Electrooxidative Allene Annulations by Mild Cobalt-Catalyzed C–H Activation

Carboxylate breaks the arene C-H bond via a hydrogen-atom-transfer mechanism in electrochemical cobalt catalysis

Combined computational and experimental studies elucidated the distinctive mechanistic features of electrochemical cobalt-catalyzed C-H oxygenation. A sequential electrochemical-chemical (EC) process was identified for the formation of an amidylcobalt(iii) intermediate. The synthesis, characterization, cyclic voltammetry studies, and stoichiometric reactions of the related amidylcobalt(iii) intermediate suggested that a second on-cycle electro-oxidation occurs on the amidylcobalt(iii) species, which leads to a formal Co(iv) intermediate. This amidylcobalt(iv) intermediate is essentially a cobalt(iii) complex with one additional single electron distributed on the coordinating heteroatoms. The radical nature of the coordinating pivalate allows the formal Co(iv) intermediate to undergo a novel carboxylate-assisted HAT mechanism to cleave the arene C-H bond, and a CMD mechanism could be excluded for a Co(iii/i) catalytic scenario. The mechanistic understanding of electrochemical cobalt-catalyzed C-H bond activation highlights the multi-tasking electro-oxidation and the underexplored reaction channels in electrochemical transition metal catalysis.

Insights into Cobalta(III/IV/II)-Electrocatalysis: Oxidation-Induced Reductive Elimination for Twofold C-H Activation

The merger of cobalt‐catalyzed C−H activation and electrosynthesis provides new avenues for resource‐economical molecular syntheses, unfortunately their reaction mechanisms remain poorly understood. Herein, we report the identification and full characterization of electrochemically generated high‐valent cobalt(III/IV) complexes as crucial intermediates in electrochemical cobalt‐catalyzed C−H oxygenations. Detailed mechanistic studies provided support for an oxidatively‐induced reductive elimination via highly‐reactive cobalt(IV) intermediates. These key insights set the stage for unprecedented cobaltaelectro two‐fold C−H/C−H activation.

Cobaltaelectro-catalyzed C-H activation for resource-economical molecular syntheses

The direct cleavage of otherwise inert C-H bonds has emerged as a sustainable approach for organic synthesis; in contrast to other approaches, these reactions result in the formation of fewer undesired by-products and do not require pre-functionalization steps. In recent years, oxidative C-H/N-H alkyne annulations and C-H oxygenations were realized by 3d metals. Unfortunately, most of these reactions require stoichiometric amounts of often toxic chemical oxidants. This protocol provides a general method for cobaltaelectro-catalyzed C-H activations of benzamides. Here, anodic oxidation obviates the need for a chemical oxidant and uses 10-20% of a more environmentally benign, inexpensive catalyst. We outline a detailed and precise description of the designed electrolytic cell for metallaelectrocatalysis, including readily available electrode materials and electrode holders. The custom-made device is further compared with the commercially available and standardized ElectraSyn 2.0 electrochemistry kit. As example applications of this approach, we describe cobaltaelectro-catalyzed C-H activation protocols for the direct C-H oxygenation of benzamides and resource-economical synthesis of isoquinolones. The cobaltaelectrocatalysis setup and reaction take about 17 h, while an additional 5 h have to be anticipated for workup and chromatographic purification. The methods described herein feature broad functional group tolerance, operational simplicity, low waste-product formation and an overall exceptional level of resource economy.

Cobalta-Electrocatalyzed C-H Activation in Biomass-Derived Glycerol: Powered by Renewable Wind and Solar Energy

Aqueous glycerol was identified as a renewable reaction medium for metalla-electrocatalyzed C-H activation powered by sustainable energy sources. The renewable solvent was employed for cobalt-catalyzed C-H/N-H functionalizations under mild conditions. The cobalta-electrocatalysis manifold occurred with high levels of chemo- and positional selectivity and allowed for electrochemical C-H activations with broad substrate scope. The resource economy of this strategy was considerably substantiated by the direct use of renewable solar and wind energy.

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.

Cobaltaelectro-catalyzed oxidative allene annulation by electro-removable hydrazides

An efficient C-H/N-H functionalization with allenes was enabled via versatile electro-oxidative cobalt catalysis. Thus, electrochemical C-H activations were accomplished with high levels of chemoselectivity and regioselectivity in an operationally simple undivided cell setup. The user-friendly nature of this protocol was highlighted by excellent functional group tolerance, an electro-reductive removable hydrazide directing group and easy scalability. Experimental mechanistic studies were indicative of a facile BIES C-H cobaltation event.

Bidentate Directing Groups: An Efficient Tool in C-H Bond Functionalization Chemistry for the Expedient Construction of C-C Bonds

During the past decades, synthetic organic chemistry discovered that directing group assisted C-H activation is a key tool for the expedient and siteselective construction of C-C bonds. Among the various directing group strategies, bidentate directing groups are now recognized as one of the most efficient devices for the selective functionalization of certain positions due to fact that its metal center permits fine, tunable, and reversible coordination. The family of bidentate directing groups permit various types of assistance to be achieved, such as N,N-dentate, N,O-dentate, and N,S-dentate auxiliaries, which are categorized based on the coordination site. In this review, we broadly discuss various C-H bond functionalization reactions for the formation of C-C bonds with the aid of bidentate directing groups.

