Ring-fused hexahydro-1,2,4,5-tetrazines: synthesis, structure, and mechanistic studies on isolable rotational isomers

We designed conformationally stable rotational isomers around the C(sp²)-C(sp³) axis at the C₃-position of hexahydro-1,2,4,5-tetrazines. Isolation of each rotamer by silica gel column chromatography was successfully achieved at room temperature. The proposed isomerization mechanism of the rotamers was supported by NMR kinetic studies.

Recent Progress in Cyclic Aryliodonium Chemistry: Syntheses and Applications

Hypervalent aryliodoumiums are intensively investigated as arylating agents. They are excellent surrogates to aryl halides, and moreover they exhibit better reactivity, which allows the corresponding arylation reactions to be performed under mild conditions. In the past decades, acyclic aryliodoniums are widely explored as arylation agents. However, the unmet need for acyclic aryliodoniums is the improvement of their notoriously low reaction economy because the coproduced aryl iodides during the arylation are often wasted. Cyclic aryliodoniums have their intrinsic advantage in terms of reaction economy, and they have started to receive considerable attention due to their valuable synthetic applications to initiate cascade reactions, which can enable the construction of complex structures, including polycycles with potential pharmaceutical and functional properties. Here, we are summarizing the recent advances made in the research field of cyclic aryliodoniums, including the nascent design of aryliodonium species and their synthetic applications. First, the general preparation of typical diphenyl iodoniums is described, followed by the construction of heterocyclic iodoniums and monoaryl iodoniums. Then, the initiated arylations coupled with subsequent domino reactions are summarized to construct polycycles. Meanwhile, the advances in cyclic aryliodoniums for building biaryls including axial atropisomers are discussed in a systematic manner. Finally, a very recent advance of cyclic aryliodoniums employed as halogen-bonding organocatalysts is described.

Diastereo- and enantioselective synthesis of biaryl aldehydes bearing both axial and central chirality

An unprecedented method for the synthesis of biaryl aldehydes bearing both axial and central chirality is presented.

Atroposelective Synthesis of Triaryl α-Pyranones with 1,2-Diaxes by N-Heterocyclic Carbene Organocatalysis

Atropisomers bearing multiple stereogenic axes are of increasing importance to the field of material science, pharmaceuticals, and catalysis. However, the atroposelective construction of multi-axis atropisomers remains rare and challenging, due to the intrinsical difficulties in the stereo-control of the multiple stereogenic axes. Herein, we demonstrate a single-step construction of a new class of 1,2-diaxially chiral triaryl α-pyranones by an N-heterocyclic carbene organocatalytic asymmetric [3+3] annulation of well-designed alkynyl acylazolium precursors and enolizable sterically hindered 2-aryl ketones. The protocol features broad substrate scope (>50 examples), excellent stereo-control (most cases >20 : 1 dr, up to 99.5 : 0.5 er), and potentially useful synthetic applications. The success of this reaction relies on the rational design of structurally matched reaction partners and the careful selection of the asymmetric catalytic system. DFT calculations have also been performed to discover and rationalize the origin of the high stereoselectivity of this reaction.

Advances in the Asymmetric Synthesis of BINOL Derivatives

BINOL derivatives have shown relevant biological activities and are important chiral ligands and catalysts. Due to these properties, their asymmetric synthesis has attracted the interest of the scientific community. In this work, we present an overview of the most efficient methods to obtain chiral BINOLs, highlighting the use of metal complexes and organocatalysts as well as kinetic resolution. Further derivatizations of BINOLs are also discussed.

