Chromium-Catalyzed Activation of Acyl C–O Bonds with Magnesium for Amidation of Esters with Nitroarenes

Intramolecular Reductive Amidation of Unactivated Esters with Nitroarenes: A Telescoped Synthesis of Tetrahydropyrrolo/Pyrido[1,2-a]quinoxalinones

A reductive amidation of unactivated esters with nitroarenes, a key step in the telescopic synthesis of tetrahydropyrrolo[1,2-a]quinoxalinones and tetrahydropyrido[1,2-a]quinoxalinones, is reported. The process involves an intermolecular base-mediated SNAr reaction, followed by intramolecular reductive amidation employing sodium dithionite. The substrate scope coupled with the demonstration of the synthesis of pharmaceuticals is reported. The key features include nitro reduction at room temperature, easy purification without chromatography, amidation of unactivated esters without any externally added activating agent, and a telescopic process.

Direct Synthesis of α-Ketoamides via Copper-Catalyzed Reductive Amidation of Nitroarenes with α-Oxocarboxylic Acids

Nitroarenes are known for their stability, low toxicity, easy availability, and cost-effectiveness, making them one of the most fundamental chemical feedstocks. The direct utilization of nitroarenes as nitrogen sources in amidation reactions offers significant advantages over using arylamines. Herein, we disclose a streamlined method for constructing α-ketoamides through the direct coupling of nitroarenes with α-oxocarboxylic acids. This transformation obviates the need for preparing, isolating, and purifying arylamines, leading to improved efficiency, cost-effectiveness, and time savings.

Deoxyfluorinated amidation and esterification of carboxylic acid by pyridinesulfonyl fluoride

Amide bond synthesis is one of the most used reactions in medicinal chemistry. We report an amide bond formation reaction through deoxyfluorinated carboxylic acids under mild conditions using 2-pyridinesulfonyl fluoride. The reaction procedure has been used in a one-pot synthesis of amides and esters via in situ generation of acyl fluoride. This one-pot synthetic method provides easy access to amides and esters. Using this method, we have sequentially synthesized a tetrapeptide and calceolarioside-B glycoside derivative with good yields.

Iron-Mediated Reductive Amidation of Triazine Esters with Nitroarenes

A reductive amidation of triazine esters with nitroarenes by using cheap iron as a reducing metal in the presence of TMSCl in DMF was developed. The reactions proceeded efficiently under transition metal-free conditions to give the corresponding amides in moderate to good yields with good functional group compatibility. Preliminary mechanistic investigations indicated that nitrosobenzene, N-phenyl hydroxylamine, azoxybenzene, azobenzene, aniline, and N-arylformamide possibly served as the intermediates of the reaction.

Borane-Pyridine: An Efficient Catalyst for Direct Amidation

Borane-pyridine acts as an efficient (5 mol%) liquid catalyst, providing improved solubility for the direct amidation of a wide range of aromatic and aliphatic carboxylic acids and amines to form secondary and tertiary carboxamides. Tolerance of potentially incompatible halo, nitro, and alkene functionalities has been demonstrated.

Reductive transamidation of tertiary amides with nitroarenes enabled by magnesium and chlorosilane

Reported here is the reductive transamidation of tertiary amides with nitroarenes promoted by main group metal magnesium and chlorosilane. The reaction uses commercially available and air-stable nitroarenes as nitrogen sources, so it can occur under transition-metal- and ligand-free conditions, thus providing a step-economic and cost-effective strategy for forming centrally important secondary amides. Several biologically interesting amide motifs are readily accessible by the Mg-promoted reductive transamidation.

Reductive cross-coupling of N-acyl pyrazole and nitroarene using tetrahydroxydiboron: synthesis of secondary amides

We report on a new method for the synthesis of amides using acyl pyrazoles and nitroarenes under reducing conditions. It was found that acyl pyrazoles react with organo-nitro compounds in the presence of B2(OH)4, giving the corresponding amides in good yields. We demonstrated that benzoyl pyrazoles having various substituents and nitroarenes with different substituents can be used to produce a range of N-substituted benzamides. The method shows good functional group tolerance and has potential application in the synthesis of a variety of organic molecules.

