Evidence for Photocatalyst Involvement in Oxidative Additions of Nickel-Catalyzed Carboxylate O-Arylations

Mechanistic insights into the visible-light-driven O-arylation of carboxylic acids catalyzed by xanthine-based nickel complexes

We present a photocatalytic protocol for the O-arylation of carboxylic acids using nickel complexes bearing C8-pyridyl xanthines. Our studies suggest that the underlying mechanism operates independently of external photosensitizers. Stoichiometric experiments and crystallographic studies characterize the catalytically relevant Ni complexes. Spectroscopic and computational investigations propose a thermally controlled Ni(i)/Ni(iii) cycle followed by a photochemical regeneration of Ni(i) species. Furthermore, the pathways leading to the hydrodehalogenation of aryl halides, the comproportionation of Ni(i) and Ni(iii) species, the dimerization of Ni(i) intermediates and the influence of the counter ion on the cross-coupling reaction are unveiled. These investigations offer a comprehensive mechanistic understanding of the photocatalytic cross-coupling reaction catalyzed by a single Ni species and highlight key aspects of nickel-catalyzed metallaphotoredox reactions.

Synthesis, Structure, and Redox Reactivity of Ni Complexes Bearing a Redox and Acid-Base Non-innocent Ligand with NiII, NiIII, and NiIV Formal Oxidation States

A series of Ni complexes bearing a redox and acid-base noninnocent tetraamido macrocyclic ligand, H4-(TAML-4) {H4-(TAML-4) = 15,15-dimethyl-5,8,13,17-tetrahydro-5,8,13,17-tetraaza-dibenzo[a,g]cyclotridecene-6,7,14,16-tetraone}, with formal oxidation states of NiII, NiIII, and NiIV were synthesized and characterized structurally and spectroscopically. The X-ray crystallographic analysis of the Ni complexes revealed a square planar geometry, and the [Ni(TAML-4)] complex with the formal oxidation state of NiIV was characterized to be [NiIII(TAML-4•+)] with the oxidation state of the NiIII ion and the one-electron oxidized TAML-4 ligand, TAML-4•+. The NiIII oxidation state and the TAML-4 radical cation ligand, TAML-4•+, were supported by X-ray absorption spectroscopy and density functional theory calculations. The reversible interconversions between [NiII(TAML-4)]2- and [NiIII(TAML-4)]- and between [NiIII(TAML-4)]- and [NiIII(TAML-4•+)] were demonstrated in spectroelectrochemical measurements as well as in chemical oxidation and reduction reactions. The reactivities of [NiIII(TAML-4)]- and [NiIII(TAML-4•+)] were then investigated in hydride transfer reactions using NADH analogs. Hydride transfer from 9,10-dihydro-10-methylacridine (AcrH2) to [NiIII(TAML-4•+)] was found to proceed via electron transfer (ET) from AcrH2 to [NiIII(TAML-4•+)] with no deuterium kinetic isotope effect (kH/kD = 1.0(2)). In contrast, hydride transfer from AcrH2 to [NiIII(TAML-4)]- proceeded much more slowly via a concerted proton-coupled electron transfer (PCET) process with kH/kD = 7.0(5). In the latter reaction, an electron and a proton were transferred to the NiIII center and the TAML-4 ligand, respectively. The mechanisms of the ET by [NiIII(TAML-4•+)] and the concerted PCET by [NiIII(TAML-4)]- were ascribed to the different redox potentials of the Ni complexes.

Activation and C-C Coupling of Aryl Iodides via Bismuth Photocatalysis

Within the emerging field of bismuth redox catalysis, the catalytic formation of C-C bonds using aryl halides would be highly desirable; yet such a process remains a synthetic challenge. Herein, we present a chemoselective bismuth-photocatalyzed activation and subsequent coupling of (hetero)aryl iodides with pyrrole derivatives to access C(sp2)-C(sp2) linkages through C-H functionalization. This unique reactivity is the result of the bismuth complex featuring two redox state-dependent interactions with light, which 1) activates the Bi(I) complex for oxidative addition via MLCT, and 2) promotes the homolytic cleavage of aryl Bi(III) intermediates through a LLCT process.