Probing the versatility of metallo-electro hybrid catalysis: enabling access towards facile C–N bond formation

Metallo-electro catalysis has emerged as sustainable alternate to conventional transition metal methodologies. This review highlights the recent advances for the formation of C–N bonds by merging transition metal catalysis with electrosynthesis.

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.

Electrochemically Enabled Double C-H Activation of Amides: Chemoselective Synthesis of Polycyclic Isoquinolinones

We developed an electrochemically enabled dehydrogenative annulation reaction of amides and alkynes for the synthesis of antitumor polycyclic isoquinolinones through a double C-H activation route. No external oxidant is required in this reaction, and electricity is used for Ru catalyst circulation. The most remarkable feature of this reaction is the effective improvement of product regioselectivity under mild electrolytic conditions in comparison with previously set strong oxidant conditions.

Electrochemistry-Enabled Ir-Catalyzed Vinylic C-H Functionalization

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.

Cobalt-Catalyzed Hydroacylative Dimerization of Allenes Leading to Skipped Dienes

A cobalt-diphosphine catalyst has been found to promote a selective 1:2 coupling reaction between aldehydes and allenes to form β,δ-dialkylidene ketones, featuring skipped diene moieties, with high regioselectivities and stereoselectivities. The reaction is distinct from previously reported, rhodium-catalyzed aldehyde-allene 1:2 coupling to afford β,γ-dialkylidene ketones bearing 1,3-diene moieties. The present hydroacylative dimerization involves a unique allene/allene oxidative cyclization mode to form a C1-C2 linkage between the allene molecules.

Cobaltaelectro-Catalyzed C-H Activation with Carbon Monoxide or Isocyanides

Electrochemical oxidative C-H/N-H activations with isocyanides have been realized with a versatile cobalt catalyst. The widely applicable cobalt catalysis manifold further enabled electrooxidative C-H/N-H carbonylations with carbon monoxide under ambient conditions. The C-H functionalizations were efficiently realized with ample scope and outstanding functional group tolerance in a user-friendly undivided cell setup.

Reaching Green: Heterocycle Synthesis by Transition Metal-Catalyzed C-H Functionalization in Sustainable Medium

Catalytic C-H functionalization has emerged as an efficient alternative to traditional coupling reactions. However, some of these reactions depend on environmentally harmful solvents, weakening the overall green nature of these methods. As organic processes consume large amount of solvents, the use of less harmful solvents enhance the sustainability of these reactions. Herein, we present an overview of transition metal-catalyzed C-H functionalization reactions for the synthesis of heterocycles in sustainable solvents based on CHEM21 solvent selection guide.

Recent Advances in Oxidative R1-H/R2-H Cross-Coupling with Hydrogen Evolution via Photo-/Electrochemistry

Photo-/electrochemical catalyzed oxidative R¹-H/R²-H cross-coupling with hydrogen evolution has become an increasingly important issue for molecular synthesis. The dream of construction of C-C/C-X bonds from readily available C-H/X-H with release of H₂ can be facilely achieved without external chemical oxidants, providing a greener model for chemical bond formation. Given the great influence of these reactions in organic chemistry, we give a summary of the state of the art in oxidative R¹-H/R²-H cross-coupling with hydrogen evolution via photo/electrochemistry, and we hope this review will stimulate the development of a greener synthetic strategy in the near future.

Catalyst-Free, Direct Electrochemical Tri- and Difluoroalkylation/Cyclization: Access to Functionalized Oxindoles and Quinolinones

The catalyst-free electrochemical di- and trifluoromethylation/cyclization of N-substituted acrylamides was realized under external oxidant-free conditions. The strategy provides expedient access to fluoroalkylated oxindoles and 3,4-dihydroquinolin-2(1 H)-ones with ample scope and broad functional group tolerance by mild, direct electrolysis of sodium sulfinates in an undivided cell. Detailed mechanistic studies provided strong support for a SET-based reaction manifold.

Electro-Oxidative C-C Alkenylation by Rhodium(III) Catalysis

Electrochemical C-C activations were accomplished by expedient oxidative rhodium(III) catalysis. Thus, oxidative C-C alkenylations proved viable with the aid of electricity, avoiding the use of toxic and/or expensive transition-metal oxidants. The chelation-assisted C-C functionalizations proceeded with ample scope and excellent levels of chemo- and position selectivities within an organometallic C-C activation manifold. Detailed mechanistic studies provided support for a kinetically relevant C-C scission, and a well-defined organometallic rhodium(III) complex was identified as a catalytically competent intermediate. The electrochemical C-C functionalization was devoid of additional electrolytes, could be conducted on a gram scale, and provided position-selective access to densely 1,2,3-substituted arenes, which are not viable by C-H activation.

