Dynamic Kinetic Asymmetric Heck Reaction for the Simultaneous Generation of Central and Axial Chirality

Enantioselective synthesis of molecules with multiple stereogenic elements

This review explores the fascinating world of molecules featuring multiple stereogenic elements, unraveling the different strategies designed over the years for their enantioselective synthesis. Specifically, (dynamic) kinetic resolutions, desymmetrisations and simultaneous installation of stereogenic elements exploiting either metal- or organo-catalysis are the principal approaches to efficiently create and control the three-dimensional shapes of these attractive molecules. Although most molecules presented in this review possess a stereogenic carbon atom in combination with a stereogenic axis, other combinations with helices or planes of chirality have started to emerge, as well as molecules displaying more than two different stereogenic elements.

Palladium-Catalyzed Asymmetric Larock Isoquinoline Synthesis to Access Axially Chiral 3,4-Disubstituted Isoquinolines

Larock isoquinoline synthesis is one of the most efficient and straightforward approaches to the construction of 3,4-disubstituted isoquinolines. However, there have been no asymmetric versions for the synthesis of axially chiral isoquinolines since their initial report in 2001. Herein, we documented the first example of an asymmetric Larock isoquinoline synthesis by employing Pd(OAc)2/Walphos SL-W002-1 as the catalyst, affording the axially chiral 3,4-disubstituted isoquinolines with up to 97.5:2.5 er and 98% yield. Density Functional Theory (DFT) calculations clearly clarified the catalytic mechanism and the origin of the experimentally observed enantioselectivity.

Symmetric Anion Mediated Dynamic Kinetic Asymmetric Knoevenagel Reaction for N-C and N-N Atropisomers Synthesis

A symmetric anion mediated dynamic kinetic asymmetric Knoevenagel reaction was established as a general and efficient method for accessing both N-C and N-N atropisomers. The resulting highly enantio-pure pyridine-2,6(1H,3H)-diones exhibit diverse structures and functional groups. The key to excellent regio- and remote enantiocontrol could be owed to the hydrogen bond between the enolate anion and triflamide block of the organocatalyst. This connected the enolate anion and iminium cation by a chiral backbone. The mechanism investigation via control experiments, correlation analysis, and density functional theory calculations further revealed how the stereochemical information was transferred from the catalyst into the axially chiral pyridine-2,6(1H,3H)-diones. The synthetic applications also demonstrated the reaction's potential.

Asymmetric Synthesis of Axially Chiral N, N-1,2-Pentatomic Heterobiaryl Diamines by an Organocatalytic Arylation Reaction

The utilization of axially chiral biaryl diamines has been widely acknowledged as highly advantageous structures for the advancement of chiral catalysts and ligands. This highlights their extensive range of applications in asymmetric catalysis and synthesis. Herein, we devised a direct arylation reactions of 5-aminopyrazoles with azonaphthalenes, utilizing chiral phosphoric acid as the catalyst. This method delivers structurally novel atroposelective N, N-1,2-azole heteroaryl diamines with high yields (up to >98 %) and good to excellent enantiomeric ratios while exhibiting a wide range of substrate compatibility.

Enantioconvergent synthesis of axially chiral amides enabled by Pd-catalyzed dynamic kinetic asymmetric aminocarbonylation

Atropisomeric biaryls bearing carbonyl groups have attracted increasing attention due to their prevalence in diverse bioactive molecules and crucial role as efficient organo-catalysts or ligands in asymmetric transformations. However, their preparation often involves tedious multiple steps, and the direct synthesis via asymmetric carbonylation has scarcely been investigated. Herein, we report an efficient palladium-catalyzed enantioconvergent aminocarbonylation of racemic heterobiaryl triflates with amines via dynamic kinetic asymmetric transformation (DyKAT). This protocol features a broad substrate scope and excellent compatibility for rapid construction of axially chiral amides in good to high yields with excellent enantioselectivities. Detailed mechanistic investigations discover that the base can impede the intramolecular hydrogen bond-assisted axis rotation of the products, thus allowing for the success to achieve high enantioselectivity. Moreover, the achieved axially chiral heterobiaryl amides can be directly utilized as N,N,N-pincer ligands in copper-catalyzed enantioselective formation of C(sp3)-N and C(sp3)-P bonds.

