From Pd(OAc)2 to Chiral Catalysts: The Discovery and Development of Bifunctional Mono-N-Protected Amino Acid Ligands for Diverse C-H Functionalization Reactions

Rhodium-Catalyzed Direct ortho-Arylation of Anilines

An efficient and broadly applicable rhodium-catalyzed direct ortho-arylation of anilines with aryl iodides relying on readily available aminophosphines as traceless directing groups is reported. Its scope and functional group compatibility were both found to be quite broad as a large variety of both aminophosphines and (hetero)aryl iodides, including complex ones, could be utilized. The ortho-arylated anilines could be obtained in high average yields, without any competing diarylation and with full regioselectivity, which constitutes a major step forward compared to other processes. The reaction is moreover not limited to aryl iodides, as an aryl bromide and a triflate could be successfully used, and could be extended to diarylation. Mechanistic studies revealed the key and unique role of the aminophosphine, acting not only as a substrate but also as a ligand for the rhodium catalyst.

Formate and CO2 Enable Reductive Carboxylation of Imines: Synthesis of Unnatural α-Amino Acids

Herein, a photocatalytic umpolung strategy for reductive carboxylation of imines for the synthesis of α-amino acids was disclosed. Carbon dioxide radical anion (CO2•-) generated from formate is the key single electron reductant in the reactions. An unprecedentedly broad substrate scope of imines with excellent reaction yields was obtained with carbon dioxide (CO2) and formate salt as carbon sources.

Ligand-Enabled Palladium-Catalyzed [3 + 2] Annulation of Aryl Iodides with Maleimides via C(sp3)-H Activation

Palladium-catalyzed intermolecular [3 + 2] annulation reactions via C-H activation represent a powerful and charming tool for assembling cyclopentanes. Herein, we have developed a strategy for the palladium-catalyzed intermolecular alkene-relayed annulation reaction of aryl iodides and maleimides via C(sp3)-H activation for the construction of polycyclic structures. In contrast to directed-group-enabled intermolecular maleimide-relayed [3 + 2] annulation reactions, this protocol stands out for its utilization of aryl iodides as substrates. Notably, monoprotected amino acids played a crucial role as ligands in this reaction, which is rarely observed in C-H activation reactions initiated with organohalides.

Asymmetric Synthesis of β,β-Disubstituted Alanines via a Sequential C(sp2)-C(sp3) Cross-Coupling-Hydrogenation Strategy

We report the development of a sequential C(sp2)-C(sp3) Suzuki cross-coupling-asymmetric hydrogenation strategy which allows access to a diverse array of valuable β,β-disubstituted alanine derivatives. This synthesis exhibits broad functional group tolerance, and permits efficient access to β-aryl-β-alkyl, and the more rarely reported β,β-dialkyl Ala derivatives with high yield and excellent enantioselectivity. This transformation has been exhibited on decagram quantity, and can be used to generate Fmoc amino acid derivatives which are useful for SPPS.

Chiral amino acids: evolution in atroposelective C-H activation

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

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

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

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

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

Chiral Osmium(II)/Salox Species Enabled Enantioselective γ-C(sp3)-H Amidation: Integrated Experimental and Computational Validation For the Ligand Design and Reaction Development

Herein, multiple types of chiral Os(II) complexes have been designed to address the appealing yet challenging asymmetric C(sp3)-H functionalization, among which the Os(II)/Salox species is found to be the most efficient for precise stereocontrol in realizing the asymmetric C(sp3)-H amidation. As exemplified by the enantioenriched pyrrolidinone synthesis, such tailored Os(II)/Salox catalyst efficiently enables an intramolecular site-/enantioselective C(sp3)-H amidation in the γ-position of dioxazolone substrates, in which benzyl, propargyl and allyl groups bearing various substituted forms are well compatible, affording the corresponding chiral γ-lactam products with good er values (up to 99 : 1) and diverse functionality (>35 examples). The unique performance advantage of the developed chiral Os(II)/Salox system in terms of the catalytic energy profile and the chiral induction has been further clarified by integrated experimental and computational studies.

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.

