Weak coordination as a powerful means for developing broadly useful C-H functionalization reactions

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.

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.

X-type silyl ligands for transition-metal catalysis

Given the critical importance of novel ligand development for transition-metal (TM) catalysis, as well as the resurgence of the field of organosilicon chemistry and silyl ligands, to summarize the topic of X-type silyl ligands for TM catalysis is highly attractive and timely. This review particularly emphasizes the unique σ-donating characteristics and trans-effects of silyl ligands, highlighting their crucial roles in enhancing the reactivity and selectivity of various catalytic reactions, including small molecule activation, Kumada cross-coupling, hydrofunctionalization, C-H functionalization, and dehydrogenative Si-O coupling reactions. Additionally, future developments in this field are also provided, which would inspire new insights and applications in catalytic synthetic chemistry.

Palladium (II)-Catalyzed C-H Activation with Bifunctional Ligands: From Curiosity to Industrialization

In 2001, our curiosity to understand the stereochemistry of C-H metalation with Pd prompted our first studies in Pd(II)-catalyzed asymmetric C-H activation (RSC Research appointment: 020 7451 2545, Grant: RG 36873, Dec. 2002). We identified four central challenges: 1. poor reactivity of simple Pd salts with native substrates; 2. few strategies to control site selectivity for remote C-H bonds; 3. the lack of chiral catalysts to achieve enantioselectivity via asymmetric C-H metalation, and 4. low practicality due to limited coupling partner scope and the use of specialized oxidants. These challenges necessitated new strategies in catalyst and reaction development. For reactivity, we developed approaches to enhance substrate-catalyst affinity together with novel bifunctional ligands which participate in and accelerate the C-H cleavage step. For site-selectivity, we introduced the concept of systematically modulating the distance and geometry between a directing template, catalyst, and substrate to selectively access remote C-H bonds. For enantioselectivity, we devised predictable stereomodels for catalyst-controlled enantioselective C-H activation based on the participation of bifunctional ligands. Finally, for practicality, we have developed varied catalytic manifolds for Pd(II) to accommodate diverse coupling partners while employing practical oxidants such as simple peroxides. These advances have culminated in numerous C-H activation reactions, setting the stage for broad industrial applications.

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.

Carbene-Assisted Arene Ring-Opening

Despite the significant achievements in dearomatization and C-H functionalization of arenes, the arene ring-opening remains a largely unmet challenge and is underdeveloped due to the high bond dissociation energy and strong resonance stabilization energy inherent in aromatic compounds. Herein, we demonstrate a novel carbene assisted strategy for arene ring-opening. The understanding of the mechanism by our DFT calculations will stimulate wide application of bulk arene chemicals for the synthesis of value-added polyconjugated chain molecules. Various aryl azide derivatives now can be directly converted into valuable polyconjugated enynes, avoiding traditional synthesis including multistep unsaturated precursors, poor selectivity control, and subsequent transition-metal catalyzed cross-coupling reactions. The simple conditions required were demonstrated in the late-stage modification of complex molecules and fused ring compounds. This chemistry expands the horizons of carbene chemistry and provides a novel pathway for arene ring-opening.

Recent Advances in the Nickel-Catalyzed Alkylation of C-H Bonds

Functionalization of C-H bonds has emerged as a powerful strategy for converting inert, nonfunctional C-H bonds into their reactive counterparts. A wide range of C-H bond functionalization reactions has become possible by the catalysis of metals, typically from the second row of transition metals. First-row transition metals can also catalyze C-H functionalization, and they have the merits of greater earth-abundance, lower cost and better environmental friendliness in comparison to their second-row counterparts. C-H bond alkylation is a particularly important C-H functionalization reaction due to its chemical significance and its applications in natural product synthesis. This review covers Ni-catalyzed C-H bond alkylation reactions using alkyl halides and olefins as alkyl sources.

Diversity-Oriented Synthesis of Indole-Fused Polycyclic Scaffolds via Rhodium-Catalyzed NH-Indole-Directed C-H Coupling of 2-Phenyl-1H-indoles with Propargylic Alcohol Derivatives

Diversity-oriented synthesis strategy for the efficient assembly of indole-fused polycyclic scaffolds via rhodium-catalyzed NH-indole-directed C-H coupling with propargylic alcohol derivatives in a regioselective manner was developed. Five 2-phenyl-1H-indole-embedded core skeletons were synthesized. In particular, three different indole-fused exo-olefin-containing polycycles were realized, which may be manipulated for further chemistry.