Synergistic Photoredox/Palladium Catalyzed Enantioconvergent Carboxylation of Racemic Heterobiaryl (Pseudo)Halides with CO2

Herein, we report a synergistic photoredox/palladium catalytic system for the efficient enantioconvergent synthesis of axially chiral esters from racemic heterobiaryl (pseudo)halides (bromides/triflates) with CO2 and alkyl bromides under mild conditions. A wide range of axially chiral esters were obtained in good to high yields with excellent enantioselectivities. Detailed mechanistic studies unveiled that the ratio of photocatalyst and palladium catalyst exhibited significant impact on the chemo- and enantioselectivities of the reaction. Kinetic studies and control experiments supported the proposed mechanism involving cascade asymmetric carboxylation followed by SN2 substitution. The achievement of high enantioselectivity relies not only on the choice of synergistic metallaphotoredox catalysts but also on the utilization of alkyl bromides, which trap the generated chiral carboxylic anions in situ, thus preventing their immediate racemization.

Enantioselective Construction of Anthracenylidene-Based Axial Chirality by Asymmetric Heck Reaction

Anthracenylidene is an intriguing structural unit with potential in various fields. The study presents a novel approach to introducing axial chirality into this all-carbon core skeleton through a remotely controlled desymmetrization strategy. A palladium-catalyzed enantioselective Heck arylation of exocyclic double bond of anthracene with two distinct substituents at the C10 position is harnessed to realize such a transformation. The judicious identification of the P-centrally chiral ligand is pivotal to ensure the competitive competence in reactivity and stereocontrol when the heteroatom handle is absent from the anthracenylidene skeleton. Both C10 mono- and disubstituted substrates were compatible for the established catalytic system, and structurally diverse anthracenylidene-based frameworks were forged with good-to-high enantiocontrol. The subsequent derivatization of the obtained products yielded a valuable array of centrally and axially chiral molecules, thus emphasizing the practicality of this chemistry. DFT calculations shed light on the catalytic mechanism and provided insights into the origin of the experimentally observed enantioselectivity for this reaction.

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.

Atroposelective catalysis

Atropisomeric compounds-stereoisomers that arise from the restricted rotation about a single bond-have attracted widespread attention in recent years due to their immense potential for applications in a variety of fields, including medicinal chemistry, catalysis and molecular nanoscience. This increased interest led to the invention of new molecular motors, the incorporation of atropisomers into drug discovery programmes and a wide range of novel atroposelective reactions, including those that simultaneously control multiple stereogenic axes. A diverse set of synthetic methodologies, which can be grouped into desymmetrizations, (dynamic) kinetic resolutions, cross-coupling reactions and de novo ring formations, is available for the catalyst-controlled stereoselective synthesis of various atropisomer classes. In this Review, we generalize the concepts for the catalyst-controlled stereoselective synthesis of atropisomers within these categories with an emphasis on recent advancements and underdeveloped atropisomeric scaffolds beyond stereogenic C(sp2)-C(sp2) axes. We also discuss more complex systems with multiple stereogenic axes or higher-order stereogenicity.

CoH-catalyzed asymmetric remote hydroalkylation of heterocyclic alkenes: a rapid approach to chiral five-membered S- and O-heterocycles

Saturated heterocycles, which incorporate S and O heteroatoms, serve as fundamental frameworks in a diverse array of natural products, bioactive compounds, and pharmaceuticals. Herein, we describe a unique cobalt-catalyzed approach integrated with a desymmetrization strategy, facilitating precise and enantioselective remote hydroalkylation of unactivated heterocyclic alkenes. This method delivers hydroalkylation products with high yields and excellent stereoselectivity, representing good efficiency in constructing alkyl chiral centers at remote C3-positions within five-membered S/O-heterocycles. Notably, the broad scope and good functional group tolerance of this asymmetric C(sp3)-C(sp3) coupling enhance its applicability.

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.

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.

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.

