Binaphthyl Scaffold: A Class of Versatile Structure in Asymmetric C–H Functionalization

Access to distal biaxial atropisomers by iridium catalyzed asymmetric C-H alkylation

Distal biaxial atropisomers are typical structures in chiral catalysts and ligands and offer a wide variety of applications in biology and materials technology, but the development of efficient synthesis of these valuable scaffolds is still in great demand. Herein, we describe a highly efficient iridium catalyzed asymmetric C-H alkylation reaction that provides a range of new distal biaxial atropisomers with excellent yields (up to 99%) and stereoselectivity (up to 99% ee and essentially one isomer). Based on this unprecedented strategy, a polycyclic skeleton with five successive chiral centers as well as C-C and C-N (or N-N) two distal chiral axes was created successfully in mild circumstances. In addition, the optically pure products bearing fluorophores show circular polarized luminescence (CPL) properties, being potential candidate materials for CPL applications.

H8-BINOL-Derived Chiral η6-Benzene Ligands: New Opportunities for the Ruthenium-Catalyzed Asymmetric C-H Activation

Given the tremendous success of (p-cymene)RuII-catalyzed C-H activation over the past 20 years, the community has long been aware that the development of chiral η6-benzene (Ben) ligands should be a potent strategy for achieving the attractive but incredibly underdeveloped ruthenium(II)-catalyzed asymmetric C-H activation. However, it has rarely been achieved due to the severe difficulty in developing proper chiral Ben ligands. In particular, designing chiral Ben ligands by connecting a benzene fragment to a chiral framework including benzene rings remained an unsolved challenge until this effort. Here we present a novel class of axially chiral Ben ligands derived from readily available (S)-5,5',6,6',7,7',8,8'-octahydro-1,1'-bi-2-naphthol ((S)-H8-BINOL) in 4-8 steps. Notably, when coordinated with ruthenium, such chiral Ben ligand containing three benzene rings only forms one of the three possible isomeric BenRuII complexes. The related chiral BenRuII catalysts could effectively catalyze the asymmetric C-H activation of N-sulfonyl ketimines with alkynes, affording a range of chiral spirocyclic sultams in up to 99 % yield with up to >99 % ee. These catalysts are expected to find broad applications in future.

Chiral Cpx Rhodium(III)-Catalyzed Enantioselective Aziridination of Unactivated Terminal Alkenes

Chiral cyclopentadienyl-rhodium(III) Cpx Rh(III) catalysis has been demonstrated to be competent for catalyzing highly enantioselective aziridination of challenging unactivated terminal alkenes and nitrene sources. The chiral Cpx Rh(III) catalysis system exhibited outstanding catalytic performance and wide functional group tolerance, yielding synthetically important and highly valuable chiral aziridines with good to excellent yields and enantioselectivities (up to 99 % yield, 93 % ee). This protocol presents a novel and effective strategy for synthesizing enantioenriched aziridines from simple alkenes. Various transformations were performed on the aziridine products, illustrating the versatility and synthetic potential of this protocol for constructing highly functionalized compounds.

Atroposelective Synthesis of Axially Chiral Diaryl Ethers by N-Heterocyclic-Carbene-Catalyzed Sequentially Desymmetric/Kinetic Resolution Process

We describe herein an N-heterocyclic-carbene-catalyzed atroposelective synthesis of axially chiral diaryl ethers. Through a sequentially enantioselective desymmetric process and a kinetic resolution process, the products could be constructed in good yields with excellent enantiopurities. Both alcohols and phenols were compatible with this catalytic system. The axially chiral carboxylic acids derived from the esters were proven to be potential chiral ligands for asymmetric synthesis, for example, Rh(III)-catalyzed enantioselective C-H functionalization.

