92%. Regioselectivity was highest (4.9:1.0) when R¹ = CH₃ and R² = H, yielding the product in 83%.

A few years later, in 2021, Mal and co-workers reported an intramolecular dehydrogenative C–N bond cyclization reaction. The benzimidazole–phenyl cyclization proceeds under 350 nm irradiation in ethylene dichloride solvent and, similarly, tolerates both electron-donating and electron-withdrawing groups. Nonetheless, the maximum product yield of 98% was achieved for unsubstituted biphenyl-benzimidazole. The fluorescence quantum yields of the synthesized compounds reached an impressive 0.96 for fluoro-substituted phenanthridine derivatives but was only 0.18 for dichloro-substituted benzimidazole-fused phenanthridine.

4.3. Homogeneous Photochemical Dehydrogenative Cyclization of C–O Bonds

In 2018, Lei and co-workers reported the first homogeneously catalyzed intramolecular C–O bond cyclization reaction (Scheme ). Using an acridinium photosensitizer from Fukuzumi’s family, i.e. Acr⁺–MesClO₄ ^– (84), in combination with Co^III(dmgH)₂ClPy (6) as the proton reduction catalyst, C–O bonds were formed between the carboxylic acid group and the aromatic ring in 2-arylbenzoic acids. The following mechanistic insights were proposed (Scheme ). Prior to irradiation, the nitrogen atom of the acetonitrile solvent coordinates via hydrogen bonding to the proton of the carboxylic acid group in the 2-phenylbenzoic acid substrate (91), forming a solvent-coordinated intermediate (92). Upon light irradiation, the acridinium photosensitizer (84) is promoted to its excited state, initiating homolytic cleavage of the O–H bond in intermediate 92, and forming a carboxyl radical intermediate (93). This process also leads to reductive quenching of the excited photosensitizer, generating its radical form. The carboxyl radical (93) undergoes intramolecular radical cyclization with the adjacent phenyl ring, forming a radical cyclized intermediate (94). At this stage, the first SET occurs from the reduced photosensitizer (84) to the Co^III(dmgH)₂ClPy complex (6), reducing it to Co­(II). A second SET follows, in which the radical intermediate 94 donates an electron to the Co­(II) species, yielding a double-reduced Co­(I) complex and a cationic cyclized intermediate (95). The Co­(I) species abstracts a proton from the carboxylic acid, forming a hydridic Co­(III)–H complex. Aromaticity restoration drives the deprotonation of the cationic intermediate 95, affording the final cyclized product (96). Finally, the Co­(III)–H complex reacts with the released proton to form dihydrogen gas, completing the catalytic cycle.

28. Proposed Mechanism for Photochemical Intramolecular Dehydrogenative Cyclization of the Carboxylic Acid Group and One of the Phenyl Rings in 2-Arylbenzoic Acids, Using a Binary Homogeneous Photocatalytic System Consisting of an Acridinium Salt Photosensitizer and a Cobalt-Based Proton Reduction Catalyst .

Interestingly, the above-mentioned reaction is regioselective (Scheme ). Cyclized product 97a exhibited a maximum regioselectivity of 15:1, while 97b achieved 12:1, both with 70% product yield. Additionally, products 98a, 98b, and 98c were obtained as single regioisomers, with yields of up to 99%, 65%, and 55%, respectively. Notably, the addition of alkaline additives such as sodium carbonate or triethylamine terminates the reaction. This is attributed to their strong reducing character, which degrades the structure of the acridinium catalyst, thereby deactivating the photosensitizer. While both electron-donating groups and electron-withdrawing groups are tolerated, substrates bearing electron-donating groups generally provide slightly higher yields. The reaction is also compatible with halogen and alcohol functional groups. However, substitution at the ortho position of the phenyl ring significantly reduces product yield due to steric hindrance.

In 2023, Sen, Pathania, and Roy demonstrated that 2-phenylbenzoic acid can also undergo intramolecular C–O bond cyclization using an organic Lewis acidic phenalenyl photocatalyst. This photocatalyst exhibits a high excited-state redox potential (E _1/2 = +2.61 V vs Ag/AgCl), enabling direct oxidation of the carboxylic acid group. Consistent with the findings of Lei and co-workers, the reaction was carried out in MeCN solvent using a cobalt-based cocatalyst. A maximum yield of 96% was obtained after 24 h. When a methyl substituent was introduced at the meta position of biaryl-2-carboxylic acid, regioselectivity of 2.3:1 was observed, along with a product yield of 74%. As in Lei’s study, this system could withstand a variety of electron-donating and electron-withdrawing groups. Interestingly, a clear difference in reaction time was noted between the two reports: replacing the acridinium-based photosensitizer (Lei et al.) with the phenalenyl photocatalyst (Sen et al.) extended the reaction time from 10 to 24 h.

