Pd(II)-Catalyzed Enantioselective C(sp3)-H Arylation of Free Carboxylic Acids

Unveiling the Mechanistic Role of Chiral Palladacycles in Pd(II)-Catalyzed Enantioselective C(sp3)-H Functionalization

Palladium-catalyzed enantioselective C(sp3)-H functionalization reactions has attracted considerable attention due to its ability for the synthesis of enantiomerically enriched molecules and stimulation of novel retrosynthetic disconnections. Understanding the reaction mechanism, especially the stereochemical process of the reaction, is crucial for the rational design of more efficient catalytic systems. Previously, we developed a Pd(II)/sulfoxide-2-hydroxypridine (SOHP) catalytic system for asymmetric C(sp3)-H functionalization reactions. In this study, we focused on unraveling the chemistry of chiral palladacycles involved in the Pd(II)-catalyzed enantioselective C(sp3)-H functionalization. We have isolated key palladacycle intermediates involved in the enantioselective β-C(sp3)-H arylation of carboxylic acids catalyzed by the Pd(II)/SOHP system. These palladacycles, exhibiting ligand-induced chirality, provided a significant opportunity to investigate the stereochemical process and the ligand effect in this asymmetric C-H functionalization. Our investigation provided direct evidence for the C-H palladation step as the enantioselectivity-determining step, which forms diastereomeric palladacycles that exhibited preservation of chirality in the functionalization step. DFT calculations provided insights into the chiral induction in palladacycle formation. This work highlights the value of chiral palladacycle chemistry in offering mechanistic insights into the Pd(II)-catalyzed asymmetric C(sp3)-H functionalization reactions.

Pd-Catalyzed Atroposelective C-H Olefination: Diverse Synthesis of Axially Chiral Biaryl-2-carboxylic Acids

Axially chiral carboxylic acids are important motifs in chiral catalysts and ligands. We herein reported the synthesis of axially chiral carboxylic acids via Pd(II)-catalyzed atroposelective C-H olefination using carboxylic acid as the native directing group. A broad range of axial chiral biaryl-2-carboxylic acids were synthesized in good yields with high enantioselectivities (up to 84% yield with 99% ee). Gram-scale reaction and further transformation reactions also provide a platform for synthetic applications of this method.

One-Step Synthesis of Chiral 9,10-Dihydrophenanthrenes via Ligand-Enabled Enantioselective Cascade β,γ-Diarylation of Acids

Pd(II)-catalyzed enantioselective C-H activation has emerged as a versatile platform for constructing point, axial, and planar chirality. Herein, we present an unexpected discovery of a Pd-catalyzed enantioselective cascade β,γ-methylene C(sp3)-H diarylation of free carboxylic acids using bidentate chiral mono-protected amino thioether ligands (MPAThio), enabling one-step synthesis of a complex chiral 9,10-dihydrophenanthrene scaffolds with high enantioselectivity. In this process, two methylene C(sp3)-H bonds and three C(sp2)-H bonds were activated, leading to the formation of four C-C bonds and three chiral centers in one pot. A plausible catalytic pathway starts with enantioselective β,γ-dehydrogenation to form chiral β,γ-cyclohexene. Intriguingly, this olefin serves as a norbornene-type reagent (presumably assisted by the carboxyl directing effect), relaying two successive Catellani arylation reactions and a C-H arylation reaction to furnish chiral 9,10-dihydrophenanthrenes along with meta-selective homocoupling products of iodoarene.

Ligand-Accelerated Pd-Catalyzed C(sp2)-H and C(sp3)-H Arylation under the Catalytic System

It is universally acknowledged that ligands can improve the reaction activity to simplify the reaction operating conditions and enrich the applicability of the reaction. Therefore, we developed N-octylglycine ligand-accelerated Pd-catalyzed ortho-arylation of benzoic acids under mild conditions with just 6 h; moreover, this N-octylglycine ligand was successfully implemented to carboxyl-directed Pd-catalyzed β-C(sp3)-H arylation and ortho-arylation of phenylacetic acids under mild conditions.