Biocatalytic Enantioselective Synthesis of Atropisomers

Atropisomeric compounds are found extensively as natural products, as ligands for asymmetric transition-metal catalysis, and increasingly as bioactive and pharmaceutically relevant targets. Their enantioselective synthesis is therefore an important ongoing research target. While a vast majority of known atropisomeric structures are (hetero)biaryls, which display hindered rotation around a C-C single bond, our group's long-standing interest in the control of molecular conformation has led to the identification and stereoselective preparation of a variety of other classes of "nonbiaryl" atropisomeric compounds displaying restricted rotation around C-C, C-N, C-O, and C-S single bonds.Biocatalytic transformations are finding increasing application in both academic and industrial contexts as a result of a significant broadening of the range of biocatalytic reactions and sources of enzymes available to the synthetic chemist. In this Account, we summarize the main biocatalytic strategies currently available for the asymmetric synthesis of biaryl, heterobiaryl, and nonbiaryl atropisomers. As is the case with more traditional synthetic approaches to these compounds, most biocatalytic methodologies for the construction of enantioenriched atropisomers follow one of two distinct strategies. The first of these is the direct asymmetric construction of atropisomeric bonds. Synthetically applicable biocatalytic methodologies for this type of transformation are limited, despite the extensive research into the biosynthesis of (hetero)biaryls by oxidative homocoupling or cross-coupling of electron-rich arenes. The second of these is the asymmetric transformation of a molecule in which the bond that will form the axis already exists, and this approach represents the majority of biocatalytic strategies available to the synthetic organic chemist. This strategy encompasses a variety of stereoselective techniques including kinetic resolution (KR), desymmetrization, dynamic kinetic resolution (DKR), and dynamic kinetic asymmetric transformation (DYKAT).Nondynamic kinetic resolution (KR) of conformationally stable biaryl derivatives has provided the earliest and most numerous examples of synthetically useful methodologies for the enantioselective preparation of atropisomeric compounds. Lipases (i.e., enzymes that mediate the formation or hydrolysis of esters) are particularly effective and have attracted broad attention. This success has led researchers to broaden the scope of lipase-mediated transformations to desymmetrization reactions, in addition to a limited number of DKR and DYKAT examples. By contrast, our group has used redox enzymes, including an engineered galactose oxidase (GOase) and commercially available ketoreductases (KREDs), to desymmetrize prochiral atropisomeric diaryl ether and biaryl derivatives. Building on this experience and our long-standing interest in dynamic conformational processes, we later harnessed intramolecular noncovalent interactions to facilitate bond rotation at ambient temperatures, which allowed the development of the efficient DKR of heterobiaryl aldehydes using KREDs. With this Account we provide an overview of the current and prospective biocatalytic strategies available to the synthetic organic chemist for the enantioselective preparation of atropisomeric molecules.

DBU Promoted Polysubstituted Arene Formation via a Michael Addition/Cyclization/Elimination Cascade Reaction

The straightforward construction of polysubstituted arenes is essential in both synthetic chemistry and medicinal chemistry. Herein, we reported a DBU promoted Michael addition/cyclization/elimination cascade reaction between vinylogous malononitrile derivatives and chlorinated nitrostyrenes for the synthesis of polysubstituted arenes. The method features mild reaction conditions, wide substrate scope and high yield. Interestingly, preliminary study of the enantioselective version of this cascade was conducted to give chiral biaryl atropisomers with up to 40% ee through center-to-axial chirality transfer strategy.

N,S-Chelating triazole-thioether palladium for the one-pot synthesis of biaryls

In this work, for the one-pot two-step coupling reaction of aryl halides with bis(pinacol)diboron, we first applied a phosphorus-free N,S-chelated triazole sulfide palladium-catalyzed system. At the same time, we also carried out careful ligand design to explore the effect of the environment around the coordinating sulfur atom on the reaction. Experiments have shown that the N2-thioether substituted 1,2,3-triazlole palladium is an optimal catalyst The reaction could also reach up to quantitative yield in 4 h with only 1 mol% catalyst. Moreover, some low-activity aryl chlorides can also be coupled with bis(pinacolato)diboron under this catalytic system. We were able to obtain biaryls containing various functional groups in good to excellent yields.

Organocatalytic Enantioselective Synthesis of Axially Chiral Molecules: Development of Strategies and Skeletons