Ba/Ti MOF: A Versatile Heterogeneous Photoredox Catalyst for Visible-Light Metallaphotocatalysis

The field of sustainable heterogeneous catalysis is evolving rapidly, with a strong emphasis on developing catalysts that enhance efficiency. Among various heterogeneous photocatalysts, metal-organic frameworks (MOFs) have gained significant attention for their exceptional performance in photocatalytic reactions. In this context, contrary to the conventional homogeneous iridium or ruthenium-based photocatalysts, which face significant challenges in terms of availability, cost, scalability, and recyclability, a new Ba/Ti MOF (ACM-4) is developed as a heterogeneous catalyst that can mimic/outperform the conventional photocatalysts, offering a more sustainable solution for efficient chemical processes. Its redox potential and triplet energy are comparable to or higher than the conventional catalysts, organic dyes, and metal semiconductors, enabling its use in both electron transfer and energy transfer applications. It facilitates a broad range of coupling reactions involving pharmaceuticals, agrochemicals, and natural products, and is compatible with various transition metals such as nickel, copper, cobalt, and palladium as co-catalysts. The effectiveness of the ACM-4 as a photocatalyst is supported by comprehensive material studies, photophysical, and recycling experiments. These significant findings underscore the potential of ACM-4 as a highly versatile and cost-effective photoredox catalyst, providing a sustainable, one-material solution for efficient chemical processes.

Multi-Catalytic Metal-Based Homogeneous-Heterogeneous Systems in Organic Chemistry

The combination of metal-based homogeneous and heterogeneous catalysts in the same reaction media is a powerful, yet relatively unexplored approach in organic chemistry. This strategy can address important limitations associated with purely homogeneous or heterogeneous catalysis such as the incompatibility of different catalytic species in solution, or the limited tunability of solid catalysts, respectively. Moreover, the facile reusability of the solid catalyst, contributes to increase the overall sustainability of the process. As a result, this semi-heterogeneous multi-catalytic approach has unlocked significant advances in organic chemistry, improving existing reactions and even enabling the discovery of novel transformations, exemplified by the formal alkane metathesis. This concept article aims to showcase the benefits of this strategy through the exploration of diverse relevant examples from the literature, hoping to spur research on new metal-based homogeneous-heterogeneous catalyst combinations that will result in reactivity challenging to achieve by conventional homogeneous or heterogeneous catalysis alone.

Femtosecond Core-Level Spectroscopy Reveals Involvement of Triplet States in the Gas-Phase Photodissociation of Fe(CO)5

Excitation of iron pentacarbonyl [Fe(CO)5], a prototypical photocatalyst, at 266 nm causes the sequential loss of two CO ligands in the gas phase, creating catalytically active, unsaturated iron carbonyls. Despite numerous studies, major aspects of its ultrafast photochemistry remain unresolved because the early excited-state dynamics have so far eluded spectroscopic observation. This has led to the long-held assumption that ultrafast dissociation of gas-phase Fe(CO)5 proceeds exclusively on the singlet manifold. Herein, we present a combined experimental-theoretical study employing ultrafast extreme ultraviolet transient absorption spectroscopy near the Fe M2,3-edge, which features spectral evolution on 100 fs and 3 ps time scales, alongside high-level electronic structure theory, which enables characterization of the molecular geometries and electronic states involved in the ultrafast photodissociation of Fe(CO)5. We assign the 100 fs evolution to spectroscopic signatures associated with intertwined structural and electronic dynamics on the singlet metal-centered states during the first CO loss and the 3 ps evolution to the competing dissociation of Fe(CO)4 along the lowest singlet and triplet surfaces to form Fe(CO)3. Calculations of transient spectra in both singlet and triplet states as well as spin-orbit coupling constants along key structural pathways provide evidence for intersystem crossing to the triplet ground state of Fe(CO)4. Thus, our work presents the first spectroscopic detection of transient excited states during ultrafast photodissociation of gas-phase Fe(CO)5 and challenges the long-standing assumption that triplet states do not play a role in the ultrafast dynamics.

