Transition-Metal Catalysis of Nucleophilic Substitution Reactions: A Radical Alternative to SN1 and SN2 Processes

Sulfinylation of Tertiary Alkyl Halides via a Halogenophilic Substitution (SN2X) Reaction

The halogenophilic SN2X reaction involving a nucleophilic attack on the X group from the front is less sensitive to backside steric hindrance. Herein, we report a mild and efficient SN2X reaction for sulfinylation of activated tertiary alkyl halides, which could provide a novel method for accessing sulfoxides decorated with a congested carbon center. Preliminary mechanistic studies indicated that the generated sulfinyl bromides would be the key electrophilic intermediates in the reaction.

Chemoselective Functionalization of Tertiary C-H Bonds of Allylic Ethers: Enantioconvergent Access to sec,tert-Vicinal Diols

While enantioenriched alcohols are highly significant in medicinal chemistry, total synthesis, and materials science, the stereoselective synthesis of tertiary alcohols with two adjacent stereocenters remains a formidable challenge. In this study, we present a dual catalysis approach utilizing photoredox and nickel catalysts to enable the unprecedented chemoselective functionalization of tertiary allylic C-H bonds in allyl ethers instead of cleaving the C-O bond. The resulting allyl-Ni intermediates can undergo coupling with various aldehydes, facilitating a novel enantioconvergent approach to access extensively functionalized homoallylic sec,tert-vicinal diols frameworks. This protocol exhibits nice tolerance towards functional groups, a broad scope of substrates, excellent diastereo- and enantioselectivity (up to 20 : 1 dr, 99 % ee). Mechanistic studies suggested that allyl-NiII acts as the nucleophilic species in the coupling reaction with carbonyls.

Cyclic Diphenylchloronium-Salt-Triggered Coupling of Sulfides with Nucleophiles: Modular Assembly of Pharmaceuticals

We report a novel coupling strategy enabled by cyclic diphenylchloronium salt that facilitates reactions between sulfides and diverse nucleophiles, including oxygen- and nitrogen-based species. The methodology efficiently produces structurally varied valuable compounds, including carbamates, carboxylic esters, aryl ethers, and alkylated amines, under mild, operationally simple conditions. The protocol's synthetic utility was highlighted through modular preparation of five important drugs and five structural analogues, demonstrating significant potential for drug discovery applications.

Through-Space 1,4-Ni/H Shift: Unlocking Migration along Coupling Partners in Olefin Borylcarbofunctionalization

Olefin migratory functionalization is a well-established strategy for the selective installation of functional groups at remote C(sp3)-H positions along an alkyl chain. However, prior research has predominantly focused on migration along the alkene component. Herein, we describe a conceptually new migratory coupling strategy for the difunctionalization of alkenes, where migration selectively occurs along the C(sp2) coupling partner rather than the alkene component, facilitated by a through-space 1,4-Ni/H shift. This approach offers a modular three-component strategy for the selective and efficient construction of densely functionalized alkyl boronates from readily accessible chemicals. Moreover, by integrating the 1,2-Ni/H shift with the 1,4-Ni/H shift, this platform has been expanded to achieve a two-fold migration along both the alkene components and the coupling partners, facilitating selective borylative remote C(sp3)─H/C(sp2)─H cross-coupling.

New reactivity of late 3d transition metal complexes in catalytic reactions of alkynes

Late 3d metals such as iron, cobalt, nickel, and copper are abundantly present in the Earth's crust and they are produced in huge quantities in the mining industry. Often, these inexpensive metals exhibit unique or special reactivities in catalytic reactions as compared with expensive noble metals such as palladium, iridium, and rhodium. The novel reactivities of 3d metal complexes originate from their unique physical and atomic properties as compared with heavier 4d/5d congeners: smaller ionic and covalent radii, contracted 3d orbitals of smaller sizes and lower energies, lower values of Pauli electronegativity, etc. This review summarizes the recent progress in late 3d transition metal-catalyzed transformations of alkynes. We organize catalytic examples according to each type of novel elementary reactivity exhibited by 3d metal complexes. Each section includes a description of the unique reactivity of the 3d metals, the atomic and theoretical basis of the reactivity and illustrations of catalytic examples: (1) single electron transfer from low-valent metal complexes to alkyl halides, (2) facile reductive elimination from nickel(III), (3) facile reductive elimination from copper(III), (4) cis-to-trans isomerization of alkenyl metal complexes after syn-insertion, (5) ligand-to-ligand hydrogen transfer, (6) hydrogen atom transfer from hydride complexes and (7) protonation of nickel metallacyclopropenes.

The Chemical Deformation of a Thermally Cured Polyimide Film Surface into Neutral 1,2,4,5-Benzentetracarbonyliron and 4,4'-Oxydianiline to Remarkably Enhance the Chemical-Mechanical Planarization Polishing Rate

The rapid advancement of 3D packaging technology has emerged as a key solution to overcome the scaling-down limitation of advanced memory and logic devices. Redistribution layer (RDL) fabrication, a critical process in 3D packaging, requires the use of polyimide (PI) films with thicknesses in the micrometer range. However, these polyimide films present surface topography variations in the range of hundreds of nanometers, necessitating chemical-mechanical planarization (CMP) to achieve nanometer-level surface flatness. Polyimide films, composed of copolymers of pyromellitimide and diphenyl ether, possess strong covalent bonds such as C-C, C-O, C=O, and C-N, leading to inherently low polishing rates during CMP. To address this challenge, the introduction of Fe(NO3)3 into CMP slurries has been proposed as a polishing rate accelerator. During CMP, this Fe(NO3)3 deformed the surface of a polyimide film into strongly positively charged 1,2,4,5-benzenetetracarbonyliron and weakly negatively charged 4,4'-oxydianiline (ODA). The chemically dominant polishing rate enhanced with the concentration of the Fe(NO3)3 due to accelerated surface interactions. However, higher Fe(NO3)3 concentrations reduce the attractive electrostatic force between the positively charged wet ceria abrasives and the negatively charged deformed surface of the polyimide film, thereby decreasing the mechanically dominant polishing rate. A comprehensive investigation of the chemical and mechanical polishing rate dynamics revealed that the optimal Fe(NO3)3 concentration to achieve the maximum polyimide film removal rate was 0.05 wt%.

