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

Synthesis of bicyclo[3.2.0]heptane lactones via a ligand-enabled Pd-catalyzed C(sp3)-H activation cascade

Bicyclo[3.2.0]heptane lactones represent an important scaffold in bioactive molecules. Herein, we report a diastereoselective synthetic disconnection to access bicyclo[3.2.0]heptane lactones from bicyclo[1.1.1]pentane carboxylic acids, which proceeds through palladium-catalyzed C-H activation and C-C cleavage processes. By using two different classes of ligands, MPAA and pyridone-amine, either all-syn arylated bicyclo[3.2.0]heptane lactones or non-arylated ones can be synthesized. The bicyclo[3.2.0]heptane lactone products were converted into multiple substituted cyclobutane, γ-lactone, and oxobicyclo[3.2.0]heptane derivatives to showcase the synthetic versatility of this method.

One-Step Synthesis of Indoloquinoline Alkaloids via the Pd-Catalyzed Larock/Ligand-Accelerated C-H Activation Cascade Reaction

Pd-catalyzed cascade annulation of 2-iodoanilines with N,3-diphenylpropiolamides was achieved, which afforded a series of indolo[2,3-c]quinolinones with good yields and functional group compatibility. Previously, the retro-synthetic routine for the indolo[2,3-c]quinolinone skeleton has often involved intramolecular disconnection of the C(sp2)-C(sp2) bond at variable positions, which necessitates the construction of an indole or quinoline reactant with multiple pre-set functionalities. Meanwhile, using the C-H activation strategy to construct indoloquinoline is limited to cases in which the reactant must be an indole bearing a nonremovable directing group. Therefore, to circumvent these limitations, we herein report a Larock/ligand-accelerated C-H activation cascade strategy to construct indolo[2,3-c]quinolinone in a one-pot fashion. This transformation for the construction of 2-amidoindole relies on the Pd-catalyzed Larock reaction, which then forms a C-N bond to afford indoloquinoline via ligand-enabled Pd-catalyzed C-H activation.

Ligand-Enabled Palladium-Catalyzed β-C(sp3)-H Biarylation of Native Amides with Cyclic Diaryliodonium Salts

Herein, we report a Pd-catalyzed β-C(sp3)-H biarylation of native amides using diaryliodonium salts. A pyridine-3-sulfonic acid ligand that might stabilize the substrate-bound palladium species was found to be essential for high catalytic activity. The reaction displayed a broad scope and showed excellent compatibility with diverse cyclic diaryliodonium salts and amide substrates. The retained iodo functionality on the product provides a versatile handle for further increasing molecular complexity.

Palladium-Catalyzed Enantioselective C-H Arylations and Alkenylations of 2-Aminobiaryls with Atmospheric Air as the Sole Oxidant

Optically active 2-aminobiaryls are valuable chiral frameworks with broad applications in catalysis, synthetic chemistry, and materials science. Here, we present a simple and practical methodology for their asymmetric synthesis via enantioselective palladium catalyzed C-H arylations or alkenylations of racemic precursors. The methodology utilizes a kinetic resolution strategy, producing two highly valuable enantioenriched axially chiral molecules: the C-C coupling product and the unreacted starting material. Notably, we have established reaction conditions that enable the in situ regeneration of the active Pd(II) catalyst using atmospheric air as the sole oxidant. Finally, we showcase the synthetic utility of this approach by preparing several derivatives relevant to the field of asymmetric catalysis.

Generating the Pd-catalyzed δ C-H chalcogenation of aliphatic picolinamides: systematically decreasing the bias

Reaching out to the distal sp3 C-H bonds remains a daunting challenge to synthetic organic chemists, primarily due to the relative inertness of the C-H bonds in alkanes. As such, most reports have envisaged the employment of sterically biased substrates, which render the other possible positions inaccessible for functionalization. Herein, we report a palladium-catalyzed highly selective δ-chalcogenation of aliphatic picolinamides, whereby both sterically biased and relatively unbiased substrates are made feasible for site-selective δ-C-H functionalization. The successful employment of the Thorpe-Ingold effect explains the reactive intermediates involved. The present protocol also provides direct access to the introduction of structural modifications on α-amino acid structural motifs, such as leucine, with high regioselectivity. Sequential hetero-di-functionalization has been carried out at δ-sp3 C-H bonds, resulting in the desymmetrization of quaternary centers. A thorough mechanistic investigation has been carried out, which provided evidence for the reaction pathway and the plausible mechanistic cycle involved.

