Palladium(II)-Catalyzed Direct ortho-CH Acylation of Anilides by Oxidative Cross-Coupling with Aldehydes using tert-Butyl Hydroperoxide as Oxidant

Factors Affecting the Formation and Transformation of the Intermediates in Pd(II)-Catalyzed Aromatic C-H Activation: A Comprehensive Study with the Pd(II)/LA Platform

How the factors affecting the formation and transformation of the intermediates in Pd(II)-catalyzed aromatic C-H activation: A comprehensive study with the Pd(II)/LA platform. Using the Pd(II)/Lewis acid (LA)-catalyzed C-H activation of electron-rich acetanilides as a platform, the C-H activation intermediates, including the precomplex η2-intermediate, the agostic hydrogen intermediate, and the palladacycle compound have been well-characterized. This work presents how the catalyst source, substrate, and solvent affect the formation of the η2-intermediate and its equilibrium with the agostic hydrogen intermediate, and the transformation of the agostic hydrogen intermediate to the palladacycle compound through C-H activation. The findings disclosed above are provided as a guideline for the catalyst design of the oxidative olefination of acetanilide with dioxygen, and the catalytic efficiency matched well with the mechanistic findings.

Synthesis of functionalized alkenes via Cu(I)-catalysed allylation of acetanilides using Morita-Baylis-Hillman bromides

An easy access to functionalized alkenes has been developed by the C-H functionalization of anilides with Morita-Baylis-Hillman (MBH) bromides in the presence of copper chloride, TBHP and acetic acid. Unsubstituted as well as ortho/meta-substituted anilides exclusively give rise to the para-allylated products, whereas para-substitution brings about the formation of ortho-allylated anilides.

Palladium-Catalyzed Ortho C-H Arylation of Unprotected Anilines: Chemo- and Regioselectivity Enabled by the Cooperating Ligand [2,2'-Bipyridin]-6(1H)-one

Metal-catalyzed C-H functionalizations on the aryl ring of anilines usually need cumbersome N-protection-deprotection strategies to ensure chemoselectivity. We describe here the Pd-catalyzed direct C-H arylation of unprotected anilines with no competition of the N-arylation product. The ligand [2,2'-bipyridin]-6(1H)-one drives the chemoselectivity by kinetic differentiation in the product-forming step, while playing a cooperating role in the C-H cleavage step. The latter is favored in an anionic intermediate where the NH moiety is deprotonated, driving the regioselectivity of the reaction toward ortho substitution.

Oxidant-Switched Palladium-Catalyzed Regioselective Mono- versus Bis-ortho-Aroylation of 1-Aryl-1H-indazoles with Aldehydes via C-H Bond Activation

A highly efficient oxidant-switched palladium-catalyzed regioselective C_(sp²)-H/C_(sp²)-H cross-dehydrogenative coupling (CDC) for direct mono/bis-ortho-aroylation of substituted 1-phenyl-1H-indazoles 1a-j with various substituted aldehydes 3a-t via C_(sp²)-H bond activation has been developed. In this study, Pd-catalyzed chelation-assisted mono- or bis-aroylation of substituted 1-phenyl-1H-indazoles depends on the type of oxidant being used for the CDC reaction. While mono-ortho-aroylation of substituted 1-phenyl-1H-indazole was obtained using dicumylperoxide (DCP) as the oxidant, the bis-ortho-aroylation product has been afforded by the use of tert-butyl hydroperoxide (TBHP). Regardless of the greater activity at the C-3 position of 1H-indazoles, the greater coordinating capacity of the N atom directed the aroylating group to the ortho position, leaving behind the nondirected metalation pathway. The Pd-catalyzed operationally simplified methodology proceeded in the presence of oxidants with either DCP or TBHP in dichloroethane as the solvent at 110 °C for 16 h, which generated a miscellaneous variety of monosubstituted o-benzoyl/acyl-1-aryl-1H-indazoles 4a-t/5a-i and bis-substituted o-benzoyl-1-aryl-1H-indazoles 6a-j in ≤88% yields. The probable mechanistic pathway involves a free radical chelation-assisted approach that could be accomplished by the addition of an in situ-generated oxidant-promoted benzoyl/acyl radical to the ortho position of 1-phenyl-1H-indazoles. A wide range of substrates, a high functional group tolerance, gram-scale synthesis, control/competitive experiments, and a variety of synthetic applications further exemplify the versatility of the developed methodology.

