Recent applications of C-H functionalization in complex natural product synthesis

Chiral amino acids: evolution in atroposelective C-H activation

This review covers the journey of chiral amino acids as ligands in atroposelective C-H bond activation/functionalization via transition metal catalysis. Herein, we intend to demonstrate how these chiral amino acids have evolved and flourished in this stimulating field. Unprotected amino acids, mono-N-protected amino acids, and di-N-protected amino acids have been devised for atroposelective C-H activation. In each section, we have briefly discuss the key successes of amino acids in the atroposelective synthesis of biaryls, heterobiaryls, and non-biaryl atropisomers and their advantages in atroposelective C-H activation.

Accessing 7,8-Dihydroquinoline-2,5-diones via Rh-Catalyzed Olefinic C-H Activation/[4+2] Cyclization

Herein, we report a rhodium-catalyzed C-H activation/[4+2] cyclization reaction between α,β-unsaturated amides and iodonium ylides for the synthesis of novel 7,8-dihydroquinoline-2,5-diones and analogues. This protocol provides a series of pyridones fused with saturated cycles with good functional group compatibility, good water and air tolerance, and good to excellent yields under mild and green reaction conditions. Additionally, scale-up synthesis can be smoothly performed with as low as 0.25 mol % catalyst loading. Recycling experiments and different transformation experiments were also carried out to demonstrate the potential synthetic utility of this protocol.

[2.2]Benzoindenophane-Based Chiral Indenyl Ligands: Design, Synthesis, and Applications in Asymmetric C-H Activation

Development of chiral indenyl ligands for asymmetric C-H activation is a longstanding challenge, and extremely few successes have been achieved. In this paper, we describe a class of readily accessible, facilely tunable and user-friendly chiral indenyl ligands featuring a [2.2]benzoindenophane skeleton via a divergent synthesis strategy. The corresponding chiral indenyl rhodium catalysts were successfully applied in the asymmetric C-H activation reaction of O-Boc hydroxybenzamide with alkenes to give various chiral dihydroisoquinolone products (up to 97 % yield, up to 98 % ee). Moreover, the asymmetric C-H activation reaction of carboxylic acids with alkynes was also successfully accomplished, providing a range of axially chiral isocoumarins (up to 99 % yield, up to 94 % ee). Notably, this represents the first example of enantioselective transition metal catalyzed C(sp2)-H activation/oxidative coupling of benzoic acids with internal alkynes to construct isocoumarins. Given many attractive features of this class of indenyl ligands, such as convenient synthesis, high tunability and exclusive face-selectivity of coordination, its applications in more catalytic asymmetric C-H activation and in other asymmetric catalysis are foreseen.

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.

Carbon-nitrogen transmutation in polycyclic arenol skeletons to access N-heteroarenes

Developing skeletal editing tools is not a trivial task, and realizing the corresponding single-atom transmutation in a ring system without altering the ring size is even more challenging. Here, we introduce a skeletal editing strategy that enables polycyclic arenols, a highly prevalent motif in bioactive molecules, to be readily converted into N-heteroarenes through carbon-nitrogen transmutation. The reaction features selective nitrogen insertion into the C-C bond of the arenol frameworks by azidative dearomatization and aryl migration, followed by ring-opening, and ring-closing (ANRORC) to achieve carbon-to-nitrogen transmutation in the aromatic framework of the arenol. Using widely available arenols as N-heteroarene precursors, this alternative approach allows the streamlined assembly of complex polycyclic heteroaromatics with broad functional group tolerance. Finally, pertinent transformations of the products, including synthesis complex biheteroarene skeletons, were conducted and exhibited significant potential in materials chemistry.

Derivatization of 2,1,3-Benzothiadiazole via Regioselective C-H Functionalization and Aryne Reactivity

Despite growing interest in 2,1,3-benzothiadiazole (BTD) as an integral component of many functional molecules, methods for the functionalization of its benzenoid ring have remained limited, and many even simply decorated BTDs have required de novo synthesis. We show that regioselective Ir-catalyzed C-H borylation allows access to versatile 5-boryl or 4,6-diboryl BTD building blocks, which undergo functionalization at the C4, C5, C6, and C7 positions. The optimization and regioselectivity of C-H borylation are discussed. A broad reaction scope is presented, encompassing ipso substitution at the C-B bond, the first examples of ortho-directed C-H functionalization of BTD, ring closing reactions to generate fused ring systems, as well as the generation and capture reactions of novel BTD-based heteroarynes. The regioselectivity of the latter is discussed with reference to the Aryne Distortion Model.

Climbing the Oxidase Phase Ladder by Using Dioxygen as the Sole Oxidant: The Case Study of Costunolide

Natural sesquiterpenoid lactones are prominent scaffolds in drug discovery. Despite the progress made in their synthesis, their extensive oxidative decoration makes their chemo- and stereoselective syntheses highly challenging. Herein, we report our effort to mimic part of the oxidase phase used in the costunolide pathway to achieve the protecting-group-free total synthesis of santamarine, dehydrocostus lactone, estafiatin, and nine more related natural sesquiterpenoid lactones by using dioxygen as the sole oxidant.