4.4. Homogeneous Photochemical Dehydrogenative Cyclization of C–S Bonds

The first homogeneous intramolecular dehydrogenative cyclization reaction between a sulfur group and a six-membered aromatic ring was reported by Lei and co-workers in 2018 (Scheme ). The reaction employs [Ru^II(bpy)₃]­(PF₆)₂ (46b) as photosensitizer and Co^III(dmgH)₂(p-Nme₂Py)Cl (100) as proton reduction catalyst. Redox potentials of the catalytic species were measured to validate the proposed photoredox mechanism: (E _1/2 red = +0.77 V vs SCE for [Ru^II(bpy)₃]­(PF₆)₂, E _1/2 red = −1.33 V vs SCE for [Ru^II(bpy)₃]­(PF₆)₂, E _1/2 red = −0.83 V vs SCE for Co^III(dmgH)₂(p-Nme₂Py)­Cl, and E _1/2 red = −1.08 V vs SCE for Co^III(dmgH)₂(p-Nme₂Py)­Cl). These findings show that the excited Ru­(II) complex (E _1/2 red = +0.77 V vs SCE for [Ru^II(bpy)₃]­(PF₆)₂) is a sufficiently strong oxidant to initiate the reaction, while the Ru­(I) complex (E _1/2 red = −1.33 V vs SCE for [Ru^II(bpy)₃]­(PF₆)₂) can reduce Co­(III) to Co­(II), thereby closing the photoredox cycle.

29. Proposed Mechanism for Photochemical Intramolecular Dehydrogenative Cyclization of N-Phenylbenzothioamide, Using a Binary Homogeneous Photocatalytic System Consisting of a Ruthenium-Based Photosensitizer and a Cobalt-Based Proton Reduction Catalyst .

In the proposed mechanism (Scheme ), the [Ru^II(bpy)₃]­(PF₆)₂ (46b) photocatalyst is first photoexcited, and a weak base such as tetrabutylammonium hydroxide (TBAOH) is added to deprotonate the initial N-phenylbenzothioamide substrate (99), forming a thioamide anion intermediate (101). The excited Ru­(II) complex then oxidizes the sulfur moiety via a first SET, generating the Ru­(I) species and a sulfur-centered radical intermediate (102). Subsequently, the Ru­(I) species donates an electron to the Co­(III) complex (100), reducing it to Co­(II) via a second SET. The sulfur radical intermediate (102) undergoes intramolecular radical cyclization with the phenyl ring to form an aryl radical intermediate (103). This radical is then oxidized by Co­(II) to form an aryl cation intermediate (104) via a third SET. The proton from the acid–base conjugate pair (formed in the first step) is then transferred to the Co­(I) complex, generating a hydridic Co­(III)–H species. Finally, the conjugate base abstracts a proton from the aryl cation (104) to produce the final aromatic cyclized product (105), and the Co­(III)–H reacts with the proton to release dihydrogen gas. The catalytic cycle restarts upon regeneration of 46b and 100.

This reaction proceeds with a wide variety of electron-donating and electron-withdrawing groups at both para and ortho positions of the aromatic ring, with maximum yields reaching 99% (Scheme ). However, strongly electron-donating (e.g., −OMe) and electron-withdrawing (e.g., −CF₃) substituents were found to significantly reduce the yield. Interestingly, TBAOH (10 mol %) was shown to promote initial deprotonation of the substrate needed for efficient Co-based hydrogen evolution. Regioselectivity was not discussed in the study.

In general, dehydrogenative photochemical cyclization reactions can be carried out at room temperature using binary homogeneous systems consisting of acridinium salts or Ir­(III)/Ru­(II) photosensitizers, in combination with Co­(III) complexes serving as proton reduction catalysts. Interestingly, light-induced C–C bond cyclization between a five-membered heterocycle and a six-membered phenyl or pyridine ring can proceed under ambient conditions without a photocatalyst. To date, heterogeneous photocatalytic systems for dehydrogenative cyclization reactions remain largely unexplored. A notable example is rutile Pt/TiO₂, reported by Yoshida and co-workers to promote dehydrogenative lactonization of diols at room temperature. Most homogeneous systems exhibit broad tolerance toward both electron-donating and electron-withdrawing substituents. In some cases, electron-donating groups are slightly preferred, whereas in other systems strong electron-withdrawing groups have been reported to lead to diminished yields. However, in the case of cyclizations involving carbon and sulfur moieties, strongly donating or withdrawing substituents have been shown to inhibit both product formation and dihydrogen evolution.

