Catalyzing Electrosynthesis: A Homogeneous Electrocatalytic Approach to Reaction Discovery

Multicomponent reactions and photo/electrochemistry join forces: atom economy meets energy efficiency

Visible-light photoredox catalysis has been regarded as an extremely powerful tool in organic chemistry, bringing the spotlight back to radical processes. The versatility of photocatalyzed reactions has already been demonstrated to be effective in providing alternative routes for cross-coupling as well as multicomponent reactions. The photocatalyst allows the generation of high-energy intermediates through light irradiation rather than using highly reactive reagents or harsh reaction conditions. In a similar vein, organic electrochemistry has experienced a fruitful renaissance as a tool for generating reactive intermediates without the need for any catalyst. Such milder approaches pose the basis toward higher selectivity and broader applicability. In photocatalyzed and electrochemical multicomponent reactions, the generation of the radical species acts as a starter of the cascade of events. This allows for diverse reactivity and the use of reagents is usually not covered by classical methods. Owing to the availability of cheaper and more standardized photo- and electrochemical reactors, as well as easily scalable flow-setups, it is not surprising that these two fields have become areas of increased research interest. Keeping these in view, this review is aimed at providing an overview of the synthetic approaches in the design of MCRs involving photoredox catalysis and/or electrochemical activation as a crucial step with particular focus on the choice of the difunctionalized reagent.

Designing Sites in Heterogeneous Catalysis: Are We Reaching Selectivities Competitive With Those of Homogeneous Catalysts?

A critical review of different prominent nanotechnologies adapted to catalysis is provided, with focus on how they contribute to the improvement of selectivity in heterogeneous catalysis. Ways to modify catalytic sites range from the use of the reversible or irreversible adsorption of molecular modifiers to the immobilization or tethering of homogeneous catalysts and the development of well-defined catalytic sites on solid surfaces. The latter covers methods for the dispersion of single-atom sites within solid supports as well as the use of complex nanostructures, and it includes the post-modification of materials via processes such as silylation and atomic layer deposition. All these methodologies exhibit both advantages and limitations, but all offer new avenues for the design of catalysts for specific applications. Because of the high cost of most nanotechnologies and the fact that the resulting materials may exhibit limited thermal or chemical stability, they may be best aimed at improving the selective synthesis of high value-added chemicals, to be incorporated in organic synthesis schemes, but other applications are being explored as well to address problems in energy production, for instance, and to design greener chemical processes. The details of each of these approaches are discussed, and representative examples are provided. We conclude with some general remarks on the future of this field.

Electrooxidative palladium- and enantioselective rhodium-catalyzed [3 + 2] spiroannulations

Despite indisputable progress in the development of electrochemical transformations, electrocatalytic annulations for the synthesis of biologically relevant three-dimensional spirocyclic compounds has as of yet not been accomplished. In sharp contrast, herein, we describe the palladaelectro-catalyzed C-H activation/[3 + 2] spiroannulation of alkynes by 1-aryl-2-naphthols. Likewise, a cationic rhodium(iii) catalyst was shown to enable electrooxidative [3 + 2] spiroannulations via formal C(sp3)-H activations. The versatile spiroannulations featured a broad substrate scope, employing electricity as a green oxidant in lieu of stoichiometric chemical oxidants under mild conditions. An array of spirocyclic enones and diverse spiropyrazolones, bearing all-carbon quaternary stereogenic centers were thereby accessed in a user-friendly undivided cell setup, with molecular hydrogen as the sole byproduct.

Electrochemical Benzylic C(sp3)-H Isothiocyanation

Selective C(sp³)-H isothiocyanation represents a significant strategy for the synthesis of isothiocyanate derivatives. We report herein an electrochemical benzylic isothiocyanation in a highly chemo- and site-selective manner under external oxidant-free conditions. The high chemoselectivity is attributed to the facile in situ isomerization of benzylic thiocyanates to isothiocyanates. Notably, the method exhibits high functional group compatibility and is suitable for late-stage functionalization of bioactive molecules.

