Reductive Denitration of Nitroarenes

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.

Palladium-Catalyzed Deuteration of Arylketone Oxime Ethers

We report herein the development of palladium-catalyzed deacylative deuteration of arylketone oxime ethers. This protocol features excellent functional group tolerance, heterocyclic compatibility, and high deuterium incorporation levels. Regioselective deuteration of some biologically important drugs and natural products are showcased via Friedel-Crafts acylation and subsequent deacylative deuteration. Vicinal meta-C-H bond functionalization (including fluorination, arylation, and alkylation) and para-C-H bond deuteration of electro-rich arenes are realized by using the ketone as both directing group and leaving group, which is distinct from aryl halide in conventional dehalogenative deuteration.

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.

Synthesis of Functionalized Pyrrolidinone Scaffolds via Smiles-Truce Cascade

Arylsulfonamides have been found to react with cyclopropane diesters under simple base treatment to give α-arylated pyrrolidinones. This one-pot process comprises three steps: nucleophilic ring-opening of the cyclopropane, reaction of the resulting enolate in a Smiles-Truce aryl transfer, and lactam formation. The reaction represents a new, operationally simple approach to biologically active pyrrolidinones and expands Smiles-Truce arylation methods to encompass sp3 electrophilic centers in cascade processes.

Denitrative Hydroxylation of Unactivated Nitroarenes

A one-step method for the conversion of nitroarenes into phenols under operationally simple, transition-metal-free conditions is described. This denitrative functionalization protocol provides a concise and economical alternative to conventional three-step synthetic sequences. Experimental and computational studies suggest that nitroarenes may be substituted by an electron-catalysed radical-nucleophilic substitution (S_RN 1) chain mechanism.

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.

Synthesis and Properties of Pyridine-Fused Triazolylidene-Palladium: Catalyst for Cross-Coupling Using Chloroarenes and Nitroarenes

The synthesis and catalytic activity of pyridine-fused triazolylidene as a novel abnormal N-heterocyclic carbene (aNHC) ligand is described. The evaluation of physical properties using X-ray crystallographic analysis and infrared spectroscopy revealed that these triazolylidenes have a high electron-donating ability toward the metal center. The application of this triazolylidene to the palladium-catalyzed cross-coupling of chloroarenes and nitroarenes with arylboronic acids showcased its ability to activate C-Cl and C-NO₂ bonds.

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.

A modular and scalable strategy, which converts 3-methylglutaric anhydride to 2-oxopiperidines bearing at least three contiguous stereocenters is described.

Rhodium-catalysed ortho-alkynylation of nitroarenes

The ortho-alkynylation of nitro-(hetero)arenes takes place in the presence of a Rh(iii) catalyst to deliver a wide variety of alkynylated nitroarenes regioselectively. These interesting products could be further derivatized by selective reduction of the nitro group or palladium-catalysed couplings. Experimental and computational mechanistic studies demonstrate that the reaction proceeds via a turnover-limiting electrophilic C-H metalation ortho to the strongly electron-withdrawing nitro group.

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.

2-Fluoro-5-nitrophenyldiazonium: A Novel Sanger-Type Reagent for the Versatile Functionalization of Alcohols

As a novel Sanger-type reagent, 2-fluoro-5-nitrophenyldiazonium tetrafluoroborate enabled the versatile functionalization of primary and secondary aliphatic alcohols. Based on a mild nucleophilic aromatic substitution of the fluorine atom under unprecedented, base-free conditions, the diazonium unit on the aromatic core of the resulting aryl-alkyl ether could be employed for such diverse transformations as radical C-H activation and cyclization, as well as palladium catalyzed cross-coupling reactions.

Ni-catalyzed reductive decyanation of nitriles with ethanol as the reductant

Ethanol, as an environmentally friendly and renewable reagent, was applied as a hydride donor to convert C–CN to C–H bonds in this nickel-catalyzed protocol.

Etherification of phenols by amines via transient diazonium intermediates

In this paper, the synthesis of alkyl aryl ethers from electron poor phenols and amines, using 1,3-propanedinitrite, is described. Due to the mild conditions, functionalized primary, secondary, and tertiary alkyl groups were successfully introduced, denoting a highly tolerant process that allows for unprotected alcohols and acetals. The reaction is thought to proceed through the formation of a diazonium intermediate that undergoes subsequent SN2 or SN1 reactions.

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).

Palladium-catalyzed denitrative Sonogashira-type cross-coupling of nitrobenzenes with terminal alkynes

Described herein is a palladium-catalyzed cross-coupling reaction between nitroarenes and terminal alkynes, offering a facile method for C(sp²)–C(sp) bond formation.

Copper-catalyzed synthesis of sulfonamides from nitroarenes via the insertion of sulfur dioxide

A three-component reaction of arylboronic acids, nitroarenes, and potassium metabisulfite under copper catalysis proceeds smoothly, giving rise to a range of sulfonamides in good to excellent yields with broad substrate scope.

Regioselective Functionalization of 9,9-Dimethyl-9-silafluorenes by Borylation, Bromination, and Nitration

Despite the utility of 9-silafluorenes as functional materials and as building blocks, methods for efficient functionalization of their backbone are rare, probably because of the presence of easily cleavable C-Si bonds. Although controlling the regioselectivity of iridium-catalyzed direct borylation of C-H bonds is difficult, we found that bromination and nitration of 2-methoxy-9-silafluorene under mild conditions occurred predominantly at the electron-rich position. The resulting product having methoxy and bromo groups can be utilized as a building block for the synthesis of unsymmetrically substituted 9-silafluorene-containing π-conjugated molecules.

Pd/NHC-catalyzed cross-coupling reactions of nitroarenes

N-Heterocyclic carbene (NHC) ligands effective for the cross-coupling of nitroarenes were identified.

Nickel-catalyzed reductive defunctionalization of esters in the absence of an external reductant: activation of C–O bonds

The nickel-catalyzed reductive cleavage of esters in the absence of an external reductant, which involves the cleavage of an inert acyl C–O bond in O-alkyl esters is reported.

Synthesis of N-arylsulfonamides through a Pd-catalyzed reduction coupling reaction of nitroarenes with sodium arylsulfinates

A novel one-step direct reductive coupling reaction between nitroarenes and sodium arylsulfinates was realized in the presence of an inexpensive Pd/C catalyst. In this procedure, readily available nitroarenes are employed as the nitrogen sources, and sodium arylsulfinates serve as both coupling partners and reductants. The method features high efficiency by using cheap Pd/C with low catalyst loading and good functional group tolerance in the absence of any additional reductants or ligands. This facile and mild synthetic method enables the high efficiency synthesis of functionalized N-arylsulfonamides from readily available substrates.