Microwave-assisted palladium catalysed C-H acylation with aldehydes: synthesis and diversification of 3-acylthiophenes

The use of MW allows the efficient palladium(II)-catalysed C-3 acylation of thiophenes with aldehydes via C(sp²)-H activation for the synthesis of (cyclo)alkyl/aryl thienyl ketones (43 examples). Compared to standard thermal conditions, the use of MW reduces the reaction time (15 to 30 min vs. 1 to 3 hours), leading to improved yields of the ketones (up to 92%). The control of positional selectivity is achieved by 2-pyridinyl and 2-pyrimidyl ortho-directing groups at C-2 of the thiophene scaffold. To show the synthetic applicability, selected ketones were subjected to further transformations, including intramolecular reactions to directly embed the directing group in the core structure of the new molecule.

Pd(II)-Catalyzed C-H Acylation of (Hetero)arenes-Recent Advances

Di(hetero)aryl ketones are important motifs present in natural products, pharmaceuticals or agrochemicals. In recent years, Pd(II)-catalyzed acylation of (hetero)arenes in the presence of an oxidant has emerged as a catalytic alternative to classical acylation methods, reducing the production of toxic metal waste. Different directing groups and acyl sources are being studied for this purpose, although further development is required to face mainly selectivity problems in order to be applied in the synthesis of more complex molecules. Selected recent developments and applications are covered in this review.

Steering Site-Selectivity in Transition Metal-Catalyzed C-H Bond Functionalization: the Challenge of Benzanilides

Selective C-H bond functionalization catalyzed by metal complexes have completely revolutionized the way in which chemical synthesis is conceived nowadays. Typically, the reactivity of a transition metal catalyst is the key to control the site-, regio- and/or stereo-selectivity of a C-H bond functionalization. Of particular interests are molecules that contain multiple C-H bonds prone to undergo C-H bond activations with very similar bond dissociation energies at different positions. This is the case of benzanilides, relevant chemical motifs that are found in many useful fine chemicals, in which two C-H sites are present in chemically different aromatic fragments. In the last years, it has been found that depending on the metal catalyst and the reaction conditions, the amide motif might behave as a directing group towards the metal-catalyzed C-H bond activation in the benzamide site or in the anilide site. The impact and the consequences of such subtle control of site-selectivity are herein reviewed with important applications in carbon-carbon and carbon-heteroatom bond forming processes. The mechanisms unraveling these unique transformations are discussed in order to provide a better understanding for future developments in the field of site-selective C-H bond functionalization with transition metal catalysts.

In silico rationalisation of selectivity and reactivity in Pd-catalysed C-H activation reactions

A computational approach has been developed to automatically generate and analyse the structures of the intermediates of palladium-catalysed carbon-hydrogen (C-H) activation reactions as well as to predict the final products. Implemented as a high-performance computing cluster tool, it has been shown to correctly choose the mechanism and rationalise regioselectivity of chosen examples from open literature reports. The developed methodology is capable of predicting reactivity of various substrates by differentiation between two major mechanisms - proton abstraction and electrophilic aromatic substitution. An attempt has been made to predict new C-H activation reactions. This methodology can also be used for the automated reaction planning, as well as a starting point for microkinetic modelling.

Recent advances in the direct functionalization of quinoxalin-2(1H)-ones

The direct C3-functionalization of quinoxalin-2(1H)-ones via C-H bond activation has recently attracted considerable attention, due to their diverse biological activities and chemical properties. This review will focus on the recent achievements, mainly including arylation, alkylation, acylation, alkoxycarbonylation, amination, amidation and phosphonation of quinoxalin-2(1H)-ones. Their mechanisms are also discussed.

Visible-Light-Mediated Dearomatisation of Indoles and Pyrroles to Pharmaceuticals and Pesticides

Dearomatisation of indole derivatives to the corresponding isatin derivatives has been achieved with the aid of visible light and oxygen. It should be noted that isatin derivatives are highly important for the synthesis of pharmaceuticals and bioactive compounds. Notably, this chemistry works excellently with N-protected and protection-free indoles. Additionally, this methodology can also be applied to dearomatise pyrrole derivatives to generate cyclic imides in a single step. Later this methodology was applied for the synthesis of four pharmaceuticals and a pesticide called dianthalexin B. Detailed mechanistic studies revealed the actual role of oxygen and photocatalyst.