Synthesis of Biaryl Carboxylic Acids through a Cascade Suzuki-Miyaura Coupling/Friedel-Crafts Alkylation/Lewis-Acid-Catalyzed Rearrangement/Aromatization Process

In this study, we present a series of 1,3-dicarbonyls that can undergo a cascade Suzuki coupling, followed by a Friedel-Crafts reaction to produce molecules containing polycyclic alcohols. These polycyclic alcohols can then be converted into biaryl carboxylic acids through ring-opening rearrangement reactions catalyzed by a Lewis acid. The Friedel-Crafts reaction exhibits selective para-positioning of the hydroxyl group and demonstrates good compatibility with functional groups with a yield of up to 82%. Substrates with substituted hydroxyl groups can also be converted into biaryl carboxylic acids through a Lewis-acid-catalyzed ring-opening rearrangement.

2,2'-Tethered Binaphthyl-Embedded One-Handed Helical Ladder Polymers: Impact of the Tether Length on Helical Geometry and Chiroptical Property

Synthetic breakthroughs diversify the molecules and polymers available to chemists. We now report the first successful synthesis of a series of optically-pure 2,2'-tethered binaphthyl-embedded helical ladder polymers based on quantitative and chemoselective ladderization by the modified alkyne benzannulations using the 4-alkoxy-2,6-dimethylphenylethynyl group as the alkyne source, inaccessible by the conventional approach lacking the 2,6-dimethyl substituents. Due to the defect-free helix formation, the circular dichroism signal increased by more than 6 times the previously reported value. The resulting helical secondary structure can be fine-tuned by controlling the binaphthyl dihedral angle in the repeating unit with variations in the 2,2'-alkylenedioxy tethering groups by one carbon atom at a time. The optimization of the helical ladder structures led to a strong circularly polarized luminescence with a high fluorescence quantum yield (28 %) and luminescence dissymmetry factor (2.6×10-3 ).

Combinatorial Ligand Assisted Simultaneous Control of Axial and Central Chirality in Highly Stereoselective C-H Allylation

The significance of stereoselective C-H bond functionalization thrives on its direct application potential to pharmaceuticals or complex chiral molecule synthesis. Complication arises when there are multiple stereogenic elements such as a center and an axis of chirality to control. Over the years cooperative assistance of multiple chiral ligands has been applied to control only chiral centers. In this work, we harness the essence of cooperative ligand approach to control two different stereogenic elements in the same molecule by atroposelective allylation to synthesize axially chiral biaryls from its racemic precursor. The crucial roles played by chiral phosphoric acid and chiral amino acid ligand in concert helped us to obtain one major stereoisomer out of four distinct possibilities.

Rhodium(I)-Catalyzed Asymmetric Hydroarylative Cyclization of 1,6-Diynes to Access Atropisomerically Labile Chiral Dienes

Axially chiral open-chained olefins are an underexplored class of atropisomers, whose enantioselective synthesis represents a daunting challenge due to their relatively low racemization barrier. We herein report rhodium(I)-catalyzed hydroarylative cyclization of 1,6-diynes with three distinct classes of arenes, enabling highly enantioselective synthesis of a broad range of axially chiral 1,3-dienes that are conformationally labile (ΔG≠ (rac)=26.6-28.0 kcal/mol). The coupling reactions in each category proceeded with excellent enantioselectivity, regioselectivity, and Z/E selectivity under mild reaction conditions. Computational studies of the coupling of quinoline N-oxide system reveal that the reaction proceeds via initial oxidative cyclization of the 1,6-diyne to give a rhodacyclic intermediate, followed by σ-bond metathesis between the arene C-H bond and the Rh-C(vinyl) bond, with subsequent C-C reductive elimination being enantio-determining and turnover-limiting. The DFT-established mechanism is consistent with the experimental studies. The coupled products of quinoline N-oxides undergo facile visible light-induced intramolecular oxygen-atom transfer, affording chiral epoxides with complete axial-to-central chirality transfer.

