Recent Advances in Catalytic Asymmetric Construction of Atropisomers

Enantioselective Nickel-Catalyzed Denitrogenative Transannulation En Route to N-N Atropisomers

Nickel-catalyzed transannulation reactions triggered by the extrusion of small gaseous molecules have emerged as a powerful strategy for the efficient construction of heterocyclic compounds. However, their use in asymmetric synthesis remains challenging because of the difficulty in controlling stereo- and regioselectivity. Herein, we report the first nickel-catalyzed asymmetric synthesis of N-N atropisomers by the denitrogenative transannulation of benzotriazones with alkynes. A broad range of N-N atropisomers was obtained with excellent regio- and enantioselectivity under mild conditions. Moreover, density functional theory (DFT) calculations provided insights into the nickel-catalyzed reaction mechanism and enantioselectivity control.

Atroposelective N-N Axes Synthesis via Electrochemical Cobalt Catalysis

Here, we disclosed an unprecedented cobalt electrocatalyzed atroposelective C-H activation and annulation for the efficient construction of diversely functionalized N-N axes in an undivided cell. A broad range of allene substrates and benzamides bearing different functionalities are compatible with generating axially chiral products with good yields and excellent enantioselectivities (up to 92% yield and 99% ee). A series of synthetic applications and control experiments were also performed, which further expanded the practicality of this strategy.

Chiral dinitrogen ligand enabled asymmetric Pd/norbornene cooperative catalysis toward the assembly of C-N axially chiral scaffolds

C - N axially chiral compounds have recently attracted significant interest among synthetic chemistry community due to their widespread application in pharmaceuticals, advanced materials and organic synthesis. Although the emerging asymmetric Catellani reaction offers great opportunity for their modular and efficient preparation, the only operative chiral NBE strategy to date requires using half stoichiometric amount of chiral NBE and 2,6-disubstituted bromoarenes as electrophiles. We herein report an efficient assembly of C-N axially chiral scaffolds through a distinct chiral ligand strategy. The crucial chiral source, a biimidazoline (BiIM) chiral dinitrogen ligand, is used in relatively low loading and permits the use of less bulky bromoarenes. The method also features the use of feedstock plain NBE, high reactivity, good enantioselectivity, ease of operation and scale-up. Applications in the preparation of chiral optoelectronic material candidates featuring two C-N chiral axes and a chiral ligand for asymmetric C-H activation have also been demonstrated.

Phosphine-Catalyzed Stereospecific and Enantioselective Desymmetrizative [3+2] Cycloaddition of MBH Carbonates and N-(2-tert-Butylphenyl)maleimides

Herein, we report an l-valine-derived amide phosphine-catalyzed [3+2] cyclization of MBH carbonates and N-(2-tert-butylphenyl)maleimides via asymmetric desymmetrization. Bicyclic N-aryl succinimide derivatives bearing three continuous chiral centers with a remote C-N atropisomeric chirality were constructed stereospecifically and enantioselectively. A wide variety of MBH carbonates could be employed in this process to deliver highly optically pure succinimide derivatives in moderate to excellent yields.

Atroposelective synthesis of biaxial bridged eight-membered terphenyls via a Co/SPDO-catalyzed aerobic oxidative coupling/desymmetrization of phenols

Bridged chiral biaryls are axially chiral compounds with a medium-sized ring connecting the two arenes. Compared with plentiful methods for the enantioselective synthesis of biaryl compounds, synthetic approaches for this subclass of bridged atropisomers are limited. Here we show an atroposelective synthesis of 1,3-diaxial bridged eight-membered terphenyl atropisomers through an Co/SPDO (spirocyclic pyrrolidine oxazoline)-catalyzed aerobic oxidative coupling/desymmetrization reaction of prochiral phenols. This catalytic desymmetric process is enabled by combination of an earth-abundant Co(OAc)2 and a unique SPDO ligand in the presence of DABCO (1,4-diaza[2.2.2]bicyclooctane). An array of diaxial bridged terphenyls embedded in an azocane can be accessed in high yields (up to 99%) with excellent enantio- (>99% ee) and diastereoselectivities (>20:1 dr).

Asymmetric synthesis of atropisomeric arylpyrazoles via direct arylation of 5-aminopyrazoles with naphthoquinones

Construction of axially chiral arylpyrazoles represents an attractive challenge due to the relatively low rotational barrier of biaryl structures containing five-membered heterocycles. This work describes the catalytic asymmetric construction of axially chiral arylpyrazoles using 5-aminopyrazoles and naphthoquinone derivatives. The chiral axis could be formed through a central-to-axial chirality relay step of the chiral phosphoric acid-catalyzed arylation reaction, which features excellent yields and enantioselectivities with a broad substrate scope under mild reaction conditions.