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.

Pd(0)-Catalyzed Intermolecular Methylene C(sp3)-H Silylation by Using N-Heterocyclic Carbene Ligands

The direct functionalization of methylene C(sp3)-H bonds is one of the greatest challenges in transition metal-catalyzed C-H activation. Although Pd(0)-catalyzed intramolecular cyclization reactions of methylene C(sp3)-H bonds have been reported, intermolecular functionalization remains to be discovered. Herein, we report the first example of a Pd(0)-catalyzed intermolecular methylene C(sp3)-H functionalization reaction. By use of a N-heterocyclic carbene ligand, the methylene C(sp3)-H bonds of 1-(benzyloxy)-2-iodobenzenes are activated and disilylated with hexamethyldisilane, affording disilylated products.

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

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

Carboxylate-Directed Pd-Catalyzed β-C(sp3)-H Arylation of N-Methyl Alanine Derivatives for Diversification of Bioactive Peptides

This study presents a Pd(II)-catalyzed method for the β-C(sp3)-H arylation of N-Cbz- or N-Fmoc-protected N-methyl alanines, providing ready access to building blocks for N-methylated peptide synthesis. For this transformation, the native carboxylate was exploited as the directing group, attributing its success to the use of a monoprotected amino-pyridine ligand. Its synthetic utility was demonstrated by facile generation of nine analogues of the naturally occurring N-methylated cyclic peptide cycloaspeptide A.

Late-Stage C-H Activation of Drug (Derivative) Molecules with Pd(ll) Catalysis

This review comprehensively analyses representative examples of Pd(II)-catalyzed late-stage C-H activation reactions and demonstrates their efficacy in converting C-H bonds at multiple positions within drug (derivative) molecules into diverse functional groups. These transformative reactions hold immense potential in medicinal chemistry, enabling the efficient and selective functionalization of specific sites within drug molecules, thereby enhancing their pharmacological activity and expanding the scope of potential drug candidates. Although notable articles have focused on late-stage C-H functionalization reactions of drug-like molecules using transition-metal catalysts, reviews specifically focusing on late-stage C-H functionalization reactions of drug (derivative) molecules using Pd(II) catalysts are required owing to their prominence as the most widely utilized metal catalysts for C-H activation and their ability to introduce a myriad of functional groups at specific C-H bonds. The utilization of Pd-catalyzed C-H activation methodologies demonstrates impressive success in introducing various functional groups, such as cyano (CN), fluorine (F), chlorine (Cl), aromatic rings, olefin, alkyl, alkyne, and hydroxyl groups, to drug (derivative) molecules with high regioselectivity and functional-group tolerance. These breakthroughs in late-stage C-H activation reactions serve as invaluable tools for drug discovery and development, thereby offering strategic options to optimize drug candidates and drive the exploration of innovative therapeutic solutions.

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.

Synthesis of β,γ-Unsaturated Aliphatic Acids via Ligand-Enabled Dehydrogenation

α,β-Dehydrogenation of aliphatic acids has been realized through both enolate and β-C-H metalation pathways. However, the synthesis of isolated β,γ-unsaturated aliphatic acids via dehydrogenation has not been achieved to date. Herein, we report the ligand-enabled β,γ-dehydrogenation of abundant and inexpensive free aliphatic acids, which provides a new synthetic disconnection as well as a versatile platform for the downstream functionalization of complex molecules at remote γ-sites. A variety of free aliphatic acids, including acyclic and cyclic systems with ring sizes from five-membered to macrocyclic, undergo efficient dehydrogenation. Notably, this protocol features good chemoselectivity in the presence of more accessible α-C-H bonds and excellent regioselectivity in fused bicyclic scaffolds. The utility of this protocol has been demonstrated by the late-stage functionalization of a series of bioactive terpene natural products at the γ-sites. Further functionalization of the β,γ-double bond allows for the installation of covalent warheads, including epoxides, aziridines, and β-lactones, into complex natural product scaffolds, which are valuable for targeted covalent drug discovery.

