Enantioselective Synthesis of Atropisomeric Vinyl Arene Compounds by Palladium Catalysis: A Carbene Strategy

A Dynamic Kinetic Resolution Approach to Axially Chiral Diaryl Ethers by Catalytic Atroposelective Transfer Hydrogenation

Diaryl ethers are widespread in biologically active compounds, ligands and catalysts. It is known that the diaryl ether skeleton may exhibit atropisomerism when both aryl rings are unsymmetrically substituted with bulky groups. Despite recent advances, only very few catalytic asymmetric methods have been developed to construct such axially chiral compounds. We describe herein a dynamic kinetic resolution approach to axially chiral diaryl ethers via a Brønsted acid catalyzed atroposelective transfer hydrogenation (ATH) reaction of dicarbaldehydes with anilines. The desired diaryl ethers could be obtained in moderate to good chemical yields (up to 79 %) and high enantioselectivities (up to 95 % ee) under standard reaction conditions. Such structural motifs are interesting precursors for further transformations and may have potential applications in the synthesis of chiral ligands or catalysts.

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.

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.

Rhodium-Catalyzed Atroposelective Access to Axially Chiral Olefins via C-H Bond Activation and Directing Group Migration

Axially chiral open-chain olefins represent an underexplored class of chiral platform. In this report, two classes of tetrasubstituted axially chiral acyclic olefins have been accessed in excellent enantioselectivity and regioselectivity via C-H activation of (hetero)arenes assisted by a migratable directing group en route to coupling with sterically hindered alkynes. The coupling of indoles bearing an N-aminocarbonyl directing group afforded C-N axially chiral acrylamides with the assistance of a racemic zinc carboxylate additive. DFT studies suggest a β-nitrogen elimination-reinsertion pathway for the directing group migration. Meanwhile, the employment of N-phenoxycarboxamide delivered C-C axially chiral enamides via migration of the oxidizing directing group. Experiments suggest that in both cases the (hetero)arene substrate adopts a well-defined orientation during the C-H activation, which in turn determines the disposition of the alkyne in migratory insertion. Synthetic applications of representative chiral olefins are demonstrated.

Combined Dynamic Kinetic Resolution and C-H Functionalization for Facile Synthesis of Non-Biaryl-Atropisomer-Type Axially Chiral Organosilanes

Although asymmetric C-H functionalization has been available for the synthesis of structurally diverse molecules, catalytic dynamic kinetic resolution (DKR) approaches to change racemic stereogenic axes remain synthetic challenges in this field. Here, a concise palladium-catalyzed DKR was combined with C-H functionalization involving olefination and alkynylation for the highly efficient synthesis of non-biaryl-atropisomer-type (NBA) axially chiral oragnosilanes. The chemistry proceeded through two different and distinct DKR: first, an atroposelective C-H olefination or alkynylation produced axially chiral vinylsilanes or alkynylsilanes as a new family of non-biaryl atropisomers (NBA), and second, the extension of this DKR strategy to twofold o,o'-C-H functionalization led to the multifunctional axially chiral organosilicon compounds with up to >99 % ee.

Construction of Axially Chiral Compounds via Central-to-Axial Chirality Conversion

Central-to-axial chirality conversion represents a fascinating class of chemical processes consisting of the destruction of stereogenic centers and the simultaneous installation of axial chiral elements, which provides efficient methods for the preparation of axially chiral compounds. Using the strategy, a wide range of axially chiral compounds, including biaryls, heterobiaryls, aromatic amides, allenes and vinyl arenes, have been synthesized with high efficiency and excellent enantioselectivity. In addition, central-to-axial chirality conversion strategy has been applied to the synthesis of natural products. The strategy has undoubtedly become and will continue to be a hot research topic in the field of asymmetric catalysis and synthesis. In this minireview, we selected some examples to introduce the developments and trends in the central-to-axial chirality conversion strategy up to April 2020.

