Nickel-Catalyzed Enantioselective Reductive Cross-Coupling of Styrenyl Aziridines

A General Copper-Box System for the Asymmetric Arylative Functionalization of Benzylic, Propargylic or Allenylic Radicals

Radical-involved arylative cross-coupling reactions have recently emerged as an attractive strategy to access valuable aryl-substituted motifs. However, there still exist several challenges such as limited scope of radical precursors/acceptors, and lack of general asymmetric catalytic systems, especially regarding the multicomponent variants. Herein, we reported a general copper-Box system for asymmetric three-component arylative radical cross-coupling of vinylarenes and 1,3-enynes, with oxime carbonates and aryl boronic acids. The reactions proceed under practical conditions in the absence or presence of visible-light irradiation, affording chiral 1,1-diarylalkanes, benzylic alkynes and allenes with good enantioselectivities. Mechanistic studies imply that the copper/Box complexes play a dual role in both radical generation and ensuing asymmetric cross-coupling. In the cases of 1,3-enynes, visible-light irradiation could improve the activity of copper/Box complex toward the initial radical generation, enabling better efficiency match between radical formation and cross-coupling.

Kinetics and Mechanism of PPh3/Ni-Catalyzed, Zn-Mediated, Aryl Chloride Homocoupling: Antagonistic Effects of ZnCl2/Cl. J Am Chem Soc 2024. [PMID: 39420638 DOI: 10.1021/jacs.4c12088] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

The Ni/PPh3-catalyzed homocoupling of aryl chlorides in DMF using Zn as the stochiometric reducing agent is one of a general class of Ni-catalyzed processes, where the mechanism has been a matter of long-standing debate. This study re-evaluates prior conclusions and insights. NMR spectroscopy is used to identify [(PPh3)2NiII(Ar)Cl] as a key intermediate and to explore the indirect roles of using Zn as the reductant. The [ZnCl2] coproduct is responsible for several features, including a sequential transmetalation pathway involving [ArZnCl]. [ZnCl2] also abstracts halide from [(PPh3)2NiCl2] to generate [NiIICl(DMF)5]+[ZnCl3(DMF)]-, and in doing so, affects the NiII + Ni0 ↔ 2 NiI speciation. [ZnCl2] thus acts as an accelerator and inhibitor, resulting in mildly sigmoidal reaction profiles. When the [ZnCl2] concentration becomes too high or the phosphine ligand concentration too low, catalysis stalls. Turnover is restored by the addition of further phosphine ligand, or chloride ion. In the presence of an exogenous chloride ion, turnover is rapid, again proceeding via [(PPh3)2NiII(Ar)Cl] but via dinuclear metathesis. The generation of [ZnCl3(DMF)]- results in mutually antagonistic effects between [ZnCl2] and [Cl]- such that turnover proceeds via one mechanism or the other, depending on which species is in excess. The intermediacy of [ArZnCl] suggests a solution to the long-standing anomaly that many other reductants were found to be much less effective than Zn in inducing turnover of Ni/PPh3 catalyzed aryl chloride homocoupling in DMF. The use of DMAc as a solvent in place of DMF inhibits stalling through the steric inhibition of mixed metalate generation.

Multicomponent Construction of Tertiary Alkylamines by Photoredox/Nickel-Catalyzed Aminoalkylation of Organohalides

Tertiary alkylamines are privileged structural motifs widely present in natural products, pharmaceutical agents, and bioactive molecules, and their efficient synthesis has been a longstanding goal in organic chemistry. The functionalization of α-amino radicals derived from abundant precursors represents an emerging approach to accessing alkylamines, but application of this strategy to obtain tertiary alkylamines remains challenging. Here, we show that dual photoredox/nickel catalysis enables aminoalkylation of organohalides (sp2- and sp3-hybridized) in combination with secondary alkylamines and aldehydes. The multicomponent process proceeds through selective generation of α-amino radicals from the reduction of in situ-generated iminium ions by photoredox catalysis, followed by nickel-catalyzed cross-coupling to build a wide array of functionally diverse tertiary alkylamines. This strategy could also be extended to unprecedented four-component reactions and their asymmetric variants to deliver enantioenriched α-aryl-substituted γ-amino acid derivatives. Taken together, this work offers a streamlined synthetic route to aliphatic tertiary amines.

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.

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.

Magic Blue-Initiated SN2-Type Ring Opening of Activated Aziridines: Friedel-Crafts-Type Alkylation of Electron-Rich Arenes/Heteroarenes

A transition metal-free, atom-economical, and highly stereospecific synthetic approach to Friedel-Crafts-type alkylation of arenes/heteroarenes has been developed. The protocol involves the catalytic aminium radical-cation salt (Magic Blue)-initiated SN2-type nucleophilic ring opening of activated aziridines with arenes/heteroarenes to give the corresponding 2,2-diarylethylamines up to 99% yield and 85% ee (for nonracemic aziridines) in a very short reaction time. Moreover, on reaction with 1,3-dimethylindole and benzofuran, aziridines undergo domino-ring-opening cyclization (DROC) to give the various biologically significant heterocyclic scaffolds in moderate to good yields.

