Copper-catalyzed atroposelective synthesis of C-O axially chiral compounds enabled by chiral 1,8-naphthyridine based ligands

Axially chiral molecular scaffolds are widely present in therapeutic agents, natural products, catalysts, and advanced materials. The construction of such molecules has garnered significant attention from academia and industry. The catalytic asymmetric synthesis of axially chiral biaryls, along with other non-biaryl axially chiral molecules, has been extensively explored in the past decade. However, the atroposelective synthesis of C-O axial chirality remains largely underdeveloped. Herein, we document a copper-catalyzed atroposelective construction of C-O axially chiral compounds using novel 1,8-naphthyridine-based chiral ligands. Mechanistic investigations have provided good evidence in support of a mechanism involving synergistic interplay between a desymmetrization reaction and kinetic resolution process. The method described in this study holds great significance for the atroposelective synthesis of C-O axially chiral compounds, with promising applications in organic chemistry. The utilization of 1,8-naphthyridine-based ligands in copper catalysis is anticipated to find broad applications in asymmetric copper(i)-catalyzed azide-alkyne cycloadditions (CuAACs) and beyond.

Copper-Catalyzed Enantioselective Desymmetric Protosilylation of Prochiral Diynes: Access to Optically Functionalized Tertiary Alcohols

In this protocol, a copper-catalyzed desymmetric protosilylation of prochiral diynes was developed. The corresponding products were obtained in moderate to high yields and enantiomeric ratios. This approach provides a simple method for synthesizing functionalized chiral tertiary alcohols in the presence of a chiral pyridine-bisimidazoline (Pybim) ligand.

Atropisomers with Axial and Point Chirality: Synthesis and Applications

Enantiopure atropisomers have become increasingly important in asymmetric synthesis and catalysis, pharmaceutical science, and material science since the discovery of inherent features of axial chirality originating from rotational restriction. Despite the advances made in this field to date, it remains highly desirable to construct structurally diverse atropisomers with potentially useful functions. We propose superposition to match axial and point chirality as a potentially useful strategy to access structurally complex and diverse building blocks for organic synthesis and pharmaceutical science because merging atropisomeric backbones with one or more extra chiral elements can topologically broaden three-dimensional environments to create complex scaffolds with multiple tunable parameters. Over the past decade, we have successfully implemented a strategic design for the superposition of axial and point chirality to develop a series of enantiopure atropisomers and have utilized the synergistic functions of these molecules to enhance chirality transfer in various catalytic asymmetric transformations.In this Account, we present several novel atropisomers with superposed axial and point chirality developed in our laboratory. In our studies, this superposition strategy was used to design and synthesize both biaryl and non-biaryl atropisomers from commercially available chiral sources. Consequently, these atropisomers were used to demonstrate the importance of the synergetic functions of axial and point chirality in specific enantioselective reactions. For example, aromatic amide-derived atropisomers, simplified as Xing-Phos arrays, were broadly employed in Ag-catalyzed [3 + 2] cycloaddition by a series of reactions of aldiminoesters with activated alkenes and imines, as well as being used as chiral solvating agents for the discrimination of optically active mandelic acid derivatives. Considering the powerful potential of non-biaryl atropisomers for asymmetric catalysis, we also explored the transition-metal-catalyzed enantioselective construction of a novel backbone of non-biaryl atropisomers (Ar-alkene, Ar-N axis) bearing both axial and point chirality for the design and synthesis of chiral ligands and functional molecules.The studies presented herein are expected to stimulate further research efforts on the development of functional atropisomers by superposition of matching axial and point chirality. In addition to tunable electron and stereohindrance effects, the synergy between matching chiral elements of axial/point chirality and functional groups is proven to be a special function that cannot be ignored for promoting reactivity and chirality-transfer efficiency in enantioselective synthesis. Consequently, our novel types of scaffolds with superposed axial and point chirality that are capable of versatile coordination with various metal catalysts in asymmetric catalysis highlight the power of the superposition of matching axial and point chirality for the construction of synthetically useful atropisomers.

Catalytic Enantioselective Construction of Chiroptical Boron-Stereogenic Compounds

