A Bird's Eye View of Atropisomers Featuring a Five-Membered Ring

Design and Synthesis of C4-Symmetric Axially Chiral β-Aryl Porphyrins and Application for Supporting Ir(III)-Catalyzed Enantioselective C-H Alkylation

A hitherto unknown class of C4-symmetric Caryl-Cβ (C3, C8, C13, C18) axially chiral porphyrins has been synthesized and the application of their iridium (Ir) complexes in catalytic asymmetric C(sp3)-H functionalization is documented. Cyclotetramerization of enantioenriched axially chiral 2-hydroxymethyl-3-naphthyl pyrroles under mild acidic conditions affords, after oxidation with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ), the C4-symmetric α,α,α,α-atropenantiomer as an only isolable diastereomer. Both regioisomeric Ir(Por*)(CO)(Cl) complexes catalyze the carbene C-H insertion reaction affording the same enantiomer, albeit with slight difference in enantioselectivity. With the optimum Ir-complex 3 e, the 2-substituted arylacetic acid derivatives were generated from diazo compounds and cyclohexadiene in excellent yields and enantioselectivities.

Bifunctional Squaramide-Catalyzed Oxidative Kinetic Resolution: Simultaneous Access to Axially Chiral Thioether and Sulfoxide

Axially chiral thioethers and sulfoxides emerge as two pivotal classes of ligands and organocatalysts, which have remarkable features in the stereoinduction of various asymmetric transformations. However, the lack of easy methods to access such molecules with diverse structures has hampered their broader utilization. Herein, an oxidative kinetic resolution for sulfides using a chiral bifunctional squaramide as the catalyst with cumene hydroperoxide as the terminal oxidant is established. This asymmetric approach provides a variety of axially chiral thioethers as well as sulfoxides bearing both axial and central chirality, with excellent diastereo- and enantioselectivities. This catalytic system also successfully extends to the kinetic resolution of benzothiophene-based sulfides. Preliminary mechanism investigation indicates that the multiple hydrogen bonding interactions between the bifunctional squaramide catalyst and substrates play a crucial role in determining the enantioselectivity and reactivity.

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.

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.

Synthetic strategies for the construction of C3-N1' bisindoles

C3-N1' bisindoles are unique structures, and the construction of these structures has drawn much attention. However, their synthesis still presents significant challenges that limit the functional group compatibility. This minireview summarizes the recent progress in the methodology for constructing C3-N1' bisindoles. There are two approaches for access to C3-N1' bisindoles: (1) direct approaches including reverse polarity techniques. (2) Stepwise approaches using designed and prefunctionalized substrates enable further functionalization by additional reactions to facilitate access to the target products. I believe that this review will allow its readers to develop novel approaches for the synthesis of C3-N1' bisindoles.

Pd-Catalyzed Asymmetric Larock Indole Synthesis to Access Axially Chiral N-Arylindoles

Larock indole synthesis is one of the most straightforward and efficient methods for the synthesis of indoles; however, there has been no asymmetric version yet for the construction of indole-based axially chiral N-arylindoles since its initial report in 1991. Herein we report the first example of an asymmetric Larock indole synthesis by employing a chiral sulfinamide phosphine (SadPhos) ligand (Ming-Phos) with palladium. It allows rapid construction of a wide range of axially chiral N-arylindole compounds in good yields up to 98:2 er. The application of this unique chiral scaffold as an organocatalyst is promising. Furthermore, a kinetic study has revealed that the alkyne migratory insertion is the rate-determining step, which has been proven by the density functional theory (DFT) calculations. Additionally, DFT studies also suggest that the N-C dihedral difference caused by the steric hindrance of the ligand contributes to enantioselectivity control.

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.