Catalytic asymmetric synthesis of planar-chiral dianthranilides via (Dynamic) kinetic resolution

Chirality constitutes an inherent attribute of nature. The catalytic asymmetric synthesis of molecules with central, axial, and helical chirality is a topic of intense interest and is becoming a mature field of research. However, due to the difficulty in synthesis and the lack of a prototype, less attention has been given to planar chirality arising from the destruction of symmetry on a single planar ring. Herein, we report the catalytic asymmetric synthesis of planar-chiral dianthranilides, a unique class of tub-shaped eight-membered cyclic dilactams. This protocol is enabled by cinchona alkaloid-catalyzed (dynamic) kinetic resolution. Under mild conditions, various C2- or C1-symmetric planar-chiral dianthranilides have been readily prepared in high yields with excellent enantioselectivity. These dianthranilides can serve as an addition to the family of planar-chiral molecules. Its synthetic value has been demonstrated by kinetic resolution of racemic amines via acyl transfer, enantiodivergent synthesis of the natural product eupolyphagin, and preliminary antitumor activity studies.

Scaffold-Oriented Asymmetric Catalysis: Conformational Modulation of Transition State Multivalency during a Catalyst-Controlled Assembly of a Pharmaceutically Relevant Atropisomer

A new class of superbasic, bifunctional peptidyl guanidine catalysts is presented, which enables the organocatalytic, atroposelective synthesis of axially chiral quinazolinediones. Computational modeling unveiled the conformational modulation of the catalyst by a novel phenyl urea N-cap, that preorganizes the structure into the active, folded state. A previously unanticipated noncovalent interaction involving a difluoroacetamide acting as a hybrid mono- or bidentate hydrogen bond donor emerged as a decisive control element inducing atroposelectivity. These discoveries spurred from a scaffold-oriented project inspired from a fascinating investigational BTK inhibitor featuring two stable chiral axes and relies on a mechanistic framework that was foreign to the extant lexicon of asymmetric catalysis.

Atropisomeric Carboxylic Acids Synthesis via Nickel-Catalyzed Enantioconvergent Carboxylation of Aza-Biaryl Triflates with CO2

Upgrading CO2 to value-added chiral molecules via catalytic asymmetric C-C bond formation is a highly important yet challenging task. Although great progress on the formation of centrally chiral carboxylic acids has been achieved, catalytic construction of axially chiral carboxylic acids with CO2 has never been reported to date. Herein, we report the first catalytic asymmetric synthesis of axially chiral carboxylic acids with CO2, which is enabled by nickel-catalyzed dynamic kinetic asymmetric reductive carboxylation of racemic aza-biaryl triflates. A variety of important axially chiral carboxylic acids, which are valuable but difficult to obtain via catalysis, are generated in an enantioconvergent version. This new methodology features good functional group tolerance, easy to scale-up, facile transformation and avoids cumbersome steps, handling organometallic reagents and using stoichiometric chiral materials. Mechanistic investigations indicate a dynamic kinetic asymmetric transformation process induced by chiral nickel catalysis.

Nickel-Catalyzed Atroposelective Cross-Electrophile Coupling of Aryl Halides: A General and Practical Route to Diverse MOP-Type Ligands

We report a highly cross- and atroposelective coupling between ortho-(chloro)arylphosphine oxides and ortho-(bromo)aryl ethers. This previously unknown asymmetric nickel-catalyzed reaction offers a direct route to highly enantioenriched axially chiral biaryl monophosphine oxides that are difficult to access by other means. These products can be readily reduced to generate chiral MOP-type ligands bearing complex skeletal backbones. The utility of these chiral ligands in asymmetric catalysis is also demonstrated.

Synthesis of C-N Axially Chiral N-Arylbenzo[g]indoles via a Central-to-Axial Chirality Conversion Strategy

Gold-catalyzed cascade cyclization of diynes for the synthesis of previously unexplored C-N axially chiral N-arylbenzo[g]indoles was described. The transformation was achieved via a central-to-axial chirality conversion strategy. The chiral conversion exhibited high efficiency. Besides single C-N chiral axis, N-arylbenzo[g]indoles bearing both C-N and C-C chiral axes were also afforded. The title compound derived monophosphine ligand was prepared and was evaluated in Pd-catalyzed asymmetric allylic substitutions, showing excellent chiral induction ability.

1,5-Hydrogen atom transfer of α-iminyl radical cations: a new platform for relay annulation for pyridine derivatives and axially chiral heterobiaryls

The exploitation of new reactive species and novel transformation modes for their synthetic applications have significantly promoted the development of synthetic organic methodology, drug discovery, and advanced functional materials. α-Iminyl radical cations, a class of distonic ions, exhibit great synthetic potential for the synthesis of valuable molecules. For their generation, radical conjugate addition to α,β-unsaturated iminium ions represents a concise yet highly challenging route, because the in situ generated species are short-lived and highly reactive and they have a high tendency to cause radical elimination (β-scission) to regenerate the more stable iminium ions. Herein, we report a new transformation mode of the α-iminyl radical cation, that is to say, 1,5-hydrogen atom transfer (1,5-HAT). Such a strategy can generate a species bearing multiple reactive sites, which serves as a platform to realize (asymmetric) relay annulations. The present iron/secondary amine synergistic catalysis causes a modular assembly of a broad spectrum of new structurally fused pyridines including axially chiral heterobiaryls, and exhibits good functional group tolerance. A series of mechanistic experiments support the α-iminyl radical cation-induced 1,5-HAT, and the formation of several radical species in the relay annulations. Various synthetic transformations of the reaction products demonstrate the usefulness of this relay annulation protocol for the synthesis of significant molecules.

