Cross-Electrophile Couplings of Activated and Sterically Hindered Halides and Alcohol Derivatives

Nickel-catalyzed reductive cross-coupling of difluoromethylated secondary alkyl bromides with organohalides

We present a highly efficient and versatile nickel-catalyzed protocol for the reductive cross-coupling of unactivated CF2H-substituted electrophiles with a wide variety of aryl and alkenyl halides. This novel approach offers high catalytic reactivity and broad functional group compatibility, enabling late-stage fluoroalkylation of drug molecules.

Benzothiazolium salts as versatile primary alcohol derivatives in Ni-catalyzed cross-electrophile arylation/vinylation

Herein, we report a Ni-catalyzed cross-electrophile coupling of aryl/vinyl halides with benzothiazolium salts derived from alcohols. Our findings demonstrate that primary alkyl benzothiazolium salts serve as effective C(sp3)-O substrates, facilitating coupling with aryl and vinyl halides. This method not only enables the formal functionalization of primary alcohols but also provides experimental support for previously established sequential alcohol halogenation and Ni-catalyzed reductive coupling platforms. Furthermore, this work introduces a new alcohol activation mode, presenting an alternative approach for engaging alcohols as coupling partners in organic synthesis.

Selective Ni(I)/Ni(III) Process for Consecutive Geminal C(sp3)-C(sp2) Bond Formation

Ni-catalyzed multicomponent cross-couplings have emerged as a powerful strategy for efficiently constructing complex molecular architectures from a diverse array of organic halides. Despite its potential, selectively forming multiple chemical bonds in a single operation, particularly in the realm of cross-electrophile coupling catalysis, remains a significant challenge. In this study, we have developed a consecutive open-shell reductive Ni catalysis, enabling the formation of two geminal C(sp3)-C(sp2) bonds from two stereoelectronically similar C(sp2)-I reactants in conjunction with a methylene electrophile. Using zirconaaziridine and elemental Mg0 as reductants, this protocol exhibits broad applicability across a wide range of (hetero)aromatic, alkenyl, and glycal halides, allowing for the rapid assembly of medicinally relevant scaffolds with excellent functional group tolerance. Further kinetic studies suggest a dual "sequential reduction" catalytic process facilitated by a zirconaaziridine-mediated redox-transmetalation process in Ni catalysis. Notably, the concerted oxidative addition of Ni(I)-I across a C(sp2)-I bond, as well as the halide atom abstraction among various C(sp3) electrophiles by an open-shell C(sp2)-Ni(I) species, can proceed with high selectivity. The use of an unsymmetrical methylene electrophile with exceptionally high reactivity in XEC resulted in the rapid accumulation of a benzylic or allylic electrophile intermediate at the outset of reaction, thereby finely controlling the coupling sequence.

Current-Regulated Selective Nickel-Catalyzed Electroreductive Cross-Electrophile Carbonylation to β/γ-Hydroxy Ketones

The nickel catalyzed multi-component cross-electrophile carbonylation which emerges as a powerful and efficient method for constructing diverse ketones has attracted increasing attention of organic chemists. However, the selectivity of this reaction poses a significant challenge. In this work, we have developed a current-regulated selective nickel-catalyzed electroreductive cross-electrophile carbonylation, which offers a direct convergent synthesis of β/γ-hydroxy ketones, which represent pivotal structural motifs found in numerous natural products, bioactive molecules, pharmaceutical compounds, and essential building blocks. A diverse range of multi-substituted β/γ-hydroxyketones can be accessed with high chemo- and regioselectivity from epoxides, aryl iodides, and a simple CO source (ClCO2Pr). This electroreductive carbonylation strategy exhibits high functional group tolerance and can be applied in late-stage derivatization of drugs and natural products. Notably, chiral epoxides can be employed as reactants with chirality retention, enabling the synthesis of asymmetric β-hydroxy ketones. Our approach demonstrates a novel electrochemical selectivity-controlled strategy in multi-component cross-electrophile coupling.

Electroreductive Cross-Coupling Reactions: Carboxylation, Deuteration, and Alkylation

ConspectusElectrochemistry has been used as a tool to drive chemical reactions for more than two centuries. With the help of an electrode and a power source, chemists are provided with a system whose potential can be precisely dialed in. The theoretically infinite redox range renders electrochemistry capable of oxidizing or reducing some of the most tenacious compounds. Indeed, electroreduction offers an alternative to generating highly active intermediates from electrophiles (e.g., halides, alkenes, etc.) in organic synthesis, which can be untouchable with traditional reduction methods. Meanwhile, the reductive coupling reactions are extensively utilized in both industrial and academic settings due to their ability to swiftly, accurately, and effectively construct C-C and C-X bonds, which present innovative approaches for synthesizing complex molecules. Nonetheless, its application is constrained by several inherent limitations: (a) the requirement for stoichiometric quantities of reducing agents, (b) scarce activation strategies for inert substrates with high reduction potentials, (c) incomplete mechanistic elucidation, and (d) challenges in the isolation of intermediates. The merging of electrochemistry and reductive coupling represents an attractive approach to address the above limitations in organic synthesis and has seen increasing use in the synthetic community over the past few years.Since 2020, our group has been dedicated to developing electroreductive cross-coupling reactions using readily available organic substrates with small molecules, such as organic halides, alkenes, arenes, CO2, and D2O, to construct high value-added organic products. Electroreductive chemistry is highly versatile and offers powerful reducing capacity and precise selectivity control, which has allowed us to develop three electrochemical modes in our lab: (1) An economically advantageous electrochemical direct reduction (EDR) strategy that emphasizes efficiency, achieves high atom utilization, and minimizes unnecessary atomic waste. (2) A class of electrochemical organo-mediated reduction (EOMR) methods that are capable of effectively controlling reaction intermediates and reaction pathways. This allows for precise modulation of reaction processes to enhance efficiency and selectivity. (3) The electrochemical metal-catalyzed reduction (EMCR) method that enables selective activation and functionalization of specific chemical bonds or functional groups under mild conditions, thereby reducing the occurrence of side reactions. We commenced our studies by establishing an organic-mediator-promoted electroreductive carboxylation of aryl and alkyl halides. This strategy was then employed for the arylcarboxylation of simple styrenes with aryl halides in a highly selective manner. Meanwhile, under direct electrolysis conditions, the carboxylation of arenes and epoxides with CO2 as the carboxyl source was achieved. Moreover, through the precise adjustment of the electroreductive conditions, we successfully accomplished the electroreductive deuteration of arenes, olefins, and unactivated alkyl halides, enabling the efficient and selective formation of D-labeled products. Finally, building on our previous understanding of alkyl halides, we developed a series of electrochemical alkylation reactions that enable the efficient formation of C(sp3)-C(sp3) bonds using alkyl halides.