Studies towards the Enantioselective Synthesis of Cryptowolinol via Pd0-Catalyzed C(sp3)-H Arylation/Parallel Kinetic Resolution

We report a model study towards the enantioselective synthesis of the dibenzopyrrocoline alkaloid (-)-cryptowolinol. The key step involves a challenging enantioselective Pd0-catalyzed C(sp3)-H arylation performed with a chiral NHC ligand, which proceeds via parallel kinetic resolution (PKR). A very efficient PKR process was achieved on a deoxygenated model substrate and was successfully transposed to a potential intermediate en route to (-)-cryptowolinol.

Free Amine-Directed Redox Neutral Ruthenium(II) Catalysis toward Regioselective Synthesis of Heterobiaryls

A regioselective coupling of ortho-heteroaryl anilines and 7-oxabenzonorbornadienes has been developed by leveraging free amine-directed redox-neutral Ru(II) catalysis. This protocol facilitates formal C-2 arylation of the indole moiety under mild conditions to offer valuable heterobiaryls in high yields. The reaction displays a broad substrate generality and scalability and retains efficacy in the presence of diverse pharmacophore scaffolds. Moreover, products bearing a free amine group were successfully employed in Mg(NTf2)2-catalyzed double Michael addition cascade, which led to the synthesis of intricate indole- and pyrrole-fused azaheterocycles.

Rhodium-catalysed additive-free carbonylation of benzamides with diethyl dicarbonate as a carbonyl source

Phthalimides are prevalent in numerous pharmaceuticals, prompting various phthalimide syntheses through C-H activation. Nevertheless, the necessity for stoichiometric additives limits their practicality and versatility. Herein, we introduced diethyl dicarbonate as a carbonyl source for an additive-free carbonylation of benzamides. This transformation signifies an operationally simple and CO-free phthalimide synthesis.

Enantioselective Divergent Syntheses of Diterpenoid Pyrones

Capitalizing a synergy between late-stage C(sp3)-H alkynylation and a series of transition metal-catalyzed alkyne functionalization reactions, we reported herein enantioselective divergent synthesis of 10 diterpenoid pyrones within 14-16 steps starting from chiral pool enoxolone, including the first enantioselective synthesis of higginsianins A, B, D, E, and metarhizin C. Our synthesis also highlights an unprecedented biomimetic oxidative rearrangement of α-pyrone into 3(2H)-furanone, as well as applications of Echavarren C(sp3)-H alkynylation reaction and Toste chiral counterion-mediated Au-catalyzed intramolecular allene hydroalkoxylation in natural product synthesis.

Trialkoxysilane-Induced Iridium-Catalyzed para-Selective C-H Bond Borylation of Arenes

An ideal approach for the construction of aryl boron compounds is to selectively replace a C-H bond in arenes with a C-B bond, and controlling regioselectivity is one of the most challenging aspects of these transformations. Herein, we report an iridium-catalyzed trialkoxysilane protecting group-assisted regioselective C-H borylation of arenes, including derivatives of benzaldehydes, acetophenones, benzoic acids, benzyl alcohols, phenols, aryl silanes, benzyl silanes, and multi-functionalized aromatic rings are all well tolerated and gave the para -selective C-H borylation products in a short time without the requirement of inert gases atmosphere. The site-selective C-H borylation can be adjustable by installing the developed trialkoxysilane protecting group on different functional groups on one aromatic ring. Importantly, the preparation process of the trialkoxychlorosilane is efficient and scalable. Mechanistic and computational studies reveal that the steric hindrance of the trialkoxysilane protecting group plays a key role in dictating the para-selectivity.

Recent advances in Rh(I)-catalyzed enantioselective C-H functionalization

Chiral carbon-carbon (C-C) and carbon-heteroatom (C-X) bonds are pervasive and very essential in natural products, bioactive molecules, and functional materials, and their catalytic construction has emerged as one of the hottest research fields in synthetic organic chemistry. The last decade has witnessed vigorous progress in Rh(I)-catalyzed asymmetric C-H functionalization as a complement to Rh(II) and Rh(III) catalysis. This review aims to provide the most comprehensive and up-to-date summary covering the recent advances in Rh(I)-catalyzed C-H activation for asymmetric functionalization. In addition to the development of diverse reactions, chiral ligand design and mechanistic investigation (inner-sphere mechanism, outer-sphere mechanism, and 1,4-Rh migration) will also be highlighted.

