Manganese-Catalyzed Oxidative Azidation of C(sp3)-H Bonds under Electrophotocatalytic Conditions

Unlocking flavin photoacid catalysis through electrophotochemistry

Molecular flavins are one of the most versatile photocatalysts. They can coordinate single and multiple electron transfer processes, gift hydrogen atoms, form reversible covalent linkages that support group transfer mechanisms, and impart photonic energy to ground state molecules, priming them for downstream reactions. But one mechanism that has not featured extensively is the ability of flavins to act as photoacids. Herein, we disclose our proof-of-concept studies showing that electrophotochemistry can transform fully oxidized flavin quinones to super-oxidized flavinium photoacids that successfully guide proton-transfer and deliver acid-catalyzed products. We also show that these species can adopt a second mechanism wherein they react with water to release hydroxyl radicals that facilitate hydrogen-atom abstraction and sp3C-H functionalization protocols. Together, this unprecedented bimodal reactivity enables electro-generated flavinium salts to affect synthetic chemistries previously unknown to flavins, greatly expanding their versatility as catalysts.

Electrophotocatalysis Versus Indirect Electrolysis: Electrochemical Selenocyclization of 3-Aza-1,5-dienes Facilitated by Energy Transfer, Direct Photolysis or N-Hydroxyphthalimide

Three hybrid electrochemical protocols, which involve the energy transfer, direct photolysis and N-hydroxyphthalimide catalyst, respectively, are presented for the selenylation/cyclization of the fragile substrates of 3-aza-1,5-dienes with diorganyl diselenides to afford 3-selenomethyl-4-pyrrolin-2-ones. The two electrophotocatalytic reactions and the indirect electrolysis one are both regioselective and external-oxidant- and transition-metal-free, and are associated with a broad substrate scope and high Se-economy, and all three methods are amenable to gram-scale syntheses, late-stage functionalizations, sunlight-induced experiments and all-solar-driven syntheses.

Photoelectrochemical Fe/Ni cocatalyzed C-C functionalization of alcohols

The simultaneous activation of reactants on the anode and cathode via paired electrocatalysis has not been extensively demonstrated. This report presents a paired oxidative and reductive catalysis based on earth-abundant iron/nickel cocatalyzed C-C functionalization of ubiquitous alcohols. A variety of alcohols (i.e., primary, secondary, tertiary, or unstrained cyclic alcohols) can be activated at very low oxidation potential of (~0.30 V vs. Ag/AgCl) via photoelectrocatalysis coupled with versatile electrophiles. This reactivity yields a wide range of structurally diverse molecules with broad functional group compatibility (more than 50 examples).

Electrochemical Allylic C(sp3)-H Isothiocyanation via [3,3]-Sigmatropic Rearrangement

The direct allylic C(sp3)-H functionalization provides a straightforward protocol for the synthesis of valuable molecules. We report herein the first chemo- and site-selective method for allylic C(sp3)-H isothiocyanation of various internal alkenes under mild electrochemical conditions. This method exhibits broad functional group tolerance and excellent selectivity and can be applied for late-stage isothiocyanation of bioactive molecules. Combined experimental and computational studies indicate that the reaction proceeds via an unexpected [3,3]-sigmatropic rearrangement.

Organophotoelectrocatalytic C(sp2)-H alkylation of heteroarenes with unactivated C(sp3)-H compounds

An organophotoelectrocatalytic method for the C(sp2)-H alkylation of heteroarenes with unactivated C(sp3)-H compounds through dehydrogenation cross-coupling has been developed. The C(sp2)-H alkylation combines organic catalysis, photochemistry and electrochemistry, avoiding the need for external metal-reagents, HAT-reagents, and oxidants. This protocol exhibits good substrate tolerance and functional group compatibility, providing a straightforward and powerful pathway to access a variety of alkylated heteroarenes under green conditions.

Organocatalyzed Photoelectrochemistry for the Generation of Acyl and Phosphoryl Radicals through Hydrogen Atom-Transfer Process

An organocatalyzed photoelectrochemical method for the generation of acyl and phosphoryl radicals from formamides, aldehydes, and phosphine oxides has been developed. This protocol utilizes 9,10-phenanthrenequinone (PQ) as both a molecular catalyst and a hydrogen atom-transfer (HAT) reagent, eliminating the requirement for external metal-based reagents, HAT reagents, and oxidants. The generated acyl radicals can be applied to a range of radical-mediated transformation reactions, including C-H carbamoylation of heteroarenes, intermolecular tandem radical cyclization of CF3-substituted N-arylacrylamides, as well as intramolecular cyclization reactions. The use of acyl radicals in these transformations offers an efficient and sustainable approach to accessing structurally diverse carbonyl compounds.

Electrochemical dual oxidative C(sp3)-H amination: switchable synthesis of imidazo-fused quinazolinones

An efficient electrochemical dual C(sp3)-H amination was developed under metal-free and chemical oxidant-free conditions. A series of imidazo[1,5-a]quinazolin-5(4H)-ones and 5-oxo-4,5-dihydroimidazo[1,5-a]quinazoline-3-carbonitriles can be obtained in high yields and the product distribution can be modulated by virtue of this method. The reaction mechanism was investigated and the corresponding intermediates were studied. The reaction features a broad substrate scope, regulation of the product distribution, mild conditions and scalable preparation.

