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

Recent Advances in Catalytic Asymmetric Construction of Atropisomers

Atropisomerism is a stereochemical behavior portrayed by three-dimensional molecules that bear rotationally restricted σ bond. Akin to the well-represented point-chiral molecules, atropisomerically chiral compounds are finding increasing utilities in many disciplines where molecular asymmetry is influential. This provides steady demand on atroposelective synthesis, where numerous synthetic pursuits have been rewarded with conceptually novel and streamlined methods while expanding the structural diversity of atropisomers. This review summarizes key achievements in stereoselective preparation of biaryl, heterobiaryl, and nonbiaryl atropisomers documented between 2015 and 2020. Emphasis is placed on the synthetic strategies for each structural class, while examples are cited to illustrate the potential applications of the accessed atropochiral targets.

Synthesis of Aminoallenes via Selenium-π-Acid-Catalyzed Cross-Coupling of N-Fluorinated Sulfonimides with Simple Alkynes

The facile synthesis of aminoallenes, accomplished by a selenium-π-acid-catalyzed cross-coupling of an N-fluorinated sulfonimide with simple, non-activated alkynes, is reported. Until now, aminoallenes were difficult to be accessed by customary means, inasmuch as pre-activated and, in part, intricate starting materials were necessary for their synthesis. In sharp contrast, the current study shows that ordinary internal alkynes can serve as simple and readily available precursors for the construction of the aminoallene motif. The operating reaction conditions tolerate numerous functional groups such as esters, nitriles, (silyl)ethers, acetals, and halogen substituents, furnishing the target compounds in up to 86 % yield.

Atropisomerism in Styrene: Synthesis, Stability, and Applications

Atropisomeric styrenes are a class of optically active compounds, the chirality of which results from restricted rotation of the C(vinyl)–C(aryl) single bond. In comparison with biaryl atropisomers, the less rigid skeleton of styrenes usually leads them to have lower rotational barriers. Although it has been overlooked for a long time, scientists have paid attention to this class of unique molecules in recent years and have developed many methods for the preparation of optically active atropisomeric styrenes. In this article, we review the development of the concept of atropisomeric styrenes, along with their isolation, asymmetric synthesis, and synthetic applications.

1 Introduction

2 The Concept of Styrene Atropisomerism

3 Early Research: Separation of Optically Active Styrenes

4 Synthesis of Optically Active Styrenes

5 Stability of the Chirality of Atropisomeric Styrenes

6 Outlook

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.

3-Thiolated pyrroles/pyrrolines: controllable synthesis and usage for the construction of thiolated fluorophores

Thiolation/cyclization of homopropargylic tosylamides allowed the selective synthesis of 3-thiolated pyrroles and pyrrolines controlled by solvents. Moreover, the desired 3-thiolated pyrroles were readily transformed to organic fluorophores benzothienopyrrole and bisthiolated boron dipyrromethene (S-BODIPY).

Stereoselective Synthesis of Chiral α-SCF3-β-Ketoesters Featuring a Quaternary Stereocenter

The development of new and efficient methods, reagents, and catalysts for the introduction of fluorine atoms or fluorinated moieties in molecular scaffolds has become a topic of paramount importance in organic synthesis. In this framework, the incorporation of the SCF3 group into organic molecule has often led to beneficial effects on the drug’s metabolic stability and bioavailability. Here we report our studies aimed to the stereoselective synthesis of chiral α-SCF3-β−ketoesters featuring a tetrasubstituted stereocenter. The use of a chiral auxiliary was crucial to synthesize enantiopure enamines that were reacted with N-trifluoromethylthio saccharin or phthalimide, to afford enantioenriched α-SCF3-tetrasubstitued β-keto esters. By using a readily available, inexpensive chiral diamine, such as trans-1,2-diaminocyclohexane, the fluorinated products could be obtained in modest to good yields, and, after the removal of the chiral auxiliary, α-substituted- α trifluoromethylthio-β−ketoesters were isolated with high enantioselectivity (up to 91% ee).

Insights into the chiral sulfide/selenide-catalyzed electrophilic carbothiolation of alkynes: mechanism and origin of axial chirality

The FMO overlap mode, ELF, and AIM analyses along the formation process of thiiranium ion intermediate have been performed for the first time to explore the nature of the electronic structural changes and origin of stereoselectivity.

Synthetic developments on the preparation of sulfides from thiol-free reagents

This critical review covers the main thiolating reagents with respect to their characteristics and reactivities. In fact, they are complementary to each other and bring different thiolation strategies, avoiding the hazardous thiol derivatives.

