Carbene-Catalyzed Asymmetric Construction of Atropisomers

A Chemically Powered Rotary Molecular Motor Based on Reversible Oxazepine Formation

While biological machines are powered mainly by chemical transformations, chemically driven artificial rotary motor systems are very limited. Here, we report an aniline-phenol-based rotary molecular motor that operates via an information ratchet mechanism. The 360° directional rotation about a single covalent bond can be chemically driven by reversible oxazepine formation. Both the oxazepine formation and hydrolysis steps are kinetically gated via dynamic kinetic resolution, arising from the kinetic bias of chiral catalysts for enantiomers. Given the 95 % ee (97.5 : 2.5) and 88 % ee (94 : 6) of the individual gating steps of motor analogues, the overall directionality ratio could be calculated to be 91.7 : 8.3 (97.5 %×94 %≈91.7 %), which means that the motor will make one mistake (backward rotation) approximately every 11 to 12 turns.

Divergent Synthesis of Axially Chiral 2-Pyranones and Fused 2-Pyridones via N-Heterocyclic Carbene-Catalyzed Atroposelective [3 + 3] Annulation

An N-heterocyclic carbene-catalyzed atroposelective [3 + 3] annulation of alkynyl acylazoliums with benzothiazole derivatives has been developed for the divergent synthesis of axially chiral triaryl 2-pyranones and fused 2-pyridones. The regioselectivity of this protocol depends on the structure of benzothiazoles with three different nucleophilic centers. The obtained axially chiral frameworks represent a new class of arylheterocycle atropisomers, which may be potentially useful in medicinal chemistry.

Biocatalytic Desymmetrization for the Atroposelective Synthesis of Axially Chiral Biaryls Using an Engineered Imine Reductase

The enzymatic atroposelective synthesis of biaryl compounds is relatively rare, despite considerable attention received by biocatalysis in the academic and industrial sectors. Imine reductases (IREDs) are an important class of enzymes that have been applied in the asymmetric synthesis of chiral amine building blocks. In this study, two IREDs (IR140 and IR189) were identified to catalyze the efficient desymmetrization of biaryls utilizing various amine donors. Further protein engineering enabled the identification of variants (IR189 M8-M9 and IR189 M13-M14) that are able to catalyze the formation of both (R) and (S) atropisomers in excellent yields and atroposelectivities (24 examples, up to 99 % ee and yield). The absolute configuration and rotational barriers were confirmed, and the reactions were readily scaled up to allow isolation of the atropisomeric product in 99 % ee and 82 % yield. The optically pure biaryl amines were further derivatized into various synthetically useful atropisomers. To shed light on the molecular recognition mechanisms, molecular dynamics (MD) simulations were performed, offering plausible explanations for the improved atroposelectivity and enzymatic activity. The current strategy expands the scope of the IRED-catalyzed synthesis of axially chiral biaryl amines, contributing significantly to the field of atroposelective biocatalysis.

NHC-Catalyzed Aldimine Umpolung/6π-Electrocyclization Cascade to Access Tetracyclic Dihydrochromeno Indoles

The umpolung of aldimines using N-heterocyclic carbenes (NHCs) is less explored compared to the established polarity reversal of aldehydes. Described herein is an NHC-catalyzed imine umpolung /6π-electrocyclization cascade, which leads to the atom- and pot-economic synthesis of biologically important dihydrochromeno indoles. For the first time, the nucleophilic aza-Breslow intermediates have been intercepted with unactivated alkynes. Preliminary mechanistic and DFT studies shed light on the role of the phenolic -OH moiety in promoting the addition of the aza-Breslow intermediate to the unactivated alkyne via an intramolecular proton transfer in a stepwise manner. DFT studies also support the regioselectivity preference for the 5-exo-dig cyclization pathway, leading to the exclusive formation of the indole products. Moreover, a comparison of Gibbs free energies provides insight into a thermodynamically preferred 6π-electrocyclization over a competing oxa-Michael pathway. Further, this strategy is applied to the formal synthesis of a Hepatitis C Virus (HCV) NS5A inhibitor in a step-economical method.

Recent advances in organocatalytic atroposelective reactions

Axial chirality is present in a variety of naturally occurring compounds, and is becoming increasingly relevant also in medicine. Many axially chiral compounds are important as catalysts in asymmetric catalysis or have chiroptical properties. This review overviews recent progress in the synthesis of axially chiral compounds via asymmetric organocatalysis. Atroposelective organocatalytic reactions are discussed according to the dominant catalyst activation mode. For covalent organocatalysis, the typical enamine and iminium modes are presented, followed by N-heterocyclic carbene-catalyzed reactions. The bulk of the review is devoted to non-covalent activation, where chiral Brønsted acids feature as the most prolific catalytic structure. The last part of the article discusses hydrogen-bond-donating catalysts and other catalyst motifs such as phase-transfer catalysts.