Evolution in the asymmetric synthesis of biaryl ethers and related atropisomers

Axially chiral biaryl ethers and related compounds hold valuable potential in natural products, medicinal chemistry, and catalysis; however, their asymmetric syntheses have always been overlooked compared to other biaryl/hetero-biaryl atropisomers. Unlike the later class molecules bearing a single chiral axis, the former category possesses a unique type of atropisomerism bearing two potential axes. Due to their great importance in diverse research domains, catalytic atropselective biaryl ether synthesis has recently witnessed an upsurge. This highlight article provides an elaborated view on the developments of catalytic synthetic methods that have been explored to achieve dual axial chirality in biaryl ethers and related scaffolds.

Rationalization of a Streptavidin Based Enantioselective Artificial Suzukiase: An Integrative Computational Approach

An Artificial Metalloenzyme (ArM) built employing the streptavidin-biotin technology has been used for the enantioselective synthesis of binaphthyls by means of asymmetric Suzuki-Miyaura cross-coupling reactions. Despite its success, it remains a challenge to understand how the length of the biotin cofactors or the introduction of mutations to streptavidin leads the preferential synthesis of one atropisomer over the other. In this study, we apply an integrated computational modeling approach, including DFT calculations, protein-ligand dockings and molecular dynamics to rationalize the impact of mutations and length of the biotion cofactor on the enantioselectivities of the biaryl product. The results unravel that the enantiomeric differences found experimentally can be rationalized by the disposition of the first intermediate, coming from the oxidative addition step, and the entrance of the second substrate. The work also showcases the difficulties facing to control the enantioselection when engineering ArM to catalyze enantioselective Suzuki-Miyaura couplings and how the combination of DFT calculations, molecular dockings and MD simulations can be used to rationalize artificial metalloenzymes.

Enantioselective Synthesis of Heteroatom-Linked Non-Biaryl Atropisomers

Atropisomers hold significant fascination, not only for their prevalence in natural compounds but also for their biological importance and wide-ranging applications as chiral materials, ligands, and organocatalysts. While biaryl and heterobiaryl atropisomers are commonly studied, the enantioselective synthesis of less abundant heteroatom-linked non-biaryl atropisomers presents a formidable challenge in modern organic synthesis. Unlike classical atropisomers, these molecules allow rotation around two bonds, resulting in low barriers to enantiomerization through concerted bond rotations. In recent years the discovery of new configurationally stable rare non-biaryl scaffolds such as aryl amines, aryl ethers and aryl sulfones as well as innovative methodologies to control their configuration have been disclosed in the literature and constitute the topic of this minireview.

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.

Palladium-catalyzed regio- and stereo-selective phosphination of cyclic biarylsulfonium salts to access atropoisomeric phosphines

A palladium-catalyzed regio- and stereo-selective phosphination of cyclic biarylsulfonium salts (racemic) with HPAr3Ar4 for straightforward synthesis of atropoisomeric phosphines (P,S-ligands) bearing a stereogenic axis or both a stereogenic axis and a P-stereogenic center is reported. The high reactivity and regio- and stereo-selectivity originate from the torsional strain release and palladium catalysis, and the construction of a P-stereogenic center is enabled by an efficient dynamic kinetic resolution. The high performance of the nascent P,S-ligands has been demonstrated in palladium-catalyzed asymmetric allylic substitutions, indicating the great potential of the present methodology.

Stable Axially Chiral Cyclohexylidenes from Catalytic Asymmetric Knoevenagel Condensation

Axially chiral cycloalkylidenes are interesting but less developed axially chiral molecules. Here, a bispidine-based chiral amine catalytic system was developed to promote efficiently the asymmetric Knoevenagel condensation of N-protected oxindoles and benzofuranones with 4-substituted cyclohexanones. A variety of alkylidenecycloalkanes with stable axial chirality were obtained in good yields and fairly good er (enantiomeric ratio). Based on the absolute configuration determination of product and DFT calculations, a possible mechanism of stereoselective induction was proposed.

Enantioselective Construction of Chiral THIQUINOL and Its Derivatives via Chiral Phosphoric Acid Catalysis

The first catalytic enantioselective construction of chiral THIQUINOL and its derivatives has been accomplished through a chiral phosphoric-acid-catalyzed direct aza-Friedel-Crafts reaction of 3,4-dihydroisoquinolines with 2-naphthols/anthracen-2-ols/phenanthren-9-ol. This method offers a powerful and straightforward synthetic route toward chiral THIQUINOL derivatives with good to excellent yields and enantioselectivities. These structural motifs are crucial chiral components for further transformations into established or potential chiral ligands and catalysis.