Cross-Electrophile Coupling: Principles, Methods, and Applications in Synthesis

Cross-electrophile coupling (XEC), defined by us as the cross-coupling of two different σ-electrophiles that is driven by catalyst reduction, has seen rapid progression in recent years. As such, this review aims to summarize the field from its beginnings up until mid-2023 and to provide comprehensive coverage on synthetic methods and current state of mechanistic understanding. Chapters are split by type of bond formed, which include C(sp3)-C(sp3), C(sp2)-C(sp2), C(sp2)-C(sp3), and C(sp2)-C(sp) bond formation. Additional chapters include alkene difunctionalization, alkyne difunctionalization, and formation of carbon-heteroatom bonds. Each chapter is generally organized with an initial summary of mechanisms followed by detailed figures and notes on methodological developments and ending with application notes in synthesis. While XEC is becoming an increasingly utilized approach in synthesis, its early stage of development means that optimal catalysts, ligands, additives, and reductants are still in flux. This review has collected data on these and various other aspects of the reactions to capture the state of the field. Finally, the data collected on the papers in this review is offered as Supporting Information for readers.

Ambient Electroreductive Carboxylation of Unactivated Alkyl Chlorides and Polyvinyl Chloride (PVC) Upgrading

Electrosynthesis of alkyl carboxylic acids upon activating stronger alkyl chlorides at low-energy cost is desired in producing carbon-rich feedstock. Carbon dioxide (CO2), a greenhouse gas, has been recognized as an ideal primary carbon source for those syntheses, and such events also mitigate the atmospheric CO2 level, which is already alarming. On the other hand, the promising upcycling of polyvinyl chloride to polyacrylate is a high energy-demanding carbon-chloride (C-Cl) bond activation process. Molecular catalysts that can efficiently perform such transformation under ambient reaction conditions are rarely known. Herein, we reveal a nickel (Ni)-pincer complex that catalyzes the electrochemical upgrading of polyvinyl chloride to polyacrylate in 95 % yield. The activities of such a Ni electrocatalyst bearing a redox-active ligand were also tested to convert diverse examples of unactivated alkyl chlorides to their corresponding carboxylic acid derivatives. Furthermore, electronic structure calculations revealed that CO2 binding occurs in a resting state to yield an η2-CO2 adduct and that the C-Cl bond activation step is the rate-determining transition state, which has an activation energy of 19.3 kcal/mol. A combination of electroanalytical methods, control experiments, and computational studies were also carried out to propose the mechanism of the electrochemical C-Cl activation process with the subsequent carboxylation step.

Modular Access to C2'-Aryl/Alkenyl Nucleosides with Electrochemical Stereoselective Cross-Coupling

Chemically modified oligonucleotides have garnered significant attention in medicinal chemistry, chemical biology, and synthetic biology due to their enhanced stability in vivo compared to naturally occurring oligonucleotides. However, current methods for synthesizing modified nucleosides, particularly at the C2'-position, are limited in terms of efficiency, modularity, and selectivity. Herein, we report a new approach for the synthesis of highly functionalized C2'-α-aryl/alkenyl nucleosides via an electrochemical nickel-catalyzed cross-coupling of 2'-bromo nucleosides with a variety of (hetero)aryl and alkenyl iodides. This method affords a diverse array of C2'- α-aryl/alkenyl nucleosides with excellent stereoselectivity, broad substrate scope, and good functional group compatibility. We further synthesized oligonucleotides incorporating C2'-aryl-modified thymidine moieties and demonstrated that their annealed double-stranded DNAs exhibit decreased melting temperatures (Tm). Additionally, oligonucleotides with C2'-aryl modifications at the 3' end showed enhanced resistance to 3'-exonuclease degradation and C2'-aryl modifications did not impede the cellular uptake process, highlighting the potential of these modified oligonucleotides for therapeutic applications.

Reductive deaminative cross-coupling of alkyl bistriflimides enabled by electrocatalysis

We present a versatile nickel-electrocatalytic deaminative cross-coupling platform for the efficient construction of C(sp3)-C(sp3) and C(sp3)-C(sp2) bonds from readily available alkyl bistriflimides. This methodology involves the assembly of two leaving groups on alkyl amines to form alkyl bistriflimides, followed by their effective coupling with a wide range of alkyl halides, alkyl pseudohalides, aryl halides, and alkenyl halides under electrochemical reductive conditions. Moreover, the successful application of electrochemical reductive relay cross-coupling and transition metal-free cross-electrophile coupling further demonstrates the versatility of alkyl bistriflimides as valuable building blocks in organic synthesis. Combined control experiments and density functional theory calculations provide insights into the reaction pathway and the crucial role of iodide in the catalytic process.

Iron-Catalyzed Cross-Electrophile Coupling for the Formation of All-Carbon Quaternary Centers

Quaternary carbon centers are desirable targets for drug discovery and complex molecule synthesis, yet the synthesis of these motifs within traditional cross-coupling paradigms remains a significant challenge due to competing β-hydride elimination pathways. In contrast, the bimolecular homolytic substitution (SH2) mechanism offers a unique and attractive alternative pathway. Metal porphyrin complexes have emerged as privileged catalysts owing to their ability to selectively form primary metal-alkyl complexes, thereby eliminating the challenges associated with tertiary alkyl complexation with a metal center. Herein, we report an iron-catalyzed cross-electrophile coupling of tertiary bromides and primary alkyl electrophiles for the formation of all-carbon quaternary centers through a biomimetic SH2 mechanism.

