Development of an air-stable nickel precatalyst for the amination of aryl chlorides, sulfamates, mesylates, and triflates

Total Synthesis of (+)-Dixiamycin C via a Late-Stage Ni(II)-Photoredox N-Arylation of Carbazoles

We report the asymmetric total synthesis of dixiamycin C (1) through the shrewd alliance of the naturally occurring monomer xiamycin A methyl ester (5) and its bromo derivative (31) following a late-stage Buchwald-Macmillan's C-N bond formation via a photoredox electron transfer approach with a less reactive carbazole nitrogen. The key step in the synthesis of monomer xiamycin A methyl ester (5) involves Buchwald's Pd(II)-mediated aerobic dehydrogenative C-N bond formation, Beckmann rearrangement, and ipso-acetylation of an electron-rich aromatic ring of an abietane core.

Transition-Metal-Catalyzed Transformations for the Synthesis of Marine Drugs

Transition metal catalysis has contributed to the discovery of novel methodologies and the preparation of natural products, as well as new chances to increase the chemical space in drug discovery programs. In the case of marine drugs, this strategy has been used to achieve selective, sustainable and efficient transformations, which cannot be obtained otherwise. In this perspective, we aim to showcase how a variety of transition metals have provided fruitful couplings in a wide variety of marine drug-like scaffolds over the past few years, by accelerating the production of these valuable molecules.

The development of cage phosphine 'DalPhos' ligands to enable nickel-catalyzed cross-couplings of (hetero)aryl electrophiles

Nickel-catalyzed cross-couplings of (hetero)aryl electrophiles with a diversity of nucleophiles (nitrogen, oxygen, carbon, and others) have evolved into competitive alternatives to well-established palladium- and copper-based protocols for the synthesis of (hetero)aryl products, including (hetero)anilines and (hetero)aryl ethers. A survey of the literature reveals that the use of cage phosphine (CgP) 'DalPhos' (DALhousie PHOSphine) bisphosphine-type ligands operating under thermal conditions currently offers the most broad substrate scope in nickel-catalyzed cross-couplings of this type, especially involving (hetero)aryl chlorides and phenol-derived electrophiles. The development and application of these DalPhos ligands is described in a ligand-specific manner that is intended to serve as a guide for the synthetic chemistry end-user.

Thermal Nickel-Catalyzed N-Arylation of NH-Sulfoximines with (Hetero)aryl Chlorides Enabled by PhPAd-DalPhos Ligation

We report a versatile method for cross-coupling of NH-sulfoximines with (hetero)aryl chlorides, as well as bromide and sulfonate electrophiles, that makes use of the air-stable, commercial precatalyst (PhPAd-DalPhos)Ni(o-tol)Cl. Under optimized conditions a diverse electrophile scope is established, including the N-arylation of the pharmaceutical Clozapine. While 5 mol % Ni and 80 °C are commonly employed in this chemistry, successful examples utilizing 1 mol % Ni and/or 25 °C are presented. Competition experiments establish the superiority of NH-sulfoximine over primary sulfonamide as nucleophiles under these conditions.

DalPhos/Nickel-Catalyzed C2-H Arylation of 1,3-Azoles Using a Dual-Base System

We report a versatile method for C2 functionalization of (benz)oxazoles and (benzo)thiazoles employing a tert-butylimino-tri(pyrrolidino)phosphorane/sodium trifluoroacetate (BTPP/NaTFA) "dual-base" system in combination with an air-stable Ni(II) precatalyst containing either CyPAd-DalPhos or PhPAd-DalPhos. These catalyst systems enable access to a reaction scope that encompasses a range of challenging oxidative addition partners, including (hetero)aryl chlorides as well as pivalates, tosylates, and other related phenol derivatives. The utility of this method is demonstrated through the derivatization of an active pharmaceutical ingredient and 5 mmol synthesis of a thiazole derivative.