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.

Kinetic resolution of 1,1'-binaphthyl-2,2'-diamine derivatives by chiral calcium phosphate-catalyzed acylation

1,1'-Binaphthyl-2,2'-diamine (BINAM) is a useful axially chiral compound. The kinetic resolution of BINAM is one of the most crucial methods for synthesizing chiral BINAM. We have developed a chiral calcium phosphate-catalyzed kinetic resolution of BINAM by utilizing an acylation reaction to produce a mono-amide. The kinetic resolution of BINAM derivatives was achieved by using isobutyric anhydride in the presence of chiral calcium phosphate and 4-morpholinopyridine with up to s = 127. 6,6'-Substituted BINAM derivatives were also applicable for this reaction. The resulting mono-acylated BINAM could be transformed into BINAM by hydrolysis under acidic conditions.

Conformation Analysis and Stereodynamics of Symmetrically ortho-Disubstituted Carvacrol Derivatives

The design and synthesis of analogs of natural products can be a valuable source of medicinal preparations for the pharmaceutical industry. In the present study, the structural elucidation of eleven derivatives of 2,4-dihalogeno substituted synthetic analogues of the natural compound carvacrol was carried out by means of NMR experiments, and of another thirteen by DFT calculations. By selective NOE experiments and the irradiation of CH signals of the isopropyl group, individual conformers were assigned as syn and anti. By comparing GIAO/B3LYP/6-311++G(d,p)-calculated and experimentally measured vicinal 3JCH spin-spin constants, this assignment was confirmed. An unusual relationship is reported for proton-carbon vicinal couplings: 3JCH (180°) < 3JCH (0°). The conformational mobility of carvacrols was studied by 2D EXSY spectra. The application of homonuclear decoupling technique (HOBS) to these spectra simplifies the spectra, improves resolution without reducing the sensitivity, and allows a systematic examination of the rotational barrier of all compounds via their CH signals of the isopropyl group in a wider temperature interval. The rate constants of the isopropyl rotation between syn and anti conformers were determined and the corresponding energy barriers (14-17 kcal/mol) were calculated. DFT calculations of the energy barriers in carvacrol derivatives allowed the determination of the steric origin of the restricted isopropyl rotation. The barrier height depends on the size of the 2- and 4-position substituents, and is independent of the derivatization of the OH group.

Iron-catalyzed stereoselective C-H alkylation for simultaneous construction of C-N axial and C-central chirality

The assembly of chiral molecules with multiple stereogenic elements is challenging, and, despite of indisputable advances, largely limited to toxic, cost-intensive and precious metal catalysts. In sharp contrast, we herein disclose a versatile C-H alkylation using a non-toxic, low-cost iron catalyst for the synthesis of substituted indoles with two chiral elements. The key for achieving excellent diastereo- and enantioselectivity was substitution on a chiral N-heterocyclic carbene ligand providing steric hindrance and extra represented by noncovalent interaction for the concomitant generation of C-N axial chirality and C-stereogenic center. Experimental and computational mechanistic studies have unraveled the origin of the catalytic efficacy and stereoselectivity.

Catalytic asymmetric construction of helicenes via transformation of biaryls

This review showcases a systematic overview of the available tools for the catalytic asymmetric transformation of biaryl substrates toward the construction of challenging enantioenriched helicenes and the conceptual aspects associated with each type of transformation. Depending on the properties of the biaryl and the nature of the process, several methodologies have been developed, including olefin metathesis, hydroarylation of alkynes, C-X (X = C, O, N) coupling, and C-H functionalization. Pioneering studies and an array of representative reactions are discussed to underscore the potential of these synthetic protocols.

Nickel-Catalyzed Chemoselective Carbomagnesiation for Atroposelective Ring-Opening Difunctionalization

There is a pressing need for methods that can connect enantioenriched organic compounds with readily accessible building blocks via asymmetric functionalization of unreactive chemical bonds in organic synthesis and medicinal chemistry. Herein, the asymmetric chemoselective cleavage of two unactivated C(Ar)-O bonds in the same molecule is disclosed for the first time through an unusual nickel-catalyzed carbomagnesiation. This reaction facilitates the evolution of a novel atroposelective ring-opening difunctionalization. Utilizing readily available dibenzo bicyclic substrates, diverse valuable axially chiral biaryls are furnished with high efficiencies. Synthetic elaborations showcase the application potential of this method. The features of this method include good atom-economy, multiple roles of the nucleophile, and a simple catalytic system that enables the precise magnesiation of an α-C(Ar)-O bond and arylation of a β-C(Ar)-O bond.