Nickel-Catalyzed Reductive Alkenylation of Enol Derivatives: A Versatile Tool for Alkene Construction

ConspectusKetone-to-alkene transformations are essential in organic synthesis, and transition-metal-catalyzed cross-coupling reactions involving enol derivatives have become powerful tools to achieve this goal. While substantial progress has been made in nucleophile-electrophile reactions, recent developments in nickel-catalyzed reductive alkenylation reactions have garnered increasing attention. These methods accommodate a broad range of functional groups such as aldehyde, ketone, amide, alcohol, alkyne, heterocycles, and organotin compounds, providing an efficient strategy to access structurally diverse alkenes. This Account primarily highlights the contributions from our laboratory to this growing field while also acknowledging key contributions from other researchers.Our early efforts in this area focused on coupling radical-active substrates, such as α-chloroboronates. This method follows the conventional radical chain mechanism, resulting in facile access to valuable allylboronates. Encouraged by these promising results, we subsequently expanded the substrate scope to encompass radical-inactive compounds. By developing new strategies for controlling cross-selectivity, we enabled the coupling of Csp3 electrophiles (e.g., alcohols and sulfonates), Csp2 electrophiles (e.g., bromoalkenylboronates and acyl fluorides), and heavier group-14 electrophiles like chlorosilanes and chlorogermanes with alkenyl triflates. These advances have provided efficient synthetic routes to a wide range of valuable products, including aliphatic alkenes, enones, dienylboronates, and silicon- and germanium-containing alkenes. Notably, these methods are particularly effective for synthesizing functionalized cycloalkenes, which are traditionally challenging to obtain through conventional methods involving alkenyl halide or organometallic couplings. We have also extended the scope of enol derivatives from triflates to acetates. These compounds are among the most accessible, stable, cost-effective, and environmentally friendly reagents, while their application in cross-coupling has been hampered by low reactivity and selectivity challenges. We showcased that by the use of a Ni(I) catalyst, alkenyl acetates could undergo reductive alkylation with a broad range of alkyl bromides, yielding diverse cyclic and acyclic aliphatic alkenes.Furthermore, our work has demonstrated that reductive coupling of enol derivatives with alkenes provides a highly appealing alternative for alkene synthesis. Particularly, this approach offers opportunity to address the regioselectivity challenges encountered in conventional alkene transformations. For instance, achieving regioselective hydrocarbonation of aliphatic 1,3-dienes has been a longstanding challenge in synthetic chemistry. By using a phosphine-nitrile ligand, we developed a nickel-catalyzed reductive alkenylation of 1,3-dienes with alkenyl triflates, delivering a diverse array of 1,4-dienes with high 1,2-branch selectivity (>20:1) while preserving the geometry of the C3-C4 double bond. Additionally, our investigations laid the foundation for enantioselective reductive alkenylation methodologies, offering new pathways for constructing enantioenriched diketones as well as complex carbo- and heterocyclic compounds. The introduced alkenyl functionality can be further diversified, enhancing molecular diversity and complexity.

Ultrasound-Assisted Remote Benzylic C(sp3)-H Alkylation of N-Fluoroamides with Enol Silanes

We have developed an ultrasound-assisted remote benzylic C(sp3)-H alkylation of N-fluorobenzamides with enol silanes; a series of β-aryl substituted propiophenones was generated in good to high yields. This reaction represents a formal C(sp3)-C(sp3) coupling and features simplicity, high efficiency, and wide substrate scope in a multiple-step sequential process, which involves N-F homolysis, 1,5-hydrogen atom transfer, benzylic radical addition, oxidation by copper(II) salt, and removal of the trimethylsilyl group. Also, DFT theoretical calculations and Marcus theory were employed to consolidate the proposed reaction mechanism.

Nickel-Catalyzed Cross-Electrophile Vinylation of α-Chloro Phosphonates

Herein, we report a general and efficient Ni-catalyzed reductive cross-coupling reaction of substituted vinyl bromides and α-chloro phosphonates to access a set of α-vinyl phosphonates using zinc as the terminal reductant. This reaction exhibits broad substrate adaptability and good functional group tolerance, which allows to afford diverse compounds including structurally complex motifs from natural products and drugs. Furthermore, the practicality was certificated through the gram-scale and transformation experiments. The preliminary mechanistic investigations support a radical chain process. The potential to realize enantiomeric control makes it more valuable for further exploration.

Cobalt-catalyzed reductive cross-coupling: a review

Transition-metal-catalyzed reductive cross-coupling is highly efficient for forming C-C bonds. It earns its limelight from its application by coupling unreactive electrophilic substrates to synthesize a variety of carbon-carbon bonds with various hybridizations (sp, sp2, and sp3), late-stage functionalization, and bioactive molecules' synthesis. Reductive cross-coupling is challenging to bring selectivity but promising approach. Cobalt is comparatively more affordable than other highly efficient metals e.g., palladium and nickel but cobalt catalysis is still facing efficacy challenges. Researchers are trying to harness the maximum out of cobalt's catalytic properties. Shortly, with efficiency achieved combined with the affordability of cobalt, it will revolutionize industrial applications. This review gives insight into the core of cobalt-catalyzed reductive cross-coupling reactions with a variety of substrates forming a range of differently hybridized coupled products.

Magnesium-promoted nickel-catalysed chlorination of aryl halides and triflates under mild conditions

In this study, we present a ligand-free nickel(II)-catalyzed halogen exchange of aromatic halides with magnesium chloride. This method effectively facilitates the retro-Finkelstein reaction for a wide range of aryl bromides, iodides and triflates, demonstrating excellent functional group tolerance. Mechanistic studies reveal that magnesium plays a crucial role in the challenging reductive elimination from Ni(II) intermediates.

Palladium-Catalyzed Cross-Electrophile Couplings of Aryl Thianthrenium Salts with Aryl Bromides via C-S Bond Activation

We report here a step-economic and cost-effective cross-electrophile coupling of aryl thianthrenium salts with widely available aryl bromides, which proceeded effectively via C-S bond activation at ambient temperature in THF in the presence of a palladium catalyst, magnesium turnings, and lithium chloride to enable the facile assembly of a wide array of structurally diverse biaryls in modest to good yields with good functional group compatibility. In addition, the gram-scale reaction could also be realized.

