Iron-Electrocatalyzed C-H Arylations: Mechanistic Insights into Oxidation-Induced Reductive Elimination for Ferraelectrocatalysis

Unlocking Catalysis Using Oxidatively Induced Reductive Elimination

Oxidatively induced reductive elimination (OIRE) has become a crucial chemical process to enable the effective formation of otherwise much less accessible products. The chemically, electrochemically, or photochemically triggered oxidation of appropriate metal complexes leading to a reductive elimination process otherwise chemically unfavorable has long been studied in stoichiometric metal-centered reactions. However, its conscious inclusion and targeted application in catalysis is a rather recent development. Ruthenium and Group 9 metal complexes are particularly representative examples of how the observed OIRE abilities can be modulated using different coupling partners. Parallels can be drawn between rhodium and iridium, which have a long history as efficient catalysts, and their more readily available 3d row metal neighbor, i. e. cobalt, with ruthenium also providing a matching reactivity. This review explores the history of OIRE reactions throughout the last few centuries, highlighting the role of Ti, Mo, Fe, Hg, Ni, Ru and group 9 transition metals as key players in C-H bond functionalization and OIRE chemistry, discussing the most relevant studies published in the field whilst pinpointing research gaps that require new theoretical and mechanistic studies, with the final goal of unlocking the full potential of OIRE chemistry in organometallic homogeneous catalysis.

Redox-Neutral, Iron-Mediated Directed C-H Activation: General Principles and Mechanistic Insights

Experimental and computational studies have been conducted and established the general principles for enabling redox-neutral C-H activation by iron(II) complexes. The idealized octahedral iron(II) dimethyl complex, (depe)2Fe(CH3)2 (depe = 1,2-bis(diethylphosphino)ethane) promoted the directed, regioselective ortho C(sp2)-H methylation of pivalophenone. The rate of the iron(II)-mediated C(sp2)-H functionalization depended on the lability of L-type phosphine ligands, the spin state of the iron center, and the size of the X-type ligands (halide, hydrocarbyl) in P4FeIIX2 complexes. The C(sp2)-H alkylation reaction proved general among multiple substrates with directing groups including carbonyl, imines and pyridines. Among these, ketones and aldehydes were identified as optimal and were compatible with various steric environments and presence of acidic α-hydrogens. With stronger nitrogen donors, higher barriers for product-forming reductive elimination were observed. The effect of orbital hybridization on the chemoselectivity of C-H activation through a σ-CAM pathway by dn>0 transition metals was also established by studying the stoichiometric reactivity of the differentially substituted (depe)2Fe(Me)R complexes (R = alkyl, aryl), where the Fe-R bond with greater s-character preferentially promoted selective C-H activation. Deuterium labeling and kinetic studies, coupled with computational analysis, supported a pathway involving phosphine dissociation and rate-determining C-H bond activation, leading to the observed products.

Revealing the Mechanistic Features of an Electrosynthetic Catalytic Reaction and the Role of Redox Mediators through DFT Calculations and Microkinetic Modeling

Organic electrosynthesis is an emerging field that provides original selectivity while adding features of atom economy, sustainability, and selectivity. Electrosynthesis is often enhanced by redox mediators or electroauxiliaries. The mechanistic understanding of organic electrosynthesis is however often limited by the low lifetime of intermediates and its difficult detection. In this work, we report a computational analysis of the mechanism of an appealing reaction previously reported by Mei and co-workers which is catalyzed by copper and employs iodide as redox mediator. Our scheme combines DFT calculations with microkinetic modeling and covers both the reaction in solution and the electrodic steps. A detailed mechanistic scheme is obtained which reproduces well experimental data and opens perspectives for the general treatment of these processes.

Mechanochemical C-H Arylation and Alkylation of Indoles Using 3 d Transition Metal and Zero-Valent Magnesium

Although the 3 d transition-metal catalyzed C-H functionalization have been extensively employed to promote the formation of valuable carbon-carbon bonds, the persistent problems, including the use of sensitive Grignard reagents and the rigorous operations (solvent-drying, inert gas protection, metal pre-activation and RMgX addition rate control), still leave great room for further development of sustainable methodologies. Herein, we report a mechanochemical technology toward in-situ preparation of highly sensitive organomagnesium reagents, and thus building two general 3 d transition-metal catalytic platforms that enables regioselective arylation and alkylation of indoles with a wide variety of halides (including those containing post transformable functionalities and heteroaromatic rings). This mechanochemical strategy also brings unique reactivity and high step-economy in producing functionalized N-free indole products.

