Esterification of Carboxylic Acids with Aryl Halides via the Merger of Paired Electrolysis and Nickel Catalysis

Tetranuclear CoII4O4 Cubane Complex: Effective Catalyst Toward Electrochemical Water Oxidation

The reaction of Co(OAc)2·6H2O with 2,2'-[{(1E,1'E)-pyridine-2,6-diyl-bis(methaneylylidene)bis(azaneylylidene)}diphenol](LH2) a multisite coordination ligand and Et3N in a 1:2:3 stoichiometric ratio forms a tetranuclear complex Co4(L)2(μ-η1:η1-OAc)2(η2-OAc)2]· 1.5 CH3OH· 1.5 CHCl3 (1). Based on X-ray diffraction investigations, complex 1 comprises a distorted Co4O4 cubane core consisting of two completely deprotonated ligands [L]2- and four acetate ligands. Two distinct types of CoII centers exist in the complex, where the Co(2) center has a distorted octahedral geometry; alternatively, Co(1) has a distorted pentagonal-bipyramidal geometry. Analysis of magnetic data in 1 shows predominant antiferromagnetic coupling (J = -2.1 cm-1), while the magnetic anisotropy is the easy-plane type (D1 = 8.8, D2 = 0.76 cm-1). Furthermore, complex 1 demonstrates an electrochemical oxygen evolution reaction (OER) with an overpotential of 325 mV and Tafel slope of 85 mV dec-1, required to attain a current density of 10 mA cm-2 and moderate stability under alkaline conditions (pH = 14). Electrochemical impedance spectroscopy studies reveal that compound 1 has a charge transfer resistance (Rct) of 2.927 Ω, which is comparatively lower than standard Co3O4 (5.242 Ω), indicating rapid charge transfer kinetics between electrode and electrolyte solution that enhances higher catalytic activity toward OER kinetics.

Dual Transition Metal Electrocatalysis: Direct Decarboxylative Alkenylation of Aliphatic Carboxylic Acids

Direct decarboxylative alkenylation of widely available aliphatic carboxylic acids with vinyl halides for the synthesis of alkenes with all substitution patterns has been accomplished by means of Ce/Ni dual transition metal electrocatalysis. The reactions employ alkyl acids as the limiting reagents and exhibit a broad scope with respect to both coupling partners. Notably, simple primary alkyl carboxylic acids could be readily engaged as carbon-centered radical precursors in the reaction. This new alkenylation protocol has been successfully demonstrated in direct modification of naturally occurring complex acids and is amenable to the enantioselective decarboxylative alkenylation of arylacetic acid. Mechanistic studies, including a series of controlled experiments and cyclic voltammetry data, allow us to probe the key intermediates and the pathway of the reaction.

Gold-Catalyzed C-O Cross-Coupling Reactions of Aryl Iodides with Silver Carboxylates

We have developed a C-O cross-coupling reaction of (hetero)aryl iodides with silver carboxylates via a AuI/AuIII catalytic cycle. The transformation featured exclusive chemoselectivity and moisture/air insensitivity. Aromatic and aliphatic (including primary, secondary, and tertiary) silver carboxylates are all suitable substrates. Moreover, this protocol worked well intermolecularly and intramolecularly. Most importantly, good yields were obtained regardless of the substrates' electronic effect and steric hindrance.

Paired electrolysis-enabled nickel-catalyzed enantioselective reductive cross-coupling between α-chloroesters and aryl bromides

Electrochemical asymmetric catalysis has emerged as a sustainable and promising approach to the production of chiral compounds and the utilization of both the anode and cathode as working electrodes would provide a unique approach for organic synthesis. However, precise matching of the rate and electric potential of anodic oxidation and cathodic reduction make such idealized electrolysis difficult to achieve. Herein, asymmetric cross-coupling between α-chloroesters and aryl bromides is probed as a model reaction, wherein alkyl radicals are generated from the α-chloroesters through a sequential oxidative electron transfer process at the anode, while the nickel catalyst is reduced to a lower oxidation state at the cathode. Radical clock studies, cyclic voltammetry analysis, and electron paramagnetic resonance experiments support the synergistic involvement of anodic and cathodic redox events. This electrolytic method provides an alternative avenue for asymmetric catalysis that could find significant utility in organic synthesis.

Electro-Synthesis of Organic Compounds with Heterogeneous Catalysis

Electro-organic synthesis has attracted a lot of attention in pharmaceutical science, medicinal chemistry, and future industrial applications in energy storage and conversion. To date, there has not been a detailed review on electro-organic synthesis with the strategy of heterogeneous catalysis. In this review, the most recent advances in synthesizing value-added chemicals by heterogeneous catalysis are summarized. An overview of electrocatalytic oxidation and reduction processes as well as paired electrocatalysis is provided, and the anodic oxidation of alcohols (monohydric and polyhydric), aldehydes, and amines are discussed. This review also provides in-depth insight into the cathodic reduction of carboxylates, carbon dioxide, CC, C≡C, and reductive coupling reactions. Moreover, the electrocatalytic paired electro-synthesis methods, including parallel paired, sequential divergent paired, and convergent paired electrolysis, are summarized. Additionally, the strategies developed to achieve high electrosynthesis efficiency and the associated challenges are also addressed. It is believed that electro-organic synthesis is a promising direction of organic electrochemistry, offering numerous opportunities to develop new organic reaction methods.