Palladium-Catalyzed Stitching of 1,3-C(sp3)-H Bonds with Dihaloarenes: Short Synthesis of (±)-Echinolactone D

Palladium-catalyzed C(sp3)-H functionalization presents an efficient strategy to construct a variety of carbon-carbon bonds. However, application of this approach toward the preparation of five-membered benzo-fused carbocycles via the most simplifying C-H activation logic has not been realized. In this Article, we report a palladium-catalyzed annulation reaction between gem-dimethyl-containing amides and 1-bromo-2-iodoarenes that effectively constructs two Calkyl-Caryl bonds and provides access to a variety of five-membered benzo-fused compounds. In this transformation, the dihaloarene is stitched to the gem-dimethyl moiety via two sequential β-C(sp3)-H arylations utilizing the differential reactivity of the 1,2-difunctionalized electrophile. This annulation reaction is enabled by a dual-ligand system comprising of an N-acyl glycine and a pyridine-3-sulfonic acid that synergistically promotes the palladium stitching and provides the bicyclic products. This method displays a broad substrate scope and shows excellent amide compatibility. We also demonstrate the synthetic potential of this annulation by synthesizing echinolactone D.

Metal-catalysed C-C bond formation at cyclopropanes

Cyclopropanes are important substructures in natural products and pharmaceuticals. Although traditional methods for their incorporation rely on cyclopropanation of an existing scaffold, the advent of transition-metal catalysis has enabled installation of functionalized cyclopropanes using cross-coupling reactions. The unique bonding and structural properties of cyclopropane render it more easily functionalized in transition-metal-catalysed cross-couplings than other C(sp³) substrates. The cyclopropane coupling partner can participate in polar cross-coupling reactions either as a nucleophile (organometallic reagents) or as an electrophile (cyclopropyl halides). More recently, single-electron transformations featuring cyclopropyl radicals have emerged. This Review will provide an overview of transition-metal-catalysed C-C bond formation reactions at cyclopropane, covering both traditional and current strategies, and the benefits and limitations of each.

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.

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.

Aniline assisted dimerization of phenylalanines: convenient synthesis of 2-aroyl-3-arylquinoline in an I2-DMSO system

We herein report an efficient synthesis of 2-aroyl-3-arylquinolines from phenylalanines and anilines. The mechanism involves I₂-mediated Strecker degradation enabled catabolism and reconstruction of amino acids and a cascade aniline-assisted annulation. Both DMSO and water act as oxygen sources in this convenient protocol.

Site- and Stereoselective C(sp3 )-H Borylation of Strained (Hetero)Cycloalkanols Enabled by Iridium Catalysis

Transition metal-catalyzed site- and stereoselective C-H activation of strained (hetero)cycloalkanes remains a formidable challenge. We herein report a carbamate-directed iridium-catalyzed asymmetric β-C(sp³ )-H borylation of cyclopropanol derivatives. A variety of densely functionalized cyclopropanols were obtained in good enantioselectivities via desymmetrization and kinetic resolution. In addition, site-selective C(sp³ )-H borylation of methine groups furnished α-borylated (hetero)cycloalkanols in moderate to good yields. The synthetic utility of the method was further shown in a gram-scale synthesis and diverse downstream transformations of borylated products.

Convenient synthesis, characterization and biological evaluation of novel 1-phenylcyclopropane carboxamide derivatives

Small, strained ring molecules of phenylcyclopropane carboxamide have rigid, defined conformations and unique electronic properties. For these reasons many groups, seek to use these subunits to form biologically active compounds. Herein we report a generally applicable approach for preparing a small cyclopropane ring containing 1-phenylcyclopropane carboxamide derivatives to a wide range of the different aromatic compounds by α-alkylation of 2-phenyl acetonitrile derivatives with 1, 2-dibromo ethane in good yields followed by the conversion of cyano group to acid group by the reaction with concentrated hydrochloric acid. This obtained acid derivative undergoes acid amine coupling with various Methyl 2-(aminophenoxy)acetate to form 1-Phenylcyclopropane Carboxamide. These compounds possess distinct effective inhibition on the proliferation of U937, pro-monocytic, human myeloid leukaemia cell line while these compounds did not show cytotoxic activity on these cells. The structure-activity relationships of these compounds are discussed.