Photoinduced cyclization reactions are typically conducted in polar solvents and are activated by irradiation with light in the 250–500 nm range. Steric effects also play a role: substituents at the para position of six-membered rings are generally favored over those at the more sterically hindered meta or ortho positions. For most dehydrogenative cyclization reactions reported to date, AQEs were not determined, making it difficult to directly compare the efficiency of different photocatalytic systems.

5. Mechanisms of Photochemical Dehydrogenations of Cycloalkanes

Aromatic hydrocarbons are produced on an industrial scale via petroleum refining. In 2019, global benzene demand exceeded 48 million tons, with revenues from benzene, toluene, and xylene reaching 31, 16, and 46 billion USD, respectively. Producing one ton of these aromatic building blocks through catalytic naphtha reforming, a process in which dehydrogenation plays a central role, results in an estimated 380 kg of CO₂ emissions. , In recent years, the dehydrogenation of homo- and N-heterocyclic hydrocarbons has garnered increased interest for applications in liquid organic hydrogen carriers (LOHCs). However, the high energy demand associated with thermal dehydrogenation presents a significant barrier to practical implementation. Photocatalytic dehydrogenation of cycloalkanes offers a promising route to both sustainable aromatic compound production and efficient hydrogen release from LOHCs. To this end, effective photochemical dehydrogenation strategies have been developed for both hydrocarbon homocycles and N-heterocycles, employing homogeneous and heterogeneous photocatalysts.

5.1. Homogeneous Photocatalysis for the Dehydrogenation of Homocycles

The first homogeneous light-driven endothermic dehydrogenation of a homocycle was reported in 1985 by Burk, Crabtree, and McGrath, who employed an Ir^IIIH₂(CF₃CO₂)­(PR₃)₂ photocatalyst under solvent-free ambient conditions (Scheme a). The dehydrogenation of cyclooctane proceeded very slowly, yielding only 28 turnovers over 7 days. Unfortunately, neither the reaction mechanism nor the quantum yield was investigated.

30. a) Photochemical Dehydrogenation of Cyclooctane Using a Homogeneous Iridium-Based Photocatalyst, b) Proposed Mechanism for Photochemical Dehydrogenation of Cyclooctane Using a Homogeneous Rhodium-Based Photocatalyst, c) Photochemical Dehydrogenation of Methylcyclohexane Using a Homogeneous Rhodium-Based Photocatalyst, and d) Photochemical Dehydrogenation of Cycloalkanes Using Tetra-n-butylammonium Decatungstate (TBADT) with a Cobaloxime Cocatalyst ,,

Subsequently, in 1989, Maguire, Boese, and Goldman elucidated the first general mechanism for cyclooctane dehydrogenation, using a trans-Rh^I(PMe₃)₂(CO)­Cl photocatalyst (Scheme b). The reaction begins with a substrate–ligand exchange, wherein CO dissociates from the Rh complex and is replaced by cyclooctane via oxidative addition. This is followed by a β-hydride elimination to generate the cyclooctene product. The reductive elimination of dihydrogen is facilitated by recoordination of the CO ligand to the Rh center. The quantum yield and TON reached 0.10 and 5000, respectively. Interestingly, the dehydrogenation of methylcyclohexane was also studied under the same conditions (Scheme c). This reaction produced four regioisomers: 1-methylcyclohexene, 3-methylcyclohexene, 4-methylcyclohexene, and methylenecyclohexene. While 1-methylcyclohexene was the most thermodynamically stable product, the highest yield (∼55%) was obtained for 4-methylcyclohexene after 22.5 h of UV irradiation.

In 2014, Beller and co-workers employed the same trans-Rh^I(PMe₃)₂(CO)Cl photocatalyst used by Maguire, Boese, and Goldman for light-driven dehydrogenation of cyclooctane, but added free bases such as 4,4′-bipyridine to increase catalytic activity. The basic additive accelerated the reaction, reducing the reaction time from 48 to 7 h and increasing the maximum product yield from 58% to 71%. However, neither the dihydrogen yield, the quantum yield, nor the possible role of bipyridines as ligands was reported.

One year later, West, Huang, and Sorensen reported the first noble-metal-free system for the acceptorless dehydrogenation of cycloalkanes, relying on cooperative base metal catalysis operating via successive HAT steps. The authors employed tetra-n-butylammonium decatungstate (TBADT) as the photocatalyst, owing to its strong C–H bond activation capability, in combination with Co­(dmgH)₂ClPy (6) to promote dihydrogen evolution (Scheme d). Under optimized conditions (CD₃CN as solvent, near-UV irradiation, room temperature, 48 h), cyclooctane was dehydrogenated to cyclooctene in 19% yield. Dehydrogenation of cyclopentane and cyclohexane to the corresponding monoalkenes proceeded with lower yields of 8% and 5%, respectively. In contrast, dehydrogenation of alcohols to ketones afforded higher yields, ranging from 43% for cyclopentanol to 83% for 1-phenylethanol. Overall, this work provides a notable example of how nature-inspired catalysis can be applied in synthesis by enabling the dehydrogenation of saturated alkanes and alcohols under mild reaction conditions, albeit with low yields for cyclic substrates at that time.