Palladium/XuPhos-catalyzed enantioselective cascade Heck/intermolecular C(sp2)-H alkylation reaction

Palladium-catalyzed enantioselective domino Heck/intramolecular C-H functionalization reaction, as a valuable strategy for creating molecular diversity, has remained a prominent challenge. Here, we describe a Pd/XuPhos catalyst for asymmetric domino Heck/intermolecular C-H alkylation of unactivated alkenes with diverse polyfluoro- and heteroarenes in a highly chemo- and enantioselective manner. This process enables efficient synthesis of various dihydrobenzofurans, indolines and indanes, which are of interest in pharmaceutical research and other areas. Late-stage modifications of the core structures of natural products are also well showcased. Moreover, synthetic transformations create a valuable platform for preparing a series of functionalized molecules. Several control experiments for mechanistic study are conducted to pursue a further understanding of the reaction.

Explainable Graph Neural Networks with Data Augmentation for Predicting pKa of C-H Acids

The pKa of C-H acids is an important parameter in the fields of organic synthesis, drug discovery, and materials science. However, the prediction of pKa is still a great challenge due to the limit of experimental data and the lack of chemical insight. Here, a new model for predicting the pKa values of C-H acids is proposed on the basis of graph neural networks (GNNs) and data augmentation. A message passing unit (MPU) was used to extract the topological and target-related information from the molecular graph data, and a readout layer was utilized to retrieve the information on the ionization site C atom. The retrieved information then was adopted to predict pKa by a fully connected network. Furthermore, to increase the diversity of the training data, a knowledge-infused data augmentation technique was established by replacing the H atoms in a molecule with substituents exhibiting different electronic effects. The MPU was pretrained with the augmented data. The efficacy of data augmentation was confirmed by visualizing the distribution of compounds with different substituents and by classifying compounds. The explainability of the model was studied by examining the change of pKa values when a specific atom was masked. This explainability was used to identify the key substituents for pKa. The model was evaluated on two data sets from the iBonD database. Dataset1 includes the experimental pKa values of C-H acids measured in DMSO, while dataset2 comprises the pKa values measured in water. The results show that the knowledge-infused data augmentation technique greatly improves the predictive accuracy of the model, especially when the number of samples is small.

Late-Stage Diversification of Phosphines by C-H Activation: A Robust Strategy for Ligand Design and Preparation

ConspectusThe advent of the twenty-first century marked a golden era in the realm of synthetic chemistry, exemplified by groundbreaking advancements in the field of C-H activation, which is a concept that quickly transitioned from mere academic fascination to an essential element within the synthetic chemist's toolkit. This methodological breakthrough has given rise to a wealth of opportunities spanning a wide range of chemical disciplines. It has facilitated the late-stage diversification of elaborate organic frameworks, encompassing the spectrum from simple methane to complex polymers, thus refining the lead optimization process and easing the production of diverse molecular analogues. Among these strides forward, the development of phosphorus(III)-directed C-H activation stands out as an increasingly significant and inventive approach for the design and synthesis of ligands, substantially redefining the contours of synthetic methodology.Phosphines, renowned for their roles as ligands and organocatalysts, have become fundamentally important in modern organic chemistry. Their efficiency as ligands is significantly affected by coordination with transition metals, which is essential for their involvement in catalytic processes, influencing both the catalytic activity and the selectivity. Historically, the fabrication of phosphines predominantly relied on synthesis employing complex, multistep procedures. Addressing this limitation, our research has delved into ligand design and synthesis through innovative catalytic P(III)-directed C-H activation strategies. In this Account, we have explored a spectrum of procedures, including direct arylation using metal catalysis, and ventured further into domains such as C-H alkylation, alkenylation, aminocarbonylation, alkynylation, borylation, and silylation. These advances have enriched the field by providing efficient methods for the late-stage diversification of biaryl-type monophosphines as well as enabled the C-H activation of triphenylphosphine and its derivatives. Moreover, we have successfully constructed libraries of diverse axially chiral binaphthyl phosphine ligands, showcasing their potency in asymmetric catalysis. Through this Account, we aim to illuminate the exciting possibilities presented by P(III)-directed C-H activation in propelling the boundaries of organic synthesis. By highlighting our pioneering work, we hope to inspire further developments in this promising field of chemistry.

Ultrafast photoinduced C-H bond formation from two small inorganic molecules

The formation of carbon-hydrogen (C-H) bonds via the reaction of small inorganic molecules is of great significance for understanding the fundamental transition from inorganic to organic matter, and thus the origin of life. Yet, the detailed mechanism of the C-H bond formation, particularly the time scale and molecular-level control of the dynamics, remain elusive. Here, we investigate the light-induced bimolecular reaction starting from a van der Waals molecular dimer composed of two small inorganic molecules, H2 and CO. Employing reaction microscopy driven by a tailored two-color light field, we identify the pathways leading to C-H photobonding thereby producing HCO+ ions, and achieve coherent control over the reaction dynamics. Using a femtosecond pump-probe scheme, we capture the ultrafast formation time, i.e., 198 ± 16 femtoseconds. The real-time visualization and coherent control of the dynamics contribute to a deeper understanding of the most fundamental bimolecular reactions responsible for C-H bond formation, thus contributing to elucidate the emergence of organic components in the universe.