Palladium-Catalyzed Electrooxidative Double C-H Arylation

The electrochemical transition metal-catalyzed cross-dehydrogenative reaction has emerged as a promising platform to achieve a sustainable and atom-economic organic synthesis that avoids hazardous oxidants and minimizes undesired byproducts and circuitous functional group operations. However, a poor mechanistic understanding still prevents the widespread adoption of this strategy. In this regard, we herein present an electrochemical palladium-catalyzed oxidative coupling strategy to access biaryls in the absence of a stoichiometric chemical oxidant. The robust palladaelectrocatalysis considerably suppresses the occurrence of homocoupling and oxygenation, being compatible even with electron-deficient arenes. Late-stage functionalization and Boscalid precursor synthesis further highlighted the practical importance of our electrolysis. Remarkably, mechanistic studies including the evaluation of the reaction order of each component by variable time normalization analysis (VTNA) and initial rate analysis, H/D exchange experiment, kinetic isotope effect, and stoichiometric organometallic experiments provided strong support for the involvement of transmetalation between two organopalladium complexes in the turnover limiting step. Therefore, matching the concentrations or lifetimes of two distinct organopalladium intermediates is revealed to be a pivot to the success of electrooxidative catalysis. Moreover, the presence of cationic copper(II) seems to contribute to the stabilization of the palladium(0) catalyst instead of playing a role in the oxidation of the catalyst.

Chelation-Assisted Iron-Catalyzed C-H Activations: Scope and Mechanism

ConspectusTo improve the resource economy of molecular syntheses, researchers have developed strategies to directly activate otherwise inert C-H bonds, thus avoiding cumbersome and costly substrate prefunctionalizations. During the past two decades, remarkable progress in coordination chemistry has set the stage for developing increasingly viable metal catalysts for C-H activations. Despite remarkable advances, C-H activations are largely dominated by precious 4d and 5d transition metal catalysts based primarily on palladium, ruthenium, iridium, and rhodium, thus decreasing the inherent sustainable nature of the C-H activation approach. Therefore, advancing catalytic reactions based on Earth-abundant and less toxic 3d transition metals, especially nontoxic and inexpensive iron, represents a desirable and attractive alternative. While research had previously focused on 8-aminoquinoline directing groups in C-H activations, we have devised easily accessible, tunable, and clickable triazoles, which feature widespread applications in bioactive compounds and drugs, among others, as peptide isosteres. Thus, in contrast to other directing groups, the triazole group is a highly desirable structural motif and functions as a bioisostere in medicine and biology, where it is exploited to mimic amide bonds.This Account summarizes the evolution of chelation-assisted iron-catalyzed C-H activations via C-H, C-H/N-H, and C-H/N-H/C-C bond cleavages, with a topical focus on the most recent contributions of our team. Thus, the triazole-enabled iron catalysis has surfaced as a transformative platform for a large variety of C-H transformations, including arylations, alkylations, alkenylations, allylations, annulations, and alkynylations, achieved through C-H activations with organometallic reagents, organohalides, alkynes, alkenes, allenes, and bicyclopropylidenes among others. Consequently, we developed widely applicable methods for the versatile preparation of decorated arenes and heteroarenes, providing access to benzamides, isoquinolones, pyrrolones, pyridones, phenones, indoles, and isoindolinones, among others. Most of these reactions employed 1,2-dichloroisobutane (DCIB) as an oxidant. Notably, chemical-oxidant-free strategies were also developed, with the major breakthroughs being the use of internal oxidants in oxidative annulations, the use of resource-economic electrocatalysis, and the development of well-defined iron(0)-mediated catalysis. In addition, a highly enantioselective inner-sphere C-H alkylation of (aza)indoles was developed by designing novel remotely decorated N-heterocyclic carbene ligands with dispersion energy donors. In addition, detailed mechanistic experiments including kinetic analyses, intermediate isolation, Mößbauer spectroscopy, and computation provided strong support for the mode of catalysis operation, enabling unprecedented C-H activations. Thereby, low-valent iron catalysts paved the way toward weakly coordinating ketones and enantioselective iron-catalyzed C-H activations through organometallic intermediates.