Diaryldiazoketones as Effective Carbene Sources for Highly Selective Rh(II)-Catalyzed Intermolecular C-H Functionalization

A novel donor/acceptor carbene intermediate has been developed using diaryldiazoketones as carbene precursors. In the presence of the chiral dirhodium catalyst, Rh2(S-TPPTTL)4, diaryldiazoketones undergo highly regio-, stereo-, and diastereoselective C-H functionalization of activated and unactivated secondary and tertiary C-H bonds. Computational studies revealed that the arylketo group behaves differently than the carboxylate acceptor group because the orientation of the arylketo group predetermines which face of the carbene will be attacked.

Remote C-H Amination and Alkylation of Camphor at C8 through Hydrogen-Atom Abstraction

Camphor continues to serve as a versatile chiral building block for chemical synthesis. We have developed a novel method to functionalize the camphor skeleton at C8 using an intramolecular hydrogen atom abstraction. The key advance involves the use of a camphor-derived aminonitrile, which is converted to the corresponding nitrogen-centered radical under photoredox conditions to effect the 1,5-hydrogen atom transfer at C8. The resulting carbon-centered radical at C8 was utilized in a C-H amination to access topologically complex proline derivatives. Furthermore, the total synthesis of several sesquiterpenoids was accomplished by engaging the radical generated at C8 in alkylation reactions.

Catalyst-Controlled Intermolecular Homobenzylic C(sp3)-H Amination for the Synthesis of β-Arylethylamines

The combination of a tailored sulfamate with a C4-symmetrical rhodium(II) tetracarboxylate allows to uncover a selective intermolecular amination of unactivated homobenzylic C(sp3)-H bonds. The reaction has a broad scope (>30 examples) and proceeds with a high level of regioselectivity with homobenzylic/benzylic ratio of up to 35:1, thereby providing a direct access to β-arylethylamines that are of utmost interest in medicinal chemistry. Computational investigations evidenced a concerted mechanism, involving an asynchronous transition state. Based on a combined activation strain model and energy decomposition analysis, the regioselectivity of the reaction was found to rely mainly on the degree of orbital interaction between the [Rh2]-nitrene and the C-H bond. The latter is facilitated at the homobenzylic position due to the establishment of specific noncovalent interactions within the catalytic pocket.

Cobalt-Catalyzed Enantioselective C-H Carbonylation towards Chiral Isoindolinones

Transition metal-catalyzed enantioselective C-H carbonylation with carbon monoxide, an essential and easily available C1 feedstock, remains challenging. Here, we disclosed an unprecedented enantioselective C-H carbonylation catalyzed by inexpensive and readily available cobalt(II) salt. The reactions proceed efficiently through desymmetrization, kinetic resolution, and parallel kinetic resolution, affording a broad range of chiral isoindolinones in good yields with excellent enantioselectivities (up to 92 % yield and 99 % ee). The synthetic potential of this method was demonstrated by asymmetric synthesis of biological active compounds, such as (S)-PD172938 and (S)-Pazinaclone. The resulting chiral isoindolinones also serve as chiral ligands in cobalt-catalyzed enantioselective C-H annulation with alkynes to construct phosphorus stereocenter.

Guggulsterone - a potent bioactive phytosteroid: synthesis, structural modification, and its improved bioactivities

Guggulsterone is a phytosteroid derived from the oleo-gum resin of the critically endangered plant Commiphora wightii. This molecule has attracted increasing attention due to its excellent biochemistry potential and the compound has consequently been evaluated in clinical trials. With a low concentration in natural resources but wide medicinal and therapeutic value, chemists have developed several synthetic routes for guggulsterone starting from various steroid precursors. Moreover, numerous studies have attempted to modify its structure to improve the biological properties. Nowadays, green and sustainable chemistry has also attracted more attention for advanced chemical processes and reactions in steroid chemistry. The present review aimed to summarize the literature and provide an update about the improvements in the chemical synthesis and structural modification of guggulsterone from the view of green chemistry. Moreover, this review encompasses the improved activities of structurally modified guggulsterone derivatives. We expect that the information provided here will be useful to researchers working in this field and on this molecule.

Convergent and Efficient Total Synthesis of (+)-Heilonine Enabled by C-H Functionalizations

We report a convergent and efficient total synthesis of the C-nor D-homo steroidal alkaloid (+)-heilonine with a hexacyclic ring system, nine stereocenters, and a trans-hydrindane moiety. Our synthesis features four selective C-H functionalizations to form key C-C bonds and stereocenters, a Stille carbonylative cross-coupling to connect the AB ring system with the DEF ring system, and a Nazarov cyclization to construct the five-membered C ring. These enabling transformations significantly reduced functional group manipulations and delivered (+)-heilonine in 11 or 13 longest linear sequence (LLS) steps.

Rh(III)-catalyzed selective mono- and dual-functionalization/cyclization of 1-aryl-5-aminopyrazoles with iodonium ylides

An efficient Rh(III)-catalyzed selective mono- and dual-C-H bond functionalization/cyclization with iodonium ylide as a single coupling partner was demonstrated, in which fused benzodiazepine skeletons were obtained in excellent yields. This method greatly improved an effective approach to dual C-H unsymmetrical functionalization.

