Dual Ligand Enabled Nondirected C-H Chalcogenation of Arenes and Heteroarenes

C-H Functionalization of Heteroarenes via Electron Donor-Acceptor Complex Photoactivation

C-H Functionalization of heteroarenes stands as a potent instrument in organic synthesis, and with the incorporation of visible light, it emerged as a transformative game-changer. In this domain, electron donor-acceptor (EDA) complex, formed through the pairing of an electron-rich substrate with an electron-accepting molecule, has garnered substantial consideration in recent years due to the related avoidance of the requirement of photocatalyst as well as oxidant. EDA complexes can undergo photoactivation under mild conditions and exhibit high functional group tolerance, making them potentially suitable for the functionalization of biologically relevant heteroarenes. This review article provides an overview of recent advancements in the field of C-H functionalization of heteroarenes via EDA complex photoactivation with literature coverage up to April, 2024.

Ligand-Enabled ortho-Selective C-H Olefination of Tertiary Aniline Derivatives

A highly ortho-selective CAr-H olefination of tertiary anilines without a directing group was developed. This reaction tolerated various substituted arenes and olefin coupling partners, affording ortho-olefination products in moderate to good yields. Preliminary mechanistic studies showed that N-Ac-d-Ala, Ag2CO3, and BQ were the key factors for tuning the regioselectivity from para to ortho. Density functional theory was used to achieve a theoretical understanding of the ortho selectivity.

Non-Directed C-H Arylation of Anisole Derivatives via Pd/S,O-Ligand Catalysis

Non-directed C-H arylation is one of the most efficient methods to synthesize biaryl compounds without the need of the prefuctionalization of starting materials, or the installment and removal of directing groups on the substrate. A direct C-H arylation of simple arenes as limiting reactants remains a challenge. Here we disclose a non-directed C-H arylation of anisole derivatives as limiting reagents with aryl iodides under mild reaction conditions. The arylated products are obtained in synthetically useful yields and the arylation of bioactive molecules is also demonstrated. Key to the success of this methodology is the use of a one-step synthesized S,O-ligand.

Recent advances in selective mono-/dichalcogenation and exclusive dichalcogenation of C(sp2)-H and C(sp3)-H bonds

Organochalcogen compounds are prevalent in numerous natural products, pharmaceuticals, agrochemicals, polymers, biological molecules and synthetic intermediates. Direct chalcogenation of C-H bonds has evolved as a step- and atom-economical method for the synthesis of chalcogen-bearing compounds. Nevertheless, direct C-H chalcogenation severely lags behind C-C, C-N and C-O bond formations. Moreover, compared with the C-H monochalcogenation, reports of selective mono-/dichalcogenation and exclusive dichalcogenation of C-H bonds are relatively scarce. The past decade has witnessed significant advancements in selective mono-/dichalcogenation and exclusive dichalcogenation of various C(sp2)-H and C(sp3)-H bonds via transition-metal-catalyzed/mediated, photocatalytic, electrochemical or metal-free approaches. In light of the significance of both mono- and dichalcogen-containing compounds in various fields of chemical science and the critical issue of chemoselectivity in organic synthesis, the present review systematically summarizes the advances in these research fields, with a special focus on elucidating scopes and mechanistic aspects. Moreover, the synthetic limitations, applications of some of these processes, the current challenges and our own perspectives on these highly active research fields are also discussed. Based on the substrate types and C-H bonds being chalcogenated, the present review is organized into four sections: (1) transition-metal-catalyzed/mediated chelation-assisted selective C-H mono-/dichalcogenation or exclusive dichalcogenation of (hetero)arenes; (2) directing group-free selective C-H mono-/dichalcogenation or exclusive dichalcogenation of electron-rich (hetero)arenes; (3) C(sp3)-H dichalcogenation; (4) dichalcogenation of both C(sp2)-H and C(sp3)-H bonds. We believe the present review will serve as an invaluable resource for future innovations and drug discovery.

Bidentate Ligand Driven Intramolecularly Te…O Bonded Organotellurium Cations from Synthesis, Stability to Catalysis

A new series of unsymmetrical phenyl tellurides derived from 2-N-(quinolin-8-yl) benzamide ligand has been synthesized in a practical manner by the copper-catalyzed method by using diaryl ditelluride and Mg as a reductant at room temperature. In order to augment the Lewis acidity of these newly formed unsymmetrical monotellurides, these have been transformed into corresponding unsymmetrical 2-N-(quinolin-8-yl)benzamide tellurium cations. Subsequently, these Lewis acidic tellurium cations were used as chalcogen bonding catalysts, enabling the synthesis of various substituted 1,2-dihydroquinolines by activating ketones with anilines under mild conditions. Moreover, the synthesized 2-N-(quinolin-8-yl)benzamide phenyl tellurium cation has also catalyzed the formation of β-amino alcohols in high regioselectivity by effectively activating epoxides at room temperature. Mechanistic insight by 1 H and 19 F NMR study, electrostatic surface potential (ESP map), control reaction in which tellurium cation reacted explosively with epoxide, suggested that the enhanced Lewis acidity of tellurium center seems responsible for efficient catalytic activities under mild conditions enabling β-amino alcohols with excellent regioselectivity and 1,2-dihydroquinolines with trifluoromethyl, nitro, and pyridylsubstitution, which were difficult to access.