Interfacing single-atom catalysis with continuous-flow organic electrosynthesis

The global warming crisis has sparked a series of environmentally cautious trends in chemistry, allowing us to rethink the way we conduct our synthesis, and to incorporate more earth-abundant materials in our catalyst design. "Single-atom catalysis" has recently appeared on the catalytic spectrum, and has truly merged the benefits that homogeneous and heterogeneous analogues have to offer. Further still, the possibility to activate these catalysts by means of a suitable electric potential could pave the way for a true integration of diverse synthetic methodologies and renewable electricity. Despite their esteemed benefits, single-atom electrocatalysts are still limited to the energy sector (hydrogen evolution reaction, oxygen reduction, etc.) and numerous examples in the literature still invoke the use of precious metals (Pd, Pt, Ir, etc.). Additionally, batch electroreactors are employed, which limit the intensification of such processes. It is of paramount importance that the field continues to grow in a more sustainable direction, seeking new ventures into the space of organic electrosynthesis and flow electroreactor technologies. In this piece, we discuss some of the progress being made with earth abundant homogeneous and heterogeneous electrocatalysts and flow electrochemistry, within the context of organic electrosynthesis, and highlight the prospects of alternatively utilizing single-atom catalysts for such applications.

Ruthenaelectro-catalyzed C-H acyloxylation for late-stage tyrosine and oligopeptide diversification

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

Traditional and sustainable approaches for the construction of C–C bonds by harnessing C–H arylation

Biaryl scaffolds are found in natural products and drug molecules and exhibit a wide range of biological activities. In past decade, the transition metal-catalyzed C–H arylation reaction came out as an effective tool for the construction of biaryl motifs. However, traditional transition metal-catalyzed C–H arylation reactions have limitations like harsh reaction conditions, narrow substrate scope, use of additives etc. and therefore encouraged synthetic chemists to look for alternate greener approaches. This review aims to draw a general overview on C–H bond arylation reactions for the formation of C–C bonds with the aid of different methodologies, majorly highlighting on greener and sustainable approaches.

Transition-metal-catalyzed C–H arylations are an effective tool for the construction of biaryl motifs in an efficient and selective manner. Here the authors provide an overview of the state-of-the-art of the field and perspectives on emerging directions toward increased sustainability.

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.

Biaryl Synthesis via Electrooxidative Transition-Metal-Catalyzed C–H Activation

The use of electricity as an inexpensive and waste-free oxidant opens up new opportunities for the development of sustainable C–H functionalization reactions. Herein we summarize recent advances in the synthesis of biaryls through electrooxidative processes involving transition-metal-catalyzed ortho-directed C–H activation. A particular focus is set on electrooxidative C–H/C–M couplings and dehydrogenative couplings.

1 Introduction

2 Electrooxidative C–H/C–M Couplings

3 Electrooxidative C–H/C–H Couplings

4 Conclusion and Outlook

Electrocatalysis as an enabling technology for organic synthesis

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.

Fe-Catalyzed Intramolecular B-H/C-H Dehydrogenative Coupling: Synthesis of Carborane-Fused Nitrogen Heterocycles

We disclose herein the first example of iron-catalyzed regioselective intramolecular C-H/B-H dehydrogenative coupling, affording unprecedented C,B-substituted carborane-fused phenanthroline derivatives. The 8-aminoquinoline type auxiliaries not only serve as the bidentate directing groups but also ingeniously become the core part of the final products. The mechanistic hypothesis includes B-H activation, directing group rotation promoted by trans effect, C-H activation, and reductive elimination.

Evolution of Earth-Abundant 3 d-Metallaelectro-Catalyzed C-H Activation: From Chelation-Assistance to C-H Functionalization without Directing Groups

Catalyzed C-H functionalizations have emerged as a transformative platform for molecular syntheses. Despite of indisputable advances, oxidative C-H activations have been largely restricted to precious transition metals and stoichiometric amounts of chemical oxidants. In contrast, we herein discuss the potential of earth-abundant, environmentally-benign 3d transition metals for C-H activation, which has recently gained major momentum. Thus, a strategy for full resource economy has been established in our group, with green electricity as a renewable redox agent, giving valuable hydrogen as the sole byproduct under redox mediator-free conditions. In this account, we detail our accomplishments in 3d metallaelectrocatalysis towards green syntheses until March 2021.