Transition-Metal-Catalyzed Directed C-H Functionalization in/on Water

Directing group assisted C-H bond functionalization using transition-metal-catalysis has emerged as a reliable synthetic tool for the construction of regioselective carbon-carbon/heteroatom bonds. Off late, "in/on water directed transition-metal-catalysis", though still underdeveloped, has appeared as one of the prominent themes in sustainable organic chemistry. This article covers the advancements, mechanistic insights and application of the sustainable directed C-H bond functionalization of (hetero)arenes in/on water in the presence of transition-metal-catalysis.

Methylene C(sp3 )-H Arylation Enables the Stereoselective Synthesis and Structure Revision of Indidene Natural Products

The divergent synthesis of two indane polyketides of the indidene family, namely (±)-indidene A (11 steps, 1.7 %) and (+)-indidene C (13 steps, 1.3 %), is reported. The synthesis of the trans-configured common indane intermediate was enabled by palladium(0)-catalyzed methylene C(sp3 )-H arylation, which was performed in both racemic and enantioselective (e.r. 99 : 1) modes. Further elaboration of this common intermediate by nickel-catalyzed dehydrogenative coupling allowed the rapid installation of the aroyl moiety of (±)-indidene A. In parallel, the biphenyl system of (±)- and (+)-indidene C was constructed by Suzuki-Miyaura coupling. These investigations led us to revise the structures of indidenes B and C.

The Recent Advances in Iron-Catalyzed C(sp3 )-H Functionalization

The use of iron as a core metal in catalysis has become a research topic of interest over the last few decades. The reasons are clear. Iron is the most abundant transition metal on Earth's crust and it is widely distributed across the world. It has been extracted and processed since the dawn of civilization. All these features render iron a noncontaminant, biocompatible, nontoxic, and inexpensive metal and therefore it constitutes the perfect candidate to replace noble metals (rhodium, palladium, platinum, iridium, etc.). Moreover, direct C-H functionalization is one of the most efficient strategies by which to introduce new functional groups into small organic molecules. The majority of organic compounds contain C(sp3 )-H bonds. Given the enormous importance of organic molecules in so many aspects of existence, the utilization and bioactivity of C(sp3 )-H bonds are of the utmost importance. This review sheds light on the substrate scope, selectivity, benefits, and limitations of iron catalysts for direct C(sp3 )-H bond activations. An overview of the use of iron catalysis in C(sp3 )-H activation protocols is summarized herein up to 2022.

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.

Rh(III) Catalyzed Redox-Neutral C-H Activation/[5 + 2] Annulation of Aroyl Hydrazides and Sulfoxonium Ylides: Synthesis of Benzodiazepinones

We herein report the Rh(III) catalyzed redox-neutral C-H activation/[5 + 2] annulation of aroyl hydrazides with sulfoxonium ylides as safe carbene precursors. The reaction shows excellent functional group tolerance, broad substrate scope, and scalability. We demonstrated the synthetic utility of the protocol via the synthesis of various diazepam drug analogues, late-stage functionalization of probenecid drug, and large scale synthesis. Finally, kinetic studies revealed C-H activation as the rate-determining step.

Cobalt-Catalyzed Enantioselective C-H Annulation of Benzylamines with Alkynes: Application to the Modular and Asymmetric Syntheses of Bioactive Molecules

The transition metal-catalyzed enantioselective C-H functionalization strategy has revolutionized the logic of natural product synthesis. However, previous applications have heavily relied on the use of noble metal catalysts such as rhodium and palladium. Herein, we report the efficient synthesis of C1-chiral 1,2-dihydroisoquinolines (DHIQs) via enantioselective C-H/N-H annulation of picolinamides with alkynes catalyzed by a more sustainable and cheaper 3d metal catalyst, cobalt(II) acetate tetrahydrate. A wide range of enantiomerically enriched DHIQs were obtained in good yields with excellent enantioselectivities (up to 98% yield and >99% ee). The robustness and synthetic potential of this method were demonstrated by the modular and asymmetric syntheses of several tetrahydroisoquinoline alkaloids, including (S)-norlaudanosine, (S)-laudanosine, (S)-xylopinine, (S)-sebiferine, and (S)-cryptostyline II, and the asymmetric syntheses of key intermediates of (+)-solifenacin, FR115427, and (+)-NPS R-568.

