Stronger Brønsted Acids: Recent Progress

Acid-Mediated Intramolecular Cyclizations of (N-Aryl)-acetylenic Sulfonamides: Synthesis of Fused and Spirocyclic Sultams

Herein, we report acid-mediated divergent annulations of (N-aryl)-alkynyl sulfonamides. The substituent at the para position of the N-aryl group determines two diverse reaction paths, leading to the selective assembly of benzo-fused sultams and spirocyclic sultams. This strategy provides a series of benzo/spiro-sultams with wide functional group compatibility and good to excellent yields under mild reaction conditions. Additionally, scale-up reaction and further transformations of the products were also carried out to demonstrate the utility of the protocol.

Consecutive chirality transfer: efficient synthesis of chiral C,O-chelated BINOL/gold(iii) complexes for asymmetric catalysis and chiral resolution of disubstituted BINOLs

A novel approach for efficient synthesis of chiral C,O-chelated BINOL/gold(iii) complexes by diastereomeric resolution using enantiopure BINOL as a chiral resolving agent was demonstrated. The BINOL/gold(iii) diastereomers with different solubility were separated by simple filtration, providing optically pure BINOL/gold(iii) complexes with up to >99 : 1 dr. By combining this with an efficient BINOL ligand dissociation process, a simple and column-free method for chiral resolution of racemic gold(iii) dichloride complexes on a gram scale was established, affording their enantiopure forms in good yields. Conversely, the resolved enantiopure gold(iii) dichloride complexes could serve as chiral resolving agents to resolve disubstituted BINOL derivatives, achieving both BINOLs and gold(iii) complexes in good to excellent yields (overall 77-96% and 76-95%, respectively) with a high optical purity of up to 99% ee. Through a consecutive chirality transfer process, the chiral information from an inexpensive chiral source was transferred to highly valuable gold(iii) complexes, followed by sterically bulky BINOL derivatives. This work would open a new synthetic strategy facilitating the development of structurally diverse chiral gold(iii) complexes and gold(iii)-mediated chiral resolution of BINOL derivatives. In addition, this new class of C,O-chelated BINOL/gold(iii) complexes achieved asymmetric carboalkoxylation of ortho-alkynylbenzaldehydes with an excellent enantioselectivity of up to 99% ee.

Bridged Biaryl Atropisomers by Organocatalyzed Kinetic Asymmetric Alcoholysis

We disclose herein an asymmetric synthesis of axially chiral oxazepine-containing bridged biaryls via CPA-catalyzed kinetic asymmetric alcoholysis. Control experiments showed that this CPA-catalyzed alcoholysis was reversible, and lowering the reaction temperature could almost suppress the reversible reaction, thus providing a series of axially chiral oxazepine-containing bridged biaryl compounds in good to excellent enantioselectivities. The gram-scale reactions and facile derivatizations of the enantioenriched products demonstrate the practical utility of this reaction.

Catalytic enantioselective synthesis of 2-pyrazolines via one-pot condensation/6π-electrocyclization: 3,5-bis(pentafluorosulfanyl)-phenylthioureas as powerful hydrogen bond donors

A new conjugate-base-stabilized carboxylic acid (CBSCA) containing a 3,5-bis(pentafluorosulfanyl)phenylthiourea functionality catalyses challenging one-pot condensations/6π-electrocyclizations of hydrazines and α,β-unsaturated ketones under mild conditions. Structurally diverse N-aryl 2-pyrazolines are obtained in good yields and enantioselectivities. The superior performance of 3,5-bis(SF5)phenylthioureas over the widely used 3,5-bis(CF3)phenylthioureas is further demonstrated in the Michael addition of dimethyl malonate to nitrostyrene, using a new Takemoto-type catalyst.

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.