In 2017, Kanai and co-workers reported a visible light-driven dehydrogenation of tetrahydronaphthalene, using a ternary photocatalytic system composed of the acridinium photosensitizer Acr⁺MesClO₄ ^– (84), a noble-metal proton reduction catalyst Pd­(BF₄)₂·4MeCN, and an organosulfur compound (107) (Scheme ). The reaction, carried out at room temperature under visible-light irradiation, begins with excitation of the photosensitizer (84), which then accepts an electron from the organosulfur compound (107) via SET, generating an organosulfur radical and a proton. Subsequently, HAT from the tetrahydronaphthalene substrate (106) to the organosulfur radical forms a tetrahydronaphthalene radical intermediate (108). This radical intermediate then undergoes oxidative addition to the Pd center, forming an oxidized Pd complex (109). The neutral photosensitizer subsequently reduces the Pd­(II) complex (109) via another SET to yield a reduced Pd complex (110), from which a 1,2-dihydronaphthalene intermediate (111) is released. Upon β-hydride elimination, the reduced Pd complex (110) regenerates, and the 1,2-dihydronaphthalene (111) enters a second catalytic cycle to afford the fully dehydrogenated product (112). Molecular dihydrogen evolves via coupling of the hydridic Pd–H species with the proton previously released from the organosulfur compound (107). The reaction achieves a product yield of up to 82% after 47 h.

31. Proposed Mechanism for Photochemical Dehydrogenation of Tetrahydronapthalene, Using a Homogeneous Photocatalytic System Consisting of an Acridinium Salt Photosensitizer, an Organosulfur Co-Catalyst, and a Palladium-Based Proton Reduction Catalyst .

One year later, in 2018, the same research group discovered a noble metal-free proton reduction catalyst by replacing Pd­(BF₄)₂·4MeCN with Ni­(NTf₂)₂·xH₂O, while keeping the ternary photocatalytic system and reaction conditions the same, i.e., using the acridinium photosensitizer Acr⁺MesClO₄ ^– (84) and the organosulfur compound (107) under visible light at ambient temperature. Remarkably, within 48 h, a product yield of up to 93% was achieved. The highest yields were achieved for methyl-monosubstituted substrates, with the maximum yield (99%) observed for methyl substitution at the meta-position of the saturated ring. Complete dehydrogenation of 2-methyl-1,2,3,4-tetrahydronaphthalene was observed, yielding up to 99%. However, quantum yield and dihydrogen gas evolution were not reported.

The aforementioned light-driven endothermic dehydrogenation reactions of homocyclic rings require the presence of an adjacent aromatic ring (Scheme ). In 2025, Kanai and co-workers made the groundbreaking discovery that monocyclic cyclohexane derivatives can be completely dehydrogenated in the absence of an aromatic ring, using a similar photocatalytic system. This reaction proceeds via a double HAT process (Scheme ). Notably, a chlorine radical is employed to activate the first saturated C–H bond in the methylcyclohexane substrate. In the initial HAT step, the photocatalyst [Ir^III{dF­(CF₃)­ppy}₂(dtbpy)]­PF₆ (114) is photoexcited and undergoes SET with tetrabutylammonium chloride (TBACl, 116), forming Ir­(II) and a chlorine radical (Scheme ). The chlorine radical then abstracts a hydrogen atom from methylcyclohexane (113), generating a methylcyclohexyl radical (118), a chloride anion, and a proton. The methylcyclohexyl radical (118) proceeds through a proton reduction mechanism analogous to that described in Scheme , now using Co^III(dmgH)₂(p-OMePy)Cl as the proton reduction catalyst (115). This step results in the elimination of atomic hydrogen, formation of a methylcyclohexene intermediate (119), and release of dihydrogen gas. The monoalkene is then further activated to its radical derivative (120) via a second HAT process involving the thiophosphoric acid catalyst (117). This radical intermediate (120) undergoes another C–H bond cleavage via the proton reduction catalyst. The resulting metal hydride reacts with a proton to release a second equivalent of dihydrogen gas. This cycle is repeated once more to yield the aromatic product toluene (121) and a third equivalent of dihydrogen gas.

