Mild CH Halogenation of Anilides and the Isolation of an Unusual Palladium(I)-Palladium(II) Species

Copper/O2-Mediated Oxidative C-C Activation of Nitriles for Selective Acylation-Bromination of Anilines

This study reports selective dual amino acylation and C-H bromination of aniline compounds enabled by Cu/O2 catalyst systems. This method involves crucial oxidation-induced C-CN bond cleavage of α-methylene nitriles to generate an acylcyanide intermediate that is facilely intercepted by anilines. After amino acylation, the Cu(II) precatalyst in combination with NBS generates Cu(III)-Br in situ that engages in selective electrophilic para- or ortho-C-H bromination. The substrate scope, mechanistic aspects, and late-stage functionalization of biologically active anilines are studied. This study shows the synthetic potential of oxidative C-CN bond activation of nitriles for the development of valuable reactions.

Regioselective ortho halogenation of N-aryl amides and ureas via oxidative halodeboronation: harnessing boron reactivity for efficient C-halogen bond installation

The installation of the C-halogen bond at the ortho position of N-aryl amides and ureas represents a tool to prepare motifs that are ubiquitous in biologically active compounds. To construct such prevalent bonds, most methods require the use of precious metals and a multistep process. Here we report a novel protocol for the long-standing challenge of regioselective ortho halogenation of N-aryl amides and ureas using an oxidative halodeboronation. By harnessing the reactivity of boron over nitrogen, we merge carbonyl-directed borylation with consecutive halodeboronation, enabling the precise introduction of the C-X bond at the desired ortho position of N-aryl amides and ureas. This method offers an efficient, practical, and scalable solution for synthesizing halogenated N-heteroarenes under mild conditions, highlighting the superiority of boron reactivity in directing the regioselectivity of the reaction.

Automated grindstone chemistry: a simple and facile way for PEG-assisted stoichiometry-controlled halogenation of phenols and anilines using N-halosuccinimides

A simple electrical mortar–pestle was used for the development of a green and facile mechanochemical route for the catalyst-free halogenation of phenols and anilines via liquid-assisted grinding using PEG-400 as the grinding auxiliary. A series of mono-, di-, and tri-halogenated phenols and anilines was synthesized in good to excellent yields within 10–15 min in a chemoselective manner by controlling the stoichiometry of N-halosuccinimides (NXS, X = Br, I, and Cl). It was observed that PEG-400 plays a key role in controlling the reactivity of the substrates and to afford better regioselectivity. Almost exclusive para-selectivity was observed for the aromatic substrates with free o- and p-positions for mono- and dihalogenations. As known, the decarboxylation (or desulfonation) was observed in the case of salicylic acids and anthranilic acids (or sulfanilic acids) leading to 2,4,6-trihalogenated products when 3 equiv of NXS was used. Simple instrumentation, metal-free approach, cost-effectiveness, atom economy, short reaction time, and mild reaction conditions are a few noticeable merits of this environmentally sustainable mechanochemical protocol.

Copper-catalyzed regioselective C2-H chlorination of indoles with para-toluenesulfonyl chloride

A copper-catalyzed, pyrimidine directed regioselective C-H chlorination of indoles with para-toluenesulfonyl chloride (TsCl) has been developed. The reactions proceeded smoothly in the presence of 20 mol% of Cu(OAc)₂ as the catalyst and TsCl as the chlorine source, delivering C2-chlorinated indoles with structural diversity in moderate to excellent yields. Mechanistic studies suggested that single electron transfer (SET) from Cu(II) to TsCl accompanied by the release of the p-toluenesulfonyl radical and the related Cu(III)Cl species might be involved in the reactions.

ZnCl2/PhI=O Mediated Selective ortho-Chlorination of Amides

Abstract:

A new ortho-chlorination system consisting of zinc(II) and hypervalent iodine(III) reagent was developed for ortho-chlorination of amides, and the desired products were obtained in moderate to good yields (38-85%). This highly facile and convenient methodology is tolerant of aromatic amide and alkyl amide with diverse substituted groups. A plausible mechanism has been illustrated, in which carbocation rearrangement and metal salt coordinate facilitated orthochlorination are involved.