Regime Switch in the Dual-Catalyzed Coupling of Alkyl Silicates with Aryl Bromides

Metallaphotoredox catalyzed cross-coupling of an arylbromide (Ar-Br) with an alkyl bis(catecholato)silicate (R-Si⊖ ) has been analyzed in depth using a continuum of analytical techniques (EPR, fluorine NMR, electrochemistry, photophysics) and modeling (micro-kinetics and DFT calculations). These studies converged on the impact of four control parameters consisting in the initial concentrations of the iridium photocatalyst ([Ir]0 ), nickel precatalyst ([Ni]0 ) and silicate ([R-Si⊖ ]0 ) as well as light intensity I0 for an efficient reaction between Ar-Br and R-Si⊖ . More precisely, two regimes were found to be possibly at play. The first one relies on an equimolar consumption of Ar-Br with R-Si⊖ smoothly leading to Ar-R, with no side-product from R-Si⊖ and a second one in which R-Si⊖ is simultaneously coupled to Ar-Br and degraded to R-H. This integrative approach could serve as a case study for the investigation of other metallaphotoredox catalysis manifolds of synthetic significance.

Dual Nickel(II)/Mechanoredox Catalysis: Mechanical-Force-Driven Aryl-Amination Reactions Using Ball Milling and Piezoelectric Materials

The combination of a nickel(II) catalyst and a mechanoredox catalyst under ball-milling conditions promotes mechanical-force-driven C-N cross-coupling reactions. In this nickel(II)/mechanoredox cocatalyst system, the modulation of the oxidation state of the nickel center, induced by piezoelectricity, is used to facilitate a highly efficient aryl-amination reaction, which is characterized by a broad substrate scope, an inexpensive combination of catalysts (NiBr2 and BaTiO3 ), short reaction times, and an almost negligible quantity of solvents. Moreover, this reaction can be readily up-scaled to the multi-gram scale, and all synthetic operations can be carried out under atmospheric conditions without the need for complicated reaction setups. Furthermore, this force-induced system is suitable for excitation-energy-accepting molecules and poorly soluble polyaromatic substrates that are incompatible with solution-based nickel(II)/photoredox cocatalysts.

Broad-Scope Amination of Aryl Sulfamates Catalyzed by a Palladium Phosphine Complex

Among phenol-derived electrophiles, aryl sulfamates are attractive substrates since they can be employed as directing groups for C-H functionalization prior to catalysis. However, their use in C-N coupling is limited only to Ni catalysis. Here, we describe a Pd-based catalyst with a broad scope for the amination of aryl sulfamates. We show that the N-methyl-2-aminobiphenyl palladacycle supported by the PCyp2ArXyl2 ligand (Cyp = cyclopentyl; ArXyl2 = 2,6-bis(2,6-dimethylphenyl)phenyl) efficiently catalyzes the C-N coupling of aryl sulfamates with a variety of nitrogen nucleophiles, including anilines, primary and secondary alkyl amines, heteroaryl amines, N-heterocycles, and primary amides. DFT calculations support that the oxidative addition of the aryl sulfamate is the rate-determining step. The C-N coupling takes place through a cationic pathway in the polar protic medium.

Transition-metal-free silylboronate-mediated cross-couplings of organic fluorides with amines

C-N bond cross-couplings are fundamental in the field of organic chemistry. Herein, silylboronate-mediated selective defluorinative cross-coupling of organic fluorides with secondary amines via a transition-metal-free strategy is disclosed. The cooperation of silylboronate and potassium tert-butoxide enables the room-temperature cross-coupling of C-F and N-H bonds, effectively avoiding the high barriers associated with thermally induced S_N2 or S_N1 amination. The significant advantage of this transformation is the selective activation of the C-F bond of the organic fluoride by silylboronate without affecting potentially cleavable C-O, C-Cl, heteroaryl C-H, or C-N bonds and CF₃ groups. Tertiary amines with aromatic, heteroaromatic, and/or aliphatic groups were efficiently synthesized in a single step using electronically and sterically varying organic fluorides and N-alkylanilines or secondary amines. The protocol is extended to the late-stage syntheses of drug candidates, including their deuterium-labeled analogs.

