C-H bond activation enables the rapid construction and late-stage diversification of functional molecules

Efficient Derivatization of a Thienobenzobisthiazole-Based π-Conjugated Polymer Through Late-Stage Functionalization Towards High-Efficiency Organic Photovoltaic Cells

Derivatization is essential for optimizing organic material properties. However, because functional groups are often introduced at an early stage of the synthesis, similar intermediates have to be repeatedly synthesized to produce derivatives, which amounts to a daunting and time-consuming task. Using thienobenzobisthiazole (TBTz) as a building unit of donor polymers for organic photovoltaics (OPVs), we demonstrate an efficient derivatization of a TBTz-based π-conjugated polymer by late-stage functionalization. In the developed synthetic route, functional groups are introduced at the last step of monomer synthesis, enabling us to easily synthesize several derivatives from a common intermediate. Ester and acyl groups are introduced into the polymer instead of the alkyl group, giving rise to deep HOMO energy levels and resulting in OPV cells with high open-circuit voltage even in the absence of halogen substituents that are typically introduced into the donor polymers. Notably, the ester-functionalized TBTz-based polymer shows a small nonradiative voltage loss (ΔVnr) of 0.19 V and has one of the highest charge generation efficiencies among the halogen-free donor polymers with similar ΔVnr, improving the critical trade-off relationship between voltage loss and charge generation. Our results provide an important guideline for the efficient development of high-performance polymers for OPVs.

Solvent-Controlled Rh(III)-Catalyzed Mono- and Dual Functionalization of Quinolyl Aldoximes with Diazo Compounds

A solvent-controlled Rh(III)-catalyzed mono- and dual-C-H bond activation/carbene migratory insertion with diazo compounds as a single coupling partner was demonstrated. The reaction proceeded under mild conditions, yielding products in good to excellent yields. These results are significant for the development of the domino multiple functionalization of C(sp2)-H bonds via a carbenoid insertion approach.

Access to Functionalized Amines and Medium N-Heterocycles via Amine-Enabled Remote C-H Alkynylation

By using weakly coordinating amines, we developed remote C-H alkynylation with precise control of reactivity and regioselectivity, enabling modification of complex drugs, natural products, and materials. The readily transformable alkyne-containing amine products would facilitate expedient delivery of molecular libraries of functionalized amines and medium N-heterocycles, which are previously elusive to access. Moreover, the introduced alkyne functionality could serve as a versatile handle to expand the diversity and synthetic application of this remote C-H functionalization.

An Air-Stable, Single-Component Iridium Precatalyst for the Borylation of C-H Bonds on Large to Miniaturized Scales

The functionalization of C-H bonds enables the modification of complex molecules, often with the intention of forming compound libraries. The borylation of aryl C-H bonds is a widely used class of C-H bond functionalization, and conventional catalyst systems for the borylation of C-H bonds consist of an iridium source and an N,N-ligand, in conjunction with pinacolborane, to form the active iridium(III) tris(boryl) catalyst. These multicomponent catalyst systems complicate borylation reactions at large and small scales, due to the air sensitivity of the most common iridium precursor [Ir(cod)OMe]2, and, particularly on small scale, the challenges associated with dispensing multiple components with differing solubilities or that are air-sensitive. We describe the discovery of an air-stable, single-component iridium precatalyst, [(tmphen)Ir(coe)2Cl], that generates the same active iridium(III) tris(boryl) catalyst and reacts with higher turnovers, comparable selectivity, and similar scope to those of known catalyst systems for the borylation of aryl and heteroaryl C-H bonds. We show how the development of this precatalyst enables reactions to be run on submicromole scale in a high-throughput experimentation format in conjunction with ChemBead technology, and with a second diversification step that illustrates the potential to diversify structures by chemical sequences involving catalytic reactions, including C-H bond functionalizations, on submicromole scales in the same reaction vessel.

Oxoammonium-Catalyzed Ether Oxidation via Hydride Abstraction: Methodology Development and Mechanistic Investigation Using Paramagnetic Relaxation Enhancement NMR

Hydride abstraction represents a promising yet underexplored approach in the functionalization of C-H bonds. In this work, we report the oxidation of α-C-H bonds of ethers via oxoammonium catalysis using 3-chloroperbenzoic acid (mCPBA) as the terminal chemical oxidant or by means of electrochemistry. Mechanistic studies revealed intricate equilibria and interconversion events between various catalytic intermediates in the presence of mCPBA, which alone however was incompetent to drive catalytic turnover. The addition of a small amount of strong acid HNTf2 or weakly coordinating salt NaSbF6 turned on catalytic turnover and promoted ether oxidation with excellent efficiency. NMR experiments leveraging paramagnetic relaxation enhancement effect allowed for quantification of open-shell catalytic intermediates in real time during the reaction course, which aided the identification of catalyst resting states and elucidation of reaction mechanisms.

Regioselective bromination of pyrrolo[1,2-a]quinoxalines

In this study, we report a novel and efficient method for the regioselective bromination of pyrrolo[1,2-a]quinoxalines using tetrabutylammonium tribromide (TBATB). This method exploits the mild nature of TBATB to obtain highly selective C3-brominated or C1, C3-dibrominated products in good yields. Notably, the reaction has a broad substrate applicability, and the C3-brominated product can be synthesized on a gram scale and can be further converted into structurally diverse pyrrolo[1,2-a]quinoxaline derivatives.