Paired electrolysis enabled annulation for the quinolyl-modification of bioactive molecules

A paired electrolysis enabled cascade annulation that enables the efficient synthesis of highly functionalized quinoline-substituted bioactive molecules from readily available starting materials is reported. Using this methodology, two goals, namely, the direct synthesis of quinolines and the introduction of quinoline moieties to bioactive molecules, can be simultaneously achieved in one simple operation. The use of electroreduction for the activation of isatin, together with the further anodic oxidation of KI to catalytically result in a cascade annulation, highlight the unique possibilities associated with electrochemical activation methods. This transformation can tolerate a wide range of functional groups and can also be used as a functionalization tactic in pharmaceutical research as well as other areas.

A paired electrolysis enabled cascade annulation that enables the efficient synthesis of highly functionalized quinoline-substituted bioactive molecules from readily available starting materials is reported.

Complementary Copper-Catalyzed and Electrochemical Aminosulfonylation of O-Homoallyl Benzimidates and N-Alkenyl Amidines with Sodium Sulfinates

A complementary copper-catalyzed and electrochemical aminosulfonylation of O-homoallyl benzimidates and N-alkenyl amidines with sodium sulfinates was developed. The terminal alkene substrate produced sulfone-containing 1,3-oxazines and tetrahydropyrimidines in the presence of Cu(OAc)₂, Ag₂CO₃, and DPP, and under similar reaction conditions, sulfonylated tetrahydro-1,3-oxazepines were prepared from 1-aryl-substituted O-homoallyl benzimidates in moderate to good yields. For certain electron-rich 1,1-diaryl-substituted alkene substrates, the corresponding tetrahydro-1,3-oxazepines could also be obtained in similar or even higher yields via a green electrochemical technique.

Photochemical and electrochemical C-N borylation of arylhydrazines

The C-N borylation of arylhydrazine hydrochlorides with bis(pinacolato)diboron was achieved under photochemical and electrochemical conditions, respectively. This novel and scalable transformation provides two efficient and mild transition-metal-free synthetic routes towards aryl boronate esters from easily available arylhydrazines.

Electroreductive synthesis of polyfunctionalized pyridin-2-ones from acetoacetanilides and carbon disulfide with oxygen evolution

A chemical reductant or a sacrificial electron donor is required in any reduction reactions, generally resulting in undesired chemical waste. Herein, we report a reductant-free reductive [3 + 2 + 1] annulation of β-keto amides with CS₂ enabled by the synergy of electro/copper/base using water as an innocuous anodic sacrifice with O₂ as a sustainable by-product. This electrochemical protocol is mild and provides access to polyfunctionalized pyridin-2-ones from simple starting materials in a single step.

Electrochemical Ring-Opening Dicarboxylation of Strained Carbon-Carbon Single Bonds with CO2: Facile Synthesis of Diacids and Derivatization into Polyesters

Diacids are important monomers in the polymer industry to construct valuable materials. Dicarboxylation of unsaturated bonds, such as alkenes and alkynes, with CO₂ has been demonstrated as a promising synthetic method. However, dicarboxylation of C─C single bonds with CO₂ has rarely been investigated. Herein we report a novel electrochemical ring-opening dicarboxylation of C─C single bonds in strained rings with CO₂. Structurally diverse glutaric acid and adipic acid derivatives were synthesized from substituted cyclopropanes and cyclobutanes in moderate to high yields. In contrast to oxidative ring openings, this is also the first realization of an electroreductive ring-opening reaction of strained rings, including commercialized ones. Control experiments suggested that radical anions and carbanions might be the key intermediates in this reaction. Moreover, this process features high step and atom economy, mild reaction conditions (1 atm, room temperature), good chemoselectivity and functional group tolerance, low electrolyte concentration, and easy derivatization of the products. Furthermore, we conducted polymerization of the corresponding diesters with diols to obtain a potential UV-shielding material with a self-healing function and a fluorine-containing polyester, whose performance tests showed promising applications.

Photons or Electrons? A Critical Comparison of Electrochemistry and Photoredox Catalysis for Organic Synthesis

Redox processes are at the heart of synthetic methods that rely on either electrochemistry or photoredox catalysis, but how do electrochemistry and photoredox catalysis compare? Both approaches provide access to high energy intermediates (e.g., radicals) that enable bond formations not constrained by the rules of ionic or 2 electron (e) mechanisms. Instead, they enable 1e mechanisms capable of bypassing electronic or steric limitations and protecting group requirements, thus enabling synthetic chemists to disconnect molecules in new and different ways. However, while providing access to similar intermediates, electrochemistry and photoredox catalysis differ in several physical chemistry principles. Understanding those differences can be key to designing new transformations and forging new bond disconnections. This review aims to highlight these differences and similarities between electrochemistry and photoredox catalysis by comparing their underlying physical chemistry principles and describing their impact on electrochemical and photochemical methods.