Catalytic, Enantioselective Sulfenofunctionalization of Alkenes: Development and Recent Advances

The last decade has witnessed a burgeoning of new methods for the enantioselective vicinal difunctionalization of alkenes initiated by electrophilic sulfenyl group transfer. The addition of sulfenium ions to alkenes results in the generation of chiral, non-racemic thiiranium ions. These highly reactive intermediates are susceptible to attack by a myriad of nucleophiles in a stereospecific ring-opening event to afford anti 1,2-sulfenofunctionalized products. The practical application of sulfenium ion transfer has been enabled by advances in the field of Lewis base catalysis. This Review will chronicle the initial discovery and characterization of thiiranium ion intermediates followed by the determination of their configurational stability and the challenges of developing enantioselective variants. Once the framework for the reactivity and stability of thiiranium ions has been established, a critical analysis of pioneering studies will be presented. Finally, a comprehensive discussion of modern synthetic applications will be categorized around the type of nucleophile employed for sulfenofunctionalization.

Chalcogenide-Catalyzed Intermolecular Electrophilic Thio- and Halofunctionalization of gem-Difluoroalkenes: Construction of Diverse Difluoroalkyl Sulfides and Halides

Thio- and halodifluoromethylated compounds are an important class of compounds in medicinal chemistry and organic synthesis. Herein, we report a facile method for the construction of these compounds via chalcogenide-catalyzed intermolecular electrophilic thio- and halofunctionalization of gem-difluoroalkenes. Simple treatment of gem-difluoroalkenes with electrophilic sulfur/halogen reagents and various O- or N-nucleophiles affords diverse multifunctionalized thio- and halodifluoromethylated compounds. This reaction features a relatively broad substrate scope, good functional group tolerance, and mild reaction conditions.

BF3·OEt2-mediated cyclization of β,γ-unsaturated oximes and hydrazones with N-(arylthio/arylseleno)succinimides: an efficient approach to synthesize isoxazoles or dihydropyrazoles

A highly efficient BF3·OEt2-mediated cyclization of β,γ-unsaturated oximes and tosylhydrazones with N-(arylthio/arylseleno)succinimides has been established for the construction of N-heterocycles in a one-step manner. This metal-free cyclization provides direct access to isoxazoles and dihydropyrazoles in good to excellent yields at room temperature. The mechanistic experiments support the formation of a cationic species PhS+ which plays a critical role in this cyclization process.

Asymmetric allylic substitution-isomerization to axially chiral enamides via hydrogen-bonding assisted central-to-axial chirality transfer

Axially chiral enamides bearing a N–C axis have been recently studied and were proposed to be valuable chiral building blocks, but a stereoselective synthesis has not been achieved. Here, we report the first enantioselective synthesis of axially chiral enamides via a highly efficient, catalytic approach. In this approach, C(sp²)–N bond formation is achieved through an iridium-catalyzed asymmetric allylation, and then in situ isomerization of the initial products through an organic base promoted 1,3-H transfer, leading to the enamide products with excellent central-to-axial transfer of chirality. Computational and experimental studies revealed that the 1,3-H transfer occurs via a stepwise deprotonation/re-protonation pathway with a chiral ion-pair intermediate. Hydrogen bonding interactions with the enamide carbonyl play a significant role in promoting both the reactivity and stereospecificity of the stepwise 1,3-H transfer. The mild and operationally simple formal N-vinylation reaction delivered a series of configurationally stable axially chiral enamides with good to excellent yields and enantioselectivities.

Axially chiral enamides bearing a N–C axis have been recently studied and were proposed to be valuable chiral building blocks, but a stereoselective synthesis has not been achieved.

Lewis-Acid-Mediated Intramolecular Trifluoromethylthiolation of Alkenes with Phenols: Access to SCF3-Containing Chromane and Dihydrobenzofuran Compounds

A Lewis-acid-mediated intramolecular trifluoromethylthiolation of alkenes with phenols that can offer direct access to SCF₃-containing chromane and dihydrobenzofuran compounds was disclosed for the first time. Numerous SCF₃-containing chromanes were obtained in moderate to good yields using γ-substituted 2-allyphenols as substrates. Meanwhile, various SCF₃-containing dihydrobenzofurans with oxa-quaternary centers were also delivered in moderate to good yields using β-substituted 2-allyphenols as substrates.

Diastereoselective synthesis of spiro-cyclopropanyl-cyclohexadienones via direct sulfide-catalyzed [2 + 1] annulation of para-quinone methides with bromides

A sulfide-catalyzed [2 + 1] annulation of para-quinone methides with diverse bromides was disclosed to access appealing spiro-cyclopropanyl-cyclohexadienones with high diastereoselectivity.