Ligand-Enabled Pd(II)-Catalyzed β-Methylene C(sp3)-H Arylation of Free Aliphatic Acids

Ligand development has enabled rapid advances in Pd(II)-catalyzed β-methyl C(sp3)-H activation of free carboxylic acids. However, there are only a handful of reports of free-acid-directed β-methylene C(sp3)-H activation, all of which are limited to intramolecular reactions. Herein, we report the first Pd(II)-catalyzed intermolecular β-methylene C(sp3)-H arylation of free aliphatic acids, which is enabled by bidentate pyridine-pyridone ligands. The bite angle of this ligand has been discovered to play a key role in promoting β-methylene C-H activation of free carboxylic acid. This new transformation provides a disconnection for alkylation of arenes with simple aliphatic acids. A variety of free aliphatic acids, including the antiasthmatic drug seratrodast, were compatible with the reported protocol.

MPAI-Ligand Accelerated Pd-Catalyzed C(sp3)-H Arylation of Free Aliphatic Acids

Here, we report the development of a class of bifunctional monoprotected amino-imidazoline (MPAI) ligands and their applications in Pd-catalyzed C(sp³)-H arylation of free aliphatic acids. The newly developed MPAI ligand allows the use of 1.0 equiv of aliphatic acids containing an alpha hydrogen for the first time.

HFIP in Organic Synthesis

1,1,1,3,3,3-Hexafluoroisopropanol (HFIP) is a polar, strongly hydrogen bond-donating solvent that has found numerous uses in organic synthesis due to its ability to stabilize ionic species, transfer protons, and engage in a range of other intermolecular interactions. The use of this solvent has exponentially increased in the past decade and has become a solvent of choice in some areas, such as C-H functionalization chemistry. In this review, following a brief history of HFIP in organic synthesis and an overview of its physical properties, literature examples of organic reactions using HFIP as a solvent or an additive are presented, emphasizing the effect of solvent of each reaction.

One-Step Synthesis of β-Alkylidene-γ-lactones via Ligand-Enabled β,γ-Dehydrogenation of Aliphatic Acids

Ligand-enabled Pd-catalyzed regioselective α,β-dehydrogenation of carbonyl compounds via β-methylene C-H activation has recently emerged as a promising transformation. Herein, we report the realization of β,γ-dehydrogenation and subsequent vinyl C-H olefination reactions of free carboxylic acids, thus providing a unique method for the structural diversification of aliphatic acids containing α-quaternary centers through sequential functionalizations of two β-C-H bonds and one γ-C-H bond. This tandem dehydrogenation-olefination-lactonization reaction offers a one-step preparation of β-alkylidene-γ-lactones, which are often difficult to prepare through conventional methods, from inexpensive and abundant free aliphatic acids. A variety of free aliphatic acids, such as isosteviol and grandiflorolic acid natural products, and olefins are compatible with the reported protocol. The newly designed bidentate oxime ether-pyridone and morpholine-pyridone ligands are crucial for this tandem reaction to proceed. Notably, these ligands also enable preferential methylene C-H activation over the previously reported, competing process of methyl C-H bond olefination.