Biaxially chiral compounds: research and development of synthesis

With the rapid advancements in asymmetric catalysis, there is a growing need for the asymmetric selective synthesis of complex and diverse molecules with chiral axes. Axially chiral molecules are not only present in natural products and drugs but also in specialized chiral ligands and catalysts. The catalytic asymmetric synthesis of axially chiral molecules has garnered significant attention within the chemical community. To date, two asymmetric catalytic methods have been established for the synthesis of biaxially chiral molecules: transition-metal catalysis and asymmetric organocatalysis. This feature article presents a summary of the research progress made in the field of asymmetric selective synthesis of biaxially chiral molecules. This review focuses on the reaction range, limitations, and reaction mechanism of the biaxially chiral molecular synthesis method, as well as the contribution and advantages of this method in the preparation of various axially chiral compounds.

Synthesis of Homochiral N-Heterocyclic Carbene-Based Nanosheets for Enhanced Asymmetric Catalysis

Utilizing ultrathin 2D metal-organic framework nanosheets (2D MONs) as supports for incorporating chiral catalysts represents a highly promising avenue in the field of asymmetric catalysis. In this study, four pairs of isostructural chiral metal-organic layers (MOLs) adorned with N-Heterocyclic carbene (NHC) groups are successfully synthesized. Notably, the obtained bulky (S)-1-Zn crystals can be readily delaminated into ultrathin MONs consisting of 1-2 layers through an ion intercalation-assisted exfoliation process. Subsequent asymmetric catalysis studies revealed that the NHC sites can be effectively activated by a proton sponge while maintaining structural integrity for the subsequent benzoin condensation. Due to their well-exposed catalytic sites, ultrathin morphology, and porous structure, the (S)-1-Zn nanosheets exhibited significantly enhanced yield and enantioselectivity compared to their bulk counterparts and organic precursors. This research highlights an efficient strategy for incorporating chiral NHC species onto 2D MONs, thereby unlocking their immense potential for heterogeneous asymmetric catalysis.

Tunable Thiazolium Carbenes for Enantioselective Radical Three-Component Dicarbofunctionalizations

Asymmetric N-heterocyclic carbene (NHC) organocatalysis is a cornerstone of synthetic organic chemistry. The emerging concept of single-electron NHC catalysis broadened the scope of C-C bond-forming reactions, facilitating the synthesis of a variety of attractive racemic compounds. However, the development of effective and selective chiral NHC catalysts for asymmetric radical-mediated reactions has been challenging. In this report, we introduce a family of highly tunable chiral thiazolium carbenes with three distinct positions for broad electronic and steric modulation featuring bulky chiral flanking groups. We demonstrate the catalytic efficacy of these chiral carbenes in an enantioselective SET-type three-component acyl-difluoroalkylation of olefins using a broad range of aldehydes and difluoroalkyl bromides. This method provides straightforward access to a diverse set of β-difluoroalkylated α-chiral ketones (65 examples) with an up to 87% yield and excellent enantioselectivities of up to >99:1 er. The utility of this methodology is further outlined by enantio- and diastereoselective late-stage modifications of pharmaceutically relevant compounds and selective twofold orthogonal acyl-difluoroalkylations of linchpin reagents.

N-Heterocyclic carbene (NHC) organocatalysis: from fundamentals to frontiers

N-Heterocyclic carbenes (NHCs) have been used as organocatalysts for a multitude of C-C and C-heteroatom bond-forming reactions. They enable diverse modalities of activating a wide range of structurally distinct substrate classes and allow access to electronically distinct intermediates. The easy tunability of the NHC scaffold contributes to its versatility. Recent years have witnessed a surge of interest in various organocatalytic reactions of NHCs, leading to the forays of NHC catalysis into the relatively newer domains such as reactions involving radical intermediates, atroposelective synthesis, umpolung of electrophiles other than aldehydes, and the use of NHCs as non-covalent templates for enantioinduction. This tutorial review provides an overview of various important structural features and reactivity modes of NHCs and delves deep into some frontiers of NHC-organocatalysis.

Organocatalytic Asymmetric Construction and Application of Axially Chiral Spiro-bisindoles

Spiro skeletons have emerged as a privileged class of chiral carriers across various research fields, including asymmetric catalysis and functional materials, due to their remarkable configurational rigidity. However, limited structural diversity of spiro frameworks significantly restricts the expansion of their applications. Here we present a new class of axially chiral spiro-bisindole frameworks and report their first enantioselective construction via a chiral phosphoric acid-catalyzed intramolecular dehydrative cyclization reaction. Unlike the classical SPINOL backbone, incorporation of indole moieties in place of phenol enhances the nucleophilicity of ketone substrates, thereby eliminating the need for a tedious pre-activation process. By leveraging the retained active sites of indole, the resulting highly enantioenriched spiro-bisindoles can be rapidly transformed into other valuable structures. More importantly, axially chiral fluorescent molecules with good asymmetry factors and quantum fluorescence efficiency are readily accessed, opening a new avenue for developing chiral fluorescent materials. Control experiments demonstrate the pivotal role of both unmasked N-H bonds in achieving good efficiency and enantiocontrol.

