Transition-Metal-Catalyzed, Coordination-Assisted Functionalization of Nonactivated C(sp3)-H Bonds

Pd-Catalyzed Picolinamide-Directed C(sp2)-H Sulfonylation of Amino Acids/Peptides with Sodium Sulfinates

This report describes a Pd-catalyzed picolinamide-directed site-selective C(sp2)-H sulfonylation of amino acids and peptides with sodium sulfinates in moderate to good yields. Sulfonylation of levodopa and dopamine drug molecules and late-stage directed peptide sulfonylation are studied for the first time. Broad substrate scope having various functionalities, late-stage drug modifications, and various post synthetic utilities such as chalcogenation, bromination, olefination, and arylation are potential advantages.

Merging Ring-Opening 1,2-Metallate Shift with Asymmetric C(sp3)-H Borylation of Aziridines

Chiral secondary alkyl amines with a vicinal quaternary stereocenter are undoubtedly important and ubiquitous subunits in natural products and pharmaceuticals. However, their asymmetric synthesis remains a formidable challenge. Herein, we merge the ring-opening 1,2-metallate shift with iridium-catalyzed enantioselective C(sp3)-H borylation of aziridines to deliver these frameworks with high enantioselectivities. We also demonstrated the synthetic application by downstream transformations, including the total synthesis of two Amaryllidaceae alkaloids, (-)-crinane and (+)-mesmebrane.

Purinyl N-directed aroylation of 6-arylpurine ribo- and 2'-deoxyribonucleosides, and mechanistic insights

The purinyl ring contains four embedded nitrogen atoms of varying basicities. Selective utilization of these ring nitrogen atoms can lead to relatively facile remote functionalization, yielding modified purinyl motifs that are otherwise not easily obtained. Herein, we report previously undescribed N-directed aroylation of 6-arylpurine ribo and the more labile 2'-deoxyribonucleosides. Kinetic isotope analysis as well as reaction with a well-defined dimeric, palladated 9-benzyl 6-arylpurine provided evidence for N-directed cyclometallation as a key step, with a plausible rate-limiting C-H bond cleavage. Radical inhibition experiments indicate the likely intermediacy of aroyl radicals. The chemistry surmounts difficulties often posed in the functionalization of polynitrogenated and polyoxygenated nucleosidic structures that possess complex reactivities and a labile glycosidic bond that is more sensitive in the 2'-deoxy substrates.

Catalytic C(sp3)-H Trifluoroethylation of Amino Acids and Carboxylic Acids

Integrating of the trifluoroethyl (-CH2CF3) group into the organic compounds by activating the distal C(sp3)-H bond is a challenging but crucial task in organic chemistry. This transformation imparts unique physicochemical properties to the compounds, such as enhanced lipophilicity, metabolic stability, and altered electronic characteristics. In this study, we unveil a new palladium-catalyzed method to directly introduce the trifluoroethyl group into amino acid and carboxylic acid derivatives. Remarkably, this method effectively activates the β-C(sp3)-H bond across various substrates at room temperature. Utilizing mesityl(2,2,2-trifluoroethyl)iodonium triflate as a trifluoroethyl source, our approach selectively targets the distal β-C(sp3)-H bonds of amino and carboxylic acids, ensuring high chemoselectivity and enabling the straightforward synthesis of a diverse array of important γ-trifluoromethyl amino acid and carboxylic acid derivatives. Furthermore, the practical applicability of this methodology is demonstrated through its scalability for gram-scale synthesis.

Sulfonamide-directed site-selective functionalization of unactivated C(sp3)-H enabled by photocatalytic sequential electron/proton transfer

The generation of alkyl radical from C(sp3)-H substrates via hydrogen atom abstraction represents a desirable yet underexplored strategy in alkylation reaction since involving common concerns remain adequately unaddressed, such as the harsh reaction conditions, limited substrate scope, and the employment of noble metal- or photo-catalysts and stoichiometric oxidants. Here, we utilize the synergistic strategy of photoredox and hydrogen atom transfer (HAT) catalysis to accomplish a general and practical functionalization of unactived C(sp3)-H centers with broad reaction scope, high functional group compatibility, and operational simplicity. A combination of validation experiments and density functional theory reveals that the N-centered radicals, generated from free N - H bond in a stepwise electron/proton transfer event, are the key intermediates that enable an intramolecular 1,5-HAT or intermolecular HAT process for nucleophilic carbon-centered radicals formation to achieve heteroarylation, alkylation, amination, cyanation, azidation, trifluoromethylthiolation, halogenation and deuteration. The practical value of this protocol is further demonstrated by the gram-scale synthesis and the late-stage functionalization of natural products and drug derivatives.