Streamlining the synthesis of amides using Nickel-based nanocatalysts

The synthesis of amides is a key technology for the preparation of fine and bulk chemicals in industry, as well as the manufacture of a plethora of daily life products. Furthermore, it constitutes a central bond-forming methodology for organic synthesis and provides the basis for the preparation of numerous biomolecules. Here, we present a robust methodology for amide synthesis compared to traditional amidation reactions: the reductive amidation of esters with nitro compounds under additives-free conditions. In the presence of a specific heterogeneous nickel-based catalyst a wide range of amides bearing different functional groups can be selectively prepared in a more step-economy way compared to previous syntheses. The potential value of this protocol is highlighted by the synthesis of drugs, as well as late-stage modifications of bioactive compounds. Based on control experiments, material characterizations, and DFT computations, we suggest metallic nickel and low-valent Ti-species to be crucial factors that makes this direct amide synthesis possible.

A sustainable metal and base-free direct amidation of esters using water as a green solvent

Herein, we report a simple and efficient synthetic approach for direct amidation of esters via C(acyl)-O bond cleavage without any additional reagents or catalysts, using only water as a green solvent. Subsequently, the reaction byproduct is recovered and utilized for the next phase of ester synthesis. This method emphasized metal-free, additive-free, and base-free characteristics making it a new, sustainable, and eco-friendly way to realize direct amide bond formation. In addition, the synthesis of the drug molecule diethyltoluamide and the Gram-scale synthesis of a representative amide are demonstrated.

B2(OH)4-Mediated Reductive Transamidation of N-Acyl Benzotriazoles with Nitro Compounds En Route to Aqueous Amide Synthesis

We herein developed a reductive transamidation reaction between N-acyl benzotriazoles (AcBt) and organic nitro compounds or NaNO₂ under mild conditions. This protocol employed the stable and readily available B₂(OH)₄ as the reducing agent and H₂O as the ideal solvent. N-Deuterated amides can be synthesized when conducting the reaction in D₂O. A reasonable reaction mechanism involving bond metathesis between the AcBt amide and amino boric acid intermediate was proposed to explain the unique nature of AcBt.

Nickel-Catalyzed Aminocarbonylation of Aryl Iodides with 1 atm CO

Reported here is a nickel-catalyzed aminocarbonylation of aromatic iodides with (hetero)aryl anilines and alkyl amines under atmospheric CO pressure. The reaction features with broad substrate scope with excellent functional group tolerance, providing an expedient method for the construction of amide analogues. Notably, amino alcohols can be selectively transformed into the corresponding amides successfully without interfering the hydroxyl group under the current standard conditions.

Mechanistic Understanding of KOtBu-Mediated Direct Amidation of Esters with Anilines: An Experimental Study and Computational Approach

A sustainable and cost-effective protocol has been reported for the synthesis of amide bonds from unactivated esters and non-nucleophilic amines promoted by potassium tert -butoxide under aerobic conditions. The reaction proceeds under relatively mild conditions, encompassing wide substrate scope. A combined experimental and quantum chemical study has been performed to shed light on the mechanism, which implied that a radical pathway is operating for the present protocol.

Palladium-Catalysed Reductive Aminocarbonylation of Aryl Bromides and Iodides with Nitroarenes

Amide functional groups are a structural feature in a vast array of beneficial organic molecules. This has resulted in a surge in new methodologies developed to enable access to this functional group using a broad range of coupling partners. Herein, we report a palladium-catalysed reductive aminocarbonylation of aryl bromides and iodides with nitroarenes to afford the respective amide products. The developed protocol employs Mo(CO)6 as a carbonyl source and a combination of Zn and TMSCl as co-reducing agents. For most substrates, the anticipated amide products were obtained in modest to high amide product yields.