Mediator-Enabled Electrocatalysis with Ligandless Copper for Anaerobic Chan-Lam Coupling Reactions

Simple copper salts serve as catalysts to effect C-X bond-forming reactions in some of the most utilized transformations in synthesis, including the oxidative coupling of aryl boronic acids and amines. However, these Chan-Lam coupling reactions have historically relied on chemical oxidants that limit their applicability beyond small-scale synthesis. Despite the success of replacing strong chemical oxidants with electrochemistry for a variety of metal-catalyzed processes, electrooxidative reactions with ligandless copper catalysts are plagued by slow electron-transfer kinetics, irreversible copper plating, and competitive substrate oxidation. Herein, we report the implementation of substoichiometric quantities of redox mediators to address limitations to Cu-catalyzed electrosynthesis. Mechanistic studies reveal that mediators serve multiple roles by (i) rapidly oxidizing low-valent Cu intermediates, (ii) stripping Cu metal from the cathode to regenerate the catalyst and reveal the active Pt surface for proton reduction, and (iii) providing anodic overcharge protection to prevent substrate oxidation. This strategy is applied to Chan-Lam coupling of aryl-, heteroaryl-, and alkylamines with arylboronic acids in the absence of chemical oxidants. Couplings under these electrochemical conditions occur with higher yields and shorter reaction times than conventional reactions in air and provide complementary substrate reactivity.

Organic Electrochemistry: Molecular Syntheses with Potential

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.

Electroreductive Nickel-Catalyzed Thiolation: Efficient Cross-Electrophile Coupling for C-S Formation

Sulfur-containing molecules are of utmost topical importance towards the effective development of pharmaceuticals and functional materials. Herein, we present an efficient and mild electrochemical thiolation by cross-electrophile coupling of alkyl bromides with functionalized bench-stable thiosulfonates to access alkyl sulfides with excellent efficacy and broad functional group tolerance. Cyclic voltammetry and potentiostatic analysis were performed to elucidate mechanistic insights into this electrocatalytic thiolation reaction.

Dehydrogenative 6π heterocyclization under visible light irradiation and mechanistic insights

A visible-light-driven oxidative 6π heterocyclization for the synthesis of structurally diverse π-conjugated polycyclic 1-aminoisoquinolines has been developed. DFT calculations demonstrated that deprotonation is the rate-determining step.

Green strategies for transition metal-catalyzed C–H activation in molecular syntheses

Sustainable strategies for the activation of inert C–H bonds towards improved resource-economy.

Facilitating Ir-Catalyzed C-H Alkynylation with Electrochemistry: Anodic Oxidation-Induced Reductive Elimination

An electrochemical approach in promoting directed C-H alkynylation with terminal alkyne via iridium catalysis is reported. This work employed anodic oxidation of Ir(III) intermediate (characterized by X-ray crystallography) to promote reductive elimination, giving the desired coupling products in good yields (up to 95%) without the addition of any other external oxidants. This transformation is suitable for various directing groups with H2 as the only by-product, which warrants a high atom economy and practical oxidative C-C bond formation under mild conditions.

Evolution of High-Valent Nickela-Electrocatalyzed C-H Activation: From Cross(-Electrophile)-Couplings to Electrooxidative C-H Transformations

C-H activation has emerged as one of the most efficient tools for the formation of carbon-carbon and carbon-heteroatom bonds, avoiding the use of prefunctionalized materials. In spite of tremendous progress in the field, stoichiometric quantities of toxic and/or costly chemical redox reagents, such as silver(I) or copper(II) salts, are largely required for oxidative C-H activations. Recently, electrosynthesis has experienced a remarkable renaissance that enables the use of storable, safe and waste-free electric current as a redox equivalent. While major recent momentum was gained in electrocatalyzed C-H activations by 4d and 5d metals, user-friendly and inexpensive nickela-electrocatalysis has until recently proven elusive for oxidative C-H activations. Herein, the early developments of nickela-electrocatalyzed reductive cross-electrophile couplings as well as net-redox-neutral cross-couplings are first introduced. The focus of this Minireview is, however, the recent emergence of nickel-catalyzed electrooxidative C-H activations until April 2020.