Ru(II)-Catalyzed Divergent C-H Alkynylation Cascade with Bifunctional α-Alcohol Haloalkynes

Native functionality directed the C-H activation cascade to enable rapid construction of molecular complexity, featuring step-economy and synthetic efficiency. Herein, by exploiting bifunctional α-alcohol haloalkynes, we developed Ru(II)-catalyzed carboxylic acid, amine, and amide assisted divergent C-H alkynylation and annulation cascade, affording polyfunctional heterocycles. Significantly, a bilateral aryl C-H polycyclization cascade of azobenzenes was achieved using the versatile haloalkynes.

Palladium-Catalyzed Directed Carbon-Carbon Bond Activation of Aryl Nitriles for Cyano Transfer

Herein, we report the C-H cyanation of indoles via a palladium-catalyzed directed C-CN activation reaction using aryl nitrile as a cyano source. The employment of the phenoxy-oriented group is the key to the cleavage of the C-CN bond. This protocol features a broad substrate scope, good efficiency, and high regioselectivity. Furthermore, the practical application of this protocol was showcased in the late-stage functionalization and synthesis of indole derivatives, which were derived from drugs and natural products, through the process of cyanation.

Expanding the Scope of Alkynes in C-H Activation: Weak Chelation-Assisted Cobalt-Catalyzed Synthesis of Indole C(4)-Acrylophenone via C-O Bond Cleavage of Propargylic Ethers

Herein, we report the facile synthesis of indole C(4)-acrylophenone using a C-H bond activation strategy. For this conversion, an unsymmetrical alkyne (phenylethynyl ether) in the presence of cobalt(III)-catalyst works efficiently. In this process, alkyne gets oxidized in the presence of in situ generated water, which is the key step for this method, for which trifluoroethanol is the water source. The pivaloyl directing group chelates effectively to generate the cobaltacycle intermediate, which was detected through high-resolution mass spectrometry (HRMS). Also, the formation of bis(2,2,2-trifluoroethyl) ether has been confirmed and quantified using 19F NMR. In addition, the applicability of obtained indole C(4)-acrylophenone product has been demonstrated by performing the Nazarov cyclization and conjugate addition to the α,β-unsaturated ketone moiety.

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.

Remote-carbonyl-directed sequential Heck/isomerization/C(sp2)-H arylation of alkenes for modular synthesis of stereodefined tetrasubstituted olefins

Modular and regio-/stereoselective syntheses of all-carbon tetrasubstituted olefins from simple alkene materials remain a challenging project. Here, we demonstrate that a remote-carbonyl-directed palladium-catalyzed Heck/isomerization/C(sp2)-H arylation sequence enables unactivated 1,1-disubstituted alkenes to undergo stereoselective terminal diarylation with aryl iodides, thus offering a concise approach to construct stereodefined tetrasubstituted olefins in generally good yields under mild conditions; diverse carbonyl groups are allowed to act as directing groups, and various aryl groups can be introduced at the desired position simply by changing aryl iodides. The stereocontrol of the protocol stems from the compatibility between the E/Z isomerization and the alkenyl C(sp2)-H arylation, where the vicinal group-directed C(sp2)-H arylation of the Z-type intermediate product thermodynamically drives the reversible E to Z isomerization. Besides, the carbonyl group not only promotes the Pd-catalyzed sequential transformations of unactivated alkenes by weak coordination, but also avoids byproducts caused by other possible β-H elimination.

Palladium-Catalyzed Weak-Chelation-Assisted C4-Nitration of Indoles with tert-Butyl Nitrite: Formal Access to Aminated Indoles

Palladium-catalyzed weak-chelation-assisted C4-selective nitration of indoles has been accomplished employing tert-butyl nitrite in the presence of oxone under molecular oxygen at a moderate temperature. Aerobic conditions, C4-selectivity, substrate scope, conversion to valuable aminated indoles, and late-stage natural product modifications are the important practical features.

Iridium(III)-catalyzed β-trifluoromethyl enone carbonyl-directed regioselective ortho-C(sp2)-H olefination

Due to the lower LUMO energy level at the β-position of α,β-unsaturated-β-trifluoromethyl enone than that of its non-fluorinated counterpart, there is a challenge to activate the sp2 C-H bond of aromatic rings. Herein, we have reported iridium(III)-catalyzed β-trifluoromethyl enone carbonyl-directed regioselective aromatic C(sp2)-H olefination with acrylates under oxidative conditions. Furthermore, coupling with natural product-derived acrylates, scale-up and product diversification have also been performed.

Synthesis of silyl indenes by ruthenium-catalyzed aldehyde- and acylsilane-enabled C-H alkylation/cyclization

A ruthenium-catalyzed C-H alkylation/cyclization sequence is presented to prepare silyl indenes with atom and step-economy. This domino reaction is triggered by acyl silane-directed C-H activation, and an aldehyde controlled the following enol cyclization/condensation other than β-H elimination. The protocol tolerates a broad substitution pattern, and the further synthetic elaboration of silyl indenes allows access to a diverse range of interesting indene and indanone derivatives.