Pd(II)-catalyzed cyclization of 2-methyl aromatic ketones with maleimides through weak chelation assisted dual C-H activation

A palladium-catalyzed dual C-H functionalization of substituted aromatic ketones and ester with maleimides leading to tricyclic heterocyclic molecules with good to excellent yield is reported. In this protocol, weak chelation of the carbonyl groups has been successfully utilized for the selective activation of the ortho-methyl C(sp3)-H bond instead of the ortho-C(sp2)-H bond in the presence of an external bidentate ligand Ac-Ile-OH. The reaction proceeds through two-fold C-H activation to generate a five-membered cyclic ring. The first C-H activation takes place selectively at the benzylic position followed by a second C-H bond activation at the meta position. The protocol demonstrates compatibility among diverse substituted aromatic ketones and ester as well as various substituted maleimides. Further derivatization of the tricyclic ketone to an alcohol exhibits the synthetic applicability of the protocol. Also, a plausible reaction mechanism has been proposed.

CuI-amidobis(phosphine) catalyzed C(sp3)-C(sp3) direct homo- and hetero-coupling of unactivated alkanes via C(sp3)-H activation

Herein, we present a CuI-dimer, [CuI{Ph2PC6H4C(O)NC6H4PPh2-o}]2, which catalyzed direct C(sp3)-H homocoupling of benzyl and cycloalkane derivatives with excellent yields and regio-selectivity. The method is very simple and tolerates various functionalities. Synergistic metal-ligand cooperativity was observed in Cu-N bond cleavage and protonation of nitrogen, and facilitates a bifunctional pathway, minimising the free energy corrugation for catalytic intermediates.

Iridium-Catalyzed Ortho-Selective C-H Borylation of Aryl Ketones with Transient Imine Ligands

Ortho-selective C-H borylation of aromatic ketones has not been extensively explored. Herein, we report the iridium-catalyzed ortho-selective C-H borylation of aromatic ketones using in situ-formed imine as the ligand. Good compatibility is observed for various substituted acetophenones and other aromatic ketones, and corresponding products are obtained with medium to excellent yields.

Rhodium-catalyzed three-component C(sp3)/C(sp2)-H activation enabled by a two-fold directing group strategy

Rh-catalyzed three-component C(sp3)/C(sp2)-H activation has been achieved through a two-directing group strategy. This protocol provides a convenient and efficient pathway for the construction of diverse 8-alkyl quinoline derivatives in one-pot. Furthermore, mechanistic studies revealed that the first C-H amidation was significantly faster than the sequential C-H alkylation.

[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.

Synthesis of CF3-Substituted N-Heterocyclic Compounds Based on C-H Activation-Initiated Formal [2 + 3] Annulation Featuring with a Latent Nucleophilic Site

Presented herein is a novel synthesis of CF3-substituted pyrrolo[1,2-a]indole derivatives based on the cascade reactions of N-alkoxycarbamoyl indoles with CF3-ynones. Mechanistically, the formation of a product involves a tandem process initiated by Rh(III)-catalyzed and N-alkoxycarbamoyl group-directed regioselective C2-H alkenylation of the indole scaffold followed by in situ removal of the directing group and intramolecular N-nucleophilic addition/annulation under one set of reaction conditions. To our knowledge, this is the first example in which a N-alkoxycarbamoyl unit initially acts as a directing group for C2-H functionalization of the indole scaffold and is then removed to provide the required reactive NH-moiety for subsequent intramolecular condensation. Moreover, the products thus obtained could be conveniently transformed into structurally and biologically attractive cycloheptenone fused indole derivatives through an acid-promoted cascade transformation. In addition, studies on the activity of selected products against human cancer cell lines demonstrated their potential as lead compounds for the development of novel anticancer drugs.

Directed C(sp3)-H Arylation of Free α-Aminophosphonates: Dual Models Exploration via Palladium Catalysis

In this report, we present the dual activation models for transient directing group-directed and amino-self-directed Pd-catalyzed α-aminophosphonate side-chain C(sp3)-H arylation. Both strategies showed facile, efficient, and single regioselectivity in the reaction between free α-aminophosphonates and aryl iodides. Furthermore, the modification of amino and late-stage functionalization of the C(sp3)-P bond from products indicates potential applications for α-aminophosphonates.

Palladium-Catalyzed Arylation of C(sp2)-H Bonds and C(sp3)-H Bonds with 4-Amino-benzotriazole as the Bidentate Directing Group

The arylation of C(sp2)-H and C(sp3)-H bonds in carboxylic acids catalyzed by Pd(II) with 4-aminobentriazole as the directing group was investigated. In addition to activation of the C(sp2)-H bond, selective arylation of alkyl carboxylic acids and amino acids in the β position can also be achieved. This strategy involved a 5,5-bicyclic Pd intermediate complex whose structure was determined by X-ray single crystal diffraction analysis. Importantly, the DG (directing group) can be easily removed under mild conditions.

RhIII-Catalyzed Direct Heteroarylation of Unactivated C(sp3)-H with N-Heteroaryl Boronates

Disclosed herein is a rhodium(III)-catalyzed direct heteroarylation reaction between unactivated aliphatic C(sp3)-H bonds in 2-alkylpyridines and heteroaryl organoboron reagents. This catalytic protocol is compatible with various heterocyclic boronates containing ortho- and meta-pyridine, pyrazoles, furan, and quinoline with strong coordination capability. The achievement of this methodology provides an efficient route to build new C(sp3)-heteroaryl bonds.