Catalytic Concerted SNAr Reactions of Fluoroarenes by an Organic Superbase

We herein propose that the catalytic concerted SNAr reaction is a powerful method to prepare functionalized aromatic scaffolds. Classic stepwise SNAr reactions involving addition/elimination processes require the use of electron-deficient aromatic halides to stabilize Meisenheimer intermediates, despite their widespread use in medicinal chemistry research. Recent efforts have been made to develop concerted SNAr reactions involving a single transition state, allowing the use of electron-rich substrates based on the use of stoichiometric amounts of strong bases or reactive nucleophiles. This study demonstrates that, without the use of such reagents, the organic superbase t-Bu-P4 efficiently catalyzes the concerted SNAr reactions of aryl fluorides regardless of their electronic nature. The key to establishing this system is the dual activation of aryl fluoride and anionic nucleophiles by the t-Bu-P4 catalyst. Furthermore, this catalysis allows excellent functional group tolerance, utilization of diverse nucleophiles, and late-stage functionalization of bioactive compound derivatives. These findings make possible diverse applications in chemical synthesis and pharmaceutical development.

Visible light-induced ruthenium(ii)-catalyzed hydroarylation of unactivated olefins

Hydroarylation reactions have emerged as a valuable tool for the direct functionalization of C-H bonds with ideal atom economy. However, common catalytic variants for these transformations largely require harsh reaction conditions, which often translate into reduced selectivites. In contrast, we herein report on a photo-induced hydroarylation of unactivated olefins at room temperature employing a readily available ruthenium(ii) catalyst. Our findings include high position- and regio-selectivity and remarkable tolerance of a wide range of functional groups, which further enabled the late-stage diversification.

Zwitterionic Acridinium Amidate: A Nitrogen-Centered Radical Catalyst for Photoinduced Direct Hydrogen Atom Transfer

The development of small organic molecules that can convert light energy into chemical energy to directly promote molecular transformation is of fundamental importance in chemical science. Herein, we report a zwitterionic acridinium amidate as a catalyst for the direct functionalization of aliphatic C-H bonds. This organic zwitterion absorbs visible light to generate the corresponding amidyl radical in the form of excited-state triplet diradical with prominent reactivity for hydrogen atom transfer to facilitate C-H alkylation with a high turnover number. The experimental and theoretical investigations revealed that the noncovalent interactions between the anionic amidate nitrogen and a pertinent hydrogen-bond donor, such as hexafluoroisopropanol, are crucial for ensuring the efficient generation of catalytically active species, thereby fully eliciting the distinct reactivity of the acridinium amidate as a photoinduced direct hydrogen atom transfer catalyst.

Theoretical Study on the Mechanism of Ru(II)-Catalyzed Intermolecular [3 + 2] Annulation between o-Toluic Acid and 3,5-Bis(trifluoromethyl)benzaldehyde: Octahedral vs Trigonal Bipyramidal

Density functional theory was utilized to investigate the mechanism of Ru(II)-catalyzed aromatic C-H activation and addition of aromatic aldehydes. The proposed catalytic cycle consists of C-H bond activation, aldehyde carbonyl insertion for C-C coupling, lactonization for the formation of the final product, product separation, and catalyst recovery. Our calculations suggest that Ru(OAc)2(PCy3) (referred to as CAT) is the most favorable active catalyst, facilitating the C-H bond activation to form a five-membered ring cycloruthenium intermediate (INT2). Subsequently, the aromatic aldehyde reactant 2a enters the Ru coordination sphere, accelerating the C-C coupling and lactonization for the formation of the final product. The involvement of acetate assists in the final product separation, while INT1 re-enters the Ru coordination sphere to initiate a new catalytic cycle. Utilizing the energetic span model, the apparent activation free energy barrier was computed to be 34.3 kcal mol-1 at 443 K. Furthermore, exploration of the reaction mechanism in the absence of phosphine ligands identified Ru(OAc)2(p-cymene) as the most favorable active catalyst. The derived apparent activation free energy barrier offers a comprehensive explanation for the experimentally observed yields. Additionally, we have examined the disparities between the octahedral and trigonal bipyramidal structures of the catalysts concerning their effects on the reaction mechanisms and apparent activation free energy barriers.

Radical-triggered translocation of C-C double bond and functional group

Multi-site functionalization of molecules provides a potent approach to accessing intricate compounds. However, simultaneous functionalization of the reactive site and the inert remote C(sp3)-H poses a formidable challenge, as chemical reactions conventionally occur at the most active site. In addition, achieving precise control over site selectivity for remote C(sp3)-H activation presents an additional hurdle. Here we report an alternative modular method for alkene difunctionalization, encompassing radical-triggered translocation of functional groups and remote C(sp3)-H desaturation via photo/cobalt dual catalysis. By systematically combining radical addition, functional group migration and cobalt-promoted hydrogen atom transfer, we successfully effectuate the translocation of the carbon-carbon double bond and another functional group with precise site selectivity and remarkable E/Z selectivity. This redox-neutral approach shows good compatibility with diverse fluoroalkyl and sulfonyl radical precursors, enabling the migration of benzoyloxy, acetoxy, formyl, cyano and heteroaryl groups. This protocol offers a resolution for the simultaneous transformation of manifold sites.

Benzoic Acid Serves as Precursor of Catalytic HAT Reagent in a Two-Molecule Photoredox System

The photoinduced regioselective HAT reactions of acetals, ethers, and alcohols using benzoic acids in a two-molecule photoredox system led to the formation of new C-C bonds with alkenes under mild conditions. Aryl carboxy radicals generated from benzoic acids in a two-molecule photoredox system can function as catalytic HAT reagents, even though an excess amount of a hydrogen donor substrate is required. Various acetals, ethers, alcohols, and alkenes can be employed in the photoreaction to provide both high yields of adducts and high recoveries of benzoic acids.