Nickela-electrocatalyzed Mild C-H Alkylations at Room Temperature

Direct alkylations of carboxylic acid derivatives are challenging and particularly nickel catalysis commonly requires high reaction temperatures and strong bases, translating into limited substrate scope. Herein, nickel-catalyzed C-H alkylations of unactivated 8-aminoquinoline amides have been realized under exceedingly mild conditions, namely at room temperature, with a mild base and a user-friendly electrochemical setup. This electrocatalyzed C-H alkylation displays high functional group tolerance and is applicable to both the primary and secondary alkylation. Based on detailed mechanistic studies, a nickel(II/III/I) catalytic manifold has been proposed.

Renewable resources for sustainable metallaelectro-catalysed C-H activation

The necessity for more sustainable industrial chemical processes has internationally been agreed upon. During the last decade, the scientific community has responded to this urgent need by developing novel sustainable methodologies targeted at molecular transformations that not only produce reduced amounts of byproducts, but also by the use of cleaner and renewable energy sources. A prime example is the electrochemical functionalization of organic molecules, by which toxic and costly chemicals can be replaced by renewable electricity. Unrivalled levels of resource economy can thereby be achieved via the merger of metal-catalyzed C-H activation with electrosynthesis. This perspective aims at highlighting the most relevant advances in metallaelectro-catalysed C-H activations, with a particular focus on the use of green solvents and sustainable wind power and solar energy until June 2020.

Electroreductive Cobalt-Catalyzed Carboxylation: Cross-Electrophile Electrocoupling with Atmospheric CO2

The chemical use of CO2 as an inexpensive, nontoxic C1 synthon is of utmost topical interest in the context of carbon capture and utilization (CCU). We present the merger of cobalt catalysis and electrochemical synthesis for mild catalytic carboxylations of allylic chlorides with CO2 . Styrylacetic acid derivatives were obtained with moderate to good yields and good functional group tolerance. The thus-obtained products are useful as versatile synthons of γ-arylbutyrolactones. Cyclic voltammetry and in operando kinetic analysis were performed to provide mechanistic insights into the electrocatalytic carboxylation with CO2 .

3d metallaelectrocatalysis for resource economical syntheses

Resource economy constitutes one of the key challenges for researchers and practitioners in academia and industries, in terms of rising demand for sustainable and green synthetic methodology. To achieve ideal levels of resource economy in molecular syntheses, novel avenues are required, which include, but are not limited to the use of naturally abundant, renewable feedstocks, solvents, metal catalysts, energy, and redox reagents. In this context, electrosyntheses create the unique possibility to replace stoichiometric amounts of oxidizing or reducing reagents as well as electron transfer events by electric current. Particularly, the merger of Earth-abundant 3d metal catalysis and electrooxidation has recently been recognized as an increasingly viable strategy to forge challenging C-C and C-heteroatom bonds for complex organic molecules in a sustainable fashion under mild reaction conditions. In this review, we highlight the key developments in 3d metallaelectrocatalysis in the context of resource economy in molecular syntheses until February 2020.

Azaruthena(II)-bicyclo[3.2.0]heptadiene: Key Intermediate for Ruthenaelectro(II/III/I)-catalyzed Alkyne Annulations

A ruthenium-catalyzed electrochemical dehydrogenative annulation reaction of imidazoles with alkynes has been established, enabling the preparation of various bridgehead N-fused [5,6]-bicyclic heteroarenes through regioselective electrochemical C-H/N-H annulation without chemical metal oxidants. Novel azaruthenabicyclo[3.2.0]heptadienes were fully characterized and identified as key intermediates. Mechanistic studies are suggestive of an oxidatively induced reductive elimination pathway within a ruthenium(II/III) regime.

Iron-Catalyzed C-H Functionalizations under Triazole-Assistance

3d transition metals-catalyzed C-H bond functionalizations represent nowadays an important tool in organic synthesis, appearing as the most promising alternative to cross-coupling reactions. Among 3d transition metals, iron found widespread application due to its availability and benign nature, and it was established as an efficient catalyst in organic synthesis. In this context, the use of ortho-orientating directing groups (DGs) turned out to be necessary for promoting selective iron-catalyzed C-H functionalization reactions. Very recently, triazoles DGs were demonstrated to be more than an excellent alternative to the commonly employed 8-aminoquinoline (AQ) DG, as a result of their modular synthesis as well as the mild reaction conditions applied for their removal. In addition, their tunable geometry and electronics allowed for new unprecedented reactivities in iron-catalyzed C-H activation methodologies that will be summarized within this review.