Enantioselective remote methylene C-H (hetero)arylation of cycloalkane carboxylic acids

Stereoselective construction of γ- and δ-stereocenters in carbonyl compounds is a pivotal objective in asymmetric synthesis. Here, we report chiral bifunctional oxazoline-pyridone ligands that enable enantioselective palladium-catalyzed remote γ-C-H (hetero)arylations of free cycloalkane carboxylic acids, which are essential carbocyclic building blocks in organic synthesis. The reaction establishes γ-tertiary and α-quaternary stereocenters simultaneously in up to >99% enantiomeric excess, providing access to a wide range of cyclic chiral synthons and bioactive molecules. The sequential enantioselective editing of two methylene C-H bonds can be achieved by using chiral ligands with opposite configuration to construct carbocycles containing three chiral centers. Enantioselective remote δ-C-H (hetero)arylation is also realized to establish δ-stereocenters that are particularly challenging to access using classical methodologies.

Palladium Catalysis Enabled Sequential C(sp3)-H/C-C Activation: Access to Vinyl γ-Lactams

A Pd(II)-catalyzed tandem reaction of aliphatic amides with vinylcyclopropanes (VCPs) was accomplished by merging C(sp3)-H and C-C activation. The reaction of VCP revealed alkenylation/cyclization, followed by ring opening via C-C cleavage, delivering vinyl γ-lactams with (E)-selectivity. The role of ligands, site-selectivity, functional group diversity, mechanistic insight, and synthetic utilities are important practical features.

Pd-Catalyzed Direct C7 Trifluoromethylation of Indolines with Umemoto's Reagent

An efficient palladium-catalyzed region-selective C7-trifluoromethylation of indolines using commercially available Umemoto's reagent was reported. The reaction utilizing Umemoto's reagent as CF3 radical precursor, pyrimidine as a removable directing group, Pd(II) as a catalyst, and Cu(II) as an oxidant furnished the required products with excellent regioselectivities and good yields. The present strategy has good region-selectivity, broad substrate scope, and scale-up application. Additionally, the present method was underlined by the direct C-1 trifluoromethylation of carbazoles. Furthermore, C7 trifluoromethylated indole can also be easily obtained via Pd-catalyzed direct C-7 trifluoromethylation/oxidation/deprotection sequential reactions.

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.

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.

Iodoarene-directed photoredox β-C(sp3)-H arylation of 1-(o-iodoaryl)alkan-1-ones with cyanoarenes via halogen atom transfer and hydrogen atom transfer

Site selective functionalization of inert remote C(sp3)-H bonds to increase molecular complexity offers vital potential for chemical synthesis and new drug development, thus it has been attracting ongoing research interest. In particular, typical β-C(sp3)-H arylation methods using chelation-assisted metal catalysis or metal-catalyzed oxidative/photochemical in situ generated allyl C(sp3)-H bond processes have been well developed. However, radical-mediated direct β-C(sp3)-H arylation of carbonyls remains elusive. Herein, we describe an iodoarene-directed photoredox β-C(sp3)-H arylation of 1-(o-iodoaryl)alkan-1-ones with cyanoarenes via halogen atom transfer (XAT) and hydrogen atom transfer (HAT). The method involves diethylaminoethyl radical-mediated generation of an aryl radical intermediate via XAT, then directed 1,5-HAT to form the remote alkyl radical intermediate and radical-radical coupling with cyanoarenes, and is applicable to a broad scope of unactivated remote C(sp3)-H bonds like β-C(sp3)-H bonds of o-iodoaryl-substituted alkanones and α-C(sp3)-H bonds of o-iodoarylamides. Experimental findings are supported by computational studies (DFT calculations), revealing that this method operates via a radical-relay stepwise mechanism involving multiple SET, XAT, 1,5-HAT and radical-radical coupling processes.

Cobalt- or rhodium-catalyzed synthesis of 1,2-dihydrophosphete oxides via C-H activation and formal phosphoryl migration

A highly stereo- and chemoselective intermolecular coupling of diverse heterocycles with dialkynylphosphine oxides has been realized via cobalt/rhodium-catalyzed C-H bond activation. This protocol provides an efficient synthetic entry to functionalized 1,2-dihydrophosphete oxides in excellent yields via the merger of C-H bond activation and formal 1,2-migration of the phosphoryl group. Compared with traditional methods of synthesis of 1,2-dihydrophosphetes that predominantly relied on stoichiometric metal reagents, this catalytic system features high efficiency, a relatively short reaction time, atom-economy, and operational simplicity. Photophysical properties of selected 1,2-dihydrophosphete oxides are also disclosed.