Mechanistic Studies and Identification of Catalyst Deactivation Pathways for Pyridine(diimine) Iron Catalyzed C(sp2)-H Borylation

The synthesis and application of aryl-substituted pyridine(diimine) iron complexes (RPDI)FeCH3 to the catalytic borylation of heteroarenes under thermal conditions is described. Improvements in catalyst design and performance were guided by precatalyst activation studies, where investigations into stoichiometric reactivities of iron borohydride (4- t Bu- iPrPDI)Fe(H2BPin) and iron furyl (4- t Bu- iPrPDI)Fe(2-methylfuryl) complexes revealed facile C(sp2)-H activation and a slower and potentially turnover-limiting C(sp2)-B formation step. Formation of the flyover dimer, [(4- t Bu- iPrPDI)Fe]2 was identified as a catalyst deactivation pathway and formally iron(0) complexes were found to be inactive for borylation. The pyridine(diimine) iron borohydride, flyover dimer and furyl complexes were characterized by X-ray diffraction and their electronic structures determined by a combination of NMR, EPR, and Mössbauer spectroscopies corroborated by DFT calculations. The role of the redox-active pyridine(diimine) ligand in catalytic C-H borylation was also investigated.

Ru(II)-Catalyzed Decarboxylative (4 + 2)-Annulation of Benzoic Acids and Benzamides with Propargyl Cyclic Carbonates

Propargyl cyclic carbonates have emerged as versatile precursors in synthetic chemistry. However, their reactivity has so far been limited to transition metal-catalyzed substitution and cyclization reactions. Herein, we illustrate the successful employment of propargyl cyclic carbonates as coupling partners in Ru(II)-catalyzed C-H annulation of benzoic acids and benzamides. This approach allowed us to access a broad range of biologically relevant isocoumarin and isoquinolinone derivatives in good to excellent yields, utilizing bench-stable and easily accessible precursors. Preliminary mechanistic studies indicated that the C-H metalation step is both reversible and rate-determining in the reaction pathway. Furthermore, the utility of the developed methodology has been illustrated by scale-up and postfunctionalization experiments.

Carbodicarbene-Stibenium Ion-Mediated Functionalization of C(sp3)-H and C(sp)-H Bonds

Main-group element-mediated C-H activation remains experimentally challenging and the development of clear concepts and design principles has been limited by the increased reactivity of relevant complexes, especially for the heavier elements. Herein, we report that the stibenium ion [(pyCDC)Sb][NTf2]3 (1) (pyCDC=bis-pyridyl carbodicarbene; NTf2=bis(trifluoromethanesulfonyl)imide) reacts with acetonitrile in the presence of the base 2,6-di-tert-butylpyridine to enable C(sp3)-H bond breaking to generate the stiba-methylene nitrile complex [(pyCDC)Sb(CH2CN)][NTf2]2 (2). Kinetic analyses were performed to elucidate the rate dependence for all the substrates involved in the reaction. Computational studies suggest that C-H activation proceeds via a mechanism in which acetonitrile first coordinates to the Sb center through the nitrogen atom in a κ1 fashion, thereby weakening the C-H bond which can then be deprotonated by base in solution. Further, we show that 1 reacts with terminal alkynes in the presence of 2,6-di-tert-butylpyridine to enable C(sp)-H bond breaking to form stiba-alkynyl adducts of the type [(pyCDC)Sb(CCR)][NTf2]2 (3 a-f). Compound 1 shows excellent specificity for the activation of the terminal C(sp)-H bond even across alkynes with diverse functionality. The resulting stiba-methylene nitrile and stiba-alkynyl adducts react with elemental iodine (I2) to produce iodoacetonitrile and iodoalkynes, while regenerating an Sb trication.

Recent Advances in Direct Regioselective C-H Chlorination at Aromatic and Aliphatic

Direct installation of key functionalities in a molecule through C-H bond activation is one of the thrust areas as well as challenging task in organic synthesis. Particularly, introduction of chlorine in a molecule imparts additional benefits for further functionalizations as well as improves the electronic behaviour such as lipophilicity and polarity towards drug development process. The chlorinated molecules have also been established as efficient biologically relevant scaffolds. Current manuscript has been focused on the direct installation of the chlorine atom at various aromatic and aliphatic positions to produce functional molecules. The key highlight of the manuscript belongs to the site selectivity (regioselectivity) for the installation of chlorine functionality. Manuscript describes the advanced methods developed for the direct C-H chlorination reactions and further simplified for the chlorination reactions at various positions including aromatic (o-, m-, and p-), benzylic, heteroaromatic, and aliphatic positions. Directing groups (DGs) and the coordination with the catalyst is the key for the enhancement of regioselectivities during direct C-H chlorination reactions.

Skeletal and Mechanistic Diversity in Ir-Catalyzed Cycloisomerizations of Allene-Tethered Pyrroles and Indoles

Pyrroles and indoles bearing N-allenyl tethers participate in a variety of iridium-catalyzed cycloisomerization processes initiated by a C-H activation step, to deliver a diversity of synthetically relevant azaheterocyclic products. By appropriate selection of the ancillary ligand and the substitution pattern of the allene, the reactions can diverge from simple intramolecular hydrocarbonations to tandem processes involving intriguing mechanistic issues. Accordingly, a wide range of heterocyclic structures ranging from dihydro-indolizines and pyridoindoles to tetrahydroindolizines, as well as cyclopropane-fused tetrahydroindolizines can be obtained. Moreover, by using chiral ligands, these cascade processes can be carried out in an enantioselective manner. DFT studies provide insights into the underlying mechanisms and justify the observed chemo- regio- and stereoselectivities.

