Formation of α-chiral centers by asymmetric β-C(sp3)-H arylation, alkenylation, and alkynylation

Pd(ii)-Catalyzed asymmetric intramolecular arylation of unbiased methylene C(sp3)–H bonds using readily accessible 3,3′-F2-BINOL as a chiral ligand

Pd(ii)-Catalyzed asymmetric intramolecular methylene C(sp³)–H arylation using readily accessible 3,3′-F₂-BINOL as a ligand is reported.

Remote C(sp3)–H functionalization via catalytic cyclometallation: beyond five-membered ring metallacycle intermediates

Despite impressive recent momentum gained in C(sp3)–H activation, achieving high regioselectivity in molecules containing different C–H bonds with similar high energy without abusing tailored substitution remains as one of the biggest challenges.

Direct β- and γ-C(sp3 )-H Alkynylation of Free Carboxylic Acids*

In this study we report the identification of a novel class of ligands for palladium-catalyzed C(sp3 )-H activation that enables the direct alkynylation of free carboxylic acid substrates. In contrast to previous synthetic methods, no introduction/removal of an exogenous directing group is required. A broad scope of acids including both α-quaternary and challenging α-non-quaternary can be used as substrates. Additionally, the alkynylation in the distal γ-position is reported. Finally, this study encompasses preliminary findings on an enantioselective variant of the title transformation as well as synthetic applications of the products obtained.

Direct Enantioselective C(sp3)-H Acylation for the Synthesis of α-Amino Ketones

A direct enantioselective acylation of α-amino C(sp³)-H bonds with carboxylic acids has been achieved via the merger of transition metal and photoredox catalysis. This straightforward protocol enables cross-coupling of a wide range of carboxylic acids, one class of feedstock chemicals, with readily available N-alkyl benzamides to produce highly valuable α-amino ketones in high enantioselectivities under mild conditions. The synthetic utility of this method is further demonstrated by gram scale synthesis and application to late-stage functionalization. This method provides an unprecedented solution to address the challenging stereocontrol in metallaphotoredox catalysis and C(sp³)-H functionalization. Mechanistic studies suggest the α-C(sp³)-H bond of the benzamide coupling partner is cleavage by photocatalytically generated bromine radicals to form α-amino alkyl radicals, which subsequently engages in nickel-catalyzed asymmetric acylation.

Ligand-Enabled γ-C(sp3 )-H Olefination of Free Carboxylic Acids

We report the ligand-enabled C-H activation/olefination of free carboxylic acids in the γ-position. Through an intramolecular Michael addition, δ-lactones are obtained as products. Two distinct ligand classes are identified that enable the challenging palladium-catalyzed activation of free carboxylic acids in the γ-position. The developed protocol features a wide range of acid substrates and olefin reaction partners and is shown to be applicable on a preparatively useful scale. Insights into the underlying reaction mechanism obtained through kinetic studies are reported.

Pd(II)-Catalyzed Enantioselective γ-C(sp3)-H Functionalizations of Free Cyclopropylmethylamines

Prized for their ability to reliably forge stereocenters with precise regiocontrol from simple and abundant starting materials, substrate-directable enantioselective reactions are widely used in modern organic synthesis. As such, enantioselective C(sp3)-H functionalization reactions directed by innate functional groups could provide new routes to introduce molecular complexity within the inert hydrocarbon moiety, but to date this approach has been met with little success. While free primary aliphatic amines are common, versatile intermediates in synthesis, they are traditionally unreactive in C(sp3)-H activation reactions. Herein we report the Pd-catalyzed enantioselective C(sp3)-H functionalization of free aliphatic amines (cyclopropylmethylamines) enabled by a chiral bidentate thioether ligand. This ligand's privileged bidentate coordination mode and thioether motif favor the generation of the requisite mono(amine)-Pd(II) intermediate, thus enabling the enantioselective C-H activation of free amines. The resulting C-Pd(II) species could engage in either Pd(II)/Pd(IV) or Pd(II)/Pd(0) catalytic cycles, enabling access to a diverse range of products through (hetero)arylation, carbonylation, and olefination reactions. Consequently, this versatile reactivity offers medicinal chemists a general strategy to rapidly prepare and functionalize biologically relevant amines.