Comparative Screening of DalPhos/Ni Catalysts in C-N Cross-couplings of (Hetero)aryl Chlorides Enables Development of Aminopyrazole Cross-couplings with Amine Base

A systematic competitive evaluation of the DalPhos ligand family in nickel-catalyzed N-arylation chemistry is reported, involving primary (linear and branched) and secondary alkylamines, as well as a primary five-membered heteroarylamine (aminopyrazole), in combination with a diverse set of test electrophiles and bases (NaOtBu, K₂ CO₃ , DBU/NaTFA). In addition to providing optimal ligand/catalyst identification, and bringing to light methodology limitations (e. g., unwanted C-O cross-coupling with NaOtBu), our survey enabled the development of the first efficient catalyst system for heteroatom-dense C-N cross-coupling of aminopyrazoles and related nucleophiles with (hetero)aryl chlorides by use of an amine 'dual-base' system.

General Method for the Amination of Aryl Halides with Primary and Secondary Alkyl Amines via Nickel Photocatalysis

The Buchwald-Hartwig C-N coupling reaction has been ranked as one of the 20 most frequently used reactions in medicinal chemistry. Owing to its much lower cost and higher reactivity toward less reactive aryl chlorides than palladium, the C-N coupling reaction catalyzed by Ni-based catalysts has received a great deal of attention. However, there appear to be no universal, practical Ni catalytic systems so far that could enable the coupling of electron-rich and electron-poor aryl halides with both primary and secondary alkyl amines. In this study, it is reported that a Ni(II)-bipyridine complex catalyzes efficient C-N coupling of aryl chlorides and bromides with various primary and secondary alkyl amines under direct excitation with light. Intramolecular C-N coupling is also demonstrated. The feasibility and applicability of the protocol in organic synthesis is attested by more than 200 examples.

Nickel-Catalyzed C-N Cross-Coupling of 4-Chloro-1,8-naphthalimides and Bulky, Primary Alkylamines at Room Temperature

4-Amino-1,8-naphthalimides, potentially useful fluorescent probes in biological applications, are prepared via Ni(cod)₂/IPr-catalyzed cross-couplings between 4-chloro-1,8-naphthalimide electrophiles and α,α,α-trisubstituted, primary alkylamines at room temperature. This method represents the first synthesis of 4-amino-1,8-naphthalimides using Ni-catalyzed C-N cross-coupling and provides the first examples of 4-amino-1,8-naphthalimides incorporating such bulky primary alkylamines, thereby highlighting the utility of Ni-catalyzed processes in synthesizing naphthalimide scaffolds that were inaccessible using established methods (S_NAr; Pd or Cu catalysis).

Mapping Dual-Base-Enabled Nickel-Catalyzed Aryl Amidations: Application in the Synthesis of 4-Quinolones

The C-N cross-coupling of (hetero)aryl (pseudo)halides with NH substrates employing nickel catalysts and organic amine bases represents an emergent strategy for the sustainable synthesis of (hetero)anilines. However, unlike protocols that rely on photoredox/electrochemical/reductant methods within Ni^I/III cycles, the reaction steps that comprise a putative Ni^0/II C-N cross-coupling cycle for a thermally promoted catalyst system using organic amine base have not been elucidated. Here we disclose an efficient new nickel-catalyzed protocol for the C-N cross-coupling of amides and 2'-(pseudo)halide-substituted acetophenones, for the first time where the (pseudo)halide is chloride or sulfonate, which makes use of the commercial bisphosphine ligand PAd2-DalPhos (L4) in combination with an organic amine base/halide scavenger, leading to 4-quinolones. Room-temperature stoichiometric experiments involving isolated Ni^{0, I, and II} species support a Ni^0/II pathway, where the combined action of DBU/NaTFA allows for room-temperature amide cross-couplings.