Mechanisms of Photoredox Catalysis Featuring Nickel-Bipyridine Complexes

Metallaphotoredox catalysis can unlock useful pathways for transforming organic reactants into desirable products, largely due to the conversion of photon energy into chemical potential to drive redox and bond transformation processes. Despite the importance of these processes for cross-coupling reactions and other transformations, their mechanistic details are only superficially understood. In this review, we have provided a detailed summary of various photoredox mechanisms that have been proposed to date for Ni-bipyridine (bpy) complexes, focusing separately on photosensitized and direct excitation reaction processes. By highlighting multiple bond transformation pathways and key findings, we depict how photoredox reaction mechanisms, which ultimately define substrate scope, are themselves defined by the ground- and excited-state geometric and electronic structures of key Ni-based intermediates. We further identify knowledge gaps to motivate future mechanistic studies and the development of synergistic research approaches spanning the physical, organic, and inorganic chemistry communities.

Mechanistic Evidence of a Ni(0/II/III) Cycle for Nickel Photoredox Amide Arylation

This work demonstrates the dominance of a Ni(0/II/III) cycle for Ni-photoredox amide arylation, which contrasts with other Ni-photoredox C-heteroatom couplings that operate via Ni(I/III) self-sustained cycles. The kinetic data gathered when using different Ni precatalysts supports an initial Ni(0)-mediated oxidative addition into the aryl bromide. Using NiCl2 as the precatalyst resulted in an observable induction period, which was found to arise from a photochemical activation event to generate Ni(0) and to be prolonged by unproductive comproportionation between the Ni(II) precatalyst and the in situ generated Ni(0) active species. Ligand exchange after oxidative addition yields a Ni(II) aryl amido complex, which was identified as the catalyst resting state for the reaction. Stoichiometric experiments showed that oxidation of this Ni(II) aryl amido intermediate was required to yield functionalized amide products. The kinetic data presented supports a rate-limiting photochemically-mediated Ni(II/III) oxidation to enable C-N reductive elimination. An alternative Ni(I/III) self-sustained manifold was discarded based on EPR and kinetic measurements. The mechanistic insights uncovered herein will inform the community on how subtle changes in Ni-photoredox reaction conditions may impact the reaction pathway, and have enabled us to include aryl chlorides as coupling partners and to reduce the Ni loading by 20-fold without any reactivity loss.

Visible-Light-Activated Nickel Thiolates for C-S Couplings

Thiolates are known as the inhibitors of metal catalysis due to their strong coordination with the metal. Herein, we reported visible-light-induced homolysis of the Ni-S bond to activate the nickel(II) thiolates for the C-S coupling, obviating the use of exogenous photocatalysts and other additives. Various aryl bromides/iodides can efficiently couple with thiols with a wide range of functional groups under mild conditions. Preliminary mechanistic studies suggested the homolysis of the Ni-S bond is the key step for couplings and nickel(0) is not involved in the process.

Aligning Metal Coordination Sites in Metal-Organic Framework-Enabled Metallaphotoredox Catalysis

Construction of catalytic metal centers, the key modules in artificial photosynthetic systems, lies at the heart to explore unpaved reactivity patterns powered by light. Here, we disclose that the amino (-NH₂) and carboxylic (-COO) functionalities, aligned in various visible-light-harvesting metal-organic frameworks (MOFs) (NH₂-UiO-66, (NH₂)₂-UiO-67, and NH₂-MIL-125), provide N/O-ligated Ni featuring different configurations and valence states. Of note, these Ni centers, in situ formed or preimplanted, demonstrated coordination units' spatial arrangement-dependent activity in cross-coupling of aryl halides and various nucleophiles. Our work provides a novel approach to construct and to regulate metal center(s) by MOFs' skeleton defined coordination environments, highlighting exclusive potential in exploring the reactivity pattern of the hosted metals.