Catalytic Asymmetric Synthesis of Atropisomers Bearing Multiple Chiral Elements: An Emerging Field

With the rapid development of asymmetric catalysis, the demand for the enantioselective synthesis of complex and diverse molecules with different chiral elements is increasing. Owing to the unique features of atropisomerism, the catalytic asymmetric synthesis of atropisomers has attracted a considerable interest from the chemical science community. In particular, introducing additional chiral elements, such as carbon centered chirality, heteroatomic chirality, planar chirality, and helical chirality, into atropisomers provides an opportunity to incorporate new properties into axially chiral compounds, thus expanding the potential applications of atropisomers. Thus, it is important to perform catalytic asymmetric transformations to synthesize atropisomers bearing multiple chiral elements. In spite of challenges in such transformations, in recent years, chemists have devised powerful strategies under asymmetric organocatalysis or metal catalysis, synthesizing a wide range of enantioenriched atropisomers bearing multiple chiral elements. Therefore, the catalytic asymmetric synthesis of atropisomers bearing multiple chiral elements has become an emerging field. This review summarizes the rapid progress in this field and indicates challenges, thereby promoting this field to a new horizon.

Catalytic Enantioselective Synthesis of Axially Chiral Imidazoles by Cation-Directed Desymmetrization

Axially chiral five-membered heterobiaryls synthesized by enantioselective catalysis typically feature large ortho-substituents or a heteroatom in the chiral axis to maintain a stable configuration. Herein we report a cation-directed catalytic enantioselective desymmetrization method that enables rapid access to axially chiral imidazoles with the basic nitrogen at the ortho position and efficiently integrates π-stacking moieties to ensure a stable axial configuration for further applications. The process is operationally simple, is highly enantioselective, and can be performed on the gram scale. The majority of the products are obtained in >90% ee, but interestingly even those with only moderate ee can readily be enriched to near optical purity by selective racemate crystallization. Together with a mild phosphine oxide reduction method, axially chiral imidazoles such as StackPhos and its derivatives are readily prepared in high yield and excellent enantioselectivity on the gram scale. The method also enables the preparation of new chiral non-phosphine-bearing imidazoles.

Dynamic Kinetic Resolution of Indole-Based Sulfenylated Heterobiaryls by Rhodium-Catalyzed Atroposelective Reductive Aldol Reaction

A highly enantio- and diastereoselective dynamic kinetic resolution (DKR) of configurationally labile 3-aryl indole-2-carbaldehydes is described. The DKR proceeds via a Rh-catalyzed intermolecular asymmetric reductive aldol reaction with acrylate esters, with simultaneous generation of three stereogenic elements. The strategy relies on the labilization of the stereogenic axis that takes place thanks to a transient Lewis acid-base interaction (LABI) between the formyl group and a thioether moiety strategically located at the ortho' position. The atropisomeric indole products present a high degree of functionalization and can be further converted to a series of axially chiral derivatives, thereby expanding their potential application in drug discovery and asymmetric catalysis.

Rhodium-catalyzed atroposelective access to trisubstituted olefins via C-H bond olefination of diverse arenes

The atroposelective synthesis of axially chiral acyclic olefins remains a daunting challenge due to their relatively lower racemization barriers, especially for trisubstituted ones. In this work, atroposelective C-H olefination has been realized for synthesis of open-chain trisubstituted olefins via C-H activation of two classes of (hetero)arenes in the coupling with sterically hindered alkynes. The employment of phenyl N-methoxycarbamates as arene reagents afforded phenol-tethered olefins, with the carbamate being a traceless directing group. The olefination of N-methoxy-2-indolylcarboxamides afforded the corresponding chiral olefin by circumventing the redox-neutral [4 + 2] annulation. The reactions proceeded with excellent Z/E selectivity, chemoselectivity, regioselectivity, and enantioselectivity in both hydroarylation systems.

Development of axially chiral urazole scaffolds for antiplant virus applications against potato virus Y

BACKGROUND

Potato virus Y (PVY) was first discovered by Smith in 1931 and is currently ranked as the fifth most significant plant virus. It can cause severe damage to plants from the family Solanaceae, which results in billions of dollars of economic loss worldwide every year. To discover new antiviral drugs, a class of multifunctional urazole derivatives bearing a stereogenic CN axis were synthesized with excellent optical purities for antiviral evaluations against PVY.

RESULTS

The absolute configurations of the axially chiral compounds exhibited obvious distinctions in antiviral bioactivities, with several of these enantio-enriched axially chiral molecules showing excellent anti-PVY activities. In particular, compound (R)-9f exhibited remarkable curative activities against PVY with a 50% maximal effective concentration (EC₅₀ ) of 224.9 μg mL^-1 , which was better than that of ningnanmycin (NNM), which had an EC₅₀ of 234.0 μg mL^-1 . And the EC₅₀ value of the protective activities of compound (R)-9f was 462.2 μg mL^-1 , which was comparable to that of NNM (442.0 μg mL^-1 ). The mechanisms of two enantiomer of the axially chiral compounds 9f were studied by both molecule docking and defensive enzyme activity tests.