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.

Solvent-Controlled Regioselective Reaction of 2-Methyleneaziridines with Acrylic/Propargylic Acids: Synthesis of Carboxylate Aziridine/Acetone Esters

Herein, we report a convenient solvent-controlled regioselective esterification to access two types of carboxylate esters without any additive or non-green activation strategy. In this transformation, 2-methyleneaziridines served as an ester reagent, providing two alternative electrophilic carbon centers. Notably, this protocol is suitable for some structure-complicated clinical molecules with a carboxylic acid group, presenting remarkable application potential.

Nickel-Catalyzed Paired Electrochemical Cross-Coupling of Aryl Halides with Nucleophiles

Electrochemistry has recently gained increased attention as a versatile strategy for achieving challenging transformations at the forefront of synthetic organic chemistry. However, most electrochemical transformations only employ one electrode (anodic oxidation or cathodic reduction) to afford the desired products, while the chemistry that occurs at the counter electrode yields stoichiometric waste. In contrast, paired electrochemical reactions can synchronously utilize the anodic and cathodic reactions to deliver the desired product, thus improving the atom economy and energy efficiency of the electrolytic process. This review gives an overview of recent advances in nickel-catalyzed paired electrochemical cross-coupling reactions of aryl/alkenyl halides with different nucleophiles.

1 Introduction

2 Nickel-Catalyzed Cross-Coupling Reactions

2.1 C–C Bond Formation

2.2 C–N Bond Formation

2.3 C–S/O Bond Formation

2.4 C–P Bond Formation

3 Conclusion

Recent advances in organic electrosynthesis employing transition metal complexes as electrocatalysts

Organic electrosynthesis has been widely used as an environmentally conscious alternative to conventional methods for redox reactions because it utilizes electric current as a traceless redox agent instead of chemical redox agents. Indirect electrolysis employing a redox catalyst has received tremendous attention, since it provides various advantages compared to direct electrolysis. With indirect electrolysis, overpotential of electron transfer can be avoided, which is inherently milder, thus wide functional group tolerance can be achieved. Additionally, chemoselectivity, regioselectivity, and stereoselectivity can be tuned by the redox catalysts used in indirect electrolysis. Furthermore, electrode passivation can be avoided by preventing the formation of polymer films on the electrode surface. Common redox catalysts include N-oxyl radicals, hypervalent iodine species, halides, amines, benzoquinones (such as DDQ and tetrachlorobenzoquinone), and transition metals. In recent years, great progress has been made in the field of indirect organic electrosynthesis using transition metals as redox catalysts for reaction classes including C-H functionalization, radical cyclization, and cross-coupling of aryl halides-each owing to the diverse reactivity and accessible oxidation states of transition metals. Although various reviews of organic electrosynthesis are available, there is a lack of articles that focus on recent research progress in the area of indirect electrolysis using transition metals, which is the impetus for this review.

Transition metal-catalyzed organic reactions in undivided electrochemical cells

Transition metal-catalyzed organic electrochemistry is a rapidly growing research area owing in part to the ability of metal catalysts to alter the selectivity of a given transformation. This conversion mainly focuses on transition metal-catalyzed anodic oxidation and cathodic reduction and great progress has been achieved in both areas. Typically, only one of the half-cell reactions is involved in the organic reaction while a sacrificial reaction occurs at the counter electrode, which is inherently wasteful since one electrode is not being used productively. Recently, transition metal-catalyzed paired electrolysis that makes use of both anodic oxidation and cathodic reduction has attracted much attention. This perspective highlights the recent progress of each type of electrochemical reaction and relatively focuses on the transition metal-catalyzed paired electrolysis, showcasing that electrochemical reactions involving transition metal catalysis have advantages over conventional reactions in terms of controlling the reaction activity and selectivity and figuring out that transition metal-catalyzed paired electrolysis is an important direction of organic electrochemistry in the future and offers numerous opportunities for new and improved organic reaction methods.

Chemoselective, Scalable Nickel-Electrocatalytic O-Arylation of Alcohols

The formation of aryl-alkyl ether bonds through cross coupling of alcohols with aryl halides represents a useful strategic departure from classical SN 2 methods. Numerous tactics relying on Pd-, Cu-, and Ni-based catalytic systems have emerged over the past several years. Herein we disclose a Ni-catalyzed electrochemically driven protocol to achieve this useful transformation with a broad substrate scope in an operationally simple way. This electrochemical method does not require strong base, exogenous expensive transition metal catalysts (e.g., Ir, Ru), and can easily be scaled up in either a batch or flow setting. Interestingly, e-etherification exhibits an enhanced substrate scope over the mechanistically related photochemical variant as it tolerates tertiary amine functional groups in the alcohol nucleophile.