Palladium-catalyzed C-C bond cleavage of N-cyclopropyl acylhydrazones

Despite their utility as directing groups, the C-C bond cleavage of cyclopropanes utilizing hydrazones has not been explored. Herein, Pd-catalyzed C-C bond cleavage reaction of N-cyclopropyl acylhydrazones, followed by cycloisomerization to yield pyrazoles, has been developed. The protocol enables the synthesis of various α-pyrazole carbonyl compounds, which have a potential of biological activity. Control experiments and DFT calculations suggest that β-carbon elimination of a stable 6-membered chelate palladium complex occurs, generating a conjugated azine as a reaction intermediate for the following cycloisomerization.

Palladium-Catalyzed C-H Olefination of Imidazo[1,2a] pyridine Carboxamide in Aqueous Ethanol under Oxygen

The advancement of sustainable chemistry and changes in the economy are strongly intertwined. Reaction time, cost savings, moderate temperatures, and generation of the fewest byproducts are frequently achieved by using catalytic processes. Herein, we report the C-H olefination of imidazo[1,2a] pyridine carboxamides with various acrylates in the presence of Pd (OAc)2 with O2 as the oxidant in aqueous ethanol rather than using non-ecofriendly solvents. The C-H activation features most user-friendly reaction conditions, excellent yield as well as plenty substrate scope and applicable for C-H deuteriation of the corresponding heteroarenes with D2O. Experimental mechanistic studies indicate that C-H activation step succeeded after formation of tetra coordinated square planer Pd-substrate adduct.

Expeditious Synthesis of Spiroindoline Derivatives via Tandem C(sp2)-H and C(sp3)-H Bond Functionalization of N-Methyl-N-nitrosoanilines

Presented herein is a novel synthesis of pharmaceutically privileged spiroindoline derivatives via cascade reactions of N-methyl-N-nitrosoanilines with diazo homophthalimides. A group of mechanistic studies disclosed that the formation of product involves an unusual reaction mode of N-methyl-N-nitrosoaniline featuring an initial C(sp2)-H bond activation/alkylation followed by a C(sp3)-H bond activation/spiroannulation. To our knowledge, this is the first example in which N-methyl-N-nitrosoaniline acts as a C3N1 synthon to accomplish formal [4+1] spiroannulation with the participation of the N-methyl unit rather than the previously reported C2N1 synthon to undergo formal [3+2] annulation without the participation of the N-methyl unit. In general, this newly developed synthetic protocol features simple and readily accessible starting materials, valuable products, unique reaction mechanism, high efficiency and atom-economy, excellent compatibility with diverse functional groups, and ready scalability.

Functional and Promiscuity Studies of Three-Residue Cyclophane Forming Enzymes Show Nonnative C-C Cross-Linked Products and Leader-Dependent Cyclization

Enzymes catalyzing peptide macrocyclization are important biochemical tools in drug discovery. The three-residue cyclophane-forming enzymes (3-CyFEs) are an emerging family of post-translational modifying enzymes that catalyze the formation of three-residue peptide cyclophanes. In this report, we introduce three additional 3-CyFEs, including ChlB, WnsB, and FnnB, that catalyze cyclophane formation on Tyr, Trp, and Phe, respectively. To understand the promiscuity of these enzymes and those previously reported (MscB, HaaB, and YxdB), we tested single amino acid substitutions at the three-residue motif of modification (Ω1X2X3, Ω1 = aromatic). Collectively, we observe that substrate promiscuity is observed at the Ω1 and X2 positions, but a greater specificity is observed for the X3 residue. Two nonnative cyclophane products were characterized showing a Phe-C3 to Arg-Cβ and His-C2 to Pro-Cβ cross-links, respectively. We also tested the leader dependence of selected 3-CyFEs and show that a predicted helix region is important for cyclophane formation. These results demonstrate the biocatalytic potential of these maturases and allow rational design of substrates to obtain a diverse array of genetically encoded 3-residue cyclophanes.

Cu(II)-Mediated Sulfonylation of (Hetero)arenes with TosMIC Using Monodentate Directing Groups

Monodentate chelation-assisted direct ortho-C-H sulfonylation of (hetero)arenes using TosMIC as the novel sulfonylating reagent has been developed. A broad range of substrates, including indolines, indoles, 2-phenylpyridines, and others were well tolerated to afford the corresponding products in moderate to good yields. Mechanistic studies revealed that the sulfonyl radical might be involved. Inspired by the above discovery, preliminary para-C-H sulfonylation of naphthalene substrate was also successfully realized. The current protocol featured with cheap metal catalysis, good functional group compatibility, and operational convenience.

Rhodium-Catalyzed Difunctionalization of Alkenes Using Cyclic 1,3-Dicarbonyl-Derived Iodonium Ylides

Herein, we introduce an iodonium ylide strategy to achieve novel α-alkylation of cyclic 1,3-dicarbonyls through harnessing C(sp3)-Rh species generated from 5-exo-trig cyclization to provide rapid access to molecular hybridization of medically important isoindolin-1-ones and cyclic 1,3-dicarbonyls from readily available substrates. This approach features mild conditions, good yield, excellent functional group tolerance, and the simultaneous formation of two new chemical bonds and one stereogenic center. Moreover, the hydroxyl group of resulting product provides a good handle for downstream transformations. Importantly, we also demonstrate this strategy can be achieved in a one-pot manner. A C(sp3)-Rh complex was prepared and proved to be the key intermediate.