Electrochemical-induced solvent-tuned selective C(sp3)-H bond activation towards the synthesis of C3-functionalized chromone derivatives

An unprecedented solvent-tuned electrochemical method for selective C(sp3)-H bond activation towards the synthesis of C3 functionalized chromone derivatives has been developed. This electrosynthesis protocol provides an efficient and green way to access various C3-functionalized chromones by avoiding traditionally employed transition metals and high temperatures. The swappable chemoselectivity was controlled mainly by altering the solvent and the current. A plausible reaction mechanism has been proposed with the help of radical capture and cyclic voltammetry experiments.

Electrochemical Benzylic C-H Amination via N-Aminopyridinium

An electrochemical protocol for benzylic C(sp3)-H aminopyridylation via direct C-H/N-H cross-coupling of alkylarenes with N-aminopyridinium triflate has been developed. This method features excellent site-selectivity, broad substrate scope, redox reagent-free and facile scalability. The generated benzylaminopyridiniums can be readily converted to benzylamines via electroreductive N-N bond cleavage.

Dual Nickel/Photoredox-Catalyzed Asymmetric Carbamoylation of Benzylic C(sp3 )-H Bonds

Radical-mediated Hydrogen Atom Abstraction of Csp3 -H bonds has become a powerful tool for the asymmetric functionalization of organic feedstocks. Here, we present an asymmetric synthesis of α-aryl amides via carbamoylation of alkylarenes with isocyanates as electrophiles. The synergistic combination of a photoredox and a chiral nickel-catalyst, enables the use of readily available and neutral reagents under mild reaction conditions and provides straightforward access to pharmacologically relevant motifs in enantiomerically pure form.

Simple, catalytic C(sp3)-H azidation using the C-H donor as the limiting reagent

C-N bonds play a critical role in pharmaceutical, agrochemical, and materials sciences, necessitating ever-better methods to forge this linkage. Here we report a simple procedure for direct C(sp3)-H azidation using iron or manganese catalysis and a nucleophilic azide source. All reagents are commercially available, the experimental procedure is simple, and we can use the C-H donor substrate as the limiting reagent, a challenge for many C-H azidation methods. Preliminary experiments are consistent with a hydrogen atom transfer (HAT)/radical ligand transfer (RLT) radical cascade mechanism and a wide variety of substrates can be azidated in moderate to high yields.

Electrochemical C-H/C-C Bond Oxygenation: A Potential Technology for Plastic Depolymerization

Herein, we provide eco-friendly and safely operated electrocatalytic methods for the selective oxidation directly or with water, air, light, metal catalyst or other mediators serving as the only oxygen supply. Heavy metals, stoichiometric chemical oxidants, or harsh conditions were drawbacks of earlier oxidative cleavage techniques. It has recently come to light that a crucial stage in the deconstruction of plastic waste and the utilization of biomass is the selective activation of inert C(sp3 )-C/H(sp3 ) bonds, which continues to be a significant obstacle in the chemical upcycling of resistant polyolefin waste. An appealing alternative to chemical oxidations using oxygen and catalysts is direct or indirect electrochemical conversion. An essential transition in the chemical and pharmaceutical industries is the electrochemical oxidation of C-H/C-C bonds. In this review, we discuss cutting-edge approaches to chemically recycle commercial plastics and feasible C-C/C-H bonds oxygenation routes for industrial scale-up.

Blue-Light Irradiated Mn(0)-Catalyzed Hydroxylation and C(sp3 )-H Functionalization of Unactivated Alkanes with C(sp2 )-H Bonds of Quinones for Alkylated Hydroxy Quinones and Parvaquone

Site-selective C(sp3 )-H functionalization of unreactive hydrocarbons is always challenging due to its inherited chemical inertness, slightly different reactivity of various C-H bonds, and intrinsically high bond dissociation energies. Here, a site-selective C-H alkylation of naphthoquinone with unactivated hydrocarbons using Mn2 (CO)10 as a catalyst under blue-light (457 nm) irradiation without any external acid or base and pre-functionalization is presented. The selective C-H functionalization of tertiary over secondary and secondary over primary C(sp3 )-H bonds in abundant chemical feedstocks was achieved, and hydroxylation of quinones was realized in situ by employing the developed methodology. This protocol provides a new catalytic system for the direct construction of high-value-added compounds, namely, parvaquone (a commercially available drug used to treat theileriosis) and its derivatives under ambient reaction conditions. Moreover, this operationally simple protocol applies to various linear-, branched-, and cyclo-alkanes with high degrees of site selectivity under blue-light irradiated conditions and could provide rapid and straightforward access to versatile methodologies for upgrading feedstock chemicals. Mechanistic insight by radical trapping, radical scavenging, EPR, and other controlled experiments well corroborated with DFT studies suggest that the reaction proceeds by a radical pathway.

Electrochemical decarboxylative alkylation of β-ketoacids with phenol derivatives

An electrochemical method for the decarboxylative alkylation of β-ketoacids with phenol derivatives has been developed. The protocol was carried out in readily available unseparated cells at room temperature in the absence of catalysts and oxidants. The corresponding aryl ketones were obtained in satisfactory yields without additional electrolytes, and were easy to produce in gram-scale synthesis. Based on control experiments and cyclic voltammetry, a plausible reaction mechanism was proposed.