Benzylic C-H Radical Sulfation by Persulfates

Sulfation is a highly valuable pathological and physiological process, yet it is often underappreciated considering the rather difficult accessibility of organosulfates. O-sulfonation (O-SO3), a conventional and still common way to make organosulfates, restricts its applicability to hydroxyl compounds and therein lies a major challenge of library construction. Here, we describe a benzylic C-H radical sulfation with persulfates via C-O bond formation. This strategy leverages modular control over the reactivity of persulfates and the stability of sulfate radicals by coutercations. K+/NH4 + stabilized sulfate radicals act as the oxidant to generate carbon-centered radicals from substrates, and activation of persulfates by n-NBu4 + provides O-O resource pool to facilitate C-OSO3 - bond formation via a bimolecular homolytic substitution (SH2) process.

Iridium-Catalyzed Stereocontrolled C(sp3)-C(sp3) Cross-Coupling of Boronic Esters and Allylic Carbonates Enabled by Boron-to-Zinc Transmetalation

The stereocontrolled C(sp3)-C(sp3) cross-coupling represents a considerable challenge of great contemporary interest. While this has been achieved through the reactions of boronate complexes with π-allyl iridium complexes, such reactions suffered from a limited substrate scope. We now report that following transmetalation from boronate complexes to organozinc reagents enables previously unreactive substrates to engage in stereocontrolled C(sp3)-C(sp3) cross-coupling. The broader substrate scope has enabled their application to the synthesis of biologically active molecules. The organozinc reagents react through a stereoinvertive coupling pathway with π-allyl iridium complexes, in contrast to reactions with other electrophiles that occur with retention of stereochemistry. The reaction uniquely combines the enantiospecific reactivity of an enantioenriched organometallic nucleophile with the enantioselective engagement of a racemic electrophile, enabling access to all stereoisomers.

Metallaphotoredox Enabled Single Carbon Atom Insertion into Alkenes for Allene Synthesis

Efficient methods for synthesizing allenes from readily available starting materials pose a persistent challenge in organic chemistry. In this work, we present a novel two-stage protocol for allene synthesis involving the single-atom insertion into alkenes, facilitated by synergistic photoredox and cobalt catalysis. Diverging from conventional methods such as the Doering-LaFlamme reaction, this photochemical rearrangement approach operates efficiently under mild conditions in a radical-based manner. The protocol exhibits a broad substrate scope and demonstrates applicability in the late-stage diversification of alkene-containing natural products and bioactive molecules. Preliminary mechanistic studies and density functional theory (DFT) calculations offer insights into the reaction pathway, indicating a radical mechanism involving fleeting cyclopropyl carbene intermediates followed by rapid ring opening to form allenes.

Synthesis of tertiary alkyl amines via photoinduced copper-catalysed nucleophilic substitution

In view of the high propensity of tertiary alkyl amines to be bioactive, the development of new methods for their synthesis is an important challenge. Transition-metal catalysis has the potential to greatly expand the scope of nucleophilic substitution reactions of alkyl electrophiles; unfortunately, in the case of alkyl amines as nucleophiles, only one success has been described so far: the selective mono-alkylation of primary amines to form secondary amines. Here, using photoinduced copper catalysis, we report the synthesis of tertiary alkyl amines from secondary amines and unactivated alkyl electrophiles, two readily available coupling partners. Utilizing an array of tools, we have analysed the mechanism of this process; specifically, we have structurally characterized the three principal copper-based intermediates that are detected during catalysis and provided support for the key steps of the proposed catalytic cycle, including the coupling of a copper(II)-amine intermediate with an alkyl radical to form a C-N bond.

Enantioselective reductive cross-couplings to forge C(sp2)-C(sp3) bonds by merging electrochemistry with nickel catalysis

Motivated by the inherent benefits of synergistically combining electrochemical methodologies with nickel catalysis, we present here a Ni-catalyzed enantioselective electroreductive cross-coupling of benzyl chlorides with aryl halides, yielding chiral 1,1-diaryl compounds with good to excellent enantioselectivity. This catalytic reaction can not only be applied to aryl chlorides/bromides, which are challenging to access by other means, but also to benzyl chlorides containing silicon groups. Additionally, the absence of a sacrificial anode lays a foundation for scalability. The combination of cyclic voltammetry analysis with electrode potential studies suggests that NiI species activate aryl halides via oxidative addition and alkyl chlorides via single electron transfer.

CF2H-synthon enables asymmetric radical difluoroalkylation for synthesis of chiral difluoromethylated amines

The difluoromethyl group is a crucial fluorinated moiety with distinctive biological properties, and the synthesis of chiral CF₂H-containing analogs has been recognized as a powerful strategy in drug design. To date, the most established method for accessing enantioenriched difluoromethyl compounds involves the enantioselective functionalization of nucleophilic and electrophilic CF₂H synthons. However, this approach is limited by lower reactivity and reduced enantioselectivity. Leveraging the unique fluorine effect, we design and synthesize a radical CF₂H synthon by incorporating isoindolinone into alkyl halides for asymmetric radical transformation. Here, we report an efficient strategy for the asymmetric construction of carbon stereocenters featuring a difluoromethyl group via nickel-catalyzed Negishi cross-coupling. This approach demonstrates mild reaction conditions and excellent enantioselectivity. Given that optically pure difluoromethylated amines and isoindolinones are key structural motifs in bioactive compounds, this strategy offers a practical solution for the efficient synthesis of CF₂H-containing chiral drug-like molecules.

A modular approach to catalytic stereoselective synthesis of chiral 1,2-diols and 1,3-diols

Optically pure 1,2-diols and 1,3-diols are the most privileged structural motifs, widely present in natural products, pharmaceuticals and chiral auxiliaries or ligands. However, their synthesis relies on the use of toxic or expensive metal catalysts or suffer from low regioselectivity. Catalytic asymmetric synthesis of optically pure 1,n-diols from bulk chemicals in a highly stereoselective and atom-economical manner remains a formidable challenge. Here, we disclose a versatile and modular method for the synthesis of enantioenriched 1,2-diols and 1,3-diols from the high-production-volume chemicals ethane-1,2-diol (MEG) and 1,3-propanediol (PDO), respectively. The key to success is to temporarily mask the diol group as an acetonide, which imparts selectivity to the key step of C(sp3)-H functionalization. Additionally, 1,n-diols containing two stereogenic centers are also prepared through diastereoselective C(sp3)-H functionalization. The late-stage functionalization of biological active compounds and the expedient synthesis of chiral ligands and pharmaceutically relevant molecules further highlight the synthetic potential of this protocol.