Conformer aggregates exhibit dual wavelength emissions on chiral binaphthyl-based triphenylethylenes and acetone detection

The study on structure-property relationship has been a significant focus in the field of organic molecular luminescence. In the present work, three chiral binaphthyl-based triphenylethylene (HTPE) derivatives were prepared through condensation reactions. Despite their similar structures, these compounds exhibited distinct luminescent properties. Diphenylmethane-derived HTPE displayed dual-state emissions, characterized by dual-wavelength emissions which were insensitive to the polarity of solvents. The dual emissions in solution state could be attributed to the different locally excited (LE) excitons. However, upon aggregation, two stable conformers were generated, probably leading to different emission peaks. In contrast, dibenzocycloheptadiene-derived HTPE aggregates showed only a single emission peak. Surprisingly, fluorene-derived HTPE exhibited obvious luminescence in neither solution nor aggregate states due to inherent π-π interactions. These conclusions were substantiated by X-ray analysis, spectroscopic analysis, and theory calculations. Application studies demonstrated that fluorescence on/off switches could be achieved through exposure to acetone. More importantly, trace amounts of acetone could be detected using luminescent materials in both organic and aqueous phases with a detection limit of 0.08 %. Thus, this work not only presents a strategy for designing chiral triphenylethylene fluorophores but also provides valuable information for dual wavelength emissions resulting from two stable conformations.

Planar Chiral Rhodium Complex Based on the Tetrahydrofluorenyl Core for Enantioselective Catalysis

A simple four-step route to a chiral tetrahydrofluorenyl rhodium catalyst from naturally occurring (-)-α-pinene was developed. Our approach does not use multistep and time-consuming procedures such as chiral HPLC or diastereomeric resolution. The key to success lies in the face-selective coordination of rhodium to the sterically hindered tetrahydrofluorenyl ligand, giving only one diastereomeric complex. This catalyst proved to be highly efficient for asymmetric C-H annulation of aryl hydroxamates with alkenes (yield up to 95%, 91% ee) at low loading (up to 0.4 mol % based on Rh).

Palladium-Catalyzed Atroposelective Kinetic C-H Olefination and Allylation for the Synthesis of C-B Axial Chirality

The direct C-H functionalization of 1,2-benzazaborines, especially asymmetric version, remains a great challenge. Here we report a palladium-catalyzed enantioselective C-H olefination and allylation reactions of 1,2-benzazaborines. This asymmetric approach is a kinetic resolution (KR), providing various C-B axially chiral 2-aryl-1,2-benzazaborines and 3-substituted 2-aryl-1,2-benzazaborines in generally high yields with excellent enantioselectivities (selectivity (S) factor up to 354). The synthetic potential of this reaction is showcased by late-stage modification of complex molecules, scale-up reaction, and applications.

Axially Chiral 2-Hydroxybiaryls by Palladium-Catalyzed Enantioselective C-H Activation

This article describes the discovery and development of a palladium-catalyzed asymmetric C-H olefination of 2-hydroxybiaryls. The strategy allows a direct assembly of optically active, axially chiral 2-substituted-2'-hydroxybiaryls from readily available precursors and demonstrates that the native hydroxy unit of the substrates can work as an efficient directing group for the C-H activation. This represents a substantial advantage over other approaches that require the preinstallation of metal coordinating units. The simplicity of the approach and versatility of the products allow a practical and efficient synthesis of a broad variety of optically active binaphthyl derivatives.

Transition-metal-catalyzed atroposelective synthesis of axially chiral styrenes

Axially chiral styrenes, a type of atropisomer analogous to biaryls, have attracted great interest because of their unique presence in natural products and asymmetric catalysis. Since 2016, a number of methodologies have been developed for the atroposelective construction of these chiral skeletons, involving both transition metal catalysis and organocatalysis. In this feature article, we aim to provide a comprehensive understanding of recent advances in the asymmetric synthesis of axially chiral styrenes catalyzed by transition metals, integrating scattered work with different catalytic systems together. This feature article is cataloged into five sections according to the strategies, including asymmetric coupling, enantioselective C-H activation, central-to-axial chirality transfer, asymmetric alkyne functionalization, and atroposelective [2+2+2] cycloaddition.