Transition metal-catalyzed organic reactions in undivided electrochemical cells

Transition metal-catalyzed organic electrochemistry is a rapidly growing research area owing in part to the ability of metal catalysts to alter the selectivity of a given transformation. This conversion mainly focuses on transition metal-catalyzed anodic oxidation and cathodic reduction and great progress has been achieved in both areas. Typically, only one of the half-cell reactions is involved in the organic reaction while a sacrificial reaction occurs at the counter electrode, which is inherently wasteful since one electrode is not being used productively. Recently, transition metal-catalyzed paired electrolysis that makes use of both anodic oxidation and cathodic reduction has attracted much attention. This perspective highlights the recent progress of each type of electrochemical reaction and relatively focuses on the transition metal-catalyzed paired electrolysis, showcasing that electrochemical reactions involving transition metal catalysis have advantages over conventional reactions in terms of controlling the reaction activity and selectivity and figuring out that transition metal-catalyzed paired electrolysis is an important direction of organic electrochemistry in the future and offers numerous opportunities for new and improved organic reaction methods.

Chiral Arylated Amines via C-N Coupling of Chiral Amines with Aryl Bromides Promoted by Light

The Buchwald-Hartwig C-N coupling reaction has found widespread applications in organic synthesis. Over the past two decades or so, many improved catalysts have been introduced, allowing various amines and aryl electrophiles to be readily used nowadays. However, there lacks a protocol that could be used to couple a wide range of chiral amines and aryl halides, without erosion of the enantiomeric excess (ee). Reported in this article is a method based on molecular Ni catalysis driven by light, which enables stereoretentive C-N coupling of optically active amines, amino alcohols, and amino acid esters with aryl bromides, with no need for any external photosensitizer. The method is effective for a wide variety of coupling partners, including those bearing functional groups sensitive to bases and nucleophiles, thus providing a viable alternative to accessing synthetically important chiral N-aryl amines, amino alcohols, and amino acids esters. Its viability is demonstrated by 92 examples with up to 99 % ee.

Recent Progress Concerning the N-Arylation of Indoles

This review summarizes the current state-of-the-art procedures in terms of the preparation of N-arylindoles. After a short introduction, the transition-metal-free procedures available for the N-arylation of indoles are briefly discussed. Then, the nickel-catalyzed and palladium-catalyzed N-arylation of indoles are both discussed. In the next section, copper-catalyzed procedures for the N-arylation of indoles are described. The final section focuses on recent findings in the field of biologically active N-arylindoles.

Low-Temperature Nickel-Catalyzed C-N Cross-Coupling via Kinetic Resolution Enabled by a Bulky and Flexible Chiral N-Heterocyclic Carbene Ligand

The transition-metal-catalyzed C-N cross-coupling has revolutionized the construction of amines. Despite the innovations of multiple generations of ligands to modulate the reactivity of the metal center, ligands for the low-temperature enantioselective amination of aryl halides remain a coveted target of catalyst engineering. Designs that promote one elementary reaction often create bottlenecks at other steps. We here report an unprecedented low-temperature (as low as -50 °C), enantioselective Ni-catalyzed C-N cross-coupling of aryl chlorides with sterically hindered secondary amines via a kinetic resolution process (s factor up to >300). A bulky yet flexible chiral N-heterocyclic carbene (NHC) ligand is leveraged to drive both oxidative addition and reductive elimination with low barriers and control the enantioselectivity. Computational studies indicate that the rotations of multiple σ-bonds on the C₂ -symmetric chiral ligand adapt to the changing needs of catalytic processes. We expect this design would be widely applicable to diverse transition states to achieve other challenging metal-catalyzed asymmetric cross-coupling reactions.

Nickelocene as an Air- and Moisture-Tolerant Precatalyst in the Regioselective Synthesis of Multisubstituted Pyridines

Ni(COD)₂-catalyzed cycloaddition reactions to access pyridines have been extensively studied. However, this catalyst typically requires drying procedures and inert-atmosphere techniques for the reactions. Herein, we report operationally simple nickel(0) catalysis to access substituted pyridines from various nitriles and 1,6-diynes without the aid of air-free techniques. The Ni-Xantphos-based catalytic manifold is tolerant to air, moisture, and heat while promoting the [2 + 2 + 2] cycloaddition reactions with high reaction yields and broad substrate scope. In addition, we disclose that not only the steric effect but also the frontier molecular orbital interactions can play a critical role in determining the regiochemical outcome of nickel-catalyzed [2 + 2 + 2] cycloaddition for the synthesis of substituted pyridines.

