Access to chiral dihydrophenanthridines via a palladium(0)-catalyzed Suzuki coupling and C-H arylation cascade reaction using new chiral-bridged biphenyl bifunctional ligands

A class of chiral-bridged biphenyl phosphine-carboxylate bifunctional ligands CB-Phos has been developed and successfully applied to Pd(0)-catalyzed single enantioselective C-H arylation and a one pot cascade reaction involving Suzuki cross-coupling and C-H arylation. The catalytic system provides a new and convenient way for the synthesis of versatile chiral dihydrophenanthridines with rich structures and broad functional group tolerance. Good to excellent yields with high enantioselectivities were generally achieved. The reaction mechanism of the cascade reaction was also preliminarily discussed.

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.

Recent advances in Pd-catalyzed asymmetric cyclization reactions

Over the past few decades, there have been major developments in transition metal-catalyzed asymmetric cyclization reactions, enabling the convenient access to a wide spectrum of structurally diverse chiral carbo- and hetero-cycles, common skeletons found in fine chemicals, natural products, pharmaceuticals, agrochemicals, and materials. In particular, a plethora of enantioselective cyclization reactions have been promoted by chiral palladium catalysts owing to their outstanding features. This review aims to collect the latest advancements in enantioselective palladium-catalyzed cyclization reactions over the past eleven years, and it is organized into thirteen sections depending on the different types of transformations involved.

Eight-Membered Palladacycle Intermediate Enabled Synthesis of Cyclic Biarylphosphonates

Transition-metal-catalyzed coupling reactions that involve the direct functionalization of insert C-H bond represent one of the most efficient strategies for forming carbon-carbon bonds. Herein, a palladium-catalyzed intramolecular C-H bond arylation of triaryl phosphates is reported to access seven-membered cyclic biarylphosphonate targets. The reaction is achieved via a unique eight-membered palladacyclic intermediate and shows good functional group compatibility. Meanwhile, the product can be readily converted into other valuable phosphate compounds.

Asymmetric (4+1) Annulations by Cascade Allylation and Transient σ-Alkyl-Pd(II) Initiated Allylic Csp3 -H Activation

A unique Pd-catalyzed approach for asymmetric (4+1) annulations via cascade allylation and transient σ-alkyl-Pd(II) initiated methylene Csp3 -H activation is reported. The enolate fragment derived from the decarboxylation of vinyl methylene carbonate is crucial to stabilize the key intermediate. These reactions enable the synthesis of various useful dihydrobenzofurans with excellent enantioselectivity, typically >95 : 5 er, and exclusive (Z)-stereoselectivity. Compared with the well-established annulations via Heck-type C-H activations, this protocol showcases a conceptually new way to generate σ-alkyl-Pd(II) species that could initiate challenging asymmetric Csp3 -H activations.

Effect of α-Substitution on the Reactivity of C(sp3)-H Bonds in Pd0-Catalyzed C-H Arylation

We report mechanistic studies on the reactivity of different α-substituted C(sp3)-H bonds, -CHnR (R = H, Me, CO2Me, CONMe2, OMe, and Ph, as well as the cyclopropyl and isopropyl derivatives -CH(CH2)2 and -CHMe2) in the context of Pd0-catalyzed C(sp3)-H arylation. Primary kinetic isotope effects, kH/kD, were determined experimentally for R = H (3.2) and Me (3.5), and these, along with the determination of reaction orders and computational studies, indicate rate-limiting C-H activation for all substituents except when R = CO2Me. This last result was confirmed experimentally (kH/kD ∼ 1). A reactivity scale for C(sp3)-H activation was then determined: CH2CO2Me > CH(CH2)2 ≥ CH2CONMe2 > CH3 ≫ CH2Ph > CH2Me > CH2OMe ≫ CHMe2. C-H activation involves AMLA/CMD transition states featuring intramolecular O → H-C H-bonding assisted by C-H → Pd agostic bonding. The "AMLA coefficient", χ, is introduced to quantify the energies associated with these interactions via natural bond orbital 2nd order perturbation theory analysis. Higher barriers correlate with lower χ values, which in turn signal a greater agostic interaction in the transition state. We believe that this reactivity scale and the underlying factors that determine this will be of use for future studies in transition-metal-catalyzed C(sp3)-H activation proceeding via the AMLA/CMD mechanism.