Radical-Based Enantioconvergent Reductive Couplings of Racemic Allenes and Aldehydes

Transition metal-catalyzed radical-based enantioconvergent reactions have become a powerful strategy to synthesize enantiopure compounds from racemic starting materials. However, existing methods primarily address precursors with central chirality, neglecting those with axial chirality. Herein, we describe the enantioconvergent reductive coupling of racemic allenes with aldehydes, facilitated by a photoredox, chromium, and cobalt triple catalysis system. This method selectively affords one product from sixteen possible regio- and stereoisomers. The protocol leverages CoIII-H mediated hydrogen atom transfer (MHAT) and Cr-catalyzed radical-polar crossover for efficient stereoablation of axial chirality and asymmetric addition, respectively. Supported by mechanistic insights from control experiments, deuterium labeling, and DFT calculations, our approach offers synthetic chemists a valuable tool for creating enantioenriched chiral homoallylic alcohols, promising to advance radical-based strategies for synthesizing complex chiral molecules.

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.

Green Synthesis of Diphenyl-Substituted Alcohols Via Radical Coupling of Aromatic Alcohols Under Transition-Metal-Free Conditions

Alcohols are common alkylating agents and starting materials alternative to harmful alkyl halides. In this study, a simple, benign and efficient pathway was developed to synthesize 1,3-diphenylpropan-1-ols via the β-alkylation of 1-phenylethanol with benzyl alcohols. Unlike conventional borrowing hydrogen processes in which alcohols were activated by transition-metal catalyzed dehydrogenation, in this work, t-BuONa was suggested to be a dual-role reagent, namely, both base and radical initiator, for the radical coupling of aromatic alcohols. The cross-coupling reaction readily proceeded under transition metal-free conditions and an inert atmosphere, affording 1,3-diphenylpropan-1-ol with an excellent yield. A good functional group tolerance in benzyl alcohols was observed, leading to the production of various phenyl-substituted propan-1-ol derivatives in moderate-to-good yields. The mechanistic studies proposed that the reaction could involve the formation of reactive radical anions by base-mediated deprotonation and single electron transfer.

Enantio- and Regioconvergent Nickel-Catalyzed Etherification of Phenols by Allylation to Access Chiral C(sp3)-O Allyl Aryl Ethers

An enantio- and regioconvergent allylation of phenols under nickel catalysis with an α-/γ-regioisomeric mixture of racemic silylated/germylated allylic chlorides is reported. The silyl/germyl group governs the regioselectivity, and the transformation affords enantiomerically enriched unsymmetrical 1,3-disubstituted allyl aryl ethers with excellent regiocontrol in good yields and excellent enantioselectivities. Notably, no nickel-mediated C-O bond activation is observed at room temperature. The synthetic value of these densely functionalized silicon-containing building blocks is demonstrated in a series of chemoselective transformations, including a [3,3]-sigmatropic rearrangement for the construction of an α-chiral silane.

Redox Activity and Potentials of Bidentate N-Ligands Commonly Applied in Nickel-Catalyzed Cross-Coupling Reactions

Bidentate N-ligands are paramount to recent advances in nickel-catalyzed cross-coupling reactions. Through comprehensive organometallic, spectroscopic, and computational studies on bi-oxazoline and imidazoline ligands, we reveal that a square planar geometry enables redox activity of these ligands in stabilizing nickel radical species. This finding contrasts with the prior assumption that bi-oxazoline lacks redox activity due to strong mesomeric donation. Moreover, we conducted systematic cyclic voltammetry (CV) analyses of bidentate pyridyl, oxazoline, and imidazoline nitrogen ligands, along with their corresponding nickel complexes. Complexation with nickel shifts the reduction potentials to a more positive region and narrows the differences in redox potentials among the ligands. Additionally, various ligands led to different degrees of bromide dissociation from singly reduced (L)Ni(Ar)(Br) complexes, reflecting varying reactivity in the subsequent activation of alkyl halides, a crucial step in cross-electrophile coupling. These insights highlight the significant electronic effects of ligands on the stability of metalloradical species and their redox potentials, which interplay with coordination geometry. Quantifying the electron-donating, π-accepting properties of these ligands, as well as their effect on catalyst speciation, provides crucial benchmarks for controlling catalytic activity and enhancing catalyst stability.

Overcoming Copper Reduction Limitation in Asymmetric Substitution: Aryl-Radical-Enabled Enantioconvergent Cyanation of Alkyl Iodides

Cu-catalyzed enantioconvergent cross-coupling of alkyl halides has emerged as a powerful strategy for synthesizing enantioenriched molecules. However, this approach is intrinsically limited by the weak reducing power of copper(I) species, which restricts the scope of compatible nucleophiles and necessitates extensive ligand optimization or the use of complex chiral scaffolds. To overcome these challenges, we introduce an aryl-radical-enabled strategy that decouples the alkyl halide activation step from the chiral Cu center. We demonstrate that merging aryl-radical-enabled iodine abstraction with Cu-catalyzed asymmetric radical functionalization enables the conversion of racemic α-iodoamides to enantioenriched alkyl nitrile products with good yield and enantioselectivity. The rational design of chiral ligands identified a new class of carboxamide-containing BOX ligands. Mechanistic studies support an aryl-radical-enabled pathway and the unique hydrogen-bonding ability in the newly designed BOX ligands. This aryl-radical-enabled asymmetric substitution reaction has the potential to significantly expand the scope of Cu-catalyzed enantioconvergent cross-coupling reactions.

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.