Construction of Axially Chiral 4-Aminoquinolines by Cycloaddition and Central-to-Axial Chirality Conversion

A two-step strategy has been established for the enantioselective synthesis of 4-aminoquinolines possessing axial chirality. This approach involves a chiral phosphoric acid-catalyzed cycloaddition, followed by a DDQ oxidation step. The method offers efficient access to a variety of 1,1'-biaryl-2,2'-amino alcohol derivatives in excellent yields and enantioselectivities (up to 98% yield and 93% ee). Furthermore, the synthetic transformation of the products was also investigated.

N-Heterocyclic Carbene Catalysis for Polycyclic Benzazepines Assembly: Regioselective Intramolecular Tandem Radical Cyclization

A variety of polycyclic benzazepines were rapidly constructed by NHC-catalyzed regioselective redox-neutral intramolecular tandem cyclization. Initial mechanistic studies revealed that a SET radical process was possibly involved.

Halogen-Bond-Assisted NHC-Catalyzed (Dynamic) Kinetic Resolution for the Atroposelective Synthesis of Heterobiaryls

We report a novel halogen-bond-assisted NHC-catalyzed (dynamic) kinetic resolution strategy for the synthesis of axially chiral heterobiaryls. A class of axially chiral quinolines are prepared efficiently in excellent enantioselectivities (≤98% ee) employing 3-5 mol % NHC catalyst. Mechanistic studies reveal the indispensability of 5-bromo-2-iodobenzaldehyde in this reaction, in which a pivotal halogen bonding interaction plays a crucial role in the process.

N-Heterocyclic Carbene Enabled Functionalization of Inert C(Sp3)-H Bonds via Hydrogen Atom Transfer (HAT) Processes

Developing methods to directly transform C(sp3) -H bonds is crucial in synthetic chemistry due to their prevalence in various organic compounds. While conventional protocols have largely relied on transition metal catalysis, recent advancements in organocatalysis, particularly with radical NHC catalysis have sparked interest in the direct functionalization of "inert" C(sp3) -H bonds for cross C-C coupling with carbonyl moieties. This strategy involves selective cleavage of C(sp3) -H bonds to generate key carbon radicals, often achieved via hydrogen atom transfer (HAT) processes. By leveraging the bond dissociation energy (BDE) and polarity effects, HAT enables the rapid functionalization of diverse C(sp3)-H substrates, such as ethers, amines, and alkanes. This mini-review summarizes the progress in carbene organocatalytic functionalization of inert C(sp3)-H bonds enabled by HAT processes, categorizing them into two sections: 1) C-H functionalization involving acyl azolium intermediates; and 2) functionalization of C-H bonds via reductive Breslow intermediates.

Ionic Hydrogen Bond-Assisted Catalytic Construction of Nitrogen Stereogenic Center via Formal Desymmetrization of Remote Diols

The control of noncarbon stereogenic centers is of profound importance owing to their enormous interest in bioactive compounds and chiral catalyst or ligand design for enantioselective synthesis. Despite various elegant approaches have been achieved for construction of S-, P-, Si- and B-stereocenters over the past decades, the catalyst-controlled strategies to govern the formation of N-stereogenic compounds have garnered less attention. Here, we disclose the first organocatalytic approach for efficient access to a wide range of nitrogen-stereogenic compounds through a desymmetrization approach. Intriguingly, the pro-chiral remote diols, which are previously not well addressed with enantiocontrol, are well differentiated by potent chiral carbene-bound acyl azolium intermediates. Preliminary studies shed insights on the critical importance of the ionic hydrogen bond (IHB) formed between the dimer aggregate of diols to afford the chiral N-oxide products that feature a tetrahedral nitrogen as the sole stereogenic element with good yields and excellent enantioselectivities. Notably, the chiral N-oxide products could offer an attractive strategy for chiral ligand design and discovery of potential antibacterial agrochemicals.

N-heterocyclic carbene-catalyzed atroposelective synthesis of N-Aryl phthalimides and maleimides via activation of carboxylic acids

Traditionally, N-aryl phthalimides are synthesized by the condensation of phthalic anhydride and aniline derivatives, usually proceeding under harsh conditions. The alternative mild and organocatalytic strategies for their synthesis are underdeveloped. Herein, we demonstrate the organocatalytic atroposelective synthesis of N-aryl phthalimides via the traditional N-CC=O disconnection under mild conditions. The in-situ acid activation of phthalamic acid and subsequent N-heterocyclic carbene (NHC)-catalyzed atroposelective amidation allowed the synthesis of well-decorated N-aryl phthalimides in excellent yields and enantioselectivities. Mechanistic studies reveal the addition of NHC to the in situ generated isoimides, thus introducing a unique mode of generating acylazoliums. Interestingly, both enantiomers of the product can be accessed from the same phthalic anhydride and aniline using the same NHC pre-catalyst. Moreover, this strategy has been extended to the atroposelective synthesis of N-aryl maleimides.

Enantioselective synthesis of 4-aryl-3,4-dihydrocoumarins via N-heterocyclic carbene catalyzed β-arylation/cyclization of α-bromoenals

4-Aryl-3,4-dihydrocoumarins are one of the most important structural motifs. Herein, we disclose an enantioselective N-heterocyclic carbene catalyzed β-arylation/cyclization of α-bromoenals with 3-aminophenols under mild conditions. The protocol allows for the rapid preparation of 4-aryl-3,4-dihydrocoumarins in acceptable yields with good enantioselectivities. The products of this reaction could be converted into chiral diarylpropanoic acid derivatives without erosion of the enantioselectivity.