32. Proposed Mechanism for Photochemical Dehydrogenation of Homocyclic Hydrocarbons Using a Homogeneous Photocatalytic System Consisting of an Iridium-Based Photosensitizer, a Cobalt-Based Proton Reduction Catalyst, a Tetrabutylammonium Chloride Additive, and a Thiophosphoric Acid Co-Catalyst .

Overall, the reaction favors weak electron-donating groups (such as Me and tBu) over electron-withdrawing groups (Scheme ). Maximum product yields of up to 73% were achieved within 24 h. Owing to the mild conditions under which this reaction operates, it holds significant promise for implementation in continuous flow systems.

5.2. Heterogeneous Photocatalysis for the Dehydrogenation of Homocycles

In 2015, Li and co-workers reported the first heterogeneously catalyzed light-driven dehydrogenation of homocyclic hydrocarbons (Scheme ). The addition of light energy enables the Pt atoms in the heterogeneous Pt/TiO₂ Mott–Schottky-type photocatalyst to abstract two hydrogen atoms from cyclohexane, forming the cyclohexene intermediate. The same mechanism then facilitates the formation of the cyclohexadiene intermediate, which rapidly undergoes further dehydrogenation to yield benzene as the final product. In total, three equivalents of dihydrogen gas are released. Notably, the substrate scope of this reaction is limited to cyclohexane. A kinetic isotope effect (KIE) of k _h/k _d = 5.8 was observed when comparing cyclohexane and cyclohexane-d ₁₂ (C₆D₁₂), indicating that C–H bond cleavage is the rate-limiting step. Importantly, the overall reaction exhibits an AQE of 6%, with 100% product selectivity and a conversion rate of 99%. These performance metrics make this photocatalytic system particularly promising for applications in liquid organic hydrogen carriers, provided the substrate scope can be expanded.

33. Proposed Mechanism for Photochemical Dehydrogenation of Cyclohexane over a Heterogeneous Pt/TiO₂ Photocatalyst .

5.3. Homogeneous Photocatalysis for the Dehydrogenation of N-Heterocycles

The first homogeneous light-driven endothermic dehydrogenation of N-heterocycles was reported by Li and co-workers in 2017. , The reaction utilized [Ru^II(bpy)₃]­Cl₂ (46a) and Co^III(dmgH)₂ClPy (6)previously applied in the dehydrogenation of hydrazobenzene and n-octane, respectivelyto fully dehydrogenate 1,1′,2,2′,3,3′,4,4′-octahydro-2,2′-biquinoline to 2,2′-biquinoline, releasing four equivalents of dihydrogen gas. Both product and hydrogen yields reached 99%. The reaction was found to favor electron-donating groups over electron-withdrawing groups at the para position. AQE and KIE were not analyzed.

A few years later, in 2022, Mata and co-workers reported the light-driven endothermic dehydrogenation of 1,2,3,4-tetrahydroquinoline using a binuclear iridium-based photocatalyst, [{Ir^III(Cp*)­(Cl)}₂(thbpym)]. The proposed mechanism involves initial ligand exchange, where the acetonitrile ligand dissociates to allow binding of the tetrahydroquinoline substrate, followed by β-hydride elimination adjacent to the nitrogen atom to form an iridium hydride complex. Upon visible light irradiation, the singlet complex is excited to its triplet state, facilitating proton transfer from the cationic substrate to the activated Ir–H complex. This yields a 3,4-dihydroquinoline intermediate and releases the first equivalent of dihydrogen gas. A second heterolytic C–H bond cleavage completes the formation of aromatic quinoline and releases a second equivalent of H₂. A maximum product yield of 99% was obtained. In line with Li and co-workers, the reaction favored electron-donating groups over electron-withdrawing groups at the para position. AQE, KIE, and hydrogen yield were not reported.

In the same year, König and co-workers reported light-driven endothermic dehydrogenation of N-heterocycles using a transition metal-based photocatalytic system (Scheme ). The system combined [Ir^III{dF­(CF₃)­ppy}₂(dtbpy)]­PF₆ (114) as photocatalyst and Ni^IIBr₂(dtbbpy) (124) as proton reduction catalyst. Notably, the bromide ligand within the nickel complex serves as an electron shuttle. Measured redox potentials support the proposed mechanism: (E _1/2 oxi = +1.21 V vs SCE for [Ir^III{dF­(CF₃)­ppy}₂(dtbpy)]­PF₆), (E _1/2 oxi = +0.66 V vs SCE for bromide anion), (E _1/2 red = −1.37 V vs SCE for [Ir^III{dF­(CF₃)­ppy}₂(dtbpy)]­PF₆), (E _1/2 red = −1.17 V vs SCE for Ni^IIBr₂(dtbbpy)).