Nickela-electrocatalyzed C-H Alkoxylation with Secondary Alcohols: Oxidation-Induced Reductive Elimination at Nickel(III)

Nickela-electrooxidative C-H alkoxylations with challenging secondary alcohols were accomplished in a fully dehydrogenative fashion, thereby avoiding stoichiometric chemical oxidants, with H2 as the only stoichiometric byproduct. The nickela-electrocatalyzed oxygenation proved viable with various (hetero)arenes, including naturally occurring secondary alcohols, without racemization. Detailed mechanistic investigation, including DFT calculations and cyclovoltammetric studies of a well-defined C-H activated nickel(III) intermediate, suggest an oxidation-induced reductive elimination at nickel(III).

Metalla-electrocatalyzed C-H Activation by Earth-Abundant 3d Metals and Beyond

To improve the efficacy of molecular syntheses, researchers wish to capitalize upon the selective modification of otherwise inert C-H bonds. The past two decades have witnessed considerable advances in coordination chemistry that have set the stage for transformative tools for C-H functionalizations. Particularly, oxidative C-H/C-H and C-H/Het-H transformations have gained major attention because they avoid all elements of substrate prefunctionalization. Despite considerable advances, oxidative C-H activations have been dominated by precious transition metal catalysts based on palladium, ruthenium, iridium, and rhodium, thus compromising the sustainable nature of the overall C-H activation approach. The same holds true for the predominant use of stoichiometric chemical oxidants for the regeneration of the active catalyst, prominently featuring hypervalent iodine(III), copper(II), and silver(I) oxidants. Thereby, stoichiometric quantities of undesired byproducts are generated, which are preventive for applications of C-H activation on scale. In contrast, the elegant merger of homogeneous metal-catalyzed C-H activation with molecular electrosynthesis bears the unique power to achieve outstanding levels of oxidant and resource economy. Thus, in contrast to classical electrosyntheses by substrate control, metalla-electrocatalysis holds huge and largely untapped potential for oxidative C-H activations with unmet site selectivities by means of catalyst control. While indirect electrolysis using precious palladium complexes has been realized, less toxic and less expensive base metal catalysts feature distinct beneficial assets toward sustainable resource economy. In this Account, I summarize the emergence of electrocatalyzed C-H activation by earth-abundant 3d base metals and beyond, with a topical focus on contributions from our laboratories through November 2019. Thus, cobalt electrocatalysis was identified as a particularly powerful platform for a wealth of C-H transformations, including C-H oxygenations and C-H nitrogenations as well as C-H activations with alkynes, alkenes, allenes, isocyanides, and carbon monoxide, among others. As complementary tools, catalysts based on nickel, copper, and very recently iron have been devised for metalla-electrocatalyzed C-H activations. Key to success were detailed mechanistic insights, prominently featuring oxidation-induced reductive elimination scenarios. Likewise, the development of methods that make use of weak O-coordination benefited from crucial insights into the catalyst's modes of action by experiment, in operando spectroscopy, and computation. Overall, metalla-electrocatalyzed C-H activations have thereby set the stage for molecular syntheses with unique levels of resource economy. These electrooxidative C-H transformations overall avoid the use of chemical oxidants and are frequently characterized by improved chemoselectivities. Hence, the ability to dial in the redox potential at the minimum level required for the desired transformation renders electrocatalysis an ideal platform for the functionalization of structurally complex molecules with sensitive functional groups. This strategy was, inter alia, successfully applied to scale-up by continuous flow and the step-economical assembly of polycyclic aromatic hydrocarbons.

Iron-Electrocatalyzed C-H Arylations: Mechanistic Insights into Oxidation-Induced Reductive Elimination for Ferraelectrocatalysis

Despite major advances, organometallic C-H transformations are dominated by precious 5d and 4d transition metals, such as iridium, palladium and rhodium. In contrast, the unique potential of less toxic Earth-abundant 3d metals has been underexplored. While iron is the most naturally abundant transition metal, its use in oxidative, organometallic C-H activation has faced major limitations due to the need for superstoichiometric amounts of corrosive, cost-intensive DCIB as the sacrificial oxidant. To fully address these restrictions, we describe herein the unprecedented merger of electrosynthesis with iron-catalyzed C-H activation through oxidation-induced reductive elimination. Thus, ferra- and manganaelectro-catalyzed C-H arylations were accomplished at mild reaction temperatures with ample scope by the action of sustainable iron catalysts, employing electricity as a benign oxidant.