Synthesis of Axially Chiral Compounds via Transition Metal-Catalyzed Atroposelective C-H Functionalization

ConspectusAxially chiral skeletons are prevalent in natural products and biologically important compounds, and they are widely utilized as privileged scaffolds in enantioselective catalysis. Consequently, the catalytic atroposelective synthesis of enantiopure atropisomers has garnered considerable attention. A variety of synthetic strategies involving metal catalysis or organocatalysis have been developed. Among these elegant approaches, transition metal-catalyzed enantioselective C-H activation has emerged as an atom- and step-economical strategy to streamline the construction of axially chiral compounds in recent years.In this Account, we discuss our efforts in the atroposelective synthesis of different types of axially chiral compounds, including biaryls, atropisomeric styrenes, and C-N atropisomers, via transition metal-catalyzed enantioselective C-H activation strategies. To this end, we have developed several approaches, including the chiral transient directing group (cTDG) strategy using catalytic Pd(OAc)2 and tert-leucine (Tle), as well as catalytic enantioselective systems involving Pd(II)/chiral phosphoric acid (CPA), Pd(II)/l-pyroglutamic acid (pGlu), Pd(0)/norbornene cooperative catalysis with a chiral biimidazoline (BiIM) ligand, and Co(II)/salicyloxazoline (Salox).At the outset, we successfully applied the cTDG strategy to access axially chiral biaryl aldehydes through Pd-catalyzed atroposelective C-H olefination, alkynylation, allylation, naphthylation, and alkylation. The efficacy of these methods has been demonstrated in the enantioselective synthesis of chiral aldehyde catalysts and natural products, such as TAN-1085, (+)-isochizandrin, and (+)-steganone. To facilitate the synthesis of biaryl atropisomers with diverse functionalities, we developed a novel Pd(II)/CPA catalytic system, which enables the preparation of various axially chiral quinolines, biaryl-2-amines, and atropisomeric biaryls bearing chalcogenoether units with high enantioselectivities. The Pd(II)/CPA system also allows for the synthesis of more challenging conjugated diene-based axially chiral styrenes.Nonbiaryl atropisomers, such as axially chiral styrenes and anilides, present synthetic challenges due to their conformational instability and higher degree of rotational freedom compared to their biaryl counterparts. We have addressed these challenges and achieved the highly efficient synthesis of atropisomeric styrenes and anilides using Pd(II)/pGlu and Pd(0)/norbornene/BiIM catalysis. In addition to palladium catalysis, cobalt(II)/Salox catalysis has also been developed for the construction of chiral biaryls, atropisomers with vicinal C-N and C-C stereogenic axes, remote distinct C-N diaxes, and chiral calix[4]arenes featuring both inherent and axial chirality. We anticipate that the enantioselective C-H activation strategy will find broad applications in the construction of synthetically useful axially chiral compounds.

Synthesis of β,β-Dinaphthyl Amino Acids: Towards Molecular Gearing in Peptides and Mini-Proteins

We report the synthesis of β,β-dinaphthyl amino acids by directed CH-activation and their subsequent utilization in peptides. The synthesis of aromatic β-branched amino acids is limited by sterical crowding for large aromatic ring systems. The amino acids are transformed into adequately protected building blocks for solid-phase peptide synthesis by using our amide weakening strategy. Our aim was to study the interactions of the side chain aryl groups in solution and solid state. Based on quantitative analysis of 1H NMR reference nuclei, we investigate conformational equilibria and compare them to established gearing systems.

Chiral Porous Organic Polymers (CPOPs): Design, Synthesis, and Applications in Asymmetric Catalysis

Since the recognition of the area of asymmetric synthesis in 2000, there has been a tremendous focus on the development of heterogeneous catalysts for asymmetric synthesis. Porous organic polymers (POPs) have emerged in recent years as inextricable materials of high physicochemical and hydrolytic stabilities, permitting infinite possibilities to modulate and tune reactivity, engineer porosity, regulate spatial environments and pore attributes, and maneuver material transport. With a diligent design of building blocks and the exploitation of organic reactions judiciously, the synthesis of POPs with BET surface areas of the order of a few thousand cm3/g has been demonstrated. The incorporation of reactive functional groups and chiral centers into the porous matrices of polymers offers opportunities to conduct asymmetric synthesis. Very high enantioselectivities of the order of 99% ee have been exemplified in the reactions mediated by chiral POPs (CPOPs). The design-driven tunability of POPs allows the development of catalytic materials for targeted applications in a tailor-made fashion. This review, while placing the development of chiral materials for asymmetric synthesis in the right perspective, delves into different design principles to pave the way for continued research on futuristic CPOP materials by a creative design, limited by one's imagination, for heretofore unprecedented results.

Ether-Directed Enantioselective C(sp2)-H Borylation for the Synthesis of Axially Chiral Biaryls

We report an ether-directed enantioselective C(sp2)-H borylation catalyzed by a chiral bidentate boryl ligand (CBL)/iridium system for constructing axially chiral biaryls. This method delivered diverse chiral biaryls with good to high enantioselectivities, accommodating varied electronic and steric substituents on the aryl rings. Gram-scale synthesis and downstream transformations of the C-B bond underscored its practicality.

Diastereoselective C(sp3)-H acetoxylation of phosphoramidites

Chiral phosphines are important ligands in asymmetric catalysis, yet their potential as directing groups for asymmetric C-H activation remains unexplored due to the oxidative nature of these reactions. We present a Pd-catalysed, P(III)-directed diastereoselective acetoxylation of phosphoramidites, with DFT calculations elucidating their unique reactivity and supporting the proposed reaction mechanism.