Nickel/Photoredox-Catalyzed Asymmetric Three-Component Cross-Coupling To Access Enantioenriched 1,1-Diaryl(heteroaryl)alkanes

An enantioselective 1,2-dicarbofunctionalization of vinyl (hetero)arenes with alkyl bromides and aryl bromides through nickel/photoredox catalysis is described. This three-component enantioselective domino alkyl arylation of vinyl (hetero)arenes could generate a diverse array of enantioenriched 1,1-diaryl(heteroaryl)alkanes with good to excellent yields (up to 88%) and high enantioselectivities (up to 99% ee). This transformation could proceed well under mild conditions with excellent chemo- and regioselectivity due to the avoidance of the use of air- and moisture-sensitive organometallic reagents and stoichiometric metal reductants. Mechanistic studies suggested that the alkyl radical is generated via NiI species or photocatalyst.

Strategies and Mechanisms of First-Row Transition Metal-Regulated Radical C-H Functionalization

Radical C-H functionalization represents a useful means of streamlining synthetic routes by avoiding substrate preactivation and allowing access to target molecules in fewer steps. The first-row transition metals (Ti, V, Cr, Mn, Fe, Co, Ni, and Cu) are Earth-abundant and can be employed to regulate radical C-H functionalization. The use of such metals is desirable because of the diverse interaction modes between first-row transition metal complexes and radical species including radical addition to the metal center, radical addition to the ligand of metal complexes, radical substitution of the metal complexes, single-electron transfer between radicals and metal complexes, hydrogen atom transfer between radicals and metal complexes, and noncovalent interaction between the radicals and metal complexes. Such interactions could improve the reactivity, diversity, and selectivity of radical transformations to allow for more challenging radical C-H functionalization reactions. This review examines the achievements in this promising area over the past decade, with a focus on the state-of-the-art while also discussing existing limitations and the enormous potential of high-value radical C-H functionalization regulated by these metals. The aim is to provide the reader with a detailed account of the strategies and mechanisms associated with such functionalization.

Visible-Light Mediated Nickel-Catalyzed Asymmetric Difunctionalizations of Alkenes

Difunctionalizations of alkenes represent one of the most straightforward protocols to build molecular complexity due to the simultaneous construction of two vicinal bonds cross π-bond of alkenes. It is extremely attractive yet challenging to control the stereochemistry outcome of this event. Over the past years, visible-light and Ni-catalyzed asymmetric difunctionalizations of alkenes provide an environmental benign and promising solution for the construction of saturated carbon centers with the control of regio- and enantioselectivity. In this Concept, the initiative and progress of regio- and enantioselective difunctionalizations of alkenes enabled by visible-light and nickel catalysis has been summarized. Moreover, further efforts and directions for the development of visible-light mediated Ni-catalyzed asymmetric difunctionalizations of alkenes has been discussed.

Dual Nickel- and Photoredox-Catalyzed Asymmetric Reductive Cross-Couplings: Just a Change of the Reduction System?

ConspectusIn recent years, nickel-catalyzed asymmetric coupling reactions have emerged as efficient methods for constructing chiral C(sp3) carbon centers. Numerous novel approaches have been reported to rapidly construct chiral carbon-carbon bonds through nickel-catalyzed asymmetric couplings between electrophiles and nucleophiles or asymmetric reductive cross-couplings of two different electrophiles. Building upon these advances, our group has been devoted to interrogating dual nickel- and photoredox-catalyzed asymmetric reductive cross-coupling reactions.In our endeavors over the past few years, we have successfully developed several dual Ni-/photoredox-catalyzed asymmetric reductive cross-coupling reactions involving organohalides. While some probably think that this system is just a change of the reduction system from traditional metal reductants to a photocatalysis system, a question that we also pondered at the beginning of our studies, both the achievable reaction types and mechanisms suggest a different conclusion: that this dual catalysis system has its own advantages in the chiral carbon-carbon bond formation. Even in certain asymmetric reactions where the photocatalysis regime functions only as a reducing system, the robust reducing capability of photocatalysts can effectively accelerate the regeneration of low-valent nickel species, thus expanding the selectable scope of chiral ligands. More importantly, in many transformations, besides reducing nickel catalysts, the photocatalysis system can also undertake the responsibility of alkyl radical formation, thereby establishing two coordinated, yet independent catalytic cycles. This catalytic mode has been proven to play a crucial role in achieving diverse asymmetric coupling reactions with great challenges.In this Account, we elucidate our understanding of this system based on our experience and findings. In the Introduction, we provide an overview of the main distinctions between this system and traditional Ni-catalyzed asymmetric reductive cross-couplings with metal reductants and the potential opportunities arising from these differences. Subsequently, we outline various chiral carbon-carbon bond-forming types obtained by this dual Ni/photoredox catalysis system and their mechanisms. In terms of chiral C(sp3)-C(sp2) bond formation, extensive discussion focuses on the asymmetric arylations of α-chloroboronates, α-trifluoromethyl alkyl bromides, α-bromophosphonates, and so on. In the realm of chiral C(sp3)-C(sp) bond formation, asymmetric alkynylations of α-bromophosphonates and α-trifluoromethyl alkyl bromides have been presented herein. Regarding C(sp3)-C(sp3) bond formation, we take the asymmetric alkylation of α-chloroboronates as a compelling example to illustrate the great efficiency of this dual catalysis system. This summary would enable a better grasp of the advantages of this dual catalysis system and clarify how the photocatalysis regime facilitates enantioselective transformations. We anticipate that this Account will offer valuable insights and contribute to the development of new methodologies in this field.

In situ copper photocatalysts triggering halide atom transfer of unactivated alkyl halides for general C(sp3)-N couplings

Direct reduction of unactivated alkyl halides for C(sp3)-N couplings under mild conditions presents a significant challenge in organic synthesis due to their low reduction potential. Herein, we introduce an in situ formed pyridyl-carbene-ligated copper (I) catalyst that is capable of abstracting halide atom and generating alkyl radicals for general C(sp3)-N couplings under visible light. Control experiments confirmed that the mono-pyridyl-carbene-ligated copper complex is the active species responsible for catalysis. Mechanistic investigations using transient absorption spectroscopy across multiple decades of timescales revealed ultrafast intersystem crossing (260 ps) of the photoexcited copper (I) complexes into their long-lived triplet excited states (>2 μs). The non-Stern-Volmer quenching dynamics of the triplets by unactivated alkyl halides suggests an association between copper (I) complexes and alkyl halides, thereby facilitating the abstraction of halide atoms via inner-sphere single electron transfer (SET), rather than outer-sphere SET, for the formation of alkyl radicals for subsequent cross couplings.