Axis-Unfixed Biphenylphosphine-Oxazoline Ligands: Design and Applications in Asymmetric Catalytic Reactions

The design and synthesis of chiral ligands plays an important role in asymmetric catalytic reactions. Over the past decades, various types of chiral phosphine-oxazolines (PHOX ligands) have been developed and have greatly advanced the field of asymmetric catalysis. Novel chiral PHOX ligand with an axis-unfixed biphenyl backbone, developed by our group, have shown interesting coordination behavior and excellent chiral inducing ability in various transition-metal-catalyzed asymmetric reactions. This personal account focuses on our developed axis-unfixed biphenylphosphine-oxazoline ligand (BiphPHOX), including an overview of its design and applications, which will provide inspiration for the exploration of novel ligands and related reactions.

A General Enantioselective C-H Arylation Using an Immobilized Recoverable Palladium Catalyst

We herein report a general and efficient enantioselective C-H arylation of aryl bromides based on the use of BozPhos as the bisphosphine ligand and SP-NHC-PdII as recoverable heterogeneous catalyst. By exploiting the "release and catch" mechanism of action of the catalytic system, we used BozPhos as a broadly applicable chiral ligand, furnishing high enantioselectivities across all types of examined substrates containing methyl, cyclopropyl and aryl C-H bonds. For each reaction, the reaction scope was investigated, giving rise to 30 enantioenriched products, obtained with high yields and enantioselectivities, and minimal palladium leaching. The developed catalytic system provides a more sustainable solution compared to homogeneous systems for the synthesis of high added-value chiral products through recycling of the precious metal.

Ni-catalyzed carbamoylation of unactivated alkenes for stereoselective construction of six-membered lactams

Nitrogen-based heterocycles have aroused widespread interest due to their reoccurrence in many pharmaceuticals. Amongst these motifs, the enantioenriched lactams are the ubiquitous scaffolds found in myriad biologically active natural products and drugs. Recently, the transition metal-catalyzed asymmetric carbamoylation has been widely employed as a straightforward arsenal for chiral lactam architecture synthesis, including β-lactam and γ-lactam. However, despite the extensive efforts, there still remains no protocol to accomplish the related δ-lactam synthesis. In this manuscript, the Ni-catalyzed enantioselective carbamoylation of unactivated alkenes by the leverage of reductive dicarbofunctionalization strategy allows for the expedient access to two types of mostly common six-membered lactams: 3,4-dihydroquinolinones and 2-piperidinone in high yield and enantioselectivity. This protocol features with good functional group tolerance, as well as broad substrate scope. The newly developed chiral 8-Quinox skeleton ligand is the key parameter for this transformation, which significantly enhances the reactivity and enantioselectivity.

Six-membered chiral lactams are common structural motifs of pharmaceuticals. Here, the authors describe a nickel-catalyzed reductive carbamoylation of alkenes to form enantioenriched six-membered lactams.

Cytotoxic Assessment of 3,3-Dichloro-β-Lactams Prepared through Microwave-Assisted Benzylic C-H Activation from Benzyl-Tethered Trichloroacetamides Catalyzed by RuCl2(PPh3)3

Natural and synthetic β-lactam derivatives constitute an interesting class of compounds due to their diverse biological activity. Mostly used as antibiotics, they were also found to have antitubercular, anticancer and antidiabetic activities, among others. In this investigation, six new 3,3-dichloro-β-lactams prepared in a previous work were evaluated for their hemolytic and cytotoxic properties. The results showed that the proposed compounds have non-hemolytic properties and exhibited an interesting cytotoxic activity toward squamous cell carcinoma (A431 cell line), which was highly dependent on the structure and concentration of these β-lactams. Among the molecules tested, 2b was the most cytotoxic, with the lowest IC₅₀ values (30–47 µg/mL) and a promising selectivity against the tumor cells compared with non-tumoral cells.