The growing importance of axially chiral architectures in different scientific domains has unveiled shortcomings in terms of efficient synthetic access and skeletal variety. This account describes our strategies in answering these challenges within the organocatalytic context where the emergence of bifunctional catalysts such as chiral phosphoric acids (CPAs) has proven invaluable in controlling the sense of axial chirality. The wide occurrence of bi(hetero)aryl skeletons in privileged structures constitutes a strong motivation to devise more effective arylation methods. Our design revolves around modulating the intrinsic nucleophilicity of aromatic amines and alcohols. The first approach involves the design of an electron-withdrawing activating group which could associate with the catalyst for reactivity enhancement and selectivity control. The resonance of arenes offers the unique mechanistic possibility to select between activating sites. C2-Azo- and nitroso-substituted naphthalenes undergo atroposelective ortho C- or N-arylation with (hetero)aromatic nucleophiles. For monocyclic benzenes, programmable charge localization leads to regioselective activation by catalytic control alone or aided by substrate design. For instance, selective addition to nitroso nitrogen enables successive annulation initiated by the amine to yield axially chiral N-arylbenzimidazoles. In a biomimetic manner, a finely tuned catalyst could direct a para-selective nucleophilic approach in the atroposelective arylation of azobenzenes. The second strategy employs electrophilic arene precursors for arylation which occurs via rearomatization with central-to-axial chirality transfer. This enabled the arylation of (imino)quinones with indoles to access phenylindole atropisomers. By adapting this chemistry with an additional oxidation event to liberate the carbonyl functionalities, aryl-o-naphthoquinone and aryl-p-quinone atropisomers were attained. Along with the development of new arylation strategies, deriving new axially chiral structures has been another consistent theme of our research program. The atroposelective functionalization of alkynes provides broad entry to atropisomeric alkenes. The monofunctionalization of alkynes through the interception of an electrophilic vinylidene-quinone-methide (VQM) intermediate with 2-naphthols yielded the new EBINOL scaffolds. By designing an internal directing group, the atroposelective dihalogenation of alkynes was realized using abundant alkali halides despite their weak nucleophilicities and poor solubilities. The atroposelective N-alkylation of alkenes was pursued to prepare multifunctionalized alkene atropisomers that could be converted into 2-arylpyrroles with chirality transfer. The synthesis of B-aryl-1,2-azaborines containing a C-B chiral axis was accomplished where the CPA catalyst effects the desymmetrization and defines the configuration of the distal C-B bond. Inspired by the axially chiral scaffold of allenes, we leveraged the developed arene activation strategy to achieve para-addition and dearomatization of judiciously designed azobenzenes, which led to structurally novel cyclohexadienylidene-based hydrazones. To complement these structures, axially chiral cyclohexadienyl oxime ethers were also attained through CPA-catalyzed condensation between hydroxylamines and spiro[4.5]trienones.

Approaches to Synthesis and Isolation of Enantiomerically Pure Biologically Active Atropisomers

Atropisomerism is a stereochemical phenomenon exhibited by molecules containing a rotationally restricted σ bond. Contrary to classical point chirality, the two atropisomeric stereoisomers exist as a dynamic mixture and can be interconverted without the requirement of breaking and reforming a bond. Although this feature increases structural complexity, atropisomers have become frequent targets in medicinal chemistry projects. Their axial chirality, e.g., from axially chiral biaryl motifs, gives access to unique 3D structures. It is often desirable to have access to both enantiomers of the atropisomers via a nonselective reaction during the early discovery phase as it allows the medicinal chemistry team to probe the structure activity relationship in both directions. However, once a single atropisomer is selected, it presents several problems. First, the pure single atropisomer may interconvert to the undesired stereoisomer under certain conditions. Second, separation of atropisomers is nontrivial and often requires expensive chiral stationary phases using chromatography or additives if a salt resolution approach is chosen. Other options can be kinetic resolution using enzymes or chiral catalysts. However, apart from the high cost often associated with the two latter methods, a maximum yield of only 50% of the desired atropisomer can be obtained. The ideal approach is to install the chiral atropisomeric axis enantioselectively or employing a dynamic kinetic resolution approach. In theory, both approaches have the potential to provide a single atropisomer in quantitative yield. This Account will discuss the successes/failures and challenges we have experienced in developing methods for resolution/separation and asymmetric synthesis of atropisomeric drug candidates in one of our early phase drug development projects. Suitability for the different methods at various stages of the drug development phase is discussed. Depending on the scale and time available, a separation of a mixture of atropisomers by chromatography was sometimes preferred, whereas asymmetric- or resolution approaches were desired for long-term supply. With the use of chromatography, the impact on separation efficiency and solvent consumption, depending on the nature of the substrate, is discussed. We hope that with this Account the readers will get a better view on the challenges medicinal and process chemists meet when designing new atropisomeric drug candidates and developing processes for manufacture of a single atropisomer.