Direct Heterocycle C-H Alkenylation via Dual Catalysis Using a Palladacycle Precatalyst: Multifactor Optimization and Scope Exploration Enabled by High-Throughput Experimentation

One of the major challenges in developing catalytic methods for C-C bond formation is the identification of generally applicable reaction conditions, particularly if multiple substrate structural classes are involved. Pd-catalyzed direct arylation reactions are powerful transformations that enable direct functionalization of C-H bonds; however, the corresponding direct alkenylation reactions, using vinyl (pseudo) halide electrophiles, are less well developed. Inspired by process development efforts toward GSK3368715, an investigational active pharmaceutical ingredient, we report that a Pd(II) palladacycle derived from tri-tert-butylphosphine and Pd(OAc)2 is an effective single-component precatalyst for a variety of direct alkenylation reactions. High-throughput experimentation identified optimal solvent/base combinations for a variety of HetAr-H substrate classes undergoing C-H activation without the need for cocatalysts or stoichiometric silver bases (e.g., Ag2CO3). We propose this reaction proceeds via a dual cooperative catalytic mechanism, where in situ-generated Pd(0) supports a canonical Pd(0)/(II) cross-coupling cycle and the palladacycle effects C-H activation via CMD in a redox-neutral cycle. In all, 192 substrate combinations were tested with a hit rate of approximately 40% and 24 isolated examples. Importantly, this method was applied to prepare a key intermediate in the synthesis of GSK3368715 on multigram scale.

Visible-Light-Induced Transition-Metal-Free Redox-Neutral Carboxylation of Remote Benzylic C(sp3)-H Bonds via 1,5-Hydrogen Atom Transfer

The direct carboxylation of the benzylic C-H bonds under mild conditions is of great importance and is quite challenging. Herein, we report an approach for the carboxylation of remote benzylic C(sp3)-H bonds by integrating the redox-neutral visible-light photoredox catalysis and the nitrogen-centered 1,5-hydrogen atom transfer. The chemical transformation progresses via consecutive single electron transfer, 1,5-hydrogen atom transfer, formation of benzylic carbanion, and nucleophilic attack on the CO2 steps, thereby enabling access to the desired carboxylation products with moderate to high yields. We also endeavor to recover the CO2 groups generated in situ intramolecularly to achieve carboxylation under a nitrogen atmosphere, resulting in moderate yields of corresponding carboxylation.

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.

Palladium K-edge X-ray Absorption Spectroscopy Studies on Controlled Ligand Systems

X-ray absorption spectroscopy (XAS) is widely used across the life and physical sciences to identify the electronic properties and structure surrounding a specific element. XAS is less often used for the characterization of organometallic compounds, especially for sensitive and highly reactive species. In this study, we used solid- and solution-phase XAS to compare a series of 25 palladium complexes in controlled ligand environments. The compounds include palladium centers in the formal I, II, III, and IV oxidation states, supported by tridentate and tetradentate macrocyclic ligands, with different halide and methyl ligand combinations. The Pd K-edge energies increased not only upon oxidizing the metal center but also upon increasing the denticity of the ligand framework, substituting sigma-donating methyl groups with chlorides, and increasing the charge of the overall metal complex by replacing charged ligands with neutral ligands. These trends were then applied to characterize compounds whose oxidation states were otherwise unconfirmed.

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.

Sterically demanding Csp2(ortho-substitution)-Csp3(tertiary) bond formation via carboxylate-directed Mizoroki-Heck reaction under extra-ligand-free conditions

Construction of the sterically demanding Csp2(oS)-Csp3(T) bond was achieved by carrying out the Pd-catalyzed carboxylate-directed Mizoroki-Heck reaction under extra-ligand-free aqueous conditions. The cooperative role of the presence of water with the absence of phosphine ligand was proposed to accelerate the migratory insertion process considerably, delivering a broad substrate scope.

B(C6 F5 )3 -Catalyzed [2+3]-Cyclative o,m-diC-H Functionalization of Phenols

Metal-free catalytic C-H functionalization is highly desired for the construction of C-C bonds. We herein report a highly chemoselective consecutive C-H [2+3]-cyclative functionalization for the simultaneous formation of two C-C bonds with construction of polycyclic phenols catalyzed by commercially available and low-cost B(C6 F5 )3 . This catalytic system tolerates a wide range of substrate scope, providing a series of 2,6,7,8-tetrahydroacenaphthylen-3-ol-type polycyclic compounds efficiently. Several derivatizations of the catalytic products have also been conducted to show the potential application of this method in synthesis of polycyclic compounds.