Ligand‐Enabled β‐C(sp 3 )−H Lactamization of Tosyl‐Protected Aliphatic Amides Using a Practical Oxidant

The development of C(sp³)−H functionalization reactions that use common protecting groups and practical oxidants remains a significant challenge. Herein we report a monoprotected aminoethyl thioether (MPAThio) ligand‐enabled β‐C(sp³)−H lactamization of tosyl‐protected aliphatic amides using tert‐butyl hydrogen peroxide (TBHP) as the sole oxidant. This protocol features exceedingly mild reaction conditions, reliable scalability, and the use of practical oxidants and protecting groups. Further derivatization of the β‐lactam products enables the synthesis of a range of biologically important motifs including β‐amino acids, γ‐amino alcohols, and azetidines.

An Artificial Metalloenzyme Based on a Copper Heteroscorpionate Enables sp3 C-H Functionalization via Intramolecular Carbene Insertion

The selective functionalization of sp³ C–H bonds is a versatile tool for the diversification of organic compounds. Combining attractive features of homogeneous and enzymatic catalysts, artificial metalloenzymes offer an ideal means to selectively modify these inert motifs. Herein, we report on a copper(I) heteroscorpionate complex embedded within streptavidin that catalyzes the intramolecular insertion of a carbene into sp³ C–H bonds. Target residues for genetic optimization of the artificial metalloenzyme were identified by quantum mechanics/molecular mechanics simulations. Double-saturation mutagenesis yielded detailed insight on the contribution of individual amino acids on the activity and the selectivity of the artificial metalloenzyme. Mutagenesis at a third position afforded a set of artificial metalloenzymes that catalyze the enantio- and regioselective formation of β- and γ-lactams with high turnovers and promising enantioselectivities.

Synthesis of 3-substituted 2-oxindoles from secondary α-bromo-propionanilides via palladium-catalyzed intramolecular cyclization

In contrast to aromatic halides, coupling reactions involving oxidative addition of alkyl halides, especially secondary or tertiary halides, to transition metals tend to be more challenging. Herein a palladium-catalyzed intramolecular cyclization of α-bromo-propionanilides has been developed, delivering a series of 3-substituted 2-oxindoles in high yields. The method features easy to prepare starting materials, broad substrate scope and excellent functional group tolerance. A detailed mechanistic investigation has been performed.

One-Pot Construction of Diverse β-Lactam Scaffolds via the Green Oxidation of Amines and Its Application to the Diastereoselective Synthesis of β-Amino Acids

In this study, a simple one-pot construction of β-lactam scaffolds was successfully achieved via 4,6-dihydroxysalicylic acid-catalyzed organocatalytic oxidation of amines to imines using molecular oxygen. Although some imines are highly unstable and difficult to isolate by conventional methods, the organocatalytic oxidation of amines described herein, followed by their direct reaction with acyl chlorides in the presence of a base, afforded a series of new β-lactam derivatives with excellent cis selectivity, which could not be synthesized and isolated by previously reported methods. Thus, this one-pot protocol will be one of the powerful methods applicable to the synthesis of various potential drug candidates and functional molecules. Furthermore, the subsequent hydrolysis of these β-lactams successfully afforded the corresponding β-amino acids as almost single diastereomers in up to 99% yields.

Pd-catalyzed cross-electrophile Coupling/C-H alkylation reaction enabled by a mediator generated via C(sp3)-H activation

Transition-metal-catalyzed cross-electrophile C(sp²)–(sp³) coupling and C–H alkylation reactions represent two efficient methods for the incorporation of an alkyl group into aromatic rings. Herein, we report a Pd-catalyzed cascade cross-electrophile coupling and C–H alkylation reaction of 2-iodo-alkoxylarenes with alkyl chlorides. Methoxy and benzyloxy groups, which are ubiquitous functional groups and common protecting groups, were utilized as crucial mediators via primary or secondary C(sp³)–H activation. The reaction provides an innovative and convenient access for the synthesis of alkylated phenol derivatives, which are widely found in bioactive compounds and organic functional materials.

