Cu-Catalyzed Cross-Coupling of Nitroarenes with Aryl Boronic Acids to Construct Diarylamines

Synthesis of Diarylamines via Nitrosonium-Initiated C-N Bond Formation

Electron-rich diarylamines, exemplified by anisole-derived amines, play pivotal roles in process chemistry, pharmaceuticals, and materials. In this study, homo-diarylamines were synthesized directly from the C-H activation of electron-rich arenes by sodium nitrate/trifluoroacetic acid and the successive treatment of iron powder. Mechanistic investigations reveal that nitrosoarene serves as the reaction intermediate, and the formation of the second C-N bond between the resulting nitrosoarene and electron-rich arene is catalyzed by the nitrosonium ion (NO+). Thus, hetero-diarylamines were synthesized using preformed nitrosoarenes and various electron-rich arenes. This reaction complements a range of cross-coupling reactions catalyzed by transition metal catalysts.

CuH-Catalyzed Reductive Coupling of Nitroarenes with Phosphine Oxides for the Direct Synthesis of Phosphamides

A CuH-catalyzed reductive coupling of nitroarenes with phosphine oxides is developed, which produces a series of phosphamides in moderate to excellent yields with good functional group tolerance. Gram-scale synthesis and late-stage modification of nitro-aromatic functional molecule niclosamide are also successfully conducted. The mechanism study shows that the nitro group is transformed after being reduced to nitroso and a nucleophilic addition procedure is involved during the reaction.

Dual role of nitroarenes as electrophiles and arylamine surrogates in Buchwald-Hartwig-type coupling for C-N bond construction

One of the most widely utilized methods for the construction of C(sp2)-N bonds is the transition-metal-catalyzed cross-coupling of aryl halides/boronic acids with amines, known as Ullmann condensation, Buchwald-Hartwig amination, and Chan-Lam coupling. However, aryl halides/boronic acids often require multi-step preparation while generating a large amount of corrosive and toxic waste, making the reaction less attractive. Herein, we present an unprecedented method for the C(sp2)-N formation via Buchwald-Hartwig-type reactions using synthetically upstream nitroarenes as the sole starting materials, thus eliminating the need for arylhalides and pre-formed arylamines. A diverse range of symmetrical di- and triarylamines were obtained in a single step from nitroarenes, and more importantly, various unsymmetrical di- and triarylamines were also highly selectively synthesized in a one-pot/two-step process. Furthermore, the success of the scale-up experiments, the late-stage functionalization of a drug intermediate, and the rapid preparation of hole-transporting material TCTA showcased the utility and practicality of this protocol in synthetic chemistry. Mechanistic studies indicate that this transformation may proceed via an arylamine intermediate generated in situ from the reduction of nitroarenes, which is followed by a denitrative Buchwald-Hartwig-type reaction with another nitroarene to form a C-N bond.

Photoinduced Reduction of Nitroarenes and Tandem C-N Cross-Coupling with Haloarenes

An efficient photocatalytic C-N cross-coupling of nitroarenes with haloarenes has been developed using simple and cheap Ni(acac)2 as a cocatalyst. The reaction is confirmed as a stepwise process: (1) metal free photoinduced reduction of nitroarenes into aniline derivatives and (2) photo- and Ni-catalyzed C-N cross-coupling of anilines with haloarenes. The reaction conditions are simple and mild, giving high-value diarylamines with good to excellent yields and good functional group tolerance.

Reductive coupling of nitro compounds with boronic acid derivatives: an overview

The purpose of this review is to summarize the current literature on reductive C-N coupling of nitro compounds and boronic acids, with special emphasis on the mechanistic features of the reactions. The metal-catalyzed reactions are discussed first. This is followed by electro-synthesis and organophosphorus-catalyzed reactions. Finally, the available examples of catalyst-free reactions will be covered at the end of this review.

Catalytic ipso-Nitration of Organosilanes Enabled by Electrophilic N-Nitrosaccharin Reagent

Nitroaromatic compounds represent one of the essential classes of molecules that are widely used as feedstock for the synthesis of intermediates, the preparation of nitro-derived pharmaceuticals, agrochemicals, and materials on both laboratory and industrial scales. We herein disclose the efficient, mild, and catalytic ipso-nitration of organotrimethylsilanes, enabled by an electrophilic N-nitrosaccharin reagent and allows chemoselective nitration under mild reaction conditions, while exhibiting remarkable substrate generality and functional group compatibility. Additionally, the reaction conditions proved to be orthogonal to other common functionalities, allowing programming of molecular complexity via successive transformations or late-stage nitration. Detailed mechanistic investigation by experimental and computational approaches strongly supported a classical electrophilic aromatic substitution (SE Ar) mechanism, which was found to proceed through a highly ordered transition state.