C-H functionalization reactions catalyzed by artificial metalloenzymes

CH functionalization, a promising frontier in modern organic chemistry, facilitates the direct conversion of inert CH bonds into many valuable functional groups. Despite its merits, traditional homogeneous catalysis, often faces challenges in efficiency, selectivity, and sustainability towards this transformation. In this context, artificial metalloenzymes (ArMs), resulting from the incorporation of a catalytically-competent metal cofactor within an evolvable protein scaffold, bridges the gap between the efficiency of enzymatic transformations and the versatility of transition metal catalysis. Accordingly, ArMs have emerged as attractive tools for various challenging catalytic transformations. Additionally, the coming of age of directed evolution has unlocked unprecedented avenues for optimizing enzymatic catalysis. Taking advantage of their genetically-encoded protein scaffold, ArMs have been evolved to catalyze various CH functionalization reactions. This review delves into the recent developments of ArM-catalyzed CH functionalization reactions, highlighting the benefits of engineering the second coordination sphere around a metal cofactor within a host protein.

Deuteration of Arenes via Pd-Catalyzed C-H Activation: A Lesson in Nondirected C-H Activation, Isotopic Labeling, and NMR Characterization

Isotopic labeling is an important tool in medicinal research, metabolomics, and for understanding reaction mechanisms. In this context, transition metal-catalyzed C-H activation has emerged as a key technology for deuterium incorporation via hydrogen isotope exchange. A detailed and easy-to-implement experimental procedure for a nondirected arene deuteration has been developed that exclusively uses commercial equipment and chemicals. The protocol is ideally suited for students and other prospective applicants who are not experts in catalysis. The degree of deuterium incorporation was analyzed via different means like mass spectrometry and 1H and 2H nuclear magnetic resonance (NMR). A hands-on understanding of quantitative NMR, as well as the influence of H/D exchange on experimental spectra, was conveyed by comparative NMR spin simulations. Students were measurably familiarized with the concepts of C-H activation, isotope effects, and basics in experimental catalysis.

Molecular Design of Naphthalene- and Carbazole-Based Monomers for Regiospecific Synthesis of Poly(arylenevinylene)s via Co-Catalyzed Hydroarylation Polyaddition

This study focuses on the development of regiospecific hydroarylation polyaddition of naphthalene- and carbazole-based monomers with diynes under mild reaction conditions at room temperature. A 1-pyrazole substituent serves as an appropriate directing group for a Co-catalyst to efficiently activate the C-H bonds of generally inactive six-membered aromatic hydrocarbons. The 1-pyrazole groups in 2,6-di(1-pyrazolyl)naphthalene adopt planar conformations and act as directing groups, resulting in a smooth hydroarylation reaction. In contrast, the reaction with 1,5-di(1-pyrazolyl)naphthalene do not proceed. The polyaddition reaction of 2,6-di(1-pyrazolyl)naphthalene selectively proceeds at 3,7-positions under mild reaction conditions at 30 °C, and yields corresponding poly(arylenevinylene) (PAV) with high molecular weight. This molecular design is also applicable to the hydroarylation polyaddition of carbazole; the polyaddition reaction of 9-(2-ethylhexyl)-3,6-di(1-pyrazolyl)carbazole selectively occurred at 2,7-positions. The optical and electronic properties of the synthesized compounds are evaluated. The obtained PAVs serve as an emitting material in organic light-emitting diode (OLED). This study aims to develop a Co-catalyzed hydroarylation polyaddition via C-H activation of generally inactive polyaromatic hydrocarbons (PAHs) under mild conditions.

Repurposing a supramolecular iridium catalyst via secondary Zn⋯O[double bond, length as m-dash]C weak interactions between the ligand and substrate leads to ortho-selective C(sp2)-H borylation of benzamides with unusual kinetics

The iridium-catalyzed C-H borylation of benzamides typically leads to meta and para selectivities using state-of-the-art iridium-based N,N-chelating bipyridine ligands. However, reaching ortho selectivity patterns requires extensive trial-and-error screening via molecular design at the ligand first coordination sphere. Herein, we demonstrate that triazolylpyridines are excellent ligands for the selective iridium-catalyzed ortho C-H borylation of tertiary benzamides and, importantly, we demonstrate the almost negligible effect of the first coordination sphere in the selectivity, which is so far unprecedented in iridium C-H bond borylations. Remarkably, the activity is dramatically enhanced by exploiting a remote Zn⋯O[double bond, length as m-dash]C weak interaction between the substrate and a rationally designed molecular-recognition site in the catalyst. Kinetic studies and DFT calculations indicate that the iridium-catalyzed C-H activation step is not rate-determining, this being unique for remotely controlled C-H functionalizations. Consequently, a previously established supramolecular iridium catalyst designed for meta-borylation of pyridines is now compatible with the ortho-borylation of benzamides, a regioselectivity switch that is counter-intuitive regarding precedents in the literature. In addition, we highlight the role of the cyclohexene additive in avoiding the formation of undesired side-products as well as accelerating the HBpin release event that precedes the catalyst regeneration step, which is highly relevant for the design of powerful and selective iridium borylating catalysts.

Access to Substituted 1,1-Diarylalkanes by Friedel-Crafts Benzylations Mediated by FeCl3-based Deep Eutectic Solvents

The development of new, more efficient Friedel-Crafts benzylation methodologies that provide access to 1,1-diarylalkanes is an important objective of interest for the production of pharmaceuticals and fine chemical products. In this regard, this study introduces a novel synthetic route to 1,1-diarylalkanes conducted in the Deep Eutectic Solvent (DES) 3 FeCl3 ⋅ 6 H2O/Gly, which serves as both a reaction medium and promoter. Under these conditions, Friedel-Crafts benzylations of various arenes bearing activating and deactivating ortho-/para-directing groups, can be performed using diverse benzylating reagents such as styrenes, alcohols, acetates, ethers, and chlorides. Importantly, highly electronically deactivated electrophiles, including those with CF3 and NO2 groups, are suitable substrates. This methodology provides a wide range of asymmetric 1,1-diarylalkanes (up to 132 examples) with generally good yields and high regioselectivities. The efficiency of this approach was demonstrated with the multigram-scale synthesis (10 mmol) of 1-phenyl-1-xylyl ethane (PXE), a liquid with great industrial applicability. Moreover, the Fe(III)-based DES could be reused for 20 consecutive cycles with no appreciable erosion of the yields.