Copper-catalyzed acylation of pyrazolones with aldehydes to afford 4-acylpyrazolones

Copper-catalyzed direct acylation of the alkenyl C-H bond in 1,2-dihydro-3H-pyrazol-3-ones has been developed, affording a series of 4-acylpyrazolones in moderate to good yields. Notably, this protocol involves readily accessible substrates and reagents, which have good functional group tolerance leading to pyrazolone derivatives under mild reaction conditions.

The synergistic effect of self-assembly and visible-light induced the oxidative C-H acylation of N-heterocyclic aromatic compounds with aldehydes

Constructing carbon-carbon bonds through oxidative cross-coupling between two hydrocarbon compounds is regarded as a foundational issue in green chemistry. High atom-economy and mild conditions are long term pursued goals in this field. Herein, we developed a visible-light mediated direct cross-coupling between N-heterocycles and aldehydes without the requirement of a photocatalyst. Several N-heterocycles afforded the acylation products with aromatic or aliphatic aldehydes in good yields. The reaction can be conducted under room temperature and easily scaled up.

Palladium-catalyzed decarboxylative, decarbonylative and dehydrogenative C(sp2)-H acylation at room temperature

Over the past few decades, an impressive array of C-H activation methodology has been developed for organic synthesis. However, due to the inherent inertness of the C-H bonds (e.g. ∼110 kcal mol^-1 for the cleavage of C(aryl)-H bonds) harsh reaction conditions have been realized to overcome high energetic transition states resulting in a limited substrate scope and functional group tolerance. Therefore, the development of mild C-H functionalization protocols is in high demand to exploit the full potential of the C-H activation strategy in the synthesis of a complex molecular framework. Although, electron-rich substrates undergo electrophilic metalation under relatively mild conditions, electron-deficient substrates proceed through a rate-limiting C-H insertion under forcing conditions at high temperature. In addition, a stoichiometric amount of toxic silver salt is frequently used in palladium catalysis to facilitate the C-H activation process which is not acceptable from the environmental and industrial standpoint. We report herein, a Pd(ii)-catalyzed decarboxylative C-H acylation of 2-arylpyridines with α-ketocarboxylic acids under mild conditions. The present protocol does not require stoichiometric silver(i) salts as additives and proceeds smoothly at ambient temperature. A novel decarbonylative C-H acylation reaction has also been accomplished using aryl glyoxals as acyl surrogates. Finally, a practical C-H acylation via a dehydrogenative pathway has been demonstrated using commercially available benzaldehydes and aqueous hydroperoxides. We also disclose that acetonitrile solvent is optimal for the acylation reaction at room temperature and has a prominent role in the reaction outcome. Control experiments suggest that the acylation reaction via decarboxylative, decarbonylative and dehydrogenative proceeds through a radical pathway. Thus we disclose a practical protocol for the sp² C-H acylation reaction.

Synthesis of unsymmetrical multi-aroyl derivatives of ferrocene using palladium catalysed oxidative C–H aroylation

Palladium catalysed oxidative C–H aroylation of ferrocene: accessing the multi-aroyl derivatives of ferrocene.

Computational Study of Ni-Catalyzed C-H Functionalization: Factors That Control the Competition of Oxidative Addition and Radical Pathways

The mechanisms of Ni-catalyzed C-H arylation, alkylation, and sulfenylation with N,N-bidentate directing groups are investigated using density functional theory (DFT) calculations. While the C-H cleavage occurs via the concerted metalation-deprotonation (CMD) mechanism in all types of reactions, the subsequent C-C and C-X bond formation steps may occur via either oxidative addition to form a Ni(IV) intermediate or radical pathways involving Ni(III) complexes generated from homolytic dissociation of disulfides/peroxides or halide-atom transfer from alkyl halides. DFT calculations revealed that radical mechanisms are preferred in reactions with sterically hindered coupling partners with relatively low bond dissociation energies (BDE) such as dicumyl peroxide, heptafluoroisopropyl iodide and diphenyl disulfide. In contrast, these radical processes are highly disfavored when generating unstable phenyl and primary alkyl radicals. In such cases, the reaction proceeds via an oxidative addition/reductive elimination mechanism involving a Ni(IV) intermediate. These theoretical insights into the substrate-controlled mechanisms in the C-H functionalizations were employed to investigate a number of experimental phenomena including substituent effects on reactivity, chemo- and regioselectivity and the effects of oxidant in the intermolecular oxidative C-H/C-H coupling reactions.