Synthesis of (Hetero)biaryls via Nickel Catalyzed Reductive Cross-Electrophile Coupling Between (Hetero)aryl Iodides and Bromides

(Hetero)biaryls are fundamental building blocks in the pharmaceutical industry and rapid access to these scaffolds is imperative for the success of numerous medicinal chemistry campaigns. Herein, a highly general, mild, and chemoselective reductive cross-electrophile coupling between (hetero)aryl iodides and heteroaryl bromides is reported. By employing more reactive (hetero)aryl halides, a broad range of successful substrates (45 examples) were identified. The reaction was also found to be chemoselective for C(sp2)-C(sp2) bond formation between (hetero)aryl iodides and bromides over (hetero)aryl chlorides, which were generally inert under the described reaction conditions. The efficiency of the procedure is also further demonstrated in parallel synthesis library format, on gram scale, as well as in the formal synthesis of Ruxolitinib, a potent JAK inhibitor. As such, we anticipate this method will find widespread utility in the assembly of (hetero)biaryls for medicinal chemistry efforts.

β-Phenethylamine Synthesis: N-Pyridinium Aziridines as Latent Dual Electrophiles

β-Phenethylamines are widely represented in biologically and pharmacologically active organic small molecules. Here, we introduce N-pyridinium aziridines as latent dual electrophiles for the synthesis of β-phenethylamines. Bromide-promoted ring opening generates β-halopyridinium amines. Selective Ni-catalyzed C-C cross-coupling between organozinc nucleophiles and the benzylic C-Br electrophile affords a diverse family of β-functionalized phenethylaminopyridinium salts, and coupling is stereoconvergent in the presence of chiral ligands. Subsequent Ni-catalyzed reductive N-N bond activation within the β-functionalized phenethylaminopyridinium salts furnishes the products of formal olefin carboamination. Other reductive N-N cleavage reactions are demonstrated to provide access to free primary amines, alkylated amines, heterocycles, and products derived from N-centered radical chemistry. The developed reaction sequence can be implemented in the context of complex molecules and natural product derivatives. Together, the described results provide a general and modular synthesis of β-phenethylamines and significantly expand the utility of N-pyridinium aziridines as linchpins in chemical synthesis.

Polyoxometalate-based iron-organic complex nanozymes with peroxidase-like activities for colorimetric detection of hydrogen peroxide and ascorbic acid

As a new type of artificial enzyme, a nanozyme is an ideal substitute for natural enzymes and has been successfully applied in many fields. However, in the application of biomolecular detection, most nanozymes have the disadvantages of long reaction times or high detection limits, prompting researchers to search for new efficient nanozymes. In this work, the enzyme-like activities of three polyoxometalate-based iron-organic complexes ([Fe(bpp)2](Mo6O19), [Fe(bpp)2]2(Mo8O26)·2CH3OH, and [Fe(bpp)2]4H[Na(Mo8O26)]3), namely, FeMo6, Fe2Mo8, and Fe4Mo8Na, were analyzed. All three polyoxometalate-based iron-organic complexes were found to be capable of catalyzing hydrogen peroxide (H2O2) to oxidize 3,3',5,5'-tetramethylbenzidine and o-phenylenediamine, resulting in visible color changes, further exhibiting peroxidase-like activity. Results showed that Fe4Mo8Na had more active sites due to its long chain structure, endowing more prominent peroxidase-like activity compared with Fe2Mo8 and FeMo6. A colorimetric sensing platform for H2O2 and ascorbic acid detection based on Fe4Mo8Na was established. The linear response range for H2O2 detection was 0.5-100 μM, and the detection limit was 0.143 μM. The linear response for ascorbic acid detection ranges from 0 to 750 μM with a detection limit of 1.07 μM. This study provides a new perspective for developing new nanozymes and expanding the sensing and detection application of nanozymes.

Merging Ring-Opening 1,2-Metallate Shift with Asymmetric C(sp3)-H Borylation of Aziridines

Chiral secondary alkyl amines with a vicinal quaternary stereocenter are undoubtedly important and ubiquitous subunits in natural products and pharmaceuticals. However, their asymmetric synthesis remains a formidable challenge. Herein, we merge the ring-opening 1,2-metallate shift with iridium-catalyzed enantioselective C(sp3)-H borylation of aziridines to deliver these frameworks with high enantioselectivities. We also demonstrated the synthetic application by downstream transformations, including the total synthesis of two Amaryllidaceae alkaloids, (-)-crinane and (+)-mesmebrane.