Enantioselective organocatalytic strategies to access noncanonical α-amino acids

Organocatalytic asymmetric synthesis has evolved over the years and continues to attract the interest of many researchers worldwide. Enantiopure noncanonical amino acids (ncAAs) are valuable building blocks in organic synthesis, medicinal chemistry, and chemical biology. They are employed in the elaboration of peptides and proteins with enhanced activities and/or improved properties compared to their natural counterparts, as chiral catalysts, in chiral ligand design, and as chiral building blocks for asymmetric syntheses of complex molecules, including natural products. The linkage of ncAA synthesis and enantioselective organocatalysis, the subject of this perspective, tries to imitate the natural biosynthetic process. Herein, we present contemporary and earlier developments in the field of organocatalytic activation of simple feedstock materials, providing potential ncAAs with diverse side chains, unique three-dimensional structures, and a high degree of functionality. These asymmetric organocatalytic strategies, useful for forging a wide range of C-C, C-H, and C-N bonds and/or combinations thereof, vary from classical name reactions, such as Ugi, Strecker, and Mannich reactions, to the most advanced concepts such as deracemisation, transamination, and carbene N-H insertion. Concurrently, we present some interesting mechanistic studies/models, providing information on the chirality transfer process. Finally, this perspective highlights, through the diversity of the amino acids (AAs) not selected by nature for protein incorporation, the most generic modes of activation, induction, and reactivity commonly used, such as chiral enamine, hydrogen bonding, Brønsted acids/bases, and phase-transfer organocatalysis, reflecting their increasingly important role in organic and applied chemistry.

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.

Pd(II)-Catalyzed enantioselective C-H olefination toward the synthesis of P-stereogenic phosphinamides

P-Stereogenic phosphorus compounds are important structural elements in chiral ligands or organocatalysts. Herein, we report a Pd(II)-catalyzed enantioselective C-H olefination toward the synthesis of P-stereogenic phosphinamides using cheap commercially available L-pGlu-OH as a chiral ligand. A broad range of P-stereogenic phosphinamides were gained in good yields with high enantioselectivities (33 examples, up to 77% yield, 99% ee) via desymmetrization and kinetic resolution.

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

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

Unified Synthesis of 2-Isocyanoallopupukeanane and 9-Isocyanopupukeanane through a "Contra-biosynthetic" Rearrangement

Herein, we describe our synthetic efforts toward the pupukeanane natural products, in which we have completed the first enantiospecific route to 2-isocyanoallopupukeanane in 10 steps (formal synthesis), enabled by a key Pd-mediated cyclization cascade. This subsequently facilitated an unprecedented bio-inspired "contra-biosynthetic" rearrangement, providing divergent access to 9-isocyanopupukeanane in 15 steps (formal synthesis). Computational studies provide insight into the nature of this rearrangement.

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

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

Palladium-Catalyzed Enantioselective Directed C(sp3)-H Functionalization Using C5-Substituted 8-Aminoquinoline Auxiliaries

8-Aminoquinoline (AQ) has proven to be a highly effective bidentate directing group for palladium-catalyzed C-H functionalization reactions. However, enantiocontrol of AQ-directed C(sp3)-H functionalization reactions has been challenging. Herein, a new protocol is presented for the Pd-catalyzed enantioselective arylation of unactivated β C(sp3)-H bonds of alkyl carboxamides with aryl iodides using a C5-iodinated 8-aminoquinolines (IQ) auxiliary in conjugation with a BINOL ligand. Additionally, a C5-aryl substituted 8-aminoquinoline auxiliary can facilitate enantioselective alkenylation and alkynylation of benzylic C(sp3)-H bonds of 3-arylpropanamides with the corresponding bromide reagents under similar conditions.

Ligand-Enabled Palladium(II)-Catalyzed γ-C(sp3)-H Arylation of Primary Aliphatic Amines

The Pd(II)/sulfoxide-2-hydroxypyridine catalytic system shows promising activity in C-H activation chemistry. In this study, we showcase how this catalytic system solves the problem of native primary amine-directed γ-C(sp3)-H arylation. Primary amines with different complexities are compatible with the established methodology, and the range of applicable substrates can be expanded to include pyridine, oxime ether, and pyridine N-oxide.