Enantioselective Synthesis of Atropisomers via Vinylidene ortho-Quinone Methides (VQMs)

Atropisomers, arising from conformational restriction, are inherently chiral due to the intersecting dissymmetric planes. Since there are numerous applications of enantiopure atropisomers in catalyst design, drug discovery, and material science, the asymmetric preparation of these highly prized molecules has become a flourishing field in synthetic chemistry. A number of catalysts, synthetic procedures, and novel concepts have been developed for the manufacture of the atropisomeric molecules. However, due to the intrinsic properties of different types of atropisomers featuring biaryl, hetero-biaryl, or non-biaryl architectures, only very few methods pass the rigorous inspection and are considered generally applicable. The development of a broadly applicable synthetic strategy for various atropisomers is a challenge. In this Account, we summarize our recent studies on the enantioselective synthesis of atropisomers using the vinylidene ortho-quinone methides (VQMs) as pluripotent intermediates.The most appealing features of VQMs are the disturbed aromaticity and axial chirality of the allene fragment. At the outset, the applications of VQMs in organic synthesis have been neglected due to their principal liabilities: ephemeral nature, extraordinary reactivity, and multireaction sites. The domestication of this transient intermediate was demonstrated by in situ catalytic asymmetric generation of VQMs, and the reactivity and selectivity were fully explored by judiciously modifying precursors and tuning catalytic systems. A variety of axially chiral heterocycles were achieved through five-, six-, seven- and nine-membered ring formation of VQM intermediates with different kinds of branched nucleophilic functional groups. The axially chiral C-N axis could be constructed from VQM intermediates via N-annulation or desymmetrization of preformed C-N scaffolds. We take advantage of the high electrophilicity of VQMs toward a series of sulfur and carbon based nucleophiles leading to atropisomeric vinyl arenes. Furthermore, chiral helical compounds were realized by cycloaddition or consecutive annulation of VQM intermediates. These achievements demonstrated that the VQMs could work as a nuclear parent for the collective synthesis of distinct and complex optically active atropisomers. Recently, we have realized the isolation and structural characterization of the elusive VQMs, which were questioned as putative intermediates for decades. The successful isolation of VQMs provided direct evidence for their existence and an unprecedented opportunity to directly investigate their reactivity. The good thermal stability and reserved reactivity of the isolated VQMs demonstrated their great potential as synthetic reagents and expanded the border of VQM chemistry.

Atroposelective Synthesis of N-Arylindoles via Enantioselective N-H Bond Insertion

We present here a rhodium-catalyzed asymmetric N-H insertion reaction, which is a concerted process revealed by DFT calculations, for the synthesis of novel axially chiral N-arylindoles by the reaction between indoles and diazonaphthoquinones. The reaction occurs at the N1 rather than C2/C3 positions of indoles, providing the chiral N-arylindoles in good yields and excellent enantiomeric ratios. Furthermore, this protocol is also amenable to the synthesis of chiral N-arylcarbazoles with excellent enantiocontrol.

Enantioselective Synthesis of Both Axially and Planar Chiral Ferrocenes via Axial-to-Planar Diastereoinduction

Ferrocenes with planar chirality have emerged as an important class of scaffolds for ligands in asymmetric catalysis; however, ferrocene molecules with polychiral structures have not been well explored. Herein, both axially and planar chiral ferrocenes were synthesized via palladium/chiral norbornene cooperative catalysis and axial-to-planar diastereoinduction. In this work, chiral norbornene was used to stereoselectively control the aromatic axial chirality, and further selectivity induced C(sp²)-H activation for ferrocene planar chirality. Based on density functional theory calculations, the catalytic model of chiral norbornene with the substrate and the axial-to-planar diastereoinduction process were confirmed.