Photoinduced copper catalyzed nitrogen-to-alkyl radical relay Sonogashira-type coupling of o-alkylbenzamides with alkynes

This report describes a copper-catalyzed, photoinduced N-to-alkyl radical relay Sonogashira-type reactions at benzylic sites in o-alkylbenzamides with alkynes. The process employs an N-to-alkyl radical mechanism, initiated through the copper-catalyzed reductive generation of nitrogen radicals. Radical translocation is facilitated by a 1,5-hydrogen atom transfer (1,5-HAT), leading to the formation of translocated carbon radicals. These radicals are then subjected to copper-catalyzed alkynylation. The methodology exhibits broad sub-strate scope and applicability to the synthesis of complex natural products.

Liquid-assisted grinding enables a direct mechanochemical functionalization of polystyrene waste

The plastic waste crisis has grave consequences for our environment, as most single-use commodity polymers remain in landfills and oceans long after their commercial lifetimes. Utilizing modern synthetic techniques to chemically modify the structure of these post-consumer plastics (e.g., upcycling) can impart new properties and added value for commercial applications. To expand beyond the abilities of current solution-state chemical processes, we demonstrate post-polymerization modification of polystyrene via solid-state mechanochemistry enabled by liquid-assisted grinding (LAG). Importantly, this emblematic trifluoromethylation study modifies discarded plastic, including dyed materials, using minimal exogenous solvent and plasticizers for improved sustainability. Ultimately, this work serves as a proof-of-concept for the direct mechanochemical post-polymerization modification of commodity polymers, and we expect future remediation of plastic waste via similar mechanochemical reactions.

Light-driven C-H activation mediated by 2D transition metal dichalcogenides

C-H bond activation enables the facile synthesis of new chemicals. While C-H activation in short-chain alkanes has been widely investigated, it remains largely unexplored for long-chain organic molecules. Here, we report light-driven C-H activation in complex organic materials mediated by 2D transition metal dichalcogenides (TMDCs) and the resultant solid-state synthesis of luminescent carbon dots in a spatially-resolved fashion. We unravel the efficient H adsorption and a lowered energy barrier of C-C coupling mediated by 2D TMDCs to promote C-H activation and carbon dots synthesis. Our results shed light on 2D materials for C-H activation in organic compounds for applications in organic chemistry, environmental remediation, and photonic materials.

Iron-Catalyzed Photoredox Alcohol α-C-H Alkylation and Tandem Intramolecular Cyclization: Facile Access to Multisubstituted 2,3-Dihydrofurans and γ-Butyrolactones

Multisubstituted furans occupy a pivotal position within the realms of synthetic chemistry and pharmacological science due to their distinctive chemical configurations and inherent properties. We herein introduce a tandem difunctionalization protocol of alcohols for the efficient synthesis of multisubstituted 2,3-dihydrofurans and γ-butyrolactones through the combination of photocatalysis and iron catalysis under mild conditions. Photoredox alcohol α-C(sp3)-H activation and Pinner-type intramolecular cyclization are two key processes. This method features significant convenience, economic benefits, and environmental friendliness.

Ruthenium(II)-Catalyzed Late-Stage Incorporation of N-Aryl Triazoles and Tetrazoles with Sulfonium Salts via C-H Activation

The late-stage functionalization of active pharmaceutical ingredients is a key challenge in medicinal chemistry. Furthermore, N-aryl triazoles and tetrazoles are important structural motifs with the potential to boost the activity of diverse drug molecules. Using easily accessible dibenzothiophenium salts for the ruthenium-catalyzed C-H arylation, these scaffolds were introduced into a variety of bioactive compounds. Our methodology uses cost-efficient ruthenium, KOAc as a mild base and gives access to a plethora of highly decorated triazole and tetrazole containing drug derivatives.

Controllable Skeletal and Peripheral Editing of Pyrroles with Vinylcarbenes

The skeletal editing of azaarenes through insertion, deletion, or swapping of single atoms has recently gained considerable momentum in chemical synthesis. Here, we describe a practical skeletal editing strategy using vinylcarbenes in situ generated from trifluoromethyl vinyl N-triftosylhydrazones, leading to the first dearomative skeletal editing of pyrroles through carbon-atom insertion. Furthermore, depending on the used catalyst and substrate, three types of peripheral editing reactions of pyrroles are also disclosed: α- or γ-selective C-H insertion, and [3+2] cycloaddition. These controllable molecular editing reactions provide a powerful platform for accessing medicinally relevant CF3-containing N-heterocyclic frameworks, such as 2,5-dihydropyridines, piperidines, azabicyclo[3.3.0]octadienes, and allylated pyrroles from readily available pyrroles. Mechanistic insights from experiments and density functional theory (DFT) calculations shed light on the origin of substrate- or catalyst-controlled chemo- and regioselectivity as well as the reaction mechanism.

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.