Photoinduced Nickel-Catalyzed Carbon-Heteroatom Coupling

Herein, we report visible light-promoted single nickel catalysis for diverse carbon-heteroatom couplings under mild conditions. This mild, general, and robust method to couple diverse nitrogen, oxygen, and sulfur nucleophiles with aryl(heteroaryl)/alkenyl iodides/bromides exhibits a wide functional group tolerance and is applicable to late-stage modification of pharmaceuticals and natural products. On the base of preliminary mechanistic studies, a Ni^I /Ni^III cycle via the generation of active Ni^I complexes that appear from homolysis of Ni^II -I rather than Ni^II -aryl bond was tentatively proposed.

Intraligand Charge Transfer Enables Visible-Light-Mediated Nickel-Catalyzed Cross-Coupling Reactions

We demonstrate that several visible-light-mediated carbon-heteroatom cross-coupling reactions can be carried out using a photoactive Ni^II precatalyst that forms in situ from a nickel salt and a bipyridine ligand decorated with two carbazole groups (Ni(Czbpy)Cl₂ ). The activation of this precatalyst towards cross-coupling reactions follows a hitherto undisclosed mechanism that is different from previously reported light-responsive nickel complexes that undergo metal-to-ligand charge transfer. Theoretical and spectroscopic investigations revealed that irradiation of Ni(Czbpy)Cl₂ with visible light causes an initial intraligand charge transfer event that triggers productive catalysis. Ligand polymerization affords a porous, recyclable organic polymer for heterogeneous nickel catalysis of cross-coupling reactions. The heterogeneous catalyst shows stable performance in a packed-bed flow reactor during a week of continuous operation.

Shining Visible Light on Reductive Elimination: Acridine-Pd-Catalyzed Cross-Coupling of Aryl Halides with Carboxylic Acids

Despite the recent tremendous progress on transition-metal/photoredox dual catalysis in organic synthesis, single transition-metal catalysis under visible-light irradiation, which can utilize light energy more efficiently, is still underdeveloped. Herein, we report the design of photosensitizing phosphinoacridine bidentate ligands for visible-light-induced transition-metal catalysis, expecting that the electron-accepting acridine moiety would create a highly reactive electron-deficient metal center toward reductive elimination via metal-to-ligand charge transfer (MLCT). Using these ligands, we have achieved a palladium-catalyzed cross-coupling reaction of aryl halides with carboxylic acids under visible-light irradiation. Electronic tuning of the phosphinoacridine ligands not only enabled the use of a variety of aryl halides as the coupling partner, including less reactive aryl chlorides, under blue light irradiation, but also realized the employment of lower-energy green and red light for the cross-coupling. Experimental mechanistic studies have proved that the reductive elimination of aryl esters is induced by photoirradiation of phosphinoacridine-ligated arylpalladium(II) carboxylate complexes. The theoretical calculation suggests that the reductive elimination in the excited state is promoted by decreasing the electron density of the Pd center through photoinduced intramolecular electron transfer, i.e., MLCT, in the transition state owing to the electron-deficient acridine scaffold. This is a very rare example of photoinduced reductive elimination on palladium(II) complexes.

Photocatalytic cyclization of nitrogen-centered radicals with carbon nitride through promoting substrate/catalyst interaction

The use of metal-free carbon nitride and light to drive catalytic transformations constitutes a sustainable strategy for organic synthesis. At the moment, enhancing the intrinsic activity of CN catalysts by tuning the interfacial coupling between catalyst and substrate remains challenging. Herein, we demonstrate that urea-derived carbon nitride catalysts with the abundant −NH₂ groups and the relative positive charged surface could effectively complex with the deprotonated anionic intermediate to improve the adsorption of organic reactants on the catalyst surface. The decreased oxidation potential and upshift in its highest occupied molecular orbital position make the electron abstraction kinetics by the catalyst more energetically favorable. The prepared catalyst is thus utilized for the photocatalytic cyclization of nitrogen-centered radicals for the synthesis of diverse pharmaceutical-related compounds (33 examples) with high activity and reusability, which shows competent performance to the homogeneous catalysts.