Regio- and Atroposelective Ring-Opening of 1H-Benzo[4,5]oxazolopyridinones

The development of new methods for regio- and stereoselective activation of C-O bonds in ethers holds significant promise for synthetic chemistry, offering advantages in terms of environmental sustainability and economic efficiency. Moreover, the C-N atropisomers represent a fascinating and crucial chiral system, extensively found in natural products, pharmaceutical leads, and the frameworks of advanced materials. In this work, we have introduced a nickel-catalyzed regio- and enantioselective carbon-oxygen arylation reaction for atroposelective synthesis of N-arylisoquinoline-1,3(2H,4H)-diones. The high regioselectivity of C-O cleavage benefits from the high stability of the in situ formed (amido)ethenolate via oxidative addition. Additionally, the self-activation of the aryl C-O bond facilitates the reaction under mild conditions, leading to outstanding enantioselectivities. The diverse post-functionalizations of the axially chiral isoquinoline-1,3(2H,4H)-diones further highlighted the utility of this protocol in preparing valuable C-N atropisomers, including the chiral phosphine ligands.

Asymmetric Sulfonylation from a Reaction of Cyclopropan-1-ol, Sulfur Dioxide, and 1-(Alkynyl)naphthalen-2-ol

Asymmetric sulfonylation from a reaction of cyclopropan-1-ol, sulfur dioxide, and 1-(alkynyl)naphthalen-2-ol in the presence of a catalytic amount of organocatalyst at room temperature is developed. Axially chiral (S)-(E)-1-(1-(alkylsulfonyl)-2-arylvinyl)naphthalen-2-ols are generated in moderate to good yields with excellent enantioselectivity and regioselectivity under mild conditions. During this transformation, γ-keto sulfinate generated in situ from cyclopropan-1-ol and sulfur dioxide acts as the key intermediate.

Conformational Sampling over Transition-Metal-Catalyzed Reaction Pathways: Toward Revealing Atroposelectivity

The Py-Conformational-Sampling (PyCoSa) technique is introduced as a systematic computational means to sample the configurational space of transition-metal-catalyzed stereoselective reactions. When applied to atroposelective Suzuki-Miyaura coupling to create axially chiral biaryl products, the results show a range of mechanistic possibilities that include multiple low-energy channels through which C-C bonds can be formed.

Palladium-Catalyzed Atroposelective Suzuki-Miyaura Coupling to Construct Axially Chiral Tetra-Substituted α-Boryl Styrenes

Palladium-catalyzed Suzuki-Miyaura (SM) coupling is a valuable method for forming C─C bonds, including those between aryl moieties. However, achieving atroposelective synthesis of axially chiral styrenes via SM coupling remains challenging. In this study, a palladium-catalyzed atroposelective Suzuki-Miyaura coupling between gem-diborylalkenes and aryl halides is presented. Using the monophosphine ligand Me-BI-DIME (L2), a range of axially chiral tetra-substituted acyclic styrenes with high yields and excellent enantioselectivities are successfully synthesized. Control experiments reveal that the gem-diboryl group significantly influences the product enantioselectivities and the coupling prefers to occur at sites with lower steric hindrance. Additionally, the alkenyl boronate group in the products proves versatile, allowing for various transformations while maintaining high optical purities.

Organocatalytic Atroposelective Construction of Pentatomic Heterobiaryl Diamines through Arylation of 5-Aminoisoxazoles with Azonaphthalenes

An efficient catalytic asymmetric Michael-type reaction of azonaphthalenes with 5-aminoisoxazoles has been developed. The reaction was based on the utilization of a chiral phosphoric acid as the catalyst, delivering a large panel of axially chiral heterobiaryl diamines in generally good yields with excellent enantioselectivities. The gram-scale reaction and postmodification of the chiral product demonstrated their potentials in the synthesis of chiral catalysts and ligands. This approach not only provides a useful method for the construction of pentatomic heterobiaryl scaffolds but also offers new members to the axially chiral diamine family with promising applications in synthetic and medicinal chemistry.

Electrochemical Cross-Coupling between N-(4-Hydroxyphenyl)-sulfonamides and 2-Naphthols: Synthesis of 2,2'-Bis(arenol)s

We herein present an electrochemical method for the dehydrogenative cross-coupling of N-(4-hydroxyphenyl)-sulfonamides and 2-naphthols. This transformation provides a direct and scalable approach to a wide range of C1-symmetric 2,2'-bis(arenol)s with moderate to high yields under mild conditions. Preliminary attempts with the asymmetric variant of this reaction were also performed with ≤55% ee for the synthesis of 2,2'-bis(arenol)s. Control experiments were conducted to propose a plausible mechanism for the reaction.

Synthesis, structure and stereodynamics of atropisomeric N-chloroamides

Atropisomeric N-chloroamides were efficiently accessed by electrophilic halogenation of ortho-substituted secondary anilides. The stereodynamics of atropisomerism in these novel scaffolds was interrogated by detailed experimental and computational studies, revealing that racemization is correlated with amide isomerization. The stereoelectronic nature of the amide was shown to significantly influence racemization rates, with potentially important implications for other C-N atropisomeric scaffolds.