Electrochemical C-H/C-C Bond Oxygenation: A Potential Technology for Plastic Depolymerization

Herein, we provide eco-friendly and safely operated electrocatalytic methods for the selective oxidation directly or with water, air, light, metal catalyst or other mediators serving as the only oxygen supply. Heavy metals, stoichiometric chemical oxidants, or harsh conditions were drawbacks of earlier oxidative cleavage techniques. It has recently come to light that a crucial stage in the deconstruction of plastic waste and the utilization of biomass is the selective activation of inert C(sp3 )-C/H(sp3 ) bonds, which continues to be a significant obstacle in the chemical upcycling of resistant polyolefin waste. An appealing alternative to chemical oxidations using oxygen and catalysts is direct or indirect electrochemical conversion. An essential transition in the chemical and pharmaceutical industries is the electrochemical oxidation of C-H/C-C bonds. In this review, we discuss cutting-edge approaches to chemically recycle commercial plastics and feasible C-C/C-H bonds oxygenation routes for industrial scale-up.

Enantioselective Rhodium-Catalyzed C-H Arylation Enables Direct Synthesis of Atropisomeric Phosphines

Atropisomeric phosphines hold considerable significance in asymmetric catalysis, yet their synthesis presents a formidable challenge owing to intricate multistep procedures. In this context, a groundbreaking methodology has been presented for their preparation. This innovative approach entails an atroposelective rhodium-catalyzed C-H activation employing aryl and heteroaryl halides, chelated by a P(III) center. The essence of this strategy lies in its ability to directly construct chiral phosphine ligands in a single step, thereby exhibiting exceptional efficiency in terms of atom and redox economy. Illustrative examples serve to demonstrate the immense potential of in situ-formed ligands in asymmetric catalysis. Mechanistic experiments have further provided invaluable insights into this transformation.

P(III)-Directed Asymmetric C-H Arylation toward Planar Chiral Ferrocenes by Palladium Catalysis

Planar chiral ferrocenyl phosphines have been employed as highly valuable ligands in metal-catalyzed asymmetric reactions. However, their preparation remains a formidable challenge due to the requirement for intricate, multistep synthetic sequences. In addressing this issue, we have developed a groundbreaking enantioselective C-H activation strategy facilitated by P(III) directing groups, enabling the efficient construction of planar chiral ferrocenyl phosphines in a single step. Our innovative approach entails the combination of a palladium catalyst, a parent ferrocenyl phosphine, and a chiral phosphoramidite ligand, leading to exceptional reactivity and enantioselectivity. Remarkably, these novel ligands exhibit remarkable efficacy in silver-catalyzed asymmetric 1,3-dipolar cycloadditions. We carried out a combination of experimental and computational studies to obtain a more comprehensive understanding of the reaction pathway and the factors contributing to enantioselectivity.

Picolinamide-assisted ortho-C-H functionalization of pyrenylglycine derivatives using aryl iodides

Chemical transformations involving the pyrenylglycine motif (an unnatural amino acid) and practical methods toward it are seldom known. This work aimed at developing a method for synthesizing novel pyrenylglycine (pyrene-based glycine) unnatural amino acid derivatives. To realize this, initially, a new pyrenylglycine substrate possessing the picolinamide moiety was assembled via the Ugi multicomponent reaction. The picolinamide moiety linked to amine substrates is a well-known bidentate directing group for accomplishing the site-selective γ-C-H functionalization of amines. Subsequently, it was aimed at using a Pd(II)-catalyzed bidentate directing group-aided γ-C-H arylation strategy for generating a wide range of unprecedented examples of C(2)-H arylated pyrenylglycines. Accordingly, pyrenylglycine possessing the picolinamide moiety was subjected to Pd(II)-catalyzed C(2)-H arylation in the non-K-region to afford a library of C(2)-arylated pyrenylglycines (π-extended pyrenes). Additionally, pyrenylglycine-based small peptides were assembled using C(2)-arylated pyrenylglycines. The X-ray structure of a representative compound was obtained, which corroborated the structure of pyrenylglycine and the regioselectivity of C(2)-H arylation of the pyrene in the non-K-region.

Asymmetric Ruthenium-Catalyzed C-H Activation by a Versatile Chiral-Amide-Directing Strategy

A versatile and readily available chiral amide directing group has been developed for the ruthenium(II)-catalyzed asymmetric C-H activation. Asymmetric C-H activation of the related chiral benzamides with various olefins, aldehydes and propargylic alcohols has been accomplished with high stereoselectivities, affording a series of chiral products including 3,4-dihydroisocoumarins (up to 96 % ee), isocoumarins (up to 92 % ee), phthalides (up to 99 % ee), chiral bicyclo[2.2.1]heptanes (>20 : 1 dr), 4-alkylidene-3,4-dihydroisocoumarins (up to 97 % ee) and allenes (>20 : 1 dr). Importantly, our methodologies enabled concise syntheses of many biologically active compounds and natural products (e.g., Montroumarin, Cyclosporone E, Cyclosporone Q, Concentricolide, Chuangxinol, and Eleutherol).