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.

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.

Mechanisms of Photoredox Catalysis Featuring Nickel-Bipyridine Complexes

Metallaphotoredox catalysis can unlock useful pathways for transforming organic reactants into desirable products, largely due to the conversion of photon energy into chemical potential to drive redox and bond transformation processes. Despite the importance of these processes for cross-coupling reactions and other transformations, their mechanistic details are only superficially understood. In this review, we have provided a detailed summary of various photoredox mechanisms that have been proposed to date for Ni-bipyridine (bpy) complexes, focusing separately on photosensitized and direct excitation reaction processes. By highlighting multiple bond transformation pathways and key findings, we depict how photoredox reaction mechanisms, which ultimately define substrate scope, are themselves defined by the ground- and excited-state geometric and electronic structures of key Ni-based intermediates. We further identify knowledge gaps to motivate future mechanistic studies and the development of synergistic research approaches spanning the physical, organic, and inorganic chemistry communities.

Three-Component Radical Cross-Coupling: Asymmetric Vicinal Sulfonyl-Esterification of Alkenes Involving Sulfur Dioxide

A novel catalytic system for radical cross-coupling reactions based on copper and chiral Pyridyl-bis(imidazole) (PyBim) ligands is described. It overcomes the challenges of chemoselectivity and enantioselectivity, achieving a highly enantioselective vicinal sulfonyl-esterification reaction of alkenes involving sulfur dioxide. This strategy involves the use of earth-abundant metal catalyst, mild reaction conditions, a broad range of substrates (84 examples), high yields (up to 97% yield), and exceptional control over enantioselectivity. The reaction system is compatible with different types of radical precursors, including O-acylhydroxylamines, cycloketone oxime esters, aryldiazonium salts, and drug molecules. Chiral ligand PyBim is identified as particularly effective in achieving the desired high enantioselectivity. Mechanistic studies reveal that copper/PyBim system plays a vital role in C─O coupling, employing an outer-sphere model. In addition, the side arm effect of ligand is observed.

Electro/Ni Dual-Catalyzed Decarboxylative C(sp3)-C(sp2) Cross-Coupling Reactions of Carboxylates and Aryl Bromide

Paired redox-neutral electrolysis offers an attractive green platform for organic synthesis by avoiding sacrificial oxidants and reductants. Carboxylates are non-toxic, stable, inexpensive, and widely available, making them ideal nucleophiles for C-C cross-coupling reactions. Here, we report the electro/Ni dual-catalyzed redox-neutral decarboxylative C(sp3)-C(sp2) cross-coupling reactions of pristine carboxylates with aryl bromides. At a cathode, a NiII(Ar)(Br) intermediate is formed through the activation of Ar-Br bond by a NiI-bipyridine catalyst and subsequent reduction. At an anode, the carboxylates, including amino acid, benzyl carboxylic acid, and 2-phenoxy propionic acid, undergo oxidative decarboxylation to form carbon-based free radicals. The combination of NiII(Ar)(Br) intermediate and carbon radical results in the formation of C(sp3)-C(sp2) cross-coupling products. The adaptation of this electrosynthesis method to flow synthesis and valuable molecule synthesis was demonstrated. The reaction mechanism was systematically studied through electrochemical voltammetry and density functional theory (DFT) computational studies. The relationships between the electrochemical properties of carboxylates and the reaction selectivity were revealed. The electro/Ni dual-catalyzed cross-coupling reactions described herein expand the chemical space of paired electrochemical C(sp3)-C(sp2) cross-coupling and represent a promising method for the construction of the C(sp3)-C(sp2) bonds because of the ubiquitous carboxylate nucleophiles and the innate scalability and flexibility of electrochemical flow-synthesis technology.

Simultaneous Stereoinvertive and Stereoselective C(sp3)-C(sp3) Cross-Coupling of Boronic Esters and Allylic Carbonates

With increasing interest in constructing more three-dimensional entities, there has been growing interest in cross-coupling reactions that forge C(sp3)-C(sp3) bonds, which leads to additional challenges as it is not just a more difficult bond to construct but issues of stereocontrol also arise. Herein, we report the stereocontrolled cross-coupling of enantioenriched boronic esters with racemic allylic carbonates enabled by iridium catalysis, leading to the formation of C(sp3)-C(sp3) bonds with single or vicinal stereogenic centers. The method shows broad substrate scope, enabling primary, secondary, and even tertiary boronic esters to be employed, and can be used to prepare any of the four possible stereoisomers of a coupled product with vicinal chiral centers. The new method, which combines the simultaneous enantiospecific reaction of a chiral nucleophile with the enantioselective reaction of a chiral electrophile in a single process, offers a solution for stereodivergent cross-coupling of two C(sp3) fragments.

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.

(Phenoxyimine)nickel-Catalyzed C(sp2)-C(sp3) Suzuki-Miyaura Cross-Coupling: Evidence for a Recovering Radical Chain Mechanism

Phenoxyimine (FI)-nickel(II)(2-tolyl)(DMAP) compounds were synthesized and evaluated as precatalysts for the C(sp2)-C(sp3) Suzuki-Miyaura cross coupling of (hetero)arylboronic acids with alkyl bromides. With 5 mol % of the optimal (MeOMeFI)Ni(Aryl)(DMAP) precatalyst, the scope of the cross-coupling reaction was established and included a variety of (hetero)arylboronic acids and alkyl bromides (>50 examples, 33-97% yield). A β-hydride elimination-reductive elimination sequence from reaction with potassium isopropoxide base, yielding a potassium (FI)nickel(0)ate, was identified as a catalyst activation pathway that is responsible for halogen atom abstraction from the alkyl bromide. A combination of NMR and EPR spectroscopies identified (FI)nickel(II)-aryl complexes as the resting state during catalysis with no evidence for long-lived organic radical or odd-electron nickel intermediates. These data establish that the radical chain is short-lived and undergoes facile termination and also support a "recovering radical chain" process whereby the (FI)nickel(II)-aryl compound continually (re)initiates the radical chain. Kinetic studies established that the rate of C(sp2)-C(sp3) product formation was proportional to the concentration of the (FI)nickel(II)-aryl resting state that captures the alkyl radical for chain propagation. The proposed mechanism involves two key and concurrently operating catalytic cycles; the first involving a nickel(I/II/III) radical propagation cycle consisting of radical capture at (FI)nickel(II)-aryl, C(sp2)-C(sp3) reductive elimination, bromine atom abstraction from C(sp3)-Br, and transmetalation; and the second involving an off-cycle catalyst recovery process by slow (FI)nickel(II)-aryl → (FI)nickel(0)ate conversion for nickel(I) regeneration.