Mild Amide Synthesis Using Nitrobenzene under Neutral Conditions

Amide synthesis is one of the most important transformations in organic chemistry due to the broad application in pharmaceutical drugs and organic materials. In this report, we describe a mild protocol for amide formation using the readily available nitroarenes as nitrogen sources and an inexpensive iron complex as a catalyst. Because of the use of the pH-neutral conditions and the avoidance of the strong oxidant or reductant, a wide range of aromatic and aliphatic aldehydes as well as nitroarenes with various functional groups could be tolerated well. A plausible mechanism is proposed based on the detailed studies, in which iron catalyst initiates the radical process and the solvent plays a key role as O-atom acceptor.

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.

Nickel-Catalyzed Arylcarbamoylation of Alkenes of N-(o-Iodoaryl)acrylamides with Nitroarenes via Reductive Aminocarbonylation: Facile Synthesis of Carbamoyl-Substituted Oxindoles

Nickel-catalyzed arylcarbamoylation reactions of alkenes of N-(o-haloaryl)acrylamides with CO and nitroarenes via reductive aminocarbonylation to produce carbamoyl-substituted oxindoles with an all-carbon quaternary stereogenic center are presented. Starting with N-(o-haloaryl)acrylamides, simple CO, and inexpensive nitroarenes and using a Ni catalyst, a dinitrogen-based ligand, a Zn reductant, a TMSCl additive, and a base system, this protocol enables the synthesis of various carbamoyl-substituted oxindoles and allows the efficient late-stage derivatization of valuable molecules.

Nickel-Catalyzed Reductive Cross-Coupling of N-Acyl and N-Sulfonyl Benzotriazoles with Diverse Nitro Compounds: Rapid Access to Amides and Sulfonamides

Herein we report a Ni-catalyzed reductive transamidation of conveniently available N-acyl benzotriazoles with alkyl, alkenyl, and aryl nitro compounds, which afforded various amides with good yields and a broad substrate scope. The same catalytic reaction conditions were also applicable for N-sulfonyl benzotriazoles, which could undergo smooth reductive coupling with nitroarenes and nitroalkanes to afford the corresponding sulfonamides.

Mo-Catalyzed One-Pot Synthesis of N-Polyheterocycles from Nitroarenes and Glycols with Recycling of the Waste Reduction Byproduct. Substituent-Tuned Photophysical Properties

A catalytic domino reduction-imine formation-intramolecular cyclization-oxidation for the general synthesis of a wide variety of biologically relevant N-polyheterocycles, such as quinoxaline- and quinoline-fused derivatives, and phenanthridines, is reported. A simple, easily available, and environmentally friendly dioxomolybdenum(VI) complex has proven to be a highly efficient and versatile catalyst for transforming a broad range of starting nitroarenes involving several redox processes. Not only is this a sustainable, step-economical as well as air- and moisture-tolerant method, but also it is worth highlighting that the waste byproduct generated in the first step of the sequence is recycled and incorporated in the final target molecule, improving the overall synthetic efficiency. Moreover, selected indoloquinoxalines have been photophysically characterized in cyclohexane and toluene with exceptional fluorescence quantum yields above 0.7 for the alkyl derivatives.

Direct Access to Amides from Nitro-Compounds via Aminocarbonylation and Amidation Reactions: A Minireview

The ubiquity of the amide bond in functional molecules including proteins, natural products, pharmaceuticals, agrochemicals and materials provides impetus to design and develop newer strategies for the generation of this linkage. Owing to growing awareness about sustainability and development of benign strategies, the traditional route of synthesis of amides via reaction between carboxylic acids and amines in the presence of stoichiometric amount of coupling reagents is tagged to be harsh and wasteful. In one of the unconventional routes, nitro compounds are used directly as amine surrogates for preparing amides mostly via aminocarbonylation and amidation reactions. Typically, such processes involves nitroarenes owing to their propensity to transform into nitroso, hydroxylamine, diazo, hydrazine or aniline intermediates in situ under the influence of suitable catalyst or oxidant. This short review provides the comprehensive overview of these reactions including insight into the scope and their mechanisms.