34. Proposed Mechanism for Photochemical Dehydrogenation of N-Heterocycles Using a Homogeneous Photocatalytic System Consisting of an Iridium-Based Photosensitizer and a Nickel-Based Proton Reduction Catalyst .

The mechanism begins with an anion exchange, allowing bromide coordination to the Ir photocatalyst (114) (Scheme ). Upon excitation, the Ir complex undergoes single electron transfer (SET) with bromide, generating a bromine radical and a reduced Ir­(II) species. The bromine radical then abstracts a hydrogen atom from the N-heterocycle substrate (122) via HAT, yielding HBr and an alkyl radical intermediate (125). A second SET reduces Ni²⁺Br₂(dtbbpy) (124) to a Ni­(I) complex, which subsequently undergoes oxidative addition with the alkyl radical to form an organonickel species (126). β-Hydride elimination then gives the monoalkene product (127) and a hydridic Ni­(II)–H complex. The Ni–H species reacts with the proton from HBr to evolve dihydrogen gas and regenerate the bromide anion. Importantly, the substrate scope of the reaction was expanded to include a pyridone derivative (123), which yielded a dehydrogenated pyridone product (128).

The rate and product yield of the above-mentioned reaction can be enhanced by the addition of weak base additives, such as dipotassium phosphate (Scheme ). KIE experiments confirmed that the HAT step involving the bromine radical is rate-limiting. The combination of regioselective control and broad functional group tolerance enabled the photochemical synthesis of 12 biologically relevant molecules. The reaction is unlikely to proceed via a radical chain mechanism, as the measured quantum yield was only 1.4%. The maximum product yield reached 92%, while the yield of dihydrogen gas was not reported.

Very recently, in 2025, Cai and co-workers published acceptorless dehydrogenation of 1,2,3,4-tetrahydroquinoline in acetone at room temperature using a combination of 4CzIPN as organophotocatalyst and Co­(dmgH)₂ClPy (6) as cocatalyst. Interestingly, changing the solvent to dichloroethane (DCE) shifted preference of the major product (Scheme ). Acetone promotes dehydrogenation of 1,2,3,4-tetrahydroquinoline toward the corresponding heteroarene, while DCE opens an alternative pathway of dehydrogenative coupling yielding biheteroarenes. According to the authors, the observed solvent dependence originates from the ability of DCE to facilitate cross-coupling of enamine and imine intermediates due to its increased acidity compared to acetone (pK _a 19.7 and 26.5 in DMSO, respectively).

35. Solvent Dependent Light-Driven Dehydrogenation of 1,2,3,4-Tetrahydroquinoline Using a Homogeneous Dual Organophotoredox Catalytic System, Consisting of 4CzIPN as Photocatalyst and a Cobaloxime Proton Reduction Catalyst .

5.4. Heterogeneous Photocatalysis for the Dehydrogenation of N-Heterocycles

The first light-driven dehydrogenation reactions of N-heterocycles under heterogeneous conditions were reported by Wang and co-workers in 2018 (Scheme ). Hexagonal borocarbonitride (h-BCN) was employed as a metal-free photocatalyst to dehydrogenate 1,2,3,4-tetrahydroisoquinolines (129) in water. The choice of h-BCN was based on its ability to facilitate dehydrogenation under visible light irradiation, as supported by a band gap of 2.7 eV and a valence band edge potential of −1.3 V vs the normal hydrogen electrode (Figure ). The oxidation potentials of the 1,2,3,4-tetrahydroisoquinoline substrates are below 1.0 V.

36. Proposed Mechanism for Photochemical Dehydrogenation of Tetrahydroisoquinolines over a Heterogeneous Hexagonal-Borocarbonitride Nanosheet Photocatalyst .

Upon light irradiation, electron–hole pairs are generated. The photogenerated holes oxidize the 1,2,3,4-tetrahydroisoquinoline (129) to yield a radical cation intermediate (130) (Scheme ). Subsequent proton loss leads to an anionic radical cation intermediate (131), which resonates with the neutral radical intermediate (132). A second hole oxidizes intermediate 132, releasing another proton and generating the 3,4-dihydroisoquinoline intermediate (133). This species is in resonance with the 1,4-dihydroisoquinoline intermediate (134), and tautomerization of the CN double bond can yield a 1,2-dihydroisoquinoline intermediate (135). Repetition of the h-BCN photocatalytic cycle ultimately affords the fully aromatic isoquinoline product (136). Overall, reduction of four protons by four electrons leads to the evolution of two equivalents of dihydrogen gas.