N-Heterocyclic carbene/palladium synergistic catalysis in organic synthesis

The cooperation of two distinct catalytic cycles to activate different reactive centers leading to a chemical transformation has been classified as synergistic catalysis. The synergistic combination of NHC with palladium catalysis has emerged as a powerful strategy in the last few years. Merging the ability of NHCs to inverse the polarity of a functional group with the unique reactivity of palladium enables transformations that cannot be accomplished by either of these catalysts alone. Despite the associated challenges, such as quenching of catalysts, reactivity mismatch etc., significant development has been achieved in the field of NHC/Pd synergistic catalysis. The recent incorporation of photoredox catalysis with NHC/Pd synergistic catalysis has further advanced this area. This review highlights the developments made in the area of NHC/Pd synergistic catalysis.

Carbene-Catalyzed Atroposelective Construction of Chiral Diaryl Ethers

Atropoisomeric chemotypes of diaryl ethers-related scaffolds are prevalent in naturally active compounds. Nevertheless, there remains considerable research to be carried out on the catalytic asymmetric synthesis of these axially chiral molecules. In this instance, we disclose an N-heterocyclic carbene (NHC)-catalyzed synthesis of axially chiral diaryl ethers via atroposelective esterification of dialdehyde-containing diaryl ethers. NHC desymmetrization produces axially chiral diaryl ether atropisomers with high yields and enantioselectivities in moderate circumstances. Chiral diaryl ether compounds may be precursors for highly functionalized diaryl ethers with bioactivity and chiral ligands for asymmetric catalysis.

Construction of Highly Functionalized 2-Styrylfurans by N-Heterocyclic Carbene/Brønsted Acid Catalysis

This research presents an original method for synthesizing styrylfurans using N-heterocyclic carbenes (NHCs) and Brønsted acid catalysis. By exploiting 2,4-dioxoesters as conjugated 1,3-dicarbonyls, we have developed a technique allowing the efficient formation of highly functionalized styrylfurans with interesting photochemical properties, through a NHC-catalyzed cross-benzoin reaction followed by a Brønsted acid-driven Paal-Knorr-like condensation. This approach permits the integration of various substituents on the furan ring, with preliminary biological studies indicating potential as fluorescent dyes.

Recent Advances in NHC-Catalyzed Chemoselective Activation of Carbonyl Compounds

N-Heterocyclic carbenes (NHCs) catalysts have been employed as effective tools in the development of various reactions, which have made notable contributions in developing diverse reaction modes and generating significant functionalized molecules. This review provides an overview of the recent advancements in the chemo- and regioselective activation of different aldehydes using NHCs, categorized into five parts based on the different activation modes. A brief conclusion and outlook is provided to stimulate the development of novel activation modes for accessing functional molecules.

Enantioselective synthesis of tricyclic oxoquinolines via NHC-catalyzed Michael-aldol-lactamization-dehydration cascade

The enantioselective synthesis of tricyclic oxoquinolines via NHC-catalyzed cascade reaction of enals with malonates bearing a 2-aminophenyl group is reported. The chiral α,β-unsaturated acylazoliums underwent a smooth Michael-aldol-lactamization-dehydration quadruple cascade with the amino malonate derivative to afford the desired tricyclic products.

Organocatalytic desymmetrization provides access to planar chiral [2.2]paracyclophanes

Planar chiral [2.2]paracyclophanes consist of two functionalized benzene rings connected by two ethylene bridges. These organic compounds have a wide range of applications in asymmetric synthesis, as both ligands and catalysts, and in materials science, as polymers, energy materials and dyes. However, these molecules can only be accessed by enantiomer separation via (a) time-consuming chiral separations and (b) kinetic resolution approaches, often with a limited substrate scope, yielding both enantiomers. Here, we report a simple, efficient, metal-free protocol for organocatalytic desymmetrization of prochiral diformyl[2.2]paracyclophanes. Our detailed experimental mechanistic study highlights differences in the origin of enantiocontrol of pseudo-para and pseudo-gem diformyl derivatives in NHC catalyzed desymmetrizations based on whether a key Breslow intermediate is irreversibly or reversibly formed in this process. This gram-scale reaction enables a wide range of follow-up derivatizations of carbonyl groups, producing various enantiomerically pure planar chiral [2.2]paracyclophane derivatives, thereby underscoring the potential of this method.

N-Heterocyclic carbene-catalyzed enantioselective synthesis of planar-chiral cyclophanes via dynamic kinetic resolution

Planar-chiral cyclophanes have gained considerable concerns for drug discovery due to their unique conformational strain and 3D structure. However, the enantioselective synthesis of planar-chiral cyclophanes is a long-standing challenge for the synthetic community. We herein describe an N-heterocyclic carbene (NHC)-catalyzed asymmetric construction of planar-chiral cyclophanes. This transformation occurs through a dynamic kinetic resolution (DKR) process to convert racemic substrates into planar-chiral macrocycle scaffolds in good to high yields with high to excellent enantioselectivities. The ansa chain length and aromatic ring substituent size is crucial to achieve the DKR approach. Controlled experiments and DFT calculations were performed to clarify the DKR process.