Enantioselective Synthesis of Heteroatom-Linked Non-Biaryl Atropisomers

Atropisomers hold significant fascination, not only for their prevalence in natural compounds but also for their biological importance and wide-ranging applications as chiral materials, ligands, and organocatalysts. While biaryl and heterobiaryl atropisomers are commonly studied, the enantioselective synthesis of less abundant heteroatom-linked non-biaryl atropisomers presents a formidable challenge in modern organic synthesis. Unlike classical atropisomers, these molecules allow rotation around two bonds, resulting in low barriers to enantiomerization through concerted bond rotations. In recent years the discovery of new configurationally stable rare non-biaryl scaffolds such as aryl amines, aryl ethers and aryl sulfones as well as innovative methodologies to control their configuration have been disclosed in the literature and constitute the topic of this minireview.

Phosphoric Acid-Catalyzed Alkene Difunctionalization of 2-Vinylpyridines via HOMO/LUMO Biactivated Diels-Alder Reaction

The difunctionalization of vinylpyridines based on the cyclization strategy remains rare and underdeveloped, in contrast to the well-developed hydrogen functionalization. Current exploration on [4 + 2] cyclization of vinylpyridines mainly relies on extremely high temperatures and the LUMO activation of vinylpyridines using boron trifluoride as a strong Lewis acid. Herein, we established a phosphoric acid-catalyzed [4 + 2] cyclization reaction of 3-vinyl-1H-indoles and 2-vinylpyridines by means of the LUMO/HOMO bifunctional activation model. This protocol features mild reaction conditions, high functional group tolerance, broad substrate compatibility, and high diastereoselectivity, enabling the efficient construction of various functionalized pyridine-substituted tetrahydrocarbazoles with prominent potential in drug discovery.

Organocatalytic Enantioselective 1,12-Addition of Alkynyl Biphenyl Quinone Methides Formed In Situ

The chemistry of quinone methides formed in situ has been flourishing in recent years. In sharp contrast, the development and utilization of biphenyl quinone methides are rare. In this study, we achieved a remote stereocontrolled 1,12-conjugate addition of biphenyl quinone methides formed in situ for the first time. In the presence of a suitable chiral phosphoric acid, alkynyl biphenyl quinone methides were generated from α-[4-(4-hydroxyphenyl)phenyl]propargyl alcohols, followed by enantioselective 1,12-conjugate addition with indole-2-carboxylates. The strategy enabled the alcohols to serve as efficient allenylation reagents, providing practical access to a broad range of axially chiral allenes bearing a (1,1'-biphenyl)-4-ol unit, which were previously less accessible. Combined with control experiments, density functional theory calculations shed light on the reaction mechanism, indicating that enantioselectivity originates from the nucleophilic addition of alkynyl biphenyl quinone methides. Notably, not only the presence of biphenyl quinone methides as versatile intermediates was confirmed but also organocatalytic enantioselective 1,12-addition was established.

Organocatalysis as an enabling tool for enantioselective ring-opening reactions of cyclopropanes

The rich reactivity profile of cyclopropanes has been extensively explored to trigger new organic transformations that enable unusual disconnective approaches to synthesize molecular motifs that are not easily reached through conventional reactions. In particular, the chemistry of cyclopropanes has received special attention in the last decade, with multiple new approaches that capitalize on the use of organocatalysis for the activation of the cyclopropane scaffold. This situation has also opened the possibility of developing enantioselective variants of many reactions that until now were only carried out in an enantiospecific or diastereoselective manner. Our group has been particularly active in this field, focusing more specifically on the use of aminocatalysis and Brønsted acid catalysis as major organocatalytic activation manifolds to trigger new unprecedented transformations involving cyclopropanes that add to the current toolbox of general methodologies available to organic chemists for the enantioselective synthesis of chiral compounds.

A New Age of Biocatalysis Enabled by Generic Activation Modes

Biocatalysis is currently undergoing a profound transformation. The field moves from relying on nature's chemical logic to a discipline that exploits generic activation modes, allowing for novel biocatalytic reactions and, in many instances, entirely new chemistry. Generic activation modes enable a wide range of reaction types and played a pivotal role in advancing the fields of organo- and photocatalysis. This perspective aims to summarize the principal activation modes harnessed in enzymes to develop new biocatalysts. Although extensively researched in the past, the highlighted activation modes, when applied within enzyme active sites, facilitate chemical transformations that have largely eluded efficient and selective catalysis. This advance is attributed to multiple tunable interactions in the substrate binding pocket that precisely control competing reaction pathways and transition states. We will highlight cases of new synthetic methodologies achieved by engineered enzymes and will provide insights into potential future developments in this rapidly evolving field.

