Nickel-Catalyzed Gas-Free Reductive Carbonylation of Aryl Thianthrenium Salts to Access Aryl Amides and Aryl Thioesters

A nickel-catalyzed site-selective reductive carbonylation of arenes via aryl thianthrenium salts is described. Using Mo(CO)6 as a convenient solid CO source and reductant and employing nitroarenes and sulfonyl chlorides as readily available nitrogen and sulfur sources, a range of aryl amides and aryl thioesters were successfully synthesized in moderate to good yields. The utility of this transformation is demonstrated through the synthesis of antimicrobial agents and the late-stage functionalization of biorelevant molecules.

Pd/NHCs-Catalyzed Denitrative/Dechlorinated N-Arylation of Nitroarenes/Chloroarenes to Hydrazine Derivatives

Palladium-catalyzed denitrative/dechlorinated C-N coupling reactions of nitroaromatics/chloroarenes with hydrazines/hydrazones were performed using sterically bulky N-heterocyclic ligands (NHC=2-aryl-5-(2,4,6-triisopropylphenyl)-2,3-imidazolylidene[1,5-a]pyridines). A range of N-arylbenzophenone hydrazones/hydrazine derivatives were obtained in good to excellent yields. This protocol provides an efficient method for the preparation of hydrazones/hydrazines via C-N coupling of inexpensive nitroarenes and chloroarenes.

Transformative reactions in nitroarene chemistry: C-N bond cleavage, skeletal editing, and N-O bond utilization

Nitroarenes are highly versatile building blocks in organic synthesis, playing a pivotal role in various reactions. Common transformations involving nitroarenes include nucleophilic aromatic substitution (SNAr) reactions, where the nitro group functions both as a potent electron-withdrawing group that activates the aromatic ring and as a leaving group facilitating the substitution. Additionally, the direct transformation of nitro groups, such as reduction-driven syntheses of amines and carboxylic acids, as well as ipso-substitution SNAr reactions, have been extensively explored. Interactions between ortho-nitro groups and neighboring substituents also provide unique opportunities for selective transformations. However, beyond these well-established processes, direct transformations of nitro groups have been relatively limited. In recent years, significant advancements have been made in alternative methodologies for nitro group transformations. This review focuses on the latest progress in novel transformations of nitroarenes, with emphasis on three major categories: (i) functional group transformations involving C-N bond cleavage in nitroarenes, (ii) skeletal editing via nitrene intermediates generated by N-O bond cleavage, and (iii) the utilization of nitroarenes as an oxygen source through N-O bond cleavage. These developments under-score the expanding utility of nitroarenes in modern organic synthesis.

Transition-metal-free photochemical reductive denitration of nitroarenes

We have developed a simple and mild photochemical process for the reductive denitration of nitroarenes under transition-metal-free conditions. This method is compatible with a broad range of functional groups, providing a practical and efficient approach for converting nitroarenes into denitrated arenes. The utility of this protocol is demonstrated through the prompt synthesis of dibenzoxepane.

Substituent-dependent [4+2] or [2+2] cycloadditions of phenylallenyl phosphine oxides with arynes

A [4+2] cycloaddition strategy to assemble phenanthren-9-yldiphenylphosphine oxides is reported. This reaction relies on the strategic use of readily available phenylallenyl phosphine oxides as dienes to participate in [4+2] cycloaddition with arynes. Notably, benzo[b][1,4]oxaphosphinin-4-iums can be controllably synthesized by simply tuning the substituents in the phosphine oxide unit through a [2+2] cycloaddition cascade.

Palladium-Catalyzed Denitrative α-Arylation of Heteroarenes with Nitroarenes via C-H and C-NO2 Bond Activations

A general approach for the α-arylation of heteroarenes with nitroarenes via denitrative coupling is reported for the first time. Various heteroarenes, including derivatives of furan, benzofuran, pyrrole, indole, thiophene, and benzothiophene, can be arylated at the α-position in moderate to good yields. Mechanistic studies demonstrate that the reaction proceeds via a CMD pathway, with C-H bond activation as the rate-determining step. Furthermore, the scalability and applicability in the synthesis of a drug molecule exemplify the utility of this protocol.

Tetrabutylammonium Bromide (TBAB)-Promoted, Pd/Cu-Catalyzed Sonogashira Coupling of N-Tosyl Aryltriazenes

Sonogashira coupling of N-tosyl aryltriazenes is reported to offer arylalkynes in yields up to 92% with the aid of tetrabutylammonium bromide (TBAB) as a dual activator for both the palladium catalyst and aryltriazenes. Common functional groups could be well tolerated, although large electronic effects from alkynes were observed. TBAB-assisted oxidative addition of palladium(0) to aryltriazene instead of in situ formed arylhalide has been proposed to initiate the catalytic cycle.