Sulfamate-Tethered Aza-Wacker Strategy for a Kasugamine Synthon

We present our preparation of a kasugamine synthon, which proceeds in 14 steps from a literature epoxide. We expect that this kasugamine derivative can be used for the total syntheses of kasugamycin, minosaminomycin, and analogue antibiotics. A key step in the synthesis is our laboratory's sulfamate-tethered aza-Wacker cyclization.

Enantioselective Intermolecular C-H Functionalization of Primary Benzylic C-H Bonds Using ((Aryl)(diazo)methyl)phosphonates

Catalyst-controlled C-H functionalization using donor/acceptor carbenes has been shown to be an efficient process capable of high levels of site control and stereocontrol. This study demonstrated that the scope of the donor/acceptor carbene C-H functionalization can be extended to systems where the acceptor group is a phosphonate. When using the optimized dirhodium catalyst, Rh2(S-di-(4-Br)TPPTTL)4, ((aryl)(diazo)methyl)phosphonates undergo highly enantioselective (84-99% ee) and site-selective (>30:1 r.r.) benzylic C-H functionalization. The phosphonate group is much more sterically demanding than the previously studied carboxylate ester group, leading to much higher selectivity for a primary site versus more sterically crowded positions. The effectiveness of this methodology has been demonstrated by the late-stage primary C-H functionalization of estrone, adapalene, (S)-naproxen, clofibrate, and gemfibrozil derivatives.

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.

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.

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

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

Ligand-Enabled Double γ-C(sp3 )-H Functionalization of Aliphatic Acids: One-Step Synthesis of γ-Arylated γ-Lactones

γ-methylene C(sp3 )-H functionalization of linear free carboxylic acids remains a significant challenge. Here in we report a Pd(II)-catalyzed tandem γ-arylation and γ-lactonization of aliphatic acids enabled by a L,X-type CarboxPyridone ligand. A wide range of γ-arylated γ-lactones are synthesized in a single step from aliphatic acids in moderate to good yield. Arylated lactones can readily be converted into disubstituted tetrahydrofurans, a prominent scaffold amongst bioactive molecules.

Cu-mediated enantioselective C-H alkynylation of ferrocenes with chiral BINOL ligands

A wide range of Cu(II)-catalyzed C-H activation reactions have been realized since 2006, however, whether a C-H metalation mechanism similar to Pd(II)-catalyzed C-H activation reaction is operating remains an open question. To address this question and ultimately develop ligand accelerated Cu(II)-catalyzed C-H activation reactions, realizing the enantioselective version and investigating the mechanism is critically important. With a modified chiral BINOL ligand, we report the first example of Cu-mediated enantioselective C-H activation reaction for the construction of planar chiral ferrocenes with high yields and stereoinduction. The key to the success of this reaction is the discovery of a ligand acceleration effect with the BINOL-based diol ligand in the directed Cu-catalyzed C-H alkynylation of ferrocene derivatives bearing an oxazoline-aniline directing group. This transformation is compatible with terminal aryl and alkyl alkynes, which are incompatible with Pd-catalyzed C-H activation reactions. This finding provides an invaluable mechanistic information in determining whether Cu(II) cleaves C-H bonds via CMD pathway in analogous manner to Pd(II) catalysts.

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

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

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

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

Pd(II)-Catalyzed atroposelective C-H olefination: synthesis of enantioenriched N-aryl peptoid atropisomers

Herein, we reported the synthesis of enantioenriched N-aryl peptoid atropisomers via Pd(II)-catalyzed atroposelective C-H olefination using the easily accessible L-pyroglutamic acid (L-pGlu-OH) as the chiral ligand. A series of optically active N-aryl peptoid atropisomers were obtained in synthetically useful yields with high enantioselectivities.