Direct Synthesis of Unprotected Indolines Through Intramolecular sp3 C-H Amination Using Nitroarenes as Aryl Nitrene Precursors

Given the prevalence of molecules containing nitro groups in organic synthesis, innovative methods to expand the reactivity of this functional group are of interest in both industrial and academic settings. In this report, a metal-free intramolecular benzylic sp3 C-H amination is disclosed using aryl nitro compounds as aryl nitrene precursors. Organosilicon reagent N,N'-bis(trimethylsilyl)-4,4'-bipyridinylidene (Si-DHBP) served as an efficient reductant in the transformation, enabling the in situ generation of aryl nitrene species for the direct, metal-free synthesis of unprotected 2-arylindolines from the corresponding nitroarene compounds.

Reductive Arylation of Nitroarenes with Chloroarenes: Reducing Conditions Enable New Reactivity from Palladium Catalysts

Palladium-catalyzed C-N bond forming reactions are a key tool in modern synthetic organic chemistry. Despite advances in catalyst design enabling the use of a variety of aryl (pseudo)halides, the necessary aniline coupling partner is often synthesized in a discrete reduction step from a nitroarene. An ideal synthetic sequence would avoid the necessity of this step while maintaining the reliable reactivity of palladium catalysis. Herein, we describe how reducing conditions enable new chemical steps and reactivity from well-studied palladium catalysts, resulting in a new, useful transformation: the reductive arylation of nitroarenes with chloroarenes to form diarylamines. Mechanistic experiments suggest that under reducing conditions, BrettPhos-palladium complexes catalyze the dual N-arylation of typically inert azoarenes─generated via the in situ reduction of nitroarenes─via two distinct mechanisms. Initial N-arylation proceeds via a novel association-reductive palladation sequence followed by reductive elimination to yield an intermediate 1,1,2-triarylhydrazine. Arylation of this intermediate by the same catalyst via a traditional amine arylation sequence forms a transient tetraarylhydrazine, unlocking reductive N-N bond cleavage to liberate the desired product. The resulting reaction allows for the synthesis of diarylamines bearing a variety of synthetically valuable functionalities and heteroaryl cores in high yield.

P(III)-Promoted Reductive Coupling of Aromatic and Aliphatic Nitro Compounds with Grignard Reagents

A phosphine-promoted reductive coupling of nitro compounds with Grignard reagents is described. Polyfunctional and pharmaceutically relevant diarylamines were generated by this reaction in moderate to high yields. Aliphatic nitro compounds that are highly challenging substrates undergo a combination of α-arylation and reductive coupling to afford the α-arylated arylamines efficiently. A series of valuable biaryl compounds with polyfluorinated and heteroaryl rings are co-generated in 56-94% yields.

Azide-Alkyne Cycloaddition Catalyzed by Copper(I) Coordination Polymers in PPM Levels Using Deep Eutectic Solvents as Reusable Reaction Media: A Waste-Minimized Sustainable Approach

Two air-stable copper(I)-halide coordination polymers 1 and 2 with NNS and NNO ligand frameworks were synthesized and successfully utilized as efficient catalysts in an important organic reaction, namely, copper-catalyzed azide-alkyne cycloaddition, which is generally conducted in a mixture of water and organic solvents. The azide-alkyne "click" reaction was successfully conducted in pure water at r.t. under aerobic conditions. Other green solvents, including ethanol and glycerol, were also effectively used. Finally, deep eutectic solvents as green and sustainable reaction media were successfully utilized. In deep eutectic solvents, complete conversion with excellent isolated yield was achieved in a short period of time (1 h) with low catalyst loading (1 mol %) at r.t. Full conversion could also be achieved within 24 h with ppm-level (50 ppm) catalyst loading at 70 °C. Optimized reaction conditions were used for the syntheses of a large number of 1,4-disubstituted 1,2,3-triazoles with various functionalities. Triazole products were easily isolated by simple filtration. The reaction media, such as water and deep eutectic solvents, were recovered and recycled in three consecutive runs. The limited waste production is reflected in a very low E-factor (0.3-2.8). Finally, the CHEM21 green metrics toolkit was employed to evaluate the sustainability credentials of different optimized protocols in various green solvents such as water, ethanol, glycerol, and deep eutectic solvents.

From N-H Nitration to Controllable Aromatic Mononitration and Dinitration-The Discovery of a Versatile and Powerful N-Nitropyrazole Nitrating Reagent

Nitroaromatics are tremendously valuable organic compounds with a long history of being used as pharmaceuticals, agrochemicals, and explosives as well as vital intermediates to a wide variety of chemicals. Consequently, the exploration of aromatic nitration has become an important endeavor in both academia and industry. Herein, we report the identification of a powerful nitrating reagent, 5-methyl-1,3-dinitro-1H-pyrazole, from the N-nitro-type reagent library constructed using a practical N-H nitration method. This nitrating reagent behaves as a controllable source of the nitronium ion, enabling mild and scalable nitration of a broad range of (hetero)arenes with good functional group tolerance. Of note, our nitration method could be controlled by manipulating the reaction conditions to furnish mononitrated or dinitrated product selectively. The value of this method in medicinal chemistry has been well established by its efficient late-stage C-H nitration of complex biorelevant molecules. Density functional theory (DFT) calculations and preliminary mechanistic studies reveal that the powerfulness and versatility of this nitrating reagent are due to the synergistic "nitro effect" and "methyl effect".