Electrocatalytic Formal C(sp2)-H Alkylations via Nickel-Catalyzed Cross-Electrophile Coupling with Versatile Arylsulfonium Salts

Producing sp3-hybridized carbon-enriched molecules is of particular interest due to their high success rate in clinical trials. The installation of aliphatic chains onto aromatic scaffolds was accomplished by nickel-catalyzed C(sp2)-C(sp3) cross-electrophile coupling with arylsulfonium salts. Thus, simple non-prefunctionalized arenes could be alkylated through the formation of aryldibenzothiophenium salts. The reaction employs an electrochemical approach to avoid potentially hazardous chemical redox agents, and importantly, the one-pot alkylation proved also viable, highlighting the robustness of our approach.

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.

B2(OH)4-Mediated Reductive Ring-Opening of N-Tosyl Aziridines by Nitroarenes: A Green and Regioselective Access to Vicinal Diamines

The nucleophilic ring-opening of aziridine derivatives provides an important synthetic tool for the preparation of various β-functionalized amines. Amines as nucleophiles are employed to prepare synthetically useful 1,2-diamines in the presence of various catalysts or activators. Herein, the B2(OH)4-mediated reductive ring-opening transformation of N-tosyl aziridines by nitroarenes was developed. This aqueous protocol employed nitroarenes as cheap and readily available amino sources and proceeds under external catalyst-free conditions. Control experiments and DFT calculations pointed to the in situ reduction of nitroarenes to aryl amines via N-aryl boramidic acid (E) and an SN1-type ring-opening of N-tosylaziridines by the resultant aryl amines with high regioselectivity.

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.

Polarizability matters in enantio-selection

The prevalence of chirality, or, handedness in biological world is a fundamental phenomenon and a characteristic hallmark of life. Thus, understanding the origin of enantio-selection, i.e., the sense and magnitude of asymmetric induction, has been a long-pursued goal in asymmetric catalysis. Herein, we demonstrated a polarizability-derived electronic effect that was shown to be capable of rationalizing a broad range of stereochemical observations made in the field of asymmetric catalysis. This effect provided a consistent enantio-control model for the prediction of major enantiomers formed in a ruthenium-catalyzed asymmetric transfer hydrogenations of ketones. Direct and quantitative linear free energy relationships between substrates' local polarizabilities and observed enantio-selectivity were also revealed in three widely known asymmetric catalytic systems covering both reductions and oxidations. This broadly applicable polarizability-based electronic effect, in conjunction with conventional wisdom mainly leveraging on steric effect considerations, should aid rational design of enantio-selective processes for better production of chiral substances.

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.

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.

Nickel-catalyzed electrophiles-controlled enantioselective reductive arylative cyclization and enantiospecific reductive alkylative cyclization of 1,6-enynes

Transition metal-catalyzed asymmetric cyclization of 1,6-enynes is a powerful tool for the construction of chiral nitrogen-containing heterocycles. Despite notable achievements, these transformations have been largely limited to the use of aryl or alkenyl metal reagents, and stereoselective or stereospecific alkylative cyclization of 1,6-enynes remains unexploited. Herein, we report Ni-catalyzed enantioselective reductive anti-arylative cyclization of 1,6-enynes with aryl iodides, providing enantioenriched six-membered carbo- and heterocycles in good yields with excellent enantioselectivities. Additionally, we have realized Ni-catalyzed enantiospecific reductive cis-alkylative cyclization of 1,6-enynes with alkyl bromides, furnishing chiral five-membered heterocycles with high regioselectivity and stereochemical fidelity. Mechanistic studies reveal that the arylative cyclization of 1,6-enynes is initiated by the oxidative addition of Ni(0) to aryl halides and the alkylative cyclization is triggered by the oxidative addition of Ni(0) to allylic acetates. The utility of this strategy is further demonstrated in the enantioselective synthesis of the antiepileptic drug Brivaracetam.

Dual Nickel- and Photoredox-Catalyzed Asymmetric Reductive Cross-Coupling To Access Chiral Secondary Benzylic Alcohols

Transition-metal-catalyzed asymmetric cross-coupling represents a powerful strategy for C-C bond formation and the synthesis of enantiomerically pure molecules. Here, we report a dual nickel/photoredox-catalyzed enantioselective reductive cross-coupling of aryl halides with α-bromobenzoates, readily generated from aliphatic aldehydes, to provide diverse chiral secondary benzylic alcohols that are important motifs in bioactive natural products and pharmaceuticals. This dual catalytic system features mild conditions, good functional group tolerance, broad substrate scope, excellent enantiocontrol, and avoidance of stoichiometric metal reductants, presenting great potential for late-stage functionalization of complex molecules.