Palladium-Catalyzed Aromatic C-H Functionalizations Utilizing Electrochemical Oxidations

Transition-metal-catalyzed electrochemical C-H functionalizations have been extensively studied as atom- and step-economical clean methods in organic synthesis. In this account, we described our efforts on the palladium-catalyzed electrochemical C-H functionalizations, including C-H halogenations of arylpyridines and benzamide derivatives using HCl/HBr and I₂ as a halogen source, a one-pot process giving teraryls via the palladium-catalyzed electrochemical C-H iodination and subsequent Suzuki-Miyaura coupling, and an iodine-mediated oxidative homo-coupling reaction of arylpyridines.

C-5 selective chlorination of 8-aminoquinoline amides using dichloromethane

An oxidant-free electrochemical regioselective chlorination of 8-aminoquinoline amides at ambient temperature in batch and continuous-flow was achieved. Inert DCM was used as the chlorinating reagent. Owing to the continuous-flow setup, the reaction scale up can be achieved conveniently with higher productivity. Moreover, this method has good position-control, and water and air tolerance. Costly quaternary ammonium salts were avoided. Radical-trapping, H/D exchange, KIE and cyclic voltammetry experiments were conducted to gain insight into the reaction mechanism.

C4-arylation and domino C4-arylation/3,2-carbonyl migration of indoles by tuning Pd catalytic modes: Pd(i)-Pd(ii) catalysis vs. Pd(ii) catalysis

Efficient C4-arylation and domino C4-arylation/3,2-carbonyl migration of indoles have been developed. The former route enables C4-arylation in a highly efficient and mild manner and the latter route provides an alternative straightforward protocol for synthesis of C2/C4 disubstituted indoles. The mechanism studies imply that the different reaction pathways were tuned by the distinct acid additives, which led to either the Pd(i)-Pd(ii) pathway or Pd(ii) catalysis.

A practical route to 2-iodoanilines via the transition-metal-free and base-free decarboxylative iodination of anthranilic acids under oxygen

A new approach for synthesizing 2-iodoanilines via the transition-metal-free and base-free decarboxylative ortho-C–H iodination of anthranilic acids with a combination of KI and I₂ as the halogen donor and catalyst under oxygen has been developed.

IrIII -Catalyzed Selective ortho-Monoiodination of Benzoic Acids with Unbiased C-H Bonds

An iridium‐catalyzed selective ortho‐monoiodination of benzoic acids with two equivalent C−H bonds is presented. A wide range of electron‐rich and electron‐poor substrates undergo the reaction under mild conditions, with >20:1 mono/di selectivity. Importantly, the C−H iodination occurs selectively ortho to the carboxylic acid moiety in substrates bearing competing coordinating directing groups. The reaction is performed at room temperature and no inert atmosphere or exclusion of moisture is required. Mechanistic investigations revealed a substrate‐dependent reversible C−H activation/protodemetalation step, a substrate‐dependent turnover‐limiting step, and the crucial role of the Ag^I additive in the deactivation of the iodination product towards further reaction.

Proton Transfer Can Govern Regioselectivity Assisted by Iron Catalysis

Ortho-selective aromatic C-H functionalization is frequently used in organic synthesis and chemical/pharmaceutical industries. However, this reaction relies heavily on the use of directing groups suffering from limited substrate scope and extra steps to put on and remove the directing/protecting groups. Herein we present the previously neglected concept that enables good to nearly complete selective ortho position. Proton transfer was utilized to tune the electron density on the aryl ring and determine the positional selectivity of electrophilic substitution. Consistently with deuteration experiments and DFT studies, this work demonstrates that acid-promoted proton transfer directs accelerated ortho-selective halogenation of NH/OH contained aromatic amines/phenols with excellent selectivity (>40 examples; up to 98:2 ortho/para selectivity). The application potential of this Fe-catalyzed method is demonstrated by the convenient synthesis of three alkaloids and tizanidine. This report raises the possibility that proton transfer could serve as the basis of developing new selective C-H functionalization reactions.