Pd(II)-Catalyzed Synthesis of Bicyclo[3.2.1] Lactones via Tandem Intramolecular β-C(sp3)-H Olefination and Lactonization of Free Carboxylic Acids

Bicyclo[3.2.1] lactones are chemical scaffolds found in numerous bioactive natural products. Herein, we detail the development of a novel palladium(II)-catalyzed tandem intramolecular β-C(sp³)-H olefination and lactonization reaction that rapidly transforms linear carboxylic acid possessing a tethered olefin into the bicyclo[3.2.1] lactone motif. This transformation features a broad substrate scope, shows excellent functional group compatibility, and can be extended to the preparation of the related seven-membered bicyclo[4.2.1] lactones. Additionally, we demonstrate the synthetic potential of this annulation by constructing the 6,6,5-tricyclic lactone core structure of the meroterpenoid cochlactone A. We anticipate that this compelling reaction may provide a novel synthetic disconnection that can be broadly applied toward the preparation of a variety of bioactive natural products.

Access to hexahydroazepinone heterocycles via palladium-catalysed C(sp3)-H alkenylation/ring-opening of cyclopropanes

In this communication, we describe the synthesis of novel hexahydroazepinone derivatives starting from two simple building blocks in presence of a readily available palladium catalyst. The reaction proceeds through a selective C(sp³)-H alkenylation/ring-opening process to obtain the seven-membered ring products in good to excellent yields on a wide variety of substrates under batch, microwave, and continuous flow conditions.

Iridium-Catalyzed Enantioselective C(sp3 )-H Borylation of Aminocyclopropanes

Transition-metal-catalyzed regio- and stereo-controllable C-H functionalization remains a formidable challenge in asymmetric catalysis. Herein, we disclose the first example of iridium-catalyzed C(sp³ )-H borylation of aminocyclopropanes by using simple imides as weakly coordinating directing groups under mild reaction conditions. The reaction proceeded via a six-membered iridacycle, affording a vast range of chiral aminocyclopropyl boronates. The current method features a broad spectrum of functional groups (36 examples) and high enantioselectivities (up to 99 %). We also demonstrated the synthetic utility by a preparative scale C-H borylation, C-B bond transformations, and conversion of the directing group.

Pd-catalysed C-H functionalisation of free carboxylic acids

Pd-catalysed C-H functionalisation of free carboxylic acids has drawn significant attention over the last few years due to the predominance of carboxylic acid moieties in pharmaceuticals and agrochemicals. But their coordinating ability was overlooked and masked by exogenous directing groups for a long time. Even other crucial roles of carboxylic acids as additives and steric inducers that directly influence the mode of a reaction have been widely neglected. This review aims to embrace all of the diverse aspects of carboxylic acids except additive and steric effects by concisely and systematically describing their versatile role in Pd-catalysed proximal and distal C-H activation reactions that could be implemented in the pharmaceutical and agrochemical industries. In addition, the mechanistic perspectives along with several recent strategies developed in the last few years discussed here will serve as educational resources for future research.

Traditional and sustainable approaches for the construction of C–C bonds by harnessing C–H arylation

Biaryl scaffolds are found in natural products and drug molecules and exhibit a wide range of biological activities. In past decade, the transition metal-catalyzed C–H arylation reaction came out as an effective tool for the construction of biaryl motifs. However, traditional transition metal-catalyzed C–H arylation reactions have limitations like harsh reaction conditions, narrow substrate scope, use of additives etc. and therefore encouraged synthetic chemists to look for alternate greener approaches. This review aims to draw a general overview on C–H bond arylation reactions for the formation of C–C bonds with the aid of different methodologies, majorly highlighting on greener and sustainable approaches.

Transition-metal-catalyzed C–H arylations are an effective tool for the construction of biaryl motifs in an efficient and selective manner. Here the authors provide an overview of the state-of-the-art of the field and perspectives on emerging directions toward increased sustainability.