Strategies for chiral separation: from racemate to enantiomer

Chiral separation has become a crucial topic for effectively utilizing superfluous racemates synthesized by chemical means and satisfying the growing requirements for producing enantiopure chiral compounds. However, the remarkably close physical and chemical properties of enantiomers present significant obstacles, making it necessary to develop novel enantioseparation methods. This review comprehensively summaries the latest developments in the main enantioseparation methods, including preparative-scale chromatography, enantioselective liquid-liquid extraction, crystallization-based methods for chiral separation, deracemization process coupling racemization and crystallization, porous material method and membrane resolution method, focusing on significant cases involving crystallization, deracemization and membranes. Notably, potential trends and future directions are suggested based on the state-of-art "coupling" strategy, which may greatly reinvigorate the existing individual methods and facilitate the emergence of cross-cutting ideas among researchers from different enantioseparation domains.

Axially Chiral 2-Hydroxybiaryls by Palladium-Catalyzed Enantioselective C-H Activation

This article describes the discovery and development of a palladium-catalyzed asymmetric C-H olefination of 2-hydroxybiaryls. The strategy allows a direct assembly of optically active, axially chiral 2-substituted-2'-hydroxybiaryls from readily available precursors and demonstrates that the native hydroxy unit of the substrates can work as an efficient directing group for the C-H activation. This represents a substantial advantage over other approaches that require the preinstallation of metal coordinating units. The simplicity of the approach and versatility of the products allow a practical and efficient synthesis of a broad variety of optically active binaphthyl derivatives.

Transition-metal-catalyzed atroposelective synthesis of axially chiral styrenes

Axially chiral styrenes, a type of atropisomer analogous to biaryls, have attracted great interest because of their unique presence in natural products and asymmetric catalysis. Since 2016, a number of methodologies have been developed for the atroposelective construction of these chiral skeletons, involving both transition metal catalysis and organocatalysis. In this feature article, we aim to provide a comprehensive understanding of recent advances in the asymmetric synthesis of axially chiral styrenes catalyzed by transition metals, integrating scattered work with different catalytic systems together. This feature article is cataloged into five sections according to the strategies, including asymmetric coupling, enantioselective C-H activation, central-to-axial chirality transfer, asymmetric alkyne functionalization, and atroposelective [2+2+2] cycloaddition.

A palladium catalyzed asymmetric desymmetrization approach to enantioenriched 1,3-disubstituted isoindolines

Herein, we report the first palladium/MPAA catalyzed enantioselective C-H activation/[4 + 1] annulation of diarylmethyltriflamide and olefins to construct chiral cis-1,3-disubstituted isoindoline derivatives. The use of a readily accessible mono-N-protected amino acid as a chiral ligand improves the efficiency and enantioselectivity of the catalytic transformation. The developed method provides access to both enantiomers of a product using either d or l-phenylalanine derivative as a chiral ligand facilitating the synthesis of both optically active 1,3-disubstituted isoindoline derivatives.

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

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

Palladium-Catalyzed Regiodivergent C-H Olefination of Imidazo[1,2a]pyridine Carboxamide and Unactivated Alkenes

Despite remarkable successes in linear and branched vinyl (hetero) arene synthesis, regiodivergent C-H olefination with a single catalytic system has remained underdeveloped. Overcoming this limitation, a Pd/MPAA-catalyzed regiodivergent C-H olefination of imidazo[1,2a] pyridine carboxamides with unactivated terminal alkenes to generate branched and linear olefinated products depending upon the electronic nature of alkenes is reported herein. Moreover, this protocol can be applied for C-H deuteriation of the corresponding heteroarenes with D2 O as deuterium source. Preliminary experimental studies combined with computational investigations (DFT studies) suggest that regiodivergent olefination can be controlled by olefin insertion and β-hydride elimination steps.

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.

Synthesis of P-Stereogenic Phosphinamides via Pd(II)-Catalyzed Enantioselective C-H Alkynylation

P-Stereogenic phosphinamides represent important structural elements in chiral organocatalysts and bioactive compounds. Herein, we report Pd(II)-catalyzed enantioselective C-H alkynylation using cheap commercially available l-pyroglutamic acid as a chiral ligand. A range of structurally diverse P-stereogenic phosphinamides was prepared in good yields with high enantioselectivities via desymmetrization and kinetic resolution. A tailor-made congested directing group, N-ethyl-N-(3-methylpyridin-2-yl)amino, was crucial for the reactivity.