Nickel-Catalyzed Asymmetric Propargyl-Aryl Cross-Electrophile Coupling

Performing asymmetric cross-coupling reactions between propargylic electrophiles and aryl nucleophiles is a well-established method to build enantioenriched benzylic alkynes. Here, a catalytic enantioselective propargyl-aryl cross-coupling between two electrophiles was achieved for the first time in a stereoconvergent manner. Propargylic chlorides were treated with aryl iodides as well as heteroaryl iodides in the presence of a chiral nickel complex, and manganese metal was used as a stoichiometric reductant, allowing for the construction of a propargyl C-aryl bond under mild conditions. An alternative dual nickel/photoredox catalytic protocol was also developed for this cross-electrophile coupling in the absence of a metal reductant. The potential utility of this conversion is demonstrated in the facile construction of stereoenriched bioactive molecule derivatives and medicinal compounds based on the diversity of acetylenic chemistry. Detailed experimental studies have revealed the key mechanistic features of this transformation.

Photoredox Nickel-Catalysed Stille Cross-Coupling Reactions

The Stille cross-coupling reaction is one of the most common strategies for the construction of C-C bonds. Despite notable strides in the advancement of the Stille reaction, persistent challenges persist in hindering its greener evolution. These challenges encompass multiple facets, such as the high cost of precious metals and ligands, the demand for various additives, and the slow reaction rate. In comparison to the dominant palladium-catalysed Stille reactions, cost-effective nickel-catalysed systems lag behind, and enantioconvergent Stille reactions of racemic stannanes remain undeveloped. Herein, we present a pioneering instance of nickel-catalysed enantioconvergent Stille cross-coupling reactions of racemic stannane reagents, resulting in the formation of C-C bonds in good to high yields with excellent stereoselectivity. This strategy provides a practical, scalable, and operationally straightforward method for the synthesis of C(sp3 )-C(sp3 ), C(sp3 )-C(sp2 ), and C(sp3 )-C(sp) bonds under exceptionally mild conditions (without additives and bases, ambient temperature). The innovative use of synergistic photoredox/nickel catalysis enables a novel single-electron transmetalation process of stannane reagents, providing a new research paradigm of Stille reactions.

Ligand-Enabled Carboamidation of Unactivated Alkenes through Enhanced Organonickel Electrophilicity

Catalytic carboamination of alkenes is a powerful synthetic tool to access valuable amine scaffolds from abundant and readily available alkenes. Although a number of synthetic approaches have been developed to achieve the rapid buildup of molecular complexity in this realm, the installation of diverse carbon and nitrogen functionalities onto unactivated alkenes remains underdeveloped. Here we present a ligand design approach to enable nickel-catalyzed three-component carboamidation that is applicable to a wide range of alkenyl amine derivatives via a tandem process involving alkyl migratory insertion and inner-sphere metal-nitrenoid transfer. With this method, various nitrogen functionalities can be installed into both internal and terminal unactivated alkenes, leading to differentially substituted diamines that would otherwise be difficult to access. Mechanistic investigations reveal that the tailored Ni(cod)(BQiPr) precatalyst modulates the electronic properties of the presumed π-alkene-nickel intermediate via the quinone ligand, leading to enhanced carbonickelation efficiency across the unactivated C═C bond. These findings establish nickel's ability to catalyze multicomponent carboamidation with a high efficiency and exquisite selectivity.

Enantioselective Reductive Cross-Couplings of Olefins by Merging Electrochemistry with Nickel Catalysis

The merger of electrochemistry and transition metal catalysis has emerged as a powerful tool to join two electrophiles in an enantioselective manner. However, the development of enantioselective electroreductive cross-couplings of olefins remains a challenge. Inspired by the advantages of the synergistic use of electrochemistry with nickel catalysis, we present here a Ni-catalyzed enantioselective electroreductive cross-coupling of acrylates with aryl halides and alkyl bromides, which affords chiral α-aryl carbonyls in good to excellent enantioselectivity. Additionally, this catalytic reaction can be applied to (hetero)aryl chlorides, which is difficult to achieve by other methods. The combination of cyclic voltammetry analysis with electrode potential studies suggests that the NiI species activates aryl halides by oxidative addition and alkyl bromides by single-electron transfer.