Palladium-Catalyzed Denitrative Synthesis of Aryl Nitriles from Nitroarenes and Organocyanides

A denitrative cyanation of nitroarenes using organocyanides and a palladium catalyst was developed. The key for this reaction was the utilization of an aminoacetonitrile as a cyano source to avoid the generation of stoichiometric metal- and halogen-containing chemical waste. A wide range of nitroarenes, including heteroarenes and pharmaceutical molecules, can be converted into aryl nitriles.

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.

Direct synthesis of haloaromatics from nitroarenes via a sequential one-pot Mo-catalyzed reduction/Sandmeyer reaction

Herein, we report the direct synthesis of a wide variety of functionalized aromatic bromides, chlorides, iodides, and fluorides from nitroarenes in a sequential one-pot operation. This protocol is based on an air- and moisture-tolerant dioxomolybdenum-catalyzed reduction of nitroaromatics, employing pinacol as a reducing agent, which enables subsequent diazotization and halogenation steps. This methodology represents a step-economical, practical, and alternative procedure for synthesizing haloaromatics directly from nitroaromatics.

Denitrative Mizoroki–Heck reaction of unactivated alkenes

A general palladium-catalyzed denitrative Mizoroki–Heck reaction of unactivated alkenes has been developed with high E/Z selectivity.

Well-Defined, Air- and Moisture-Stable Palladium-Imidazo[1,5-a]pyridin-3-ylidene Complexes: A Versatile Catalyst Platform for Cross-Coupling Reactions by L-Shaped NHC Ligands

We describe the development of [(NHC)Pd(cinnamyl)Cl] complexes of ImPy (ImPy = imidazo[1,5-a]pyridin-3-ylidene) as a versatile class of precatalysts for cross-coupling reactions. These precatalysts feature fast activation to monoligated Pd(0) with 1:1 Pd to ligand ratio in a rigid imidazo[1,5-a]pyridin-3-ylidene template. Steric matching of the C5-substituent and N2-wingtip in the catalytic pocket of the catalyst framework led to the discovery of ImPyMesDipp as a highly reactive imidazo[1,5-a]pyridin-3-ylidene ligand for Pd-catalyzed cross-coupling of nitroarenes by challenging C-NO2 activation. Kinetic studies demonstrate fast activation and high reactivity of this class of well-defined ImPy-Pd catalysts. Structural studies provide full characteristics of this new class of imidazo[1,5-a]pyridin-3-ylidene ligands. Computational studies establish electronic properties of sterically-restricted imidazo[1,5-a]pyridin-3-ylidene ligands. Finally, a scalable synthesis of C5-substituted imidazo[1,5-a]pyridin-3-ylidene ligands through Ni-catalyzed Kumada cross-coupling is disclosed. The method obviates chromatographic purification at any of the steps, resulting in a facile and modular access to ImPy ligands. We anticipate that well-defined [Pd-ImPy] complexes will find broad utility in organic synthesis and catalysis for activation of unreactive bonds.

Construction of Heterobiaryl Skeletons through Pd-Catalyzed Cross-Coupling of Nitroarenes and Heterocyclic Arylborononate Esters with a Sterically Demanding NHC Ligand

The palladium-catalyzed Suzuki-Miyaura cross-couplings of nitroarenes and heteroarylboronate esters has been developed. A number of heterobiaryl compounds containing pyridine, pyrimidine, quinoline, furan, thiophene, and pyrazole were prepared using [Pd(cinnamyl)Cl]₂/2-aryl-5-(2,4,6-triisopropylphenyl)-2,3-imidazolylidene[1,5-a]pyridines as the catalysts in good to excellent yields. The synthetic practicality of this approach is demonstrated through the synthesis of druglike molecules.

Palladium-catalyzed denitrative N-arylation of nitroarenes with pyrroles, indoles, and carbazole

We have developed an efficient palladium-catalyzed denitrative N-arylation via cross-coupling of N–H heteroarenes with nitroarenes, one of the most inexpensive and fundamental feedstocks in the chemical industry. A variety of...

Leveraging the 1,3-azadiene-anhydride reaction for the synthesis of functionalized piperidines bearing up to five contiguous stereocenters

A modular and scalable strategy, which remodels 3-methylglutaric anhydride to 2-oxopiperidines bearing at least three contiguous stereocenters is described. The approach relies on the chemoselective and stereocontrolled annulation of 1,3-azadienes with the anhydride component. The resulting acid-tethered allylic 2-oxopiperidines are then engaged in several selective fragment growth processes, including catalytic denitrative alkenylation, halolactonization, and Vilsmeier-Haack functionalization.

Radical-induced denitration of N-(p-nitrophenyl)propiolamides coupled with dearomatization: access to phosphonylated/trifluoromethylated azaspiro[4.5]-trienones

A robust dearomative denitration of nitroarene derivatives induced by a radical ipso-cyclization process has been developed, delivering valuable phosphonated or trifluoromethylated azaspiro[4.5]trienones with good functional group tolerance. This represents a convenient and powerful approach to activate nitroarenes in a radical manner.