Highly acidic N-triflylphosphoramides as chiral Brønsted acid catalysts: the effect of weak hydrogen bonds and multiple acceptors on complex structures and aggregation

N-Triflylphosphoramides (NTPAs) represent an important catalyst class in asymmetric catalysis due to their multiple hydrogen bond acceptor sites and acidity, which is increased by several orders of magnitude compared to conventional chiral phosphoric acids (CPAs). Thus, NTPAs allow for several challenging transformations, which are not accessible with CPAs. However, detailed evidence on their hydrogen bonding situation, complex structures and aggregation is still lacking. Therefore, this study covers the hydrogen bonding behavior and structural features of binary NTPA/imine complexes compared to their CPA counterparts. Deviating from the single-well potential hydrogen bonds commonly observed in CPA/imine complexes, the NTPA/imine complexes exhibit a tautomeric equilibrium between two proton positions. Low-temperature NMR at 180 K supported by computer simulations indicates a OHN hydrogen bond between the phosphoramide oxygen and the imine, instead of the mostly proposed NHN H-bond. Furthermore, this study finds no evidence for the existence of dimeric NTPA/NTPA/imine complexes as previously suggested for CPA systems, both synthetically and through NMR studies.

Atropisomeric Carboxylic Acids Synthesis via Nickel-Catalyzed Enantioconvergent Carboxylation of Aza-Biaryl Triflates with CO2

Upgrading CO2 to value-added chiral molecules via catalytic asymmetric C-C bond formation is a highly important yet challenging task. Although great progress on the formation of centrally chiral carboxylic acids has been achieved, catalytic construction of axially chiral carboxylic acids with CO2 has never been reported to date. Herein, we report the first catalytic asymmetric synthesis of axially chiral carboxylic acids with CO2, which is enabled by nickel-catalyzed dynamic kinetic asymmetric reductive carboxylation of racemic aza-biaryl triflates. A variety of important axially chiral carboxylic acids, which are valuable but difficult to obtain via catalysis, are generated in an enantioconvergent version. This new methodology features good functional group tolerance, easy to scale-up, facile transformation and avoids cumbersome steps, handling organometallic reagents and using stoichiometric chiral materials. Mechanistic investigations indicate a dynamic kinetic asymmetric transformation process induced by chiral nickel catalysis.

B(C6F5)3/Chiral Phosphoric Acid Catalyzed Asymmetric Aza-Diels-Alder Reaction of Imines and Unactivated Dienes

Herein, we report an asymmetric aza-Diels-Alder reaction of quinoxalinones or benzoxazinones with unactivated dienes by utilizing a B(C6F5)3/chiral phosphoric acid catalyst to construct chiral six-membered N-heterocycles. Various quinoxalinones or benzoxazinones with electron-withdrawing and electron-donating groups and unactivated dienes were tolerated (up to 99% yield and 99% ee) in the methodology with only 2 mol % catalyst loading. Moreover, the luminescence mechanism and photophysical properties of the product were tested and used for anticounterfeiting of QR codes.

Harnessing Multistep Chalcogen Bonding Activation in the α-Stereoselective Synthesis of Iminoglycosides

The use of noncovalent interactions (NCIs) has received significant attention as a pivotal synthetic handle. Recently, the exploitation of unconventional NCIs has gained considerable traction in challenging reaction manifolds such as glycosylation due to their capacity to facilitate entry into difficult-to-access sugars and glycomimetics. While investigations involving oxacyclic pyrano- or furanoside scaffolds are relatively common, methods that allow the selective synthesis of biologically important iminosugars are comparatively rare. Here, we report the capacity of a phosphonochalcogenide (PCH) to catalyze the stereoselective α-iminoglycosylation of iminoglycals with a wide array of glycosyl acceptors with remarkable protecting group tolerance. Mechanistic studies have illuminated the counterintuitive role of the catalyst in serially activating both the glycosyl donor and acceptor in the up/downstream stages of the reaction through chalcogen bonding (ChB). The dynamic interaction of chalcogens with substrates opens up new mechanistic opportunities based on iterative ChB catalyst engagement and disengagement in multiple elementary steps.