Metal-Catalyzed Enantioconvergent Transformations

Enantioconvergent catalysis has expanded asymmetric synthesis to new methodologies able to convert racemic compounds into a single enantiomer. This review covers recent advances in transition-metal-catalyzed transformations, such as radical-based cross-coupling of racemic alkyl electrophiles with nucleophiles or racemic alkylmetals with electrophiles and reductive cross-coupling of two electrophiles mainly under Ni/bis(oxazoline) catalysis. C-H functionalization of racemic electrophiles or nucleophiles can be performed in an enantioconvergent manner. Hydroalkylation of alkenes, allenes, and acetylenes is an alternative to cross-coupling reactions. Hydrogen autotransfer has been applied to amination of racemic alcohols and C-C bond forming reactions (Guerbet reaction). Other metal-catalyzed reactions involve addition of racemic allylic systems to carbonyl compounds, propargylation of alcohols and phenols, amination of racemic 3-bromooxindoles, allenylation of carbonyl compounds with racemic allenolates or propargyl bromides, and hydroxylation of racemic 1,3-dicarbonyl compounds.

Catalytic Synthesis of β-(Hetero)arylethylamines: Modern Strategies and Advances

β-(Hetero)arylethylamines appear in a myriad of pharmaceuticals due to their broad spectrum of biological properties, making them prime candidates for drug discovery. Conventional methods for their preparation often require engineered substrates that limit the flexibility of the synthetic routes and the diversity of compounds that can be accessed. Consequently, methods that provide rapid and versatile access to those scaffolds remain limited. To overcome these challenges, synthetic chemists have designed innovative and modular strategies to access the β-(hetero)arylethylamine motif, paving the way for their more extensive use in future pharmaceuticals. This review outlines recent progresses in the synthesis of (hetero)arylethylamines and emphasizes how these innovations have enabled new levels of molecular complexity, selectivity, and practicality.

Nickel-Catalyzed Reductive Cross-Coupling of Aziridines and Allylic Chlorides

A nickel-catalyzed reductive cross-coupling of aziridines and allylic chlorides was realized by using manganese metal as the reducing agent. This protocol afforded a convenient approach to obtain β-allyl-substituted arylethylamines bearing various functional groups. The utility of this reaction was also demonstrated by scale-up preparation and diverse transformations, including the synthesis of Baclofen and several bioactive molecular motifs.

Decarboxylative Cross-Electrophile Coupling of (Hetero)Aromatic Bromides and NHP Esters

Aryl bromides are known to be challenging substrates in the decarboxylative cross-electrophile coupling with redox-active NHP esters-the majority of such processes utilize aryl iodides. Herein, we describe the development of conditions that are suitable for the decarboxylative cross-electrophile coupling of NHP esters and a wide range of (hetero)aryl bromides. The key advances that allowed for the use of aryl bromides in this reaction are (1) the identification of ligand L3 as an optimal ligand for the use of electron-neutral and deficient aryl bromides and (2) the significant improvement in yield that iodide salts and excess heterogenous zinc impart to this reaction. A wide variety of NHP esters perform well under the optimized conditions, including methyl, primary, secondary, and several strained tertiary systems. Likewise, a variety of aromatic and heteroaromatic bromides relevant to medicinal chemistry perform well in this transformation, including an aryl bromide precursor to the known drug dapagliflozin.

Nickel-Catalyzed Cross-Electrophile Ring Opening/gem-Difluoroallylation of Aziridines

Herein we report a nickel-catalyzed regioselective cross-electrophile ring opening reaction of sulfonyl-protected aziridines with trifluoromethyl-substituted alkenes as the gem-difluoroallylating agents, providing a new and efficient entry to prepare gem-difluorobishomoallylic sulfonamides. Moreover, the scaffold of 6-fluoro-1,2,3,4-tetrahydropyridine can be constructed starting from the ring opening products via NaH-mediated intramolecular defluorinative nucleophilic vinylic substitution.

Expeditious preparation of β-sec-alkyl vicinal amino alcohols used for chiral ligand synthesis

An economical route providing quick access to chiral β-amino alcohols bearing one β-sec-alkyl group was developed. This protocol starts with commercially available and cheap chiral sources such as derivatives of L-serine and L-threonine. A series of vicinal amino alcohols with high optical purity were prepared in good yields through 4 or 6 operationally simple steps. Two different strategies (three routes) were designed for the synthesis of amino alcohols bearing β-sec-alkyl groups with various steric hindrance.

Nickel/biimidazole-catalyzed electrochemical enantioselective reductive cross-coupling of aryl aziridines with aryl iodides

Here, we report an asymmetric electrochemical organonickel-catalyzed reductive cross-coupling of aryl aziridines with aryl iodides in an undivided cell, affording β-phenethylamines in good to excellent enantioselectivity with broad functional group tolerance. The combination of cyclic voltammetry analysis of the catalyst reduction potential as well as an electrode potential study provides a convenient route for reaction optimization. Overall, the high efficiency of this method is credited to the electroreduction-mediated turnover of the nickel catalyst instead of a metal reductant-mediated turnover. Mechanistic studies suggest a radical pathway is involved in the ring opening of aziridines. The statistical analysis serves to compare the different design requirements for photochemically and electrochemically mediated reactions under this type of mechanistic manifold.