Decarboxylative tandem C-N coupling with nitroarenes via SH2 mechanism

Aromatic tertiary amines are one of the most important classes of organic compounds in organic chemistry and drug discovery. It is difficult to efficiently construct tertiary amines from primary amines via classical nucleophilic substitution due to consecutive overalkylation. In this paper, we have developed a radical tandem C-N coupling strategy to efficiently construct aromatic tertiary amines from commercially available carboxylic acids and nitroarenes. A variety of aromatic tertiary amines can be furnished in good yields (up to 98%) with excellent functional group compatibility under mild reaction conditions. The use of two different carboxylic acids also allows for the concise synthesis of nonsymmetric aromatic tertiary amines in satisfactory yields. Mechanistic studies suggest the intermediacy of the arylamine–(TPP)Fe(III) species and might provide a possible evidence for an S_H2 (bimolecular homolytic substitution) pathway in the critical C-N bond formation step.

Aromatic tertiary amines are versatile building blocks in organic synthesis. In this article, the authors report on an iron-catalysed reaction for the decarboxylative C-N coupling from carboxylic acids and nitroarenes, leading to non-symmetric tertiary aromatic amines.

Light-Promoted Low-Valent-Tungsten-Catalyzed Ambient Temperature Amination of Boronic Acids with Nitroaromatics

Triggering C-N bond formation with nitroaromatics and boronic acids at mild conditions is highly desirable, since most prior works were carried out under harsh conditions and sometimes suffered from poor chemo- or regioselectivity. Herein, a low-valent-tungsten-catalyzed reaction that enables the ambient temperature amination of boronic acids with nitroaromatics is disclosed. With readily available W(CO)₆ as a precatalyst under external-photosensitizer-free conditions, nitroaromatics smoothly undergo C-N coupling reactions with their boronic acid partners, delivering structurally diverse secondary amines in good yields (>50 examples, yields up to 96%). This methodology is both scalable and highly chemoselective and engages both aliphatic and aromatic boronic acid partners. The catalysis is initiated by the deoxygenation of nitroaromatics by a trans-[W(CO)₄(PPh₃)₂] (trans-W, PPh₃ = triphenylphosphine) complex, which forms in situ via ligand replacement.

Application of Raw and Chemically Modified Biomasses for Heterogeneous Cu-Catalysed Conversion of Aryl boronic Acids to Phenols Derivatives

This work describes the application of raw and chemically modified cellulose and sugarcane bagasse for ipso-hydroxylation of aryl boronic acids in environmentally friendly reaction conditions. The catalytic efficiency of five support-[Cu] materials was compared in forming phenols from aryl boronic acids. Our investigation highlights that the CEDA-[Cu] material (6-deoxy-6-aminoethyleneamino cellulose loaded with Cu) leads to the best results under very mild reaction conditions. The optimized catalytic sequence, allowing a facile transformation of boronic acids to phenols, required the mandatory and joint presence of the support, Cu2O, and KOH at room temperature. CEDA-[Cu] was characterized using 13C solid-state NMR, ICP, and FTIR. The use of CEDA-[Cu] accounts for the efficacious synthesis of variously substituted phenol derivatives and presents very good recyclability after five catalytic cycles.

Iron-catalyzed one-pot cyclization and amination of 2-alkynylthioanisoles using nitrosobenzenes as the amine source

A direct strategy for the synthesis of 3-phenylaminobenzothiophene via iron-catalyzed cyclization of 2-alkynylthioanisoles and nitrosoarenes is presented in this work.

Ruthenium-catalyzed reaction of diazoquinones with arylamines to synthesize diarylamines

The diarylamine scaffold is common in bioactive molecules. Herein, we report a Ru(ii)-catalyzed C–N cross-coupling reaction of diazoquinones with arylamines, which provides access to a range of functionalized diarylamines in 43–97% yields.

Copper-promoted cross-coupling of nitroarenes with 4-alkyl-1,4-dihydropyridines using a peroxide-driven radical reductive strategy

Direct radical-mediated reductive coupling of nitroarenes with 4-alkyl-1,4-dihydropyridines to build the C(sp3)–N bond using 4-alkyl-1,4-dihydropyridines as internal reducing agents and alkyl sources is presented.