Carbon nitride catalysts with positively charged surfaces and abundant −NH2 are found to be effective photocatalysts for dihydropyrazole synthesis. A surface-mediated mechanism where deprotonated intermediates interact with the surface is proposed.

Kinetics of a Ni/Ir-Photocatalyzed Coupling of ArBr with RBr: Intermediacy of ArNiII(L)Br and Rate/Selectivity Factors

The Ni/Ir-photocatalyzed coupling of an aryl bromide (ArBr) with an alkyl bromide (RBr) has been analyzed using in situ LED-¹⁹F NMR spectroscopy. Four components (light, [ArBr], [Ni], [Ir]) are found to control the rate of ArBr consumption, but not the product selectivity, while two components ([(TMS)₃SiH], [RBr]) independently control the product selectivity, but not the rate. A major resting state of nickel has been identified as ArNi^II(L)Br, and ¹³C-isotopic entrainment is used to show that the complex undergoes Ir-photocatalyzed conversion to products (Ar-R, Ar-H, Ar-solvent) in competition with the release of ArBr. A range of competing absorption and quenching effects lead to complex correlations between the Ir and Ni catalyst loadings and the reaction rate. Differences in the Ir/Ni Beer–Lambert absorption profiles allow the rate to be increased by the use of a shorter-wavelength light source without compromising the selectivity. A minimal kinetic model for the process allows simulation of the reaction and provides insights for optimization of these processes in the laboratory.

Variable Temperature LED-NMR: Rapid Insights into a Photocatalytic Mechanism from Reaction Progress Kinetic Analysis

A multitude of techniques are available to obtain a useful understanding of photocatalytic mechanisms. The combination of LED illumination with nuclear magnetic resonance spectroscopy (LED-NMR) provides a rapid, convenient means to directly monitor a photocatalytic reaction in situ. Herein, we describe a study of the mechanism of an enantioselective intermolecular [2 + 2] photocycloaddition catalyzed by a chiral Ir photocatalyst using LED-NMR. The data-rich output of this experiment is suitable for same-excess and variable time normalization analyses (VTNA). Together, these identified an unexpected change in mechanism between reactions conducted at ambient and cryogenic temperatures. At -78 °C, the kinetic data are consistent with the triplet rebound mechanism we previously proposed for this reaction, involving sensitization of maleimide and rapid reaction with a hydrogen-bound quinoline within the solvent cage. At room temperature, the cycloaddition instead proceeds through intracomplex energy transfer to the hydrogen-bound quinolone. These results highlight the potential sensitivity of photocatalytic reaction mechanisms to the precise reaction conditions and the further utility of LED-NMR as a fast, data-rich tool for their interrogation that compares favorably to conventional ex situ kinetic analyses.

Dual Photoredox Ni/Benzophenone Catalysis: A Study of the NiII Precatalyst Photoreduction Step

