Catalytic Enantioselective Synthesis of C–N Atropisomeric Heterobiaryls

Regio- and Stereoselective α-Allylation with Enolates Prepared from N-C Axially Chiral Thiolactam and Lactam

The reaction of allyl bromide derivatives with the enolate prepared from enantioenriched N-C axially chiral N-(2,5-di-tert-butylphenyl)-3,4-dihydroquinolin-2-one (lactam) and -thione (thiolactam) proceeded in a completely regio- and stereoselective manner to afford SN2 and SN2'-like products, respectively. Furthermore, through the conversion of thiolactam to lactam, the regiodivergent and stereoselective synthesis of N-C axially chiral lactams bearing a chiral tertiary α-carbon was achieved.

Atropisomeric Foldamers

This concept article explores the emerging role of atropisomerism in foldamer chemistry, a field focussed on oligomers that adopt well-defined conformations through non-covalent interactions. Atropisomerism introduces a novel dimension to foldamer design by restricting rotational freedom around single bonds to dictate molecular shape with precision. Despite the prevalence of atropisomeric bonds in organic synthesis, their application within foldamers remains underexplored. Here, we discuss key developments in both backbone and sidechain atropisomerism, and suggest future directions for atropisomeric foldamers in the context of a recent surge in atropselective synthetic methods. We propose that judicious use of atropisomerism may serve as a transformative tool in the construction of shape-defined macromolecules.

Atroposelective Chan-Evans-Lam Amination

The synthetic control of atropoisomerism along C-N bonds is a major challenge, and methods that allow C-N atroposelective bond formation are rare. This is a problem because each atropoisomer can feature starkly differentiated biological properties. Yet, among the three most practical and applicable classical amination methods available: 1) the Cu-catalyzed Ullmann-Goldberg reaction, 2) the Pd-catalyzed Buchwald-Hartwig reaction, and 3) the Cu-catalyzed Chan-Evans-Lam reaction, none has truly been rendered atroposelective at the newly formed C-N bond. The first ever Chan-Evans-Lam atroposelective amination is herein described with a simple copper catalyst and newly designed PyrOx chiral ligand. This method should find important applications in asymmetric synthesis, in particular for medicinal chemistry.

Organocatalytic atroposelective synthesis of axially chiral N,N'-pyrrolylindoles via de novo indole formation

The first organocatalytic atroposelective synthesis of axially chiral N,N'-pyrrolylindoles based on o-alkynylanilines was successfully established via de novo indole formation catalyzed by chiral phosphoric acid (CPA). This new synthetic strategy introduced CPA-catalyzed asymmetric 5-endo-dig cyclization of new well-designed o-alkynylanilines containing a pyrrolyl unit, resulting in a wide range of axially chiral N,N'-pyrrolylindoles in high yields with exclusive regioselectivity and excellent enantioselectivity (up to 99% yield, >20 : 1 rr, 95 : 5 er). Considering the potential biological significance of N-N atropisomers, preliminary biological activity studies were performed and revealed that these structurally important N,N'-pyrrolylindoles had a low IC50 value with promising impressive cytotoxicity against several kinds of cancer cell lines. DFT studies reveal that the N-nucleophilic cyclization mediated by CPA is the rate- and stereo-determining step, in which ligand-substrate dispersion interactions facilitate the axial chirality of the target products.

Chirality Transfer Intramolecular Pauson-Khand Reaction with N-C Axially Chiral Sulfonamides Bearing an Ene-Yne Structure

An intramolecular Pauson-Khand reaction with enantioenriched N-C axially chiral N-allyl-N-(2-alkynylphenyl)sulfonamide derivatives proceeded with complete chirality transfer from axial chirality (P configuration) to central chirality (R configuration), affording chiral nitrogen-containing tricyclic compounds (tetrahydrocyclopentaquinolin-2-one derivatives).

Enantioselective Synthesis of N-N Biaryl Atropisomers through Iridium(I)-Catalyzed C-H Alkylation with Acrylates

Enantioselective synthesis of N-N biaryl atropisomers is an emerging area but remains underexplored. The development of efficient synthesis of N-N biaryl atropisomers is in great demand. Herein, the construction of N-N biaryl atropisomers through iridium-catalyzed asymmetric C-H alkylation is reported for the first time. In the presence of readily available Ir precursor and Xyl-BINAP, a variety of axially chiral molecules based on indole-pyrrole skeleton were obtained in good yields (up to 98 %) with excellent enantioselectivity (up to 99 % ee). In addition, N-N bispyrrole atropisomers could also be synthesized in excellent yields and enantioselectivity. This method features perfect atom economy, wide substrate scope, and multifunctionalized products allowing diverse transformations.