Atroposelective Synthesis of Axially Chiral Diaryl Ethers by N-Heterocyclic-Carbene-Catalyzed Sequentially Desymmetric/Kinetic Resolution Process

We describe herein an N-heterocyclic-carbene-catalyzed atroposelective synthesis of axially chiral diaryl ethers. Through a sequentially enantioselective desymmetric process and a kinetic resolution process, the products could be constructed in good yields with excellent enantiopurities. Both alcohols and phenols were compatible with this catalytic system. The axially chiral carboxylic acids derived from the esters were proven to be potential chiral ligands for asymmetric synthesis, for example, Rh(III)-catalyzed enantioselective C-H functionalization.

N-Heterocyclic Carbene-Catalyzed Regio- and Enantioselective C7-Alkylation of 4-Aminoindoles with α-Bromoenals

The first carbene-catalyzed regio- and enantioselective indole C7-alkylation reaction between 4-aminoindoles and α-bromoenals is disclosed. The corresponding indole products could be obtained in moderate to good yields with good to excellent enantioselectivities. The evaluation of antibacterial activity against Psa revealed that nine of the C7-functionalized indoles exhibited superior inhibitory activities compared to the positive controls TC and BT. Our approach provides an efficient strategy to introduce a chiral chain into the C7 position of indole compounds, with potential applications evaluated in pesticide development.

Atroposelective Synthesis of Aldehydes via Alcohol Dehydrogenase-Catalyzed Stereodivergent Desymmetrization

Axially chiral aldehydes have emerged recently as a unique class of motifs for drug design. However, few biocatalytic strategies have been reported to construct structurally diverse atropisomeric aldehydes. Herein, we describe the characterization of alcohol dehydrogenases to catalyze atroposelective desymmetrization of the biaryl dialdehydes. Investigations into the interactions between the substrate and key residues of the enzymes revealed the distinct origin of atroposelectivity. A panel of 13 atropisomeric monoaldehydes was synthesized with moderate to high enantioselectivity (up to >99% ee) and yields (up to 99%). Further derivatization allows enhancement of the diversity and application potential of the atropisomeric compounds. This study effectively expands the scope of enzymatic synthesis of atropisomeric aldehydes and provides insights into the binding modes and recognition mechanisms of such molecules.

Interrupted N-Heterocyclic Carbene-Catalyzed Radical Coupling Strategy: A Versatile Platform for Alkylation and Arylation of [60]Fullerene

An interrupted N-heterocyclic carbene-catalyzed radical coupling strategy is disclosed for efficient alkylation and arylation of [60]fullerene. This novel and general strategy bridges the gap between organocatalytic radical cross-coupling and functionalization of fullerenes. Readily available feedstocks, remarkably broad substrate scope and functional group compatibility, and convenient late-stage nanomodification of complex molecules make this strategy with incomparable diversity and practicality in the synthesis of monoalkylated and -arylated fullerenes.

N-Heterocyclic Carbene-Catalyzed Highly Enantioselective Macrolactonization to Access Planar-Chiral Macrocycles

An N-heterocyclic carbene (NHC)-catalyzed atroposelective macrolactonization has been disclosed. This approach affords planar-chiral macrocycles in high yields with excellent enantioselectivities over a broad substrate scope. Controlled experiments suggest that the enantioselectivity might arise from the cation-n interaction between the acyl azolium and the electron-rich moiety in the substrate. This mechanism is supported by density functional theory calculations, which also suggest an important π-π interaction in stabilizing the transition state.

Enantioselective Synthesis of Axially Chiral Diaryl Ethers via NHC Catalyzed Desymmetrization and Following Resolution

Axially chiral diaryl ethers are present in numerous natural products and bioactive molecules. However, only few catalytic enantioselective approaches have been established to access diaryl ether atropisomers. Herein, we report the N-heterocyclic carbene-catalyzed enantioselective synthesis of axially chiral diaryl ethers via desymmetrization of prochiral 2-aryloxyisophthalaldehydes with aliphatic alcohols, phenol derivatives, and heteroaromatic amines. This reaction features mild reaction conditions, good functional group tolerance, broad substrate scope and excellent enantioselectivity. The utility of this methodology is illustrated by late-stage functionalization, gram-scale synthesis, and diverse enantioretentive transformations. Control experiments and DFT calculations support the association of NHC-catalyzed desymmetrization with following kinetic resolution to enhance the enantioselectivity.