Nickel-Catalyzed Enantioselective Electrochemical Reductive Cross-Coupling of Aryl Aziridines with Alkenyl Bromides

An electrochemically driven nickel-catalyzed enantioselective reductive cross-coupling of aryl aziridines with alkenyl bromides has been developed, affording enantioenriched β-aryl homoallylic amines with excellent E-selectivity. This electroreductive strategy proceeds in the absence of heterogeneous metal reductants and sacrificial anodes by employing constant current electrolysis in an undivided cell with triethylamine as a terminal reductant. The reaction features mild conditions, remarkable stereocontrol, broad substrate scope, and excellent functional group compatibility, which was illustrated by the late-stage functionalization of bioactive molecules. Mechanistic studies indicate that this transformation conforms with a stereoconvergent mechanism in which the aziridine is activated through a nucleophilic halide ring-opening process.

Nickel-Catalyzed Ligand-Controlled Selective Reductive Cyclization/Cross-Couplings

ConspectusThe use of quaternary stereocenters during lead candidate optimization continues to grow because of improved physiochemical and pharmacokinetic profiles of compounds with higher sp³ fraction. Pd-catalyzed redox-neutral alkene difunctionalization involving carbopalladation of alkenes followed by nucleophilic-trapping σ-alkyl-palladium intermediates has been developed as an efficient method to construct quaternary stereocenters. However, the low chemoselectivity and air sensitivity of organometallic nucleophiles, as well as their low availability and accessibility, limit the scope of application of this elegant strategy. Recently, Ni-catalyzed reductive cross-coupling has evolved into a privileged strategy to easily construct valuable C(sp³)-C bonds. Despite great progress, the enantioselective coupling of C(sp³) electrophiles still relies on activated or functionalized alkyl precursors, which are often unstable and require multiple steps to prepare. Therefore, Ni-catalyzed reductive difunctionalization of alkenes via selective cyclization/cross-coupling was developed. This strategy not only offers a robust and practical alternative for traditional redox-neutral alkene difunctionalization but also provides strategic complementarity for reductive cross-coupling of activated alkyl electrophiles. In this Account, we summarize the latest results from our laboratory on this topic. These findings mainly include our explorations in modulating the enantioselectivity and cyclization mode of reductive cyclization/cross-couplings.We will first discuss Ni-catalyzed enantioselective reductive cyclization/cross-coupling to construct valuable chiral heterocycles with quaternary stereocenters and focus on the effects of ligands, reductants, and additives and their roles in reductive cross-coupling. A wide range of electrophiles have been explored, including aryl halides, vinyl halides, alkynyl halides, gem-difluoroalkenes, CO₂, trifluoromethyl alkenes, and cyano electrophiles. The synthetic potential of this approach has also been demonstrated in the synthesis of biologically active natural products and drug molecules. Second, we will detail how to tune the steric effects of nickel catalysts by modifying bipyridine ligands for regiodivergent cyclization/cross-couplings. Specifically, the use of bidentate ligands favors exo-selective cyclization/cross-coupling, while the use of a carboxylic acid-modified bipyridine ligand permits endo-selective cyclization/cross-coupling. We will also show how to activate the amide substrate by altering the electronic and steric properties of substituents on the nitrogen, thereby enabling the nucleophilic addition of aryl halides to amide carbonyls. Further investigation of ligand properties has led to tunable cyclization/cross-couplings (addition to the amide carbonyl vs 7-endo-cyclization) for the divergent synthesis of pharmacologically important 2-benzazepine frameworks. Finally, we serendipitously discover that modifying the ligands of nickel catalysts and changing the oxidation state of nickel can control the migratory aptitude of different groups, thus providing a switchable skeletal rearrangement strategy. This transformation is of high synthetic value because it represents a conceptually unprecedented new approach to C-C bond activation. Thus, this Account not only summarizes synthetic methods that allow the formation of valuable chiral heterocycles with quaternary stereocenters using a wide variety of electrophiles but also provides insight into the relationship between ligand structure, substrate, and cyclization selectivity.

Synthesis of Unprotected β-Arylethylamines by Iron(II)-Catalyzed 1,2-Aminoarylation of Alkenes in Hexafluoroisopropanol

β-Arylethylamines are prevalent structural motifs in molecules exhibiting biological activity. Here we report a sequential one-pot protocol for the 1,2-aminoarylation of alkenes with hydroxylammonium triflate salts and (hetero)arenes. Unlike existing methods, this reaction provides a direct entry to unprotected β-arylethylamines with remarkable functional group tolerance, allowing key drug-oriented functional groups to be installed in a two-step process. The use of hexafluoroisopropanol as a solvent in combination with an iron(II) catalyst proved essential to reaching high-value nitrogen-containing molecules.