A sustainable metal-free and additive-free olefination route to N-heteroazaarenes from methyl-substituted heterocycles and amines

A green and sustainable metal-free, additive-free olefination approach is proposed for the facile synthesis of various unsaturated N-heteroazaarenes from simple methyl-substituted heteroarenes and amines. The developed protocol employs only air as the sole oxidant and provides a useful strategy for obtaining various E-selective conjugated heterocyclic olefins. This provides a useful strategy for application in generating grams of a variety of unsaturated N-heteroazaarenes (up to 20.33 grams) and the synthetic imaging agents of STB-8 (2.40 gram) with high regioselectivity in one pot.

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.

Catalytic atroposelective synthesis of heterobiaryls with vicinal C-C and N-N diaxes via dynamic kinetic resolution

Reported herein is a highly efficient dynamic kinetic resolution protocol for the atroposelective synthesis of heterobiaryls with vicinal C-C and N-N diaxes. Atropisomers bearing vicinal diaxes mainly exist in o-triaryls, while that of biaryls is highly challenging in terms of the concerted rotation and deplanarization effects. The combination of C-C biaryl with N-N nonbiaryl delivers a novel class of vicinal-diaxis heterobiaryls. For their atroposelective synthesis, the dynamic kinetic resolution enabled by either quinine-catalyzed allylation or isothiourea-catalyzed acylation has been developed, allowing the preparation of a wide range of vicinal-axis heterobiaryls in good yields with excellent enantioselectivities. Atropisomerization experiments revealed that the C-C bond rotation led to diastereomers, and the N-N bond rotation offered enantiomers. Besides, this protocol could be extended to kinetic resolution by employing substrates with a more hindered axis.

Atroposelective Ir-Catalyzed C-H Borylation of Phthalazine Heterobiaryls

The atroposelective iridium-catalyzed borylation of menthyloxy-substituted phthalazine heterobiaryls with diborons is reported. Utilizing [Ir(OMe)(COD)]2/2-aminopyridine as a rarely used efficient catalyst system, the heterobiaryls were selectively borylated in the 2-position of the carbocycle, exclusively yielding only one of the atropisomers, depending on the substitution of the phthalazine with (+)-menthyl or (-)-menthyl moieties. Exemplary further functionalization of a borylated atropisomer demonstrated that nickel-catalyzed Suzuki-Miyaura cross-coupling with an aryl halide was able to provide stereoretention to a certain degree (up to 75% de).

Atroposelective PIII /PV =O Redox Catalysis for the Isoquinoline-Forming Staudinger-aza-Wittig Reaction

Herein, we describe the feasibility of atroposelective PIII /PV =O redox organocatalysis by the Staudinger-aza-Wittig reaction. The formation of isoquinoline heterocycles thereby enables the synthesis of a broad range of valuable atropisomers under mild conditions with enantioselectivities of up to 98 : 2 e.r. Readily prepared azido cinnamate substrates convert in high yield with stereocontrol by a chiral phosphine catalyst, which is regenerated using a silane reductant under Brønsted acid co-catalysis. The reaction provides access to diversified aryl isoquinolines, as well as benzoisoquinoline and naphthyridine atropisomers. The products are expeditiously transformed into N-oxides, naphthol and triaryl phosphine variants of prevalent catalysts and ligands. With dinitrogen release and aromatization as ideal driving forces, it is anticipated that atroposelective redox organocatalysis provides access to a multitude of aromatic heterocycles with precise control over their configuration.

Copper-Catalyzed Aryne Insertion into the Carbon-Iodine Bond of Heteroaryl Iodides

Aryne insertions into the carbon-iodine bond of heteroaryl iodides has been achieved for the first time. This novel reaction provides an efficient pathway for the synthesis of valuable building blocks 2-iodoheterobiaryls from heteroaryl iodides and o-silylaryl triflates in excellent regioselectivity. The copper(I) catalyst, which bears a N-heterocyclic carbene (NHC) ligand, is essential to accomplish the reaction. Control reactions and DFT calculations indicate that the coordination of copper, as a Lewis acid, with nitrogen atoms of heteroaryl iodides mediates the insertion of arynes into heteroaryl carbon-iodine bonds.