Enhancing Substrate-Metal Catalyst Affinity via Hydrogen Bonding: Pd(II)-Catalyzed β-C(sp3)-H Bromination of Free Carboxylic Acids

The achievement of sufficient substrate-metal catalyst affinity is a fundamental challenge for the development of synthetically useful C-H activation reactions of weakly coordinating native substrates. While hydrogen bonding has been harnessed to bias site selectivity in existing C(sp²)-H activation reactions, the potential for designing catalysts with hydrogen bond donors (HBDs) to enhance catalyst-substrate affinity and, thereby, facilitate otherwise unreactive C(sp³)-H activation remains to be demonstrated. Herein, we report the discovery of a ligand scaffold containing a remote amide motif that can form a favorable meta-macrocyclic hydrogen bonding interaction with the aliphatic acid substrate. The utility of this ligand scaffold is demonstrated through the development of an unprecedented C(sp³)-H bromination of α-tertiary and α-quaternary free carboxylic acids, which proceeds in exceedingly high mono-selectivity. The geometric relationship between the NHAc hydrogen bond donor and the coordinating quinoline ligand is crucial for forming the meta-macrocyclophane-like hydrogen bonding interaction, which provides a guideline for the future design of catalysts employing secondary interactions.

Copper-Catalyzed Consecutive Ullmann, Decarboxylation, Oxidation, and Dehydration Reaction for Synthesis of Pyrrolo or Pyrido[1,2-a]imidazo[1,2-c]quinazolines

A protocol for a copper-catalyzed intermolecular cross-coupling cascade between 2-(2-bromoaryl)-1H-benzo[d]imidazole analogues and proline or pipecolic acid has been developed. The developed protocol allows access to a variety of synthetically useful N-fused pyrrolo or pyrido[1,2-a]imidazo[1,2-c]quinazoline scaffolds with high efficiency and good functional group compatibility. Proline or pipecolic acid plays a dual role in the reaction: as ligand and reactants. A mechanistically consecutive approach for the Ullmann coupling, decarboxylation, oxidation, and dehydration reaction process was presented.

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

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

Synthesis of 1,3-Dienes via Ligand-Enabled Sequential Dehydrogenation of Aliphatic Acids

1,3-Dienes are common scaffolds in biologically active natural products as well as building blocks for chemical synthesis. Developing efficient methods for the synthesis of diverse 1,3-dienes from simple starting materials is therefore highly desirable. Herein, we report a Pd(II)-catalyzed sequential dehydrogenation reaction of free aliphatic acids via β-methylene C-H activation, which enables one-step synthesis of diverse E,E-1,3-dienes. Free aliphatic acids of varying complexities, including the antiasthmatic drug seratrodast, were found to be compatible with the reported protocol. Considering the high lability of 1,3-dienes and lack of protecting strategies, dehydrogenation of aliphatic acids to reveal 1,3-dienes at the late stage of synthesis offers an appealing strategy for the synthesis of complex molecules containing such motifs.

Formal γ-C-H Functionalization of Cyclobutyl Ketones: Synthesis of cis-1,3-Difunctionalized Cyclobutanes

1,3-Difunctionalized cyclobutanes are an emerging scaffold in medicinal chemistry that can confer beneficial pharmacological properties to small-molecule drug candidates. However, the diastereocontrolled synthesis of these compounds typically requires complicated synthetic routes, indicating a need for novel methods. Here, we report a sequential C-H/C-C functionalization strategy for the stereospecific synthesis of cis-γ-functionalized cyclobutyl ketones from readily available cyclobutyl aryl ketones. Specifically, a bicyclo[1.1.1]pentan-2-ol intermediate is generated from the parent cyclobutyl ketone via an optimized Norrish-Yang procedure. This intermediate then undergoes a ligand-enabled, palladium-catalyzed C-C cleavage/functionalization to produce valuable cis-γ-(hetero)arylated, alkenylated, and alkynylated cyclobutyl aryl ketones, the benzoyl moiety of which can subsequently be converted to a wide range of functional groups including amides and esters.

