Palladium(II)-catalyzed enantioselective C(sp³)-H activation using a chiral hydroxamic acid ligand

Pd(II)-Catalyzed Desymmetrizing gem-Dimethyl C(sp3)-H Alkenylation/Aza-Wacker Cyclization Directed by PIP Auxiliary

Desymmetrization of gem-dimethyl groups has been developed as an efficient pathway to achieve asymmetric C(sp3)-H functionalization. Herein, we described a Pd(II)-catalyzed desymmetrizing gem-dimethyl C(sp3)-H alkenylation/aza-Wacker cyclization directed by a bidentate 2-pyridinylisopropyl auxiliary. Chiral α-methyl γ-lactams were obtained in good yields (up to 82%) and high enantioselectivities (up to 91.5% ee).

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.

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.

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.

Synthesis of the Isodityrosine Moiety of Seongsanamide A-D and Its Derivatives

The concise and highly convergent synthesis of the isodityrosine unit of seongsanamide A-D and its derivatives bearing a diaryl ether moiety is described. In this work, the synthetic strategy features palladium-catalyzed C(sp³)-H functionalization and a Cu/ligand-catalyzed coupling reaction. We report a practical protocol for the palladium-catalyzed mono-arylation of β-methyl C(sp³)-H of an alanine derivative bearing a 2-thiomethylaniline auxiliary. The reaction is compatible with a variety of functional groups, providing practical access to numerous β-aryl-α-amino acids; these acids can be converted into various tyrosine and dihydroxyphenylalanine (DOPA) derivatives. Then, a CuI/N,N-dimethylglycine-catalyzed arylation of the already synthesized DOPA derivatives with aryl iodides is described for the synthesis of isodityrosine derivatives.

Cobalt-Catalyzed Diastereo- and Enantioselective Carbon-Carbon Bond Forming Reactions of Cyclobutenes

Catalytic enantioselective functionalization of cyclobutenes constitutes a general and modular strategy for construction of enantioenriched complex cyclobutanes bearing multiple stereogenic centers, as chiral four-membered rings are common motifs in biologically active molecules and versatile intermediates in organic synthesis. However, enantioselective synthesis of cyclobutanes through such a strategy remained significantly limited. Herein, we report a series of unprecedented cobalt-catalyzed carbon-carbon bond forming reactions of cyclobutenes that are initiated through enantioselective carbometalation. The protocols feature diastereo- and enantioselective introduction of allyl, alkynyl, and functionalized alkyl groups. Mechanistic studies indicated an unusual 1,3-cobalt migration and subsequent β-carbon elimination cascade process occurred in the allyl addition. These new discoveries established a new elementary process for cobalt catalysis and an extension of diversity of nucleophiles for enantioselective transformations of cyclobutenes.

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.

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.

Enantioselective Synthesis of Both Axially and Planar Chiral Ferrocenes via Axial-to-Planar Diastereoinduction

Ferrocenes with planar chirality have emerged as an important class of scaffolds for ligands in asymmetric catalysis; however, ferrocene molecules with polychiral structures have not been well explored. Herein, both axially and planar chiral ferrocenes were synthesized via palladium/chiral norbornene cooperative catalysis and axial-to-planar diastereoinduction. In this work, chiral norbornene was used to stereoselectively control the aromatic axial chirality, and further selectivity induced C(sp²)-H activation for ferrocene planar chirality. Based on density functional theory calculations, the catalytic model of chiral norbornene with the substrate and the axial-to-planar diastereoinduction process were confirmed.

Incorporation of a cyclobutyl substituent in molecules by transition metal-catalyzed cross-coupling reactions

In this review, the incorporation of a cyclobutyl substituent in molecules, by transition metal-catalyzed cross-coupling, is described by only considering the formation of C-C bonds. Three main strategies are used to introduce a cyclobutyl substituent in molecules by involving either electrophilic or nucleophilic cyclobutane derivatives.

Asymmetric hydrogenation of TIPS-protected oximes with chiral boranes

An enantioselective metal-free hydrogenation of TIPS-protected oximes has been successfully realized for the first time by using chiral borane catalysts derived from chiral dienes and Piers' borane. A variety of hydroxylamine derivatives were afforded in 84-99% yields with 33-68% ees.

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.