Palladium-catalyzed direct asymmetric C-H bond functionalization enabled by the directing group strategy

In the past decade, selective C-C and C-heteroatom bond construction through palladium-catalyzed direct C-H bond functionalization has been extensively studied by employing a variety of directing groups. Within this category, direct asymmetric C(sp2)-H and C(sp3)-H activation for the construction of highly enantiomerically enriched skeletons still progressed at a slow pace. This minireview briefly introduces the major advances in the field for palladium-catalyzed direct asymmetric C-H bond functionalization via the directing group strategy.

PdII -Catalyzed Enantioselective C(sp3 )-H Arylation of Cyclobutyl Ketones Using a Chiral Transient Directing Group

The use of chiral transient directing groups (TDGs) is a promising approach for developing PdII -catalyzed enantioselective C(sp3 )-H activation reactions. However, this strategy is challenging because the stereogenic center on the TDG is often far from the C-H bond, and both TDG covalently attached to the substrate and free TDG are capable of coordinating to PdII centers, which can result in a mixture of reactive complexes. We report a PdII -catalyzed enantioselective β-C(sp3 )-H arylation reaction of aliphatic ketones using a chiral TDG. A chiral trisubstituted cyclobutane was efficiently synthesized from a mono-substituted cyclobutane through sequential C-H arylation reactions, thus demonstrating the utility of this method for accessing structurally complex products from simple starting materials. The use of an electron-deficient pyridone ligand is crucial for the observed enantioselectivity. Interestingly, employing different silver salts can reverse the enantioselectivity.

Silanol: a bifunctional group for peptide synthesis and late-stage functionalization

Chemical modification of a specific amino acid residue on peptides represents an efficient strategy to improve their pharmacokinetics and facilitates the potential to achieve post-synthetic diversification of peptides. Herein, we reported the first Pd-catalyzed late-stage ortho-olefination of Tyr residues on peptides with high chemo- and site-selectivity, by employing the easily attached and removable silanol as a bifunctional protecting group and directing group. Up to hexapeptides with variation on amino acid sequences or locations of the Tyr residue and different olefins were compatible with this protocol, which enriched the chemical toolbox for late-stage modification via C(sp2)-H functionalization. Furthermore, the orthogonal protection strategies of Tyr were also developed and could be applied to SPPS.

Desymmetrization of unactivated bis-alkenes via chiral Brønsted acid-catalysed hydroamination

Although great success has been achieved in catalytic asymmetric hydroamination of unactivated alkenes using transition metal catalysis and organocatalysis, the development of catalytic desymmetrising hydroamination of such alkenes remains a tough challenge in terms of attaining a high level of stereocontrol over both remote sites and reaction centers at the same time. To address this problem, here we report a highly efficient and practical desymmetrising hydroamination of unactivated alkenes catalysed by chiral Brønsted acids with both high diastereoselectivity and enantioselectivity. This method features a remarkably broad alkene scope, ranging from mono-substituted and gem-/1,2-disubstituted to the challenging tri- and tetra-substituted alkenes, to provide access to a variety of diversely functionalized chiral pyrrolidines bearing two congested tertiary or quaternary stereocenters with excellent efficiency under mild and user-friendly synthetic conditions. The key to success is indirect activation of unactivated alkenes by chiral Brønsted acids via a concerted hydroamination mechanism.

Ligand-Enabled Pd(II)-Catalyzed C(sp3)-H Lactonization Using Molecular Oxygen as Oxidant

Pd(II)-catalyzed C-H lactonization of o-methyl benzoic acid substrates has been achieved using molecular oxygen as the oxidant. This finding provides a rare example of C-H oxygenation through Pd(II)/Pd(0) catalysis as well as a method to construct biologically important benzolactone scaffolds. The use of a gas mixture of 5% oxygen in nitrogen demonstrated the possibility for its application in pharmaceutical manufacturing.