Synthesis of Axially Chiral Styrenes through Pd-Catalyzed Asymmetric C-H Olefination Enabled by an Amino Amide Transient Directing Group

The atroposelective synthesis of axially chiral styrenes remains a formidable challenge due to their relatively lower rotational barriers compared to the biaryl atropoisomers. Herein, we describe the construction of axially chiral styrenes through Pd^II -catalyzed atroposelective C-H olefination, using a bulky amino amide as a transient chiral auxiliary. Various axially chiral styrenes were produced with good yields and high enantioselectivity (up to 95 % yield and 99 % ee). Carboxylic acid derivatives of the resulting axially chiral styrenes showed superior enantiocontrol over the biaryl counterparts in Co^III -catalyzed enantioselective C(sp³ )-H amidation of thioamide. Mechanistic studies suggest that C-H cleavage is the enantioselectivity-determining step.

Enantioselective Construction of Axially Chiral Amino Sulfide Vinyl Arenes by Chiral Sulfide-Catalyzed Electrophilic Carbothiolation of Alkynes

The enantioselective construction of axially chiral compounds by electrophilic carbothiolation of alkynes is disclosed for the first time. This enantioselective transformation is enabled by the use of a Ts-protected bifunctional sulfide catalyst and Ms-protected ortho-alkynylaryl amines (Ts=tosyl; Ms=mesyl). Both electrophilic arylthiolating and electrophilic trifluoromethylthiolating reagents are suitable for this reaction. The obtained products of axially chiral vinyl-aryl amino sulfides can be easily converted into biaryl amino sulfides, biaryl amino sulfoxides, biaryl amines, vinyl-aryl amines, and other valuable difunctionalized compounds.

Catalytic Kinetic Resolution by Enantioselective Aromatization: Conversion of Racemic Intermediates of the Barton-Zard Reaction into Enantioenriched 3-Arylpyrroles

Racemic 3,4-dihydro-2H-pyrroles, hypothetical intermediates of the Barton-Zard reaction, were synthesized in a highly diastereoselective manner and fully characterized for the first time. Kinetic resolution of the dihydropyrroles with a quinine-derived thiourea afforded the (+)-3-arylpyrrole products and recovered (+)-3,4-dihydro-2H-pyrroles with high efficiency (s-factor up to 153). The resolved (+)-3,4-dihydro-2H-pyrroles underwent subsequent aromatization with a quinidine-derived thiourea catalyst to afford (-)-3-arylpyrroles with excellent central-to-axial chirality transfer. In contrast to the well-accepted Barton-Zard mechanism, the aromatization of the 3,4-dihydro-2H-pyrroles in the presence of a bifunctional catalyst is believed to proceed by an unprecedented sequence involving syn elimination of HNO₂ and aromatization.

Rapid Synthesis of N-Tosylhydrazones under Solvent-Free Conditions and Their Potential Application Against Human Triple-Negative Breast Cancer

Some N-tosylhydrazone derivatives were effectively synthesized under solvent-free conditions by using a grinding method at room temperature. The short reaction time, clean and mild process with simple workup and easy purification of the target compounds were salient features of the present protocol, which enables straightforward access to N-tosylhydrazones. Among the tosylhydrazone derivatives evaluated, compound 3 l exhibits excellent apoptosis-promoting and anticancer potential against triple-negative breast cancer (TNBC) cell lines. This research shows that our synthesized compound 3 l may be a desirable and effective therapeutic drug against TNBC.

Atropoenantioselective Redox-Neutral Amination of Biaryl Compounds through Borrowing Hydrogen and Dynamic Kinetic Resolution

We report herein a novel atropoenantioselective redox-neutral amination of biaryl compounds triggered by a cascade of borrowing hydrogen and dynamic kinetic resolution under the cooperative catalysis of a chiral iridium complex and an achiral Brønsted acid. This protocol features broad substrate scope and good functional-group tolerance, and allows the rapid assembly of axially chiral biaryl compounds in good to high yields and with high to excellent enantioselectivity.

Organocatalytic Atroposelective Arylation of 2-Naphthylamines as a Practical Approach to Axially Chiral Biaryl Amino Alcohols

The first phosphoric acid catalyzed direct arylation of 2-naphthylamines with iminoquinones for the atroposelective synthesis of axially chiral biaryl amino alcohols has been developed. This reaction constitutes a highly functional-group-tolerant route for the rapid construction of enantioenriched axially chiral biaryl amino alcohols, and is a rare example of 2-naphthylamines acting as nucleophiles in an organocatalytic enantioselective transformation. Furthermore, the products, which feature various halogen atoms, provide access to structurally diverse axially chiral amino alcohols through further transformations.