Annulative π-Extension by Cp*Co(III)-Catalyzed Ketone-Directed peri-Annulation: An Approach to Access Fused Arenes

A masked-bay-region selective first-row transition-metal Cp*Co(III)-catalyzed annulative π-extension of arene-derived ketones is achieved to afford K-region-functionalized benzo[e]pyrenes, benzotetraphenes, and pyrenes. Comprehensive density functional theory studies buttress the mechanistic pathway comprising key steps like peri-C-H activation, alkyne 1,2-migratory insertion, and nucleophilic attack toward ketone, this attack being the rate-determining step. In addition, π-conjugated 1,1'-bipyrenes, potential photocatalyst pyrene-quinones, and putative n-type semiconductor cyano group-containing dibenzo[de,qr]tetracenes are also accessed.

Expedient C-H allylation of sulfoxonium ylides: merging C-H and C-C/C-het bond activation

Sulfoxonium ylide chelation-assisted C-H allylation of arenes has been accomplished utilizing strained vinyl carbo/heterocycles as the allyl surrogates via sequential C-H and C-C/het bond activation. Broad substrate scope, Co-catalysis, selectivity, and late-stage drug mutation are the important practical features.

CH bond activation in aromatic ketones mediated by iridium-tris(pyrazolyl)borate complexes

Reaction of complex [TpMe2Ir(η4-CH2C(Me)C(Me)C2)] (1) with a series of aromatic ketones at 130 °C renders, by means of a selective ortho-CH activation, Ir(III)-metallacycles 2-5, which display an Ir-H bond. When [TpMe2Ir(C6H5)2N2] (6) is treated with 2-(trifluoromethyl)acetophenone and 2-fluoroacetophenone at 80 °C, the formation of dimeric (7) and trimeric architectures (8) is achieved through the meta- and para-CH activation of the aromatic ketone, respectively. The generation of complexes 2-5 is proposed to occur by the initial formation of Ir(III) η1-ketone adducts as key intermediates, followed by aromatic CH activations and the release of a butadiene ligand. The formation of complexes 7 and 8 involves an assisted process in which a metal center activation of the less sterically hindered C-H bond of the aromatic ketone takes place (releasing a benzene molecule), followed by the coordination of the carbonyl group, which generates the respective dimeric and trimeric structures. Complexes 7 and 8 are efficient catalysts for the transfer hydrogenation of ketones and aldehydes using isopropanol as the hydrogen source. All complexes have been fully characterized by NMR spectroscopy, FT-IR, elemental analysis and, in the cases of 7 and 8, X-ray crystallography. Details of the reaction conditions, isolation of the products, and proposals for the pathways of formation of complexes 2-5 and 7-8 are discussed.

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.

Merging Visible Light Photocatalysis and P(III)-Directed C-H Activation by a Single Catalyst: Modular Assembly of P-Alkyne Hybrid Ligands

Metal-catalyzed C-H activation strategies provide an efficient approach for synthesis by minimizing atom, step, and redox economy. Developing milder, greener, and more effective protocols for these strategies is always highly desirable to the scientific community. In this study, the utilization of a single rhodium complex enabled the visible-light-induced late-stage C-H activation of biaryl-type phosphines with alkynyl bromides, employing inherent phosphorus atoms as directing groups. This chemistry combines P(III)-directed C-H activation with visible light photocatalysis, under exogenous photosensitizer-free conditions, offering a unique platform for ligand design and preparation. Furthermore, this study also explores the asymmetric catalysis and coordination chemistry of the resulting P-alkyne hybrid ligands with specific transition metals. Experimental results and density functional theory calculations demonstrate the mechanistic intricacies of this transformation.

Rhodaelectro-Catalyzed Decarboxylative Cross-Dehydrogenative Coupling of Indole-3-carboxylic Acids and Olefins via Weakly Coordinating Carboxyl Groups

A rhodaelectro-catalyzed C2-H selectively decarboxylative alkenylation of 3-carboxy-1H-indoles employing electricity as the traceless terminal oxidant has been accomplished. The weakly coordinating carboxyl group serves as the traceless directing groups. External oxidant-free in an undivided cell with constant current in aqueous solution ensures the decarboxylative C-H alkenylation to be viable and sustainable.

Ruthenium-Catalyzed Aminocarbonylation with Isocyanates Through Weak Coordinating Groups

Introducing amide functional groups under mild conditions has growing importance owing to the prevalence of such moiety in biologically active molecules. Herein, we disclose a mild protocol for the directed ruthenium-catalyzed C-H aminocarbonylation with isocyanates as the amidating agents developed through high-throughput experimentation (HTE). The redox-neutral and base-free reaction is guided by weakly Lewis basic functional groups, including anilides, lactams and carbamates to access anthranilamide derivatives. The synthetic utility of this transformation is reflected by large-scale synthesis and late-stage functionalization.

