Enantioselective Rh-Catalyzed Azide-Internal-Alkyne Cycloaddition for the Construction of Axially Chiral 1,2,3-Triazoles

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.

Catalytic Asymmetric Dearomatization of 2,3-Disubstituted Indoles by a [4 + 2] Cycloaddition Reaction with In Situ Generated Vinylidene ortho-Quinone Methides: Access to Polycyclic Fused Indolines

A protocol of enantioselective dearomatization of 2,3-disubstituted indoles by an organocatalytic intermolecular (4 + 2) cycloaddition reaction with in situ generated vinylidene ortho-quinone methide has been documented. A wide range of polycyclic 2,3-fused indolines containing vicinal quaternary carbon stereocenters was readily prepared in high yields and with excellent diastereo- and enantioselectivities.

Enantioselective copper-catalyzed azidation/click cascade reaction for access to chiral 1,2,3-triazoles

Chiral 1,2,3-triazoles are highly attractive motifs in various fields. However, achieving catalytic asymmetric click reactions of azides and alkynes for chiral triazole synthesis remains a significant challenge, mainly due to the limited catalytic systems and substrate scope. Herein, we report an enantioselective azidation/click cascade reaction of N-propargyl-β-ketoamides with a readily available and potent azido transfer reagent via copper catalysis, which affords a variety of chiral 1,2,3-triazoles with up to 99% yield and 95% ee under mild conditions. Notably, chiral 1,5-disubstituted triazoles that have not been accessed by previous asymmetric click reactions are also prepared with good functional group tolerance.

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.

Synthesis of chiral Cu(II) complexes from pro-chiral Schiff base ligand and investigation of their catalytic activity in the asymmetric synthesis of 1,2,3-triazoles

A pro-chiral Schiff base ligand (HL) was synthesized by the reaction of 2-amino-2-ethyl-1,3-propanediol and pyridine-2-carbaldehyde in methanol. The reaction of HL with CuCl2·2H2O and CuBr2 in methanol gave neutral mononuclear Cu(II) complexes with general formula of [Cu(HL)Cl2] (1) and [Cu(HL)Br2] (2), respectively. By slow evaporation of the methanolic solutions of 1 and 2, their enantiomers were isolated in crystalline format. The formation of pure chiral crystals in the racemic mixture was amply authenticated by single crystal X-ray analysis, which indicated that S-[Cu(HL)Cl2], R-[Cu(HL)Cl2], and S-[Cu(HL)Br2] are crystallized in chiral P212121 space group of orthorhombic system. Preferential crystallization was used to isolate the R and S enantiomers as single crystals and the isolated compounds were also studied by CD analysis. Structural studies indicated that the origin of the chirality in these compounds is related to the coordination mode of the employed pro-chiral ligand (HL) because one of its carbon atoms has been converted to a chiral center in the synthesized complexes. Subsequently, these complexes were used in click synthesis of a β-hydroxy-1,2,3-triazole and the results of catalytic studies indicated that 1 and 2 can act as enantioselective catalysts for the asymmetric synthesis of β-hydroxy-1,2,3-triazole product under mild condition. This study illustrates the significant capacity of the use of pro-chiral ligands in preparing chiral catalysts based on complexes which can also be considered as an effective approach to cheap chiral catalysts from achiral reagents.

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.

Synthesis of Chiral Biphenyl Monophosphines as Ligands in Enantioselective Suzuki-Miyaura Coupling

Herein, we describe our design and synthesis of novel chiral monophosphine ligands by the short-step addition of chiral lactates as side chains to the well-known ligand SPhos/RuPhos. The new chiral ligands were shown to be highly efficient in palladium-catalyzed Suzuki-Miyaura coupling, providing a series of axially chiral biphenyl products in high yield and high enantioselectivity. Furthermore, the gram-scale reaction and the diverse conversions of the products demonstrated the potential utility of the approach.

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.

Ir/Zn-cocatalyzed chemo- and atroposelective [2+2+2] cycloaddition for construction of C─N axially chiral indoles and pyrroles

Here, an Ir/Zn-cocatalyzed atroposelective [2+2+2] cycloaddition of 1,6-diynes and ynamines was developed, forging various functionalized C─N axially chiral indoles and pyrroles in generally good to excellent yields (up to 99%), excellent chemoselectivities, and high enantioselectivities (up to 98% enantiomeric excess) with wide substrate scope. This cocatalyzed strategy not only provided an alternative promising and reliable way for asymmetric alkyne [2+2+2] cyclotrimerization in an easy handle but also settled the issues of previous [Rh(COD)2]BF4-catalyzed system on the construction of C─N axial chirality such as complex operations, limited substrate scope, and low efficiency. In addition, control experiments and theoretical calculations disclosed that Zn(OTf)2 markedly reduced the barrier of migration insertion to significantly increase reaction efficiency, which was distinctly different from previous work on the Lewis acid for improving reaction yield through accelerating oxidative addition and reductive elimination.

Organocatalytic Synthesis of Furan-Embedded Styrene Atropisomers

Herein, we report the first synthesis of furan-embedded styrene atropisomers via the reaction between 1-(aryl-ethynyl)-naphthalen-2-ol and γ-hydroxyenone. The reaction proceeds through in situ furan formation from γ-hydroxyenone. The styrene derivatives were obtained in moderate to good yields with high diastereoselectivities with a catalytic amount of PTSA. Few applications such as triazole formation and cross-coupling reactions have been demonstrated. A preliminary catalytic asymmetric version was also reported.