Picomole-Scale Transition Metal Electrocatalysis Screening Platform for Discovery of Mild C-C Coupling and C-H Arylation through in Situ Anodically Generated Cationic Pd

Development of new transition-metal-catalyzed electrochemistry promises to improve overall synthetic efficiency. Here, we describe the first integrated platform for online screening of electrochemical transition-metal catalysis. It utilizes the intrinsic electrochemical capabilities of nanoelectrospray ionization mass spectrometry (nano-ESI-MS) and picomole-scale anodic corrosion of a Pd electrode to generate and evaluate highly efficient cationic catalysts for mild electrocatalysis. We demonstrate the power of the novel electrocatalysis platform by (1) identifying electrolytic Pd-catalyzed Suzuki coupling at room temperature, (2) discovering Pd-catalyzed electrochemical C-H arylation in the absence of external oxidant or additive, (3) developing electrolyzed Suzuki coupling/C-H arylation cascades, and (4) achieving late-stage functionalization of two drug molecules by the newly developed mild electrocatalytic C-H arylation. More importantly, the scale-up reactions confirm that new electrochemical pathways discovered by nano-ESI can be implemented under the conventional electrolytic reaction conditions. This approach enables in situ mechanistic studies by capturing various intermediates including transient transition metal species by MS, and thus uncovering the critical role of anodically generated cationic Pd catalyst in promoting otherwise sluggish transmetalation in C-H arylation. The anodically generated cationic Pd with superior catalytic efficiency and novel online electrochemical screening platform hold great potential for discovering mild transition-metal-catalyzed reactions.

Electrochemical Sulfonylation-Induced Lactonization of Alkenes: Synthesis of Sulfonyl Phthalides

An electrochemical cascade sulfonylation and lactonization process of alkenes and a most widely used arylsulfonylation reagent─sulfonyl hydrazines─was developed for the first time. This electrochemical sulfonyl lactonization avoided the use of toxic metal catalysts or stoichiometric oxidants and was carried out under mild conditions. The target product γ-sulfonylated phthalides with broad and excellent substrate tolerance were achieved.

An electrocatalytic three-component reaction for the synthesis of phosphoroselenoates

Phosphoroselenoates are important organic molecules because they have found widespread applications in many fields.

Site-selective electrochemical thiocyanation of benzylic C–H bonds

Direct benzylic C(sp3)−H thiocyanation is explored as a straightforward strategy toward the synthesis of thiocyanate derivatives. We report herein an electrochemical protocol for site-selective benzylic C(sp3)−H thiocyanation under mild reaction...

Aqueous electrochemically-triggered atom transfer radical polymerization

Simplified electrochemical atom transfer radical polymerization (seATRP) using Cu^II–N-propyl pyridineimine complexes (Cu^II(NPPI)₂) is reported for the first time. In aqueous solution, using oligo(ethylene glycol) methyl ether methacrylate (OEGMA), standard electrolysis conditions yield POEGMA with good control over molecular weight distribution (Đ_m < 1.35). Interestingly, the polymerizations are not under complete electrochemical control, as monomer conversion continues when electrolysis is halted. Alternatively, it is shown that the extent and rate of polymerization depends upon an initial period of electrolysis. Thus, it is proposed that seATRP using Cu^II(NPPI)₂ follows an electrochemically-triggered, rather than electrochemically mediated, ATRP mechanism, which distinguishes them from other Cu^IIL complexes that have been previously reported in the literature.

Simplified electrochemical atom transfer radical polymerization (seATRP) using Cu^II-pyridineimine complexes is reported and follows a previously unreported electrochemically triggered mechanism.

Atropoenantioselective Palladaelectro-Catalyzed Anilide C–H Olefinations Viable with Natural Sunlight as Sustainable Power Source

Enantioselective electrocatalyzed transformations represent a major challenge. We herein achieved atropoenantioselective pallada-electrocatalyzed C–H olefinations and C–H allylations with high efficacy and enantioselectivity under exceedingly mild reaction conditions. With (S)-5-oxoproline as the chiral ligand, activated and non-activated olefins were suitable substrates for the electro-C–H activations. Dual catalysis was devised in terms of electro-C–H olefination, along with catalytic hydrogenation. Challenging enantiomerically-enriched chiral anilide scaffolds were thereby obtained with high levels of enantio-control in the absence of toxic and cost-intensive silver salts. The resource-economy of the transformation was even improved by directly employing renewable solar energy.