CONCLUSION

Mechanistic studies demonstrated that the axially chiral configurations of the compounds played significant roles in the molecule PVY-CP (PVY Coat Protein) interactions and could enhance the activities of the defense enzymes. The (S)-9f showed only one carbon-hydrogen bond and one π-cation interaction between the chiral molecule and the PVY-CP amino acid sites. In contrast, the (R)-enantiomer of 9f exhibited three hydrogen bonding interactions between the carbonyl groups and the PVY-CP active sites of ARG157 and GLN158. The current study provides significant information on the roles that axial chiralities play in plant protection against viruses, which will facilitate the development of novel green pesticides bearing axial chiralities with excellent optical purities. © 2023 Society of Chemical Industry.

Synthesis and Atropisomeric Properties of Benzoazepine-Fused Isoindoles

Atropisomeric, bench-stable benzoazepine-fused isoindoles were synthesized via oxidation from isoindoline precursors. Using the isoindoles 5d-f as models, the stereochemistry and conformational folding of the systems were examined. Chiral UHPLC was used to analyze the rate of racemization and calculate the Gibbs free energy of enantiomerization (ΔG^‡_Enant). X-ray crystallography, ¹H NMR spectroscopy, and DFT calculations were used to elucidate the three axes of chirality and clarify the structural factors contributing to ΔG^‡_Enant. Tandem rotation around the axes of chirality precludes the formation of diastereomers, with rotational restriction of the C_aryl-N_sulfonamide bond determined as the moderator of atropisomeric stability in the system, affected primarily by steric hindrance as well as by π-stacking interactions facilitated by the folded conformation of the sulfonamide over the isoindole moiety.

Catalytic Asymmetric Synthesis of Atropisomeric N-Aryl 1,2,4-Triazoles

The atroposelective synthesis of N-aryl 1,2,4-triazoles was developed. A cyclodehydration reaction was rendered asymmetric with the use of a chiral phosphoric acid catalyst to afford atropisomeric N-aryl 1,2,4-triazoles in up to 91:9 er. Recrystallization of the isolated heterocycle further enriched the atropisomeric ratio of several analogs to 99:1 er or greater. A divergent and substrate-dependent reaction pathway yielding a different heterocyclic product is also disclosed.

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.

Organocatalytic Enantioselective Synthesis of Axially Chiral N,N'-Bisindoles

This study establishes the first organocatalytic enantioselective synthesis of axially chiral N,N'-bisindoles via chiral phosphoric acid-catalyzed formal (3+2) cycloadditions of indole-based enaminones as novel platform molecules with 2,3-diketoesters, where de novo indole-ring formation is involved. Using this new strategy, various axially chiral N,N'-bisindoles were synthesized in good yields and with excellent enantioselectivities (up to 87 % yield and 96 % ee). More importantly, this class of axially chiral N,N'-bisindoles exhibited some degree of cytotoxicity toward cancer cells and was derived into axially chiral phosphine ligands with high catalytic activity. This study provides a new strategy for enantioselective synthesis of axially chiral N,N'-bisindoles using asymmetric organocatalysis and is the first to realize the applications of such scaffolds in medicinal chemistry and asymmetric catalysis.

Atropisomerism transforming anti-cancer drug discovery

Atropisomerism is a stereochemical phenomenon that describes how groups are arranged in space as a result of their impeded rotation around a single bond. It is one of the frequently underappreciated conformational kinds of chirality. A significant role for atropisomers in drug discovery and development has been established via substantial study on the characteristics of molecules exhibiting this form of chirality. According to studies on the target selectivity of anti-cancer drugs, it was identified that atropisomers of specific compounds could be examined to modulate the selectivity of promiscuous inhibitors, which are a key target in cancer therapy. Conversely, it was discovered that these deliberate rigidifications of possible molecules along an axis of chirality gave an abundant possibility of acquiring more tailored anti-cancer action. Atropisomerism plays a significant role in altering pharmacodynamic and pharmacokinetic properties and thereby the success of any proposed drug candidate. It is thus necessary to anticipate the impact of stereogenic centres in such compounds on cancer drug development. Hence, herein we review atropisomeric anti-cancer moieties which have been investigated based on their target proteins, origin and isomerism. The insights offered herein would be extremely useful in anti-cancer drug design, pave way for new avenues to development promising potent agents to combat this life-threatening disease.