Nickel-Catalyzed Reductive Dicarbofunctionalization of 1,3-Dienes with Aziridines and (Het)Aryl Electrophiles

A three-component reductive 1,4-dicarbofunctionalization of 1,3-dienes with aziridines and (het)aryl electrophiles was realized. The combination of Ni catalysis with Mn powder as a final reductant enables the direct formation of a Csp2-Csp3 and a Csp3-Csp3 bond at one time. This protocol afforded a convenient approach to the synthesis of β-arylethylamines bearing various functionals and heterocyclic groups. The utility of this reaction was also demonstrated by scale-up preparation, late-stage modification of bioactive molecules, and diverse transformations.

Magnesium-Mediated Cross-Electrophile Couplings of Aryl 2-Pyridyl Esters with Aryl Bromides for Ketone Synthesis through In Situ-Formed Arylmagnesium Intermediates

Aryl 2-pyridyl esters could efficiently undergo cross-electrophile couplings with aryl bromides with the aid of magnesium as a reducing metal in the absence of a transition-metal catalyst, leading to the unsymmetrical diaryl ketones in modest to good yields with wide functionality compatibility. In addition, the reaction could be easily scaled up and applied in the late-stage modification of biologically active molecules. Preliminary mechanistic study showed that the coupling reaction presumably proceeds through the in situ formation of arylmagnesium reagents as key intermediates.

Nickel-Catalyzed Reductive Amidation of Aryl Fluorosulfates with Isocyanates: Synthesis of Amides via C-O Bond Cleavage

With the assistance of nickel as catalyst, 2,2'-bipyridine (bpy) as ligand, and manganese as reducing metal, the reductive amidation of isocyanates with readily accessible aryl fluorosulfates could be successfully accomplished. The reactions proceeded effectively via C-O bond activation in DMF at room temperature, enabling the facile synthesis of a range of structurally diverse amides in moderate to high yields with broad functionality compatibility. In addition, the synthetic usefulness of the method was further demonstrated by applying the reaction in scale-up synthesis and the late-stage functionalization of complex molecules with biological activities.

Organic Bases as the Organic Electron Donors (OED) Promoted Reductive Coupling of Diarylhalomethanes: Halogens Controlled Construction of Tetraarylethylenes and Tetraarylethanes

Using the organic base as the organic electron donors, the reductive coupling of diaryhalomethanes was smoothly achieved under transition-metal-free reaction conditions, giving a series of synthetically important tetraarylethylenes and tetraarylethanes in high yields. The mechanistic study revealed that the organic bases acting as the electron donor initiated the generation of a radical intermediate, realizing the construction of tetraarylethylene and tetraarylethane skeletons.

Cross-Electrophile Couplings of Benzyl Sulfonium Salts with Thiosulfonates via C-S Bond Activation

A zinc-mediated cross-electrophile coupling of benzyl sulfonium salts with thiosulfonates via C-S bond cleavage was achieved. The reductive thiolation proceeded well under transition metal-free conditions to afford the desired benzyl sulfides in good yields, exhibiting both broad substrate scope and good functionality tolerance. In addition, the reaction could be applied to the use of selenosulfonate as an effective selenylation agent and be subjected to scale-up synthesis.

Nickel-Catalyzed Enantioselective C(sp3)-C(sp3) Cross-Electrophile Coupling of N-Sulfonyl Styrenyl Aziridines with Alkyl Bromides

Herein, we report the first example of a highly enantioselective alkylative aziridine ring opening. Under the catalysis of a chiral nickel/pyridine-imidazoline complex, asymmetric C(sp3)-C(sp3) cross-electrophile coupling between racemic N-sulfonyl styrenyl aziridines and readily available primary alkyl bromides furnishes a variety of highly enantioenriched phenethylamine derivatives with complete regiocontrol and good functional group tolerance. Preliminary mechanistic studies support a reaction pathway consisting of regioselective iodolysis of aziridines in situ and subsequent enantioconvergent coupling of the generated β-amino benzyl iodides with alkyl bromides.

Radical C-Glycosylation Using Photoexcitable Unprotected Glycosyl Borate

We have developed radical C-glycosylation using photoexcitable unprotected glycosyl borate. The direct excitation of glycosyl borate under visible light irradiation enabled the generation of anomeric radical without any photoredox catalysts. The in situ generated anomeric radical was applicable to the radical addition such as Giese-type addition and Minisci-type reaction to introduce alkyl and heteroaryl groups at the anomeric position. In addition, the radical-radical coupling between the glycosyl borate and acyl imidazolide provided unprotected acyl C-glycosides.

Nickel-Catalyzed Desulfurative Cross-Coupling of Aryl Iodides with Heteroaromatic Thioethers via C-S Bond Cleavage

Herein, we present a Ni-catalyzed direct cross-coupling of heteroaromatic thioethers with aryl iodides via selective C(sp2)-S bond cleavage under reductive conditions, thereby providing various biaryl frameworks with high efficiency. Mechanistic studies suggested Mo(CO)6 played a crucial role in facilitating the activation of the C(sp2)-S bond. This protocol demonstrated a wide substrate scope, operational simplicity, and good functional group compatibility. Furthermore, the utility of this reaction was highlighted by facile scale-up and sequential modification of heteroaryl frameworks.

Synthesis of Stereodefined Enones from the Cross-Electrophile Coupling of Activated Acrylic Acids with Alkyl Bromides

We report a one-pot synthesis of (E)-trisubstituted enones from acrylic acids through the in situ generation of a 2-pyridyl ester and subsequent cross-electrophile coupling with a nickel catalyst under reducing conditions. The scope of trisubstituted enones is broad and compatible with functionality that can be challenging in established olefination techniques. We highlight conditions necessary to suppress undesired side reactions from the α,β-unsaturated carbonyl and improve cross-electrophile coupling approaches to prepare enones.

Nickel-catalyzed selective disulfide formation by reductive cross-coupling of thiosulfonates

Developing innovative methodologies for disulfide preparation is of importance in contemporary organic chemistry. Despite significant advancements in nickel-catalyzed reductive cross-coupling reactions for forming carbon-carbon and carbon-heteroatom bonds, the synthesis of S-S bonds remains a considerable challenge. In this context, we present a novel approach utilizing nickel catalysts for the reductive cross-coupling of thiosulfonates. This method operates under mild conditions, offering a convenient and efficient pathway to synthesize a wide range of both symmetrical and unsymmetrical disulfides from readily available, bench-stable thiosulfonates with exceptional selectivity. Notably, this approach is highly versatile, allowing for the late-stage modification of pharmaceuticals and the preparation of various targeted compounds. A comprehensive mechanistic investigation has been conducted to substantiate the proposed hypothesis.

