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

Stereocontrol in Conformationally Stable C(sp2)─C(sp3) Atropisomers

We report a two-step sequence for the enantioselective construction of a rare family of stable C(sp2)─C(sp3) atropisomers featuring two additional stereogenic centers. The process begins with an organocatalyzed dihydrobenzofurannulation that establishes and controls the stereochemistry of two carbon centers, followed by a highly diastereoselective functionalization that significantly enhances the barrier to diastereomerization of the C(sp2)─C(sp3) bond, effectively locking and stabilizing its configuration.

Mechanism and Origin of Nickel-Catalyzed Decarbonylative Construction of C(sp2)-C(sp3) Bonds from Carboxylic Acids and Their Derivatives

Nickel-catalyzed arylation of carboxylic acids provides a ligand-controlled chemoselectivity-switchable method for the construction of C(sp2)-C(sp3) bonds. Here, we employed density functional theory to provide a detailed understanding of the mechanism and origin of nickel-catalyzed ligand-controlled carbonyl transformation. This reaction generates decarbonylation products through oxidative addition, activation of C-C bonds, decarbonylation, binding of alkyl radicals with Ni(III) complexes, and final reduction elimination step. The activation of C-C bonds in aromatic carboxylate esters is more favorable than C-O bond activation because of the interaction between the nickel catalyst and the π orbitals of the substrate's aromatic moiety during C-C bond activation. The induction effect of the ligand and the carbonyl group together determines the transfer tendency of the carbonyl group.

Catalytic Enantioconvergent Alkylation Reactions That Construct Chiral Quaternary Carbon Centers and Tune C(sp3)-C(sp2) Bond Rotation

In molecules with central chiral centers functionalized with aryl groups, the rotation of the single bond between the aryl group and the central chiral center may be hindered. Enantioconvergent alkylation reactions of racemic diastereomers to construct all-carbon quaternary stereocenters resulting in easing of the rotation around the axes are described. We demonstrated that the rotation of the bond between a central chiral carbon center and a dihydrobenzofuranone can be tuned by selection of either a tertiary carbon bearing a hydrogen or a quaternary carbon at the central chiral center. The products have the benefits of a central chiral center and a flexibly rotatable bond, which can facilitate interactions with other molecules.

Organocatalytic Asymmetric Synthesis of C-N Atropisomers with Pyrrole, Oxindole and Succinimide Scaffold

An asymmetric synthesis of C-N atropisomers with pyrrole, oxindole and succinimide moities was developed via organocatalytic desymmetric Michael addition of 3-pyrrolyloxindole with prochiral N-aryl maleimides. The C-N atropisomers were obtained in acceptable yields with high diastero- and enantioselectivities (>20 : 1 dr, up to >99 % ee). C-N Rotational energy barrier has also been determined.

Recent advances in organocatalytic atroposelective reactions

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

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.

High Rotational Barrier Atropisomers

Atropisomers have attracted a great deal of attention lately due to their numerous applications in organic synthesis and to their employment in drug discovery. However, the synthetic arsenal at our disposal with which to access them remains limited. The research described herein is two-pronged; we both demonstrate the use of MCR chemistry as a synthetic strategy for the de novo synthesis of a class of atropisomers having high barriers to rotation with the simultaneous insertion of multiple chiral elements and we study these unprecedented molecular systems by employing a combination of crystallography, NMR and DFT calculations. By fully exploiting the synthetic capabilities of our chemistry, we have been able to monitor a range of different types of interaction, i. e. π-π, CH-π, heteroatom-π and CD-π, in order to conduct structure-property studies. The results could be applied both to atroposelective synthesis and in drug discovery.

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.

Towards structurally versatile mesoionic N-heterocyclic olefin ligands and their coordination to palladium, gold, and boron hydride

We have developed an efficient and versatile approach for the synthesis of a family of 1,2,3-triazole-based mesoionic N-heterocyclic olefin (mNHO) ligands and investigated their coordination to palladium, gold, and boron hydride experimentally and computationally. We reacted mNHOs obtained through deprotonation of the corresponding methylated and ethylated 1,3,4-triaryl-1,2,3-triazolium salts with [Pd(allyl)Cl]2 to give the corresponding [Pd(η3-allyl)Cl(mNHO)] coordination complexes. 13C NMR data revealed the strong σ-donor character of the mNHO ligands, consistent with the calculated bond orders and atom-condensed charges. Furthermore, we also synthesized [AuCl(mNHO)] and a BH3-mNHO adduct by reacting the triazolium salts with AuCl(SMe2) and BH3·THF, respectively. The BH3-mNHO adduct was tested in the reduction of select aldehydes and ketones to alcohols.