A tautomerized ligand enabled meta selective C-H borylation of phenol

Remote meta selective C-H functionalization of aromatic compounds remains a challenging problem in chemical synthesis. Here, we report an iridium catalyst bearing a bidentate pyridine-pyridone (PY-PYRI) ligand framework that efficiently catalyzes this meta selective borylation reaction. We demonstrate that the developed concept can be employed to introduce a boron functionality at the remote meta position of phenols, phenol containing bioactive and drug molecules, which was an extraordinary challenge. Moreover, we have demonstrated that the method can also be applied for the remote C6 borylation of indole derivatives including tryptophan that was the key synthetic precursor for the total synthesis of Verruculogen and Fumitremorgin A alkaloids. The inspiration of this catalytic concept was started from the O-Si secondary interaction, which by means of several more detailed control experiments and detailed computational investigations revealed that an unprecedented Bpin shift occurs during the transformation of iridium bis(boryl) complex to iridium tris(boryl) complex, which eventually control the remote meta selectivity by means of the dispersion between the designed ligand and steering silane group.

DFT Studies on the Mechanisms of Carboamination/Diamination of Unactivated Alkenes Mediated by Pd(IV) Intermediates

Density functional theory (DFT) calculations have been employed to investigate the mechanism of carboamination and diamination of unactivated alkenes mediated by Pd(IV) intermediates. Both reactions share a common Pd(IV) intermediate, serving as the starting point for either the carboamination or the diamination pathway. The formation of this Pd(IV) intermediate encompasses a transition state that substantially impacts the turnover frequency (TOF) of catalytic cycles, with an apparent activation free-energy barrier of 26.1 kcal mol-1. Carboamination of unactivated alkenes proceeds through the coordination of a toluene molecule, C-H activation, inner reductive elimination, and the separation of the carboamination product from this intermediate, while diamination of unactivated alkenes involves the formation of the ion nucleophile, SN2 attack, and the separation of the diamination product. A comparison of the free-energy profiles for carboamination and diamination of unactivated alkenes can elucidate the origin of the chemoselectivity, and Bader's atoms in molecules (AIM) wave function analyses have been performed to analyze the contributions of the outer C-N bonding in the diamination process.

Mechanism and origins of cobalt-catalyzed ligand-controlled regiodivergent C-H functionalization of aldehydes with enynes

The influence of the P-M-P bite angle in diphosphine ligands on selectivity has been observed in various catalytic reactions. A better understanding of the ligand bite angle concept is important for the rational design of efficient catalytic systems. In the present work, the mechanism of cobalt-catalyzed C-H functionalization of aldehydes with enynes and how the diphosphine ligands alter regioselectivity were investigated by density functional theory (DFT) calculations. The catalytic cycle is initiated by the oxidative cyclization of enynes rather than the oxidative addition of aldehydes. Regioselectivity arises from competing σ-bond metathesis and migratory insertion steps, in which the steric effects of diphosphine ligands are the dominant factors influencing the activation barriers. The calculations indicate that σ-bond metathesis is more challenging and its feasibility is highly dependent on the ligand bite angle. The improved mechanistic understanding will enable further design of transition-metal-catalyzed selective cyclization reactions.

Rh(III)-Catalyzed C-H Activation of Benzamides and Chemodivergent Annulation with Benzoxazinanones: Substrate Controlled Selectivity

Decarboxylative annulation of propargyl carbamates with benzamides has been realized via rhodium-catalyzed C-H bond activation under mild conditions, delivering two distinct classes of heterocycles in high efficiency and selectivity under substrate control. This protocol provides a direct synthetic method for the preparation of functionalized 1,8-naphthyridines and isoindolinones.

A redox-neutral weak carbonyl chelation assisted C4-H allylation of indoles with vinylcyclopropanes

A weak acyl chelation-assisted distal C4-H allylation of indoles has been accomplished using vinylcyclopropanes as an allylating agent under redox-neutral ruthenium(II) catalysis. The regioselectivity, removable directing group, substrate scope and diastereoselectivity are the important practical features.

Redox-Neutral Site-Selective C-H Allylation and Iodolactonization of Benzoic Acids Using Morita-Baylis-Hillman Adducts in Water

A Ru-catalyzed carboxylate directed C-H allylation and iodolactonization of benzoic acids has been accomplished with Morita-Baylis-Hillman adducts as the coupling partner in environmentally benign water as solvent. The redox-neutral conditions, use of water as a solvent, substrate scope, functional group tolerance, and mutation of natural products and drug molecules are the important practical features.

Weakly coordinating tert-amide assisted Rh(III)-catalyzed C4-cyanation of indoles: application in photophysical studies

A rhodium(III)-catalyzed indole C4-selective cyanation is described using the bench-stable, user-friendly electrophilic cyanation agent N-cyano-N-phenyl-p-toluenesulfonamide (NCTS) as a coupling partner. A suitably positioned weakly coordinating tert-amide group was utilized for this site selectivity. The developed protocol proceeded with a broad scope. [Cp*Rh(MeCN)3][SbF6]2 was found to be an effective Rh(III) catalyst for this transformation. An initial study was carried out to know the photophysical properties of the C4-cyanated indole frameworks.