Regioselective C(sp2)-C(sp3) Coupling Mediated by Classical and Rollover Cyclometalation

By taking advantage of a sequence of oxidative addition/reductive elimination reactions, Pt(II) cyclometalated derivatives are able to promote a rare C(sp2)-C(sp3) bond coupling, resulting in the production of novel methyl-substituted pyridines and bipyridines. Starting from 6-phenyl-2,2'-bipyridine, the step-by-step full sequence of reactions has been followed, leading to the unprecedented 3-methyl-6-phenyl-2,2'-bipyridine, which was isolated and fully characterized. The synthesis involves the following steps: (1) rollover cyclometalation to give the starting complex [Pt(N^C)(DMSO)Me]; (2) the synthesis of a more electron-rich complex [Pt(N^C)(PPh3)Me] by the substitution of DMSO with triphenylphosphine; (3) oxidative addition with methyl iodide to give the Pt(IV) complex [Pt(N^C)(PPh3)(Me)2(I)]; (4) iodide abstraction with silver tetrafluoborate to give an unstable pentacoordinate intermediate, which rapidly evolves through a carbon-carbon reductive coupling, forming a new C(sp3)-C(sp2) bond; (5) finally, the extrusion and characterization of the newly formed 3-methyl-6-phenyl-2,2'-bipyridine. The reaction has been therefore extended to a well-known classical cyclometalating ligand, 2-phenylpyridine, demonstrating that the method is not restricted to rollover derivatives. Following the same step-by-step procedure, 2-phenylpyridine was converted to 2-o-tolyl-pyridine, displaying the potential application of the method to the larger family of classical cyclometalated complexes. The application of this protocol may be useful to convert an array of heterocyclic compounds to their methyl- or alkyl-substituted analogs.

Palladium-Catalyzed C(sp3)-H Nitrooxylation of Aliphatic Carboxamides with Practical Oxidants

Here we report the palladium-catalyzed β-C(sp3)-H nitrooxylation of aliphatic carboxamides using a modified quinoline auxiliary. Notably, Al(NO3)3·9H2O was used as a nitrate source as well as a practical oxidant. The 5-chloro-8-aminoquinoline auxiliary was nitrated in situ during the reaction, which may enhance its directing ability and help its removal. The reaction has a broad substrate scope with a variety of aliphatic carboxamides. The multiple substituted auxiliary can be easily removed and recovered. Two C-H-insertion palladacycle intermediates were isolated and characterized to elucidate the mechanism.

Terminal C(sp3)-H borylation through intermolecular radical sampling

Hydrogen atom transfer (HAT) processes can overcome the strong bond dissociation energies (BDEs) of inert C(sp3)-H bonds and thereby convert feedstock alkanes into value-added fine chemicals. Nevertheless, the high reactivity of HAT reagents, coupled with the small differences among various C(sp3)-H bond strengths, renders site-selective transformations of straight-chain alkanes a great challenge. Here, we present a photocatalytic intermolecular radical sampling process for the iron-catalyzed borylation of terminal C(sp3)-H bonds in substrates with small steric hindrance, including unbranched alkanes. Mechanistic investigations have revealed that the reaction proceeds through a reversible HAT process, followed by a selective borylation of carbon radicals. A boron-sulfoxide complex may contribute to the high terminal regioselectivity observed.

Rhodium-catalyzed annulation of hydrazines with vinylene carbonate to synthesize unsubstituted 1-aminoindole derivatives

Herein, we describe rhodium-catalysed C-H bond activation for [3 + 2] annulation using hydrazide and vinylene carbonate, providing an efficient method for synthesising unsubstituted 1-aminoindole compounds. Characterised by high yields, mild reaction conditions, and no need for external oxidants, this transformation demonstrates excellent regioselectivity and a wide tolerance for various functional groups.

Discovery of Natural Ah Receptor Antagonists from Salvia miltiorrhiza Bunge and Synthesis of Analogs for Tumor Immunotherapy

IDO/TDO/Kyn/AhR signaling plays a crucial role in regulating innate and adaptive immunity, and targeting Ah receptor (AhR) inhibition can potentially redirect immune cells toward an antitumoral phenotype. Therefore, AhR is an attractive drug target for novel small molecule cancer immunotherapies. In this study, natural products tanshinolic A-D (1-4), the first adducts composed of ortho-naphthoquinone-type tanshinone and phenolic acid featuring a unique 1,4-benzodioxan hemiacetal structure, were isolated and characterized from the roots of Salvia miltiorrhiza Bunge. Luciferase reporter gene assay revealed that these adducts exhibited significant AhR inhibitory activity. A linear strategy was developed to construct a cis-3,4-disubstituted 1,4-benzodioxan hemiacetal structure. Encouragingly, in both in vitro and in vivo experiments, (±)-13e demonstrated the ability to inhibit tumor cell proliferation, promote INF-γ secretion in CD8+ T cells, and inhibit PD-1/PD-L1 signal transduction, which could exert tumor inhibition properties by inhibiting AhR activity, positioning it as a promising candidate for tumor immunotherapy.