Electrophotocatalytic hydrogenation of imines and reductive functionalization of aryl halides

The open-shell catalytically active species, like radical cations or radical anions, generated by one-electron transfer of precatalysts are widely used in energy-consuming redox reactions, but their excited-state lifetimes are usually short. Here, a closed-shell thioxanthone-hydrogen anion species (3), which can be photochemically converted to a potent and long-lived reductant, is generated under electrochemical conditions, enabling the electrophotocatalytic hydrogenation. Notably, TfOH can regulate the redox potential of the active species in this system. In the presence of TfOH, precatalyst (1) reduction can occur at low potential, so that competitive H2 evolution can be inhibited, thus effectively promoting the hydrogenation of imines. In the absence of TfOH, the reducing ability of the system can reach a potency even comparable to that of Na0 or Li0, thereby allowing the hydrogenation, borylation, stannylation and (hetero)arylation of aryl halides to construct C-H, C-B, C-Sn, and C-C bonds.

Photo- and electro-chemical strategies for the activations of strong chemical bonds

The employment of light and/or electricity - alternatively to conventional thermal energy - unlocks new reactivity paradigms as tools for chemical substrate activations. This leads to the development of new synthetic reactions and a vast expansion of chemical spaces. This review summarizes recent developments in photo- and/or electrochemical activation strategies for the functionalization of strong bonds - particularly carbon-heteroatom (C-X) bonds - via: (1) direct photoexcitation by high energy UV light; (2) activation via photoredox catalysis under irradiation with relatively lower energy UVA or blue light; (3) electrochemical reduction; (4) combination of photocatalysis and electrochemistry. Based on the types of the targeted C-X bonds, various transformations ranging from hydrodefunctionalization to cross-coupling are covered with detailed discussions of their reaction mechanisms.

The Recent Advances in Iron-Catalyzed C(sp3 )-H Functionalization

The use of iron as a core metal in catalysis has become a research topic of interest over the last few decades. The reasons are clear. Iron is the most abundant transition metal on Earth's crust and it is widely distributed across the world. It has been extracted and processed since the dawn of civilization. All these features render iron a noncontaminant, biocompatible, nontoxic, and inexpensive metal and therefore it constitutes the perfect candidate to replace noble metals (rhodium, palladium, platinum, iridium, etc.). Moreover, direct C-H functionalization is one of the most efficient strategies by which to introduce new functional groups into small organic molecules. The majority of organic compounds contain C(sp3 )-H bonds. Given the enormous importance of organic molecules in so many aspects of existence, the utilization and bioactivity of C(sp3 )-H bonds are of the utmost importance. This review sheds light on the substrate scope, selectivity, benefits, and limitations of iron catalysts for direct C(sp3 )-H bond activations. An overview of the use of iron catalysis in C(sp3 )-H activation protocols is summarized herein up to 2022.

Accelerated Electrophotocatalytic C(sp3 )-H Heteroarylation Enabled by an Efficient Continuous-Flow Reactor

Electrophotocatalytic transformations are garnering attention in organic synthesis, particularly for accessing reactive intermediates under mild conditions. Moving these methodologies to continuous-flow systems, or flow ElectroPhotoCatalysis (f-EPC), showcases potential for scalable processes due to enhanced irradiation, increased electrode surface, and improved mixing of the reaction mixture. Traditional methods sequentially link photochemical and electrochemical reactions, using flow reactors connected in series, yet struggle to accommodate reactive transient species. In this study, we introduce a new flow reactor concept for electrophotocatalysis (EPC) that simultaneously utilizes photons and electrons. The reactor is designed with a transparent electrode and employs cost-effective materials. We used this technology to develop an efficient process for electrophotocatalytic heteroarylation of C(sp3 )-H bonds. Importantly, the same setup can also facilitate purely electrochemical and photochemical transformations. This reactor represents a significant advancement in electrophotocatalysis, providing a framework for its application in flow for complex synthetic transformations.

Selective C(sp3)-H arylation/alkylation of alkanes enabled by paired electrocatalysis

We report a combination of electrocatalysis and photoredox catalysis to perform selective C(sp3)-H arylation/alkylation of alkanes, in which a binary catalytic system based on earth-abundant iron and nickel is applied. Reaction selectivity between two-component C(sp3)-H arylation and three-component C(sp3)-H alkylation is tuned by modulating the applied current and light source. Importantly, an ultra-low anodic potential (~0.23 V vs. Ag/AgCl) is applied in this protocol, thus enabling compatibility with a variety of functional groups (>70 examples). The robustness of the method is further demonstrated on a preparative scale and applied to late-stage diversification of natural products and pharmaceutical derivatives.