Enantioselective Construction of C-SCF3 Stereocenters via Nickel Catalyzed Asymmetric Negishi Coupling Reaction

The construction of the SCF3-containing 1,1-diaryl tertiary carbon stereocenters with high enantioselectivities is reported via a nickel-catalyzed asymmetric C-C coupling strategy. This method demonstrates simple operations, mild conditions and excellent functional group tolerance, with newly designed SCF3-containing synthon, which can be easily obtained from commercially available benzyl bromide and trifluoromethylthio anion in a two-step manner. Further substrate exploration indicated that the reaction system could be extended to diverse perfluoroalkyl sulfide (SC2F5, SC3F7, SC4F9, SCF2CO2Et)-substituted 1,1-diaryl compounds with excellent enantioselectivities. The synthetic utility of this transformation was further demonstrated by convenient derivatization to optical SCF3-containing analogues of bioactive compounds without an apparent decrease in enantioselectivity.

Enantioselective Construction of Quaternary Stereocenters via Cooperative Photoredox/Fe/Chiral Primary Amine Triple Catalysis

The catalytic and enantioselective construction of quaternary (all-carbon substituents) stereocenters poses a formidable challenge in organic synthesis due to the hindrance caused by steric factors. One conceptually viable and potentially versatile approach is the coupling of a C-C bond through an outer-sphere mechanism, accompanied by the realization of enantiocontrol through cooperative catalysis; however, examples of such processes are yet to be identified. Herein, we present such a method for creating different compounds with quaternary stereocenters by photoredox/Fe/chiral primary amine triple catalysis. This approach facilitates the connection of an unactivated alkyl source with a tertiary alkyl moiety, which is also rare. The scalable process exhibits mild conditions, does not necessitate the use of a base, and possesses a good functional-group tolerance. Preliminary investigations into the underlying mechanisms have provided valuable insights into the reaction pathway.

Copper-Catalyzed Enantioconvergent Radical N-Alkylation of Diverse (Hetero)aromatic Amines

The 3d transition metal-catalyzed enantioconvergent radical cross-coupling provides a powerful tool for chiral molecule synthesis. In the classic mechanism, the bond formation relies on the interaction between nucleophile-sequestered metal complexes and radicals, limiting the nucleophile scope to sterically uncongested ones. The coupling of sterically congested nucleophiles poses a significant challenge due to difficulties in transmetalation, restricting the reaction generality. Here, we describe a probable outer-sphere nucleophilic attack mechanism that circumvents the challenging transmetalation associated with sterically congested nucleophiles. This strategy enables a general copper-catalyzed enantioconvergent radical N-alkylation of aromatic amines with secondary/tertiary alkyl halides and exhibits catalyst-controlled stereoselectivity. It accommodates diverse aromatic amines, especially bulky secondary and primary ones to deliver value-added chiral amines (>110 examples). It is expected to inspire the coupling of more nucleophiles, particularly challenging sterically congested ones, and accelerate reaction generality.

Ni-catalyzed enantioconvergent deoxygenative reductive cross-coupling of unactivated alkyl alcohols and aryl bromides

Transition metal-catalyzed enantioconvergent cross-coupling of an alkyl precursor presents a promising method for producing enantioenriched C(sp3) molecules. Because alkyl alcohol is a ubiquitous and abundant family of feedstock in nature, the direct reductive coupling of alkyl alcohol and aryl halide enables efficient access to valuable compounds. Although several strategies have been developed to overcome the high bond dissociation energy of the C - O bond, the asymmetric pattern remains unknown. In this report, we describe the realization of an enantioconvergent deoxygenative reductive cross-coupling of unactivated alkyl alcohol (β-hydroxy ketone) and aryl bromide in the presence of an NHC activating agent. The approach can accommodate substituents of various sizes and functional groups, and its synthetic potency is demonstrated through a gram scale reaction and derivatizations into other compound families. Finally, we apply our convergent method to the efficient asymmetric synthesis of four β-aryl ketones that are natural products or bioactive compounds.

Nickel-Catalyzed Enantioconvergent and Diastereoselective Allenylation of Alkyl Electrophiles: Simultaneous Control of Central and Axial Chirality

In recent years, remarkable progress has been described in the development of methods that simultaneously control vicinal stereochemistry, wherein both stereochemical elements are central chirality; in contrast, methods that control central and axial chirality are comparatively rare. Herein we report that a chiral nickel catalyst achieves the enantioconvergent and diastereoselective coupling of racemic secondary alkyl electrophiles with prochiral 1,3-enynes (in the presence of a hydrosilane) to generate chiral tetrasubstituted allenes that bear an adjacent stereogenic center. A carbon-carbon and a carbon-hydrogen bond are formed in this process, which provides good stereoselectivity and is compatible with an array of functional groups.

Electrochemical Ni-Catalyzed Decarboxylative C(sp3 )-N Cross-Electrophile Coupling

A new electrochemical transformation is presented that enables chemists to couple simple alkyl carboxylic acid derivatives with an electrophilic amine reagent to construct C(sp3 )-N bond. The success of this reaction hinges on the merging of cooperative electrochemical reduction with nickel catalysis. The chemistry exhibits a high degree of practicality, showcasing its wide applicability with 1°, 2°, 3° carboxylic acids and remarkable compatibility with diverse functional groups, even in the realm of late-stage functionalization. Furthermore, extensive mechanistic studies have unveiled the engagement of alkyl radicals and iminyl radicals; and elucidated the multifaceted roles played by i Pr2 O, Ni catalyst, and electricity.