Late-stage diversification of bacterial natural products through biocatalysis

Bacterial natural products (BNPs) are very important sources of leads for drug development and chemical novelty. The possibility to perform late-stage diversification of BNPs using biocatalysis is an attractive alternative route other than total chemical synthesis or metal complexation reactions. Although biocatalysis is gaining popularity as a green chemistry methodology, a vast majority of orphan sequenced genomic data related to metabolic pathways for BNP biosynthesis and its tailoring enzymes are underexplored. In this review, we report a systematic overview of biotransformations of 21 molecules, which include derivatization by halogenation, esterification, reduction, oxidation, alkylation and nitration reactions, as well as degradation products as their sub-derivatives. These BNPs were grouped based on their biological activities into antibacterial (5), antifungal (5), anticancer (5), immunosuppressive (2) and quorum sensing modulating (4) compounds. This study summarized 73 derivatives and 16 degradation sub-derivatives originating from 12 BNPs. The highest number of biocatalytic reactions was observed for drugs that are already in clinical use: 28 reactions for the antibacterial drug vancomycin, followed by 18 reactions reported for the immunosuppressive drug rapamycin. The most common biocatalysts include oxidoreductases, transferases, lipases, isomerases and haloperoxidases. This review highlights biocatalytic routes for the late-stage diversification reactions of BNPs, which potentially help to recognize the structural optimizations of bioactive scaffolds for the generation of new biomolecules, eventually leading to drug development.

Visible-Light-Induced C-H Cyanoalkylation of Azauracils with Cycloketone Oxime Esters via Catalytic EDA Complex

Photoexcitation electron donor-acceptor (EDA) complexes provide an effective approach to produce radicals under mild conditions, while the catalytic version of EDA complex photoactivation remains scarce. Herein, we report a visible-light-induced organophotocatalytic pathway for the cyanoalkylation of azauracils using inexpensive and readily available 1,4-diazabicyclo[2.2.2]octane (DABCO) as a catalytic electron donor. This synthetic method exhibits exceptional compatibility with various functional groups and presents 34 examples in high yields. The efficient cyanoalkylation offers an environmentally friendly and sustainable route toward enhancing the structural and functional diversity of azauracils.

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.

Fluorescent coumarin-alkynes for labeling of amino acids and peptides via manganese(I)-catalyzed C-H alkenylation

The late-stage fluorescent labeling of structurally complex peptides bears immense potential for molecular imaging. Herein, we report on a manganese(I)-catalyzed peptide C-H alkenylation under exceedingly mild conditions with natural fluorophores as coumarin- and chromone-derivatives. The robustness and efficiency of the manganese(I) catalysis regime was reflected by a broad functional group tolerance and low catalyst loading in a resource- and atom-economical fashion.

Reaction strategies for the meta-selective functionalization of pyridine through dearomatization

The pyridine moiety is a crucial structural component in various pharmaceuticals. While the direct ortho- and para-functionalization of pyridines is relatively straightforward, the meta-selective C-H functionalization remains a significant challenge. This review highlights dearomatization strategies as a key area of interest in expanding the application of meta-C-H functionalization of pyridines. Dearomatization enables the meta-functionalization through various catalytic methods that directly generate dearomatization products, and some products can be rearomatized back to pyridine derivatives. Furthermore, this article also covers the dearomatization of multiple positions of pyridine in the synthesis of polycyclic compounds. It offers a comprehensive overview of the latest advancements in dearomatization at different positions of pyridine, aiming to provide a valuable resource for researchers in this field. It also highlights the advantages and limitations of existing technologies, aiming to inform a broader audience about this important field and foster its future development.

TO-UGDA: target-oriented unsupervised graph domain adaptation

Graph domain adaptation (GDA) aims to address the challenge of limited label data in the target graph domain. Existing methods such as UDAGCN, GRADE, DEAL, and COCO for different-level (node-level, graph-level) adaptation tasks exhibit variations in domain feature extraction, and most of them solely rely on representation alignment to transfer label information from a labeled source domain to an unlabeled target domain. However, this approach can be influenced by irrelevant information and usually ignores the conditional shift of the downstream predictor. To effectively address this issue, we introduce a target-oriented unsupervised graph domain adaptive framework for graph adaptation called TO-UGDA. Particularly, domain-invariant feature representations are extracted using graph information bottleneck. The discrepancy between two domains is minimized using an adversarial alignment strategy to obtain a unified feature distribution. Additionally, the meta pseudo-label is introduced to enhance downstream adaptation and improve the model's generalizability. Through extensive experimentation on real-world graph datasets, it is proved that the proposed framework achieves excellent performance across various node-level and graph-level adaptation tasks.

C(sp3)-C(sp3) Coupling of Cycloalkanes and Alkyl Halides via Dual Photocatalytic Hydrogen Atom Transfer and Nickel Catalysis

Functionalization of C(sp3)-H bonds represents the most straightforward and atom-economical transformation in organic synthesis. An innovative approach integrating photocatalytic hydrogen atom transfer (HAT) and transition metal catalysis has made significant progress in the coupling of α-heterosubstituted C-H bonds with alkyl halides. However, unactivated alkanes were ineffective as a result of the preponderance of byproduct formation. Herein, we demonstrate direct HAT and nickel catalysis in the coupling of cycloalkanes and benzyl bromides/primary alkyl iodides. Additionally, tetrabutylammonium decatungstate (TBADT) was recovered and recycled.