The combination of Ni^IIX₂ salts with a bipyridine-type ligand and aromatic carbonyl-based chromophores has emerged as a benchmark precatalytic system to efficiently conduct cross-couplings mediated by light. Mechanistic studies have led to two scenarios in which Ni⁰ is proposed as the catalytic species. Nonetheless, in none of these studies has a Ni^II to Ni⁰ photoreduction been evidenced. By exploiting UV-visible, nuclear magnetic resonance, resonance Raman, electron paramagnetic resonance, and dynamic light scattering spectroscopies and also transmission electron microscopy, we report that, when photolyzed by UVA in alcohols, the structurally defined [Ni^II₂(μ-OH₂)(dtbbpy)₂(BPCO₂)₄] complex 1 integrating a benzophenone chromophore is reduced into a diamagnetic Ni^I dimer of the general formula [Ni^I₂(dtbbpy)₂(BPCO₂)₂]. In marked contrast, in THF, photolysis led to the fast formation of Ni⁰, which accumulates in the form of metallic ultrathin Ni nanosheets characterized by a mean size of ∼100 nm and a surface plasmon resonance at 505 nm. Finally, it is shown that 1 combined with UVA irradiation catalyzes cross-couplings, that is, C(sp³)-H arylation of THF and O-arylation of methanol. These results are discussed in light of the mechanisms proposed for these cross-couplings with a focus on the oxidation state of the catalytic species.

Comparative Evaluation of Light-Driven Catalysis: A Framework for Standardized Reporting of Data

Light-driven homogeneous and heterogeneous catalysis require a complex interplay between light absorption, charge separation, charge transfer, and catalytic turnover. Optical and irradiation parameters as well as reaction engineering aspects play major roles in controlling catalytic performance. This multitude of factors makes it difficult to objectively compare light-driven catalysts and provide an unbiased performance assessment. This Scientific Perspective highlights the importance of collecting and reporting experimental data in homogeneous and heterogeneous light-driven catalysis. A critical analysis of the benefits and limitations of the commonly used experimental indicators is provided. Data collection and reporting according to FAIR principles is discussed in the context of future automated data analysis. The authors propose a minimum dataset as a basis for unified collecting and reporting of experimental data in homogeneous and heterogeneous light-driven catalysis. The community is encouraged to support the future development of this parameter list through an open online repository.

An Integrated Carbon Nitride-Nickel Photocatalyst for the Amination of Aryl Halides Using Sodium Azide

The synthesis of primary anilines via sustainable methods remains a challenge in organic synthesis. We report a photocatalytic protocol for the selective synthesis of primary anilines via cross-coupling of a wide range of aryl/heteroaryl halides with sodium azide using a photocatalyst powder consisting of nickel(II) deposited on mesoporous carbon nitride (Ni-mpg-CNx ). This heterogeneous photocatalyst contains a high surface area with a visible light-absorbing and adaptive "built-in" solid-state ligand for the integrated catalytic Ni site. The method displays a high functional group tolerance, requires mild reaction conditions, and benefits from easy recovery and reuse of the photocatalyst powder. Thereby, it overcomes the need of complex ligand scaffolds required in homogeneous catalysis, precious metals and elevated temperatures/pressures in existing protocols of primary anilines synthesis. The reported heterogeneous Ni-mpg-CNx holds potential for applications in the academic and industrial synthesis of anilines and exploration of other photocatalytic transformations.

Combining Brønsted base and photocatalysis into conjugated microporous polymers: Visible light-induced oxidation of thiols into disulfides with oxygen

Numerous applications in visible light photocatalysis have been found over conjugated microporous polymers (CMPs) whose function could be rationally designed at the molecular level. In this context, the oxidation of thiols into disulfides entails proton and electron transfer and thus requires both Brønsted base and photocatalysis, which could be both combined into CMPs. With carbazole as a Brønsted base and an electron donor, CMPs were constructed to implement the synergistic deprotonation and oxidation of thiols into disulfides in ethanol (C₂H₅OH). Gratifyingly, the bifunctional CMPs could activate molecular oxygen (O₂) to superoxide anion (O₂^•-) and promote the blue light-induced selective oxidation of thiols into symmetrical disulfides with high efficiency in C₂H₅OH. More remarkably, the highly selective formation of unsymmetrical disulfides could also be achieved without adding a Brønsted base. This work highlights the feasibility of combining cooperative photocatalysis into CMPs for versatile chemical transformations.