Catalytic Enantioselective Primary C-H Borylation for Acyclic All-Carbon Quaternary Stereocenters

Creating a perfect catalyst to operate enzyme-like chiral recognition has been a long-sought aim. A challenging example in this context is constructing acyclic all-carbon quaternary stereogenic centers by transition metal-catalyzed enantioselective C-H activation. We now report highly enantioselective iridium-catalyzed primary C-H borylation of α-all-carbon substituted 2,2-dimethyl amides enabled by a tailor-made chiral bidentate boryl ligand (CBL). The success of the current transformation is attributed to the CBL/iridium catalyst, which has a confined chiral pocket. This protocol provides a diverse array of acyclic all-carbon quaternary stereocenters with excellent enantiocontrol and distinct structural features. Computational study reveals that steric hindrance of CBL could regulate the type of dominant orbital interaction between the catalyst and substrate, which is crucial to conferring high chiral induction.

Sulfur-Directed α-C(sp3)-H Amidation of Pyrrolidines with Dioxazolones under Rhodium Catalysis

Site-selective functionalization of saturated N-heterocycles such as pyrrolidines is a central topic in organic synthesis and drug discovery. We herein report the sulfur-assisted rhodium(III)-catalyzed sp3 C-H amidation of pyrrolidines with dioxazolones as amidating agents. The amenability of the thioamide directing group is elucidated by a series of control experiments.

Oxidants Controlled C-H Bond Functionalization of N-Aryltetrahydroisoquinolines: The Construction of the Quaternary Carbon Center and Cleavage of the C-N Bond

Initiated by triarylamine radical cation salt (TBPA), the direct C-H bond functionalization of α-N-aryltetrahydroisoquinoline esters was smoothly realized, giving a series of α-hydroxylated derivatives with a quaternary carbon center in good yields. Differently, in the presence of tert-butyl nitrite (TBN), the C-N single bond was cleaved to keto esters. The mechanistic study revealed that these reactions were mediated by a similar mechanism, in which the N-nitrosation might provide a driving force to the C-N bond cleavage.

Eight-Membered Palladacycle Intermediate Enabled Synthesis of Cyclic Biarylphosphonates

Transition-metal-catalyzed coupling reactions that involve the direct functionalization of insert C-H bond represent one of the most efficient strategies for forming carbon-carbon bonds. Herein, a palladium-catalyzed intramolecular C-H bond arylation of triaryl phosphates is reported to access seven-membered cyclic biarylphosphonate targets. The reaction is achieved via a unique eight-membered palladacyclic intermediate and shows good functional group compatibility. Meanwhile, the product can be readily converted into other valuable phosphate compounds.

Palladium-Catalyzed Chelation-Assisted Aldehyde C-H Bond Activation of Quinoline-8-carbaldehydes: Synthesis of Amides from Aldehydes with Anilines and Other Amines

A palladium-catalyzed chelation-assisted direct aldehyde C-H bond amidation of quinoline-8-carbaldehydes with an amine was developed under mild reaction conditions. A wide range of amides were obtained in good to excellent yields from aldehyde with a variety of aniline derivatives and aliphatic amines. Our methodology was successfully applied to synthesize known DNA intercalating agents and can be easily scaled up to a gram scale.

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.

Biorelevant Weakly Coordinating Directing Group Assisted C-H Alkenylation with Cyclopropanols via Sequential C-H/C-C Activation

A weakly coordinating biorelevant intrinsic directing group (DG) assisted site-selective C-H alkenylation via sequential C-H/C-C bond activation has been accomplished under Ru(II)-catalysis using readily accessible cyclopropyl alcohol as an alkenyl surrogate. Utilization of an intrinsic DG, exclusive regioselectivity, functional group diversity, late-stage natural product and drug mutations are the important practical features.

Organo-Photoredox Catalyzed C(sp3 )-H Bond Arylation of Aliphatic Amides

A C(sp3 )-H bond arylation of aliphatic amides has been achieved via organophotoredox catalysis. The reaction could be realized at room temperature with visible light source and metal-free catalyst. Quinuclidine is employed as an efficient HAT reagent and a range of aliphatic amides is employed as both substrate and solvent in the reaction. This photocatalyzed transformation provides a convenient protocol to afford a board range of N-benzyl amides.