Atroposelective Synthesis of Triaryl α-Pyranones with 1,2-Diaxes by N-Heterocyclic Carbene Organocatalysis

Atropisomers bearing multiple stereogenic axes are of increasing importance to the field of material science, pharmaceuticals, and catalysis. However, the atroposelective construction of multi-axis atropisomers remains rare and challenging, due to the intrinsical difficulties in the stereo-control of the multiple stereogenic axes. Herein, we demonstrate a single-step construction of a new class of 1,2-diaxially chiral triaryl α-pyranones by an N-heterocyclic carbene organocatalytic asymmetric [3+3] annulation of well-designed alkynyl acylazolium precursors and enolizable sterically hindered 2-aryl ketones. The protocol features broad substrate scope (>50 examples), excellent stereo-control (most cases >20 : 1 dr, up to 99.5 : 0.5 er), and potentially useful synthetic applications. The success of this reaction relies on the rational design of structurally matched reaction partners and the careful selection of the asymmetric catalytic system. DFT calculations have also been performed to discover and rationalize the origin of the high stereoselectivity of this reaction.

Advances in Atroposelectively De Novo Synthesis of Axially Chiral Heterobiaryl Scaffolds

Axially chiral heterobiaryl frameworks are privileged structures in many natural products, pharmaceutically active molecules, and chiral ligands. Therefore, a variety of approaches for constructing these skeletons have been developed. Among them, de novo synthesis, due to its highly convergent and superior atom economy, serves as a promising strategy to access these challenging scaffolds including C-N, C-C, and N-N chiral axes. So far, several elegant reviews on the synthesis of axially chiral heterobiaryl skeletons have been disclosed, however, atroposelective construction of the heterobiaryl subunits by de novo synthesis was rarely covered. Herein, we summarized the recent advances in the catalytic asymmetric synthesis of the axially chiral heterobiaryl scaffold via de novo synthetic strategies. The related mechanism, scope, and applications were also included.

Organocatalytic Atroposelective Synthesis of Indole Derivatives Bearing Axial Chirality: Strategies and Applications

Catalytic atroposelective syntheses of axially chiral compounds have stimulated extensive interest in multiple communities, such as synthetic chemistry, biochemistry, and materials science, because of the intriguing characteristics of atropisomerism. In particular, atropisomeric indole derivatives, which contain a kind of five-membered heterocyclic framework, are widely distributed in a number of natural alkaloids, biologically relevant compounds, chiral ligands, and chiral organocatalysts. Hence, the catalytic atroposelective synthesis of indole derivatives bearing axial chirality is of considerable importance and has become an emerging focus of research. However, there are substantial challenges associated with the atroposelective synthesis of indole derivatives, including remote ortho-substituents around the chiral axis, a lower barrier for rotation, and a weaker configurational stability than that of atropisomeric six-membered biaryls. Therefore, the development of effective strategies toward the catalytic atroposelective synthesis of indole derivatives has become an urgent task.In order to tackle these challenges and to accomplish the task, our group devised a unique strategy of designing indole-derived platform molecules and developing organocatalytic enantioselective transformations of such platform molecules to synthesize atropisomeric indole derivatives; asymmetric organocatalysis has tremendous advantages and was the research area recognized by the Nobel Prize in Chemistry in 2021. This Account summarizes our endeavors in the organocatalytic atroposelective synthesis of indole derivatives bearing axial chirality. In brief, we devised and developed a series of indole-derived platform molecules, such as indolylmethanols, (hetero)aryl indoles, oxindole-based styrenes, N-aminoindoles, and indole-based homophthalic anhydrides, by introducing different functional groups onto the indole ring to achieve new reactivity and modulate the reactive site of the indole ring. As a result, these indole-derived platform molecules possess versatile and unique reactivity and are capable of undergoing a variety of organocatalytic enantioselective transformations for preparing structurally diversified indole derivatives with axial chirality.We used these strategies to accomplish the atroposelective synthesis of plenty of indole derivatives with axial chirality, including (hetero)aryl indoles, alkene-indoles, oxindole-based styrenes, N-pyrrolylindoles, and isochromenone-indoles. In addition, we gave a thorough and detailed understanding of the designed reaction by investigating the reaction pathway and activation mode. More importantly, we studied the biological activity of some products and performed catalyst design on the basis of atropisomeric indole moieties, which are helpful for disclosing more applications of indole derivatives bearing axial chirality.In the future, the organocatalytic atroposelective synthesis of indole derivatives bearing axial chirality will indubitably remain a frontier topic in the research area of asymmetric catalysis and chiral indole chemistry despite challenging issues, for instance, the atroposelective synthesis of novel indole derivatives bearing an unconventional chiral axis, the development of atropisomeric indole derivatives into powerful catalysts or ligands, and the discovery of atroposelective indole derivatives as potent drug candidates. We hope our efforts summarized in this Account will encourage chemists worldwide to devise innovative strategies toward solving the challenging issues that remain in this field, thus promoting its development to a higher level.