A breath of sunshine: oxygenic photosynthesis by functional molecular architectures

The conversion of light into chemical energy is the game-changer enabling technology for the energetic transition to renewable and clean solar fuels. The photochemistry of interest includes the overall reductive/oxidative splitting of water into hydrogen and oxygen and alternatives based on the reductive conversion of carbon dioxide or nitrogen, as primary sources of energy-rich products. Devices capable of performing such transformations are based on the integration of three sequential core functions: light absorption, photo-induced charge separation, and the photo-activated breaking/making of molecular bonds via specific catalytic routes. The key to success does not rely simply on the individual components' performance, but on their optimized integration in terms of type, number, geometry, spacing, and linkers dictating the photosynthetic architecture. Natural photosynthesis has evolved along this concept, by integrating each functional component in one specialized "body" (from the Greek word "soma") to enable the conversion of light quanta with high efficiency. Therefore, the natural "quantasome" represents the key paradigm to inspire man-made constructs for artificial photosynthesis. The case study presented in this perspective article deals with the design of artificial photosynthetic systems for water oxidation and oxygen production, engineered as molecular architectures then rendered on electrodic surfaces. Water oxidation to oxygen is indeed the pervasive oxidative reaction used by photosynthetic organisms, as the source of reducing equivalents (electrons and protons) to be delivered for the processing of high-energy products. Considering the vast and abundant supply of water (including seawater) as a renewable source on our planet, this is also a very appealing option for photosynthetic energy devices. We will showcase the progress in the last 15 years (2009-2023) in the strategies for integrating functional building blocks as molecular photosensitizers, multi-redox water oxidation catalysts and semiconductor materials, highlighting how additional components such as redox mediators, hydrophilic/hydrophobic pendants, and protective layers can impact on the overall photosynthetic performance. Emerging directions consider the modular tuning of the multi-component device, in order to target a diversity of photocatalytic oxidations, expanding the scope of the primary electron and proton sources while enhancing the added-value of the oxidation product beyond oxygen: the selective photooxidation of organics combines the green chemistry vision with renewable energy schemes and is expected to explode in coming years.

Organomediated electrochemical fluorosulfonylation of aryl triflates via selective C-O bond cleavage

Although aryl triflates are essential building blocks in organic synthesis, the applications as aryl radical precursors are limited. Herein, we report an organomediated electrochemical strategy for the generation of aryl radicals from aryl triflates, providing a useful method for the synthesis of aryl sulfonyl fluorides from feedstock phenol derivatives under very mild conditions. Mechanistic studies indicate that key to success is to use catalytic amounts of 9, 10-dicyanoanthracene as an organic mediator, enabling to selectively active aryl triflates to form aryl radicals via orbital-symmetry-matching electron transfer, realizing the anticipated C-O bond cleavage by overcoming the competitive S-O bond cleavage. The transition-metal-catalyst-free protocol shows good functional group tolerance, and may overcome the shortages of known methods for aryl sulfonyl fluoride synthesis. Furthermore, this method has been used for the modification and formal synthesis of bioactive molecules or tetraphenylethylene (TPE) derivative with improved quantum yield of fluorescence.

Photoelectrocatalyzed undirected C-H trifluoromethylation of arenes: catalyst evaluation and scope

During the last few years, photoelectrocatalysis has evolved as an increasingly viable tool for molecular synthesis. Despite several recent reports on the undirected C-H functionalization of arenes, thus far, a detailed comparison of different catalysts is still missing. To address this, more than a dozen different mediators were employed in the trifluoromethylation of (hetero-)arenes to compare them in their efficacies.

Selective Radical Transfer in a Series of Nonheme Iron(III) Complexes

A series of nonheme iron complexes, FeIII(BNPAPh2O)(Lax)(Leq) (Lax/eq = N3-, NCS-, NCO-, and Cl-) have been synthesized using the previously reported BNPAPh2O- ligand. The ferrous analogs FeII(BNPAPh2O)(Lax) (Lax = N3-, NCS-, and NCO-) were also prepared. The complexes were structurally characterized using single crystal X-ray diffraction, which shows that all the FeIII complexes are six-coordinate, with one anionic ligand (Lax) in the H-bonding axial site and the other anionic ligand (Leq) in the equatorial plane, cis to the Lax ligand. The reaction of FeIII(BNPAPh2O-)(Lax)(Leq) with Ph3C• shows that one ligand is selectively transferred in each case. A selectivity trend emerges that shows •N3 is the most favored for transfer in each case to the carbon radical, whereas Cl• is the least favored. The NCO and NCS ligands showed an intermediate propensity for radical transfer, with NCS > NCO. The overall order of selectivity is N3 > NCS > NCO > Cl. In addition, we also demonstrated that H-bonding has a small effect on governing product selectivity by using a non-H-bonded ligand (DPAPh2O-). This study demonstrates the inherent radical transfer selectivity of nonhydroxo-ligated nonheme iron(III) complexes, which could be useful for efforts in synthetic and (bio)catalytic C-H functionalization.

Photoelectrochemical oxidative C(sp3)-H borylation of unactivated hydrocarbons

Organoboron compounds are of high significance in organic synthesis due to the unique versatility of boryl substituents to access further modifications. The high demand for the incorporation of boryl moieties into molecular structures has witnessed significant progress, particularly in the C(sp3)-H borylation of hydrocarbons. Taking advantage of special characteristics of photo/electrochemistry, we herein describe the development of an oxidative C(sp3)-H borylation reaction under metal- and oxidant-free conditions, enabled by photoelectrochemical strategy. The reaction exhibits broad substrate scope (>57 examples), and includes the use of simple alkanes, halides, silanes, ketones, esters and nitriles as viable substrates. Notably, unconventional regioselectivity of C(sp3)-H borylation is achieved, with the coupling site of C(sp3)-H borylation selectively located in the distal methyl group. Our method is operationally simple and easily scalable, and offers a feasible approach for the one-step synthesis of high-value organoboron building blocks from simple hydrocarbons, which would provide ample opportunities for drug discovery.