Mechanistic Diversity of Low-Valent Chromium Catalysis: Cross-Coupling and Hydrofunctionalization

ConspectusTransition-metal catalysis has traditionally been dominated by precious metals because of their high reactivity toward chemical transformations. As a cost-effective alternative, catalysis by earth-abundant group 6 metal chromium is underdeveloped, and its reactivity remains largely unexplored, although the industrially important Phillips catalyst, which is composed of Cr as the active metal, is currently used to supply almost 40% of the total world demand for high-density polyethylene. Cr has traditionally served in organoreagents with high-valent states (≥2+), which are typified by reactions involving Nozaki-Hiyama-Kishi (NHK) and Takai-Utimoto one-electron transfer processes. Given that low-valent metals usually facilitate the process of oxidative addition (OA), studying the catalysis of Cr in the low-valent state provides the opportunity to develop new transformations. However, probably because of the low stability of reactive low-valent Cr or the lack of catalytic activity of structurally stable complexes, there has been limited success with respect to developing catalysis promoted by low-valent Cr. In recent years, our group has probed the reactivity of low-valent Cr in catalysis by adopting a strategy of forming reactive Cr in situ. In this Account, we detail our efforts to study the catalytic behavior and mechanism of low-valent Cr in challenging transformations, such as the cleavage of chemically inert bonds for the cross-coupling and hydrofunctionalization of arenes and nitro motifs, by developing strategies to address the prominent selectivity issues. We highlight the finding that low-valent Cr, being formed in situ, possesses the intriguing ability to promote the catalytic cleavage of unactivated C-O, C-N, and C-H bonds to achieve the Kumada couplings and even to enable challenging cross-coupling between two unactivated C(aryl)-O/C(aryl)-N bonds. During these catalytic processes, Cr usually adopts a high-spin state to interact with chemicals, allowing for insertion into unactivated σ-bonds. The OA catalytic model involving a two-electron process for the cleavage of unactivated bonds has rarely been considered for Cr. We highlight the finding that Cr allows for the breakage of two chemically inert bonds in one catalytic cycle. This ability is intriguing because most transition metals are suitable only for the cleavage of one unactivated bond in catalysis. Mechanisms involving two-electron OA for Cr are unusual, with processes involving one-electron transfer more often proposed, as exemplified in the NHK reactions. These reactions provide efficient strategies for forming functionalized benzaldehydes, amides, anilines, and amines, usually with high levels of selectivity. We hope that this account will extend the scope of cognition to Cr catalysis.

Chromium-Catalyzed Selective Dimerization/Hydroboration of Allenes to Access Boryl-Functionalized Skipped (E,Z)-Dienes

A chromium-catalyzed dimerization/hydroboration of allenes is developed to access synthetically versatile boryl-functionalized skipped dienes with a catalyst generated in situ from CrCl₂ and a pyridine-2,6-diimine ligand ^mes PDI. A variety of allenes reacted with pinacolborane (HBpin) to afford the corresponding boryl-functionalized (E,Z)-1,4-dienes in high yields and with excellent selectivity. Electron paramagnetic resonance (EPR) spectroscopic studies suggest that this chromium-catalyzed reaction probably proceeds through a chromium(I) hydride intermediate.

PCl3-mediated transesterification and aminolysis of tert-butyl esters via acid chloride formation

A PCl3-mediated conversion of tert-butyl esters into esters and amides in one-pot under air is developed. This novel protocol is highlighted by the synthesis of skeletons of bioactive molecules and gram-scale reactions. Mechanistic studies revealed that this transformation involves the formation of an acid chloride in situ, which is followed by reactions with alcohols or amines to afford the desired products.