Palladium-Catalyzed ortho C-H Allylation of Tertiary Anilines

Allyl groups could be transformed into various functional groups. A novel and highly regioselective approach for the Pd-catalyzed ortho C-H allylation of tertiary anilines has been developed. Various tertiary anilines and substitution groups were well-tolerated, and allylated linezolid, ibuprofen, naproxen, ketoprofen, and dehydroabietic acid derivatives were easily prepared. Notably, this new method overcomes the limitations of classical amide-directing groups, as the amide groups are well-tolerated in the reaction. Preliminary mechanistic studies revealed that a dual-ligand effect may be involved in achieving excellent ortho selectivity in this reaction facilitated by N-Bz-Gly and Ag2CO3. Further studies indicate that FeCl3 is necessary as a Lewis acid to activate allyl bromide.

Ligand-Enabled Nondirected and Regioselective Arylation of Internal Alkenes with Simple Arenes

Regioselective functionalization of internal alkenes has become a highly efficient approach for preparing stereochemically defined multi-substituted olefins. Unlike traditional methods that require directing groups, activating groups, or active chemical bonds (e.g., halide, pseudo halide, organometallic reagent, etc.), there remains a strong demand for nondirected and selective functionalization of unactivated alkenes with simple coupling partners, both in academic research or industrial applications. Herein, we report the development of a pyridone-oxazoline (Pyoox) type ligand that combines the features of both pyridone and pyridine-oxazoline in assisting Pd-catalyzed olefination. This ligand enables the activation of simple (hetero) arenes and internal alkenes within a single reaction system. A nondirected and regioselective arylation from simple raw materials has been achieved, providing a straightforward route to various trisubstituted olefins in moderate to excellent yields, with excellent regio-/stereocontrol. Experimental and computational studies on mechanisms offer insight into the distinctive properties and performance of this ligand-promoted catalysis. The synthetic utility of this method is further demonstrated by the simplified synthesis and late-stage diversification of bioactive molecules.

Synthesis of benzoheterocycles by palladium-catalyzed migratory cyclization through an unexpected reaction cascade

Migratory functionalization of C-H bonds through metal migration from carbon to carbon under transition metal catalysis is a process of significant academic and industrial interest. Herein, a palladium-catalyzed migratory cyclization of α-bromoalkene derivatives ArXCBr=CH2, in which X denotes a phosphorus (P(O)R), silicon (SiR2), sulfur (SO2), carbon (C(O)), nitrogen (NTs), or oxygen-based moiety, affording various benzoheterocyclic compounds has been developed. Mechanistic investigations have demonstrated that the cyclization reaction proceeds through an unexpected cascade, with trans-1,2-palladium migration between sp2 carbons being a key step of catalytic cycle. To the best of our knowledge, this type of metal migration has not been reported previously.

Arene and Heteroarene Functionalization Enabled by Organic Photoredox Catalysis

ConspectusAromatic functionalization reactions are some of the most fundamental transformations in organic chemistry and have been a mainstay of chemical synthesis for over a century. Reactions such as electrophilic and nucleophilic aromatic substitution (EAS and SNAr, respectively) represent the two most fundamental reaction classes for arene elaboration and still today typify the most utilized methods for aromatic functionalization. Despite the reliable reactivity accessed by these venerable transformations, the chemical space that can be accessed by EAS and SNAr reactions is inherently limited due to the electronic requirements of the substrate. In the case of EAS, highly active electrophiles are paired with electron-neutral to electron-rich (hetero)arenes. For SNAr, highly electron-deficient (hetero)arenes that possess appropriate nucleofuges (halides, -NO2, etc.) are required for reactivity. The inherent limitations on (hetero)arene reactivity presented an opportunity to develop alternative reactivity to access increased chemical space to expand the arsenal of reactions available to synthetic chemists.For the past decade, our research has concentrated on developing novel methods for arene functionalization, with a particular focus on electron-neutral and electron-rich arenes and applying these methods to late-stage functionalization. Specifically, electron-rich arenes undergo single electron oxidation by a photoredox catalyst under irradiation, forming arene cation radicals. These cation radicals act as key intermediates in various transformations. While electron-rich arenes are typically unreactive toward nucleophiles, arene cation radicals are highly reactive and capable of engaging with common nucleophiles.This Account details the dichotomy of reactivity accessed via arene cation radicals: C-H functionalization by nucleophiles under aerobic conditions or cation radical accelerated nucleophilic aromatic substitution (CRA-SNAr) in anaerobic settings. Based on experimental and computational studies, we propose that reversible nucleophilic addition to arene cation radicals can occur at the ipso-, para-, or ortho-positions relative to the most electron-releasing group. Under aerobic conditions, intermediates formed by para- or ortho-addition typically undergo an additional irreversible oxidation step, resulting in C-H functionalization as the major outcome. Conversely, in the absence of an external oxidant, C-H functionalization is not observed, and ipso-addition predominates, releasing an alcohol or HF nucleofuge, leading to SNAr products. Building on the success of these arene functionalization transformations, we also explored their applications to positron emission tomography (PET) radiotracer development. Both C-H functionalization and SNAr with 18F- and 11CN- have been applied to radiofluorination and radiocyanation of arenes, respectively. Applications of the radiotracers synthesized by these methods have been demonstrated in preclinical and clinical models.