DFT Study on the Mechanism of the Palladium-Catalyzed [3 + 2] Annulation of Aromatic Amides with Maleimides via Benzylic and meta-C-H Bond Activation: Role of the External Ligand Ac-Gly-OH

The mechanism of the Ac-Gly-OH-assisted palladium-catalyzed [3 + 2] annulation of aromatic amides with maleimides is investigated using density functional theory calculations. The results show that the reaction undergoes the sequential steps of N-H bond deprotonation, first benzylic C-H bond activation, maleimide insertion, second meta-C-H bond activation, reductive elimination, and oxidation. The external ligand Ac-Gly-OH acts as the internal base for hydrogen abstraction in the first benzylic C-H bond activation. The maleimide insertion step is found to be the rate-determining step. Based on the nearly same energetic span of the two pathways to generate the enantio products, the computational results are consistent with the experimental observation that the terminal [3 + 2] annulation products are racemic when using an achiral ligand. These calculation results disclose the detailed reaction mechanism and shed light on some experimental ambiguities.

Late-Stage C-H Activation of Drug (Derivative) Molecules with Pd(ll) Catalysis

This review comprehensively analyses representative examples of Pd(II)-catalyzed late-stage C-H activation reactions and demonstrates their efficacy in converting C-H bonds at multiple positions within drug (derivative) molecules into diverse functional groups. These transformative reactions hold immense potential in medicinal chemistry, enabling the efficient and selective functionalization of specific sites within drug molecules, thereby enhancing their pharmacological activity and expanding the scope of potential drug candidates. Although notable articles have focused on late-stage C-H functionalization reactions of drug-like molecules using transition-metal catalysts, reviews specifically focusing on late-stage C-H functionalization reactions of drug (derivative) molecules using Pd(II) catalysts are required owing to their prominence as the most widely utilized metal catalysts for C-H activation and their ability to introduce a myriad of functional groups at specific C-H bonds. The utilization of Pd-catalyzed C-H activation methodologies demonstrates impressive success in introducing various functional groups, such as cyano (CN), fluorine (F), chlorine (Cl), aromatic rings, olefin, alkyl, alkyne, and hydroxyl groups, to drug (derivative) molecules with high regioselectivity and functional-group tolerance. These breakthroughs in late-stage C-H activation reactions serve as invaluable tools for drug discovery and development, thereby offering strategic options to optimize drug candidates and drive the exploration of innovative therapeutic solutions.

Transition-metal catalyzed C-H activation as a means of synthesizing complex natural products

Over the past few decades, the advent of C-H activation has led to a rethink among chemists about the synthetic strategies employed for multi-step transformations. Indeed, deploying innovative and masterful tricks against the numerous classical organic transformations has been the need of the hour. Despite this, the immense importance of C-H activation remains unfulfilled unless the methodology can be deployed for large-scale industrial processes and towards the concise, step-economic synthesis of prodigious natural products and pharmaceutical drugs. Lately, the growing potential of C-H activation methodology has indeed driven the pioneers of synthetic organic chemists into finding more efficient methods to accelerate the synthesis of such complex molecular scaffolds. This review aims to draw a general overview of the various C-H activation procedures that have been adopted for synthesizing these vast majority of structurally complicated natural products. Our objective lies in drawing a complete picture and taking the readers through the synthesis of a series of such complex organic compounds by simplified techniques, making it step-economic on a larger scale and thus instigating the readers to trigger the use of such methodology and uncover new, unique patterns for future synthesis of such natural products.

Dinuclear Gold-Catalyzed para-Selective C-H Arylation of Undirected Arenes by Noncovalent Interactions

The regioselectivity of C-H functionalization is commonly achieved by directing groups, electronic factors, or steric hindrance, which facilitate the identification of reaction sites. However, such strategies are less effective for reactants such as simple monofluoroarenes due to their relatively low reactivity and the modest steric demands of the fluorine atom. Herein, we present an undirected gold-catalyzed para-C-H arylation of a wide array of monofluoroarenes using air-stable aryl silanes and germanes at room temperature. A high para-regioselectivity (up to 98 : 2) can be realized by utilizing a dinuclear dppm(AuOTs)2 (dppm=bis(diphenylphosphino)methane) as the catalyst and hexafluorobenzene as the solvent. This provides a general and practical protocol for the concise construction of structurally diverse para-arylated monofluoroarenes through C-H activation manner. It features excellent functional group tolerance and a broad substrate scope (>80 examples). Besides, this strategy is also robust for other simple monosubstituted arenes and heteroarenes. Our mechanistic studies and theoretical calculations suggest that para-C-H selectivity arises from highly electrophilic and structurally flexible dinuclear Ar-Au(III)-Au(I) species, coupled with noncovalent interaction induced by hexafluorobenzene.