Metal-free enaminone C-N bond cyanation for the stereoselective synthesis of (E)- and (Z)-β-cyano enones

A highly practical method for C-CN bond formation by C-N bond cleavage on enaminones leading to the efficient synthesis of β-cyano enones is developed. The reactions take place efficiently to provide (E)-β-cyano enones with only a molecular iodine catalyst. In addition, the additional employment of oxalic acid enables the selective synthesis of (Z)-β-cyano enones.

Cross-Coupling Reactions of Nitroarenes

Cross-coupling reactions are powerful synthetic tools to construct diverse chemical bonds often found in, for example, advanced materials and pharmaceuticals. Since their discovery, haloarenes have habitually been used as electrophilic coupling partners both in academic and industrial contexts. However, concerning the efficiency and the often-negative environmental impact of haloarene-based cross-coupling processes, more readily available, inexpensive, and environmentally friendly electrophiles have been explored.Nitroarenes, for example, are obtained from the facile nitration of aromatic compounds and, thus, represent one of the most easy-to-access feedstock electrophiles. Furthermore, their electron-deficient arene core can be functionalized easily and site-selectively through a wide variety of reactions. Yet, despite these advantages and even though the direct transformation of the NO₂ group would be an attractive option in cross-coupling chemistry, it has so far remained difficult to convert nitroarenes via a cleavage of the Ar-NO₂ bond given the inherent reactivity (or the lack thereof) of the nitro group. Such denitrative conversion has been performed by a conventional sequence of reduction, diazotization, and Sandmeyer reactions, which severely lacks efficiency and generality.This Account summarizes our recent research progress on cross-coupling reactions that employ nitroarenes as electrophiles. First, we developed the Suzuki-Miyaura coupling of nitroarenes using a palladium/BrettPhos catalyst. This reaction proceeds via an (at the time) unprecedented oxidative addition of the Ar-NO₂ bond, which was supported by experimental results and theoretical calculations. A widely accepted catalytic cycle for Pd-catalyzed cross-couplings has since been extended to include nitroarenes as electrophiles, which significantly increases substrate generality. Second, this denitrative coupling protocol was applied to various bond-forming reactions, namely, Buchwald-Hartwig amination, etherification, and hydrogenation reactions. Such diversification has enhanced the utility of nitroarenes as cross-coupling partners. To develop each reaction, it was necessary to modify the reaction conditions as required to overcome the obstacles deriving from nitro functionality including transmetalation and side reactions, as well as oxidative addition. Third, we designed a new Pd/NHC catalyst that exhibits higher activity than Pd/BrettPhos. The improved performance of Pd/NHC system was supported by its strong electron-donicity and structural robustness, and it allows the reduction of the catalyst loading significantly, thus increasing the efficacy and practicality of this method.The field of nitroarene-based cross-coupling has just started to flourish. In addition to our original work, several research groups have already adopted Pd/BrettPhos or Pd/NHC catalysts to develop new denitrative functionalizations. The utility of nitroarenes in the context of organic synthesis should be now revisited.

Alkylative Dearomatization by Using an Unactivated Aryl Nitro Group as a Leaving Group: Access to Diversified Alkylated Spiro[5.5]trienones

The cleavage of an unactivated aryl nitro group triggered by alkyl radicals enables a dearomative cyclization, affording diversified alkylated spiro[5.5]trienones in good yields. Using readily available compounds (toluene and analogues, alkanes, ethers, ketones, etc.) as alkylating reagents, various alkyls have been implanted into the spirocycles via C(sp³)-H and Ar-NO₂ bond activation with high functional group tolerance. This protocol provides a distinct method for the activation of the aryl nitro group.

Palladium catalyzed C–C and C–N bond forming reactions: an update on the synthesis of pharmaceuticals from 2015–2020

Some of the most prominent and promising catalysts in organic synthesis for the requisite construction of C–C and C–N bonds are palladium (Pd) catalysts, which play a pivotal role in pharmaceutical and medicinal chemistry.

Build-up of double carbohelicenes using nitroarenes: dual role of the nitro functionality as an activating and leaving group

The construction of double carbohelicenes is highly fascinating yet challenging work. Disclosed herein is a streamlined and simplified synthetic route to double carbohelicenes starting from nitroarenes through sequential nitro-activated ortho-C-H arylation, denitrative alkenylation and intramolecular cyclodehydrogenation. In this synthetic strategy, the nitro group plays a dual role namely as a leaving group for the denitrative alkenylation and as an activating group for ortho-C-H arylation, which is distinct from those of aryl halides in a conventional coupling reaction. In this work, the palladium-catalyzed Heck-type alkenylation of nitroarenes has been presented, in which the conventionally inert Ar-NO2 bond is cleaved. This work provides a novel synthetic strategy for polycyclic aromatic hydrocarbons (PAHs).