Organocatalytic diastereo- and atroposelective construction of N-N axially chiral pyrroles and indoles

The construction of N-N axially chiral motifs is an important research topic, owing to their wide occurrence in natural products, pharmaceuticals and chiral ligands. One efficient method is the atroposelective dihydropyrimidin-4-one formation. We present herein a direct catalytic synthesis of N-N atropisomers with simultaneous creation of contiguous axial and central chirality by oxidative NHC (N-heterocyclic carbenes) catalyzed (3 + 3) cycloaddition. Using our method, we are able to synthesize structurally diverse N-N axially chiral pyrroles and indoles with vicinal central chirality or bearing a 2,3-dihydropyrimidin-4-one moiety in moderate to good yields and excellent enantioselectivities. Further synthetic transformations of the obtained axially chiral pyrroles and indoles derivative products are demonstrated. The reaction mechanism and the origin of enantioselectivity are understood through DFT calculations.

Amide C-N bonds activation by A new variant of bifunctional N-heterocyclic carbene

We report an organocatalyst that combines a triazolium N-heterocyclic carbene (NHC) with a squaramide as a hydrogen-bonding donor (HBD), which can effectively catalyze the atroposelective ring-opening of biaryl lactams via a unique amide C-N bond cleavage mode. The free carbene species attacks the amide carbonyl, forming an axially chiral acyl-azolium intermediate. Various axially chiral biaryl amines can be accessed by this methodology with up to 99% ee and 99% yield. By using mercaptan as a catalyst turnover agent, the resulting thioester synthon can be transformed into several interesting atropisomers. Both control experiments and theoretical calculations reveal the crucial role of the hybrid NHC-HBD skeleton, which activates the amide via H-bonding and brings it spatially close to the carbene centre. This discovery illustrates the potential of the NHC-HBD chimera and demonstrates a complementary strategy for amide bond activation and manipulation.

Enantioselective Synthesis of C-O Axially Chiral Diaryl Ethers by NHC-Catalyzed Atroposelective Desymmetrization

Axially chiral diaryl ethers, a distinguished class of atropisomers possessing unique dual C-O axis, hold immense potential for diverse research domains. In contrast to the catalytic enantioselective synthesis of conventional single axis bearing atropisomers, the atroposelective synthesis of axially chiral ethers containing flexible C-O axis remains a significant challenge. Herein, we demonstrate the first N-heterocyclic carbene (NHC)-catalyzed synthesis of axially chiral diaryl ethers via atroposelective esterification of dialdehyde-containing diaryl ethers. Mechanistically, the reaction proceeds via NHC-catalyzed desymmetrization strategy to afford the corresponding axially chiral diaryl ether atropisomers in good yields and high enantioselectivities under mild conditions. The derivatization of the synthesized product expands the utility of present strategy via access to a library of C-O axially chiral compounds. The temperature dependency and preliminary investigations on the racemization barrier of C-O bonds are also presented.

Cobalt-Catalyzed Asymmetric Reductive Alkenylation and Arylation of Heterobiaryl Tosylates: Kinetic Resolution or Dynamic Kinetic Resolution?

Herein, we report a cobalt-catalyzed atroposelective reductive cross-coupling of racemic heterobiaryl tosylates with a C(sp2)-X type electrophile. Both aryl and alkenyl halides are competent precursors for this reaction, providing a variety of heterobiaryls as the products in a highly enantioselective manner with high functionality tolerance. The related asymmetric arylation and alkenylation are discovered to proceed with divergent mechanisms. The reaction pathway changes from kinetic resolution (KR) when alkenyl bromides and aryl iodides bearing strong electron-withdrawing substitution on the para-position are employed as the starting materials to an enantioconvergent transformation via dynamic KR of configurationally labile cobaltacycles when relatively electron-rich aryl iodides are used. The change of the reaction mechanisms turns out to arise from the relative rates of two competing elementary steps, which are the epimerization of the cyclic Co(I) intermediates and their trapping by the coupling electrophiles of the C(sp2)-type via oxidative addition.

NHC-Catalyzed Reaction of Carboxylic Acids Using Allene Ketones as Substrates and Activating Reagents

We present a new reaction between carboxylic acids and allene ketones mediated by N-heterocyclic carbene (NHC) catalysts, which exhibit, in principle, nearly perfect atom economy. In this new approach, allene ketones act as both an activating reagent and a reactant. All atoms in the substrates end up in the product without the need for coupling reagents. The present study aims to encourage further explorations of NHC catalytic reactions with alternative activation strategies and better atom economy.

NHC-Catalyzed Enantioselective Synthesis of Tetracyclic δ-Lactones by (4 + 2) Annulation of ortho-Quinodimethanes with Activated Ketones

The N-heterocyclic carbene (NHC)-catalyzed generation of ortho-quinodimethanes (o-QDMs) from 9H-fluorene-1-carbaldehydes followed by the interception with activated ketones resulting in the enantioselective synthesis of tetracyclic δ-lactones is presented. High diastereoselectivity of products, remote C(sp3)-H functionalization, broad substrate scope, and mild reaction conditions are the notable features of the present (4 + 2) annulation.

NHC-catalyzed radical acylation of cycloalkyl silyl peroxides to access 1,6-,1,7-, and 1,8-diketones

An unprecedented N-heterocyclic carbene (NHC)-catalyzed radical acylation of cycloalkyl silyl peroxides was developed using readily available aldehydes as the acylating agents. This protocol provides an exceptionally useful method for the efficient and rapid synthesis of long-chain 1,6-/1,7-/1,8-diketones, especially unsymmetrical ones. This strategy also has the advantages of mild conditions, good functional group compatibility, and potential applications in the late-stage functionalization of aldehydes with bioactive fragments and in the construction of long-chain complex bioactive molecules.