Nickel(II)/BINOL-catalyzed enantioselective C-H activation via desymmetrization and kinetic resolution

The field of nickel catalysis has witnessed remarkable growth in recent years. However, the use of nickel catalysts in enantioselective C-H activation remains a daunting challenge because of their variable oxidation states, intricate coordination chemistry, and unpredictable reactivity patterns. Herein, we report an enantioselective C-H activation reaction catalyzed by commercially available and air-stable nickel(II) catalyst. Readily available and simple (S)-BINOL is used as a chiral ligand. This operationally simple protocol enables the synthesis of planar chiral metallocenes in high yields with excellent enantioselectivity through desymmetrization and kinetic resolution. Air-stable planar chiral nickelacycle intermediates are first synthesized via enantioselective C-H nickelation and shown to be possible intermediates of the reaction. Deuterium-labeling studies, alongside the characterization and transformation of chiral nickel(II) species, suggest that C-H cleavage is the enantio-determining step. Moreover, the large-scale synthesis and diverse synthetic transformations underscore the practicality of this protocol.

Site-Selective Distal C(sp3)-H Bromination of Aliphatic Amines as a Gateway for Forging Nitrogen-Containing sp3 Architectures

Herein, we disclose a new strategy that rapidly and reliably incorporates bromine atoms at distal, secondary C(sp3)-H sites in aliphatic amines with an excellent and predictable site-selectivity pattern. The resulting halogenated building blocks serve as versatile linchpins to enable a series of carbon-carbon and carbon-heteroatom bond-formations at remote C(sp3) sites, thus offering a new modular and unified platform that expediates the access to advanced sp3 architectures possessing valuable nitrogen-containing saturated heterocycles of interest in medicinal chemistry settings.

Nickel-Catalyzed Cross-Coupling of Aryl Diazonium Salts with Aryl Bromides

Herein, we present a one-pot method for the direct cross-coupling of aryl diazonium salts and aryl bromides in an economical way that avoids the use of sensitive organometallic reagents. The reaction is accomplished with the assistance of nickel catalysts, ligands, magnesium turnings, lithium chloride, and triethylamine, avoiding the use of pre-activated organometallic reagents.

Late-Stage C(sp2)-C(sp3) Diversification via Nickel Oxidative Addition Complexes

Herein, we describe nickel oxidative addition complexes (Ni-OACs) of drug-like molecules as a platform to rapidly generate lead candidates with enhanced C(sp3) fraction. The potential of Ni-OACs to access new chemical space has been assessed not only in C(sp2)-C(sp3) couplings but also in additional bond formations without recourse to specialized ligands and with improved generality when compared to Ni-catalyzed reactions. The development of an automated diversification process further illustrates the robustness of Ni-OACs, thus offering a new gateway to expedite the design-make-test-analyze (DMTA) cycle in drug discovery.

Zinc Promoted Cross-Electrophile Sulfonylation to Access Alkyl-Alkyl Sulfones

The transition metal-catalyzed multi-component cross-electrophile sulfonylation, which incorporates SO2 as a linker within organic frameworks, has proven to be a powerful, efficient, and cost-effective means of synthesizing challenging alkyl-alkyl sulfones. Transition metal catalysts play a crucial role in this method by transferring electrons from reductants to electrophilic organohalides, thereby causing undesirable side reactions such as homocoupling, protodehalogenation, β-hydride elimination, etc. It is worth noting that tertiary alkyl halides have rarely been demonstrated to be compatible with current methods owing to various undesired side reactions. In this work, a zinc-promoted cross-electrophile sulfonylation is developed through a radical-polar crossover pathway. This approach enables the synthesis of various alkyl-alkyl sulfones, including 1°-1°, 2°-1°, 3°-1°, 2°-2°, and 3°-2° types, from inexpensive and readily available alkyl halides. Various functional groups are well tolerated in the work, resulting in yields of up to 93%. Additionally, this protocol has been successfully applied to intramolecular sulfonylation and homo-sulfonylation reactions. The insights gained from this work shall be useful for the further development of cross-electrophile sulfonylation to access alkyl-alkyl sulfones.

Bis(pinacolato)diboron-Enabled Ni-Catalyzed Reductive Arylation/Vinylation of Alkyl Electrophiles

Herein, the use of economically and environmentally friendly bis(pinacolato)diboron (B2Pin2) is described as a non-metallic reductant in mediating Ni-catalyzed C(sp3)-C(sp2) reductive cross-coupling of alkyl electrophiles with aryl/vinyl halides. This method exhibits excellent suitability for heteroaryl halides and alkyl halides/Katritzky salts. The present study is compatible with an in situ halogenation of alcohol method, allowing for selective mono-functionalization of diols and bio-relevant alcohols (e.g., carbohydrates). The use of B2Pin2 shows potential for easy scalability without introducing additional metal impurities into the products. It is observed for the first time in the realm of cross-electrophile coupling chemistry that B2Pin2 can sever as a reductant to reduce NiII to Ni0. This mechanistic insight may inspire the development of new reductive bond-forming methodologies that can otherwise be difficult to achieve with a metal reductant.

Aliphatic Hydrosilanes via Nickel-Catalyzed Reductive Csp3-Si Coupling of Primary Alkyl Bromides and Chlorohydrosilanes

The reductive C-Si coupling of chlorosilanes offers efficient access to organosilanes, but its potential for constructing aliphatic ones remains largely unexplored. This manuscript presents a nickel-catalyzed Csp3-Si coupling reaction of unactivated alkyl-Br and R2Si(H)Cl. This work establishes a new approach for synthesizing highly functionalized aliphatic hydrosilanes from readily available chemical feedstocks. The reaction is easily scalable and can accommodate various functional groups, including carboxylic acids, which are usually incompatible with basic conditions.