Design and Asymmetric Control of Orientational Chirality by Using the Combination of C(sp2)-C(sp) Levers and Achiral N-Protecting Group

New chiral targets of orientational chirality have been designed and asymmetrically synthesized by taking advantage of N-sulfinyl imine-directed nucleophilic addition/oxidation, Suzuki-Miyaura, and Sonogashira cross-coupling reactions. Orientation of single isomers has been selectively controlled by using aryl/alkynyl levers [C(sp2)-C(sp) axis] and tBuSO2- protecting group on nitrogen as proven by X-ray diffraction analysis. The key structural characteristic of resulting orientational products is shown by remote through-space blocking manner. Seventeen examples of multi-step synthesis were obtained with modest to good chemical yields and complete orientational selectivity.

C(sp)-C(sp) Lever-Based Targets of Orientational Chirality: Design and Asymmetric Synthesis

In this study, the design and asymmetric synthesis of a series of chiral targets of orientational chirality were conducted by taking advantage of N-sulfinylimine-assisted nucleophilic addition and modified Sonogashira catalytic coupling systems. Orientational isomers were controlled completely using alkynyl/alkynyl levers [C(sp)-C(sp) axis] with absolute configuration assignment determined by X-ray structural analysis. The key structural element of the resulting orientational chirality is uniquely characterized by remote through-space blocking. Forty examples of multi-step synthesis were performed, with modest to good yields and excellent orientational selectivity. Several chiral orientational amino targets are attached with scaffolds of natural and medicinal products, showing potential pharmaceutical and medical applications in the future.

Recent advances in catalytic asymmetric synthesis

Asymmetric catalysis stands at the forefront of modern chemistry, serving as a cornerstone for the efficient creation of enantiopure chiral molecules characterized by their high selectivity. In this review, we delve into the realm of asymmetric catalytic reactions, which spans various methodologies, each contributing to the broader landscape of the enantioselective synthesis of chiral molecules. Transition metals play a central role as catalysts for a wide range of transformations with chiral ligands such as phosphines, N-heterocyclic carbenes (NHCs), etc., facilitating the formation of chiral C-C and C-X bonds, enabling precise control over stereochemistry. Enantioselective photocatalytic reactions leverage the power of light as a driving force for the synthesis of chiral molecules. Asymmetric electrocatalysis has emerged as a sustainable approach, being both atom-efficient and environmentally friendly, while offering a versatile toolkit for enantioselective reductions and oxidations. Biocatalysis relies on nature's most efficient catalysts, i.e., enzymes, to provide exquisite selectivity, as well as a high tolerance for diverse functional groups under mild conditions. Thus, enzymatic optical resolution, kinetic resolution and dynamic kinetic resolution have revolutionized the production of enantiopure compounds. Enantioselective organocatalysis uses metal-free organocatalysts, consisting of modular chiral phosphorus, sulfur and nitrogen components, facilitating remarkably efficient and diverse enantioselective transformations. Additionally, unlocking traditionally unreactive C-H bonds through selective functionalization has expanded the arsenal of catalytic asymmetric synthesis, enabling the efficient and atom-economical construction of enantiopure chiral molecules. Incorporating flow chemistry into asymmetric catalysis has been transformative, as continuous flow systems provide precise control over reaction conditions, enhancing the efficiency and facilitating optimization. Researchers are increasingly adopting hybrid approaches that combine multiple strategies synergistically to tackle complex synthetic challenges. This convergence holds great promise, propelling the field of asymmetric catalysis forward and facilitating the efficient construction of complex molecules in enantiopure form. As these methodologies evolve and complement one another, they push the boundaries of what can be accomplished in catalytic asymmetric synthesis, leading to the discovery of novel, highly selective transformations which may lead to groundbreaking applications across various industries.

Atropisomerism about aryl-C(sp3) bonds: chemically driven rotational pathway in cannabidiol derivatives

The conformational behaviour arising from the restricted C(sp2)-C(sp3) axis in ortho O-substituted naphthylcyclohexane and naphthylcyclohexene oxide derivatives of cannabidiol was examined by means of VT-NMR experiments and DFT calculations. Atropisomeric compounds with barriers in the range of 91.1 to 95.1 kJ mol-1 were obtained at 298 K. Two possible transition states (TS1 and TS2) were located, one is more stable depending on the chemical modification made on the monoterpene ring close to the pivot bond. Extended analysis of TS structures to previously reported phenyl derivatives bearing the same O-substituent led to similar rotational pathways according to the series: through TS1 in arylcylohexenes and TS2 in arylcyclohexanes. Likewise, conversion of arylcyclohexenes into both series affects the rotation speed by decelerating it, and the nature of the aryl ring seems to have a very minor effect on this phenomenon.