Copper-Catalyzed Enantioselective C(sp3 )-SCF3 Coupling of Carbon-Centered Benzyl Radicals with (Me4 N)SCF3

In contrast with the well-established C(sp2 )-SCF3 cross-coupling to forge the Ar-SCF3 bond, the corresponding enantioselective coupling of readily available alkyl electrophiles to forge chiral C(sp3 )-SCF3 bond has remained largely unexplored. We herein disclose a copper-catalyzed enantioselective radical C(sp3 )-SCF3 coupling of a range of secondary/tertiary benzyl radicals with the easily available (Me4 N)SCF3 reagent. The key to the success lies in the utilization of chiral phosphino-oxazoline-derived anionic N,N,P-ligands through tuning electronic and steric effects for the simultaneous control of the reaction initiation and enantioselectivity. This strategy can successfully realize two types of asymmetric radical reactions, including enantioconvergent C(sp3 )-SCF3 cross-coupling of racemic benzyl halides and three-component 1,2-carbotrifluoromethylthiolation of arylated alkenes under mild reaction conditions. It therefore provides a highly flexible platform for the rapid assembly of an array of enantioenriched SCF3 -containing molecules of interest in organic synthesis and medicinal chemistry.

Interception of Transient Allyl Radicals with Low-Valent Allylpalladium Chemistry: Tandem Pd(0/II/I)-Pd(0/II/I/II) Cycles in Photoredox/Pd Dual-Catalytic Enantioselective C(sp3)-C(sp3) Homocoupling

We present comprehensive computational and experimental studies on the mechanism of an asymmetric photoredox/Pd dual-catalytic reductive C(sp3)-C(sp3) homocoupling of allylic electrophiles. In stark contrast to the canonical assumption that photoredox promotes bond formation via facile reductive elimination from high-valent metal-organic species, our computational analysis revealed an intriguing low-valent allylpalladium pathway that features tandem operation of Pd(0/II/I)-Pd(0/II/I/II) cycles. Specifically, we propose that (i) the photoredox/Pd system enables the in situ generation of allyl radicals from low-valent Pd(I)-allyl species, and (ii) effective interception of the fleeting allyl radical by the chiral Pd(I)-allyl species results in the formation of an enantioenriched product. Notably, the cooperation of the two pathways highlights the bifunctional role of Pd(I)-allyl species in the generation and interception of transient allyl radicals. Moreover, the mechanism implies divergent substrate-activation modes in this homocoupling reaction, suggesting a theoretical possibility for cross-coupling. Combined, the current study offers a novel mechanistic hypothesis for photoredox/Pd dual catalysis and highlights the use of low-valent allylpalladium as a means to efficiently intercept radicals for selective asymmetric bond constructions.

Enantioselective C(sp2)-C(sp3) Bond Construction by Ni Catalysis

ConspectusAfter decades of palladium dominating the realm of transition-metal-catalyzed cross-coupling, recent years have witnessed exciting advances in the development of new nickel-catalyzed cross-coupling reactions to form C(sp3) centers. Nickel possesses distinct properties compared with palladium, such as facile single-electron transfer to C(sp3) electrophiles and rapid C-C reductive elimination from NiIII. These properties, among others, make nickel particularly well-suited for reductive cross-coupling (RCC) in which two electrophiles are coupled and an exogenous reductant is used to turn over the metal catalyst. Ni-catalyzed RCCs use readily available and stable electrophiles as starting materials and exhibit good functional group tolerance, which makes them appealing for applications in the synthesis of complex molecules. Building upon the foundational work in Ni-catalyzed RCCs by the groups of Kumada, Durandetti, Weix, and others, as well as the advancements in Ni-catalyzed enantioselective redox-neutral cross-couplings led by Fu and co-workers, we initiated a program to explore the feasibility of developing highly enantioselective Ni-catalyzed RCCs. Our research has also been driven by a keen interest in unraveling the factors contributing to enantioinduction and electrophile activation as we seek new avenues for advancing our understanding and further developing these reactions.In the first part of this Account, we organize our reported methods on the basis of the identity of the C(sp3) electrophiles, including benzylic chlorides, N-hydroxyphthalimide (NHP) esters, and α-chloro esters and nitriles. We highlight how the selection of specific chiral ligands plays a pivotal role in achieving high cross-selectivity and enantioselectivity. In addition, we show that reduction can be accomplished not only with heterogeneous reductants, such as Mn0, but also with the soluble organic reductant tetrakis(dimethylamino)ethylene (TDAE), as well as electrochemically. The use of homogeneous reductants, such as TDAE, is well suited for studying the mechanism of the transformation. Although this Account primarily focuses on RCCs, we also highlight our work using trifluoroborate (BF3K) salts as radical precursors for enantioselective dual-Ni/photoredox systems.At the end of this Account, we summarize the relevant mechanistic studies of two closely related asymmetric reductive alkenylation reactions developed in our laboratory and provide a context between our work and related mechanistic studies by others. We discuss how the ligand properties influence the rates and mechanisms of electrophile activation and how understanding the mode of C(sp3) radical generation can be used to optimize the yield of an RCC. Our research endeavors to offer insights on the intricate mechanisms at play in asymmetric Ni-catalyzed RCCs with the goal of using the rate of electrophile activation to improve the substrate scope of enantioselective RCCs. We anticipate that the insights we share in this Account can provide guidance for the development of new methods in this field.

Micro-flow heteroatom alkylation via TfOH-mediated rapid in situ generation of carbocations and subsequent nucleophile addition

A rapid nucleophilic substitution reaction was developed using carbocations generated from diarylmethanol and trifluoromethanesulfonic acid. Undesired reactions caused by the carbocations were suppressed, presumably due to the rapid and uniform generation of carbocations and the subsequent rapid and uniform distribution of nucleophiles by the micro-flow technology.

Ligand-Controlled On-Off Switch of a Silicon-Tethered Directing Group Enabling the Regiodivergent Hydroalkylation of Vinylsilanes under Ni-H Catalysis

A regiodivergent Ni-H-catalyzed hydroalkylation of vinylsilanes is described. Depending on the ancillary ligand at the nickel catalyst, the regioselectivity can be steered by a directing group attached to the silicon atom. The mild protocols allow for the selective synthesis of either branched or linear alkylsilanes. An example of a vinylgermane is also reported. The method features a broad scope with high functional-group tolerance and follows a radical mechanism.