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Highly ortho-selective halogenations of anilines and carbazoles

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Lewis acids being able to accelerate EAS reactions

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Proton shift found to be crucial for the regioselectivity

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Practical iron sulfonate catalysis being scaled up to 100 g

Rhodium-Catalyzed ortho-Bromination of O-Phenyl Carbamates Accelerated by a Secondary Amide-Pendant Cyclopentadienyl Ligand

It has been established that a newly developed cyclopentadienyl rhodium(III) [Cp^A Rh^III ] complex, bearing an acidic secondary amide moiety on the Cp ring, is able to catalyze the ortho-bromination of O-phenyl carbamates with N-bromosuccinimide (NBS) at room temperature. The presence of the acidic secondary amide moiety on the Cp^A ligand accelerates the bromination by the hydrogen bond between the acidic NH group of the Cp^A ligand and the carbonyl group of NBS.

Rhodium(III)-Catalyzed Regioselective C(sp2)-H Functionalization of 7-Arylpyrazolo[1,5-a]pyrimidines with Dioxazolones as Amidating Agents

Rh(III)-catalyzed C-H functionalization of 7-arylpyrazolo[1,5-a]pyrimidines was developed wherein the pyrazolo[1,5-a]pyrimidine moiety is reported for the first time to direct the C-H bond activation. Various 7-arylpyrazolo[1,5-a]pyrimidines underwent smooth C-H amidation with alkyl-, aryl-, and heteroaryl-substituted dioxazolones to afford the products in moderate to good yields. Mechanistic studies suggest that a six-membered rhodacycle intermediate involving N1 might play a key role in the regioselective catalytic cycle.

C-H Functionalizations by Palladium Carboxylates: The Acid Effect

Finding optimal reaction conditions is usually complex, requires many experiments, and is therefore demanding in terms of human, financial, and environmental resources. This work provides a simple workflow for easier design of popular palladium-catalyzed C-H functionalization reactions, where the active palladium catalysts contain carboxylate ligands. The key factor for optimizing reaction conditions is to find a balance between two opposing effects of the carboxylic acid in the reaction mixture: generation of more reactive palladium catalyst versus deactivation of a substrate by its protonation.

The surprisingly facile formation of Pd(i)-phosphido complexes from ortho-biphenylphosphines and palladium acetate

The widely-used ortho-biphenylphosphine ligands SPhos and RuPhos not only undergo facile orthometallation with palladium acetate, yielding strained, four-membered dimeric palladacycles but more surprisingly, in the presence of alcoholic solvents, along with the less encumbered analogue MePhos, yield unusual dinuclear Pd(i) complexes, in which the Pd-centers are bridged by both a phosphide ligand and by the arene of a coordinated phosphine donor.

Cobalt(ii)-catalyzed regioselective C-H halogenation of anilides

A cobalt-catalyzed regioselective C-H halogenation methodology is reported herein. The highlight of this work is the highly selective C-H functionalization of anilides, which results in high-yielding, versatile, and practical halogenated products. Thereby, brominations, chlorinations and iodinations of many electron-rich and electron-deficient anilides were achieved in a highly selective fashion. Mechanistic studies with respect to the pathway of the reaction are also described.

Efficient Electrocatalysis for the Preparation of (Hetero)aryl Chlorides and Vinyl Chloride with 1,2-Dichloroethane

Although the application of 1,2-dichloroethane (DCE) as a chlorinating reagent in organic synthesis with the concomitant release of vinyl chloride as a useful byproduct is a fantastic idea, it still presents a tremendous challenge and has not yet been achieved because of the harsh dehydrochlorination conditions and the sluggish C-H chlorination process. Here we report a bifunctional electrocatalysis strategy for the catalytic dehydrochlorination of DCE at the cathode simultaneously with anodic oxidative aromatic chlorination using the released HCl as the chloride source for the efficient synthesis of value-added (hetero)aryl chlorides. The mildness and practicality of the protocol was further demonstrated by the efficient late-stage chlorination of bioactive molecules.