Preparation of Optically Active Biphenyl Compounds via an Albumin- Mediated Asymmetric Nitroaldol Reaction

Abstract:

Human serum albumin (HSA) was found to catalyze the asymmetric nitroaldol reaction of biphenyl aldehydes with nitromethane to afford the corresponding optically active 2-nitro alcohols. Careful optimization of the conditions for the reaction of 4-phenylbenzaldehyde with nitromethane in water at a neutral pH improved both the reactivity and the enantioselectivity. Finally, the reaction of 4-phenylbenzaldehyde (56 mg, 0.30 mmol) in nitromethane (2.8 mL) and water (1.1 mL) using HSA (68 mg) at 5 °C for 240 h gave (R)-1-([1,1'-biphenyl]-4-yl)-2-nitroethanol in 71% yield (52 mg), with an ee up to 85% ee. Subsequent recrystallization improved the ee up to 95%. The reaction was useful in a preparative-scale operation, and the biocatalyst could be reused several times. The procedure was also applicable to other substrates with different substitution patterns. Although the nitroaldol reaction of 2-phenylbenzaldehyde with nitromethane proceeded with low enantioselectivity to afford the corresponding (R)-2-nitroalcohols (35% ee), the reactions of the substrates bearing Br, Me, OMe, or CN group at the 4'-position of the benzene ring gave the corresponding optically active compounds with high enantioselectivities (80-88% ee).

Catalytic asymmetric hydrometallation of cyclobutenes with salicylaldehydes

Chiral, substituted cyclobutanes are common motifs in bioactive compounds and intermediates in organic synthesis but few asymmetric routes for their synthesis are known.

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

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

Iron-catalyzed regioselective synthesis of (E)-vinyl sulfones mediated by unprotected hydroxylamines

An Fe-catalyzed unprotected hydroxylamine mediated Heck-type coupling between sulfinic acids and alkenes furnished structurally important (E)-vinyl sulfones with moderate to good yields, high atom-economy and regioselectivity.

Mechanistic study of cobalt(I)-catalyzed asymmetric coupling of ethylene and enynes to functionalized cyclobutanes

Density functional theory (DFT) calculations have been performed to gain insight into the reaction mechanism of the Co(I)-catalyzed asymmetric [2 + 2] cycloaddition reaction of enyne 1a with ethylene 2 to give the functionalized cyclobutene E-4a possessing a chiral, all-carbon quaternary center in the ring framework (Science, 361, 68-72). This study reveals that the whole catalysis can be characterized via three stages: (i) oxidative dimerization followed by reductive elimination gives the intermediate IM3, (ii) the alkenyl-Co(III) metallacycloheptene IM6 formation with the addition of another equivalent ethylene via an oxidative dimerization process, (iii) β-Hydrogen elimination and reductive elimination from IM6 to result in the final product E-4a and regenerate the active speices IM1 for the next catalytic cycle. Each stage is kinetically and thermodynamically feasible for experimental realization under mild conditions, and the formation of the alkenyl-Co(III) metallacycloheptene IM6, with a barrier of 27.2 kcal mol^-1 (i.e., IM2 → TS4), should be the rate-determining step (RDS) during the whole catalysis. In addition, the origins of enantioselectivity and regioselectivity of the product are discussed.

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.

Sequential Photocatalytic Reactions for the Diastereoselective Synthesis of Cyclobutane Scaffolds

The synthesis of densely functionalized cyclobutanes containing all-carbon quaternary stereocenters in high regio- and diastereoselectivity remains synthetically challenging. Herein, we show that this can be achieved by using a sequential photocatalysis strategy, wherein 3-chloromaleimides undergo triplet sensitized [2 + 2] photocycloadditions with alkynes or alkenes followed by photoredox-catalyzed dechlorinative C-C bond forming reactions to install quaternary stereocenters. This allows the rapid assembly of structurally complex and sterically congested 3-azabicyclo[3.2.0]heptane scaffolds from readily available starting materials.

Catalytic Reduction of Oximes to Hydroxylamines: Current Methods, Challenges and Opportunities

Catalytic reduction of oximes represents a direct efficient approach to synthesize valuable hydroxylamine derivatives. However this transformation presents significant challenges: oximes are hard to reduce and, if reactive, reductive cleavage of the weak N−O bond often leads to primary amine side products. The first suitable systems involved the use of platinum‐based heterogeneous catalysts with hydrogen as reductant and stoichiometric amounts of a strong Brønsted acid. More recently metal‐free and transition‐metal‐based homogeneous catalysts have been developed, which display the highest turnovers (up to 4000). In the asymmetric variants, the E/Z‐geometry of the oxime double bond affects significantly the stereoselectivity, sometimes requiring extra synthetic efforts in substrate preparation. This minireview provides an overview of the advances and limitations in catalytic oxime to hydroxylamine reduction. Emphasis is put on highlighting and comparing the practical aspects of the existing methods, such as their reaction conditions and substrate scope. Additionally, future directions for improving this young research area are suggested.