Pd(II)-Catalyzed Enantioselective C(sp3)-H Arylation of Cyclopropanes and Cyclobutanes Guided by Tertiary Alkylamines

Strained aminomethyl-cycloalkanes are a recurrent scaffold in medicinal chemistry due to their unique structural features that give rise to a range of biological properties. Here, we report a palladium-catalyzed enantioselective C(sp³)-H arylation of aminomethyl-cyclopropanes and -cyclobutanes with aryl boronic acids. A range of native tertiary alkylamine groups are able to direct C-H cleavage and forge carbon-aryl bonds on the strained cycloalkanes framework as single diastereomers and with excellent enantiomeric ratios. Central to the success of this strategy is the use of a simple N-acetyl amino acid ligand, which not only controls the enantioselectivity but also promotes γ-C-H activation of over other pathways. Computational analysis of the cyclopalladation step provides an understanding of how enantioselective C-H cleavage occurs and revealed distinct transition structures to our previous work on enantioselective desymmetrization of N-isobutyl tertiary alkylamines. This straightforward and operationally simple method simplifies the construction of functionalized aminomethyl-strained cycloalkanes, which we believe will find widespread use in academic and industrial settings relating to the synthesis of biologically active small molecules.

Diastereoselective palladium-catalyzed functionalization of prochiral C(sp3)-H bonds of aliphatic and alicyclic compounds

We highlight the reported developments of the palladium-catalyzed C-H activation and functionalization of the inactive/unreactive prochiral C(sp³)-H bonds of aliphatic and alicyclic compounds. There exist numerous classical methods for generating contiguous stereogenic centers in a compound with a high degree of stereocontrol. Along similar lines, the Pd(II)-catalyzed, directing group-aided functionalization of inactive prochiral/diastereotopic C(sp³)-H bonds have been exploited to accomplish the stereoselective construction of stereo-arrays in organic compounds. We present a concise discussion on how specific strategies consisting of Pd(II)-catalyzed, directing group-aided C(sp³)-H functionalization have been utilized to generate two or more stereogenic centers in aliphatic and alicyclic compounds.

Palladium-Catalyzed Enantioselective β-C(sp3)-H Activation Reactions of Aliphatic Acids: A Retrosynthetic Surrogate for Enolate Alkylation and Conjugate Addition

Enolate alkylation and conjugate addition into an α,β-unsaturated system have served as long-standing strategic disconnections for the installation of α- or β-substituents on carbonyl-containing compounds. At the onset of our efforts to develop C-H activation reactions for organic synthesis, we set our eye toward developing asymmetric β-C-H activation reactions of aliphatic acids with the perspective that this bond-forming event could serve as a more flexible retrosynthetic surrogate for both canonical carbonyl-related asymmetric transformations.In this Account, we describe our early efforts using strongly coordinating chiral oxazolines to probe reaction mechanism and the stereochemical nature of the C-H cleavage transition state. The characterization of key reactive intermediates through X-ray crystallography and computational studies suggested a transition state with C-H and Pd-OAc bonds being approximately coplanar for optimum interaction. We then moved forward to develop more practical, weakly coordinating monodentate amide directing groups, a necessary advance toward achieving the β-C-H activation of weakly coordinating native carboxylic acids. Throughout this journey, gradual deconvolution between a substrate's directing effect and its intimate interplay with ligand properties has culminated in the design of new ligand classes that ultimately allowed the competency of native carboxylic acids in β-C-H activation. These efforts established the importance of ligand acceleration in Pd-catalyzed C-H activation, where the substrate's weak coordination is responsible for positioning the catalyst for C-H cleavage, while the direct participation from the bifunctional ligand is responsible for enthalpically stabilizing the C-H cleavage transition state.Building upon these principles, we developed five classes of chiral ligands (MPAA, MPAQ, MPAO, MPAThio, MPAAM) to enable enantioselective β-C-H activation reactions, including carbon-carbon and carbon-heteroatom bond formation. The accumulated data from our developed enantioselective C-H activation reactions indicate that ligands possessing point chirality are most effective for imparting stereoinduction in the C-H activation step, the application of which enabled the desymmetrization and subsequent C-H functionalization of enantiotopic carbon and protons across a range of weakly coordinating arylamides and, more recently, free carboxylic acids. Progress in ligand design, in conjunction with the enabling nature of alkali metal countercations, led to the realization of a suite of β-methyl and now methylene C(sp3)-H activation reactions. These advancements also enabled the use of economical oxidants, such as peroxides and molecular oxygen, to facilitate catalyst turnover. In the future, continued progress in designing more efficient bifunctional chiral ligands is likely to provide a myriad of enantioselective β-C-H activation reactions of readily available native substrates.