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

The functionalization of unactivated carbon-hydrogen bonds is a transformative strategy for the rapid construction of molecular complexity given the ubiquitous presence of C-H bonds in organic molecules. It represents a powerful tool for accelerating the synthesis of natural products and bioactive compounds while reducing the environmental and economic costs of synthesis. At the same time, the ubiquity and strength of C-H bonds also present major challenges toward the realization of transformations that are both highly selective and efficient. The development of practical C-H functionalization reactions has thus remained a compelling yet elusive goal in organic chemistry for over a century.Specifically, the capability to form useful new C-C, C-N, C-O, and C-X bonds via direct C-H functionalization would have wide-ranging impacts in organic synthesis. Palladium is especially attractive as a catalyst for such C-H functionalizations because of the diverse reactivity of intermediate palladium-carbon bonds. Early efforts using cyclopalladation with Pd(OAc)₂ and related salts led to the development of many Pd-catalyzed C-H functionalization reactions. However, Pd(OAc)₂ and other simple Pd salts perform only racemic transformations, which prompted a long search for effective chiral catalysts dating back to the 1970s. Pd salts also have low reactivity with synthetically useful substrates. To address these issues, effective and reliable ligands capable of accelerating and improving the selectivity of Pd-catalyzed C-H functionalizations are needed.In this Account, we highlight the discovery and development of bifunctional mono-N-protected amino acid (MPAA) ligands, which make great strides toward addressing these two challenges. MPAAs enable numerous Pd(II)-catalyzed C(sp²)-H and C(sp³)-H functionalization reactions of synthetically relevant substrates under operationally practical conditions with excellent stereoselectivity when applicable. Mechanistic studies indicate that MPAAs operate as unique bifunctional ligands for C-H activation in which both the carboxylate and amide are coordinated to Pd. The N-acyl group plays an active role in the C-H cleavage step, greatly accelerating C-H activation. The rigid MPAA chelation also results in a predictable transfer of chiral information from a single chiral center on the ligand to the substrate and permits the development of a rational stereomodel to predict the stereochemical outcome of enantioselective reactions.We also describe the application of MPAA-enabled C-H functionalization in total synthesis and provide an outlook for future development in this area. We anticipate that MPAAs and related next-generation ligands will continue to stimulate development in the field of Pd-catalyzed C-H functionalization.

Ligand-Enabled Monoselective β-C(sp3)-H Acyloxylation of Free Carboxylic Acids Using a Practical Oxidant

The development of C-H activation reactions that use inexpensive and practical oxidants remains a significant challenge. Until our recent disclosure of the β-lactonization of free aliphatic acids, the use of peroxides in C-H activation reactions directed by weakly coordinating native functional groups was unreported. Herein, we report C(sp³)-H β-acetoxylation and γ-, δ-, and ε-lactonization reactions of free carboxylic acids enabled by a novel cyclopentane-based mono-N-protected β-amino acid ligand. Notably, tert-butyl hydrogen peroxide is used as the sole oxidant for these reactions. This reaction has several key advantages over other C-H activation protocols: (1) exclusive monoselectivity was observed in the presence of two α-methyl groups; (2) aliphatic carboxylic acids containing α-hydrogens are compatible with this protocol; (3) lactonization of free acids, affording γ-, δ-, or ε-lactones, has been achieved for the first time.

Asymmetric construction of quaternary α-nitro amides by palladium-catalyzed C(sp3)–H arylation

A Pd-catalyzed enantioselective C(sp³)–H arylation of N-(o-Br-aryl) anilides has been disclosed for the preparation of optically active quaternary amino acid derivatives. Quaternary α-nitro amides are constructed with stereoselectivities of up to 98% ee.

Remote azidation of C(sp3)–H bonds to synthesize δ-azido sulfonamidesviairon-catalyzed radical relay

A selective remote C–H azidation was developed with an iron catalyst playing dual roles as a radical initiator and terminator.

Weak coordinated nitrogen functionality enabled regioselective C–H alkynylation via Pd(ii)/mono-N-protected amino acid catalysis

By using mono-N-protected amino acid (MPAA) ligand under Pd(ii) catalysis, regioselective C–H alkynylation of weak coordinated nitrogen functionalities was developed with great efficiency, proceeding via 5, 6 and 7-membered palladacycle intermediates.

Catalytic C(sp3)-H bond activation in tertiary alkylamines

The development of robust catalytic methods to assemble tertiary alkylamines provides a continual challenge to chemical synthesis. In this regard, transformation of a traditionally unreactive C-H bond, proximal to the nitrogen atom, into a versatile chemical entity would be a powerful strategy for introducing functional complexity to tertiary alkylamines. A practical and selective metal-catalysed C(sp³)-H activation facilitated by the tertiary alkylamine functionality, however, remains an unsolved problem. Here, we report a Pd(II)-catalysed protocol that appends arene feedstocks to tertiary alkylamines via C(sp³)-H functionalization. A simple ligand for Pd(II) orchestrates the C-H activation step in favour of deleterious pathways. The reaction can use both simple and complex starting materials to produce a range of multifaceted γ-aryl tertiary alkylamines and can be rendered enantioselective. The enabling features of this transformation should be attractive to practitioners of synthetic and medicinal chemistry as well as in other areas that use biologically active alkylamines.