P(═O)R2-Directed Asymmetric Catalytic C-H Olefination Leading to C-N Axially Chiral Targets

A novel P(═O)R2-directed asymmetric catalytic olefination has been developed, enabling efficient access to carbon-nitrogen axially chiral products with excellent yields (up to 92%) and enantioselectivity (up to 99% enantiomeric excess). The synergistic coordination of phosphine oxide functionality and l-pGlu-OH with the Pd metal center, serving as an efficient directing group and chiral ligand, was key to the success of this C-H functionalization system. The reaction demonstrated a broad substrate scope, yielding 33 distinct C-N axial products. The absolute configuration of the products was unambiguously confirmed via X-ray diffraction analysis. Additionally, three representative applications were showcased, involving reduction and oxidation to produce chiral phosphines and related derivatives. A plausible catalytic cycle mechanism has been proposed, supported by detailed experimental studies. Aggregates in the system were identified by aggregation-induced polarization experiments.

Cobalt(III)-Catalyzed Enantioselective C-H Functionalization: Ligand Innovation and Reaction Development

ConspectusIn contrast to precious transition metals, such as palladium and rhodium, the development of novel chiral ligands for enantioselective C-H functionalizations catalyzed by earth-abundant, cost-effective, and environmentally friendly 3d metals poses substantial challenges, primarily due to the variable oxidation states, intricate coordination patterns, and limited mechanistic insights. In this Account, we summarize our research endeavors in the development of three novel types of Co(III) catalysis: pseudotetrahedral achiral Cp*Co(III)/chiral carbonyl acid (CCA) catalysis, in situ-generated chiral octahedral cobalt(III) via cobalt/salicyloxazoline (Salox) catalysis, and Co(II)/chiral phosphoric acid (CPA) cooperative catalysis, achieved through strategic chiral ligand design. Our initial objective was to achieve enantioselective C-H functionalization catalyzed by achiral Cp*Co(III) catalysts with external chiral ligands, aiming to circumvent the laborious preparation of chiral CpxCo(III) complexes. To this end, we developed several CCA ligands, incorporating non-covalent interactions (NCIs) as a crucial design element. Next, to address the limitations associated with the lengthy synthesis of Cp-ligated Co(III) complexes and the difficulties of modification, we explored the concept of the in situ generation of Co(III) catalysis using commercially available cobalt(II) salts with tailor-made chiral ligands. This exploration led to the development of two innovative catalytic systems, namely, Co(II)/Salox catalysis and Co(II)/CCA sequential catalysis. The Co(II)/Salox catalysis emerged as a versatile strategy, demonstrating excellent enantioselectivities across a range of asymmetric C-H functionalization reactions to construct various chiral molecules with central, axial, planar, and inherent chirality. The facile synthesis in a single step, along with ease of modification, further enhances the versatility and applicability of this approach. Moreover, we successfully applied cobalt/Salox catalysis in electro- and photochemical-catalyzed enantioselective C-H functionalization, using electrons or oxygen as traceless oxidant, thereby eliminating the need for stoichiometric chemical oxidants. Through mechanistic studies and reaction developments, we elucidated the detailed ligand structure-enantioselectivity relationships in cobalt/Salox catalysis, which are expected to inform future research endeavors. Finally, the Co(II)/CPA cooperative catalysis enabled the synthesis of chiral spiro-γ-lactams through sequential C-H olefination/asymmetric [4 + 1] spirocyclization. Mechanistically, the establishment of stereochemistry occurs during the cyclization step, where the CPA ligand serves as both a neutral ligand and a chiral Brønsted acid, with stereoinduction independent of the C-H cleavage step. We anticipate that the insights and advancements detailed in this Account will inspire further innovations in ligand development and drive progress in the exploration of 3d metal-catalyzed asymmetric C-H functionalization reactions.

Palladium-Catalyzed Cross-Coupling Reactions of Carboranes with Alkenes via Selective B-H Bond Activation

A palladium-catalyzed Heck-type cross-coupling reaction of carboranes with alkenes in 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) was realized. This reaction shows good B(9) selectivity for m-carboranes and is also suitable for o- and p-carborane. Meanwhile, a series of mono-, di-, and trisubstituted alkenes were compatible substrates to afford the alkenylated products in 16-89% yields. The 1,2-bis(carboranyl)ethylene was first synthesized by the reaction of vinyltrimethylsilane and m-carborane. Further transformations of the C═C bond in the product were examined by hydroboration oxidation, oxidation, hydroboration, and bromination reaction to generate corresponding B(9)-functionalized m-carboranes.