Electrochemical Oxidative Clean Halogenation Using HX/NaX with Hydrogen Evolution

Organic halides (R-X) are prevalent structural motifs in pharmaceutical molecules and key building blocks for the synthesis of fine chemicals. Although a number of routes are available in the literature for the synthesis of organic halides, these methods often require stoichiometric additives or oxidants, metal catalysts, leaving or directing groups, or toxic halogenating agents. In addition, the necessity of employing different, often tailor-made, catalytic systems for each type of substrate also limits the applicability of these methods. Herein, we report a clean halogenation by electrochemical oxidation with NaX/HX. A series of organic halides were prepared under metal catalyst- and exogenous-oxidant-free reaction conditions. It is worth noting that this reaction has a broad substrate scope; various heteroarenes, arenes, alkenes, alkynes, and even aliphatic hydrocarbons could be applied. Most importantly, the reaction could also be performed on a 200-mmol scale with the same efficiency (86%, 50.9 g pure product).

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Metal catalyst free and exogenous oxidant free

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Commercially available, nontoxic, and green halogenating agents

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Broad substrate scope

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200-mmol-scale synthesis

Nickel-catalyzed regioselective C–H halogenation of electron-deficient arenes

A straightforward Ni(ii)-catalyzed general strategy was developed for the ortho-halogenation of electron-deficient arenes with easily available halogenating reagents N-halosuccinimides (NXS; X = Br, Cl and I).

Cobalt catalyzed regioselective C–H methylation/acetoxylation of anilides: new routes for C–C and C–O bond formation

The first cobalt-catalyzed regioselective C–H methylation/acetoxylation reactions of anilides using peroxides and acetic acid respectively as attacking reagents have been developed herein.

Ruthenium(II)-enabled para-selective C-H difluoromethylation of anilides and their derivatives

Transition-metal-catalyzed direct site-selective functionalization of arene C–H bonds has emerged as an innovative approach for building the core structure of pharmaceutical agents and other versatile complex compounds. However, para-selective C–H functionalization has seldom been explored, only a few examples, such as steric-hindered arenes, electron-rich arenes, and substrates with a directing group, have been reported to date. Here we describe the development of a ruthenium-enabled para-selective C–H difluoromethylation of anilides, indolines, and tetrahydroquinolines. This reaction tolerates various substituted arenes, affording para-difluoromethylation products in moderate to good yields. Results of a preliminary study of the mechanism indicate that chelation-assisted cycloruthenation might play a role in the selective activation of para-C_Ar–H bonds. Furthermore, this method provides a direct approach for the synthesis of fluorinated drug derivatives, which has important application for drug discovery and development.

Selective para-functionalization of substituted arenes is a formidable challenge in homogeneous catalysis. Here, the authors achieved the para-selective C-H difluoromethylation of anilides, indolines and tetrahydroquinolines with a ruthenium catalyst in good yields and apply it to the synthesis of bioactive compounds.

Palladium-catalyzed ortho-halogenations of acetanilides with N-halosuccinimides via direct sp2 C-H bond activation in ball mills

A solvent-free palladium-catalyzed ortho-iodination of acetanilides using N-iodosuccinimide as the iodine source has been developed under ball-milling conditions. This present method avoids the use of hazardous organic solvents, high reaction temperature, and long reaction time and provides a highly efficient methodology to realize the regioselective functionalization of acetanilides in yields up to 94% in a ball mill. Furthermore, the current methodology can be extended to the synthesis of ortho-brominated and ortho-chlorinated products in good yields by using the corresponding N-halosuccinimides.

Revisiting 1-chloro-1,2-benziodoxol-3-one: efficient ortho-chlorination of aryls under aqueous conditions

The ortho-selective amide directing group assisted chlorination of aryls with the easily accessible 1-chloro-1,2-benziodoxol-3-one as oxidant as well as chlorinating reagent is reported in the absence of a radical initiator. The open air, aqueous conditions and recyclable reagent demonstrate the green principles involved in the reaction.