Preparative Scale Applications of C-H Activation in Medicinal Chemistry

C-H activation is an attractive methodology to increase molecular complexity without requiring substrate prefunctionalization. In contrast to well-established cross-coupling methods, C-H activation is less explored on large scales and its use in the production of pharmaceuticals faces substantial hurdles. However, the inherent advantages, such as shorter synthetic routes and simpler starting materials, motivate medicinal chemists and process chemists to overcome these challenges, and exploit C-H activation steps for the synthesis of pharmaceutically relevant compounds. In this review, we will cover examples of drugs/drug candidates where C-H activation has been implemented on a preparative synthetic scale (range between 355 mg and 130 kg). The optimization processes will be described, and each example will be examined in terms of its advantages and disadvantages, providing the reader with an in-depth understanding of the challenges and potential of C-H activation methodologies in the production of pharmaceuticals.

Organo-photoredox-Catalyzed Selective Mono- and Bis-C-H Alkylation of Electron-Rich (Hetero)Arenes

Herein, we disclose a simple strategy for the C-H alkylation of electron-rich (hetero)arenes with alkyl bromides employing visible-light-mediated organo-photocatalytic SET processes. The generality of this method has been evidenced by the inclusion of a variety of alkyl radicals (α-alkyl-carbonyl, benzyl, cyanomethyl) as well as diverse biologically active electron-rich arenes and (hetero)arenes under mild conditions. The extent of alkylation with alkyl bromides was found to be controlled by introducing Zn(OAc)2 as a bromide scavenger, ensuring the blocking of potential bromo-arene byproduct formation under photoredox conditions. In addition, a sequential C-H alkylation strategy for selective bis-alkylation has also been developed via chronological incorporation of different alkyl radical precursors in one pot quite efficiently.

Strategic application of C-H oxidation in natural product total synthesis

The oxidation of unactivated C-H bonds has emerged as an effective tactic in natural product synthesis and has altered how chemists approach the synthesis of complex molecules. The use of C-H oxidation methods has simplified the process of synthesis planning by expanding the choice of starting materials, limiting functional group interconversion and protecting group manipulations, and enabling late-stage diversification. In this Review, we propose classifications for C-H oxidations on the basis of their strategic purpose: type 1, which installs functionality that is used to establish the carbon skeleton of the target; type 2, which is used to construct a heterocyclic ring; and type 3, which installs peripheral functional groups. The reactions are further divided based on whether they are directed or undirected. For each classification, examples from recent literature are analysed. Finally, we provide two case studies of syntheses from our laboratory that were streamlined by the judicious use of C-H oxidation reactions.

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.

Hydrogen-bond-acceptor ligands enable distal C(sp3)-H arylation of free alcohols

The functionalization of C-H bonds in organic molecules is one of the most direct approaches for chemical synthesis. Recent advances in catalysis have allowed native chemical groups such as carboxylic acids, ketones and amines to control and direct C(sp3)-H activation1-4. However, alcohols, among the most common functionalities in organic chemistry5, have remained intractable because of their low affinity for late transition-metal catalysts6,7. Here we describe ligands that enable alcohol-directed arylation of δ-C(sp3)-H bonds. We use charge balance and a secondary-coordination-sphere hydrogen-bonding interaction-evidenced by structure-activity relationship studies, computational modelling and crystallographic data-to stabilize L-type hydroxyl coordination to palladium, thereby facilitating the assembly of the key C-H cleavage transition state. In contrast to previous studies in C-H activation, in which secondary interactions were used to control selectivity in the context of established reactivity8-13, this report demonstrates the feasibility of using secondary interactions to enable challenging, previously unknown reactivity by enhancing substrate-catalyst affinity.

Conceptual advances in nucleophilic organophosphine-promoted transformations

Catalysis by trivalent nucleophilic organophosphines has emerged as an essential tool in organic synthesis. Several new organic transformations promoted by phosphines substantiate and complement the existing synthetic chemistry tools. Mere design of the substrate and reagent combinations has introduced new modes of reactivity patterns, which are otherwise difficult to achieve. These design considerations have led to the rapid build-up of complex molecular entities and laid a solid foundation to synthesise bioactive natural products and pharmaceuticals. This article presents an overview of some of the conceptual advances, including our contributions to nucleophilic organophosphine chemistry. The scope, limitations, mechanistic insights, and applications of these metal-free transformations are discussed elaborately.

Isoxazole as a nitrile synthon: en routes to the ortho-alkenylated isoxazole and benzonitrile with allyl sulfone catalyzed by Ru(II)

A Ru(II) catalyzed regioselective Heck-type C-H olefination of isoxazole with unactivated allyl phenyl sulfone is revealed. The solvent DCM offers dual sp2-sp2 C-H activation via an N-directed strategy, leading to ortho-olefinated isoxazoles with exclusive E-selectivity. On the other hand, in DCE solvent, isoxazole serves as the nitrile synthon and leads to o-olefinated benzonitrile. At a higher temperature (110 °C) in DCE, after the ortho-olefination Ru(II) mediated cleavage of isoxazoles delivered the nitrile functionality.