Dehydrogenative Cross-Coupling of α,β-Unsaturated Compounds with Unactivated Olefins via Co(III) Catalysis

An oxidative cross-coupling of α,β-unsaturated compounds with unactivated alkenes via cobalt-catalyzed vinylic C-H activation has been developed. The present catalytic reaction was examined with various differently functionalized unsaturated compounds and unactivated olefins. In these reactions, highly valuable amide functionalized butadienes and indenones were prepared in good to excellent yields. A possible reaction mechanism is proposed involving directed olefinic C-H activation through a base-assisted deprotonation pathway.

Regioselective Iridium-Catalyzed C8-H Borylation of 4-Quinolones via Transient O-Borylated Quinolines

The quinolone-quinoline tautomerization is harnessed to effect the regioselective C8-borylation of biologically important 4-quinolones by using [Ir(OMe)(cod)]2 as the catalyst precursor, the silica-supported monodentate phosphine Si-SMAP as the ligand, and B2 pin2 as the boron source. Initially, O-borylation of the quinoline tautomer takes place. Critically, the newly formed 4-(pinBO)-quinolines then undergo N-directed selective Ir-catalyzed borylation at C8. Hydrolysis of the OBpin moiety on workup returns the system to the quinolone tautomer. The C8-borylated quinolines were converted to their corresponding potassium trifluoroborate (BF3 K) salts and to their C8-chlorinated quinolone derivatives. The two-step C-H borylation-chlorination reaction sequence resulted in various C8-Cl quinolones in good yields. Conversion to C8-OH-, C8-NH2 -, and C8-Ar-substituted quinolones was also feasible by using this methodology.

Fusion of Multiple Spectra for Investigating Chemical Bonding Properties via Machine Learning

Chemical bonding properties are crucial to understanding the chemical behavior of molecules. Spectroscopy is a versatile technical tool to study various microscopic properties, but its interpretation suffers from human biases and the loss of high-dimensional information. Here, we present a machine learning approach to predict diverse bonding properties, including the bond dissociation energy, bond length, and α-C connectivity of hydroxyls in organic molecules, by fusing multiple spectra with different physical mechanisms. Combining nuclear magnetic resonance and vibrational spectroscopy exhibits higher prediction accuracy than what they did separately. On the hold-out test data set, the models achieve a mean absolute error of 1.243 kcal/mol and 1.041 × 10-4 Å for BDE and bond length and an accuracy of 95.09% for hydroxyl α-C connectivity. Our models demonstrate strong extrapolation capabilities when they are transferred to different molecules, external electric fields, and solvation environments. These end-to-end models pave the way to investigating chemical bonding properties by using spectroscopic observables.

Macrocyclization via remote meta-selective C-H olefination using a practical indolyl template

The synthesis of macrocyclic compounds with different sizes and linkages remains a great challenge via transition metal-catalysed intramolecular C-H activation. Herein, we disclose an efficient macrocyclization strategy via Pd-catalysed remote meta-C-H olefination using a practical indolyl template. This approach was successfully employed to access macrolides and coumarins. In addition, the intermolecular meta-C-H olefination also worked well and was exemplified by the synthesis of antitumor drug belinostat from inexpensive and readily available benzenesulfonyl chloride. Notably, catalytic copper acetate and molecular oxygen were used in place of silver salts as oxidants. Furthermore, for the first time, the formation of a macrocyclophane cyclopalladated intermediate was detected through in situ Fourier-transform infrared monitoring experiments and ESI-MS.

Synthesis of Naphthalimides through Tandem Pd(II)-Catalyzed C(sp3)-H Oxidation and Diels-Alder Reaction Using a Transient Directing Group Strategy

Naphthalimides have found extensive applications in materials science and pharmaceuticals. It is still highly desirable to develop efficient methods for the synthesis of naphthalimides with structural diversity. In this work, we developed a new approach for the synthesis of naphthalimides via a tandem reaction of o-methylbenzaldehydes and maleimides. The tandem reaction involves Pd(II)-catalyzed benzylic C(sp³)-H oxidation using an amino acid as the transient directing group and Diels-Alder reaction. The subsequent dehydration forms naphthalimides. The reaction introduces the imide moiety and constructs a benzene ring simultaneously, allowing for easy access to a range of naphthalimides with a variety of substituents.