Decarboxylative/Oxidative Amidation of Aryl α-Ketocarboxylic Acids with Nitroarenes and Nitroso Compounds in Aqueous Medium

The decarboxylative/oxidative amidation of aryl α-ketocarboxylic acids with 5-aryl-3-nitroisoxazole-4-carboxylates and substituted dinitrobenzenes under oxidative aqueous conditions to afford N-aryl amides is described. The reaction is suggested to proceed via a radical pathway in which a benzoyl nitroxyl radical, the key intermediate formed from reaction between nitroarene and benzoyl radical from glyoxalic acid, couples with hydroxyl radical from water to produce amide. Mechanistic insight allowed the scope of the strategy to be expanded to the synthesis of amides via reaction between aryl α-ketocarboxylic acids and nitroso compounds.

Non-Classical Amide Bond Formation: Transamidation and Amidation of Activated Amides and Esters by Selective N–C/O–C Cleavage

In the past several years, tremendous advances have been made in non-classical routes for amide bond formation that involve transamidation and amidation reactions of activated amides and esters. These new methods enable the formation of extremely valuable amide bonds via transition-metal-catalyzed, transition-metal-free, or metal-free pathways by exploiting chemoselective acyl C–X (X = N, O) cleavage under mild conditions. In a broadest sense, these reactions overcome the formidable challenge of activating C–N/C–O bonds of amides or esters by rationally tackling nN → π*C=O delocalization in amides and nO → π*C=O donation in esters. In this account, we summarize the recent remarkable advances in the development of new methods for the synthesis of amides with a focus on (1) transition-metal/NHC-catalyzed C–N/C–O bond activation, (2) transition-metal-free highly selective cleavage of C–N/C–O bonds, (3) the development of new acyl-transfer reagents, and (4) other emerging methods.

1 Introduction

2 Transamidation of Amides

2.1 Transamidation by Metal–NHC Catalysis (Pd–NHC, Ni–NHC)

2.2 Transition-Metal-Free Transamidation via Tetrahedral Intermediates

2.3 Reductive Transamidation

2.4 New Acyl-Transfer Reagents

2.5 Tandem Transamidations

3 Amidation of Esters

3.1 Amidation of Esters by Metal–NHC Catalysis (Pd–NHC, Ni–NHC)

3.2 Transition-Metal-Free Amidation of Esters via Tetrahedral Intermediates

3.3 Reductive Amidation of Esters

4 Transamidations of Amides by Other Mechanisms

5 Conclusions and Outlook

Solvent-Free Iron(III) Chloride-Catalyzed Direct Amidation of Esters

Amide functional groups are prominent in a broad range of organic compounds with diverse beneficial applications. In this work, we report the synthesis of these functional groups via an iron(iii) chloride-catalyzed direct amidation of esters. The reactions are conducted under solvent-free conditions and found to be compatible with a range of amine and ester substrates generating the desired amides in short reaction times and good to excellent yields at a catalyst loading of 15 mol%.

Step and redox efficient nitroarene to indole synthesis

A step and redox efficient nitroarene to indole synthesis was herein developed, in sharp contrast to the rich literature on the construction of indoles. Elemental Zinc was found to be best terminal reductant.

Direct Amidation of Carboxylic Acids with Nitroarenes

N-Aryl amides are an important class of compounds in pharmaceutical and agrochemical chemistry. Rapid and low-cost synthesis of N-aryl amides remains in high demand. Herein, we disclose an operationally simple process to access N-aryl amides directly from readily available nitroarenes and carboxylic acids as coupling substrates. This method involves the in situ activation of carboxylic acids to acyloxyphosphonium salt for one-pot amidation, without the need for isolation of the corresponding synthetic intermediates. Furthermore, the ease of preparation and workup allow the quick and efficient synthesis of a wide range of N-aryl amides, including several amide-based druglike and agrochemical molecules.