Both substituent positions (R¹ and R²) of the initial substrates (137) and (138) tolerate a wide range of electron-donating and electron-withdrawing groups (Scheme ). An hydroxy group at the meta position (138) afforded the highest product yield of 95%. The dihydrogen gas yield was not reported. Additionally, the authors found that the presence of nitrogen atoms in the h-BCN photocatalyst enhances dehydrogenation efficiency by decreasing the reaction’s endothermicity. The quantum yield of the reaction is only 2%, indicating that a radical chain mechanism is unlikely. This reaction can also be extended to a variety of other N-heterocycles, including hydroquinolines, hydroisoquinolines, and indolines.

Several other heterogeneous photocatalysts for the light-driven dehydrogenation of N-heterocycles have been reported (Table ). These systems are typically activated by irradiation at wavelengths between 400 and 500 nm and are employed in polar solvents such as acetonitrile–water mixtures, isopropanol, and pyridine. Notably, Rh-based photocatalysts generally require longer reaction times compared to others such as MoS₂/ZnIn₂S₄ and MIL-101@CdS–Ni. All reactions are performed under an inert atmosphere, except those using the h-BCN catalyst developed by Wang and co-workers. Across these studies, a wide range of electron-donating and electron-withdrawing substituents have been tested, revealing mixed trends in substituent effects. While some systems demonstrate robust functional group tolerance, , another achieves optimal yields only with the unsubstituted substrate. Collectively, these results suggest that substituent effects can be strategically exploited to enhance product yields, although in some catalysts increased steric demands may attenuate activity. Selectivity and conversion data are often not reported. − Importantly, these photocatalytic systems exhibit a broad substrate scope, demonstrating activity for tetrahydroquinolines, tetrahydroisoquinolines, and indolines.

4. Various Types of Photochemical Dehydrogenations of Tetrahydroisoquinoline and Tetrahydroquinoline , over Heterogeneous Photocatalysts under Ambient Conditions .

Photocatalyst	Light source	Solvent	MPY (%)	MHY (%)	FGP	T (°C), Atm, t (h)	Ref.
MoS2/ZnIn2S4	300 W Xe arc lamp, 500 nm	CH3CN:H2O (2:1, v/v)	85	95	/	25, N2, 12
Rh/TiO2	LEDs, 453 nm	i PrOH	99	99	EDG	25, Ar, 48
MIL-101@CdS-Ni	300 W Xe lamp, 450 nm	Pyridine	88	/	–H	25, Ar, 12

^aMPY = maximum product yield by different functional groups, MHY = maximum dihydrogen gas yield, FGP = functional group preference, EDG = electron-donating groups.

Taken together, cycloalkane dehydrogenations can be catalyzed using unary homogeneous photoactive Rh­(I) complexes, as well as binary homogeneous systems that combine Ir­(III) complexes or Fukuzumi-type acridinium salts as photosensitizers with Co­(III) or Ni­(II) complexes as proton-reduction catalysts. These homogeneous systems benefit from the presence of weak alkaline additives, which help reduce reaction times and increase both product and dihydrogen yields. Alternatively, heterogeneous catalysts, typically consisting of a semiconductive support (e.g., TiO₂) loaded with a transition metal active site such as Pt or Rh, can also be employed. Both homogeneous and heterogeneous systems tolerate a broad range of electron-withdrawing and electron-donating groups, although the latter generally result in higher product and dihydrogen yields. Cycloalkane dehydrogenations are commonly performed under inert atmosphere, at temperatures between 20 and 30 °C, using light irradiation in the 420–500 nm range and polar solvents. Interestingly, under comparable conditions (ambient temperatures and inert atmosphere), reaction times for both homo- and heterocycle dehydrogenations tend to be shorter in heterogeneous systems than in homogeneous ones. Unfortunately, most studies do not report values for the AQE or KIE, limiting direct comparison of catalytic efficiencies.

6. Conclusions and Outlook

The substantial energy requirements of thermal dehydrogenation reactions have spurred increasing research interest in photochemistry as a strategy to overcome their inherent thermodynamic limitations. In this review, we explore various types of light-driven dehydrogenation reactions to develop a deeper understanding of the underlying mechanisms and identify emerging trends in the literature. Our scope encompasses four categories of acceptorless dehydrogenation reactions, namely: (i) dehydrogenation of linear sp³-hybridized bonds, (ii) dehydrogenative coupling, (iii) dehydrogenative cyclization, and (iv) dehydrogenation of homo- and N-heterocyclic hydrocarbons.