Stereodynamic Strategies to Induce and Enrich Chirality of Atropisomers at a Late Stage

Enantiomers, where chirality arises from restricted rotation around a single bond, are atropisomers. Due to the unique nature of the origins of their chirality, synthetic strategies to access these compounds in an enantioselective manner differ from those used to prepare enantioenriched compounds containing point chirality arising from an unsymmetrically substituted carbon center. In particular stereodynamic transformations, such as dynamic kinetic resolutions, thermodynamic dynamic resolutions, and deracemizations, which rely on the ability to racemize or interconvert enantiomers, are a promising set of transformations to prepare optically pure compounds in the late stage of a synthetic sequence. Translation of these synthetic approaches from compounds with point chirality to atropisomers requires an expanded toolbox for epimerization/racemization and provides an opportunity to develop a new conceptual framework for the enantioselective synthesis of these compounds.

Chiral PSiSi-Ligand Enabled Iridium-Catalyzed Atroposelective Intermolecular C-H Silylation

Catalytic enantioselective intermolecular C-H silylation offers an efficient approach for the rapid construction of chiral organosilicon compounds, but remains a significant challenge. Herein, a new type of chiral silyl ligand is developed, which enables the first iridium-catalyzed atroposelective intermolecular C-H silylation reaction of 2-arylisoquinolines. This protocol features mild reaction conditions, high atom economy, and remarkable yield with excellent stereoselectivity (up to 99 % yield, 99 % ee), delivering enantioenriched axially chiral silane platform molecules with facile convertibility. Key to the success of this unprecedented transformation relies on a novel chiral PSiSi-ligand, which facilitates the intermolecular C-H silylation process with perfect chem-, regio- and stereo-control via a multi-coordinated silyl iridium complex.

Dynamic Kinetic Resolution of 2-(Quinolin-8-yl)Benzaldehydes: Atroposelective Iridium-Catalyzed Transfer Hydrogenative Allylation

An atroposelective Ir-catalyzed dynamic kinetic resolution (DKR) of 2-(quinolin-8-yl)benzaldehydes/1-naphthaldehydes by transfer hydrogenative coupling of allyl acetate is disclosed. The allylation reaction takes place with simultaneous installation of central and axial chirality, reaching high diastereoselectivities and excellent enantiomeric excesses when ortho-cyclometalated iridium-DM-BINAP is used as the catalyst. The racemization of the substrates occurs through a designed transient Lewis acid-base interaction between the quinoline nitrogen atom and the aldehyde carbonyl group.

Construction of C-B axial chirality via dynamic kinetic asymmetric cross-coupling mediated by tetracoordinate boron

Catalytic dynamic kinetic asymmetric transformation (DyKAT) provides a powerful tool to access chiral stereoisomers from racemic substrates. Such transformation has been widely employed on the construction of central chirality, however, the application in axial chirality remains underexplored because its equilibrium of substrate enantiomers is limited to five-membered metalacyclic intermediate. Here we report a tetracoordinate boron-directed dynamic kinetic asymmetric cross-coupling of racemic, configurationally stable 3-bromo-2,1-azaborines with boronic acid derivatives. A series of challenging C-B axially chiral compounds were prepared with generally good to excellent enantioselectivities. Moreover, this transformation can also be extended to prepare atropisomers bearing adjacent C-B and C-C diaxes with excellent diastereo- and enantio-control. The key to the success relies on the rational design of a reversible tetracoordinate boron intermediate, which is supported by theoretical calculations that dramatically reduces the rotational barrier of the original C-B axis and achieves the goal of DyKAT.

Ruthenium(II)-Catalyzed Sterically Hindered C-H Acyloxylation to Synthesize Biaryl Isoquinoline Derivatives via Peresters

A novel C-H acyloxylation method of 1-(1-naphthalen-1-yl)isoquinoline derivatives with peresters in the presence of [Ru(p-cymene)Cl₂]₂ has been developed. The combination of ruthenium(II), AgBF₄, CoI₂, and 2,2,6,6-tetramethyl-1-piperidinyloxy is found to be an effective catalytic system to provide various biaryl compounds in satisfactory yields within minutes. Notably, steric hindrance is a very important determinant of the reaction.