Red edge effect and chromoselective photocatalysis with amorphous covalent triazine-based frameworks

Chromoselective photocatalysis offers an intriguing opportunity to enable a specific reaction pathway out of a potentially possible multiplicity for a given substrate by using a sensitizer that converts the energy of incident photon into the redox potential of the corresponding magnitude. Several sensitizers possessing different discrete redox potentials (high/low) upon excitation with photons of specific wavelength (short/long) have been reported. Herein, we report design of molecular structures of two-dimensional amorphous covalent triazine-based frameworks (CTFs) possessing intraband states close to the valence band with strong red edge effect (REE). REE enables generation of a continuum of excited sites characterized by their own redox potentials, with the magnitude proportional to the wavelength of incident photons. Separation of charge carriers in such materials depends strongly on the wavelength of incident light and is the primary parameter that defines efficacy of the materials in photocatalytic bromination of electron rich aromatic compounds. In dual Ni-photocatalysis, excitation of electrons from the intraband states to the conduction band of the CTF with 625 nm photons enables selective formation of C‒N cross-coupling products from arylhalides and pyrrolidine, while an undesirable dehalogenation process is completely suppressed.

NiO‐MnOx/Polyaniline/Graphite Electrodes for Urea Electrocatalysis: Synergetic Effect between Polymorphs of MnOx and NiO

In this work, we are enhancing the catalytic activity of the urea electrooxidation (UEO) process by using the composite of polyaniline (Pani), nickel oxide (NiO), and polymorphs of manganese oxide (MnOx) based on a graphite electrode. The hydro‐gel method was used to prepare catalyst suspension with different ratios from NiO and MnOx. The chemical structures and surface morphology were characterized by employing the IR, XRD, and SEM/EDEX techniques. The catalytic activity of four modified electrocatalysts G/Pani/NiMn1, G/Pani/NiMn2, G/Pani/NiMn3, G/Pani/NiMn4 was investigated using cyclic voltammetry, chronoamperometry, and electrochemical impedance. The kinetic parameters such as diffusion coefficient, Tafel slope, charge transfer coefficient, and surface coverage were calculated to choose the best electrocatalysts toward UEO in alkaline solution. The anodic current of new electrodes achieved about 16 mA.cm−2at potential of 550 mV (vs. Ag/AgCl). Density functional theory studies (DFT) have been carried out to assess the adsorption energy between polyaniline (Pani) and the metal oxides.

Mesoporous Graphitic Carbon Nitride as a Heterogeneous Organic Photocatalyst in the Dual Catalytic Arylation of Alkyl Bis(catecholato)silicates

Mesoporous graphitic carbon nitride (mpg-CN) is introduced as a heterogeneous photocatalyst to perform dual photoredox- and nickel-catalyzed cross-coupling reactions between alkyl bis(catecholato)silicates as radical precursors and aryl or alkenyl bromides. The synergy between this recyclable photocatalyst and the broadly applied homogeneous nickel complex [Ni(dtbbpy)Br₂] gives access to C(sp²)-C(sp³) cross-coupling products in a sustainable fashion. The recycled mpg-CN photocatalyst was analyzed by time-resolved emission spectroscopy and EPR spectroscopy.

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.

Visible light-induced Ni-catalyzed C–heteroatom cross-coupling of aryl halides via LMCT with DBU to access a Ni(I)/Ni(III) cycle

Cross-coupling of aryl halides with nucleophiles is a synthetically attractive strategy to construct C–heteroatom bonds. Here we report a highly efficient photoinduced Ni-catalyzed method for the C–heteroatom cross-coupling of aryl...

Recent Progress on Photocatalytic Synthesis of Ester Derivatives and Reaction Mechanisms

Esters and their derivatives are distributed widely in natural products, pharmaceuticals, fine chemicals and other fields. Esters are important building blocks in pharmaceuticals such as clopidogrel, methylphenidate, fenofibrate, travoprost, prasugrel, oseltamivir, eszopiclone and fluticasone. Therefore, esterification reaction becomes more and more popular in the photochemical field. In this review, we highlight three types of reactions to synthesize esters using photochemical strategies. The reaction mechanisms involve mainly single electron transfer, energy transfer or other radical procedures.