Catalytic asymmetric synthesis of medium-sized bridged biaryls

Despite the persistent presence of medium-sized (seven- to nine-membered) scaffolds in natural products and biologically active molecules, their asymmetric syntheses have always been considered a formidable task; therefore, they have remained underdeveloped when compared to the enantioselective synthesis of five- and six-membered ring scaffolds. One important class of such medium-sized ring frameworks includes seven- to nine-membered biaryl bridged carbo- and heterocycles. These medium-ring-sized biaryl frameworks possess more configurational stability than the related smaller ring structures and are common features of valuable natural products, bioactive compounds, chiral catalysts, and molecular motors. Due to these exciting properties and broad applications, over the last few years, the catalytic enantioselective synthesis of medium-sized bridged biaryls has seen an upsurge. This highlight article describes the development of organocatalysed and transition-metal catalysed transformations for procuring seven-, eight-, and nine-membered bridged biaryls bearing a chiral axis/one or more asymmetric carbon centres.

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.

Directed Evolution of Flavin-Dependent Halogenases for Site- and Atroposelective Halogenation of 3-Aryl-4(3H)-Quinazolinones via Kinetic or Dynamic Kinetic Resolution

In this study, we engineer a variant of the flavin-dependent halogenase RebH that catalyzes site- and atroposelective halogenation of 3-aryl-4(3H)-quinazolinones via kinetic or dynamic kinetic resolution. The required directed evolution uses a combination of random and site-saturation mutagenesis, substrate walking using two probe substrates, and a two-tiered screening approach involving the analysis of variant conversion and then enantioselectivity of improved variants. The resulting variant, 3-T, provides >99:1 e.r. for the (M)-atropisomer of the major brominated product, 25-fold improved conversion, and 91-fold improved site selectivity relative to the parent enzyme on the probe substrate used in the final rounds of evolution. This high activity and selectivity translate well to several additional substrates with varied steric and electronic properties. Computational modeling and docking simulations are used to rationalize the effects of key mutations on substrate binding. Given the range of substrates that have been used for atroposelective synthesis via electrophilic halogenation in the literature, these results suggest that flavin-dependent halogenases (FDHs) could find many additional applications for atroposelective catalysis. More broadly, this study highlights how RebH can be engineered to accept structurally diverse substrates that enable its use for enantioselective catalysis.

A Modular Approach to Atropisomeric Bisphosphines of Diversified Electronic Density on Phosphorus Atoms

The series of C2-symmetric biaryl core-based non-racemic bisphosphines possessing substituents of different electronic properties: both EDG and EWG were obtained in a short sequence of good yielding transformations, started from commercial 1,3-dimethyl-2-nitrobenzene. Several different approaches leading to the desirable ligands were practically evaluated. Notably, the synthesis of the entire series of ligands could be performed with the utilization of a single early-stage precursor DIDAB (6,6’-diiodo-2,2’,4,4’-tetramethylbiphenyl-3,3’-diamine), which could be easily obtained in enantiomerically pure form. The obtained compounds at concentrations of 50 and 200 µM showed various biological activity against normal human dermal fibroblast, ranging from inactivity through time-dependent action and ending up with high toxicity.

Conformational enantiodiscrimination for asymmetric construction of atropisomers

Molecular conformations induced by the rotation about single bonds play a crucial role in chemical transformations. Revealing the relationship between the conformations of chiral catalysts and the enantiodiscrimination is a formidable challenge due to the great difficulty in isolating the conformers. Herein, we report a chiral catalytic system composed of an achiral catalytically active unit and an axially chiral 1,1'-bi-2-naphthol (BINOL) unit which are connected via a C-O single bond. The two conformers of the catalyst induced by the rotation about the C-O bond, are determined via single-crystal X-ray diffraction and found to respectively lead to the formation of highly important axially chiral 1,1'-binaphthyl-2,2'-diamine (BINAM) and 2-amino-2'-hydroxy-1,1'-binaphthyl (NOBIN) derivatives in high yields (up to 98%), with excellent enantioselectivities (up to 98:2 e.r.) and opposite absolute configurations. The results highlight the importance of conformational dynamics of chiral catalysts in asymmetric catalysis.

Copper-Catalyzed Enantioselective Hydrosilylation of Allenes to Access Axially Chiral (Cyclohexylidene)ethyl Silanes

A novel strategy of copper-catalyzed regio- and enantioselective hydrosilylation of 4-substituted vinylidenecyclohexanes with silanes was developed. In this protocol, various allenes and silanes were used to afford the corresponding (cyclohexylidene)ethyl silanes in moderate to high yields with good enantioselectivities.