Interrogating the configurational stability of atropisomers

Atropisomers are molecules whose stereogenicity arises from restricted rotation about a single bond. They are of current importance because of their applications in catalysis, medicine and materials science. The defining feature of atropisomeric molecules is that their stereoisomers are related to one another by bond rotation: as a result, evaluating their configurational stability (i.e., the rate at which their stereoisomers interconvert) is central to any work in this area. Important atropisomeric scaffolds include C-C linked biaryls, such as the ligand BINAP and the drug vancomycin, and C-N linked amine derivatives such as the drug telenzepine. This article focuses on the three most widely used experimental methods that are available to measure the rate of racemization in atropisomers, namely: (i) kinetic analysis of the racemization of an enantioenriched sample, (ii) dynamic HPLC and (iii) variable-temperature NMR. For each technique, an explanation of the theory is set out, followed by a detailed experimental procedure. A discussion is also included of which technique to try when confronted with a new molecular structure whose properties are not yet known. None of the three procedures require complex experimental techniques, and all can be performed by using standard analytical equipment (NMR and HPLC). The time taken to determine a racemization rate depends on which experimental method is required, but for a new compound it is generally possible to measure a racemization rate in <1 d.

Atroposelective Brønsted Acid-Catalyzed Photocyclization to Access Chiral N-Aryl Quinolones with Low Rotational Barriers

Herein, a novel route to atropisomeric N-aryl quinolones with low rotational barriers is demonstrated, leveraging a dual photochemical/organocatalytic approach to the required ring closure in up to 94% yield and up to >99% ee. The use of a continuous flow system allows for impurity suppression and enables rapid scale-up to a decagram scale.

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

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

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.

The Asymmetric Buchwald-Hartwig Amination Reaction

Over the past few decades, the Buchwald-Hartwig reaction has emerged as a powerful tool for forging C-N bonds, and has been vital to the pharmaceuticals, materials, and catalysis fields. However, asymmetric Buchwald-Hartwig amination reactions for constructing centered chirality, planar chirality, and axial chirality remain in their infancy owing to limited substrate scope and laggard ligand design. The recent surge in interest in the synthesis of C-N/N-N atropisomers, has witnessed a renaissance in asymmetric Buchwald-Hartwig amination chemistry as the first practical protocol for the preparation of C-N atropisomers. This review highlights reported asymmetric Buchwald-Hartwig amination protocols and provides a brief overview of their chemical practicality.

Catalytic Enantioselective Synthesis of N-C Axially Chiral N-(2,6-Disubstituted-phenyl)sulfonamides through Chiral Pd-Catalyzed N-Allylation

Recently, catalytic enantioselective syntheses of N-C axially chiral compounds have been reported by many groups. Most N-C axially chiral compounds prepared through a catalytic asymmetric reaction possess carboxamide or nitrogen-containing aromatic heterocycle skeletons. On the other hand, although N-C axially chiral sulfonamide derivatives are known, their catalytic enantioselective synthesis is relatively underexplored. We found that the reaction (Tsuji-Trost allylation) of allyl acetate with secondary sulfonamides bearing a 2-arylethynyl-6-methylphenyl group on the nitrogen atom proceeds with good enantioselectivity (up to 92% ee) in the presence of (S,S)-Trost ligand-(allyl-PdCl)₂ catalyst, affording rotationally stable N-C axially chiral N-allylated sulfonamides. Furthermore, the absolute stereochemistry of the major enantiomer was determined by X-ray single crystal structural analysis and the origin of the enantioselectivity was considered.

Copper- and Chiral Nitroxide-Catalyzed Oxidative Kinetic Resolution of Axially Chiral N-Arylpyrroles

A readily prepared C₂-symmetric, α-hydrogen-substituted chiral hydroxylamine serves as a precatalyst to generate a chiral nitroxide in situ. This chiral nitroxide catalyst in combination with a copper co-catalyst functions as an oxidant for an unprecedented enantioselective oxidative kinetic resolution (OKR) of racemic axially chiral N-arylpyrrole alcohols using atmospheric oxygen as an environmentally friendly terminal oxidant. The OKR process provides the axially chiral N-arylpyrroles in er up to 3.5:96.5 and with s factors up to 24.

Rotational Behavior of N-(5-Substituted-pyrimidin-2-yl)anilines: Relayed Electronic Effect in Two N-Ar Bond Rotations

N-Methyl-2-methoxymethylanilines 1 bearing various 5-substituted-pyrimidin-2-yl groups were prepared, and their rotational behaviors were explored in detail. It was revealed that the rotational barriers around two N-Ar bonds increase in proportion to the electron-withdrawing ability of substituents X at the 5-position.

Atropselective synthesis of N-aryl pyridones via dynamic kinetic resolution enabled by non-covalent interactions

The dynamic kinetic resolution of C-N atropisomeric pyridones was achieved via asymmetric phase-transfer catalysis, exploiting a rotational barrier-lowering hydrogen bond in the starting materials. X-ray and NMR experiments revealed the presence of a barrier-raising ground state CH⋯π interaction in the product, supported by DFT calculations. A co-crystal of the quinidine-derived phase-transfer catalyst and substrate reveals key substrate-catalyst non-covalent interactions.