ortho-C-H Acetoxylation of Cubane Enabling Access to Cubane Analogues of Pharmaceutically Relevant Scaffolds

A novel method of introducing an oxygen functionality into a cubane core was developed using a transition-metal-catalyzed directed acetoxylation methodology via C-H activation. The obtained compounds were derivatized into cubane analogues of pharmaceutically relevant structural motifs, namely, acetylsalicylic acid and coumarin motifs, which could potentially act as bioisosteres of these scaffolds.

Catalytic enantioselective synthesis of benzocyclobutenols and cyclobutanols via a sequential reduction/C-H functionalization

We report here a sequential enantioselective reduction/C-H functionalization to install contiguous stereogenic carbon centers of benzocyclobutenols and cyclobutanols. This strategy features a practical enantioselective reduction of a ketone and a diastereospecific iridium-catalyzed C-H silylation. Further transformations have been explored, including controllable regioselective ring-opening reactions. In addition, this strategy has been utilized for the synthesis of three natural products, phyllostoxin (proposed structure), grandisol and fragranol.

Iridium-Catalyzed Acid-Assisted Hydrogenation of Oximes to Hydroxylamines

We found that cyclometalated cyclopentadienyl iridium(III) complexes are uniquely efficient catalysts in homogeneous hydrogenation of oximes to hydroxylamine products. A stable iridium C,N-chelation is crucial, with alkoxy-substituted aryl ketimine ligands providing the best catalytic performance. Several Ir-complexes were mapped by X-ray crystal analysis in order to collect steric parameters that might guide a rational design of even more active catalysts. A broad range of oximes and oxime ethers were activated with stoichiometric amounts of methanesulfonic acid and reduced at room temperature, remarkably without cleavage of the fragile N-O bond. The exquisite functional group compatibility of our hydrogenation system was further demonstrated by additive tests. Experimental mechanistic investigations support an ionic hydrogenation platform, and suggest a role for the Brønsted acid beyond a proton source. Our studies provide deep understanding of this novel acidic hydrogenation and may facilitate its improvement and application to other challenging substrates.

A catalytic asymmetric cross-coupling approach to the synthesis of cyclobutanes

Stereodefined four-membered rings are common motifs in bioactive molecules and versatile intermediates in organic synthesis. However, the synthesis of complex, chiral cyclobutanes is a largely unsolved problem and there is a need for general and modular synthetic methods. Here we report a series of asymmetric cross-coupling reactions between cyclobutenes and arylboronic acids which are initiated by Rh-catalysed asymmetric carbometallation. After the initial carborhodation, Rh-cyclobutyl intermediates undergo chain-walking or C-H insertion so that overall a variety of additions such as reductive Heck reactions, 1,5-addition and homoallylic substitution are observed. The synthetic applicability of these highly stereoselective transformations is demonstrated in the concise syntheses of the drug candidates Belaperidone and PF-04862853. We anticipate this approach will be widely adopted by synthetic and medicinal chemists. While the carbometallation approach reported here is exemplified with Rh and arylboronic acids, it is likely to be applicable to other metals and nucleophiles.

Transition-Metal-Catalyzed C–H Arylation Using Organoboron Reagents

Aryl rings are ubiquitous in the core of numerous natural product and industrially important molecules and thus their facile synthesis is of major interest in the scientific community and industry. Although multiple strategies enable access to these skeletons, metal-catalyzed C–H activation is promising due to its remarkable efficiency. Commercially available organoboron reagents, a prominent arylating partner in the cross-coupling domain, have also been utilized for direct arylation. Organoborons are bench-stable, inexpensive, and readily available coupling partners that promise regioselectivity, chemodivergence, cost-efficiency, and atom-economy without requiring harsh and forcing conditions. This critical, short review presents a summary of all major studies of arylation using organoborons in transition-metal catalysis since 2005.