Atropisomeric Phosphine Ligands Bearing C-N Axial Chirality: Applications in Enantioselective Suzuki-Miyaura Cross-Coupling Towards the Assembly of Tetra-ortho-Substituted Biaryls

Biaryl phosphines bearing C_(Ar)-C_(Ar) axial chirality are commonly known and have been successfully applied in many asymmetric catalyses. Nevertheless, the development of a chiral ligand having an axially chiral C_(Ar)-N backbone remains elusive due to its undesirable less restricted rotational barrier. In fact, it is highly attractive to overcome this challenge in ligand development as the incorporation of an N-donor component at the chiral axis is more favorable toward the transient metal coordination, and thus, a better outcome of stereocommunication is anticipated to the approaching substrates. Herein, we present a rational design of a new collection of chiral phosphines featuring a C-N axial chirality and their applications in enantioselective Suzuki-Miyaura cross-coupling for accessing highly steric hindered tetra-ortho-substituted biaryls (26 examples up to 98:2 er). It is worth noting that the embodied carbazolyl framework is crucial to succeed the reaction, by the fruitful steric relief of bulky substrate coordination and transmetalation via a fleeting Pd-N jumping to Pd-π fashion. DFT calculation reveals an interesting Pd-arene-walking characteristic across the carbazolyl plane for attaining a lower energy-preferred route in a catalytic cycle. The theoretical study successfully predicts the stereooutcome and matches the enantioselectivity with the experimental results.

Counterion-Mediated Enantioconvergent Synthesis of Axially Chiral Medium Rings

There are few enantioconvergent reactions in which racemic substrates bearing multiple stereochemical features are converted into products with high levels of diastereo- and enantiocontrol. Here, we disclose a process for the highly enantio- and diastereoselective syntheses of medium ring lactams via an intramolecular counterion-directed C-alkylation reaction. The treatment of racemic biaryl anilides that exist as a complex mixture of enantiomers and diastereoisomeric conformers by virtue of multiple axes of restricted rotation with a quinidine-derived ammonium salt under basic conditions affords medium ring lactams bearing elements of both axial and point chirality via an enolate-driven configurational relaxation process. Thermal equilibration of the syn- and anti-product diasteroisomers has demonstrated that the barriers to bowl inversion are >124 kJ mol^-1. We propose that the chiral ammonium salt differentiates between a complex and rapidly equilibrating mixture of enolate and rotational isomers, ultimately leading to highly enantioselective alkylative ring closure. This dynamic and enantioconvergent process offers an operationally simple approach to the synthesis of valuable chiral medium ring lactams for which there are few catalytic and enantioselective approaches.

Transition-Metal-Free Cascade Approach for the Synthesis of Functionalized Biaryls by SNAr of Arylhydroxylamines with Arylsulfonium Salts

We report a transition-metal-free protocol for the synthesis of functionalized biaryls through nucleophilic aromatic substitution (S_NAr) of arylhydroxylamines to arylsulfonium salts. With this protocol, structurally diverse functionalized biaryls were obtained smoothly in moderate to good yields. Merits of this transformation include mild reaction conditions, broad substrate scope, great functional group tolerance, feasibility of a one-pot procedure, and ease of handing and scale-up.

Expedient Access to Polyaromatic Biaryls by Unconventional Ag-Catalyzed Cycloaromatization of Alkynylthiophenes and Au-Catalyzed Double C-H Activation

An unconventional approach for the regioselective synthesis of polyaromatic biaryls via site-selective Ag-catalyzed twofold electrophilic cycloisomerization followed by Au-catalyzed double C-H activation is described. The developed process allows the synthesis of highly decorated biaryls with excellent regioselectivity. As revealed by DFT computations, the reaction represents a rare example of C1-C5 endo-exo and C1-C6 endo-endo cycloaromatization. The formation of the 6-membered ring is predicted to be the fruit of an uncommon SEAr on a vinyl carbocation.