Ligand-Controlled Cobalt-Catalyzed Regio-, Enantio-, and Diastereoselective Oxyheterocyclic Alkene Hydroalkylation

Metal-hydride-catalyzed alkene hydroalkylation has been developed as an efficient method for C(sp3)-C(sp3) coupling with broad substrate availability and high functional group compatibility. However, auxiliary groups, a conjugated group or a chelation-directing group, are commonly required to attain high regio- and enantioselectivities. Herein, we reported a ligand-controlled cobalt-hydride-catalyzed regio-, enantio-, and diastereoselective oxyheterocyclic alkene hydroalkylation without chelation-directing groups. This reaction enables the hydroalkylation of conjugated and unconjugated oxyheterocyclic alkenes to deliver C2- or C3-alkylated tetrahydrofuran or tetrahydropyran in uniformly good yields and with high regio- and enantioselectivities. In addition, hydroalkylation of C2-substituted 2,5-dihydrofuran resulted in the simultaneous construction of 1,3-distereocenters, providing convenient access to polysubstituted tetrahydrofuran with multiple enantioenriched C(sp3) centers.

Hydroxytrifluoroethylation and Trifluoroacetylation Reactions via SET Processes

Hydroxytrifluoroethyl and trifluoroacetyl groups are of utmost importance in biologically active compounds, but methods to tether these motifs to organic architectures have been limited. Typically, the preparation of these compounds relied on the use of strong bases or multistep routes. The renaissance of radical chemistry in photocatalytic, transition metal mediated, and hydrogen atom transfer (HAT) processes have allowed the installation of these medicinally relevant fluorinated motifs. This review provides an overview of the methods available for the direct synthesis of hydroxytrifluoroethyl- and trifluoroacetyl-derived compounds governed by single-electron transfer processes.

A General Copper Catalytic System for Suzuki-Miyaura Cross-Coupling of Unactivated Secondary and Primary Alkyl Halides with Arylborons

Suzuki-Miyaura cross-couplings (SMC) are powerful tools for the construction of carbon-carbon bonds. However, the couplings of sp3-hybridized alkyl halides with arylborons often encounter several problematic issues such as sluggish oxidation addition of alkyl halides and competitive β-hydride elimination side pathways of metal-alkyl species. In precedent reports, copper is mainly utilized for the coupling of sp2-aryl halides, and the cross-couplings with unactivated alkyl halides are far less reported. Herein, we demonstrate that a high-efficiency copper system enabled the coupling of arylborons with various unactivated secondary and primary alkyl halides including bromides, iodides, and even robust chlorides. The present system features broad scope, excellent functionality tolerance, scalability, and practicality. Moreover, the current system could be applied for the late-stage functionalization of complex molecules in moderate to high efficiency.

Sterically demanding Csp2(ortho-substitution)-Csp3(tertiary) bond formation via carboxylate-directed Mizoroki-Heck reaction under extra-ligand-free conditions

Construction of the sterically demanding Csp2(oS)-Csp3(T) bond was achieved by carrying out the Pd-catalyzed carboxylate-directed Mizoroki-Heck reaction under extra-ligand-free aqueous conditions. The cooperative role of the presence of water with the absence of phosphine ligand was proposed to accelerate the migratory insertion process considerably, delivering a broad substrate scope.

Catalytic Asymmetric α-Alkylation of Ketones with Unactivated Alkyl Halides

A catalytic, enantioselective method for direct α-alkylation of ketones with unactivated alkyl halides is realized by employing an α-enolizable ketone in a nickel-catalyzed C(sp3)-C(sp3) cross-coupling reaction. The key to the success is attributed to a unique bimetallic ligand. A variety of acyclic ketones and unactivated alkyl iodides can serve as suitable substrates under mild conditions to generate chiral ketones with α-quaternary carbon stereocenters in high yields with good enantioselectivities. A range of transformations based on the ketone moiety are also demonstrated to show the potential application of this method. Preliminary mechanistic studies support a dinickel-catalyzed cross-coupling mechanism.

Redox-Active Thiocarbonyl Auxiliaries in Ni-Catalyzed Cross-Couplings of Aliphatic Alcohols

ConspectusThe Barton-McCombie deoxygenation reaction first established the use of O-alkyl thiocarbonyl derivatives as powerful redox-active agents for C(sp3)-O reduction. In recent years, first-row transition metals capable of engaging with alkyl radical intermediates generated from O-alkyl thiocarbonyl derivatives using alternative stoichiometric radical precursors have been developed. Given the ability of select Ni catalysts to both participate in single-electron oxidative addition pathways and intercept alkyl radical intermediates, our group has investigated the use of O-alkyl thiocarbonyl derivatives as electrophiles in novel cross-coupling reactions. After describing related work in this area, this Account will first summarize our entry point into this field. Here, we used the cyclopropane ring as a reporter of leaving group reactivity to aid in the design and optimization of a novel redox-active O-thiocarbamate leaving group for C(sp3)-O arylation. Motivation for this pursuit was driven by the propensity of the cyclopropane ring to undergo ring opening under polar (2e) oxidative addition pathways or to be maintained under single-electron (1e) conditions. Using these guiding principles, we developed a method for the deoxygenative arylation of cyclopropanol derivatives using a Ni catalyst without the need for a stoichiometric external reductant or photocatalyst. We next summarize our evaluation of an alternative redox-active O-thiocarbonyl imidazole auxiliary in a related deoxygenative cross-coupling. This work demonstrated an extension of our initial approach to the deoxygenative arylation of primary and secondary aliphatic alcohol derivatives. A brief mechanistic investigation revealed that this reaction likely proceeds via a distinct mechanism involving direct homolytic C(sp3)-O bond cleavage. We conclude this Account with a summary of work aimed toward a unique approach for thiocarboxylic acid derivative synthesis. This project was inspired by the efficiency of thionoester generation under most of the reaction conditions evaluated in our prior investigations. Using alcohol, amine, or thiol starting materials, which were activated with convenient thiocarbonyl sources in a single step, we optimized for a Ni-catalyzed cross-coupling capable of providing access to a range of thionoester, thioamide, or dithioester products. In summary, our work has revealed the potential of redox-active thiocarbonyl auxiliaries in Ni-catalyzed cross-couplings with C(sp3)-O electrophiles. We anticipate that the continued investigation of aliphatic thiocarbonyl derivatives as radical precursors with alternative single-electron inputs will be an area of continued growth in the years to come.