Asymmetric rhodium-catalyzed click cycloaddition to access C-N axially chiral N-triazolyl indoles

The copper-catalyzed azide-alkyne cycloaddition (CuAAC) reaction is regarded as a prime example of "click chemistry", but the asymmetric click cycloaddition of internal alkynes still remains challenging. A new asymmetric Rh-catalyzed click cycloaddition of N-alkynylindoles with azides was developed, providing atroposelective access to C-N axially chiral triazolyl indoles, a new type of heterobiaryl, with excellent yields and enantioselectivity. This asymmetric approach is efficient, mild, robust and atom-economic, and features very broad substrate scope with easily available Tol-BINAP ligands.

Copper-Catalyzed Tandem Cyclization/Direct C(sp2)-H Annulation of Azide-Ynamides via α-Imino Copper Carbenes: Access to Azepino[2,3-b:4,5-b']diindoles

A novel copper-catalyzed tandem cyclization/direct C(sp²)-H annulation of phenyl azide-ynamides via α-imino copper carbenes has been developed, which provides a concise and flexible approach for the construction of a range of valuable azepino[2,3-b:4,5-b']diindoles in mostly good to excellent yields with high chemoselectivities. This tandem reaction also exhibits a broad substrate scope, excellent functional group tolerance, simple operation, and mild reaction conditions.

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.

Iridium-catalysed thioether-directed regioselective cycloaddition of internal alkynes with azides

We report herein a cationic iridium-catalysed thioether-directed alkyne-azide cycloaddition reaction. Diverse 2-alkynyl phenyl sulfides can undergo cycloaddition with different azides in a regioselective fashion. The reaction features high efficiency, a short reaction time, and a broad substrate scope, providing modular access to complex S-containing triazoles.

I2 -Catalyzed Cycloisomerization of Ynamides: Chemoselective and Divergent Access to Indole Derivatives

Herein, an I₂ -catalyzed unprecedented cycloisomerization of ynamides is developed, furnishing various functionalized bis(indole) derivatives in generally good to excellent yields with wide substrate scope and excellent atom-economy. This protocol not only represents the first molecular-iodine-catalyzed tandem complex alkyne cycloisomerizations, but also constitutes the first chemoselective cycloisomerization of tryptamine-ynamides involving distinctively different C(sp³ )-C(sp³ ) bond cleavage and rearrangement. Moreover, chiral tetrahydropyridine frameworks containing two stereocenters are obtained with moderate to excellent diastereoselectivities and excellent enantioselectivities. Meanwhile, cycloisomerization and aromatization of ynamides produce pyrrolyl indoles with high efficiency enabled by I₂ . Additionally, control experiments and theoretical calculations reveal that this reaction probably undergoes a tandem 5-exo-dig cyclization/rearrangement process.

Construction of Axial Chirality via Click Chemistry: Rh-Catalyzed Enantioselective Synthesis of 1-Triazolyl-2-Naphthylamines

A modular and practical click chemistry for atroposelective synthesis of 1-triazolyl-2-naphthylamines is developed. In this protocol, a variety of aromatic or aliphatic azides, and 1-alkynyl-2-naphthylamines could be assembled into valuable 1-triazlyl-2-naphthylamine scaffolds via a [3 + 2] cycloaddition under Rh-catalysis. This asymmetric click technology features easily accessible starting materials, mild reaction conditions, facile scalability, and good enantioselectivity. The good thermostability of products showcases great applicable potential, and the synthetic transformations further expand the molecular diversity of atropisomers.

Asymmetric Hydrophosphinylation of Alkynes: Facile Access to Axially Chiral Styrene-Phosphines

A Cu/CPA co-catalytic system has been developed for achieving the direct hydrophosphinylation of alkynes with phosphine oxides in delivering novel axially chiral phosphorus-containing alkenes in high yields and excellent enantioselectivities (up to 99 % yield and 99 % ee). DFT calculations were performed to elucidate the reaction pathway and the origin of enantiocontrol. This streamlined and modular methodology establishes a new platform for the design and application of new axially chiral styrene-phosphine ligands.

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.

Versatile Synthetic Platform for 1,2,3-Triazole Chemistry

1,2,3-Triazole scaffolds are not obtained in nature, but they are still intensely investigated by synthetic chemists in various fields due to their excellent properties and green synthetic routes. This review will provide a library of all synthetic routes used in the past 21 years to synthesize 1,2,3-triazoles and their derivatives using various metal catalysts (such as Cu, Ni, Ru, Ir, Rh, Pd, Au, Ag, Zn, and Sm), organocatalysts, metal-free as well as solvent- and catalyst-free neat syntheses, along with their mechanistic cycles, recyclability studies, solvent systems, and reaction condition effects on regioselectivity. Constant developments indicate that 1,2,3-triazoles will help lead to future organic synthesis and are useful for creating molecular libraries of various functionalized 1,2,3-triazoles.

Highly Site-Selective Oxidative Cyclization of Ene-ynamides via Non-Noble-Metal Catalysis: Access to Functionalized Lactams

Herein, an unprecedented non-noble-metal-catalyzed oxidation/cyclization of ene-ynamides is developed, allowing the synthesis of diversely functionalized lactams in moderate to good yields with excellent diastereoselectivities without the observation of typical cyclopropanation products. In combination with Ellman's tert-butylsulfinimine chemistry, chiral γ-lactams containing three contiguous stereocenters are obtained with high diastereo- and enantioselectivity. Moreover, density functional theory (DFT) calculations indicate that this protocol probably undergoes a carbon cation or proton transfer process.