Stereoselective Cyclopropanation of Enamides via C―C Bond Cleavage of Cyclopropenes

This work describes a straightforward protocol for the stereoselective synthesis of vinylcyclopropylamides in high E/Z and syn/anti ratios by cyclopropanation of N-tosyl substituted enamides with cyclopropenes in the presence of...

Coordination-assisted, transition-metal-catalyzed enantioselective desymmetric C–H functionalization

Recent advances in transition-metal-catalyzed enantioselective desymmetric C–H functionalization are summarized.

Convenient and Flexible Syntheses of gem-Dimethyl Carboxylic Triggers via Mono-Selective β-C(sp3)-H Arylation of Pivalic Acid with ortho-Substituted Aryl Iodides

This work presents a palladium(II)-catalyzed mono-selective C(sp3)-H arylation of pivalic acid for rapid construction of an important library of 3-aryl-2,2-dimethylpropanoic acids, especially those ortho-substituted-aryl compounds. The strategy greatly streamlines the...

Catalytic asymmetric synthesis of α-stereogenic carboxylic acids: recent advances

Chiral carboxylic acids bearing an α-stereogenic center constitute the backbone of many natural products and therapeutic reagents as well as privileged chiral ligands and catalysts. Hence, it is not surprising that a large number of elegant catalytic asymmetric strategies have been developed toward the efficient synthesis of α-chiral carboxylic acids, such as α-hydroxy acids and α-amino acids. In this review, the recent advances in asymmetric synthesis of α-stereogenic free carboxylic acids via organocatalysis and transition metal catalysis are summarized (mainly from 2010 to 2020). The content is organized by the reaction type of the carboxyl source involved, including asymmetric functionalization of substituted carboxylic acids, cyclic anhydrides, α-keto acids, substituted α,β-unsaturated acids and so on. We hope that this review will motivate further interest in catalytic asymmetric synthesis of chiral α-substituted carboxylic acids.

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

Transition-metal-catalyzed, coordination-assisted C(sp³)-H functionalization has revolutionized synthetic planning over the past few decades as the use of these directing groups has allowed for increased access to many strategic positions in organic molecules. Nonetheless, several challenges remain preeminent, such as the requirement for high temperatures, the difficulty in removing or converting directing groups, and, although many metals provide some reactivity, the difficulty in employing metals outside of palladium. This review aims to give a comprehensive overview of coordination-assisted, transition-metal-catalyzed, direct functionalization of nonactivated C(sp³)-H bonds by covering the literature since 2004 in order to demonstrate the current state-of-the-art methods as well as the current limitations. For clarity, this review has been divided into nine sections by the transition metal catalyst with subdivisions by the type of bond formation. Synthetic applications and reaction mechanism are discussed where appropriate.

Ligand-controlled divergent dehydrogenative reactions of carboxylic acids via C-H activation

Dehydrogenative transformations of alkyl chains to alkenes through methylene carbon-hydrogen (C–H) activation remain a substantial challenge. We report two classes of pyridine-pyridone ligands that enable divergent dehydrogenation reactions through palladium-catalyzed β-methylene C–H activation of carboxylic acids, leading to the direct syntheses of α,β-unsaturated carboxylic acids or γ-alkylidene butenolides. The directed nature of this pair of reactions allows chemoselective dehydrogenation of carboxylic acids in the presence of other enolizable functionalities such as ketones, providing chemoselectivity that is not possible by means of existing carbonyl desaturation protocols. Product inhibition is overcome through ligand-promoted preferential activation of C(sp3)–H bonds rather than C(sp2)–H bonds or a sequence of dehydrogenation and vinyl C–H alkynylation. The dehydrogenation reaction is compatible with molecular oxygen as the terminal oxidant.