Recent Advances in Radical C-H Activation/Radical Cross-Coupling

Research and industrial interest in radical C-H activation/radical cross-coupling chemistry has continuously grown over the past few decades. These reactions offer fascinating and unconventional approaches toward connecting molecular fragments with high atom- and step-economy that are often complementary to traditional methods. Success in this area of research was made possible through the development of photocatalysis and first-row transition metal catalysis along with the use of peroxides as radical initiators. This Review provides a brief and concise overview of the current status and latest methodologies using radicals or radical cations as key intermediates produced via radical C-H activation. This Review includes radical addition, radical cascade cyclization, radical/radical cross-coupling, coupling of radicals with M-R groups, and coupling of radical cations with nucleophiles (Nu).

Palladium-catalyzed direct sulfonylation of C–H bonds with the insertion of sulfur dioxide

A palladium-catalyzed direct sulfonylation of C–H bonds with the insertion of sulfur dioxide under mild conditions is reported. The sulfonylative couplings with the insertion of sulfur dioxide into C–H bonds are effective, and two classes of sulfonylative products are formed in moderate to good yields by the combination of radical chemistry and palladium-catalyzed C–H activation.

Rh(iii)-Catalyzed direct C-7 amination of indolines with anthranils

An effective and fascinating method has been developed for direct C-7 amination of indolines under Rh(iii) catalysis with anthranils.

Palladium nanoparticles supported on organofunctionalized kaolin as an efficient heterogeneous catalyst for directed C–H functionalization of arylpyrazoles

A heterogeneous catalyst system based on the immobilization of Pd⁰ nanoparticles onto organofunctionalized kaolin is reported with a view to introducing new synthetic routes of directed C–H functionalization of arylpyrazoles.

Transition-Metal-Catalyzed C-H Bond Addition to Carbonyls, Imines, and Related Polarized π Bonds

The transition-metal-catalyzed addition of C-H bonds to carbonyls, imines, and related polarized π bonds has emerged as a particularly efficient and powerful approach for the construction of an incredibly diverse array of heteroatom-substituted products. Readily available and stable inputs are typically employed, and reactions often proceed with very high functional group compatibility and without the production of waste byproducts. Additionally, many transition-metal-catalyzed C-H bond additions to polarized π bonds occur within cascade reaction sequences to provide rapid access to a diverse array of different heterocyclic as well as carbocyclic products. This review highlights the diversity of transformations that have been achieved, catalysts that have been used, and types of products that have been prepared through the transition-metal-catalyzed addition of C-H bonds to carbonyls, imines, and related polarized π bonds.

NHPI and palladium cocatalyzed aerobic oxidative acylation of arenes through a radical process

Molecular oxygen, the most environmentally friendly oxidant, was used as the terminal oxidant for palladium-catalyzed radical oxidative acylation of arenes.

Mild metal-catalyzed C–H activation: examples and concepts

C–H Activation reactions that proceed under mild conditions are more attractive for applications in complex molecule synthesis. Mild C–H transformations reported since 2011 are reviewed and the different concepts and strategies that have enabled their mildness are discussed.

V. Synthesis of new fused heterocyclic aromatic hydrocarbons via C–S and C–C bond formation by C–H bond activation in the presence of new Pd(ii) Schiff's base complexes

Pd(ii) Schiff base macrocyclic complexes are used as photocatalysts with high stability, C–S bond and intramolecular C–H bond activation under visible light irradiation.

Cp*Rh(iii)-catalyzed electrophilic amination of arylboronic acids with azo compounds for synthesis of arylhydrazides

A [Cp*Rh(iii)]-catalyzed electrophilic amination of arylboronic acids with azocarboxylates for the synthesis of mono- and di-arylhydrazides was developed.

Copper-catalyzed oxidative coupling between quinazoline 3-oxides and unactivated aldehydes: an efficient approach to functionalized quinazolines

The first copper-catalyzed oxidative coupling between quinazoline 3-oxides and unactivated aldehydes was described.