Electrochemical Late-Stage Functionalization

Late-stage functionalization (LSF) constitutes a powerful strategy for the assembly or diversification of novel molecular entities with improved physicochemical or biological activities. LSF can thus greatly accelerate the development of medicinally relevant compounds, crop protecting agents, and functional materials. Electrochemical molecular synthesis has emerged as an environmentally friendly platform for the transformation of organic compounds. Over the past decade, electrochemical late-stage functionalization (eLSF) has gained major momentum, which is summarized herein up to February 2023.

Organoelectrophotocatalytic Generation of Acyl Radicals from Formamides and Aldehydes: Access to Acylated 3-CF3-2-Oxindoles

Organoelectrophotocatalytic generation of acyl radicals from formamides and aldehydes to synthesize acylated 3-CF3-2-oxindoles has been developed. This protocol features a monocatalytic system using 9,10-phenanthrenequinone (PQ) both as a catalyst and as a hydrogen atom transfer (HAT) reagent, which avoids the use of an external HAT reagent, metal reagent, and oxidant. A variety of acylated 3-CF3-2-oxindoles have been obtained in satisfactory yields from CF3-substituted N-arylacrylamides via a tandem radical cyclization.

Ketyl Radical Coupling Enabled by Polycyclic Aromatic Hydrocarbon Electrophotocatalysts

Herein, we report a new class of electrophotocatalysts, polycyclic aromatic hydrocarbons, that promote the reduction of unactivated carbonyl compounds to generate versatile ketyl radical intermediates. This catalytic platform enables previously challenging intermolecular ketyl radical coupling reactions, including those that classic reductants (e.g., SmI2/HMPA) have failed to promote. More broadly, this study outlines an approach to fundamentally expand the array of reactive radical intermediates that can be generated via electrophotocatalysis by obviating the need for rapid mesolytic cleavage following substrate reduction.

Photoelectrochemical Asymmetric Catalysis Enables Enantioselective Heteroarylcyanation of Alkenes via C-H Functionalization

The asymmetric difunctionalization of alkenes, a method transforming readily accessible alkenes into enantioenriched chiral structures of high value, has long been a focal point of organic synthesis. Despite tremendous efforts in this domain, it remains a considerable challenge to devise enantioselective oxidative dicarbofunctionalization of alkenes, even though these transformations can utilize stable and unfunctionalized functional group donors. In this context, we report herein a photoelectrocatalytic method for the enantioselective heteroarylcyanation of aryl alkenes, which employs unfunctionalized heteroarenes through C-H functionalization. The photoelectrochemical asymmetric catalysis (PEAC) method combines photoredox catalysis and asymmetric electrocatalysis to facilitate the formation of two C-C bonds operating via hydrogen (H2) evolution and obviating the need for external chemical oxidants.

Polymer up-cycling by mangana-electrocatalytic C(sp3)-H azidation without directing groups

The chemical up-cycling of polymers into value-added materials offers a unique opportunity to place plastic waste in a new value chain towards a circular economy. Herein, we report the selective up-cycling of polystyrenes and polyolefins to C(sp3)-H azidated materials under electrocatalytic conditions. The functionalized polymers were obtained with high retention of mass average molecular mass and high functionalization through chemo-selective mangana-electrocatalysis. Our strategy proved to be broadly applicable to a variety of homo- and copolymers. Polyethylene, polypropylene as well as post-consumer polystyrene materials were functionalized by this approach, thereby avoiding the use of hypervalent-iodine reagents in stoichiometric quantities by means of electrocatalysis. This study, hence, represents a chemical oxidant-free polymer functionalization by electro-oxidation. The electrocatalysis proved to be scalable, which highlights its unique feature for a green hydrogen economy by means of the hydrogen evolution reaction (HER).

Switchable Direct Oxygenative Arylation of C(sp3)-H Bonds via Electrophotocatalysis

A metal-free electrophotochemical C(sp3)-H arylation was developed under mild conditions. This method enables a switchable synthesis of diaryl alcohols and diaryl alkanes from inactive benzylic carbons. More importantly, a cheap and safe mediator N-chlorosuccinimide (NCS) was developed, which was employed for the hydrogen atom transfer (HAT) process of the benzylic C-H bond. In addition, this active radical was captured and identified by electron paramagnetic resonance (EPR).

Electrochemically Driven Hydrogen Atom Transfer Catalysis: A Tool for C(sp3)/Si-H Functionalization and Hydrofunctionalization of Alkenes

Electrochemically driven hydrogen atom transfer (HAT) catalysis provides a complementary approach for the transformation of redox-inactive substrates that would be inaccessible to conventional electron transfer (ET) catalysis. Moreover, electrochemically driven HAT catalysis could promote organic transformations with either hydrogen atom abstraction or donation as the key step. It provides a versatile and effective tool for the direct functionalization of C(sp³)-H/Si-H bonds and the hydrofunctionalization of alkenes. Despite these attractive properties, electrochemically driven HAT catalysis has been largely overlooked due to the lack of understanding of both the catalytic mechanism and how catalyst selection should occur. In this Review, we give an overview of the HAT catalysis applications in the direct C(sp³)-H/Si-H functionalization and hydrofunctionalization of alkenes. The mechanistic pathways, physical properties of the HAT mediators, and state-of-the-art examples are described and discussed.