Copper/iron controlled regioselective 1,2-carboazidation of 1,3-dienes with acetonitrile and azidotrimethylsilane

Carboazidation and diazidation were carried out on 1,3-diene compounds using TMSN3 as the azide source and MeCN as the cyanoalkylation reagent. This method exhibits excellent functional group tolerance, a broad substrate range, and a short reaction time, providing an effective pathway for synthesizing valuable azides. Our report introduces an unprecedented strategy for the carboazidation and diazidation of 1,3-dienes, with mechanism studies indicating that the reaction involves a radical pathway.

Metallo-azapeptides: Controlled Metal Chelation to Peptide Backbone Nitrogen

We present the first approach to controlled metal chelation of peptide backbones, where the anchoring site is an aza-amino acid nitrogen and the directionality of chelation events is dictated by the acidity of neighboring NHs. Selective backbone chelation precludes the need for metal-binding side chains and/or free N- or C-termini in peptides. We show that the presence and location of an aza-amino acid impact complex formation and report the first X-ray crystal structures of azapeptides bound to palladium and nickel. Evidence of atropisomerism in metallo-azapeptides is also presented.

Enantioselective α-C(sp3)-H Borylation of Masked Primary Alcohols Enabled by Iridium Catalysis

Functional group-directed site- and enantioselective C(sp3)-H functionalization of alcohols or masked alcohols represents a formidable challenge. We herein report the first example of iridium-catalyzed asymmetric α-C(sp3)-H borylation of primary alcohol-derived carbamates by the judicious choice of directing groups. A variety of chiral borylated carbamates were obtained with good to high enantioselectivities. We also demonstrated the synthetic utility by taking advantage of the highly transformable feature of C-B bonds and the leaving ability of carbamates.

Nickel-Catalyzed Atroposelective Carbo-Carboxylation of Alkynes with CO2: En Route to Axially Chiral Carboxylic Acids

Precise synthesis of carboxylic acids via catalytic carboxylation with CO2 is highly appealing. Although considerable advancements have been achieved in difunctionalizing carboxylation of unsaturated hydrocarbons, the asymmetric variants are conspicuously underdeveloped, particularly in addressing axially chiral alkenes. Herein, we report the first catalytic atroposelective carboxylation of alkynes with CO2. A variety of valuable axially chiral carboxylic acids are obtained with good yields and high chemo-, regio-, Z/E and enantio-selectivities. Notably, an unexpected anti-selective carbo-carboxylation is observed in the sp2-hybrid carbo-electrophile-initiated reductive carboxylation of alkynes. Mechanistic studies including DFT calculation elucidate the origin of chiral induction and anti-selectivity in vinyl-carboxylation of alkynes.

Visible-light-driven net-1,2-hydrogen atom transfer of amidyl radicals to access β-amido ketone derivatives

Hydrogen atom transfer (HAT) processes provide an important strategy for selective C-H functionalization. Compared with the popularity of 1,5-HAT processes, however, net-1,2-HAT reactions have been reported less frequently. Herein, we report a unique visible-light-mediated net-1,2-HAT of amidyl radicals for the synthesis of β-amido ketone derivatives. Single-electron transfer (SET) to N-aryloxy amides generates nitrogen-centered radicals (N˙), which undergo a rare net-1,2-HAT to form carbon-centered radicals (C˙). The C-centered radicals are then captured by silyl enol ethers on the way to β-amido ketones. A series of β-amido ketone derivatives (33 examples, up to 97% yield) were prepared with good functional group tolerance demonstrating the synthetic utility of this method. Mechanistic studies, including EPR, radical trapping experiments, deuterium labeling and KIE measurements, suggest an intramolecular radical net-1,2-HAT pathway.

Access to Functionalized Amines and Medium N-Heterocycles via Amine-Enabled Remote C-H Alkynylation

By using weakly coordinating amines, we developed remote C-H alkynylation with precise control of reactivity and regioselectivity, enabling modification of complex drugs, natural products, and materials. The readily transformable alkyne-containing amine products would facilitate expedient delivery of molecular libraries of functionalized amines and medium N-heterocycles, which are previously elusive to access. Moreover, the introduced alkyne functionality could serve as a versatile handle to expand the diversity and synthetic application of this remote C-H functionalization.

Application of the LADA Strategy for the Synthesis of Styrylalanine through Photocatalytic Addition to Alkenylboronic Acids

Unnatural amino acids (UAAs) are highly valuable building blocks in organic synthesis, pharmaceuticals, and material sciences. Previously, we developed the LADA strategy for the synthesis of UAAs. Herein, we further expanded the scope of this strategy to alkenylboronic acids. Interestingly, both photoinduced single-electron transfer (SET) and energy transfer (EnT) processes were involved in this reaction, and the E/Z ratio of the products could be changed upon reaction condition.

Rhodaelectro-Catalyzed Synthesis of Pyrano[3,4-b]indol-1(9H)-ones via the Double Dehydrogenative Heck Reaction between Indole-2-carboxylic Acids and Alkenes

A rhodaelectro-catalyzed double dehydrogenative Heck reaction of indole-2-carboxylic acids with alkenes has been developed for the synthesis of pyrano[3,4-b]indol-1(9H)-ones. The weakly coordinating carboxyl group is utilized twice as a directing group to activate the C-H bonds throughout the reaction. This reaction precedes an acceptorless dehydrogenation under exogenous oxidant-free conditions in an undivided cell with a constant current.