The construction of main group heteroatom-stereogenic compounds is of great importance due to their intriguing chemical, physical, biological, and stereoelectronic properties. Despite that organoboron compounds are widely used in organic chemistry, the creation of a tetrahedral boron-stereogenic center in one enantiomeric form remains highly challenging. Given the labile nature of ligands attached to the tetracoordinate boron atom, only a handful of enantioenriched boron-stereogenic compounds have been reported via resolution or a chiral substrate-induced diastereoselective approach. To date catalytic asymmetric synthesis of boron-stereogenic compounds has remained unknown. Here, we demonstrate the first catalytic enantioselective construction of boron-stereogenic compounds via an asymmetric copper-catalyzed azide-alkyne cycloaddition (CuAAC) reaction. This enantioselective CuAAC reaction not only gives access to a wide range of novel highly functionalized boron-stereogenic heterocycles in high yields with good to excellent enantioselectivities but also produces optically active terminal alkyne and triazole moieties with various potential application prospects. Further transformation of the chiral tetracoordinate boron compounds delivers several complex heterocyclic entities bearing boron-stereogenic centers without the loss of enantiopurity. Moreover, the X-ray structure, the barrier to racemization, and the HOMO/LUMO gap of selected tetracoordinate boron compounds are investigated. Notably, these novel N,N π-conjugated boron-stereogenic compounds exhibit bright fluorescence. The optical properties, including circular dichroism, quantum yield, and circular polarized luminescence spectroscopies, are examined. These features expand the chemical space of the chiroptical boron-based dye platform, which could have great potential applications in chiral optoelectronic materials.

Highly Diastereoselective Hydrosilane-Assisted Rhodium-Catalyzed Spiro-Type Cycloisomerization of Succinimide and Pyrazolone-Based Functional 1,6-Dienes

Organosilicon compounds are important reagents and synthetic intermediates that play a key role in the construction of new materials and complex products. Here we show a highly diastereoselective rhodium-catalyzed cycloisomerization of 1,6-dienes, in which the use of (EtO)₃ SiH accelerates the intramolecular cyclization reaction to afford a novel spiro-fused succinimide and pyrazolone derivatives in moderate to excellent yields as a single diastereoisomer. The proposed mechanism involves an active Rh-H species from the hydrosilane that is the H-donor in this spiro-type cycloisomerization reaction.

Enantioselective Nickel-Catalyzed Alkyne-Azide Cycloaddition by Dynamic Kinetic Resolution

The triazole heterocycle has been widely adopted as an isostere for the amide bond. Many native amides are α-chiral, being derived from amino acids. This makes α-N-chiral triazoles attractive building blocks. This report describes the first enantioselective triazole synthesis that proceeds via nickel-catalyzed alkyne-azide cycloaddition (NiAAC). This dynamic kinetic resolution is enabled by a spontaneous [3,3]-sigmatropic rearrangement of the allylic azide. The 1,4,5-trisubstituted triazole products, derived from internal alkynes, are complementary to those commonly obtained by the related CuAAC reaction. Initial mechanistic experiments indicate that the NiAAC reaction proceeds through a monometallic Ni complex, which is distinct from the CuAAC manifold.

The Discovery of Multifunctional Chiral P Ligands for the Catalytic Construction of Quaternary Carbon/Silicon and Multiple Stereogenic Centers

The development of highly effective chiral ligands is a key topic in enhancing the catalytic activity and selectivity in metal-catalyzed asymmetric synthesis. Traditionally, the difficulty of ligand synthesis, insufficient accuracy in controlling the stereoselectivity, and poor universality of the systems often become obstacles in this field. Using the concept of nonequivalent coordination to the metal, our group has designed and synthesized a series of new chiral catalysts to access various carbon/silicon and/or multiple stereogenic centers containing products with excellent chemo-, diastereo-, and enantioselectivity.In this Account, we summarize a series of new phosphine ligands with multiple stereogenic centers that have been developed in our laboratory. These ligands exhibited good to excellent performance in the transition-metal-catalyzed enantioselective construction of quaternary carbon/silicon and multiple stereogenic centers. In the first section, notable examples of the design and synthesis of new chiral ligands by non-covalent interaction-based multisite activation are described. The integrations of axial chirality, atom-centered chirality, and chiral anions and multifunctional groups into a single scaffold are individually highlighted, as represented by Ar-BINMOLs and their derivative ligands, HZNU-Phos, Fei-Phos, and Xing-Phos. In the second, third, and fourth sections, the enantioselective construction of quaternary carbon stereocenters, multiple stereogenic centers, and silicon stereogenic centers using our newly developed chiral ligands is summarized. These sections refer to detailed reaction information in the chiral-ligand-controlled asymmetric catalysis based on the concept of nonequivalent coordination with multisite activation. Accordingly, a wide array of transition metal and main-group metal catalysts has been applied to the enantioselective synthesis of chiral heterocycles, amino acid derivatives, cyclic ketones, alkenes, and organosilicon compounds bearing one to five stereocenters.This Account shows that this new model of multifunctional ligand-controlled catalysts exhibits excellent stereocontrol and catalytic efficiency, especially in a stereodivergent and atom-economical fashion. Furthermore, a brief mechanistic understanding of the origin of enantioselectivity from our newly developed chiral catalyst systems could inspire further development of new ligands and enhancement of enantioselective synthesis by asymmetric metal catalysis.