Transition-metal-catalyzed atroposelective synthesis of axially chiral styrenes

Axially chiral styrenes, a type of atropisomer analogous to biaryls, have attracted great interest because of their unique presence in natural products and asymmetric catalysis. Since 2016, a number of methodologies have been developed for the atroposelective construction of these chiral skeletons, involving both transition metal catalysis and organocatalysis. In this feature article, we aim to provide a comprehensive understanding of recent advances in the asymmetric synthesis of axially chiral styrenes catalyzed by transition metals, integrating scattered work with different catalytic systems together. This feature article is cataloged into five sections according to the strategies, including asymmetric coupling, enantioselective C-H activation, central-to-axial chirality transfer, asymmetric alkyne functionalization, and atroposelective [2+2+2] cycloaddition.

Feed-Forward Neural Network for Predicting Enantioselectivity of the Asymmetric Negishi Reaction

Density functional theory (DFT) is a powerful tool to model transition state (TS) energies to predict selectivity in chemical synthesis. However, a successful multistep synthesis campaign must navigate energetically narrow differences in pathways that create some limits to rapid and unambiguous application of DFT to these problems. While powerful data science techniques may provide a complementary approach to overcome this problem, doing so with the relatively small data sets that are widespread in organic synthesis presents a significant challenge. Herein, we show that a small data set can be labeled with features from DFT TS calculations to train a feed-forward neural network for predicting enantioselectivity of a Negishi cross-coupling reaction with P-chiral hindered phosphines. This approach to modeling enantioselectivity is compared with conventional approaches, including exclusive use of DFT energies and data science approaches, using features from ligands or ground states with neural network architectures.

Axially chiral styrene-based organocatalysts and their application in asymmetric cascade Michael/cyclization reaction

An axially chiral styrene-based organocatalyst, featuring a combination of axially chiral styrene-based structure and a pyrrole ring, has been designed and synthesized. This catalyst demonstrates remarkable capabilities in producing a wide range of densely substituted spirooxindoles that feature an alkyne-substituted quaternary stereogenic center. These spirooxindoles are generated through mild cascade Michael/cyclization reactions, resulting in high conversion rates and exceptional enantioselectivity. Our catalytic model, based on experiments, X-ray structure analysis and DFT calculations suggests that chiral matched π-π interactions and multiple H-bonds between the organocatalyst and substrates play significant roles in controlling the stereoselectivity of the reaction.

N-Heterocyclic Carbene-Catalyzed Atroposelective Synthesis of Axially Chiral α-Carbolinones via Heterocycle Formation

An NHC-catalyzed atroposelective synthesis of axially chiral α-carbolinones from α,β-unsaturated iminoindole derivatives and α-chloroaldehydes was developed. The reaction proceeds through a cascade process including [4 + 2] annulation and then oxidative dehydrogenation with concomitant central-to-axial chirality conversion under mild conditions. The developed method opens a new avenue to efficiently access axially chiral α-carbolinones in moderate to good enantioselectivities.

Diastereo- and atroposelective synthesis of N-arylpyrroles enabled by light-induced phosphoric acid catalysis

The C-N axially chiral N-arylpyrrole motifs are privileged scaffolds in numerous biologically active molecules and natural products, as well as in chiral ligands/catalysts. Asymmetric synthesis of N-arylpyrroles, however, is still challenging, and the simultaneous creation of contiguous C-N axial and central chirality remains unknown. Herein, a diastereo- and atroposelective synthesis of N-arylpyrroles enabled by light-induced phosphoric acid catalysis has been developed. The key transformation is a one-pot, three-component oxo-diarylation reaction, which simultaneously creates a C-N axial chirality and a central quaternary stereogenic center. A broad range of unactivated alkynes were readily employed as a reaction partner in this transformation, and the N-arylpyrrole products are obtained in good yields, with excellent enantioselectivities and very good diastereoselectivities. Notably, the N-arylpyrrole skeletons represent interesting structural motifs that could be used as chiral ligands and catalysts in asymmetric catalysis.

A Chiral Relay Race: Stereoselective Synthesis of Axially Chiral Biaryl Diketones through Ring-Opening of Optical Dihydrophenan-threne-9,10-diols

We report herein a point-to-axial chirality transfer reaction of optical dihydrophenanthrene-9,10-diols for the synthesis of axially chiral diketones. Two sets of conditions, namely a basic tBuOK/air atmosphere and an acidic NaClO/n-Bu4NHSO4, were developed to oxidatively cleave the C-C bond, resulting in the formation of axially chiral biaryl diketones. Finally, brief synthetic applications of the obtained chiral aryl diketones were demonstrated.