Atroposelective hydroarylation of biaryl phosphines directed by phosphorus centres

Prized for their ability to generate chemical complexity rapidly, catalytic carbon-hydrogen (C-H) activation and functionalization reactions have enabled a paradigm shift in the standard logic of synthetic chemistry. Directing group strategies have been used extensively in C-H activation reactions to control regio- and enantioselectivity with transition metal catalysts. However, current methods rely heavily on coordination with nitrogen and/or oxygen atoms in molecules and have therefore been found to exhibit limited generality in asymmetric syntheses. Here, we report enantioselective C-H activation with unsaturated hydrocarbons directed by phosphorus centres to rapidly construct libraries of axially chiral phosphines through dynamic kinetic resolution. High reactivity and enantioselectivity are derived from modular assembly of an iridium catalyst with an endogenous phosphorus atom and an exogenous chiral phosphorus ligand, as confirmed by detailed experimental and computational studies. This reaction mode significantly expands the pool of enantiomerically enriched functional phosphines, some of which have shown excellent efficiency for asymmetric catalysis.

Ligand-Enabled γ-C(sp3)-H Hydroxylation of Free Amines with Aqueous Hydrogen Peroxide

Selective oxidation of the γ-C-H bonds from abundant amine feedstocks via palladium catalysis is a valuable transformation in synthesis and medicinal chemistry. Despite advances on this topic in the past decade, there remain two significant limitations: C-H activation of aliphatic amines requires an exogenous directing group except for sterically hindered α-tertiary amines, and a practical catalytic system for C(sp3)-H hydroxylation using a green oxidant, such as oxygen or aqueous hydrogen peroxide, has not been developed to date. Herein, we report a ligand-enabled selective γ-C(sp3)-H hydroxylation using sustainable aqueous hydrogen peroxide (7.5-10%, w/w). Enabled by a CarboxPyridone ligand, a series of primary amines (1°), piperidines, and morpholines (2°) were hydroxylated at the γ-position with excellent monoselectivity. This method provides an avenue for the synthesis of a wide range of amines, including γ-amino alcohols, β-amino acids, and azetidines. The retention of chirality in the reaction allows rapid access to chiral amines starting from the abundant chiral amine pool.

Synthesis of 13C-methyl-labeled amino acids and their incorporation into proteins in mammalian cells

Isotopic labeling of methyl-substituted proteinogenic amino acids with 13C has transformed applications of solution-based NMR spectroscopy and allowed the study of much larger and more complex proteins than previously possible with 15N labeling. Procedures are well-established for producing methyl-labeled proteins expressed in bacteria, with efficient incorporation of 13C-methyl labeled metabolic precursors to enable the isotopic labeling of Ile, Val, and Leu methyl groups. Recently, similar methodology has been applied to enable 13C-methyl labeling of Ile, Val, and Leu in yeast, extending the approach to proteins that do not readily fold when produced in bacteria. Mammalian or insect cells are nonetheless preferable for production of many human proteins, yet 13C-methyl labeling using similar metabolic precursors is not feasible as these cells lack the requisite biosynthetic machinery. Herein, we report versatile and high-yielding synthetic routes to 13C methyl-labeled amino acids based on palladium-catalyzed C(sp3)-H functionalization. We demonstrate the efficient incorporation of two of the synthesized amino acids, 13C-γ2-Ile and 13C-γ1,γ2-Val, into human receptor extracellular domains with multiple disulfides using suspension-cultured HEK293 cells. Production costs are reasonable, even at moderate expression levels of 2-3 mg purified protein per liter of medium, and the method can be extended to label other methyl groups, such as 13C-δ1-Ile and 13C-δ1,δ2-Leu. In summary, we demonstrate the cost-effective production of methyl-labeled proteins in mammalian cells by incorporation of 13C methyl-labeled amino acids generated de novo by a versatile synthetic route.

Aryl Chloride-Directed Enantioselective C(sp2)-H Borylation Enabled by Iridium Catalysis

We herein report the iridium-catalyzed enantioselective C-H borylation of aryl chlorides. A variety of prochiral biaryl compounds could be well-tolerated, affording a vast array of axially chiral biaryls with high enantioselectivities. The current method exhibits a high turnover number (TON) of 7000, which represents the highest in functional-group-directed asymmetric C-H activation. The high TON was attributed to a weak catalyst-substrate interaction that was caused by mismatched chirality between catalyst and substrate. We also demonstrated the synthetic application of the current method by C-B, ortho-C-H, and C-Cl bond functionalization, including programmed Suzuki-Miyaura coupling for the synthesis of axially chiral polyarenes.

Palladium-Catalyzed Atroposelective Kinetic C-H Olefination and Allylation for the Synthesis of C-B Axial Chirality

The direct C-H functionalization of 1,2-benzazaborines, especially asymmetric version, remains a great challenge. Here we report a palladium-catalyzed enantioselective C-H olefination and allylation reactions of 1,2-benzazaborines. This asymmetric approach is a kinetic resolution (KR), providing various C-B axially chiral 2-aryl-1,2-benzazaborines and 3-substituted 2-aryl-1,2-benzazaborines in generally high yields with excellent enantioselectivities (selectivity (S) factor up to 354). The synthetic potential of this reaction is showcased by late-stage modification of complex molecules, scale-up reaction, and applications.