Cobalt-Catalyzed Enantioselective Reductive Arylation, Heteroarylation, and Alkenylation of Michael Acceptors via an Elementary Mechanism of 1,4-Addition

Cobalt complexes with chiral quinox ligands effectively promote the enantioselective conjugate addition of enones using aryl, heteroaryl, and alkenyl halides and sulfonates. Additionally, a cobalt complex with a strongly donating diphosphine, BenzP*, successfully catalyzes the asymmetric reductive arylation and alkenylation of α,β-unsaturated amides. Both catalytic systems show broad scopes and tolerance of sensitive functional groups. Both reactions can be scaled up with low loadings of cobalt catalysts. Experimental results and density functional theory (DFT) calculations suggest a new mechanism of elementary 1,4-addition of aryl cobalt(I) complexes.

N-Alkoxyphthalimides as Nitrogen Electrophiles to Construct C-N Bonds via Reductive Cross-Coupling

N-Alkoxyphthalimides, one kind of phthalimide derivative, have great importance in synthesis, mainly used as free radical precursors. While the phthalimide unit, for a long time, was treated as part of the waste stream. Construction of C-N bonds has always been a hot spot, especially in reductive cross-coupling. Herein, a nickel-catalyzed reductive cross-coupling reaction of N-methoxyphthalimides with alkyl halides is described, where N-methoxyphthalimides serve as nitrogen electrophiles. This tactic provides a new approach to construct C-N bonds under mild neutral conditions. Alkyl chlorides, bromides, iodides, and sulfonates are all fit to this transformation. Moreover, the reaction could tolerate a broad substrate scope, especially base-sensitive functional groups (boron or silicon groups), as well as competitive nucleophilic groups (phenols and amides), which are incompatible with traditional Gabriel synthesis under basic conditions, demonstrating a complementary role of this work to Gabriel synthesis.

Nickel-Catalyzed Atroposelective Cross-Electrophile Coupling of Aryl Halides: A General and Practical Route to Diverse MOP-Type Ligands

We report a highly cross- and atroposelective coupling between ortho-(chloro)arylphosphine oxides and ortho-(bromo)aryl ethers. This previously unknown asymmetric nickel-catalyzed reaction offers a direct route to highly enantioenriched axially chiral biaryl monophosphine oxides that are difficult to access by other means. These products can be readily reduced to generate chiral MOP-type ligands bearing complex skeletal backbones. The utility of these chiral ligands in asymmetric catalysis is also demonstrated.

Regio- and Enantioselective Hydromethylation of 3-Pyrrolines and Glycals Enabled by Cobalt Catalysis

Enantioenriched 3-methylpyrrolidine, with its unique chiral nitrogen-containing core skeleton, exists widely in various functional molecules, including natural products, bioactive compounds, and pharmaceuticals. Traditional methods for synthesizing these valuable methyl-substituted heterocycles often involve enzymatic processes or complex procedures with chiral auxiliaries, limiting the substrate scope and efficiency. Efficient catalytic methylation, especially in an enantioselective manner, has been a long-standing challenge in chemical synthesis. Herein, we present a novel approach for the remote and stereoselective installation of a methyl group onto N-heterocycles, leveraging a CoH-catalyzed asymmetric hydromethylation strategy. By effectively combining a commercial cobalt precursor with a modified bisoxazoline (BOX) ligand, a variety of easily accessible 3-pyrrolines can be converted to valuable enantiopure 3-(isotopic labeling)methylpyrrolidine compounds with outstanding enantioselectivity. This efficient protocol streamlines the two-step synthesis of enantioenriched 3-methylpyrrolidine, which previously required up to five or six steps under harsh conditions or expensive starting materials.

Metallaphotoredox deuteroalkylation utilizing thianthrenium salts

Deuterium labeling compounds play a crucial role in organic and pharmaceutical chemistry. The synthesis of such compounds typically involves deuterated building blocks, allowing for the incorporation of deuterium atoms and functional groups into a target molecule in a single step. Unfortunately, the limited availability of synthetic approaches to deuterated synthons has impeded progress in this field. Here, we present an approach utilizing alkyl-substituted thianthrenium salts that efficiently and selectively introduce deuterium at the α position of alkyl chains through a pH-dependent HIE process, using D2O as the deuterium source. The resulting α-deuterated alkyl thianthrenium salts, which bear two deuterium atoms, exhibit excellent selectivity and deuterium incorporation in electrophilic substitution reactions. Through in situ formation of isotopically labelled alkyl halides, these thianthrenium salts demonstrate excellent compatibility in a series of metallaphotoredox cross-electrophile coupling with (hetero)aryl, alkenyl, alkyl bromides, and other alkyl thianthrenium salts. Our technique allows for a wide range of substrates, high deuterium incorporation, and precise control over the site of deuterium insertion within a molecule such as the benzyl position, allylic position, or any alkyl chain in between, as well as neighboring heteroatoms. This makes it invaluable for synthesizing various deuterium-labeled compounds, especially those with pharmaceutical significance.

CoH-catalyzed asymmetric remote hydroalkylation of heterocyclic alkenes: a rapid approach to chiral five-membered S- and O-heterocycles

Saturated heterocycles, which incorporate S and O heteroatoms, serve as fundamental frameworks in a diverse array of natural products, bioactive compounds, and pharmaceuticals. Herein, we describe a unique cobalt-catalyzed approach integrated with a desymmetrization strategy, facilitating precise and enantioselective remote hydroalkylation of unactivated heterocyclic alkenes. This method delivers hydroalkylation products with high yields and excellent stereoselectivity, representing good efficiency in constructing alkyl chiral centers at remote C3-positions within five-membered S/O-heterocycles. Notably, the broad scope and good functional group tolerance of this asymmetric C(sp3)-C(sp3) coupling enhance its applicability.

Cross-selective Deoxygenative Coupling of Aliphatic Alcohols: Installation of Methyl Groups including Isotopic Labels by Nickel Catalysis

Nickel-catalyzed cross-electrophile coupling reactions of two aliphatic alcohol derivatives remain a challenge. Herein, we report a nickel-catalyzed reductive methylation reaction of aliphatic mesylates with methyl tosylate. This reaction provides straightforward access to compounds bearing aliphatic methyl groups from alkyl alcohol derivatives. Isotopically labelled substrates and reagents can be employed in the reaction to provide perdeuterated and 13C-labelled products. This transformation can be achieved by employing stoichiometric Mn reductant or electrochemically. Additionally, mechanistic experiments show that alkyl iodides are key intermediates in the transformation which undergo a stereoablative reaction via radical intermediates.