Atroposelective Access to Dihydropyridinones with C-N Axial and Point Chirality via NHC-Catalyzed [3 + 3] Annulation

An N-heterocyclic carbene-catalyzed atroposelective [3 + 3] annulation of enals with 2-aminomaleate derivatives is described. A series of substituted dihydropyridones bearing both C-N axis and point chirality were synthesized with good diastereo- and enantioselectivity under mild conditions. This efficient strategy successfully superpositions an extra point chiral element with an axial backbone, and the generated structurally interesting atropisomers may have potential application in drug discovery.

Atroposelective Synthesis of N-N Axially Chiral Bipyrroles via Rhodium-Catalyzed C-H Insertion Reaction

An atroposelective synthesis of bipyrroles with an axially chiral N-N bond has been established via a rhodium-catalyzed C-H bond insertion reaction to provide the desired atropisomers in good yields (up to 97% yield) with good to excellent enantioselectivities (up to 99% ee).

Copper-Catalyzed Asymmetric Functionalization of Vinyl Radicals for the Access to Vinylarene Atropisomers

A novel asymmetric radical strategy for the straightforward synthesis of atropisomerically chiral vinyl arenes has been established herein, proceeding through copper-catalyzed atroposelective cyanation/azidation of aryl-substituted vinyl radicals. Critical to the success of the radical relay process is the atroposelective capture of the highly reactive vinyl radicals with chiral L*Cu(II) cyanide or azide species. Moreover, these axially chiral vinylarene products can be easily transformed into atropisomerically enriched amides and amines, enantiomerically enriched benzyl nitriles via an axis-to-center chirality transfer process, and an atropisomerically pure organocatalyst for the chemo-, diastereo-, and enantioselective (4 + 2) cyclization reaction.

Catalytic Asymmetric Synthesis of Axially Chiral Diaryl Ethers through Enantioselective Desymmetrization

Axially chiral diaryl ethers are a type of unique atropisomers bearing two potential axes, which have potential applications in a variety of research fields. However, the catalytic enantioselective synthesis of these diaryl ether atropisomers is largely underexplored when compared to the catalytic asymmetric synthesis of biaryl or other types of atropisomers. Herein, we report a highly efficient catalytic asymmetric synthesis of diaryl ether atropisomers through an organocatalyzed enantioselective desymmetrization protocol. The chiral phosphoric acid-catalyzed asymmetric electrophilic aromatic aminations of the symmetrical 1,3-benzenediamine type substrates afforded a series of diaryl ether atropisomers in excellent yields and enantioselectivities. The facile construction of heterocycles by the utilizations of the 1,2-benzenediamine moiety in the products provided access to a variety of structurally diverse and novel azaarene-containing diaryl ether atropisomers.

Alkynyl Acylazolium: a Versatile 1,3-Bielectrophilic 3C-Synthon in NHC-Organocatalysis

N-Heterocyclic carbene (NHC) organocatalysis has emerged as a powerful tool in the field of modern organic synthesis especially in the asymmetric construction of various cyclic skeletons. As an emerging NHC-bound 1,3-bielectrophilic intermediate, alkynyl acylazolium has drawn substantial attention in recent years, and has been used as a versatile 3C-synthon in synthesizing valuable organic molecules since its discovery. In this review, focused on the different pathways for the formation of alkynyl acylazoliums from different precursors like alkynoic esters, alkynoic acids and ynals, the recent advances in the transformations and applications of alkynyl acylazoliums pioneered or developed over the last decade under NHC-catalysis were summarized comprehensively. At the same time, the outlook for further investigation and exploration of novel reaction modes for alkynyl acylazoliums in the future was also discussed.

Towards Axially Chiral Pyrazole-Based Phosphorus Scaffolds by Dipeptide-Phosphonium Salt Catalysis

Given the comparatively lower rotational barriers, the catalytic asymmetric construction of axially chiral biaryl structures, especially those containing a five-membered heterocycle, still remains a challenge. Herein, we described a general and modular protocol to access atropisomeric arylpyrazole scaffolds containing a phosphorus unit by a dipeptide phosphonium salt catalyzed reaction involving an oxidative central-to-axial chirality conversion. This reaction features excellent yields and enantioselectivities, broad substrate scope, and a low catalyst loading, delivering axially chiral phosphine compounds.

Design and synthesis of axially chiral aryl-pyrroloindoles via the strategy of organocatalytic asymmetric (2 + 3) cyclization

The catalytic asymmetric construction of axially chiral indole-based frameworks is an important area of research due to the unique characteristics of such frameworks. Nevertheless, research in this area is still in its infancy and has some challenges, such as designing and constructing new classes of axially chiral indole-based scaffolds and developing their applications in chiral catalysts, ligands, etc. To overcome these challenges, we present herein the design and atroposelective synthesis of aryl-pyrroloindoles as a new class of axially chiral indole-based scaffolds via the strategy of organocatalytic asymmetric (2 + 3) cyclization between 3-arylindoles and propargylic alcohols. More importantly, this new class of axially chiral scaffolds was derived into phosphines, which served as efficient chiral ligands in palladium-catalyzed asymmetric reactions. Moreover, theoretical calculations provided an in-depth understanding of the reaction mechanism. This work offers a new strategy for constructing axially chiral indole-based scaffolds, which are promising for finding more applications in asymmetric catalysis.