Electrochemistry has recently gained increased attention as a versatile strategy for achieving challenging transformations at the forefront of synthetic organic chemistry. Electrochemistry's unique ability to generate highly reactive radical and radical ion intermediates in a controlled fashion under mild conditions has inspired the development of a number of new electrochemical methodologies for the preparation of valuable chemical motifs. Particularly, recent developments in electrosynthesis have featured an increased use of redox-active electrocatalysts to further enhance control over the selective formation and downstream reactivity of these reactive intermediates. Furthermore, electrocatalytic mediators enable synthetic transformations to proceed in a manner that is mechanistically distinct from purely chemical methods, allowing for the subversion of kinetic and thermodynamic obstacles encountered in conventional organic synthesis. This review highlights key innovations within the past decade in the area of synthetic electrocatalysis, with emphasis on the mechanisms and catalyst design principles underpinning these advancements. A host of oxidative and reductive electrocatalytic methodologies are discussed and are grouped according to the classification of the synthetic transformation and the nature of the electrocatalyst.

Electrocatalysis as an enabling technology for organic synthesis.

Electrochemical synthesis of organic compounds has emerged as an attractive and environmentally benign alternative to conventional approaches for oxidation and reduction of organic compounds that utilizes electric current instead of chemical oxidants and reductants. As such, many useful transformations have been developed, including the Kolbe reaction, the Simons fluorination process, the Monsanto adiponitrile process, and the Shono oxidation, to name a few. Electrochemical C-H functionalization represents one of the most promising reaction types among many electrochemical transformations, since this process avoids prefunctionalization of substrates and provides novel retrosynthetic disconnections. However, site-selective anodic oxidation of C-H bonds is still a fundamental challenge due to the high oxidation potentials of C-H bonds compared to organic solvents and common functional groups. To overcome this issue, indirect electrolysis via the action of a mediator (a redox catalyst) is regularly employed, by which the selectivity can be controlled following reaction of said mediator with the substrate. Since the redox potentials of transition metal complexes can be easily tuned by modification of the ligand, the synergistic use of electrochemistry and transition metal catalysis to achieve site-selective C-H functionalization is an attractive strategy. In this Account, we summarize and contextualize our recent efforts toward transition metal-catalyzed electrochemical C-H functionalization proximal to a suitable directing group. We have developed C-H oxygenation, acylation, alkylation, and halogenation reactions in which a Pd(II) species is oxidized to a Pd(III) or Pd(IV) intermediate by anodic oxidation, followed by reductive elimination to form the corresponding C-O, C-C, and C-X bonds. Importantly, improved monofunctionalization selectivity is achieved in the Pd-catalyzed C(sp3)-H oxygenation compared to conventional approaches using PhI(OAc)2 as the chemical oxidant. Physical separators are sometimes used to prevent the electrochemical deposition of Pd black on the cathode resulting from reduction of high valent Pd species. We skirted this issue through the development a Cu-catalyzed electrochemical C(sp2)-H amination using n-Bu4NI as a redox cocatalyst in an undivided cell. In addition, we developed Ir-catalyzed electrochemical vinylic C-H functionalization of acrylic acids with alkynes in an undivided cell, affording various substituted α-pyrones in good to excellent yield. More importantly, chemical oxidants, including Ag2CO3, Cu(OAc)2, and PhI(OAc)2, resulted in much lower yields in the absence of electrical current under otherwise identical conditions. As elaborated below, progress in the area of electrochemical transition metal-catalyzed synthesis provides an effective platform for environmentally friendly and sustainable selective chemical transformations.

Site-Selective C–H Functionalization via Synergistic Use of Electrochemistry and Transition Metal Catalysis

Efficient and selective molecular syntheses are paramount to inter alia biomolecular chemistry and material sciences as well as for practitioners in chemical, agrochemical, and pharmaceutical industries. Organic electrosynthesis has undergone a considerable renaissance and has thus in recent years emerged as an increasingly viable platform for the sustainable molecular assembly. In stark contrast to early strategies by innate reactivity, electrochemistry was recently merged with modern concepts of organic synthesis, such as transition-metal-catalyzed transformations for inter alia C-H functionalization and asymmetric catalysis. Herein, we highlight the unique potential of organic electrosynthesis for sustainable synthesis and catalysis, showcasing key aspects of exceptional selectivities, the synergism with photocatalysis, or dual electrocatalysis, and novel mechanisms in metallaelectrocatalysis until February of 2021.

Organic Electrochemistry: Molecular Syntheses with Potential.

As the breadth of radical chemistry grows, new means to promote and regulate single-electron redox activities play increasingly important roles in driving modern synthetic innovation. In this regard, photochemistry and electrochemistry-both considered as niche fields for decades-have seen an explosive renewal of interest in recent years and gradually have become a cornerstone of organic chemistry. In this Outlook article, we examine the current state-of-the-art in the areas of electrochemistry and photochemistry, as well as the nascent area of electrophotochemistry. These techniques employ external stimuli to activate organic molecules and imbue privileged control of reaction progress and selectivity that is challenging to traditional chemical methods. Thus, they provide alternative entries to known and new reactive intermediates and enable distinct synthetic strategies that were previously unimaginable. Of the many hallmarks, electro- and photochemistry are often classified as "green" technologies, promoting organic reactions under mild conditions without the necessity for potent and wasteful oxidants and reductants. This Outlook reviews the most recent growth of these fields with special emphasis on conceptual advances that have given rise to enhanced accessibility to the tools of the modern chemical trade.

New Redox Strategies in Organic Synthesis by Means of Electrochemistry and Photochemistry

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.

Recent advances in organic electrosynthesis employing transition metal complexes as electrocatalysts

A novel strategy for asymmetric Shono-type oxidative cross-coupling has been developed by merging copper catalysis and electrochemistry, affording C1-alkynylated tetrahydroisoquinolines with good to excellent enantioselectivity. The use of TEMPO as a co-catalytic redox mediator is crucial not only for oxidizing a tetrahydroisoquinoline to an iminium ion species but also for decreasing the oxidation potential of the reaction. A novel bisoxazoline ligand is also reported.

CuII/TEMPO‐Catalyzed Enantioselective C(sp3)–H Alkynylation of Tertiary Cyclic Amines through Shono‐Type Oxidation

Synergistic use of electrochemistry and organometallic catalysis has emerged as a powerful tool for site-selective C–H functionalization, yet this type of transformation has thus far mainly been limited to arene C–H functionalization. Herein, we report the development of electrochemical vinylic C–H functionalization of acrylic acids with alkynes. In this reaction an iridium catalyst enables C–H/O–H functionalization for alkyne annulation, affording α-pyrones with good to excellent yields in an undivided cell. Preliminary mechanistic studies show that anodic oxidation is crucial for releasing the product and regeneration of an Ir(III) intermediate from a diene-Ir(I) complex, which is a coordinatively saturated, 18-electron complex. Importantly, common chemical oxidants such as Ag(I) or Cu(II) did not give significant amounts of the desired product in the absence of electrical current under otherwise identical conditions.

Electrochemistry-Enabled Ir-Catalyzed Vinylic C-H Functionalization.

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CuII/TEMPO‐Catalyzed Enantioselective C(sp3)–H Alkynylation of Tertiary Cyclic Amines through Shono‐Type Oxidation

Electrochemistry-Enabled Ir-Catalyzed Vinylic C-H Functionalization.