Ru(ii)-catalyzed regioselective oxidative Heck reaction with internal olefins that tolerated strongly coordinating heterocycles

The oxidative Heck reaction of strongly coordinating heterocycles with internal olefins often led to elusive reactivity and regioselectivity. Herein, by judicious choice of X-type directing groups under Ru(ii) catalysis, we achieved the regioselective oxidative Heck reaction of strongly coordinating heterocycles with sterically demanding internal olefins. It was postulated that the "match/mismatch effect" of sterically demanding internal olefins as coupling partners and subsequent kinetically favoured Michael addition or oxidative aromatization act as driving forces to facilitate the desired reactivity and site-selectivity.

Rhodium-catalyzed atropodivergent hydroamination of alkynes by leveraging two potential enantiodetermining steps

A pair of enantiomers is known to have different biological activities. Two catalysts with opposite chirality are nearly always required to deliver both enantiomeric products. In this work, chiral rhodium(III) cyclopentadienyl complexes are repurposed as efficient catalysts for enantiodivergent and atroposelective hydroamination of sterically hindered alkynes. Products with opposite chirality have been both obtained using the same or closely analogous chiral catalyst in good efficiency and excellent enantioselectivity, and the enantiodivergence was mainly enabled by an achiral carboxylic acid and its silver salt. Mechanistic studies revealed the origin of the enantiodivergence ascribable to the switch of the enantiodetermining step (alkyne insertion versus protonolysis) under acid control, which constitutes a previously unidentified working mode of enantiodivergence by leveraging two elementary steps.

Recent advances in the synthesis of N-heterocycles from α-amino acids mediated by iodine

The synthesis of N-heterocycles has received extensive attention from scientists considering their important role in organic synthesis, pharmaceuticals, and materials chemistry. α-Amino acids (α-AAs), both natural and non-natural, are structurally diverse, containing basic amino groups, acidic carboxyl groups, and various side-chain R groups in a single molecule. Given their structural properties and wide range of sources, they have undoubtedly become suitable synthetic building blocks for organic synthesis. However, conventional transformations of amino acids (AAs) focus on the amino and carboxyl groups independently. Conversions for these two prominent functional groups generally do not affect both their alpha positions and their branched chains. Over the past decade, with the application of iodine (I2) in the field of heterocyclic synthesis, the use of α-AAs for diverse construction of complex N-heterocyclic structures has gained increasing attention. This synthetic strategy relies on the I2-mediated Strecker degradation, which introduces α-AAs as electrophilic carbon species into the domino reaction sequence via decarboxylation and deamination processes. In this review, we have summarized recent advances in this emerging area.

Ligand-Controlled Selective Synthesis of Indoles and Benzofurans from Secondary Anilines

A ligand-controlled method for the selective synthesis of indoles and benzofurans from secondary anilines has been discovered. A six-ring palladacycle intermediate may be involved in this process by olefins as a transient directing mediator to give indoles. The indole/benzofuran ratio can be easily tuned by the MPAA ligand. Various substituted secondary anilines were well-tolerated, affording the corresponding products in moderate to good yields. Indole-derived drugs such as JWH-081, BB-22, and ML-098 could be obtained using our new method to achieve their precursors. Preliminary mechanistic studies indicated that BQ is the key factor in avoiding the β-H elimination of insertion species.

Photoinduced Radical Approach for Desulfurative Alkylation of Cysteine Derivatives to Make Unnatural Amino Acids

Unnatural amino acids (UAAs) are highly valuable molecules in organic synthesis, pharmaceutical sciences, and material science. Herein, we present a photocatalytic radical approach for desulfurative alkylation of cysteine derivatives with arenethiol as the hydrogen atom transfer catalyst for making UAAs and peptides. The formate salt, acting as the hydrogen atom donor, in situ generates the highly reductive CO2 radical anion species, which is the key to unlocking the C-S bond cleavage process with a simple benzoyl protecting group. No photocatalyst is required for the radical initiation and propagation, which makes such a visible-light-induced process mild, efficient, and sustainable.

An Approach for Highly Enantioselective Synthesis of meta-Disubstituted [n]Paracyclophanes

Atroposelective synthesis of meta-disubstituted [n]paracyclophanes is a difficult task in organic chemistry. We describe a facile approach for the synthesis of meta-disubstituted [n]paracyclophanes using Pd-catalyzed enantioselective C-H olefination and sequential reductive cleavage. A wide range of [n]paracyclophanes was obtained with excellent enantioselectivity. Thermodynamic analysis revealed that the rotational barrier of meta-disubstituted [n]paracyclophanes was lower than that of para-disubstituted [n]paracyclophanes. The synthesized planar-chiral [14]paracyclophane showed a bright fluorescence emission and impressive circularly polarized luminescence activity.