Pd(II) and Rh(I) Catalytic Precursors for Arene Alkenylation: Comparative Evaluation of Reactivity and Mechanism Based on Experimental and Computational Studies

We combine experimental and computational investigations to compare and understand catalytic arene alkenylation using the Pd(II) and Rh(I) precursors Pd(OAc)₂ and [(η²-C₂H₄)₂Rh(μ-OAc)]₂ with arene, olefin, and Cu(II) carboxylate at elevated temperatures (>120 °C). Under specific conditions, previous computational and experimental efforts have identified heterotrimetallic cyclic PdCu₂(η²-C₂H₄)₃(μ-OPiv)₆ and [(η²-C₂H₄)₂Rh(μ-OPiv)₂]₂(μ-Cu) (OPiv = pivalate) species as likely active catalysts for these processes. Further studies of catalyst speciation suggest a complicated equilibrium between Cu(II)-containing complexes containing one Rh or Pd atom with complexes containing two Rh or Pd atoms. At 120 °C, Rh catalysis produces styrene >20-fold more rapidly than Pd. Also, at 120 °C, Rh is ∼98% selective for styrene formation, while Pd is ∼82% selective. Our studies indicate that Pd catalysis has a higher predilection toward olefin functionalization to form undesired vinyl ester, while Rh catalysis is more selective for arene/olefin coupling. However, at elevated temperatures, Pd converts vinyl ester and arene to vinyl arene, which is proposed to occur through low-valent Pd(0) clusters that are formed in situ. Regardless of arene functionality, the regioselectivity for alkenylation of mono-substituted arenes with the Rh catalyst gives an approximate 2:1 meta/para ratio with minimal ortho C-H activation. In contrast, Pd selectivity is significantly influenced by arene electronics, with electron-rich arenes giving an approximate 1:2:2 ortho/meta/para ratio, while the electron-deficient (α,α,α)-trifluorotoluene gives a 3:1 meta/para ratio with minimal ortho functionalization. Kinetic intermolecular arene ethenylation competition experiments find that Rh reacts most rapidly with benzene, and the rate of mono-substituted arene alkenylation does not correlate with arene electronics. In contrast, with Pd catalysis, electron-rich arenes react more rapidly than benzene, while electron-deficient arenes react less rapidly than benzene. These experimental findings, in combination with computational results, are consistent with the arene C-H activation step for Pd catalysis involving significant η¹-arenium character due to Pd-mediated electrophilic aromatic substitution character. In contrast, the mechanism for Rh catalysis is not sensitive to arene-substituent electronics, which we propose indicates less electrophilic aromatic substitution character for the Rh-mediated arene C-H activation.

C-H Glycosylation of Native Carboxylic Acids: Discovery of Antidiabetic SGLT-2 Inhibitors

C-Glycosides are critical motifs embedded in many bioactive natural products. The inert C-glycosides are privileged structures for developing therapeutic agents owing to their high chemical and metabolic stability. Despite the comprehensive strategies and tactics established in the past few decades, highly efficient C-glycoside syntheses via C-C coupling with excellent regio-, chemo-, and stereoselectivity are still needed. Here, we report the efficient Pd-catalyzed glycosylation of C-H bonds promoted by weak coordination with native carboxylic acids without external directing groups to install various glycals to the structurally diverse aglycon parts. Mechanistic evidence points to the participation of a glycal radical donor in the C-H coupling reaction. The method has been applied to a wide range of substrates (over 60 examples), including many marketed drug molecules. Natural product- or drug-like scaffolds with compelling bioactivities have been constructed using a late-stage diversification strategy. Remarkably, a new potent sodium-glucose cotransporter-2 inhibitor with antidiabetic potential has been discovered, and the pharmacokinetic/pharmacodynamic profiles of drug molecules have been changed using our C-H glycosylation approach. The method developed here provides a powerful tool for efficiently synthesizing C-glycosides to facilitate drug discovery.

Distal meta-C-H functionalization of α-substituted cinnamates

Development of a novel strategy for the palladium-catalyzed selective meta-C-H activation of α-substituted cinnamates and their heterocyclic analogues with various alkenes using nitrile as a directing group (DG) has been described. Importantly, we introduced naphthoquinone, benzoquinones, maleimides and sulfolene as coupling partners in the meta-C-H activation reaction for the first time. Notably, allylation, acetoxylation and cyanation were also achieved through distal meta-C-H functionalization. This novel protocol also includes the coupling of various olefin-tethered bioactive molecules with high selectivity.

One-Pot Synthesis of Benzo[c]phenanthridine Alkaloids from 7-Azabenzonorbornadienes and Aryl Nitrones

An efficient synthesis of benzo[c]phenanthridine alkaloids via a synergistic combination of C-C bond formation and a cycloaromatization reaction is described. Aryl nitrones react with 7-azabenzonorbornadienes in the presence of a Rh(III) catalyst, providing pharmaceutically useful benzo[c]phenanthridine derivatives in good to moderate yields. Using this methodology, highly useful alkaloids such as norfagaronine, norchelerythrine, decarine, norsanguinarine, and nornitidine were prepared in a single step.