Copper-catalyzed enantioselective Sonogashira-type oxidative cross-coupling of unactivated C(sp3)-H bonds with alkynes

Transition metal-catalyzed enantioselective Sonogashira-type oxidative C(sp³)-C(sp) coupling of unactivated C(sp³)-H bonds with terminal alkynes has remained a prominent challenge. The difficulties mainly stem from the regiocontrol in unactivated C(sp³)-H bond functionalization and the inhibition of readily occurring Glaser homocoupling of terminal alkynes. Here, we report a copper/chiral cinchona alkaloid-based N,N,P-ligand catalyst for asymmetric oxidative cross-coupling of unactivated C(sp³)-H bonds with terminal alkynes in a highly regio-, chemo-, and enantioselective manner. The use of N-fluoroamide as a mild oxidant is essential to site-selectively generate alkyl radical species while efficiently avoiding Glaser homocoupling. This reaction accommodates a range of (hetero)aryl and alkyl alkynes; (hetero)benzylic and propargylic C(sp³)-H bonds are all applicable. This process allows expedient access to chiral alkynyl amides/aldehydes. More importantly, it also provides a versatile tool for the construction of chiral C(sp³)-C(sp), C(sp³)-C(sp²), and C(sp³)-C(sp³) bonds when allied with follow-up transformations.

Chemoselective formal β-functionalization of substituted aliphatic amides enabled by a facile stereoselective oxidation event

Aliphatic C-H functionalization is a topic of current intense interest in organic synthesis. Herein, we report that a facile and stereoselective dehydrogenation event enables the functionalization of aliphatic amides at different positions in a one-pot fashion. Derivatives of relevant pharmaceuticals were formally functionalized in the β-position in late-stage manner. A single-step synthesis of incrustoporine from a simple precursor further showcases the potential utility of this approach.

Understanding the Activity and Enantioselectivity of Acetyl-Protected Aminoethyl Quinoline Ligands in Palladium-Catalyzed β-C(sp3)-H Bond Arylation Reactions

Chiral acetyl-protected aminoalkyl quinoline (APAQ) ligands were recently discovered to afford highly active and enantioselective palladium catalysts for the arylation of methylene C(sp³)-H bonds, and herein, we investigate the origins of these heightened properties. Unprecedented amide-bridged APAQ-Pd dimers were predicted by density functional theory (DFT) calculations and were confirmed by single-crystal X-ray diffraction studies. Comparison of structural features between APAQ-Pd complexes and an acetyl-protected aminoethylpyridine APAPy-Pd complex strongly suggests that the high activity of the former originates from the presence of the quinoline ring, which slows the formation of the off-cycle palladium dimer. Furthermore, steric topographic maps for a representative subset of monomeric, monoligated palladium complexes allowed us to draw a unique parallel between the three-dimensional structures of these catalysts and their reported asymmetric induction in β-C(sp³)-H bond arylation reactions. Finally, cooperative noncovalent interactions present between the APAQ ligand and the substrate were identified as a crucial factor for imparting selectivity between chemically equivalent methylenic C(sp³)-H bonds prior to concerted metalation deprotonation activation.

Ruthenium-Catalyzed Enantioselective C-H Functionalization: A Practical Access to Optically Active Indoline Derivatives

Ru(II)-catalyzed enantioselective C-H activation/hydroarylation has been developed for the first time, allowing for highly enantioselective synthesis of indoline derivatives via catalytic C-H activation. Commercially available Ru(II) arene complexes and chiral α-methylamines were employed as highly enantioselective catalysts. Based on a sterically rigidified chiral transient directing group, multisubstituted indolines were produced in up to 92% yield with 96% ee. Further transformation of the resulting 4-formylindoline enables access to an optically active tricyclic compound that is of potential biological and pharmaceutical interest.