Late-stage meta-C-H alkylation of pharmaceuticals to modulate biological properties and expedite molecular optimisation in a single step

Catalysed C-H activation has emerged as a transformative platform for molecular synthesis and provides new opportunities in drug discovery by late-stage functionalisation (LSF) of complex molecules. Notably, small aliphatic motifs have gained significant interest in medicinal chemistry for their beneficial properties and applications as sp3-rich functional group bioisosteres. In this context, we disclose a versatile strategy with broad applicability for the ruthenium-catalysed late-stage meta-C(sp2)-H alkylation of pharmaceuticals. This general protocol leverages numerous directing groups inherently part of bioactive scaffolds to selectivity install a variety of medicinally relevant bifunctional alkyl units within drug compounds. Our strategy enables the direct modification of unprotected lead structures to quickly generate an array of pharmaceutically useful analogues without resorting to de novo syntheses. Moreover, productive late-stage modulation of key biological characteristics of drug candidates upon remote C-H alkylation proves viable, highlighting the major benefits of our approach to offer in drug development programmes.

Solution-Phase Late-Stage Chemoselective Photocatalytic Removal of Sulfonyl and Phenacyl Groups in Peptides

Herein, BPC catalyzed visible-light-triggered target-specific late-stage solution phase desulfonylation from tryptophan in oligopeptides is portrayed by overcoming the isolation issue up to octamers. This robust and mild method is highly predictable and chemoselective, tolerating myriad of functional groups in aza-heteroaromatics and peptides. Interestingly, reductive desulfonylation is also amenable to biologically significant reactive histidine and tyrosine side chains, signifying the versatility of the strategy. Additional efficacy of BPC is demonstrated by solution phase phenacyl deprotection from C-terminal in peptides. Furthermore, excellent catalyst loading of 0.5 mol% and recyclability demonstrate the practical utility and applicability of this strategy.

On-Surface Synthesis of Non-Benzenoid Nanographenes Embedding Azulene and Stone-Wales Topologies

The incorporation of non-benzenoid motifs in graphene nanostructures significantly impacts their properties, making them attractive for applications in carbon-based electronics. However, understanding how specific non-benzenoid structures influence their properties remains limited, and further investigations are needed to fully comprehend their implications. Here, we report an on-surface synthetic strategy toward fabricating non-benzenoid nanographenes containing different combinations of pentagonal and heptagonal rings. Their structure and electronic properties were investigated via scanning tunneling microscopy and spectroscopy, complemented by computational investigations. After thermal activation of the precursor P on the Au(111) surface, we detected two major nanographene products. Nanographene Aa-a embeds two azulene units formed through oxidative ring-closure of methyl substituents, while Aa-s contains one azulene unit and one Stone-Wales defect, formed by the combination of oxidative ring-closure and skeletal ring-rearrangement reactions. Aa-a exhibits an antiferromagnetic ground state with the highest magnetic exchange coupling reported up to date for a non-benzenoid containing nanographene, coexisting with side-products with closed shell configurations resulted from the combination of ring-closure and ring-rearragement reactions (Ba-a , Ba-s , Bs-a and Bs-s ). Our results provide insights into the single gold atom assisted synthesis of novel NGs containing non-benzenoid motifs and their tailored electronic/magnetic properties.

Cobalt-Catalyzed Enantio- and Regioselective C(sp3 )-H Alkenylation of Thioamides

An enantioselective cobalt-catalyzed C(sp3 )-H alkenylation of thioamides with but-2-ynoate ester coupling partners employing thioamide directing groups is presented. The method is operationally simple and requires only mild reaction conditions, while providing alkenylated products as single regioisomers in excellent yields (up to 85 %) and high enantiomeric excess [up to 91 : 9 enantiomeric ratio (er), or up to >99 : 1 er after a single recrystallization]. Diverse downstream derivatizations of the products are demonstrated, delivering a range of enantioenriched constructs. Extensive computational studies using density functional theory provide insight into the detailed reaction mechanism, origin of enantiocontrol, and the unusual regioselectivity of the alkenylation reaction.

Heterogeneous Photocatalytic Strategies for C(sp3 )-H Activation

Activation of ubiquitous C(sp3 )-H bonds is extremely attractive but remains a great challenge. Heterogeneous photocatalysis offers a promising and sustainable approach for C(sp3 )-H activation and has been fast developing in the past decade. This Minireview focuses on mechanism and strategies for heterogeneous photocatalytic C(sp3 )-H activation. After introducing mechanistic insights, heterogeneous photocatalytic strategies for C(sp3 )-H activation including precise design of active sites, regulation of reactive radical species, improving charge separation and reactor innovations are discussed. In addition, recent advances in C(sp3 )-H activation of hydrocarbons, alcohols, ethers, amines and amides by heterogeneous photocatalysis are summarized. Lastly, challenges and opportunities are outlined to encourage more efforts for the development of this exciting and promising field.

2-Aminophenanthroline Ligands Enable Mild, Undirected, Iridium-Catalyzed Borylation of Alkyl C-H Bonds

The catalytic, undirected borylation of alkyl C-H bonds typically occurs at high reaction temperatures or with excess substrate, or both, because of the low reactivity of alkyl C-H bonds. Here we report a new iridium system comprising 2-anilino-1,10-phenanthroline as the ligand that catalyzes the borylation of alkyl C-H bonds with little to no induction period and with high reaction rates. This superior activation and reactivity profile of 2-aminophenanthroline-ligated catalysts leads to broader reaction scope, including reactions of sensitive substrates, such as epoxides and glycosidic acetals, enhanced diastereoselectivity, and higher yields of borylated products. These catalysts also enable the borylation of alkanes, amines, and ethers at room temperature for the first time. Mechanistic studies imply that facile N-borylation occurs under the reaction conditions and that iridium complexes containing N-boryl aminophenanthrolines are competent precatalysts for the reaction.

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.