Palladium-Mediated C(sp3)-H Bond Activation of N-Methyl-N-(pyridin-2-yl)benzamide: Direct Arylation/Alkylation and Mechanistic Investigation

Herein, we present a facile synthetic methodology to produce a range of N-(CH₂-aryl/alkyl)-substituted N-(pyridin-2-yl)benzamides via palladium-mediated C(sp³)-H bond activation. The N-methyl-N-(pyridin-2-yl)benzamide precursor was first reacted with palladium(II) acetate in a stoichiometric manner to obtain the key dinuclear palladacycle intermediates, whose structures were elucidated by mass spectrometric, NMR spectroscopic, and X-ray crystallographic studies in detail. The subsequent C(sp³)-H bond functionalizations on the N-methyl group of the starting substrate show facile productions of the corresponding N-(CH₂-aryl/alkyl)-substituted N-(pyridin-2-yl)benzamides with good functional group tolerance. A plausible mechanism was proposed based on density functional theory calculations in conjunction with kinetic isotope effect experiments. Finally, the synthetic transformation from the prepared N-(CH₂-aryl)-N-(pyridin-2-yl)benzamides through debenzoylation to N-(CH₂-aryl)-2-aminopyridine was successfully demonstrated.

Transannular C-H functionalization of cycloalkane carboxylic acids

Cyclic organic molecules are common among natural products and pharmaceuticals1,2. In fact, the overwhelming majority of small-molecule pharmaceuticals contain at least one ring system, as they provide control over molecular shape, often increasing oral bioavailability while providing enhanced control over the activity, specificity and physical properties of drug candidates3-5. Consequently, new methods for the direct site and diastereoselective synthesis of functionalized carbocycles are highly desirable. In principle, molecular editing by C-H activation offers an ideal route to these compounds. However, the site-selective C-H functionalization of cycloalkanes remains challenging because of the strain encountered in transannular C-H palladation. Here we report that two classes of ligands-quinuclidine-pyridones (L1, L2) and sulfonamide-pyridones (L3)-enable transannular γ-methylene C-H arylation of small- to medium-sized cycloalkane carboxylic acids, with ring sizes ranging from cyclobutane to cyclooctane. Excellent γ-regioselectivity was observed in the presence of multiple β-C-H bonds. This advance marks a major step towards achieving molecular editing of saturated carbocycles: a class of scaffolds that are important in synthetic and medicinal chemistry3-5. The utility of this protocol is demonstrated by two-step formal syntheses of a series of patented biologically active small molecules, prior syntheses of which required up to 11 steps6.

N-Phthalimide as a Site-Protecting and Stereodirecting Group in Rhodium-Catalyzed C-H Functionalization with Donor/Acceptor Carbenes

The rhodium-catalyzed enantioselective C-H functionalization of unactivated C-H bonds by means of donor/acceptor carbene-induced C-H insertion was extended to substrates containing nitrogen functionality. The rhodium-stabilized donor/acceptor carbenes were generated by rhodium-catalyzed decomposition of aryldiazoacetates. The phthalimido group was the optimum nitrogen protecting group. C-H functionalization at the most sterically accessible methylene site was achieved using Rh₂(S-2-Cl-5-BrTPCP)₄ as catalyst, whereas Rh₂(S-TPPTTL)₄ was the most effective catalyst for C-H functionalization at tertiary C-H bonds and for the desymmetrization of N-phthalimidocyclohexane.

Co(III)-Catalyzed three-component assembling of N-(2-pyrimidyl) indoles with dienes and formaldehyde

A highly regio- and chemoselective three-component assembling of N-pyrimidyl indoles with dienes and formaldehyde in the presence of a Co(III) catalyst was demonstrated. The scope of the reaction was investigated with a variety of indole derivatives to synthesize substituted homoallylic alcohols. Both butadiene and isoprene units were compatible with the reaction. To understand the reaction mechanism, various investigations were carried out, and suggested the plausibility of a reaction mechanism involving C-H bond activation as a key step.