Resurgence and advancement of photochemical hydrogen atom transfer processes in selective alkane functionalizations

The selective functionalization of alkanes has long been recognized as a prominent challenge and an arduous task in organic synthesis. Hydrogen atom transfer (HAT) processes enable the direct generation of reactive alkyl radicals from feedstock alkanes and have been successfully employed in industrial applications such as the methane chlorination process, etc. Nevertheless, challenges in the regulation of radical generation and reaction pathways have created substantial obstacles in the development of diversified alkane functionalizations. In recent years, the application of photoredox catalysis has provided exciting opportunities for alkane C-H functionalization under extremely mild conditions to trigger HAT processes and achieve radical-mediated functionalizations in a more selective manner. Considerable efforts have been devoted to building more efficient and cost-effective photocatalytic systems for sustainable transformations. In this perspective, we highlight the recent development of photocatalytic systems and provide our views on current challenges and future opportunities in this field.

Photoredox-HAT Catalysis for Primary Amine α-C-H Alkylation: Mechanistic Insight with Transient Absorption Spectroscopy

The synergistic use of (organo)photoredox catalysts with hydrogen-atom transfer (HAT) cocatalysts has emerged as a powerful strategy for innate C(sp³)-H bond functionalization, particularly for C-H bonds α- to nitrogen. Azide ion (N₃^-) was recently identified as an effective HAT catalyst for the challenging α-C-H alkylation of unprotected, primary alkylamines, in combination with dicyanoarene photocatalysts such as 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN). Here, time-resolved transient absorption spectroscopy over sub-picosecond to microsecond timescales provides kinetic and mechanistic details of the photoredox catalytic cycle in acetonitrile solution. Direct observation of the electron transfer from N₃^- to photoexcited 4CzIPN reveals the participation of the S₁ excited electronic state of the organic photocatalyst as an electron acceptor, but the N₃^• radical product of this reaction is not observed. Instead, both time-resolved infrared and UV-visible spectroscopic measurements implicate rapid association of N₃^• with N₃^- (a favorable process in acetonitrile) to form the N₆^•- radical anion. Electronic structure calculations indicate that N₃^• is the active participant in the HAT reaction, suggesting a role for N₆^•- as a reservoir that regulates the concentration of N₃^•.

Blue Light Irradiated Metal-, Oxidant-, and Base-Free Cross-Dehydrogenative Coupling of C(sp2)-H and N-H Bonds: Amination of Naphthoquinones with Amines

Herein, we report a blue-light-driven amination of C(sp²)-H bond of naphthoquinones and quinones with the N-H bond of primary and secondary amines for the synthesis of 2-amino-naphthoquinones and 2-amino-quinones. The coupling of naphthoquinones with a wide array of aliphatic, aromatic, chiral, primary, and secondary amines having electron donating (-CH₃, -OCH₃, -SCH₃), withdrawing (-F, -Cl, -Br, -I), and CO₂H, -OH, -NH₂ groups with acidic protons selectively occurred to afford C-N coupled 2-amino-naphthoquinones in 60-99% yields and hydrogen gas as a byproduct in methanol solvent without using any additional reagents, additives, and oxidant under the blue light irradiation. Mechanistic insight by DFT computation, controlled experiments, kinetic isotopic effect, and substitution effect of the substrates suggest that the reaction proceeds by radical pathway in which naphthoquinone forms a highly oxidizing naphthoquinonyl biradical upon irradiation of blue light (457 nm). Consequently, electron transfer from electron-rich amine to an oxidizing naphthoquinonyl biradical leads to a naphthoquinonyl radical anion and aminyl radical cation, followed by proton transfer and delocalization leading to a carbon-centered naphthoquinonyl radical. The cross-coupling of naphthoquinonyl carbon-centered and aminyl nitrogen radicals forms a C-N bond, with subsequent elimination of hydrogen gas (which was also confirmed by GC-TCD), affording 2-amino-1,4-naphthoquinone under metal-, reagent-, base-, and oxidant-free conditions.

Contemporary photoelectrochemical strategies and reactions in organic synthesis

In recent years, with the development of organic synthetic chemistry, a variety of organic synthetic methods have been discovered and applied in practical production. Photochemistry and electrochemistry have been widely used in organic synthesis recently due to their advantages such as mild conditions and green and environmental protection and have now been developed into two of the most massive synthetic strategies in the field of organic synthesis. In order to further enhance the potential of photochemistry and electrochemistry and to overcome the limitations of each, organic synthetic chemists have worked to combine the two synthetic strategies together to develop photoelectrochemistry as a new synthetic method. Photoelectrochemistry achieves the complementary advantages and disadvantages of photochemistry and electrochemistry, avoids the problem of using stoichiometric oxidants or reductants in photochemistry and easy dimerization in electrochemistry, generates highly reactive reaction intermediates under mild conditions, and achieves reactions that are difficult to accomplish by single photochemistry or electrochemistry. This review summarizes the research progress in the field of photoelectrochemistry from the perspective of photoelectro-chemical catalysts in recent years, analyzes the catalytic mechanism of various catalysts in detail, and gives a brief outlook on the research direction and development prospects in this field.