Enantioselective C(sp3)-C(sp3) Reductive Cross-Electrophile Coupling of Unactivated Alkyl Halides with α-Chloroboronates via Dual Nickel/Photoredox Catalysis

Substantial advances in enantioconvergent C(sp³)-C(sp³) bond formations have been made with nickel-catalyzed cross-coupling of racemic alkyl electrophiles with organometallic reagents or nickel-hydride-catalyzed hydrocarbonation of alkenes. Herein, we report an unprecedented enantioselective C(sp³)-C(sp³) reductive cross-coupling by the direct utilization of two different alkyl halides with dual nickel/photoredox catalysis system. This highly selective coupling of racemic α-chloroboronates and unactivated alkyl iodides furnishes chiral secondary alkyl boronic esters, which serve as useful and important intermediates in the realm of organic synthesis and enable a desirable protocol to fast construction of enantioenriched complex molecules.

Paired electrolysis-enabled nickel-catalyzed enantioselective reductive cross-coupling between α-chloroesters and aryl bromides

Electrochemical asymmetric catalysis has emerged as a sustainable and promising approach to the production of chiral compounds and the utilization of both the anode and cathode as working electrodes would provide a unique approach for organic synthesis. However, precise matching of the rate and electric potential of anodic oxidation and cathodic reduction make such idealized electrolysis difficult to achieve. Herein, asymmetric cross-coupling between α-chloroesters and aryl bromides is probed as a model reaction, wherein alkyl radicals are generated from the α-chloroesters through a sequential oxidative electron transfer process at the anode, while the nickel catalyst is reduced to a lower oxidation state at the cathode. Radical clock studies, cyclic voltammetry analysis, and electron paramagnetic resonance experiments support the synergistic involvement of anodic and cathodic redox events. This electrolytic method provides an alternative avenue for asymmetric catalysis that could find significant utility in organic synthesis.

Ni/Photoredox-Catalyzed C(sp3)-C(sp3) Coupling between Aziridines and Acetals as Alcohol-Derived Alkyl Radical Precursors

Aziridines are readily available C(sp³) precursors that afford valuable β-functionalized amines upon ring opening. In this article, we report a Ni/photoredox methodology for C(sp³)-C(sp³) cross-coupling between aziridines and methyl/1°/2° aliphatic alcohols activated as benzaldehyde dialkyl acetals. Orthogonal activation modes of each alkyl coupling partner facilitate cross-selectivity in the C(sp³)-C(sp³) bond-forming reaction: the benzaldehyde dialkyl acetal is activated via hydrogen atom abstraction and β-scission via a bromine radical (generated in situ from single-electron oxidation of bromide), whereas the aziridine is activated at the Ni center via reduction. We demonstrate that an Ni(II) azametallacycle, conventionally proposed in aziridine cross-coupling, is not an intermediate in the productive cross-coupling. Rather, stoichiometric organometallic and linear free energy relationship studies indicate that aziridine activation proceeds via Ni(I) oxidative addition, a previously unexplored elementary step.

Nickel-Catalyzed Enantioselective Reductive Alkyl-Carbamoylation of Internal Alkenes

Herein, we leverage the Ni-catalyzed enantioselective reductive dicarbofunctionalization of internal alkenes with alkyl iodides to enable the synthesis of chiral pyrrolidinones bearing vicinal stereogenic centers. The application of newly developed 1-Nap Quinim is critical for formation of two contiguous stereocenters in high yield, enantioselectivity, and diastereoselectivity. This catalytic system also improves both the yield and enantioselectivity in the synthesis of α,α-dialkylated γ-lactams. Computational studies reveal that the enantiodetermining step proceeds with a carbamoyl-NiI intermediate that is reduced by the Mn reductant prior to intramolecular migratory insertion. The presence of the t-butyl group of the Quinim ligand leads to an unfavorable distortion of the substrate in the TS that leads to the minor enantiomer. Calculations also support an improvement in enantioselectivity with 1-Nap Quinim compared to p-tol Quinim.

Nickel-Catalyzed, Manganese-Assisted Denitrogenative Cross-Electrophile-Coupling of Benzotriazinones with Alkyl Halides for ortho-Alkylated Benzamides

A nickel-catalyzed denitrogenative cross-electrophile-coupling of benzotriazinones with unactivated alkyl halides (X = Cl, Br, I) in the presence of manganese powder as a reductant has been developed. The reaction furnishes ortho-alkylated secondary benzamides in modest to good yields under mild conditions. The scope of the reaction is demonstrated with 25 examples, showing good tolerance of steric hindrance and common functional groups, thus providing an efficient protocol to ortho-alkylated benzamide derivatives without the use of preprepared organometallic reagents.