Torsional Strain Enabled Ring-Opening Polymerization towards Axially Chiral Semiaromatic Polyesters with Chemical Recyclability

The development of new chemically recyclable polymers via monomer design would provide a transformative strategy to address the energy crisis and plastic pollution problem. Biaryl-fused cyclic esters were targeted to generate axially chiral polymers, which would impart new material performance. To overcome the non-polymerizability of the biaryl-fused monomer DBO, a cyclic ester Me-DBO installed with dimethyl substitution was prepared to enable its polymerizability via enhancing torsional strain. Impressively, Me-DBO readily went through well-controlled ring-opening polymerization, producing polymer P(Me-DBO) with high glass transition temperature (Tg >100 °C). Intriguingly, mixing these complementary enantiopure polymers containing axial chirality promoted a transformation from amorphous to crystalline material, affording a semicrystalline stereocomplex with a melting transition temperature more than 300 °C. P(Me-DBO) were capable of depolymerizing back to Me-DBO in high efficiency, highlighting an excellent recyclability.

Bioactive atropisomers: Unraveling design strategies and synthetic routes for drug discovery

Atropisomerism, an expression of axial chirality caused by limited bond rotation, is a prominent aspect within the field of medicinal chemistry. It has been shown that atropisomers of a wide range of compounds, including established FDA-approved drugs and experimental molecules, display markedly different biological activities. The time-dependent reversal of chirality in atropisomers poses complexity and obstacles in the process of drug discovery and development. Nonetheless, recent progress in understanding atropisomerism and enhanced characterization methods have greatly assisted medicinal chemists in the effective development of atropisomeric drug molecules. This article provides a comprehensive review of their special design thoughts, synthetic routes, and biological activities, serving as a reference for the synthesis and biological evaluation of bioactive atropisomers in the future.

Synthesis of axially chiral diaryl ethers via NHC-catalyzed atroposelective esterification

Axially chiral diaryl ethers bearing two potential axes find unique applications in bioactive molecules and catalysis. However, only very few catalytic methods have been developed to construct structurally diverse diaryl ethers. We herein describe an NHC-catalyzed atroposelective esterification of prochiral dialdehydes, leading to the construction of enantioenriched axially chiral diaryl ethers. Mechanistic studies indicate that the matched kinetic resolutions play an essential role in the challenging chiral induction of flexible dual-axial chirality by removing minor enantiomers via over-functionalization. This protocol features mild conditions, excellent enantioselectivity, broad substrate scope, and applicability to late-stage functionalization, and provides a modular platform for the synthesis of axially chiral diaryl ethers and their derivatives.

Catalytic Atroposelective Synthesis of Axially Chiral Heterobiaryl Oxime Ethers via the One-Step Dynamic Kinetic Condensation Reaction

The catalytic atroposelective synthesis of axially chiral heterobiaryls was first developed through the direct one-step dynamic kinetic condensation reaction with the simple transformation of the C═O bond to the C═N bond, delivering a series of novel axially chiral heterobiaryl oxime ethers.

A Torquoselective Thermal 6π-Electrocyclization Approach to 1,4-Cyclohexadienes via Solvent-Aided Proton Transfer: Experimental and Theoretical Studies

The thermal 6π-electrocyclization of hexatriene typically delivers 1,3-cyclohexadiene (1,3-CHD). However, there is only limited success in directly synthesizing 1,4-cyclohexadiene (1,4-CHD) using such an approach, probably due to the difficulty in realizing thermally-forbidden 1,3-hydride shift after electrocyclic ring closure. The present study shows that by heating (2E,4E,6E)-hexatrienes bearing ester or ketone substituents at the C1-position in a mixture of toluene/MeOH or EtOH (2 : 1) solvents at 90-100 °C, 1,4-CHDs can be selectively synthesized. This is achieved through a torquoselective disrotatory 6π-electrocyclic ring closure followed by a proton-transfer process. The success of this method depends on the polar protic solvent-assisted intramolecular proton transfer from 1,3-CHD to 1,4-CHD, which has been confirmed by deuterium-labeling experiments. There are no reports to date for such a solvent-assisted isomerization. Density functional theory (DFT) studies have suggested that forming 1,3-CHD and subsequent isomerization is a thermodynamically feasible process, regardless of the functional groups involved. Two possible successive polar solvent-assisted proton-transfer pathways have been identified for isomerization.

Developing Biarylhemiboronic Esters for Biaryl Atropisomer Synthesis via Dynamic Kinetic Atroposelective Suzuki-Miyaura Cross-Coupling

We herein introduce biarylhemiboronic esters as a new type of bridged biaryl reagent for asymmetric synthesis of axially chiral biaryl structures, and the palladium-catalyzed asymmetric Suzuki-Miyaura cross-coupling of biarylhemiboronic esters is developed. This dynamic kinetic atroposelective coupling reaction exhibits high enantioselectivity, good functional group tolerance, and a broad substrate scope. The synthetic application of the current method was demonstrated by transformations of the product and a programmed synthesis of chiral polyarene. Preliminary mechanistic studies suggested that the reaction proceeded via an enantio-determining dynamic kinetic atroposelective transmetalation step.