Palladium/Benzoic Acid-Catalyzed Regio- and Stereoselective Polymerization of Internal Diynes and Diols through C(sp3)-H Activation

The C-H activation has been a hot research area in organic chemistry, and the most difficult one is the C(sp³)-H activation. Although the C-H activation has been introduced to the research of synthetic polymer chemistry, the polymerization developed based on C(sp³)-H activation is rarely reported, which will enrich the tools for the preparation of functional polymers. In this work, palladium/benzoic acid catalyzed polymerization of internal diynes and diols through C(sp³)-H activation was successfully established. Regio- and stereoregular functional poly(allylic ether)s with 100% E-isomers and high weight average molecular weights (M_w up to 33200) were prepared in excellent yield (98%). The reaction mechanism was unveiled with the assistance of density functional theory calculations. Furthermore, the thin films of polymers display high refractive indices and low optical dispersions. The polymer containing tetraphenylethene moiety displays the aggregation-enhanced emission feature and could be used to generate 2D fluorescent photopattern. Thus, this work not only establishes a powerful polymerization based on C(sp³)-H activation, but also furnishes functional polymers for diverse applications.

A Computational Mechanistic Study of Pd(II)-Catalyzed Enantioselective C(sp3)-H Borylation: Roles of APAO Ligands

A computational mechanistic study has been performed on Pd(II)-catalyzed enantioselective reactions involving acetyl-protected aminomethyl oxazolines (APAO) ligands that significantly improved reactivity and selectivity in C(sp³)-H borylation. The results support a mechanism including initiation of C(sp³)-H bond activation generating a five-membered palladacycle and ligand exchange, followed by HPO₄^2--promoted transmetalation. These resulting Pd(II) complexes further undergo sequential reductive elimination by coordination of APAO ligands and protonation to afford the enantiomeric products and deliver Pd(0) complexes, which will then proceed by oxidation and deprotonation to regenerate the catalyst. The C(sp³)-H activation is found to be the rate- and enantioselectivity-determining step, in which the APAO ligand acts as the proton acceptor to form the two enantioselectivity models. The results demonstrate that the diverse APAO ligands control the enantioselectivity by differentiating the distortion and interaction between the major and minor pathways.

Enantio- and Diastereoswitchable C-H Arylation of Methylene Groups in Cycloalkanes

A complementary set of chiral N,N-ligands enables the Pd-catalyzed β-C-H arylation of unbiased internal methylene groups in good yield and with high levels of enantio- and diastereoinduction. Both the dia- and enantioselectivity can be reversed, thus allowing the selective arylation of any of the four β-C-H bonds in cycloalkanecarboxamides of various ring sizes. The method is applicable to a broad range of aryl iodides with electron-withdrawing and -donating substituents in the o-, m-, or p-position.

Nickel-Catalyzed Decarboxylative Alkenylation of Anhydrides with Vinyl Triflates or Halides

Decarboxylative cross-coupling of aliphatic acid anhydrides with vinyl triflates or halides was accomplished via nickel catalysis. This methodology works well with a broad array of substrates and features abundant functional group tolerance. Notably, our approach addresses the issue of safe and environmental installation of methyl or ethyl group into molecular scaffolds. The method possesses high chemoselectivity toward alkyl groups when aliphatic/aromatic mixed anhydrides are involved. Furthermore, diverse ketones could be modified with our strategy.

Directed, Palladium(II)-Catalyzed Enantioselective anti-Carboboration of Alkenyl Carbonyl Compounds

A substrate-directed enantioselective anti-carboboration reaction of alkenes has been developed, wherein a carbon-based nucleophile and a boron moiety are installed across the C=C bond through a 5-membered palladacycle intermediate. A preliminary result also shows it is possible to extend this reaction to alkenes that are more distal from the directing group and react via a 6-membered palladacycle. The reaction is promoted by a palladium(II) catalyst and a monodentate oxazoline ligand. A range of enantioenriched secondary alkylboronate products were obtained with moderate to high enantioselectivity that could be further upgraded by recrystallization. This work represents an efficient method to synthesize versatile and valuable alkylboronate building blocks. Building on an earlier mechanistic proposal by Peng, He, and Chen, a revised model is proposed to account for the stereoconvergent nature of this transformation.

Primary α-tertiary amine synthesis via α-C-H functionalization

A quinone-mediated general synthetic platform for the construction of primary α-tertiary amines from abundant primary α-branched amine starting materials is described. This procedure pivots on the efficient in situ generation of reactive ketimine intermediates and subsequent reaction with carbon-centered nucleophiles such as organomagnesium and organolithium reagents, and TMSCN, creating quaternary centers. Furthermore, extension to reverse polarity photoredox catalysis enables reactivity with electrophiles, via a nucleophilic α-amino radical intermediate. This efficient, broadly applicable and scalable amine-to-amine synthetic platform was successfully applied to library and API synthesis and in the functionalization of drug molecules.