Photoredox/NHC Dual Catalysis Enabled de Novo Synthesis of α-Amino Acids Derivatives

Herein, we report a mild and operationally simple photoredox/NHC dual catalysis strategy for the α-carboxylation of tertiary amine C(sp3)-H bonds using diethyl pyrocarbonate. This method offers a novel approach for synthesizing α-amino acid derivatives. The protocol features a broad substrate scope, accommodating both N-aryl tetrahydroisoquinolines (THIQ) and N-methyl aniline and is scalable to gram quantities. Additionally, it is suitable for the late-stage derivatization of certain pharmaceutical compounds.

Pd-catalyzed regioselective activation of C(sp2)-H and C(sp3)-H bonds

Differentiating between two highly similar C-H bonds in a given molecule remains a fundamental challenge in synthetic organic chemistry. Directing group assisted strategies for the functionalisation of proximal C-H bonds has been known for the last few decades. However, distal C-H bond functionalisation is strenuous and requires distinctly specialised techniques. In this review, we summarise the advancement in Pd-catalysed distal C(sp2)-H and C(sp3)-H bond activation through various redox manifolds including Pd(0)/Pd(II), Pd(II)/Pd(IV) and Pd(II)/Pd(0). Distal C-H functionalisation, where a Pd-catalyst is directly involved in the C-H activation step, either through assistance of an external directing group or directed by an inherent functionality or functional group incorporated at the site of the Pd-C bond is covered. The purpose of this review is to portray the current state of art in Pd-catalysed distal C(sp2)-H and C(sp3)-H functionalisation reactions, their mechanism and application in the late-stage functionalisation of medicinal compounds along with highlighting its limitations, thus leaving the field open for further synthetic adjustment.

Dative Bonding Activation Enables Precise Functionalization of the Remote B-H Bond of nido-Carborane Clusters

The innovation of synthetic strategies for selective B-H functionalization is a pivotal objective in the realm of boron cluster chemistry. However, the precise, efficient, and rapid functionalization of a B-H bond of carboranes that is distant from the existing functional groups remains intractable owing to the limited approaches for site-selective control from the established methods. Herein, we report a dative bonding activation strategy for the selective functionalization of a nonclassical remote B-H site of nido-carboranes. By leveraging the electronic effects brought by the exopolyhedral B(9)-dative bond, a cross-nucleophile B-H/S-H coupling protocol of the distal B(5)-H bond has been established. The dative bond not only amplifies the subtle reactivity difference among B-H bonds but also significantly changes the reactive sites, further infusing nido-carboranes with additional structural diversity. This reaction paradigm features mild conditions, rapid conversion, efficient production, broad scope, and excellent group tolerance, thus enabling the applicability to an array of complex bioactive molecules. The efficient and scalable reaction platform is amenable to the modular construction of photofunctional molecules and boron delivery agents for boron neutron capture therapy. This work not only provides an unprecedented solution for the selective diversification of distal B-H sites in nido-carboranes but also holds the potential for expediting the discovery of novel carborane-based functional molecules.

Cobalt-catalyzed three-component assembly of aromatic oximes with substituted dienes and formaldehyde

A cobalt-catalyzed three-component assembly of substituted aryl oximes with dienes and formaldehyde via C-H bond activation is described. This protocol affords highly regio- and chemoselective substituted homoallylic alcohols with moderate-to-excellent yields. The scope of this protocol has been extensively explored with various substituted aryl ketoximes and aldoximes. Butadiene and internally substituted dienes are also well compatible for this transformation. A plausible reaction mechanism is proposed to account for the present reaction and is supported by deuterium labeling studies.

Dual Ligand Enabled Pd-Catalyzed Ortho-Alkylation of Iodoarenes

The synthesis of complex polysubstituted aromatic molecules from simple precursors is a central goal in organic chemistry. In this study, we developed an approach for the ortho-alkylation of iodoarenes utilizing a dual ligand catalytic system. By combining Pd/olefin ligand cooperative catalysis with bulky trialkylphosphine ligand-promoted C(sp2)-I reductive elimination, we have established an ortho-alkylative Catellani-type reaction with the aryl-iodine bond reconstruction as the final step, which opens new synthetic opportunities within the Catellani-type reactions. Through in-depth mechanistic investigations, we have isolated and characterized key organopalladium intermediates, revealing the synergistic interaction of the dual ligands in merging the Catellani-type process with C(sp2)-I reductive elimination. The present study showcases the unique advantages of Pd/olefin ligand catalysis and emphasizes the effectiveness of the dual ligand system in expanding the chemical space of the Catellani chemistry.

Catalytic Asymmetric Synthesis of Unnatural Axially Chiral Biaryl δ-Amino Acid Derivatives via a Chiral Phenanthroline-Potassium Catalyst-Enabled Dynamic Kinetic Resolution

Axially chiral biaryl δ-amino acids possess significantly different conformational properties and chiral environment from centrally chiral amino acids, therefore, have drawn considerable attention in the fields of synthetic and medicinal chemistry. Herein, a novel chiral phenanthroline-potassium catalyst has been developed by constructing a well-organized axially chiral ligand composed of one 1,10-phenanthroline unit and two axially chiral 1,1'-bi-2-naphthol (BINOL) units. In the presence of this catalyst, good to excellent yields and enantioselectivities (up to 99 % yield, 98 : 2 er) have been achieved in the ring-opening alcoholytic dynamic kinetic resolution of a variety of biaryl lactams, thereby providing an efficient protocol for catalytic asymmetric synthesis of unnatural axially chiral biaryl δ-amino acid derivatives.