Mechanism and Origin of Stereoselectivity for the NHC-Catalyzed Desymmetrization Reaction for the Synthesis of Axially Chiral Biaryl Aldehydes

Organocatalytic desymmetrization reaction is a powerful tool for constructing axial chirality, but the theoretical study on the origin of stereoselectivity still lags behind even now. In this work, the N-heterocyclic carbene (NHC)-catalyzed desymmetrization reaction of biaryl frameworks for the synthesis of axially chiral aldehydes has been selected and theoretically investigated by using density functional theory (DFT). The fundamental pathway involves several steps, i.e., desymmetrization, formation of Breslow oxidation, esterification, and NHC regeneration. The desymmetrization and formation of Breslow processes have been identified as stereoselectivity-determining and rate-determining steps. Further weak interaction analyses proved that the C-H···O hydrogen bond and C-H···π interactions are responsible for the stability of the key stereoselective desymmetrization transition states. This research contributes to understanding the nature of NHC-catalyzed desymmetrization reactions for the synthesis of axially chiral compounds.

Asymmetric 1,2-diaxial synthesis of bi-(hetero)aryl benzofulvene atropisomers via transient directing group-assisted dehydrogenative coupling

The efficient cross-dehydrogenative coupling of electronically rich and sterically congested benzofulvene with bi-(hetero)aryl moieties to construct an axially chiral benzofulvene core remains a formidable task. In this study, we describe a highly efficient and practical palladium-catalyzed approach for atroposelective bi-(hetero)aryl benzofulvene synthesis, achieving excellent enantioselectivity with moderate yields. This protocol offers a remarkable opportunity for the direct regio- and enantioselective conversion of C-H bonds of benzofulvene to C-C bonds. Furthermore, the protocol permits the incorporation of benzofulvene with a 4-phenyl coumarin core, enabling access to a novel class of axially chiral coumarins.

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.

Deracemization of Atropisomeric Biaryls Enabled by Copper Catalysis

Atropisomeric biaryls have found crucial applications in versatile chiral catalysts as well as in ligands for transition metals. Herein, we have developed an efficient crystallization-induced deracemization (CID) method to access chiral biaryls from their racemates with a chiral ammonium salt under copper catalysis including BINOL, NOBIN, and BINAM derivatives. After being significantly accelerated by its bidentate diamine ligand, the copper catalyst exhibits high efficiency and selectivity in racemizing biaryl skeletons, and the cocrystal complex would be enantioselectively formed together with chiral ammonium salt, which on acid-quenching would directly deliver chiral biaryl without further chromatographic purification. This CID process is easily scalable, and the chiral ammonium salt was nicely recoverable. Ligand effect studies showed that bulky alkyl substitution was an indispensable element to ensure efficient racemization, which probably proceeds via a radical-cation intermediate and further allows axial rotation by forming a delocalized radical.

A New Series of Sandwich-Type 5,5'-Biterphenylenes: Synthetic Challenge, Structural Uniqueness and Photodynamics

A new series of biaryls, bi-linear-terphenylenes (BLTPs), were prepared using the tert-butyllithium-mediated cyclization as the key synthetic step. The three-dimensional structures of the studied compounds were verified using X-ray crystallography and DFT calculations. Tetraaryl(ethynyl)-substituted BLTPs are highly crowded molecules, and the internal rotation around the central C-C bond is restricted due to a high barrier (>50 kcal/mol). These structures contain several aryl/terphenylenyl/aryl sandwiches, where the through-space π-π (TSPP) interactions are strongly reflected in the shielding of 1 H NMR chemical shifts, reduction of oxidation potentials, increasing aromaticity of the central six-membered ring and decreasing antiaromaticity of the four-membered rings in a terphenylenyl moiety based on NICS(0) and iso-chemical shielding surfaces. Despite the restricted C-C bond associated intramolecular TSPP interactions for BLTPs in the ground state, to our surprise, the electronic coupling between two linear terphenylenes (LTPs) in BLTPs in the excited state is weak, so that the excited-state behavior is dominated by the corresponding monomeric LTPs. In other words, all BLTPs undergo ultrafast relaxation dynamics via strong exciton-vibration coupling, acting as a blue-light absorber with essentially no emission.

Theoretical insights into the enantiodivergence induced by chiral phosphoric acid catalysis with a Lewis acid for the synthesis of N-N axially chiral atropisomers

A detailed theoretical mechanistic investigation on chiral phosphoric acid (CPA)-catalyzed Paal-Knorr reactions, in the presence and absence of a Lewis acid, for the synthesis of N-N axially chiral atropisomers is described herein. Density functional theory (DFT) studies elucidate that in the absence of a Lewis acid, CPA catalyzes both the initial cyclization and the subsequent dehydroxylation processes, ambiguously identified as the rate-determining step in the reactions. Conversely, when a Lewis acid participates in the reaction, it facilitates the second dehydroxylation process with a significantly lower energy barrier, thereby reversing the rate-determining step to the initial cyclization step. It is noteworthy that in the case of N-aminoindoles, both the S-configurational transition state TS1 in the cyclization step and TS2 in the dehydroxylation process are favourable. In contrast, for the synthesis of a bispyrrole, the R-configurational TS1 and the S-configurational TS2 are dominant. Therefore, the enantiodivergence observed is essentially induced by the reversed rate-determining steps in the absence or presence of a Lewis acid in the case of a bispyrrole. Furthermore, the non-covalent interaction (NCI) and atoms-in-molecules (AIM) analysis of the TS structures reveal that the non-covalent interactions play a pivotal role in determining the enantiodivergence observed in these reactions.