Assisted Tandem Pd Catalysis Enables Regiodivergent Heck Arylation of Transiently Generated Substituted Enol Ethers

Two complementary regiodivergent Pd-catalyzed assisted tandem [isomerization/Heck arylation] reactions are reported. They provide access to a broad array of acyclic trisubstituted vinyl ethers starting from readily available alkenyl ethers. In both cases, the isomerization is conducted with a [Pd-H] precatalyst supported by tris-tert-butyl phosphine ligands. When the catalyst is modified by the addition of a chelating bisphosphine ligand (dppp), an organic base (Cy2NMe), sodium acetate, and aryl triflates are used as electrophiles, the α-arylation pathway is promoted preferentially. The β-arylation pathway is favored for electron-deficient and electron-neutral aryl halides when the catalyst is simply modified by the addition of an excess of an organic base (Et3N) after completion of the isomerization reaction. Electron-rich aryl halides lead to reduced levels of regiocontrol. The moderate stereoselectivity obtained are proposed to reflect the absence of stereocontrol in the isomerization step. Computational analyses suggest that migratory insertion is selectivity-determining for both the arylations. For the β-selective arylation, an energy decomposition analysis underscored that electronic factors favor α-regioselectivity and steric effects favor β-regioselectivity. Preliminary investigations show that high levels of stereoselectivity can be achieved for the α-selective arylation by ligand control. Complementarily, reaction conditions for postcatalytic stereo-correction have also been identified for each catalytic system.

Arylboration of Enecarbamates for the Synthesis of Borylated Saturated N-Heterocycles

Two catalytic systems have been developed for the arylboration of endocyclic enecarbamates to deliver synthetically versatile borylated saturated N-heterocycles in good regio- and diastereoselectivities. A Cu/Pd dual catalytic reaction enables the synthesis of borylated, α-arylated azetidines, while a Ni-catalysed arylboration reaction efficiently functionalizes 5-, 6-, and 7-membered enecarbamates. In the case of the Cu/Pd-system, a remarkable additive effect was identified that allowed for broader scope. The products are synthetically useful, as demonstrated by manipulations of the boronic ester to access biologically active compounds.

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.

Pd-Catalyzed Cross-Couplings: On the Importance of the Catalyst Quantity Descriptors, mol % and ppm

This Review examines parts per million (ppm) palladium concentrations in catalytic cross-coupling reactions and their relationship with mole percentage (mol %). Most studies in catalytic cross-coupling chemistry have historically focused on the concentration ratio between (pre)catalyst and the limiting reagent (substrate), expressed as mol %. Several recent papers have outlined the use of “ppm level” palladium as an alternative means of describing catalytic cross-coupling reaction systems. This led us to delve deeper into the literature to assess whether “ppm level” palladium is a practically useful descriptor of catalyst quantities in palladium-catalyzed cross-coupling reactions. Indeed, we conjectured that many reactions could, unknowingly, have employed low “ppm levels” of palladium (pre)catalyst, and generally, what would the spread of ppm palladium look like across a selection of studies reported across the vast array of the cross-coupling chemistry literature. In a few selected examples, we have examined other metal catalyst systems for comparison with palladium.

Correia C, Ros A, Hornillos V, Iglesias-Sigüenza J, Fernández R, Lassaletta JM. Pd-Catalyzed Dynamic Kinetic Asymmetric Cross-Coupling of Heterobiaryl Bromides with N-Tosylhydrazones

A dynamic kinetic asymmetric Pd-catalyzed cross-coupling reaction of heterobiaryl bromides with ketone N-tosylhydrazones for the synthesis of heterobiaryl styrenes is described. The combination of Pd(dba)₂ as a precatalyst with a TADDOL-derived phosphoramidite ligand provides the corresponding coupling products in good yields and high enantioselectivities under mild conditions. Racemization-free N-oxidation and N-alkylation of the products allowed us to obtain appealing functionalized axially chiral heterobiaryl derivatives.

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.

Organocatalytic Enantioselective Construction of Spiroketal Lactones Bearing Axial and Central Chirality via an Asymmetric Domino Reaction

The catalytic asymmetric synthesis of chiral compounds with multiple stereogenic elements via a single catalytic process is challenging. This paper proposes a domino asymmetric electrophilic halocyclization strategy for constructing heterocycloalkenyl atropisomeric spiroketal lactones. A single catalyst was utilized to realize two independent stereodetermining steps. Various spiroketal lactones containing both chiral axes and chiral centers were prepared in excellent yields with excellent enantioselectivity and diastereoselective (up to 99% ee and >20:1 dr).