Transition-Metal-Catalyzed C-H Bond Activation for the Formation of C-C Bonds in Complex Molecules

Site-predictable and chemoselective C-H bond functionalization reactions offer synthetically powerful strategies for the step-economic diversification of both feedstock and fine chemicals. Many transition-metal-catalyzed methods have emerged for the selective activation and functionalization of C-H bonds. However, challenges of regio- and chemoselectivity have emerged with application to highly complex molecules bearing significant functional group density and diversity. As molecular complexity increases within molecular structures the risks of catalyst intolerance and limited applicability grow with the number of functional groups and potentially Lewis basic heteroatoms. Given the abundance of C-H bonds within highly complex and already diversified molecules such as pharmaceuticals, natural products, and materials, design and selection of reaction conditions and tolerant catalysts has proved critical for successful direct functionalization. As such, innovations within transition-metal-catalyzed C-H bond functionalization for the direct formation of carbon-carbon bonds have been discovered and developed to overcome these challenges and limitations. This review highlights progress made for the direct metal-catalyzed C-C bond forming reactions including alkylation, methylation, arylation, and olefination of C-H bonds within complex targets.

Pd-Catalyzed Asymmetric Oxidative C-H/C-H Cross-Coupling Reaction between Ferrocenes and Azoles

The asymmetric C-H bond functionalization reaction is one of the most efficient and straightforward methods for the synthesis of optically active molecules. Herein, our work discovered a Pd-catalyzed asymmetric oxidative C-H/C-H cross-coupling reaction of ferrocenes with azoles such as oxazoles and thiazoles. Mono-N-protected amino acid as chiral ligands with palladium(II) has been demonstrated as an effective catalytic system in a weakly azine-directed asymmetric C-H bond functionalization reaction. This method offers a powerful strategy for constructing various substituted planar chiral ferrocenes via a dual C-H bond activation pathway in medium yields (up to 70%) with good enantioselectivity (up to 95.3:4.7 er) under mild conditions.

Copper-catalyzed asymmetric C(sp2)-H arylation for the synthesis of P- and axially chiral phosphorus compounds

Transition metal-catalyzed C-H bond functionalization is an important method in organic synthesis, but the development of methods that are lower cost and have a less environmental impact is desirable. Here, a Cu-catalyzed asymmetric C(sp²)-H arylation is reported. With diaryliodonium salts as arylating reagents, a range of ortho-arylated P-chiral phosphonic diamides were obtained in moderate to excellent yields with high enantioselectivities (up to 92% ee). Meanwhile, enantioselective C-3 arylation of diarylphosphine oxide indoles was also realized under similar conditions to construct axial chirality.

Synthesis of planar chiral ferrocenes via enantioselective remote C-H activation

Planar chiral ferrocenes are widely studied structures in asymmetric catalysis, materials science and medicinal chemistry. Although synthetic methods for 1,2-disubstituted planar chiral ferrocenes are well known, methods for the direct construction of 1,3-disubstituted planar chiral ferrocenes remain elusive. Here we report a modular platform for the construction of planar chirality in 1,3-disubstituted ferrocenes/ruthenocenes via an enantioselective relay remote C-H activation strategy. This method demonstrates a mechanism for remote enantiocontrol via enantiodetermining initial C‒H activation at the C2 position, enabled by a chiral mono-N-protected natural amino-acid ligand, and subsequent relay to the remote C3 position by a bridgehead-substituted norbornene mediator. A wide variety of 1,3-disubstituted planar chiral metallocenes are prepared with high enantioselectivity (96‒99% e.e.). The reaction shows good functional-group tolerance and high step-economy, and aryl iodides/bromides are compatible as coupling partners. The resulting metallocenes can be readily derivatized to yield planar chiral ligands and catalysts for asymmetric catalysis as well as building blocks for other applications.

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

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

Simple Ether-Directed Enantioselective C(sp3 )-H Borylation of Cyclopropanes Enabled by Iridium Catalysis

Reported here is an efficient and simple ether-directed iridium-catalyzed enantioselective C(sp³ )-H borylation of cyclopropanes. Various functional groups were well-tolerated, affording a vast array of chiral cyclopropanes with high enantioselectivities. We also demonstrated that the turnover numbers of the current reaction could be up to 335.