Design and Catalytic Asymmetric Synthesis of Furan-Indole Compounds Bearing both Axial and Central Chirality

In the chemistry community, catalytic asymmetric synthesis of furan-based compounds bearing both axial and central chirality has proven to be a significant but challenging issue owing to the importance and difficulty in constructing such frameworks. In this work, we have realized the first catalytic asymmetric synthesis of five-five-membered furan-based compounds bearing both axial and central chirality via organocatalytic asymmetric (2+4) annulation of achiral furan-indoles with 2,3-indolyldimethanols with uncommon regioselectivity. By this strategy, furan-indole compounds bearing both axial and central chirality were synthesized in high yields with excellent regio-, diastereo-, and enantioselectivities. Moreover, theoretical calculations were conducted to provide an in-depth understanding of the reaction pathway, activation mode, and the origin of the selectivity.

Enantioselective Rhodium-Catalyzed C-H Arylation Enables Direct Synthesis of Atropisomeric Phosphines

Atropisomeric phosphines hold considerable significance in asymmetric catalysis, yet their synthesis presents a formidable challenge owing to intricate multistep procedures. In this context, a groundbreaking methodology has been presented for their preparation. This innovative approach entails an atroposelective rhodium-catalyzed C-H activation employing aryl and heteroaryl halides, chelated by a P(III) center. The essence of this strategy lies in its ability to directly construct chiral phosphine ligands in a single step, thereby exhibiting exceptional efficiency in terms of atom and redox economy. Illustrative examples serve to demonstrate the immense potential of in situ-formed ligands in asymmetric catalysis. Mechanistic experiments have further provided invaluable insights into this transformation.

Organocatalytic Enantioselective Synthesis of Seven-Membered Ring with Inherent Chirality

Inherent chirality is used to describe chiral cyclic molecules devoid of central, axial, planar, or helical chirality and has tremendous applications in chiral recognition and enantioselective synthesis. Catalytic and divergent syntheses of inherently chiral molecules have attracted increasing interest from chemists. Herein, we report the enantioselective synthesis of inherently chiral tribenzocycloheptene derivatives via chiral phosphoric acid (CPA)-catalyzed condensation of cyclic ketones and hydroxylamines. This chemistry paves the way to accessing the less stable derivatives of 7-membered rings with inherent chirality. A series of chiral tribenzocycloheptene oxime ethers was synthesized in good yields (up to 97 %) with excellent enantioselectivities (up to 99 % ee).

Atroposelective Synthesis of Axially Chiral Naphthylpyrroles by a Catalytic Asymmetric 1,3-Dipolar Cycloaddition/Aromatization Sequence

A straightforward methodology for the enantioselective preparation of axially chiral 2-naphthylpyrroles has been developed. This protocol is based on a CuI/Fesulphos-catalyzed highly enantioselective 1,3-dipolar cycloaddition of an azomethine ylide followed by pyrrolidine alkylation and pyrrolidine to pyrrole oxidation. The mild conditions employed in the DDQ/blue light-mediated aromatization process facilitate an effective central-to-axial chirality transfer affording the corresponding pyrroles with high atroposelectivity.

Atroposelective Total Synthesis of Cihunamide B

A short, atroposelective synthesis of cihunamide B (1) is reported. The feature of this report is the decagram-scale SNAr reaction of l-tryptophan derivatives, followed by atroposelective Larock macrocyclization. This strategy allowed the construction of a Trp-Trp cross-linkage with unprecedented atropisomerism. The atroposelectivity of this Larock macrocyclization has been investigated through a combination of experimental and computational chemistry, yielding detailed insights into the synthesis of biaryl linkages. It also enabled the concise synthesis of cihunamide B (1), which is expected to be a potential antibacterial agent.

Nitro-Enabled Atroposelective Dynamic Kinetic Resolution of 2-Arylindoles by Phase-Transfer Catalysis

This study presents the atroposelective alkylation of 2-arylindoles catalyzed by a substituted cinchonium salt as a phase-transfer catalyst. Under the optimized reaction conditions, various substrates are employed to yield products with high enantioselectivity. The presence of an ortho-nitro group at the aromatic ring is essential for high atroposelectivity, because it facilitates favorable interactions between the catalyst and substrate. The origin of the enantioselectivity reveals favorable π-π interactions for both enantiomers and unfavorable steric strains for undesired enantiomers.