Catalytic Asymmetric Construction of Axially and Centrally Chiral Heterobiaryls by Minisci Reaction

Axially chiral biaryls and heterobiaryls constitute the most represented subclass of atropisomers with prevalence in natural products, bioactive compounds, privileged chiral ligand/catalysts, and optically pure materials. Despite many ionic protocols for their construction, radical-based variants represent another highly desirable and intriguing strategy but are far less developed. Moreover, efficient synthesis of axially chiral heterobiaryl molecules, especially ones having multiple heteroatoms and other types of chiral elements, through radical routes remains extremely limited. We herein disclose the first catalytic asymmetric, metal-free construction of axially and centrally chiral heterobiaryls by Minisci reaction of 5-arylpyrimidines and α-amino acid-derived redox-active esters. This is enabled by the use of 4CzIPN as an organic photoredox catalyst in conjunction with a chiral phosphoric acid catalyst. The reaction achieved a variety of interesting 5-arylpyrimidines featuring the union of an axially chiral heterobiaryl and a centrally chiral α-branched amine with generally excellent regio-, diastereo-, and enantioselectivity (up to 82% yield; >19:1 dr; >99% ee). This finding also builds up a new platform for the development of desymmetrization methods via radical-involved atroposelective functionalization at heteroarene of prochiral heterobiaryls.

DCID-Mediated Heck Cross-Coupling of Phenols via C-O Bond activation

In this work a Palladium-catalyzed Heck cross-coupling of phenols using dichloroimidazolidinedione (DCID) as new reagent for the activation of C-O bond has been developed for the first time. Substituted phenols...

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.

Twofold C-H Activation-Based Enantio- and Diastereoselective C-H Arylation Using Diarylacetylenes as Rare Arylating Reagents

C-H bond activation has been established as an attractive strategy to access axially chiral biaryls, and the most straightforward method is direct C-H arylation of arenes. However, the arylating source has been limited to several classes of reactive and bulky reagents. Reported herein is rhodium-catalyzed 1:2 coupling of diarylphosphinic amides and diarylacetylenes for enantio- and diastereoselective construction of biaryls with both central and axial chirality. This twofold C-H activation reaction stays contrast to the previously explored Miura-Satoh type 1:2 coupling of arenes and alkynes in terms of chemoselectivity and proceeded under mild conditions with the alkyne acting as a rare arylating reagent. Both C-H activation events are stereo-determining and are under catalyst control, with the 2^nd C-H activation being diastereo-determining in a remote fashion. Analysis of the stereochemistry of the major and side products suggests moderate enantioselectivity of the initial C-H activation-desymmetrization process. However, the minor (R) rhodium vinyl intermediate is consumed more readily in undesired protonolysis, eventually resulting in high enantio- and diastereoselectivity of the major product.

Enantioselective Synthesis of Axially Chiral Benzothiophene/Benzofuran-Fused Biaryls by N-Heterocyclic Carbene Catalyzed Arene Formation

Axially chiral biaryl scaffolds are prevalent in natural products, chiral ligands, and organocatalysts. However, N-heterocyclic carbene (NHC) catalyzed de novo construction of an aromatic ring with concomitant axial chirality induction for the synthesis of biaryl atropisomers is far less developed, and the efficient synthesis of axially chiral tetra-ortho-substituted biaryls remains an unsolved problem under NHC catalysis. Reported here is an NHC-catalyzed de novo synthesis of axially chiral benzothiophene/benzofuran-fused biaryls from enals and 2-benzyl-benzothiophene/benzofuran-3-carbaldehydes through a [2+4] annulation, decarboxylation, and oxidative aromatization cascade with central-to-axial chirality conversion. The developed method provides efficient and general access to novel axially chiral benzothiophene/benzofuran-fused biaryls in high enantioselectivities and works well for the synthesis of tetra-ortho-substituted biaryls.