Ruthenium(ii)-catalyzed electrooxidative [4+2] annulation of benzylic alcohols with internal alkynes: entry to isocoumarins

A new ruthenium(ii)-catalyzed electrooxidative [4+2] annulation of primary benzylic alcohols with internal alkynes is described, which enables benzylic alcohols as weakly directing group precursors to access isocoumarins via multiple C-H functionalization. The reaction works well with a broad substrate scope, tolerates a wide range of functional groups, and incorporates practically the isocoumarin unit into diverse bioactive molecules. Mechanistic studies indicate that activation of aryl C(sp²)-H bonds is achieved through the generation of the active benzoyloxy-Ru(ii) intermediates via oxidation of benzylic alcohols using an electrooxidative Ru(ii) catalyst.

Electrochemical Transition-Metal-Catalyzed C-H Bond Functionalization: Electricity as Clean Surrogates of Chemical Oxidants

Transition-metal-catalyzed C-H activation has attracted much attention from the organic synthetic community because it obviates the need to prefunctionalize substrates. However, superstoichiometric chemical oxidants, such as copper- or silver-based metal oxidants, benzoquinones, organic peroxides, K₂ S₂ O₈ , hypervalent iodine, and O₂ , are required for most of the reactions. Thus, the development of environmentally benign and user-friendly C-H bond activation protocols, in the absence of chemical oxidants, are urgently desired. The inherent advantages and unique characteristics of organic electrosynthesis make fill this gap. Herein, recent progress in this area (until the end of September 2018) is summarized for different transition metals to highlight the potential sustainability of electro-organic chemistry.

Synthesis of 1,4,5,6-tetrahydropyridazines and pyridazines via transition-metal-free (4 + 2) cycloaddition of alkoxyallenes with 1,2-diaza-1,3-dienes

We developed an economical and practical protocol for the synthesis of 1,4,5,6-tetrahydropyridazines. A diverse range of alkoxyallenes and 1,2-diaza-1,3-dienes undergo (4 + 2) cycloaddition to generate the desired products in excellent yields. The high efficiency, wide substrate scope and good functional group tolerance of this process, coupled with operational simplicity, render the method synthetically attractive. The utility of the cycloaddition is also demonstrated by the preparation of various pyridazines from 1,4,5,6-tetrahydropyridazines.

We developed an economical and practical protocol for the synthesis of 1,4,5,6-tetrahydropyridazines and pyridazines via cyclization of alkoxyallenes and 1,2-diaza-1,3-dienes.

Recent advances in cobalt-catalysed C–H functionalizations

Ready availability, low cost and low toxicity of cobalt salts have redirected the attention of researchers away from noble metals, such as Pd, Rh, and Ir, towards Co in the field of C–H functionalisation.

3d Transition Metals for C-H Activation

C-H activation has surfaced as an increasingly powerful tool for molecular sciences, with notable applications to material sciences, crop protection, drug discovery, and pharmaceutical industries, among others. Despite major advances, the vast majority of these C-H functionalizations required precious 4d or 5d transition metal catalysts. Given the cost-effective and sustainable nature of earth-abundant first row transition metals, the development of less toxic, inexpensive 3d metal catalysts for C-H activation has gained considerable recent momentum as a significantly more environmentally-benign and economically-attractive alternative. Herein, we provide a comprehensive overview on first row transition metal catalysts for C-H activation until summer 2018.

Nickel-Catalyzed Electrooxidative C-H Amination: Support for Nickel(IV)

Nickel-catalyzed electrochemical C-H aminations were accomplished by chemo- and position-selective C-H activation with ample scope. Detailed mechanistic studies highlighted a facile C-H cleavage with unique chemo-selectivity, while cyclovoltammetric analysis provided support for a nickel(II/III/IV) manifold.

Iridium-Catalyzed Electrooxidative C-H Activation by Chemoselective Redox-Catalyst Cooperation

Iridium-catalyzed electrochemical C-H activation was accomplished within a cooperative catalysis manifold, setting the stage for electrooxidative C-H alkenylations through weak O-coordination. The iridium-electrocatalyzed C-H activation featured high functional-group tolerance through assistance of a metal-free redox mediator through indirect electrolysis. Detailed mechanistic insights provided strong support for an organometallic C-H cleavage and a synergistic iridium(III/I)/redox catalyst regime, enabling the use of sustainable electricity as the terminal oxidant with improved selectivity features.

Electrochemical ruthenium-catalyzed alkyne annulations by C–H/Het–H activation of aryl carbamates or phenols in protic media

Electrooxidative peri-C–H activation was accomplished by versatile ruthenium(ii) catalysis in terms of C–H/N–H and C–H/O–H functionalization. The sustainable electrocatalysis exploited electricity, thereby avoiding the use of toxic transition metals as sacrificial oxidants.