Synthesis of Axially Chiral Biaryls through Cobalt(II)-Catalyzed Atroposelective C-H Arylation

Highly efficient synthesis of axially chiral biaryl amines through cobalt-catalyzed atroposelective C-H arylation using easily accessible cobalt(II) salt and salicyloxazoline ligand has been reported. This methodology provides a straightforward and sustainable access to a broad range of enantioenriched biaryl-2-amines in good yields (up to 99 %) with excellent enantioselectivities (up to 99 % ee). The synthetic utility of the unprecedented method is highlighted by its scalability and diverse transformations.

Enantioselective C-H Arylation for Axially Chiral Heterobiaryls

An enantioselective palladium-catalyzed C-H arylation of functionalized pyrazoles/triazoles/imidazoles is developed, affording a variety of axially chiral ortho-nitro/formyl-substituted heterobiaryls with excellent enantioselectivities and good yields. The method features a deuterated P-chiral phosphorus ligand CD3-AntPhos, a broad substrate scope of functionalized heterobiaryls, mild reaction conditions, and low palladium loadings.

Enantioselective Synthesis of 1-Aminoindanes via [3 + 2] Annulation of Aldimines with Alkenes by Scandium-Catalyzed C-H Activation

Multisubstituted chiral 1-aminoindanes are important components in many pharmaceuticals and bioactive molecules. Therefore, the development of efficient and selective methods for the synthesis of chiral 1-aminoindanes is of great interest and importance. In principle, the asymmetric [3 + 2] annulation of aldimines with alkenes through C-H activation is the most atom-efficient and straightforward route for the construction of chiral 1-aminoindanes, but such a transformation has remained undeveloped to date probably due to the lack of suitable catalysts. Herein, we report for the first time the enantioselective [3 + 2] annulation of a wide range of aromatic aldimines and alkenes via ortho-C(sp2)-H activation by chiral half-sandwich scandium catalysts, which provides a straightforward route for the synthesis of multisubstituted chiral 1-aminoindanes. This protocol features 100% atom-efficiency, broad functional group compatibility, and high regio-, diastereo-, and enantioselectivity (up to >19:1 dr and 99:1 er). Remarkably, by fine-tuning the sterics of the chiral ligand around the catalyst metal center, the diastereodivergent asymmetric [3 + 2] annulation of aldimines and styrenes has been achieved with a high level of diastereo- and enantioselectivity, offering an efficient method for the synthesis of both the trans and cis diastereomers of a novel class of chiral 1-aminoindane derivatives containing two contiguous stereocenters from the same set of starting materials. Moreover, the asymmetric [3 + 2] annulation of aldimines with aliphatic α-olefins, norbornene, and 1,3-dienes has also been achieved.

Force-Triggered Atropisomerization of a Parallel Diarylethene to Its Antiparallel Diastereomers

This paper describes a mechanical approach to inducing the atropisomerization of a parallel diarylethene into its antiparallel diastereomers exhibiting distinct chemical reactivity. A congested parallel diarylethene mechanophore in the (R_a,S_a)-configuration with mirror symmetry is atropisomerized to its antiparallel diastereomers with C₂ symmetry under ultrasound-induced force field. The resulting stereochemistry-converted material gains symmetry-allowed reactivity toward conrotatory photocyclization.

Recent Advancement in H8-BINOL Catalyzed Asymmetric Methodologies

H₈-BINOL, a partially reduced form of BINOL, is widely employed in a broad array of organocatalyzed asymmetric methodologies. Over the last 25 years, asymmetric organocatalysis has witnessed an incredible improvement, and an advancement still continues to get a single enantio-enriched product. The broad-spectrum applications of H₈-BINOL organocatalyst in C-C bond formation, C-heteroatom bond construction, name reactions, pericyclic reactions, and one pot and multicomponent reaction are attracting the attention of researchers. A diversified unique H₈-BINOL-based catalyst has been synthesized and screened for catalytic activity. In this Review we frame out the H₈-BINOL catalyzed novel discoveries from the last two decades.