Asymmetric pallada-electrocatalyzed C–H activation of achiral anilides were accomplished by catalyst control with high levels of enantioselectivity. Dual catalysis was devised, while photovoltaic cells could be used to empower the electrocatalysis.

Electrochemically-promoted synthesis of benzo[b]thiophene-1,1-dioxides via strained quaternary spirocyclization

We report an approach for the synthesis of benzothiophene motifs under electrochemical conditions by the reaction of sulfonhydrazides with internal alkynes. Upon the formation of a quaternary spirocyclization intermediate by the selective ipso-addition instead of an ortho-attack, the S-migration process was rationalized to lead to the products. Computational studies revealed the selectivity and the compatibility of drug molecules showcased the potential application of the protocols.

We report an approach for the synthesis of benzothiophene motifs under electrochemical conditions by the reaction of sulfonhydrazides with internal alkynes.

Non-directed Pd-catalysed electrooxidative olefination of arenes

The Fujiwara–Moritani reaction is a powerful tool for the olefination of arenes by Pd-catalysed C–H activation. However, the need for superstoichiometric amounts of toxic chemical oxidants makes the reaction unattractive from an environmental and atom-economical view. Herein, we report the first non-directed and regioselective olefination of simple arenes via an electrooxidative Fujiwara–Moritani reaction. The versatility of this operator-friendly approach was demonstrated by a broad substrate scope which includes arenes, heteroarenes and a variety of olefins. Electroanalytical studies suggest the involvement of a Pd(ii)/Pd(iv) catalytic cycle via a Pd(iii) intermediate.

The Fujiwara–Moritani reaction using electric current is a powerful tool for the olefination of arenes by Pd-catalysed C–H activation.

Electrochemical oxidative cyclization of alkenes, boronic acids, and dichalcogenides to access chalcogenated boronic esters and 1,3-diols

A sustainable, environmentally benign electrochemical oxidative three-component cyclization of allylic alcohols, boronic acids, and dichalcogenides under metal-free and oxidant-free conditions has been developed.

Electrochemical Enantioselective Dihydroxylation Reaction of N-Alkenyl Nucleobases for the Construction of Chiral Acyclic Nucleosides

A simple and sustainable KI-mediated electrochemical enantioselective dihydroxylation reaction of N-alkenyl nucleobases was developed for the first time in an undivided cell. A series of chiral acyclic nucleosides bearing two...

Efficient one-step synthesis of 3-(indol-2-yl)quinoxalin-2(1H)-ones via electrochemical oxidative cross-dehydrogenative coupling

A novel and efficient synthetic strategy for 3-(indol-2-yl)quinoxalin-2(1H)-ones was developed via electrochemical oxidative cross-dehydrogenative coupling of indoles and quinoxalin-2(1H)-ones.

Nickel-Catalyzed Decarboxylative Cross-Coupling of Indole-3-acetic Acids with Aryl Bromides by Convergent Paired Electrolysis

Herein, nickel-catalyzed decarboxylative cross-coupling of indole-3-acetic acids with aryl bromides by convergent paired electrolysis was developed in an undivided cell. This protocol features good functional group tolerance, chemical redox agent-...

Ruthenaelectro-Catalyzed C–H Acyloxylation for Late-Stage Tyrosine and Oligopeptide Diversification

Ruthenaelectro(ii/iv)-catalyzed intermolecular C–H acyloxylations of phenols have been developed by guidance of experimental, CV and computational insights. The use of electricity bypassed the need for stoichiometric chemical oxidants. The sustainable electrocatalysis strategy was characterized by ample scope, and its unique robustness enabled the late-stage C–H diversification of tyrosine-derived peptides.

Ruthenaelectro(ii/iv)-catalyzed intermolecular C–H acyloxylations of oligopeptides have been developed by the guidance of key experimental, CV and computational insights.