Late-Stage β-C(sp3)-H Deuteration of Carboxylic Acids

Carboxylic acids are highly abundant in bioactive molecules. In this study, we describe the late-stage β-C(sp³)-H deuteration of free carboxylic acids. On the basis of the finding that C-H activation with our catalysts is reversible, the de-deuteration process was first optimized. The resulting method uses ethylenediamine-based ligands and can be used to achieve the desired deuteration when using a deuterated solvent. The reported method allows for the functionalization of a wide range of free carboxylic acids with diverse substitution patterns, as well as the late-stage deuteration of bioactive molecules and related frameworks and enables the functionalization of nonactivated methylene β-C(sp³)-H bonds for the first time.

Progress and perspectives on directing group-assisted palladium-catalysed C-H functionalisation of amino acids and peptides

Peptide modifications can unlock a variety of compounds with structural diversity and abundant biological activity. In nature, peptide modifications, such as functionalisation at the side-chain position of amino acids, are performed using post-translational modification enzymes or incorporation of unnatural amino acids. However, accessing these modifications remains a challenge for organic chemists. During the past decades, selective C-H activation/functionalisation has attracted considerable attention in synthetic organic chemistry as a pathway to peptide modification. Various directing group strategies have been discovered that assist selective C-H activation. In particular, bidentate directing groups that enable tuneable and reversible coordination are now recognised as one of the most efficient methods for the site-selective C-H activation and functionalisation of numerous families of organic compounds. Synthetic peptide chemists have harnessed bidentate directing group strategies for selective functionalisation of the β- and γ-positions of amino acids. This method has been expanded and recognised as an effective device for the late stage macrocyclisation and total synthesis of complex peptide natural products. In this review, we discuss various β-, γ-, and δ-C(sp³)-H bond functionalisation reactions of amino acids for the formation of C-X bonds with the aid of directing groups and their application in late-stage macrocyclisation and the total synthesis of complex peptide natural products.

Advancing the Logic of Chemical Synthesis: C-H Activation as Strategic and Tactical Disconnections for C-C Bond Construction

The design of synthetic routes by retrosynthetic logic is decisively influenced by the transformations available. Transition-metal-catalyzed C-H activation has emerged as a powerful strategy for C-C bond formation, with myriad methods developed for diverse substrates and coupling partners. However, its uptake in total synthesis has been tepid, partially due to their apparent synthetic intractability, as well as a lack of comprehensive guidelines for implementation. This Review addresses these issues and offers a guide to identify retrosynthetic opportunities to generate C-C bonds by C-H activation processes. By comparing total syntheses accomplished using traditional approaches and recent C-H activation methods, this Review demonstrates how C-H activation enabled C-C bond construction has led to more efficient retrosynthetic strategies, as well as the execution of previously unattainable tactical maneuvers. Finally, shortcomings of existing processes are highlighted; this Review illustrates how some highlighted total syntheses can be further economized by adopting next-generation ligand-enabled approaches.

Synthesis of Cyclic Anhydrides via Ligand-Enabled C-H Carbonylation of Simple Aliphatic Acids

The development of C(sp³ )-H functionalizations of free carboxylic acids has provided a wide range of versatile C-C and C-Y (Y=heteroatom) bond-forming reactions. Additionally, C-H functionalizations have lent themselves to the one-step preparation of a number of valuable synthetic motifs that are often difficult to prepare through conventional methods. Herein, we report a β- or γ-C(sp³ )-H carbonylation of free carboxylic acids using Mo(CO)₆ as a convenient solid CO source and enabled by a bidentate ligand, leading to convenient syntheses of cyclic anhydrides. Among these, the succinic anhydride products are versatile stepping stones for the mono-selective introduction of various functional groups at the β position of the parent acids by decarboxylative functionalizations, thus providing a divergent strategy to synthesize a myriad of carboxylic acids inaccessible by previous β-C-H activation reactions. The enantioselective carbonylation of free cyclopropanecarboxylic acids has also been achieved using a chiral bidentate thioether ligand.