Synthesis of Multisubstituted Allylic Alcohols via a Nickel-Catalyzed Cross-Electrophile Ring-Opening Reaction

Herein we report a nickel-catalyzed cross-electrophile ring-opening reaction of vinyl epoxides wherein aryl iodides, alkyl iodides, and benzyl chlorides can all serve as the electrophilic coupling partners, providing a new approach to preparing multisubstituted allylic alcohols. This new method features broad substrate scope (76 examples), good step-economy, and high L/B- and E/Z selectivity as well as mild reaction conditions.

Enantioselective Reductive N-Cyclization-Alkylation Reaction of Alkene-Tethered Oxime Esters and Alkyl Iodides by Nickel Catalysis

Asymmetric cross-electrophile difunctionalization of tethered alkenes has become a powerful tool for the production of chiral cyclic scaffolds; however, the current studies all focus on carbocyclization reactions. Herein, we report an N-cyclization-alkylation reaction and thus showcase the potential of heterocyclization for accessing new enantioenriched cyclic architectures. This work establishes a new approach for enantioselective aza-Heck cyclization/cross-coupling sequence, which remains a long-standing unsolved challenge for the synthetic community. The reaction proceeds with primary, secondary, and a few tertiary alkyl iodides, and the use of newly defined ligands gave highly enantioenriched pyrrolines with improved molecular diversity under mild conditions. The presence of imine functionality allows for further structural variations.

N-Aminopyridinium reagents as traceless activating groups in the synthesis of N-Aryl aziridines

N-functionalized aziridines, which are both useful intermediates and important synthetic targets, can be envisioned as arising from the addition of nitrenes (i.e., NR fragments) to olefinic substrates. The exceptional reactivity of most nitrenes, in particular with respect to unimolecular decomposition, prevents general application of nitrene-transfer to the synthesis of N-functionalized aziridines. Here we demonstrate N-aryl aziridine synthesis via 1) olefin aziridination with N-aminopyridinium reagents to afford N-pyridinium aziridines followed by 2) Ni-catalyzed C-N cross-coupling of the N-pyridinium aziridines with aryl boronic acids. The N-pyridinium aziridine intermediates also participate in ring-opening chemistry with a variety of nucleophiles to afford 1,2-aminofunctionalization products. Mechanistic investigations indicate aziridine cross-coupling proceeds via a noncanonical mechanism involving initial aziridine opening promoted by the bromide counterion of the Ni catalyst, C-N cross-coupling, and finally aziridine reclosure. Together, these results provide new opportunities to achieve selective incorporation of generic aryl nitrene equivalents in organic molecules.

Nickel/Photoredox-Catalyzed Enantioselective Reductive Cross-Coupling between Vinyl Bromides and Benzyl Chlorides

A visible-light-promoted nickel/photoredox-catalyzed reductive cross-coupling reaction between vinyl bromides and benzyl chlorides is reported. A diverse array of enantioenriched allylic centers containing products could be achieved in good yields (up to 90%) and high enantioselectivities (up to 95% ee). The mechanistic studies show that this reductive cross-coupling involves a radical pathway.

Ligand-Based Control of Nickel Catalysts: Switching Chemoselectivity from One-Electron to Two-Electron Pathways in Competing Reactions of 4-Halotetrahydropyrans

Development of nickel-catalyzed transformations would be facilitated by an improved ability to predict which ligands promote and suppress competing mechanisms. We evaluate ligand-based modulation of catalyst preference for one- or two-electron pathways employing 4-halotetrahydropyrans as model substrates that can undergo divergent reaction pathways. Chemoselectivity for one- or two-electron oxidative addition is predicted by ligand class. Phosphine-ligated nickel catalysts favor closed-shell oxidative addition. In contrast, nitrogen-ligated nickel catalysts prefer the one-electron pathway, initiating with halogen atom transfer.

Nickel-Catalyzed Cross-Electrophile Coupling Reactions between Allylic Acetates and gem-Difluorovinyl Tosylate

A nickel-catalyzed cross-electrophile coupling of allylic acetates and gem-difluorovinyl tosylate is presented, which first achieves allylic gem-difluoroolefins via C(sp³)-C(sp²) cross-electrophile coupling. In addition, this protocol was performed under mild reaction conditions, affording a variety of allylic gem-difluorovinyl arenes in moderate to good yields. Moreover, both linear and branched allylic acetate could produce a linear cross-coupling product exclusively. Mechanistic studies reveal that the reaction involves two different Ni(0)/Ni(II) catalytic cycles.