Late-stage modification of bioactive compounds: Improving druggability through efficient molecular editing

Synthetic chemistry plays an indispensable role in drug discovery, contributing to hit compounds identification, lead compounds optimization, candidate drugs preparation, and so on. As Nobel Prize laureate James Black emphasized, "the most fruitful basis for the discovery of a new drug is to start with an old drug"1. Late-stage modification or functionalization of drugs, natural products and bioactive compounds have garnered significant interest due to its ability to introduce diverse elements into bioactive compounds promptly. Such modifications alter the chemical space and physiochemical properties of these compounds, ultimately influencing their potency and druggability. To enrich a toolbox of chemical modification methods for drug discovery, this review focuses on the incorporation of halogen, oxygen, and nitrogen-the ubiquitous elements in pharmacophore components of the marketed drugs-through late-stage modification in recent two decades, and discusses the state and challenges faced in these fields. We also emphasize that increasing cooperation between chemists and pharmacists may be conducive to the rapid discovery of new activities of the functionalized molecules. Ultimately, we hope this review would serve as a valuable resource, facilitating the application of late-stage modification in the construction of novel molecules and inspiring innovative concepts for designing and building new drugs.

Promoting the formation of metal-carboxylate coordination to modulate the dimensionality of ultrastable lead halide hybrids

Crystal engineering of metal halide hybrids is critical to investigate their structure-property relationship and advance their photophysical applications, but there have been limited efforts to employ coordination chemistry to precisely control the dimensionality of metal halide sublattices. Herein, we present a coordination-assembly synthetic strategy developed for the rational modulation of lead halide dimensionality, realizing the transition from 2D to 3D architectures. This manipulation is achieved by utilizing three organocarboxylates featuring the identical cyclohexane backbone unit. Specifically, the 1,4-cyclohexanedicarboxylate and 1,2,4,5-cyclohexanetetracarboxylate ligands facilitate the formation of quasi-2D layered structures, characterized by weakly corrugated and strongly corrugated lead halide layers, respectively. Importantly, the introduction of the 1,2,3,4,5,6-cyclohexanehexacarboxylate ligand results in coordination architectures featuring 3D lead chloride/bromide sublattices. The formation of the 3D coordination architectures templated by the 1,2,3,4,5,6-cyclohexanehexacarboxylate ligand affords extended wavelength coverage and superior carrier transport properties compared to their quasi-2D layered analogues. Importantly, both the 2D and 3D lead halide-based coordination polymers exhibit high aqueous stability over a wide pH range, outperforming the conventional ionic-bound lead halides. Notably, the chemically stable 3D lead bromide exhibits efficient photocatalytic ethylbenzene oxidation with the conversion rate of 498 μmol g-1 h-1, substantially higher than its 2D lead bromide counterparts. This work highlights the important role of coordination chemistry in the rational design of metal halide hybrids, which is crucial for advancing their photophysical properties and applications.

Visible-light-mediated direct C3 alkylation of quinoxalin-2(1H)-ones using alkanes

Due to the high C-H bond dissociation energy of alkanes, the utilization of alkanes as alkyl radical precursors for C-H functionalization of heteroarenes is synthetically captivating but practically challenging, especially under metal- and photocatalyst-free conditions. We report herein a mild and practical visible-light-mediated method for C-H alkylation of quinoxalin-2(1H)-ones using trifluoroacetic acid as a hydrogen atom transfer reagent and air as an oxidant. This mild protocol was performed under metal- and photocatalyst-free circumstances and presented good functional-group tolerance as well as a broad substrate scope.

Site-Selective Electrochemical Arene C-H Amination

Here we present the discovery and development of a highly selective aromatic C-H amination reaction. This electrochemical strategy involves a cathodic reduction process that generates highly electrophilic dicationic N-centered radicals that can efficiently engage in aromatic C-H functionalization and channel the regioselectivity of the aromatic substitution. The nitrogen-radical cation-pi interaction with arenes used throughout nature leads to a charge transfer mechanism, with subsequent aromatic C-N bond formation. This electrochemical process generates aryl DABCOnium salts in excellent yields and regioselectivities (single regioisomer in most cases). The scope of the reaction on arene is broad where various functionalities such as aryl halides (bromides, chlorides, fluorides), carbonyls (ketones, esters, imides), sulfonamides, and heteroarenes (pyridines, bipyridines, and terpyridines) are well tolerated. Moreover, we disclose the synthetic utility of the aryl DABCOnium salt adducts leading to the direct access of diverse aryl piperazines and the chemoselective cleavage of the exocyclic aryl C(sp2)-N bond over electrophilic C(sp3)-N+ bonds via photoredox catalysis to afford synthetically useful aryl radicals that can engage in aryl C-C and C-P bond formation.

Cu(II) Promoted C(sp3 )-H Activation in Unactivated Cycloalkanes: Oxo-Alkylation of Styrenes to Synthesize β-Disubstituted Ketones

We report the Cu(II) catalyzed synthesis of β-disubstituted ketones from styrene via oxo-alkylation with unactivated cycloalkanes as the alkylating agent in presence of tert-butylhydroperoxide (TBHP) and 1-methylimidazole as oxidant and base respectively. β-disubstituted ketones are known to be synthesized by using either expensive Ru/Ir complexes, or low-cost metal complexes (e. g., Fe, Mn) with activated species like aldehyde, acid, alcohol, or phthalimide derivatives as the alkylating agent, however, use of unactivated cycloalkanes directly as the alkylating agent remains challenging. A wide range of aliphatic C-H substrates as well as various olefinic arenes and heteroarene (35 substrates including 14 new substrates) are well-tolerated in this method. Hammett analysis shed more light on the substitution effect in the olefinic part on the overall mechanism. Furthermore, the controlled experiments, kinetic isotope effect study, and theoretical calculations (DFT) enable us to gain deeper insight of mechanistic intricacies of this new simple and atom-economic methodology.

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.

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.