Cobalt-catalyzed atroposelective C-H activation/annulation to access N-N axially chiral frameworks

The N-N atropisomer, as an important and intriguing chiral system, was widely present in natural products, pharmaceutical lead compounds, and advanced material skeletons. The anisotropic structural characteristics caused by its special axial rotation have always been one of the challenges that chemists strive to overcome. Herein, we report an efficient method for the enantioselective synthesis of N-N axially chiral frameworks via a cobalt-catalyzed atroposelective C-H activation/annulation process. The reaction proceeds under mild conditions by using Co(OAc)2·4H2O as the catalyst with a chiral salicyl-oxazoline (Salox) ligand and O2 as an oxidant, affording a variety of N-N axially chiral products with high yields and enantioselectivities. This protocol provides an efficient approach for the facile construction of N-N atropisomers and further expands the range of of N-N axially chiral derivatives. Additionally, under the conditions of electrocatalysis, the desired N-N axially chiral products were also successfully achieved with good to excellent efficiencies and enantioselectivities.

Recent Advances in C-H Functionalisation through Indirect Hydrogen Atom Transfer

The functionalisation of C-H bonds has been an enormous achievement in synthetic methodology, enabling new retrosynthetic disconnections and affording simple synthetic equivalents for synthons. Hydrogen atom transfer (HAT) is a key method for forming alkyl radicals from C-H substrates. Classic reactions, including the Barton nitrite ester reaction and Hofmann-Löffler-Freytag reaction, among others, provided early examples of HAT. However, recent developments in photoredox catalysis and electrochemistry have made HAT a powerful synthetic tool capable of introducing a wide range of functional groups into C-H bonds. Moreover, greater mechanistic insights into HAT have stimulated the development of increasingly site-selective protocols. Site-selectivity can be achieved through the tuning of electron density at certain C-H bonds using additives, a judicious choice of HAT reagent, and a solvent system. Herein, we describe the latest methods for functionalizing C-H/Si-H/Ge-H bonds using indirect HAT between 2018-2023, as well as a critical discussion of new HAT reagents, mechanistic aspects, substrate scopes, and background contexts of the protocols.

C-H Bonds as Functional Groups: Simultaneous Generation of Multiple Stereocenters by Enantioselective Hydroxylation at Unactivated Tertiary C-H Bonds

Enantioselective C-H oxidation is a standing chemical challenge foreseen as a powerful tool to transform readily available organic molecules into precious oxygenated building blocks. Here, we describe a catalytic enantioselective hydroxylation of tertiary C-H bonds in cyclohexane scaffolds with H2O2, an evolved manganese catalyst that provides structural complementary to the substrate similarly to the lock-and-key recognition operating in enzymatic active sites. Theoretical calculations unveil that enantioselectivity is governed by the precise fitting of the substrate scaffold into the catalytic site, through a network of complementary weak non-covalent interactions. Stereoretentive C(sp3)-H hydroxylation results in a single-step generation of multiple stereogenic centers (up to 4) that can be orthogonally manipulated by conventional methods providing rapid access, from a single precursor to a variety of chiral scaffolds.

A General Enantioselective C-H Arylation Using an Immobilized Recoverable Palladium Catalyst

We herein report a general and efficient enantioselective C-H arylation of aryl bromides based on the use of BozPhos as the bisphosphine ligand and SP-NHC-PdII as recoverable heterogeneous catalyst. By exploiting the "release and catch" mechanism of action of the catalytic system, we used BozPhos as a broadly applicable chiral ligand, furnishing high enantioselectivities across all types of examined substrates containing methyl, cyclopropyl and aryl C-H bonds. For each reaction, the reaction scope was investigated, giving rise to 30 enantioenriched products, obtained with high yields and enantioselectivities, and minimal palladium leaching. The developed catalytic system provides a more sustainable solution compared to homogeneous systems for the synthesis of high added-value chiral products through recycling of the precious metal.

Photoinduced meta-Selective C-H Oxygenation of Arenes

The merger of photocatalysis and transition-metal catalysis has recently emerged as an adaptable platform for the development of innovative and environmentally benign synthetic methodologies. In contrast to classical transformation by Pd complexes, photoredox Pd catalysis operates through a radical pathway in the absence of a radical initiator. Using the synergistic merger of photoredox and Pd catalysis, we have developed a highly efficient, regioselective, and general meta-oxygenation protocol for diverse arenes under mild reaction conditions. The protocol showcases the meta-oxygenation of phenylacetic acids and biphenyl carboxylic acids/alcohols and is also amenable for a series of sulfonyls and phosphonyl-tethered arenes, irrespective of the nature and position of the substituents. Unlike thermal C-H acetoxylation which operates through the Pd^II/Pd^IV catalytic cycle, this metallaphotocatalytic C-H activation involves Pd^II/Pd^III/Pd^IV intermediacy. The radical nature of the protocol is established through radical quenching experiments and EPR analysis of the reaction mixture. Furthermore, the catalytic path of this photoinduced transformation is established through control reactions, absorption spectroscopy, luminescence quenching, and kinetic studies.