Controlling Reactivity and Selectivity in the Nondirected C-H Activation of Arenes with Palladium

ConspectusAromatic structures are widespread motifs throughout organic chemistry, and C-H activation has been recognized as a major tool for enabling their sustainable and efficient functionalization. Through C-H activation, arenes can be modified without the need for prefunctionalization, leading to inherent atom- and step-economic advantages over traditional methods. However, for the development of synthetically useful methods, several hurdles have to be overcome. The strength of C-H bonds necessitates the development of sufficiently reactive catalysts, while the presence of multiple C-H bonds within a substrate poses challenges in terms of site-selectivity. Traditionally these challenges have been addressed by substrate control. By attaching different directing groups (DGs), the reactivity of the respective arene was significantly enhanced and the DG guided the metal in close proximity to specific C-H bonds, resulting in high site-selectivity. However, the introduction and removal of the DG add additional steps to the synthetic sequence, and the scope of the reaction is limited to a specific substrate class. The development of complementary nondirected methods that can be applied to a broad range of arenes without the necessity to carry a specific functional group that coordinates to Pd (referred to as simple arenes) is therefore highly desirable. However, the intrinsically lower reactivity of such substrates and the absence of a selectivity-determining DG pose significant challenges that can be solved only by the development of highly efficient catalysts. Consequently, the field of nondirected C-H activation, especially with respect to Pd-catalyzed methods, remained comparatively underdeveloped when we initiated our research program in 2017. At that time, state-of-the-art methods required the arene to be used in large excess, precluding its use in late-stage functionalization. Since organopalladium species are among the most versatile synthetic intermediates, we realized that developing a system, which can effectively and selectively activate C-H bonds in simple arenes with the arene as the limiting reagent, would be a powerful tool in synthetic organic chemistry. This account summarizes our groups' research toward the development and application of catalytic systems offering this desired reactivity and focuses explicitly on Pd-catalyzed nondirected C-H functionalization reactions of arenes, where the arene is employed as a limiting reagent. After an introduction that summarizes the state of Pd-catalyzed C-H activation of arenes before 2017 and the associated challenges, experimental and mechanistic details about the development of the first arene-limited, nondirected C-H functionalization of simple arenes with palladium will be discussed. This reactivity was enabled by the identification and combination of two complementary ligands, an N-heterocycle and an amino acid-derived ligand. Afterward we will discuss the expansion of this dual-ligand approach to further arene-limited transformations. Finally, we describe two methodologies that originated from the observations we made during our studies, namely, the late-stage deuteration of simple arenes and a highly selective olefination method that uses noncovalent interactions to induce meta selectivity.

Accelerating explicit solvent models of heterogeneous catalysts with machine learning interatomic potentials

Realistically modelling how solvents affect catalytic reactions is a longstanding challenge due to its prohibitive computational cost. Typically, an explicit atomistic treatment of the solvent molecules is needed together with molecular dynamics (MD) simulations and enhanced sampling methods. Here, we demonstrate the utility of machine learning interatomic potentials (MLIPs), coupled with active learning, to enable fast and accurate explicit solvent modelling of adsorption and reactions on heterogeneous catalysts. MLIPs trained on-the-fly were able to accelerate ab initio MD simulations by up to 4 orders of magnitude while reproducing with high fidelity the geometrical features of water in the bulk and at metal-water interfaces. Using these ML-accelerated simulations, we accurately predicted key catalytic quantities such as the adsorption energies of CO*, OH*, COH*, HCO*, and OCCHO* on Cu surfaces and the free energy barriers of C-H scission of ethylene glycol over Cu and Pd surfaces, as validated with ab initio calculations. We envision that such simulations will pave the way towards detailed and realistic studies of solvated catalysts at large time- and length-scales.