Cu(I)-Catalyzed Asymmetric Arylation of Pyrroles with Diaryliodonium Salts toward the Synthesis of N-N Atropisomers

We report herein that copper(I) catalysis using a bis(phosphine) dioxide ligand can catalyze the desymmetric C-H arylation of prochiral bipyrroles. More than 50 nitrogen-nitrogen atropisomers were achieved in good to excellent yields with excellent enantioselectivities (≤97% yield, ≤98% ee). The reaction proceeds under mild conditions with good functional group compatibility on arenes and diaryliodonium salts. Moreover, this principle enables iterative arylation of the bipyrroles to enantioselectively arylate different positions during the catalysis of copper.

Organocatalytic Atroposelective Synthesis of N-N Axially Chiral Indoles and Pyrroles by De Novo Ring Formation

The first highly atroposelective construction of N-N axially chiral indole scaffolds was established via a new strategy of de novo ring formation. This strategy makes use of the organocatalytic asymmetric Paal-Knorr reaction of well-designed N-aminoindoles with 1,4-diketones, thus affording N-pyrrolylindoles in high yields and with excellent atroposelectivities (up to 98 % yield, 96 % ee). In addition, this strategy is applicable for the atroposelective synthesis of N-N axially chiral bispyrroles (up to 98 % yield, 97 % ee). More importantly, such N-N axially chiral heterocycles can be converted into chiral organocatalysts with applications in asymmetric catalysis, and some molecules display potent anticancer activity. This work not only provides a new strategy for the atroposelective synthesis of N-N axially chiral molecules but also offers new members of the N-N atropisomer family with promising applications in synthetic and medicinal chemistry.

Asymmetric Azide-Alkyne Cycloaddition with Ir(I)/Squaramide Cooperative Catalysis: Atroposelective Synthesis of Axially Chiral Aryltriazoles

An Ir(I)/squaramide cooperative catalytic strategy for atroposelective synthesis of axially chiral aryltriazoles has been developed for the first time. Diverse structurally novel aryltriazole skeletons that cannot be accessed by traditional click reactions were synthesized in good yields with excellent enantioselectivity. Both enantiomers were easily obtained from a pair of diastereoisomeric natural quinidine- and quinine-derived squaramides. A significant Ir(I)/squaramide coordination activation, but no self-quenching phenomenon was observed in this metal/organo cooperative catalytic system.

Organocatalytic Enantioselective Construction of Conformationally Stable C(sp2)-C(sp3) Atropisomers

Nonbiaryl atropisomers are molecules defined by a stereogenic axis featuring at least one nonarene moiety. Among these, scaffolds bearing a conformationally stable C(sp²)-C(sp³) stereogenic axis have been observed in natural compounds; however, their enantioselective synthesis remains almost completely unexplored. Herein we disclose a new class of chiral C(sp²)-C(sp³) atropisomers obtained with high levels of stereoselectivity (up to 99% ee) by means of an organocatalytic asymmetric methodology. Multiple molecular motifs could be embedded in this class of C(sp²)-C(sp³) atropisomers, showing a broad and general protocol. Experimental data provide strong evidence of the conformational stability of the C(sp²)-C(sp³) stereogenic axis (up to t_1/2^25 °C >1000 y) in the obtained compounds and show kinetic control over this rare stereogenic element. This, coupled with density functional theory calculations, suggests that the observed stereoselectivity arises from a Curtin-Hammett scenario establishing an equilibrium of intermediates. Furthermore, the experimental investigation led to evidence of the operating principle of central-to-axial chirality conversions.