Nickel(II)/BINOL-catalyzed enantioselective C-H activation via desymmetrization and kinetic resolution

The field of nickel catalysis has witnessed remarkable growth in recent years. However, the use of nickel catalysts in enantioselective C-H activation remains a daunting challenge because of their variable oxidation states, intricate coordination chemistry, and unpredictable reactivity patterns. Herein, we report an enantioselective C-H activation reaction catalyzed by commercially available and air-stable nickel(II) catalyst. Readily available and simple (S)-BINOL is used as a chiral ligand. This operationally simple protocol enables the synthesis of planar chiral metallocenes in high yields with excellent enantioselectivity through desymmetrization and kinetic resolution. Air-stable planar chiral nickelacycle intermediates are first synthesized via enantioselective C-H nickelation and shown to be possible intermediates of the reaction. Deuterium-labeling studies, alongside the characterization and transformation of chiral nickel(II) species, suggest that C-H cleavage is the enantio-determining step. Moreover, the large-scale synthesis and diverse synthetic transformations underscore the practicality of this protocol.

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

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

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

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

Rhodium-Catalyzed Direct ortho-Arylation of Anilines

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

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

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

Chiral amino acids: evolution in atroposelective C-H activation

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

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

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

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

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

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

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

Palladium (II)-Catalyzed C-H Activation with Bifunctional Ligands: From Curiosity to Industrialization

In 2001, our curiosity to understand the stereochemistry of C-H metalation with Pd prompted our first studies in Pd(II)-catalyzed asymmetric C-H activation (RSC Research appointment: 020 7451 2545, Grant: RG 36873, Dec. 2002). We identified four central challenges: 1. poor reactivity of simple Pd salts with native substrates; 2. few strategies to control site selectivity for remote C-H bonds; 3. the lack of chiral catalysts to achieve enantioselectivity via asymmetric C-H metalation, and 4. low practicality due to limited coupling partner scope and the use of specialized oxidants. These challenges necessitated new strategies in catalyst and reaction development. For reactivity, we developed approaches to enhance substrate-catalyst affinity together with novel bifunctional ligands which participate in and accelerate the C-H cleavage step. For site-selectivity, we introduced the concept of systematically modulating the distance and geometry between a directing template, catalyst, and substrate to selectively access remote C-H bonds. For enantioselectivity, we devised predictable stereomodels for catalyst-controlled enantioselective C-H activation based on the participation of bifunctional ligands. Finally, for practicality, we have developed varied catalytic manifolds for Pd(II) to accommodate diverse coupling partners while employing practical oxidants such as simple peroxides. These advances have culminated in numerous C-H activation reactions, setting the stage for broad industrial applications.

Enantioselective organocatalytic strategies to access noncanonical α-amino acids

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

Fun With Unusual Functional Groups: Sulfamates, Phosphoramidates, and Di-tert-butyl Silanols

Compared to ubiquitous functional groups such as alcohols, carboxylic acids, amines, and amides, which serve as central "actors" in most organic reactions, sulfamates, phosphoramidates, and di-tert-butyl silanols have historically been viewed as "extras". Largely considered functional group curiosities rather than launch-points of vital reactivity, the chemistry of these moieties is under-developed. Our research program has uncovered new facets of reactivity of each of these functional groups, and we are optimistic that the chemistry of these fascinating molecules can be developed into truly general transformations, useful for chemists across multiple disciplines. In the ensuing sections, I will describe our efforts to develop new reactions with these "unusual" functional groups, namely sulfamates, phosphoramidates, and di-tert-butyl silanols.

Remote-carbonyl-directed sequential Heck/isomerization/C(sp2)-H arylation of alkenes for modular synthesis of stereodefined tetrasubstituted olefins

Modular and regio-/stereoselective syntheses of all-carbon tetrasubstituted olefins from simple alkene materials remain a challenging project. Here, we demonstrate that a remote-carbonyl-directed palladium-catalyzed Heck/isomerization/C(sp2)-H arylation sequence enables unactivated 1,1-disubstituted alkenes to undergo stereoselective terminal diarylation with aryl iodides, thus offering a concise approach to construct stereodefined tetrasubstituted olefins in generally good yields under mild conditions; diverse carbonyl groups are allowed to act as directing groups, and various aryl groups can be introduced at the desired position simply by changing aryl iodides. The stereocontrol of the protocol stems from the compatibility between the E/Z isomerization and the alkenyl C(sp2)-H arylation, where the vicinal group-directed C(sp2)-H arylation of the Z-type intermediate product thermodynamically drives the reversible E to Z isomerization. Besides, the carbonyl group not only promotes the Pd-catalyzed sequential transformations of unactivated alkenes by weak coordination, but also avoids byproducts caused by other possible β-H elimination.