Epoxide Electroreduction

Selective hydrogenation of epoxides would be a direct and powerful approach for alcohol synthesis, but it has proven to be elusive. Here, electrochemically epoxide hydrogenation using electrons and protons as reductants is reported. A wide range of primary, secondary, and tertiary alcohols can be achieved through selective Markovnikov or anti-Markovnikov ring opening in the absence of transition metals. Mechanistic investigations revealed that the regioselectivity is controlled by the thermodynamic stabilities of the in situ generated benzyl radicals for aryl-substituted epoxides and the kinetic tendency for Markovnikov selective ring opening for alkyl-substituted epoxides.

Electrochemical Synthesis of Cyclic Diaryl Phosphinamides via Intramolecular sp2 C-H Phosphinamidation

We developed an oxidant- and transition-metal-free approach to construct six-membered cyclic phosphinamides via an intramolecular electrochemical C-H phosphinamidation process. With ⁿBu₄NBr as the catalyst and electrolyte, cyclic phosphinamides bearing a variety of functional groups (22 examples) were readily accessed under mild conditions. Meanwhile, this protocol provided an alternative route to organic electroluminescent materials and P-N ligands.

Nickel-Catalyzed Chemo- and Regioselective Benzylarylation of Unactivated Alkenes with o-Bromobenzyl Chlorides

Chemo- and regioselectively nickel-catalyzed reductive benzylarylation of unactivated alkenes with o-bromobenzyl chlorides is disclosed herein, in which electrophiles participate through a single-component double-site approach. Moreover, its utility is underscored by the concise synthesis of bioactive Indane compounds and postreaction functionalizations leading to structurally diverse scaffolds. Preliminary mechanistic investigations suggest a radical chain reaction mechanism.

Synthesis of active pharmaceutical ingredients using electrochemical methods: keys to improve sustainability

Organic electrochemistry is receiving renewed attention as a green and cost-efficient synthetic technology. Electrochemical methods promote redox transformations by electron exchange between electrodes and species in solution, thus avoiding the use of stoichiometric amounts of oxidizing or reducing agents. The rapid development of electroorganic synthesis over the past decades has enabled the preparation of molecules of increasing complexity. Redox steps that involve hazardous or waste-generating reagents during the synthesis of active pharmaceutical ingredients or their intermediates can be substituted by electrochemical procedures. In addition to enhance sustainability, increased selectivity toward the target compound has been achieved in some cases. Electroorganic synthesis can be safely and readily scaled up to production quantities. For this pupose, utilization of flow electrolysis cells is fundamental. Despite these advantages, the application of electrochemical methods does not guarantee superior sustainability when compared with conventional protocols. The utilization of large amounts of supporting electrolytes, enviromentally unfriendly solvents or sacrificial electrodes may turn electrochemistry unfavorable in some cases. It is therefore crucial to carefully select and optimize the electrolysis conditions and carry out green metrics analysis of the process to ensure that turning a process electrochemical is advantageous.

A Strategy for Site- and Chemoselective C-H Alkenylation through Osmaelectrooxidative Catalysis

Herein, we disclose osmaelectrocatalyzed C-H activations that set the stage for electrooxidative alkyne annulations by benzoic acids. The osmium electrocatalysis enables site- and chemoselective electrooxidative C-H activations with unique levels of selectivity. The isolation of unprecedented osmium(0) and osmium(II) intermediates, along with crystallographic characterization and analyses by spectrometric and spectroscopic in operando techniques delineate a synergistic osmium redox catalyst regime. Detailed mechanistic studies revealed a facile C-H cleavage, which allows for an ample substrate scope, providing provide robust and user-friendly access to annulated heterocycles.

Electrochemical Difunctionalization of Terminal Alkynes: Access to 1,4-Dicarbonyl Compounds

1,4-Dicarbonyl compounds are versatile scaffolds for the heterocycle synthesis, including the Paal-Knorr reaction. Herein, a feasible electrosynthesis method to access 1,4-dicarbonyl compounds has been developed from simple alkynes and 1,3-dicarbonyl compounds. When the undivided cell is combined with the constant current mode, aryl alkynes containing numerous medicinal motifs with 1,3-dicarbonyl esters or ketones react smoothly. External oxidant and catalyst-free conditions conform to the requirements of green synthesis.