Cobalt-Catalyzed Enantioselective C-H Arylation of Indoles

Atropoisomeric (hetero)biaryls are scaffolds with increasing importance in the pharmaceutical and agrochemical industries. Although it is the most obvious disconnection to construct such compounds, the direct enantioselective C-H arylation through the concomitant induction of the chiral information remains extremely challenging and uncommon. Herein, the unprecedented earth-abundant 3d-metal-catalyzed atroposelective direct arylation is reported, furnishing rare atropoisomeric C2-arylated indoles. Kinetic studies and DFT computation revealed an uncommon mechanism for this asymmetric transformation, with the oxidative addition being the rate- and enantio-determining step. Excellent stereoselectivities were reached (up to 96% ee), while using an unusual N-heterocyclic carbene ligand bearing an essential remote substituent. Attractive dispersion interactions along with positive C-H---π interactions exerted by the ligand were identified as key factors to guarantee the excellent enantioselection.

Catalytic asymmetric synthesis of 3,4'-indole-pyrazole derivatives featuring axially chiral bis-pentatomic heteroaryls

The atroposelective synthesis of bis-pentatomic heteroaryl systems is challenging due to the low rotation barrier and configurational instability of the 5,5-ring system. 3,4'-Indole-pyrazole is a bis-pentatomic heteroaryl scaffold existing in...

Synthesis of axially chiral N-aryl benzimidazoles via chiral phosphoric acid catalyzed enantioselective oxidative aromatization

Hantzsch ester and benzothiazoline have been extensively used as H2 donors in enantioselective transfer hydrogenation of C=N bond during the past several decades. However, the corresponding Hantzsch pyridine and benzothiazoles...

Ready Access to Densely Substituted Furans Using Tsuji-Wacker-Type Cyclization

A competent method for the construction of highly substituted furans catalyzed by Pd(II) and Cu(II) chloride has been developed. The method provides easy access to di-, tri-, and tetrasubstituted furans from corresponding diols with relatively mild conditions in a unified strategy. The developed method has been successfully tested with more than 25 substrates, which resulted in furans of multiple substitution patterns with up to 84% isolated yields.

Rational Design of Axially Chiral Styrene-Based Organocatalysts and Their Application in Catalytic Asymmetric (2+4) Cyclizations

A new class of axially chiral styrene-based thiourea tertiary amine catalysts, which have unique characteristics such as an efficient synthetic route, multiple chiral elements, and multiple activating groups, has been rationally designed. These new chiral catalysts have proven to be efficient organocatalysts, enabling the chemo-, diastereo-, and enantioselective (2+4) cyclization of 2-benzothiazolimines with homophthalic anhydrides in good yields (up to 96 %) with excellent stereoselectivities (all >95:5 dr, up to 98 % ee). More importantly, theoretical calculations elucidated the important role of an axially chiral styrene moiety in controlling both the reactivity and enantioselectivity. This work not only represents the first design of styrene-based chiral thiourea tertiary amine catalysts and the first catalytic asymmetric (2+4) cyclization of 2-benzothiazolimines, but also gives an in-depth understanding of axially chiral styrene-based organocatalysts.

Nitrosobenzene-Enabled Chiral Phosphoric Acid Catalyzed Enantioselective Construction of Atropisomeric N-Arylbenzimidazoles

Described herein is an imidazole ring formation strategy for the synthesis of axially chiral N-arylbenzimidazoles by means of chiral phosphoric acid catalysis. Two sets of conditions were developed to transform two classes of 2-naphthylamine derivatives into structurally diverse N-arylbenzimidazole atropisomers with excellent chemo- and regioselectivity as well as high levels of enantiocontrol. It is worth reflecting on the unique roles played by the nitroso group in this domino reaction. It functions as a linchpin by first offering an electrophilic site (N) for the initial C-N bond formation while the resulting amine performs the nucleophilic addition to form the second C-N bond. Additionally, it could facilitate the final oxidative aromatization as an oxidant. The atropisomeric products could be conveniently elaborated to a series of axially chiral derivatives, enabling the exploitation of N-arylbenzimidazoles for their potential utilities in asymmetric catalysis.