Aspartyl β-Turn-Based Dirhodium(II) Metallopeptides for Benzylic C(sp3)-H Amination: Enantioselectivity and X-ray Structural Analysis

Amination of C(sp3)-H bonds is a powerful tool to introduce nitrogen into complex organic frameworks in a direct manner. Despite significant advances in catalyst design, full site- and enantiocontrol in complex molecular regimes remain elusive using established catalyst systems. To address these challenges, we herein describe a new class of peptide-based dirhodium(II) complexes derived from aspartic acid-containing β-turn-forming tetramers. This highly modular system can serve as a platform for the rapid generation of new chiral dirhodium(II) catalyst libraries, as illustrated by the facile synthesis of a series of 38 catalysts. Critically, we present the first crystal structure of a dirhodium(II) tetra-aspartate complex, which unveils retention of the β-turn conformation of the peptidyl ligand; a well-defined hydrogen-bonding network is evident, along with a near-C4 symmetry that renders the rhodium centers inequivalent. The utility of this catalyst platform is illustrated by the enantioselective amination of benzylic C(sp3)-H bonds, in which state-of-the-art levels of enantioselectivity up to 95.5:4.5 er are obtained, even for substrates that present challenges with previously reported catalyst systems. Additionally, we found these complexes to be competent catalysts for the intermolecular amination of N-alkylamides via insertion into the C(sp3)-H bond α to the amide nitrogen, yielding differentially protected 1,1-diamines. Of note, this type of insertion was also observed to occur on the amide functionalities of the catalyst itself in the absence of the substrate but did not appear to be detrimental to reaction outcomes when the substrate was present.

Strategies Toward the Catalyst-Free α-C-H Functionalizations of Tertiary Amines

α-C-H functionalization of tertiary amines has been a highly studied field for the past two decades because several important nitrogen containing heterocycles or compounds can be synthesized through this strategy. Though transition metal catalysts and some metal-free catalysts are mainly used for these reactions, a few catalyst-free reactions have recently been efficiently performed. Catalyst-free reactions are cost-effective, less sensitive to air/moisture, easier to operate, have a simple purification process, and are relatively environment-friendly. In this article, we have summarized all the α-C-H functionalization reactions of tertiary amines performed without using any external catalysts. The content of this article will undoubtedly encourage readers to do more work in this area.

Helicity-modulated remote C-H functionalization

Remote C-H functionalization is highly important for the conversion and utilization of arenes, but the conventional routes are comprehensively developed with the assistance of transition metal catalysts or templates. We report a facile metal/template-free electrochemical strategy for remote C-H functionalization in a helical system, where aromatic or aliphatic hydrogen act as a directing group to promote the alkoxylation at the opposite site of the helical skeleton by generating a unique helical "back-biting" environment. Such helicity-modulated C-H functionalization is prevalent for carbo[n]helicenes (n = 6 to 9, primitive or substituted) and hetero[6]helicenes and also occurs when the aryl hydrogen on the first position is replaced by a methyl group or a phenyl group. Thus, the relatively inert helicene skeleton can be precisely furnished with a rich array of alkoxy pendants with tunable functional moieties. Notably, the selective decoration of a methoxy group on N-methylated aza[6]helicene close or distant to the nitrogen atom leads to distinct luminescence variation upon changing the solvents.

A three component 1,3-difunctionalization of vinyl diazo esters enabled by a cobalt catalyzed C-H activation/carbene migratory insertion

We report herein, a modular, regioselective 1,3-oxyarylation of vinyl diazo esters via a Co-catalyzed C-H activation/carbene migratory insertion cascade. The transformation involves the formation of C-C and C-O bonds in a one-pot fashion and displays a broad substrate scope with respect to both, vinyl diazo esters as well as benzamides. The coupled products were subjected to hydrogenation to access elusive allyl alcohol scaffolds. Mechanistic investigations reveal interesting insights on the mode of transformation, involving C-H activation, carbene migratory insertion of the diazo compound followed by a radical addition as the key steps of the transformation.

Unlocking the Nucleophilicity of Strong Alkyl C-H Bonds via Cu/Cr Catalysis

Direct functionalization of inert C-H bonds is one of the most attractive yet challenging strategies for constructing molecules in organic chemistry. Herein, we disclose an unprecedented and Earth abundant Cu/Cr catalytic system in which unreactive alkyl C-H bonds are transformed into nucleophilic alkyl-Cr(III) species at room temperature, enabling carbonyl addition reactions with strong alkyl C-H bonds. Various aryl alkyl alcohols are furnished under mild reaction conditions even on a gram scale. Moreover, this new radical-to-polar crossover approach is further applied to the 1,1-difunctionalization of aldehydes with alkanes and different nucleophiles. Mechanistic investigations reveal that the aldehyde not only acts as a reactant but also serves as a photosensitizer to recycle the Cu and Cr catalysts.