Highly Enantioselective Construction of the Polycyclic Piperazin-2-ones via NHC-Catalyzed [12 + 2] Cycloadditions

NHC-catalyzed diastereoselective and enantioselective [12 + 2] higher-order cycloadditions of 5H-benzo[a]-pyrrolizine-3-carbaldehydes with cyclic sulfonic imines are described. Various optically pure polycyclic piperazin-2-ones were successfully constructed under mild reaction conditions with excellent diastereoselectivities and enantioselectivities. The generated chiral polycyclic piperazin-2-ones could be transformed to planar-chiral nine-membered cyclic amides via squaramide-catalyzed asymmetric ring-expansion reactions.

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.

Higher-Order Cycloaddition Reactions for the Construction of Polycyclic Aromatic and Polycyclic Heteroaromatic Compounds

Cycloadditions are an important class of reactions in materials science for the construction of polycyclic aromatic and polycyclic heteroaromatic compounds. Recently, cycloadditions have been expanded beyond the "classical" group of cycloadditions involving six π-electrons, and it is now possible to control cycloadditions for an extended number of π-electrons by applying organocatalysis. This novel field of cycloadditions-termed higher-order cycloadditions-allows new synthetic methodologies to construct polycyclic carbo- and heteroaromatic compounds in two or three dimensions. This concept presents higher-order cycloadditions as a method for accessing two- and three-dimensional azulenes and cyclazines, as well as three dimensional indenes, as polycyclic aromatic and polycyclic heteroaromatic compounds.

Ring-fused hexahydro-1,2,4,5-tetrazines: synthesis, structure, and mechanistic studies on isolable rotational isomers

We designed conformationally stable rotational isomers around the C(sp²)-C(sp³) axis at the C₃-position of hexahydro-1,2,4,5-tetrazines. Isolation of each rotamer by silica gel column chromatography was successfully achieved at room temperature. The proposed isomerization mechanism of the rotamers was supported by NMR kinetic studies.

A new chiral phenomenon of orientational chirality, its synthetic control and computational study

A new type of chirality, orientational chirality, consisting of a tetrahedron center and a remotely anchored blocker, has been discovered. The key structural element of this chirality is characterized by multiple orientations directed by a through-space functional group. The multi-step synthesis of orientational chiral targets was conducted by taking advantage of asymmetric nucleophilic addition, Suzuki-Miyaura cross-coupling and Sonogashira coupling. An unprecedented catalytic species showing a five-membered ring consisting of C (sp²)-Br-Pd-C (sp²) bonds was isolated during performing Suzuki-Miyaura cross-coupling. X-ray diffraction analysis confirmed the species structure and absolute configuration of chiral orientation products. Based on X-ray structures, a model was proposed for the new chirality phenomenon to differentiate the present molecular framework from previous others. DFT computational study presented the relative stability of individual orientatiomers. This discovery would be anticipated to result in a new stereochemistry branch and to have a broad impact on chemical, biomedical, and material sciences in the future.

Role of Cinchona Alkaloids in the Enantio- and Diastereoselective Synthesis of Axially Chiral Compounds

Asymmetric synthesis using organic catalysts has evolved since it was first realized and defined. Nowadays, it can be considered a valid alternative to transition metal catalysis for synthesizing chiral molecules. According to the literature, the number of asymmetric organocatalytic processes associated with atropisomer synthesis has rapidly increased over the past 10 years because organocatalysis addresses the challenges posed by the most widespread strategies used for preparing axially chiral molecules with satisfactory results.These strategies, useful to prepare a wide range of C-C, C-heteroatom, and N-N atropisomers, vary from kinetic resolution to direct arylation, desymmetrization, and central-to-axial chirality conversion. In this field, our contribution focuses on determining novel methods for synthesizing atropisomers, during which, in most cases, the construction of one or more stereogenic centers other than the stereogenic axis occurred. To efficiently address this challenge, we exploited the ability of catalysts based on a cinchona alkaloid scaffold to realize enantioselective organic transformations. Desymmetrization of N-(2-tert-butylphenyl) maleimides was one of the first strategies that we pursued for preparing C-N atropisomers. The main principle is based on the presence of a rotationally hindered C-N single bond owing to the presence of a large tert-butyl group. Following the peculiar reactivity of this type of substrate as a powerful electrophile and dienophile, we realized several transformations.First, we investigated the vinylogous Michael addition of 3-substituted cyclohexenones, where a stereogenic axis and two contiguous stereocenters were concomitantly and remotely formed and stereocontrolled using a primary amine catalyst. Subsequently, we realized desymmetrization via an organocatalytic Diels-Alder reaction of activated unsaturated ketones that enabled highly atropselective transformation with efficient diastereoselectivity, thereby simultaneously controlling four stereogenic elements. Employing chiral organic bases allowed us to realize efficient desymmetrizations using carbon nucleophiles, such as 1,3-dicarbonyl compounds, cyanoacetates, and oxindoles. These reactions, performed with different types of catalysts, highlighted the versatility of organocatalysis as a powerful strategy for atropselective desymmetrization of pro-axially chiral maleimides.Hereafter, we studied the Friedel-Crafts alkylation of naphthols with indenones, a powerful method for enantioselective synthesis of conformationally restricted diastereoisomeric indanones. We realized the first axially chiral selective Knoevenagel condensation using cinchona alkaloid primary amine as the catalyst. This reaction provided a powerful method to access enantioenriched olefins containing the oxindole core. Subsequently, we initiated an intense program for the computational investigation of the reaction mechanism of our atropselective processes. An understanding of the catalytic activity for vinylogous atropselective desymmetrization as well as of the role played by the acidic cocatalyst used for the experimental work was achieved.Recently, we have garnered interest in the novel frontiers of atropselective synthesis. As observed in recent publications, there is considerable interest in the development of methods for preparing N-N atropisomers, an emerging topic in the field of atropselective synthesis. We focused on the synthesis of hydrazide atropisomers by developing a one-pot sequential catalysis protocol based on two sequential organocatalytic reactions that provided high stereocontrol of two contiguous stereogenic elements.