Dirhodium(ii)-catalysed cycloisomerization of azaenyne: rapid assembly of centrally and axially chiral isoindazole frameworks

Described herein is a dirhodium(ii)-catalyzed asymmetric cycloisomerization reaction of azaenyne through a cap-tether synergistic modulation strategy, which represents the first catalytic asymmetric cycloisomerization of azaenyne. This reaction is highly challenging because of its inherent strong background reaction leading to racemate formation and the high capability of coordination of the nitrogen atom resulting in catalyst deactivation. Varieties of centrally chiral isoindazole derivatives could be prepared in up to 99 : 1 d.r., 99 : 1 er and 99% yield and diverse enantiomerically enriched atropisomers bearing two five-membered heteroaryls have been accessed by using an oxidative central-to-axial chirality transfer strategy. The tethered nitrogen atom incorporated into the starting materials enabled easy late-modifications of the centrally and axially chiral products via C–H functionalizations, which further demonstrated the appealing synthetic utilities of this powerful asymmetric cyclization.

Rh(ii)-catalyzed asymmetric cycloisomerization of azaenyne through a cap-tether synergistic modulation strategy was described. Diverse centrally and axially chiral isoindazoles were prepared and directed C–H late-stage modifications were developed.

Catalytic Atroposelective C7 Functionalisation of Indolines and Indoles

Axially chiral atropisomeric compounds are widely applied in asymmetric catalysis and medicinal chemistry. In particular, axially chiral indole‐ and indoline‐based frameworks have been recognised as important heterobiaryl classes because they are the core units of bioactive natural alkaloids, chiral ligands and bioactive compounds. Among them, the synthesis of C7‐substituted indole biaryls and the analogous indoline derivatives is particularly challenging, and methods for their efficient synthesis are in high demand. Transition‐metal catalysis is considered one of the most efficient methods to construct atropisomers. Here, we report the enantioselective synthesis of C7‐indolino‐ and C7‐indolo biaryl atropisomers by means of C−H functionalisation catalysed by chiral RhJasCp complexes.

Asymmetric Synthesis of Axially Chiral Naphthyl-C3-indoles via a Palladium-Catalyzed Cacchi Reaction

Atropoisomeric biaryl motifs are widely found in natural products and bioactive compounds as well as chiral catalysts and ligands. Various efficient approaches have been disclosed for the construction of chiral six-six biaryl skeletons. In contrast, the enantioselective synthesis of axially chiral arylindoles through the strategy of de novo construction, other than the asymmetric functionalization of indoles, remain a challenging task. Herein we report an efficient Pd(0)/(S)-Segphos-catalyzed atroposelective Cacchi reaction of 2-alkynylanilines with sterically congested naphthyl halides, which afforded an array of naphthyl-C3-indoles in high yields with good to excellent atroposelectivities. The addition of water and the modulation of the manipulation procedure by premixing the palladium complex and the naphthyl halide were the keys to success. The conformational stability of the obtained axially chiral naphthyl-C3-indole containing a synthetically more-valuable free NH moiety is revealed through kinetic experiments.

Enantioselective Synthesis of Atropisomeric Biaryls by Pd-Catalyzed Asymmetric Buchwald-Hartwig Amination

N-C Biaryl atropisomers are prevalent in natural products and bioactive drug molecules. However, the enantioselective synthesis of such molecules has not developed significantly. Particularly, the enantioselective synthesis of N-C biaryl atropisomers by stereoselective metal-catalyzed aryl amination remains unprecedented. Herein, a Pd-catalyzed cross-coupling strategy is presented for the synthesis of N-C axially chiral biaryl molecules. A broad spectrum of N-C axially chiral compounds was obtained with excellent enantioselectivities (up to 99 % ee) and good yields (up to 98 %). The practicality of this reaction was validated in the synthesis of useful biological molecules.