A Cheap and Efficient Oxidant (n-Bu)4NNO3-Enabled C(sp2)- and C(sp3)-H Olefination at Room Temperature

To further promote the widely practical application of C-H activation, developing green and mild reaction conditions has invariably been the objective of researchers, especially when it comes to remote C-H activation reactions. Herein, we report a new cheap and powerful (n-Bu)₄NNO₃ oxidant. This oxidant is efficient and universal for Pd(II)-catalyzed sp² and sp³ C-H olefination and allows the reaction to be carried out at room temperature. Because of this, we attempted to make C-H functionalization more economical and environmentally benign.

Iridium catalysed C2 site-selective methylation of indoles using a pivaloyl directing group through weak chelation-assistance

Here we present an iridium catalysed C2-selective methylation of indoles using methyltrifluoroborate as a source of methyl group. The iridium catalyst selectively discriminates the indole C2 and C4 C-H bonds by coordination with a pivaloyl directing group.

Transition metal-catalyzed C-H/C-C activation and coupling with 1,3-diyne

This review provides a broad overview of the recent developments in the field of transition metal-catalyzed C-H/C-C bond activation and coupling with 1,3-diyne for assembling alkynylated heterocycles, bis-heterocycles, and 1,3-enynes. Transition metal-catalyzed inert bond (C-H/C-C) activation has been the focus of attention among synthetic chemists in recent times. Enormous developments have taken place in C-H/C-C bond activation chemistry in the last two decades. In recent years the use of 2π-unsaturated units as coupling partners for the synthesis of heterocycles through C-H/C-C bond activation and annulation sequence has received immense attention. Among the unsaturated units employed for assembling heterocycles, the use of 1,3-diynes has garnered significant attention due to its ability to render bis-heterocycles in a straightforward manner. The C-H bond activation and coupling with 1,3-diyne has been very much explored in recent years. However, the development of strategies for the use of 1,3-diynes in the analogous C-C bond activation chemistry is less explored. Earlier methods employed to assemble bis-heterocycle used heterocycles that were preformed and pre-functionalized via transition metal-catalyzed coupling reactions. The expensive pre-functionalized halo-heterocycles and sensitive and expensive heterocyclic metal reagents limit its broad application. However, the transition metal-catalyzed C-H activation obviates the need for expensive heterocyclic metal reagents and pre-functionalized halo-heterocycles. The C-H bond activation strategy makes use of C-H bonds as functional groups for effecting the transformation. This renders the overall synthetic sequence both step and cost economic. Hence, this strategy of C-H activation and subsequent reaction with 1,3-diyne could be used for the larger-scale synthesis of chemicals in the pharmaceutical industry. Despite these advances, there is still the possibility of exploration of earth-abundant and cost-effective first-row transition metals (Ni, Cu, Mn. Fe, etc.) for the synthesis of bis-heterocycles. Moreover, the Cp*-ligand-free, simple metal-salt-mediated synthesis of bis-heterocycles is also less explored. Thus, more exploration of reaction conditions for the Cp*-free synthesis of bis-heterocycles is called for. We hope this review will inspire scientists to investigate these unexplored domains.

Propargyl Alcohols as Bifunctional Reagents for Divergent Annulations of Biphenylamines via Dual C-H Functionalization/Dual Oxidative Cyclization

The C-H functionalization strategy provides access to valuable molecules that previously required convoluted synthetic attempts. Dual C-H unsymmetrical functionalization, with a single bifunctional reagent, is an effective tactic. Propargyl alcohols (PAs), despite containing a reactive C≡C bond, have not been explored as building blocks via oxidative cleavage. Annulations via C-H activation are a versatile and synthetically attractive strategy. We disclose PA as a new bifunctional reagent for unsymmetrical dual C-H functionalization of biphenylamine for regioselectively annulated outcomes. On tuning the conditions, the annulation bifurcated towards an unusual dual oxidative cyclization. This method accommodates a wide range of PAs and showcases late-stage diversification of some natural products.

Azolation of Benzylic C-H Bonds via Photoredox-Catalyzed Carbocation Generation

A visible-light photoredox-catalyzed method is reported that enables the coupling between benzylic C-H substrates and N-H azoles. Classically, medicinally relevant N-benzyl azoles are produced via harsh substitution conditions between the azole and a benzyl electrophile in the presence of strong bases at high temperatures. Use of C-H bonds as the alkylating partner streamlines the preparation of these important motifs. In this work, we report the use of N-alkoxypyridinium salts as a critically enabling reagent for the development of a general C(sp3)-H azolation. The platform enables the alkylation of electron-deficient, -neutral, and -rich azoles with a range of C-H bonds, most notably secondary and tertiary partners. Moreover, the protocol is mild enough to tolerate benzyl electrophiles, thus offering an orthogonal approach to existing SN2 and cross-coupling methods.