Direct C(3)5-H Polyfluoroarylation of 2-Amino/alkoxy Pyridines Enabled by a Transient and Electron-deficient Palladium Intermediate

Herein, we present an unprecedented azine-limited C5-H polyfluoroarylation of 2-aminopyridines enabled by a transient and electron-deficient perfluoroaryl-Pd species via C-H/C-H coupling. The protocol further allows C3(5)-H polyfluoroarylation of 2-alkoxypyridines guided by sterics and electronics for the first time. The late-stage C-H functionalization of drugs, drug derivatives, and natural product derivatives and synthesis of C5-aryl drug derivatives further demonstrated the method's utility. The preliminary mechanistic studies reveal that the synergistic combination of the bulky yet electrophilic perfluoroaryl-Pd species and the partial nucleophilicity of the C5-position of 2-amino/alkoxy-pyridines is the origin of reactivity and selectivity. Importantly, the first experimental evidence for the role of diisopropyl sulfide is provided.

Palladium mono-N-protected amino acid complexes: experimental validation of the ligand cooperation model in C-H activation

Mechanistic proposals for the C-H activation reaction enabled by mono-N-protected amino acid ligands (MPAAs) have been supported by DFT calculations. The direct experimental observation of the ligand-assisted C-H activation has not yet been reported due to the lack of well-defined isolated palladium complexes with MPAAs that can serve as models. In this work, palladium complexes bearing chelating MPAAs (NBu₄)[Pd(κ²-N,O-AcN-CHR-COO)(C₆F₅)py] (Ac = MeC(O); R = H, Me) and [Pd(κ²-N,O-MeNH-CH₂-COO)(C₆F₅)py] have been isolated and characterized. Their evolution in a solution containing toluene leads to the C-H activation of the arene and the formation of the C₆F₅-C₆H₄Me coupling products. This process takes place only for the ligands with an acyl protecting group, showing the cooperating role of this group in a complex with a chelating MPAA, therefore experimentally validating this working model. The carboxylate group is inefficient in this C-H activation.

Redox-neutral carbon-heteroatom bond formation under photoredox catalysis

Recently, visible-light-mediated photoredox catalysis has been emerging as one of the fastest growing fields in organic chemistry because of its low cost, easy availability and environmental benignness. In the past five years, a new yet challenging trend, visible-light-induced redox-neutral carbon-heteroatom bond formation reaction involving presumed radical intermediates, has been flourishing rapidly. Although mostly transition metal-based photoredox catalysts were reported, a few organophotoredox catalysts have also shown efficacy towards carbon-heteroatom bond formation reactions. This review intends to summarize the recent research progress in redox-neutral carbon-heteroatom bond formations based on active intermediate(s) involved under photoredox catalysis.

Dual-Ligand Catalyst for the Nondirected C-H Olefination of Heteroarenes

Pd(II)-catalyzed nondirected C-H functionalization of heteroarenes is a significant challenge for the following reasons: poor reactivity of electron-deficient heterocycles and the unproductive coordination of Lewis basic nitrogen atoms. Existing methodologies using palladium catalysis often employ a large excess of heterocycle substrates to overcome these hurdles. Despite recent advances in nondirected functionalization of arenes that allow them to be used as limiting reagents, the reaction conditions are incompatible with electron-deficient heteroarenes. Herein we report a dual-ligand catalyst that enables Pd(II)-catalyzed nondirected C-H olefination of heteroarenes without using a large excess of substrate. In general, the use of 1-2 equiv of substrates was sufficient to obtain synthetically useful yields. The reactivity was rationalized by the synergy between two types of ligands: a bidentate pyridine-pyridone ligand promotes C-H cleavage; the monodentate heterocycle substrate acts as a second ligand to form a cationic Pd(II) complex that has high affinity for arenes. The proposed dual-ligand cooperation is supported by a combination of X-ray, kinetics, and control experiments.

Transition Metal-Catalyzed C-H Functionalization Through Electrocatalysis

Electrochemically promoted transition metal-catalyzed C-H functionalization has emerged as a promising area of research over the last few decades. However, development in this field is still at an early stage compared to traditional functionalization reactions using chemical-based oxidizing agents. Recent reports have shown increased attention on electrochemically promoted metal-catalyzed C-H functionalization. From the standpoint of sustainability, environmental friendliness, and cost effectiveness, electrochemically promoted oxidation of a metal catalyst offers a mild, efficient, and atom-economical alternative to traditional chemical oxidants. This Review discusses advances in the field of transition metal-electrocatalyzed C-H functionalization over the past decade and describes how the unique features of electricity enable metal-catalyzed C-H functionalization in an economic and sustainable way.

Copper-Catalyzed Regioselective Remote C-H Bond Chalcogenation of Aromatic Amine Derivatives without Using Any Large Template

A mild and convenient strategy has been developed for the para-selective chalcogenation of anilide scaffolds via C-H bond functionalization. This methodology employs one of the most earth-abundant and inexpensive Cu(II) catalysts and a commercially available simple aryl chalcogen source without any complex directing template, exogenous ligand, acid/base, oxidant, or other additives. The key feature of this methodology is an impressive regioselectivity along with a wide range of functional group tolerance with good to excellent yields under aerobic conditions.