Highly Fluorescent Bipyrrole-Based Tetra-BF2 Flag-Hinge Chromophores: Achieving Multicolor and Circularly Polarized Luminescence

This study reports the facile syntheses of tetra-boron difluoride (tetra-BF₂ ) complexes, flag-hinge-like molecules that exhibit intense green-to-orange luminescence in solution and yellow-to-red emission in the solid states. Single-crystal structure analysis and density functional theory calculations suggested a bent structure of this series of compounds. The complexes also exhibited excellent optical properties, with quantum yields reaching 100 % and a large Stokes shift. These properties were attributed to the altered bending angle of the molecule in the S₁ excited state. As the rotational motion was suppressed around the 2,2'-bipyrrole axis, atropisomers with axial chirality were formed, which are optically resolvable into (R) and (S)-enantiomers through a chiral column. The atropisomers thus function as circularly polarized luminescent (CPL) materials, in which the color (green, green-yellow, and yellow) can be varied by controlling the aggregation state. This rational design of multi-BF₂ complexes can potentially realize novel photofunctional materials.

Atroposelective Synthesis of Axially Chiral C2-Arylindoles via Rhodium-Catalyzed Asymmetric C-H Bond Insertion

A highly efficient rhodium-catalyzed formal C-H insertion reaction between indoles and 1-diazonaphthoquinones has been established, providing a novel protocol for the atroposelective synthesis of axially chiral C2-arylindoles (up to 99:1 er) under mild reaction conditions. Typically, only 1 mol % of Rh₂(S-PTTL)₄ is used and the chelation group is not needed for this conversion.

Simultaneous Construction of C-N Axial and Central Chirality via Silver-Catalyzed Desymmetrizative [3 + 2] Cycloaddition of Prochiral N-Aryl Maleimides with Activated Isocyanides

Herein, we report an unprecedented strategy for the simultaneous construction of a remote C-N stereogenic axis and three contiguous stereogenic carbon centers via silver-catalyzed desymmetrizative [3 + 2] cycloaddition of prochiral N-aryl maleimides with activated isocyanides. This method features operational simplicity, wide substrate scope, high efficiency, and good to excellent stereoselectivity. Notably, it represents the first example of catalytic enantioselective synthesis of C-N atropisomers with the use of activated isocyanides.

Transition Metal-Catalyzed Biaryl Atropisomer Synthesis via a Torsional Strain Promoted Ring-Opening Reaction

ConspectusArising from the restricted rotation of a single bond caused by steric or electronic effects, atropisomerism is one of the few fundamental categories for molecules to manifest their three-dimensional characters into which axially chiral biaryl compounds fall. Despite the widespread occurrence of axially chiral skeletons in natural products, bioactive molecules, and chiral ligands/organocatalysts, catalytic asymmetric methods for the synthesis of these structures still lag behind demand. Major challenges for the preparation of these chiral biaryls include accessing highly sterically hindered variants while controlling the stereoselectivity. A couple of useful strategies have emerged for the direct asymmetric synthesis of these molecules in the last two decades.Recently, we have engaged in catalytic asymmetric synthesis of biaryl atropisomers via transition metal catalysis, including asymmetric ring-openings of dibenzo cyclic compounds. During these studies, we serendipitously discovered that the two substituents adjacent to the axis cause these dibenzo cyclic molecules to be distorted to minimize steric repulsion. The distorted compounds display higher reactivity in the ring-opening reactions than the non-distorted molecules. In other words, torsional strain can promote a ring-opening reaction. On the basis of this concept, we have successfully realized the catalytic asymmetric ring-opening reaction of cyclic diaryliodoniums, dibenzo silanes, and 9H-fluoren-9-ols, which delivered several differently substituted ortho tetra-substituted biaryl atropisomers in high enantioselectivity. The torsional strain not only activates the substrates toward ring-opening under mild conditions but also changes the chemoselectivity of bond-breaking events. In the palladium-catalyzed carboxylation of S-aryl dibenzothiophenium, the torsional strain inversed the bond selectivity from exocyclic C-S bond cleavage to the ring-opening reaction.In this Account, we summarize our studies on copper-, rhodium-, or palladium-catalyzed asymmetric ring-opening reactions of dibenzo cyclic compounds as a useful collection of methods for the straightforward preparation of ortho tetra-substituted biaryl atropisomers with high enantiopurity on the basis of the above-mentioned torsional strain-promoted ring-opening coupling strategy. In the last part, the torsional strain energies are also discussed with the aid of density functional theory (DFT) calculations.