1 Introduction

2 Arylation without Directing Group Assistance

2.1 Palladium Catalysis

2.2 Iron Catalysis

2.3 Gold Catalysis

3 Arylation with Directing Group Assistance

3.1 Palladium Catalysis

3.2 Ruthenium Catalysis

3.3 Rhodium Catalysis

3.4 Nickel Catalysis

3.5 Cobalt Catalysis

3.6 Copper Catalysis

4 Conclusion

Kinetic Resolution of Allyltriflamides through a Pd-Catalyzed C-H Functionalization with Allenes: Asymmetric Assembly of Tetrahydropyridines

Enantioenriched, six-membered azacycles are essential structural motifs in many products of pharmaceutical or agrochemical interest. Here we report a simple and practical method for enantioselective assembly of tetrahydropyridines, which is paired to a kinetic resolution of α-branched allyltriflamides. The reaction consists of a formal (4+2) cycloaddition between the allylamine derivatives and allenes and is initiated by a palladium(II)-catalyzed C-H activation process. Both the chiral allylamide precursors and the tetrahydropyridine adducts were successfully obtained in high yields, with excellent enantioselectivity (up to 99% ee) and selectivity values of up to 127.

Site-selective and diastereoselective functionalization of α-amino acid and peptide derivatives via palladium-catalyzed sp3 C–H activation

This review introduces palladium-catalyzed C–H functionalization of amino acids and peptides.

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.

Tailored quinones support high-turnover Pd catalysts for oxidative C-H arylation with O2

Palladium(II)-catalyzed carbon-hydrogen (C-H) oxidation reactions could streamline the synthesis of pharmaceuticals, agrochemicals, and other complex organic molecules. Existing methods, however, commonly exhibit poor catalyst performance with high palladium (Pd) loading (e.g., 10 mole %) and a need for (super)stoichiometric quantities of undesirable oxidants, such as benzoquinone and silver(I) salts. The present study probes the mechanism of a representative Pd-catalyzed oxidative C-H arylation reaction and elucidates mechanistic features that undermine catalyst performance, including substrate-consuming side reactions and sequestration of the catalyst as an inactive species. Systematic tuning of the quinone cocatalyst overcomes these deleterious features. Use of 2,5-di-tert-butyl-p-benzoquinone enables efficient use of molecular oxygen as the oxidant, high reaction yields, and >1900 turnovers by the Pd catalyst.

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.

Palladium(II)-catalyzed asymmetric C-H carbonylation to diverse isoquinoline derivatives bearing all-carbon quaternary stereocenters

Enantioselective synthesis of tetrahydroisoquinolines bearing an all-carbon quaternary stereogenic center, was achieved via asymmetric C-H activation with high enantioselectivities (up to 93% ee). Fair substrate tolerance was indicated throughout the scope investigation and no evident loss of enantioselectivity was exhibited in late-stage derivatization. This study provides incentives for the construction of diverse chiral isoquinoline derivatives, which are prevalent among pharmaceuticals, natural products, etc.

Synthesis of Hydroxamic Acid Derivatives Using Blocked (Masked) O-Isocyanate Precursors

Hydroxamic acids are present in a several pharmaceuticals and agrochemicals. Synthetic strategies providing access to hydroxamic acid derivatives remain limited, typically requiring the use of nucleophilic hydroxylamine reagents. Herein, a synthesis of hydroxamates from unactivated carboxylic acids is reported making use of rare blocked (masked) O-substituted isocyanates. The applicability of this transformation was highlighted by targeting the synthesis of vorinostat and belinostat derivatives.

Ligand-Accelerated Palladium(II)-Catalyzed Enantioselective Amination of C(sp2)-H Bonds

The first example of the Pd(II)-catalyzed enantioselective amination of aryl C-H bonds is reported. The key to the successful realization of this asymmetric catalytic transformation was the identification of mono-N-protected α-amino-O-methylhydroxamic acid (MPAHA) ligands, which promote reactivity under mild conditions and control enantioselectivity. The counteranions in the solvent medium, hexafluoroacetylacetate and acetate, were also found to play key roles in stereocontrol and reactivity enhancement.

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.

C(sp3)-H arylation to construct all-syn cyclobutane-based heterobicyclic systems: a novel fragment collection

All-syn fused cyclobutanes remain an elusive chemotype and thus present an interesting synthetic challenge. Herein, we report the successful application of Pd-catalysed C(sp3)-H arylation of cyclobutane compounds to generate all-syn heterobicyclic fragments using an innovative 'inside-out' approach. Through this strategy we generate a virtual collection of 90 fragments, which we demonstrate to have enhanced three-dimensionality and superior fragment-like properties compared to existing collections.

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.

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.