Nickel-Catalyzed N-Arylation of NH-Sulfoximines with Aryl Halides via Paired Electrolysis

A novel strategy for the N-arylation of NH-sulfoximines has been developed by merging nickel catalysis and electrochemistry (in an undivided cell), thereby providing a practical method for the construction of sulfoximine derivatives. Paired electrolysis is employed in this protocol, so a sacrificial anode is not required. Owing to the mild reaction conditions, excellent functional group tolerance and yield are achieved. A preliminary mechanistic study indicates that the anodic oxidation of a Ni^II species is crucial to promote the reductive elimination of a C-N bond from the resulting Ni^III species at room temperature.

Nickel-Catalyzed N-Arylation of Fluoroalkylamines

The Ni-catalyzed N-arylation of β-fluoroalkylamines with broad scope is reported for the first time. Use of the air-stable pre-catalyst (PAd2-DalPhos)Ni(o-tol)Cl allows for reactions to be conducted at room temperature (25 °C, NaOtBu), or by use of a commercially available dual-base system (100 °C, DBU/NaOTf), to circumvent decomposition of the N-(β-fluoroalkyl)aniline product. The mild protocols disclosed herein feature broad (hetero)aryl (pseudo)halide scope (X=Cl, Br, I, and for the first time phenol-derived electrophiles), encompassing base-sensitive substrates and enantioretentive transformations, in a manner that is unmatched by any previously reported catalyst system.

Nickel(II) Nanoparticles Immobilized on EDTA-Modified Fe3O4@SiO2 Nanospheres as Efficient and Recyclable Catalysts for Ligand-Free Suzuki-Miyaura Coupling of Aryl Carbamates and Sulfamates

A highly efficient and air-, thermal-, and moisture-stable nickel-based catalyst with excellent magnetic properties supported on silica-coated magnetic Fe₃O₄ nanoparticles was successfully synthesized. It was well characterized by Fourier transform infrared spectroscopy, powder X-ray diffraction, transmission electron microscopy, field emission scanning electron microscopy, thermogravimetric analysis, dynamic light scattering (DLS), X-ray photoelectron spectroscopy, vibration sample magnetometry, energy-dispersive X-ray analysis, inductively coupled plasma analysis, and nitrogen adsorption-desorption isotherm analysis. The Suzuki-Miyaura coupling reaction between aryl carbamates and/or sulfamates with arylboronic acids was selected to demonstrate the catalytic activity and efficiency of the as-prepared magnetic nanocatalyst. Using the mentioned heterogeneous nanocatalyst in such reactions generated corresponding products in good to excellent yields in which the catalyst could easily be recovered from the reaction mixture with an external magnetic field to reuse directly for the next several cycles without significant loss of its activity.

Oxidative Addition of Aryl Halides to a Triphosphine Ni(0) Center to Form Pentacoordinate Ni(II) Aryl Species

Oxidative addition of aryl halides to Ni(0) is a ubiquitous elementary step in cross-coupling and related reactions, usually producing a square-planar Ni(II)-aryl intermediate. Here we show that a triphosphine ligand supports oxidative addition at a tris-ligated Ni(0) center to cleanly form stable five-coordinate Ni(II)-aryl compounds. Kinetic and computational studies support a concerted, two-electron mechanism rather than radical halogen abstraction. These results support the idea that oxidative addition to triphosphine Ni(0) species may be more generally involved in Ni/phosphine catalytic systems.

Direct Amination of Aromatic C-H Bonds with Free Amines

Aromatic amines belong to a highly important class of organic compounds which are found in various natural products, functional materials, and pharmaceutical agents. Their prevalence has sparked continuing interest in the development of highly efficient and environmentally benign synthetic strategies for the construction of these compounds. Cross-dehydrogenative coupling reactions between two unmodified C(X)-H bonds have recently emerged as a versatile and powerful strategy for the fabrication of new C(X)-C(X) bonds. In this context, several procedures have been reported for the synthesis of aromatic amines through the direct amination of aromatic C-H bonds with free amines. This review highlights recent advances and progress in this appealing research arena, with special emphasis on the mechanistic features of the reactions.