Nickel-Catalyzed C-Heteroatom Cross-Coupling Reactions under Mild Conditions via Facilitated Reductive Elimination

The formation of C-heteroatom bonds represents an important type of bond-forming reaction in organic synthesis and often provides a fast and efficient access to privileged structures found in pharmaceuticals, agrochemical and materials. In contrast to conventional Pd- or Cu-catalyzed C-heteroatom cross-couplings under high-temperature conditions, recent advances in homo- and heterogeneous Ni-catalyzed C-heteroatom formations under mild conditions are particularly attractive from the standpoint of sustainability and practicability. The generation of Ni^III and excited Ni^II intermediates facilitate the reductive elimination step to achieve mild cross-couplings. This review provides an overview of the state-of-the-art approaches for mild C-heteroatom bond formations and highlights the developments in photoredox and nickel dual catalysis involving SET and energy transfer processes; photoexcited nickel catalysis; electro and nickel dual catalysis; heterogeneous photoredox and nickel dual catalysis involving graphitic carbon nitride (mpg-CN), metal organic frameworks (MOFs) or semiconductor quantum dots (QDs); as well as more conventional zinc and nickel dual catalyzed reactions.

Visible-Light Promoted C-O Bond Formation with an Integrated Carbon Nitride-Nickel Heterogeneous Photocatalyst

Ni-deposited mesoporous graphitic carbon nitride (Ni-mpg-CNx) is introduced as an inexpensive, robust, easily synthesizable and recyclable material that functions as an integrated dual photocatalytic system. This material overcomes the need of expensive photosensitizers, organic ligands and additives as well as limitations of catalyst deactivation in the existing photo/Ni dual catalytic cross-coupling reactions. The dual catalytic Ni-mpg-CNx is demonstrated for C-O coupling between aryl halides and aliphatic alcohols under mild condition. The reaction affords the ether product in good-to-excellent yields (60-92 %) with broad substrate scope, including heteroaryl and aryl halides bearing electron-withdrawing, -donating and neutral groups. The heterogeneous Ni-mpg-CNx can be easily recovered from the reaction mixture and reused over multiple cycles without loss of activity. The findings highlight exciting opportunities for dual catalysis promoted by a fully heterogeneous system.

Visible-Light Promoted C-O Bond Formation with an Integrated Carbon Nitride-Nickel Heterogeneous Photocatalyst

Ni-deposited mesoporous graphitic carbon nitride (Ni-mpg-CNx ) is introduced as an inexpensive, robust, easily synthesizable and recyclable material that functions as an integrated dual photocatalytic system. This material overcomes the need of expensive photosensitizers, organic ligands and additives as well as limitations of catalyst deactivation in the existing photo/Ni dual catalytic cross-coupling reactions. The dual catalytic Ni-mpg-CNx is demonstrated for C-O coupling between aryl halides and aliphatic alcohols under mild condition. The reaction affords the ether product in good-to-excellent yields (60-92 %) with broad substrate scope, including heteroaryl and aryl halides bearing electron-withdrawing, -donating and neutral groups. The heterogeneous Ni-mpg-CNx can be easily recovered from the reaction mixture and reused over multiple cycles without loss of activity. The findings highlight exciting opportunities for dual catalysis promoted by a fully heterogeneous system.

Continuous flow synthesis of arylhydrazines via nickel/photoredox coupling of tert-butyl carbazate with aryl halides

Nickel/photoredox catalyzed C-N couplings of hydrazine-derived nucleophiles provide a powerful alternative to Pd-catalyzed methods. This continuous-flow photochemical protocol, optimized using design of experiments, achieves these couplings in short residence times, with high selectivity. A range of (hetero)aryl bromides and chlorides are compatible and understanding of process stability/reactor fouling has been discerned.