A cascade Pd-catalyzed cross-electrophile coupling and C–H alkylation reaction of 2-iodo-alkoxylarenes with alkyl chlorides has been developed by using an ortho-methoxy or benzyloxy group as a mediator via C(sp³)–H activation.

Recent advances in aminative difunctionalization of alkenes

Alkenes are versatile building blocks in modern organic synthesis. In the difunctionalization reactions of alkenes, two functional groups can be simultaneously introduced into the π system. This is an efficient strategy for the synthesis of multifunctional compounds with complex structures and has the advantages of atom and step economy. Nitrogen-containing organic compounds are widely found in natural products and synthetic compounds, such as dyes, pesticides, medicines, artificial resins, and so on. Many natural products with high biological activity and a broad range of drugs have nitrogen-containing functional groups. The research on the construction methods of C-N bonds has always been one of the most important tasks in organic synthesis, especially in drug synthesis, and the synthetic methods starting from simple and easily available raw materials have been a topic of interest to chemists. The aminative difunctionalization of alkenes can efficiently construct C-N bonds, and at the same time, prepare some compounds that usually require multiple steps of reaction. It is one of the most effective strategies for the simple and efficient synthesis of functionalized nitrogen-containing compounds. This review outlines the major developments focusing on the transition metal-catalyzed or metal-free diamination, aminohalogenation, aminocarbonation, amino-oxidation and aminoboronation reactions of alkenes from 2015-2020.

Pd-Catalyzed ipso,meta-Dimethylation of ortho-Substituted Iodoarenes via a Base-Controlled C-H Activation Cascade with Dimethyl Carbonate as the Methyl Source

A methyl group can have a profound impact on the pharmacological properties of organic molecules. Hence, developing methylation methods and methylating reagents is essential in medicinal chemistry. We report a palladium-catalyzed dimethylation reaction of ortho-substituted iodoarenes using dimethyl carbonate as a methyl source. In the presence of K₂CO₃ as a base, iodoarenes are dimethylated at the ipso- and meta-positions of the iodo group, which represents a novel strategy for meta-C-H methylation. With KOAc as the base, subsequent oxidative C(sp³)-H/C(sp³)-H coupling occurs; in this case, the overall transformation achieves triple C-H activation to form three new C-C bonds. These reactions allow expedient access to 2,6-dimethylated phenols, 2,3-dihydrobenzofurans, and indanes, which are ubiquitous structural motifs and essential synthetic intermediates of biologically and pharmacologically active compounds.

Palladium(0)-Catalyzed Enantioselective Intramolecular Arylation of Enantiotopic Secondary C-H Bonds

The enantioselective functionalization of nonactivated enantiotopic secondary C-H bonds is one of the greatest challenges in transition-metal-catalyzed C-H activation proceeding by an inner-sphere mechanism. Such reactions have remained elusive within the realm of Pd⁰ catalysis. Reported here is the unique reactivity profile of the IBiox ligand family in the Pd⁰ -catalyzed intramolecular arylation of such nonactivated secondary C-H bonds. Chiral C₂ -symmetric IBiox ligands led to high enantioselectivities for a broad range of valuable indane products containing a tertiary stereocenter, as well as the arylation of secondary C-H bonds adjacent to amides. Depending on the amide substituents and upon control of reaction time, indanes containing labile tertiary stereocenters were also obtained with high enantioselectivities. Analysis of the steric maps of the IBiox ligands indicated that the level of enantioselectivity correlates with the difference between the two most occupied and the two less occupied space quadrants, and provided a blueprint for the design of even more efficient ligands.

Chiral Catalysts for Pd0 -Catalyzed Enantioselective C-H Activation

In the past few decades, processes that involve transition-metal catalysis have represented a major part of the synthetic chemist's toolbox. Recently, the interest has shifted from the well-established cross-coupling reactions to C-H bond functionalization, thus making it a current frontier of transition-metal-catalyzed reactions. Constant progress in this field has led to the discovery of enantioselective methods to generate and control various types of stereogenic elements, thereby demonstrating its high value to generate scalemic chiral molecules. The present review is dedicated to enantioselective Pd⁰ -catalyzed C-H activation, which may be considered as an evolution of Pd⁰ -catalyzed cross-couplings, with a focus on the different chiral ligands and catalysts that enable these transformations.