Multifunctional P-ligand-controlled “silicon-centered” selectivity in Rh/Cu-catalyzed Si–C bond cleavage of silacyclobutanes

A newly developed Ar-BINMOL-Phos-controlled Rh/Cu-catalyzed (4+2) annulation of silacylcobutanes with arylpropiolate-type internal alkynes gave silicon-stereogenic 1-sila-2-cyclohexene derivatives with good ee and excellent chemoselectivity.

Enantioselective synthesis of P-chiral tertiary phosphine oxides with an ethynyl group via Cu(i)-catalyzed azide-alkyne cycloaddition

We report the highly enantioselective synthesis of P-chiral tertiary phosphine oxides featuring an ethynyl group via Cu(i)-catalyzed azide-alkyne cycloaddition. Newly developed chiral pyridinebisoxazolines (PYBOX) bearing a bulky C4 shielding group play an important role in achieving excellent enantioselectivity while suppressing side bis-triazoles formation in desymmetrizing prochiral diethynylphosphine oxides. Notably, by tuning the size of the C4 shielding group, it is possible to achieve excellent remote enantiofacial control in desymmetrizing phosphole oxide-diynes with the prochiral P-center farther from the ethynyl group by four covalent bonds. Time-dependent enantioselectivity is observed for these desymmetric CuAAC reactions, suggesting a synergic combination of a desymmetrization and a kinetic resolution, and our ligands prove to be better than unmodified PYBOX in both steps. This finding contributes to a highly enantioselective kinetic resolution of racemic ethynylphosphine oxides. The resulting chiral ethynylphosphine oxides are versatile P-chiral synthons, which can undergo a number of diversifying reactions to enrich structural diversity.

Coetaneous catalytic kinetic resolution of alkynes and azides through asymmetric triazole formation

A non-enzymatic simultaneous (coined coetaneous) kinetic resolution of a racemic alkyne and racemic azide, utilising an asymmetric CuAAC reaction is reported. The use of a CuCl (R,R)-Ph-Pybox catalyst system effects a simultaneous kinetic resolution of two racemic starting materials to give one major triazolic diastereoisomer in the ratio 74:12:4:10 (dr 84:16, 90% ee maj). The corresponding control reaction using an achiral copper catalyst gives the four possible diastereoisomers in a 23:27:23:27 ratio, demonstrating minimal inherent substrate control.

Stereoselective Synthesis of Mechanically Planar Chiral Rotaxanes

Chiral interlocked molecules in which the mechanical bond provides the sole stereogenic unit are typically produced with no control over the mechanical stereochemistry. Here we report a stereoselective approach to mechanically planar chiral rotaxanes in up to 98:2 d.r. using a readily available α-amino acid-derived azide. Symmetrization of the covalent stereocenter yields a rotaxane in which the mechanical bond provides the only stereogenic element.

Catalytic enantioselective synthesis of α-chiral azides

The catalytic asymmetric synthesis of α-chiral azides is of current interest and three synthetic strategies have been developed. This review summarizes the recent progress in this research area, discusses the advantages and limitations of each strategy, and outlines synthetic opportunities for future research.

Azide–alkyne cycloaddition reactions in water via recyclable heterogeneous Cu catalysts: reverse phase silica gel and thermoresponsive hydrogels

Functionalized reverse phase silica gel and thermoresponsive hydrogels were synthesized as heterogeneous catalysts supports. Cu(i) and Cu(ii) catalysts immobilized onto two types of supports were prepared and characterized. The copper catalyzed azide–alkyne cycloaddition was performed in water via a one-pot reaction and yielded good results. These catalysts are air stable and reusable over multiple uses.

Azide–alkyne cycloaddition reactions were performed via copper catalysts immobilized onto two types of supports in water.

Bimetallic copper and zinc-catalyzed oxidative cycloaddition of 3-aminopyridazines and nitriles: a direct synthesis of 1,2,4-triazolo[1,5-b]pyridazines via C–N and N–N bond-forming process

One-pot formation of 1,2,4-triazolo[1,5-b]pyridazine derivatives is presented in this manuscript, in which the targets are offered via cooperative Cu(i)/Zn(ii)-catalyzed tandem C–N addition and I₂/KI-mediated intramolecular N–N bond formation.

Recent Advances in Recoverable Systems for the Copper-Catalyzed Azide-Alkyne Cycloaddition Reaction (CuAAC)

The explosively-growing applications of the Cu-catalyzed Huisgen 1,3-dipolar cycloaddition reaction between organic azides and alkynes (CuAAC) have stimulated an impressive number of reports, in the last years, focusing on recoverable variants of the homogeneous or quasi-homogeneous catalysts. Recent advances in the field are reviewed, with particular emphasis on systems immobilized onto polymeric organic or inorganic supports.