Orientational Chirality, Its Asymmetric Control, and Computational Study

Orientational chirality was discovered and characterized by a C(sp)-C(sp3) axis-anchored chiral center and a remotely anchored blocker. X-ray structural analysis proved that orientatiomers are stabilized by through-space functional groups, making it possible for 1 R- or S-chiral center to exhibit 3 orientational isomers simply by rotating operations. A new model system was proposed, fundamentally different from the traditional Felkin-Ahn-type or Cram-type models. In these traditional models, chiral C(sp3) center and blocking C(sp2) carbons are connected adjacently, and there exist 6 energy barriers during rotating along the C(sp2)-C(sp3) axis. In comparison, the present orientational chirality model shows that a chiral C(sp)-C(sp3) carbon is remotely located from a blocking group. Thus, it is focused on the steric dialog between a chiral C(sp3) center and a remotely anchored functional group. There exist 3 energy barriers for either (R)- or (S)-C(sp)-C(sp3) stereogenicity in the new model. Chiral amide auxiliary was proven to be an excellent chiral auxiliary in controlling rotations of orientatiomers to give complete stereoselectivity. The asymmetric synthesis of individual orientatiomers was conducted via multistep synthesis by taking advantage of the Suzuki-Miyaura cross-coupling and Sonogashira coupling reactions. Density functional theory computational study presented optimized conformers and relative energies for individual orientatiomers. This discovery would be anticipated to result in a new stereochemistry topic and have a broad impact on chemical, biomedical, and material sciences in the future.

Asymmetric higher-order [10 + n] cycloadditions of palladium-containing 10π-cycloaddends

We uncovered an asymmetric higher-order [10 + 2] cycloaddition reaction between diverse activated alkenes and a new type of π-allylpalladium complex-containing dipole-type 10π-cycloaddend, which was generated in situ from 2-methylene-1-indanols via a dehydrative insertion and deprotonation strategy under double activation of Pd(0) and phosphoric acid. A similar strategy was applied to an asymmetric higher-order [10 + 8] cycloaddition reaction or [10 + 4] cycloaddition reaction by using a heptafulvene derivative or a cyclic enone, respectively, as the acceptor. A variety of polycyclic frameworks imbedding an indene core were generally furnished in moderate to excellent yields with high levels of enantioselectivity by employing a newly designed chiral phosphoramidite ligand.

Enantioselective Synthesis of Atropisomeric Biaryl Phosphorus Compounds by Chiral-Phosphonium-Salt-Enabled Cascade Arene Formation

Axially chiral biaryl monophosphorus molecules, exemplified by atropisomeric 1,1'-biaryl aminophosphines, are significant motifs in numerous chiral ligands/catalysts. Developing efficient methods for preparing phosphorus compounds with these privileged motifs is an important endeavor in synthetic chemistry. Herein, we develop an effective, modular method by a chiral-phosphonium-salt-catalyzed novel cascade between phosphorus-containing nitroolefins and α,α-dicyanoolefins, leading to a great diversity of atropisomeric biaryls bearing phosphorus groups in high yields with excellent stereoselectivities. The reaction features include a Thorpe-type cycloaddition/oxidative hydroxylation/aromatization cascade pathway with a central-to-axial chirality transfer process. Insight gained from our studies is expected to advance general efforts towards the catalytic synthesis of atropisomeric biaryl phosphorus compounds, offering a platform for developing new efficient chiral ligands and catalysts.