Selective Electroreductive Hydroboration of Olefins with B2pin2

Organoboron showed great potential in the synthesis of various high-value chemical compounds. Direct hydroboration of olefins has been witnessed over time as a mainstream method for the synthesis of organoboron compounds. In this work, an electroreductive anti-Markovnikov hydroboration approach of olefins with readily available B₂pin₂ to synthesize valuable organoboron compounds with high chemo- and regioselectivities under metal catalyst-free conditions was reported. This protocol exhibited broad substrate scope and good functional-group tolerance on styrenes and heteroaromatic olefins, providing synthetically useful alkylborons with high efficiency and even various deuterium borylation products with good D-incorporation when CD₃CN was employed as solvent. Furthermore, gram-scale reactions and extensive functional derivatization further highlighted the potential of this method.

Enantioselective Intermolecular Radical Amidation and Amination of Benzylic C-H Bonds via Dual Copper and Photocatalysis

A method for direct access to enantioenriched benzylic amides and carbamate-protected primary benzylamines by C-H functionalization is reported. The C-H substrate is used as limiting reagent with only a small excess of the unactivated amide or carbamate nucleophile. The enantioselective intermolecular dehydrogenative C-N bond formation is enabled by a combination of a chiral copper catalyst, a photocatalyst, and an oxidant, and it takes place under mild conditions, which allow for a broad substrate scope. The method is compatible with late-stage C-H functionalization, and it provides easy access to ¹⁵ N-labeled amides and amines starting from cheap ¹⁵ NH₄ Cl.

Iron Catalysis of C(sp3)-H Azidation Using a Heteroarene Radical Cation Strategy

The Fe^III(phen)₃ catalysis of the benzylic C(sp³)-H azidation of indoles has been investigated. The Fe(III) complex can selectively oxidize indoles to form arene radical cations, which are transformed into benzylic C(sp³) radical intermediates. This strategy exhibits a difference in reactivity between N-heteroarenes and benzene, which is difficult to achieve via direct hydrogen abstraction approaches. Various biorelevant azide precursors were constructed, highlighting the utility of this mild first-row transition-metal catalyst system.

Photoelectrochemical C-H Activation Through a Quinacridone Dye Enabling Proton-Coupled Electron Transfer

Dye-sensitized photoanodes for C-H activation in organic substrates are assembled by vacuum sublimation of a commercially available quinacridone (QNC) dye in the form of nanosized rods onto fluorine-doped tin oxide (FTO), TiO₂ , and SnO₂ slides. The photoanodes display extended absorption in the visible range (450-600 nm) and ultrafast photoinduced electron injection (<1 ps, as revealed by transient absorption spectroscopy) of the QNC dye into the semiconductor. The proton-coupled electron-transfer reactivity of QNC is exploited for generating a nitrogen-based radical as its oxidized form, which is competent in C-H bond activation. The key reactivity parameter is the bond-dissociation free energy (BDFE) associated with the N⋅/N-H couple in QNC of 80.5±2.3 kcal mol^-1 , which enables hydrogen atom abstraction from allylic or benzylic C-H moieties. A photoelectrochemical response is indeed observed for organic substrates characterized by C-H bonds with BDFE below the 80.5 kcal mol^-1 threshold, such as γ-terpinene, xanthene, or dihydroanthracene. This work provides a rational, mechanistically oriented route to the design of dye-sensitized photoelectrodes for selective organic transformations.

Electrochemical C(sp3)-H functionalization of ethers via hydrogen-atom transfer by means of cathodic reduction

The chemo- and stereoselective electrochemical allylation/alkylation of ethers is presented via a C(sp³)-H activation event. The electrosynthetic protocol enables the realization of a large library of functionalized ethers (35 examples) in high yields (up to 84%) via cathodic activation of a new type of redox-active carbonate (RAC), capable of triggering HAT (Hydrogen-Atom-Transfer) events through the generation of electrophilic oxy radicals. The process displayed high functional group tolerance and mild reaction conditions. A mechanistic elucidation via voltammetric analysis completes the study.

Mn-catalyzed electrooxidative radical phosphorylation of 2-isocyanobiaryls

As an efficient and green synthesis method, the electrocatalysis hydrogen evolution coupling reaction has been widely used by chemists to realize the combining of two nucleophiles. In this work, an alternative method to synthesize 6-phosphorylated phenanthridines has been developed by synergistically utilizing electrocatalysis and Mn catalysis, with moderate to relatively good yields achieved. Mild and oxidant-free conditions make this synthetic method applicable in various settings.