Ligand-Controlled, Nickel-Catalyzed Stereodivergent Construction of 1,3-Nonadjacent Stereocenters

In contrast to the asymmetric synthesis of molecules with a single stereocenter or 1,2-adjacent stereocenters, the simultaneous construction of acyclic 1,3-nonadjacent stereocenters via a single catalyst in an enantioselective and diastereoselective manner remains a formidable challenge. Here, we demonstrate the enantioselective and diastereodivergent construction of 1,3-nonadjacent stereocenters through Ni-catalyzed reductive cyclization/cross-coupling of alkene-tethered aryl bromides and α-bromoamides, which represents the major remaining stereochemical challenge of cyclization/difunctionalization of alkenes. Using Ming-Phos as ligand, a diverse set of oxindoles containing 1,3-nonadjacent stereocenters were obtained with high levels of enantio- and diastereoselectivity. Mechanistic experiments and density functional theory calculations indicate that magnesium salt plays a key role in controlling the diastereoselectivity. Furthermore, another set of complementary stereoisomeric products were constructed from the same set of starting materials using Ph-Phox as ligand.

From Quaternary Carbon to Tertiary C(sp3)-Si and C(sp3)-Ge Bonds: Decyanative Coupling of Malononitriles with Chlorosilanes and Chlorogermanes Enabled by Ni/Ti Dual Catalysis

Transition-metal-catalyzed C-Si/Ge cross-coupling offers promising avenues for the synthesis of organosilanes/organogermanes, yet it is fraught with long-standing challenges. A Ni/Ti-catalyzed strategy is reported here, allowing the use of disubstituted malononitriles as tertiary C(sp3) coupling partners to couple with chlorosilanes and chlorogermanes, respectively. This method enables the catalytic cleavage of the C(sp3)-CN bond of the quaternary carbon followed by the formation of C(sp3)-Si/C(sp3)-Ge bonds from ubiquitously available starting materials. The efficiency and generality are showcased by a broad scope for both of the coupling partners, therefore holding the potential to synthesize structurally diverse quaternary organosilanes and organogermanes that were difficult to access previously.

Nickel-catalysed highly regioselective synthesis of β-acyl naphthalenes under reductive conditions

Over the past decade, significant progress has been made in the direct C-H acylation of naphthalenes, occurring at the α or β-positions to yield valuable ketones through Friedel-Crafts acylation or transition-metal-catalysed carbonylative coupling reactions. Nevertheless, highly regioselective acylation of naphthalenes remains a formidable challenge. Herein, we developed a nickel-catalysed reductive ring-opening reaction of 7-oxabenzonorbornadienes with acyl chlorides as the electrophilic coupling partner, providing a new method for the exclusive preparation of β-acyl naphthalenes.

Cobalt-catalyzed cross-electrophile coupling of alkynyl sulfides with unactivated chlorosilanes

Herein, we disclose a highly efficient cobalt-catalyzed cross-electrophile alkynylation of a broad range of unactivated chlorosilanes with alkynyl sulfides as a stable and practical alkynyl electrophiles. Strategically, employing easily synthesized alkynyl sulfides as alkynyl precursors allows access to various alkynylsilanes in good to excellent yields. Notably, this method avoids the utilization of strong bases, noble metal catalysts, high temperature and forcing reaction conditions, thus presenting apparent advantages, such as broad substrate scope (72 examples, up to 97% yield), high Csp-S chemo-selectivity and excellent functional group compatibility (Ar-X, X = Cl, Br, I, OTf, OTs). Moreover, the utilities of this method are also illustrated by downstream transformations and late-stage modification of structurally complex natural products and pharmaceuticals. Mechanistic studies elucidated that the cobalt catalyst initially reacted with alkynyl sulfides, and the activation of chlorosilanes occurred via an SN2 process instead of a radical pathway.

Alcohols as Substrates in Transition-Metal-Catalyzed Arylation, Alkylation, and Related Reactions

Alcohols are abundant and attractive feedstock molecules for organic synthesis. Many methods for their functionalization require them to first be converted into a more activated derivative, while recent years have seen a vast increase in the number of complexity-building transformations that directly harness unprotected alcohols. This Review discusses how transition metal catalysis can be used toward this goal. These transformations are broadly classified into three categories. Deoxygenative functionalizations, representing derivatization of the C-O bond, enable the alcohol to act as a leaving group toward the formation of new C-C bonds. Etherifications, characterized by derivatization of the O-H bond, represent classical reactivity that has been modernized to include mild reaction conditions, diverse reaction partners, and high selectivities. Lastly, chain functionalization reactions are described, wherein the alcohol group acts as a mediator in formal C-H functionalization reactions of the alkyl backbone. Each of these three classes of transformation will be discussed in context of intermolecular arylation, alkylation, and related reactions, illustrating how catalysis can enable alcohols to be directly harnessed for organic synthesis.

Effect of 6,6'-Substituents on Bipyridine-Ligated Ni Catalysts for Cross-Electrophile Coupling

A family of 4,4'-tBu2-2,2'-bipyridine (tBubpy) ligands with substituents in either the 6-position, 4,4'-tBu2-6-Me-bpy (tBubpyMe), or 6 and 6'-positions, 4,4'-tBu2-6,6'-R2-bpy (tBubpyR2; R = Me, iPr, sBu, Ph, or Mes), was synthesized. These ligands were used to prepare Ni complexes in the 0, I, and II oxidation states. We observed that the substituents in the 6 and 6'-positions of the tBubpy ligand impact the properties of the Ni complexes. For example, bulkier substituents in the 6,6'-positions of tBubpy better stabilized (tBubpyR2)NiICl species and resulted in cleaner reduction from (tBubpyR2)NiIICl2. However, bulkier substituents hindered or prevented coordination of tBubpyR2 ligands to Ni0(cod)2. In addition, by using complexes of the type (tBubpyMe)NiCl2 and (tBubpyR2)NiCl2 as precatalysts for different XEC reactions, we demonstrated that the 6 or 6,6' substituents lead to major differences in catalytic performance. Specifically, while (tBubpyMe)NiIICl2 is one of the most active catalysts reported to date for XEC and can facilitate XEC reactions at room temperature, lower turnover frequencies were observed for catalysts containing tBubpyR2 ligands. A detailed study on the catalytic intermediates (tBubpy)Ni(Ar)I and (tBubpyMe2)Ni(Ar)I revealed several factors that likely contributed to the differences in catalytic activity. For example, whereas complexes of the type (tBubpy)Ni(Ar)I are low spin and relatively stable, complexes of the type (tBubpyMe2)Ni(Ar)I are high-spin and less stable. Further, (tBubpyMe2)Ni(Ar)I captures primary and benzylic alkyl radicals more slowly than (tBubpy)Ni(Ar)I, consistent with the lower activity of the former in catalysis. Our findings will assist in the design of tailor-made ligands for Ni-catalyzed transformations.