Three distinct trends have emerged in the development of catalysts for light-driven dehydrogenation reactions. First, homogeneous unary photoactive Rh­(I) catalysts have been shown to promote the dehydrogenation of both linear and cyclic sp³-hybridized C–C bonds in hydrocarbon substrates via oxidative C–H activation followed by β-hydride elimination. Second, binary homogeneous photocatalytic systems, typically composed of a Ru­(II), Ir­(III), or metal-free acridinium salt photosensitizer combined with a transition-metal-based proton reduction catalyst (e.g., Co­(III) or Ni­(II)), enable all four types of dehydrogenation reactions via single-electron transfer mechanisms involving a range of bonds (C–C, C–O, C–N, and C–S). Third, heterogeneous catalysts employed in light-driven dehydrogenation reactions often consist of a semiconductive support material (e.g., TiO₂, ZnO, CdS) integrated with a transition-metal-based active site (e.g., Pd or Pt), operating through Mott–Schottky-type photoinduced charge separation.

Light-driven dehydrogenation has emerged as a rapidly growing field, enabling the selective activation of saturated C–H, N–H, and O–H bonds under mild conditions. Substantial progress over the past 15 years suggests that synthesis of olefins, pharmaceuticals, and fine chemicals could be driven by light rather than heat, offering an efficient fully electric approach that reduces carbon emissions compared with fossil-based processes and avoids energy losses associated with converting electricity into heat. Despite these advances, the number of reactions that proceed at room temperature under solvent-free conditions remains limited. In homogeneous systems, base additives are commonly used to enhance reactivity, however, such solvents and additives generate waste streams and complicate product separation.

For a limited set of linear and cyclic sp³-hybridized substrates, solvent- and additive-free light-driven dehydrogenation has been achieved using noble-metal catalysis. In this context, linear sp³-hybridized substrates require temperatures of 80–100 °C, whereas cyclic sp³-hybridized substrates can be dehydrogenated at room temperature. Light-driven dehydrogenation of these cyclic hydrocarbons is particularly attractive for dihydrogen storage, as it can reduce the high energy demand associated with releasing dihydrogen from liquid organic hydrogen carriers by harnessing light. Although photocatalytic dehydrogenation of cyclohexane to benzene can be achieved at room temperature, dehydrogenation of polycyclic hydrocarbons has so far been limited to partially aromatic substrates such as tetralin or tetrahydro­(iso)­quinoline. Optimizing these reactions to convert fully saturated compounds into their aromatic counterparts under solvent-free conditions would significantly improve the energy efficiency of dihydrogen storage in liquid organic hydrogen carriers.

Coupling and cyclization are key transformations in the synthesis of fine chemicals and pharmaceuticals. Most reported light-driven coupling and cyclization reactions are performed in polar solvents at room temperature. The waste footprint of these processes could be reduced by exploring environmentally benign alternatives, including sustainable solvents or solvent-free conditions. In addition, the dihydrogen stream generated in these reactions should be valorized. To date, solvent-free light-driven coupling reactions have been achieved only under heterogeneous conditions. For light-driven cyclization reactions, only a limited number of heterogeneous systems have been reported, yet these heterogeneous approaches have the potential to eliminate solvent dependence.

With this review, we aim to enhance understanding of the interactions and mechanisms that govern light-driven dehydrogenation reactions, thereby supporting the rational design of new photocatalytic systems that may reduce dependence on precious or geopolitically sensitive metals such as Ru, Rh, Ir, Pd, Pt, and Co. Furthermore, while mechanistic proposals exist for most of the systems discussed, additional experimental (e.g., apparent quantum efficiencies and quantum yields) or computational studies will be essential to further elucidate the interactions driving photoinduced dehydrogenation and to deepen mechanistic insight across the field.

Biographies

Caroline J. Verhoef holds MSc degrees from the University of Amsterdam in Social Sciences (Sociology) and Chemistry (Molecular Sciences, joint degree with the Vrije Universiteit Amsterdam). After finishing her MSc thesis in computational and physical organic chemistry under supervision of Prof. Chris Slootweg at the University of Amsterdam, she commenced her PhD in the same group in 2024. Her current research focuses on hydrogenation and dehydrogenation reactions for large-scale storage of dihydrogen gas in chemical carriers.

Runxuan Ma received his BSc degree in Chemistry and Biotechnology from Jacobs University in Bremen in 2023. Next, he continued his study career at the University of Amsterdam and the Vrije Universiteit Amsterdam (joint degree), where he is currently doing a research internship under the supervision of Prof.dr. Joost Reek on supramolecular homogeneous photocatalysis.

J. Chris Slootweg is full professor of circular chemistry at the University of Amsterdam. The mission of his laboratory is to educate students at the intersection of fundamental physical organic chemistry, main-group chemistry, and circular chemistry.

‡.

C.J.V., R.M.: These authors contributed equally.

The authors declare no competing financial interest.