Diastereoselective Synthesis of β-Lactams by Ligand-Controlled Stereodivergent Intramolecular Tsuji-Trost Allylation

The diastereoselective synthesis of highly substituted β-lactams by intramolecular Tsuji–Trost allylation is reported. Judicious selection of the ligand on palladium allows selective access to either the trans isomer (in generally good to excellent yield with very high diastereomeric excess) or cis isomer (with yields and diastereoselectivity ranging from modest to excellent depending on the substrate). The reaction proceeds under exceedingly mild conditions (rt, no additives) with a broad range of substrates, which are readily accessible by the Ugi reaction.

Enantiodivergent Formation of C-P Bonds: Synthesis of P-Chiral Phosphines and Methylphosphonate Oligonucleotides

Phosphorus Incorporation (PI, abbreviated Π) reagents for the modular, scalable, and stereospecific synthesis of chiral phosphines and methylphosphonate nucleotides are reported. Synthesized from trans-limonene oxide, this reagent class displays an unexpected reactivity profile and enables access to chemical space distinct from that of the Phosphorus-Sulfur Incorporation reagents previously disclosed. Here, the adaptable phosphorus(V) scaffold enables sequential addition of carbon nucleophiles to produce a variety of enantiopure C-P building blocks. Addition of three carbon nucleophiles to Π, followed by stereospecific reduction, affords useful P-chiral phosphines; introduction instead of a single methyl group reveals the first stereospecific synthesis of methylphosphonate oligonucleotide precursors. While both Π enantiomers are available, only one isomer is required-the order of nucleophile addition controls the absolute stereochemistry of the final product through a unique enantiodivergent design.

The Fascinating Chemistry of α-Haloamides

The aim of this review is to highlight the rich chemistry of α-haloamides originally mainly used to discover new C-N, C-O and C-S bond forming reactions, and later widely employed in C-C cross-coupling reactions with C(sp3), C(sp2) and C(sp) coupling partners. Radical-mediated transformations of α-haloamides bearing a suitable located unsaturated bond has proven to be a straightforward alternative to access diverse cyclic compounds by means of either radical initiators, transition metal redox catalysis or visible light photoredox catalysis. On the other hand, cycloadditions with α-halohydroxamate-based azaoxyallyl cations have garnered significant attention. Moreover, in view of the important role in life and materials science of difluoroalkylated compounds, a wide range of catalysts has been developed for the efficient incorporation of difluoroacetamido moieties into activated as well as unactivated substrates.

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.

Divergent Enantioselective Synthesis of (Nor)illudalane Sesquiterpenes via Pd0-Catalyzed Asymmetric C(sp3)-H Activation

A divergent enantioselective synthesis of (nor)illudalane sesquiterpenes was designed by using a Pd⁰-catalyzed asymmetric C(sp³)-H arylation as a key step to control the isolated, highly symmetric quaternary stereocenter of the target molecules. A matched combination of chiral substrate and catalyst proved optimal to reach good levels of stereoselectivity. This approach enabled the synthesis of three (nor)illudalanes, including ( S)-deliquinone and ( S)-russujaponol F, which are synthesized for the first time in enantioenriched form.

Assembling a Hybrid Pd Catalyst from a Chiral Anionic CoIII Complex and Ligand for Asymmetric C(sp3 )-H Functionalization

An unusual hybrid palladium catalyst containing an anionic chiral Co^III complex and a chiral phosphoramidite ligand shows a high capacity for catalyzing asymmetric thioamide-directed C(sp³ )-H arylation and delivers excellent yield and enantioselectivity (up to 99 % yield, 99 % ee). Significant synergy between the chiral ligand and the anion in terms of stereochemical control was observed. Mechanistic investigations have revealed both the nature of the C-H activation and the origin of the enantioselectivity.