Computational Exploration of Dirhodium Complex-Catalyzed Selective Intermolecular Amination of Tertiary vs. Benzylic C-H Bonds

The mechanism and origins of site-selectivity of Rh₂(S-tfpttl)₄-catalyzed C(sp³)-H bond aminations were studied using density functional theory (DFT) calculations. The synergistic combination of the dirhodium complex Rh₂(S-tfpttl)₄ with tert-butylphenol sulfamate TBPhsNH₂ composes a pocket that can access both tertiary and benzylic C-H bonds. The nonactivated tertiary C-H bond was selectively aminated in the presence of an electronically activated benzylic C-H bond. Both singlet and triplet energy surfaces were investigated in this study. The computational results suggest that the triplet stepwise pathway is more favorable than the singlet concerted pathway. In the hydrogen atom abstraction by Rh-nitrene species, which is the rate- and site-selectivity-determining step, there is an attractive π-π stacking interaction between the phenyl group of the substrate and the phthalimido group of the ligand in the tertiary C-H activation transition structure. By contrast, such attractive interaction is absent in the benzylic C-H amination transition structure. Therefore, the DFT computational results clearly demonstrate how the synergistic combination of the dirhodium complex with sulfamate overrides the intrinsic preference for benzylic C-H amination to achieve the amination of the nonactivated tertiary C-H bond.

Organoelectrophotocatalytic C-H Silylation of Heteroarenes

An organoelectrophotocatalytic approach for the C-H silylation of heteroarenes through dehydrogenation cross-coupling with H₂ evolution has been developed. The organoelectrophotocatalytic strategy is carried out under a simple and efficient monocatalytic system by employing 9,10-phenanthrenequinone both as an organocatalyst and as a hydrogen atom transfer (HAT) reagent, which avoids the need for an external HAT reagent, an oxidant, or a metal reagent. A variety of heteroarenes can be compatible in satisfactory yields with excellent regioselectivity.

Electrophotocatalytic oxygenation of multiple adjacent C-H bonds

Oxygen-containing functional groups are nearly ubiquitous in complex small molecules. The installation of multiple C-O bonds by the concurrent oxygenation of contiguous C-H bonds in a selective fashion would be highly desirable but has largely been the purview of biosynthesis. Multiple, concurrent C-H bond oxygenation reactions by synthetic means presents a challenge1-6, particularly because of the risk of overoxidation. Here we report the selective oxygenation of two or three contiguous C-H bonds by dehydrogenation and oxygenation, enabling the conversion of simple alkylarenes or trifluoroacetamides to their corresponding di- or triacetoxylates. The method achieves such transformations by the repeated operation of a potent oxidative catalyst, but under conditions that are sufficiently selective to avoid destructive overoxidation. These reactions are achieved using electrophotocatalysis7, a process that harnesses the energy of both light and electricity to promote chemical reactions. Notably, the judicious choice of acid allows for the selective synthesis of either di- or trioxygenated products.

Photocatalytic Late-Stage C-H Functionalization

The emergence of modern photocatalysis, characterized by mildness and selectivity, has significantly spurred innovative late-stage C-H functionalization approaches that make use of low energy photons as a controllable energy source. Compared to traditional late-stage functionalization strategies, photocatalysis paves the way toward complementary and/or previously unattainable regio- and chemoselectivities. Merging the compelling benefits of photocatalysis with the late-stage functionalization workflow offers a potentially unmatched arsenal to tackle drug development campaigns and beyond. This Review highlights the photocatalytic late-stage C-H functionalization strategies of small-molecule drugs, agrochemicals, and natural products, classified according to the targeted C-H bond and the newly formed one. Emphasis is devoted to identifying, describing, and comparing the main mechanistic scenarios. The Review draws a critical comparison between established ionic chemistry and photocatalyzed radical-based manifolds. The Review aims to establish the current state-of-the-art and illustrate the key unsolved challenges to be addressed in the future. The authors aim to introduce the general readership to the main approaches toward photocatalytic late-stage C-H functionalization, and specialist practitioners to the critical evaluation of the current methodologies, potential for improvement, and future uncharted directions.

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

The development of an efficient and straightforward method for decarboxylative coupling using common alkyl carboxylic acid is of great value. However, decarboxylative coupling with nucleophiles always needs stoichiometric chemical oxidants or substrate prefunctionalization. Herein, we report a protocol for Cu-catalyzed enantioselective decarboxylative cyanation via the merger of photocatalysis and electrochemistry. CeCl₃ and Cu/BOX were used as co-catalysts to promote the decarboxylation and cyanation, and both catalysts were regenerated via anodic oxidation. This method establishes a proof of concept enantioselective transformation via photoelectrocatalysis. Studies by DFT calculations provided mechanistic insight on enantioselectivity control.

Electrochemical synthesis of vicinal azidoacetamides

Vicinal diamines are an important structural motif in bioactive natural products and pharmaceutical intermediates. Herein, an environmentally friendly and efficient electrochemical approach to azidoacetamides, as one variant of vicinal diamines, has been developed. This reaction features mild conditions and broad substrate scope, without the use of any chemical oxidant or transition-metal catalysts. The obtained vicinal azidoacetamides could be conveniently converted into various other vicinal diamine derivatives.

Manganese-Mediated Electrochemical Dearomatization of Indoles To Access 2-Azido Spirocyclic Indolines

An efficient and environmentally friendly electrochemical protocol for dearomatization of indoles was developed, delivering a series of azido-containing spirocyclic indolines with good functional group tolerance. This dearomatization process is proposed to result from the oxidation of Mn^II-N₃ species, supported by cyclic voltammetry experiments. Moreover, synthetic transformations can provide an alternative approach to a range of functionalized indolines.