Synthesis of 1H-indazoles by an electrochemical radical Csp2-H/N-H cyclization of arylhydrazones

The development of efficient and sustainable C-N bond-forming reactions to N-heterocyclic frameworks has been a long-standing interest in organic synthesis. In this work, we develop an electrochemical radical C_sp²-H/N-H cyclization of arylhydrazones to 1H-indazoles. The electrochemical anodic oxidation approach was adopted to synthesize a variety of 1H-indazole derivatives in moderate to good yields. HFIP was not only employed as a solvent or the proton donor, but also can promote the formation of N free radicals. This synthetic methodology is operationally simple, and less expensive electrodes would be suitable for this chemistry.

Electrochemical Synthesis of 1,2,3-Thiadiazoles from α-Phenylhydrazones

We have developed an electrochemical approach for the synthesis of fully substituted 1,2,3-thiadiazoles from α-phenylhydrazones at room temperature, which is very challenging and complementary to the conventional thermal reactions. The key step involves anodic oxidation of phenylhydrazone derivatives at a constant current followed by N,S-heterocyclization. The protocol is remarkable in that it is free of a base and free of an external oxidant and can be converted to a gram scale for postsynthetic drug development with functional thiadiazoles. Most importantly, the electrochemical transformation reflected efficient electro-oxidation with an operationally friendly easy procedure with ample functional molecules. Cyclic voltammograms support the mechanism of this electro-oxidative cyclization process.

Nickel-catalyzed electrochemical carboxylation of unactivated aryl and alkyl halides with CO2

Electrochemical catalytic reductive cross couplings are powerful and sustainable methods to construct C-C bonds by using electron as the clean reductant. However, activated substrates are used in most cases. Herein, we report a general and practical electro-reductive Ni-catalytic system, realizing the electrocatalytic carboxylation of unactivated aryl chlorides and alkyl bromides with CO₂. A variety of unactivated aryl bromides, iodides and sulfonates can also undergo such a reaction smoothly. Notably, we also realize the catalytic electrochemical carboxylation of aryl (pseudo)halides with CO₂ avoiding the use of sacrificial electrodes. Moreover, this sustainable and economic strategy with electron as the clean reductant features mild conditions, inexpensive catalyst, safe and cheap electrodes, good functional group tolerance and broad substrate scope. Mechanistic investigations indicate that the reaction might proceed via oxidative addition of aryl halides to Ni(0) complex, the reduction of aryl-Ni(II) adduct to the Ni(I) species and following carboxylation with CO₂.

PhB(OH)2-Promoted Electrochemical Sulfuration-Formyloxylation of Styrenes and Selectfluor-Mediated Oxidation-Olefination

We report a PhB(OH)₂-promoted electrochemical sulfuration-formyloxylation reaction of styrenes employing commercially available thiophenols/thiols as thiolating agents. Specifically, metal catalysts and external chemical oxidants are not needed in the reaction for the formation of β-formyloxy sulfides, and these sulfides can be further converted to (E)-vinyl sulfones via the Selectfluor-mediated oxidation-olefination. Notably, on the basis of this electrochemical oxidation strategy, β-hydroxy sulfide, β-formyloxy sulfoxide, β-formyloxy sulfone, and (E)-vinyl sulfoxide can also be easily prepared.

Covalent Organic Framework (COF)-Based Hybrids for Electrocatalysis: Recent Advances and Perspectives

Developing highly efficient electrocatalysts for renewable energy conversion and environment purification has long been a research priority in the past 15 years. Covalent organic frameworks (COFs) have emerged as a burgeoning family of organic materials internally connected by covalent bonds and have been explored as promising candidates in electrocatalysis. The reticular geometry of COFs can provide an excellent platform for precise incorporation of the active sites in the framework, and the fine-tuning hierarchical porous architectures can enable efficient accessibility of the active sites and mass transportation. Considerable advances are made in rational design and controllable fabrication of COF-based organic-inorganic hybrids, that containing organic frameworks and inorganic electroactive species to induce novel physicochemical properties, and take advantage of the synergistic effect for targeted electrocatalysis with the hybrid system. Branches of COF-based hybrids containing a diversity form of metals, metal compounds, as well as metal-free carbons have come to the fore as highly promising electrocatalysts. This review aims to provide a systematic and profound understanding of the design principles behind the COF-based hybrids for electrocatalysis applications. Particularly, the structure-activity relationship and the synergistic effects in the COF-based hybrid systems are discussed to shed some light on the future design of next-generation electrocatalysts.