Electrochemically Driven para-Selective C(sp2)-H Alkylation Enabled by Activation of Alkyl Halides without Sacrificial Anodes

With alkyl halides (I, Br, Cl) as a coupling partner, an electrochemically driven strategy for para-selective C(sp2)-H alkylation of electron-deficient arenes (aryl esters, aldehydes, nitriles, and ketones) has been achieved to access diverse alkylated arenes in one step. The reaction enables the activation of alkyl halides in the absence of sacrificial anodes, achieving the formation of C(sp2)-C(sp3) bonds under mild electrolytic conditions. The utility of this protocol is reflected in high site selectivity, broad substrate scope, and scalable.

Ru-Mg promoted reductive cross-coupling of allyl bromides and alkenes to synthesize 1,7-octadienes with an all-carbon quaternary center

A Ru-promoted reductive cross-coupling of allyl bromides and electron-deficient alkenes to provide terminal 1,7-octadienes with magnesium as a reductant is reported herein. This approach enables the facile construction of a series of complex terminal 1,7-octadienes with an all-carbon quaternary center under mild reaction conditions, and the synthetic utility of the current method has been demonstrated by a gram scale synthesis. Preliminary mechanism investigations suggested that a radical pathway might not be involved in this transformation.

Asymmetric Cross-Coupling of Aldehydes with Diverse Carbonyl or Iminyl Compounds by Photoredox-Mediated Cobalt Catalysis

The asymmetric cross-coupling of unsaturated bonds, hampered by their comparable polarity and reactivity, as well as the scarcity of efficient catalytic systems capable of diastereo- and enantiocontrol, presents a significant hurdle in organic synthesis. In this study, we introduce a highly adaptable photochemical cobalt catalysis framework that facilitates chemo- and stereoselective reductive cross-couplings between common aldehydes with a broad array of carbonyl and iminyl compounds, including N-acylhydrazones, aryl ketones, aldehydes, and α-keto esters. Our methodology hinges on a synergistic mechanism driven by photoredox-induced single-electron reduction and subsequent radical-radical coupling, all precisely guided by a chiral cobalt catalyst. Various optically enriched β-amino alcohols and unsymmetrical 1,2-diol derivatives (80 examples) have been synthesized with good yields (up to 90% yield) and high stereoselectivities (up to >20:1 dr, 99% ee). Of particular note, this approach accomplishes unattainable photochemical asymmetric transformations of aldehydes with disparate carbonyl partners without reliance on any external photosensitizer, thereby further emphasizing its versatility and cost-efficiency.

Insights into Recent Nickel-Catalyzed Reductive and Redox C-C Coupling of Electrophiles, C(sp3)-H Bonds and Alkenes

ConspectusTransition metal-catalyzed reductive cross-coupling of two carbon electrophiles, also known as cross-electrophile coupling (XEC), has transformed the landscape of C-C coupling chemistry. Nickel catalysts, in particular, have demonstrated exceptional performance in facilitating XEC reactions, allowing for diverse elegant transformations by employing various electrophiles to forge C-C bonds. Nevertheless, several crucial challenges remain to be addressed. First, the intrinsic chemoselectivity between two structurally similar electrophiles in Ni-catalyzed C(sp3)-C(sp3) and C(sp2)-C(sp2) cross-coupling has not been well understood; this necessitates an excess of one of the coupling partners to achieve synthetically useful outcomes. Second, the substitution of economically and environmentally benign nonmetal reductants for Zn/Mn can help scale up XEC reactions and avoid trace metals in pharmaceutical products, but research in this direction has progressed slowly. Finally, it is highly warranted to leverage mechanistic insights from Ni-catalyzed XEC to develop innovative thermoredox coupling protocols, specifically designed to tackle challenges associated with difficult substrates such as C(sp3)-H bonds and unactivated alkenes.In this Account, we address the aforementioned issues by reviewing our recent work on the reductive coupling of C-X and C-O electrophiles, the thermoredox strategy for coupling associated with C(sp3)-H bonds and unactivated alkenes, and the use of diboron esters as nonmetal reductants to achieve reductive coupling. We focus on the mechanistic perspectives of the transformations, particularly how the key C-NiIII-C intermediates are generated, in order to explain the chemoselective and regioselective coupling results. The Account consists of four sections. First, we discuss the Zn/Mn-mediated chemoselective C(sp2)-C(sp2) and C(sp3)-C(sp3) bond formations based on the coupling of selected alkyl/aryl, allyl/benzyl, and other electrophiles. Second, we describe the use of diboron esters as versatile reductants to achieve C(sp3)-C(sp3) and C(sp3)-C(sp2) couplings, with an emphasis on the mechanistic consideration for the construction of C(sp3)-C(sp2) bonds. Third, we discuss leveraging C(sp3)-O bonds for effective C(sp3)-C bond formation via in situ halogenation of alcohols as well as the reductive preparation of α-vinylated and -arylated unusual amino esters. In the final section, we illustrate the thermoredox functionalization of challenging C(sp3)-H bonds with aryl and alkyl halides to afford C(sp3)-C bonds by taking advantage of the compatibility of Zn with the oxidant di-tert-butylperoxide (DTBP). Furthermore, we discuss a Ni-catalyzed and SiH/DTBP-mediated hydrodimerization of terminal alkenes to selectively forge head-to-head and methyl branched C(sp3)-C(sp3) bonds. This process, conducted in the presence or absence of catalytic CuBr2, provides a solution to a long-standing challenge: site-selective hydrocoupling of unactivated alkenes to produce challenging C(sp3)-C(sp3) bonds.