Stereocontrol via Propeller Chirality in FLP-Catalyzed Asymmetric Hydrogenation

Utilization of chiral frustrated Lewis pairs as catalysts in enantioselective hydrogenation of unsaturated molecules represents a promising approach in asymmetric synthesis. In our effort to improve our current understanding of the factors governing the stereoselectivity in these catalytic processes, herein we examined the mechanism of direct hydrogenation of aromatic enamines catalyzed by a binaphthyl-based chiral amino-borane. Our computational analysis reveals that only one particular conformer of the key borohydride reaction intermediate can be regarded as a reactive form of this species. This borohydride conformer has a well-defined chiral propeller shape, which induces facial selectivity in the hydride transfer to pro-chiral iminium intermediates. The propeller chirality of the reactive borohydride conformer is generated by the axially chiral binaphthyl scaffold of the amino-borane catalyst through stabilizing π-π stacking interactions. This new computational insight can be readily used to interpret the high degree of stereoinduction observed for these reactions. We expect that the concept of chirality relay could be further exploited in catalyst design endeavors.

Enantioselective Cross-[4 + 2]-Cycloaddition/Decarboxylation of 2-Pyrones by Cooperative Catalysis of the Pd(0)/NHC Complex and Chiral Phosphoric Acid

Here, we describe a cooperative Pd(0)/chiral phosphoric acid catalytic system that allows us to realize the first chemo-, regio-, and enantioselective sequential cross-[4 + 2]-cycloaddition/decarboxylation reaction between 2-pyrones and unactivated acyclic 1,3-dienes. The key to the success of this transformation is the utilization of an achiral N-heterocyclic carbene (NHC) as the ligand and a newly developed chiral phosphoric acid as the cocatalyst. Experimental investigations and computational studies support the idea that the Pd(0)/NHC complex acts as a π-Lewis base to increase the nucleophilicity of 1,3-dienes via η2 coordination, while the chiral phosphoric acid simultaneously increases the electrophilicity of 2-pyrones by hydrogen bonding. By this synergistic catalysis, the sequential cross-[4 + 2]-cycloaddition and decarboxylation reaction proceeds efficiently, enabling the preparation of a wide range of chiral vinyl-substituted 1,3-cyclohexadienes in good yields and enantioselectivities. The synthetic utility of this reaction is demonstrated by synthetic transformations of the product to various valuable chiral six-membered carbocycles.

Organocatalytic enantioselective synthesis of Csp2-N atropisomers via formal Csp2-O bond amination

Compared with well-developed construction of Csp2-Csp2 atropisomers, the synthesis of Csp2-N atropisomers remains in its infancy, which is recognized as both appealing and challenging. Herein, we achieved the first organocatalyzed asymmetric synthesis of Csp2-N atropisomers by formal Csp2-O amination. With the aid of a suitable acid, 3-alkynyl-3-hydroxyisoindolinones reacted smoothly with 1-methylnaphthalen-2-ols to afford a wide range of atropisomers by selective formation of the Csp2-N axis. Particularly, both the kinetic (Z)-products and the thermodynamic (E)-products could be selectively formed. Furthermore, the rarely used combination of two chiral Brønsted acid catalysts achieved excellent enantiocontrol, which is intriguing and unusual in organocatalysis. Based on control experiments and DFT calculations, a cascade dehydration/addition/rearrangement process was proposed. More importantly, this work provided a new plat-form for direct atroposelective construction of the chiral Csp2-N axis.

Mechanism and Origin of Stereoselectivity for the NHC-Catalyzed Desymmetrization Reaction for the Synthesis of Axially Chiral Biaryl Aldehydes

Organocatalytic desymmetrization reaction is a powerful tool for constructing axial chirality, but the theoretical study on the origin of stereoselectivity still lags behind even now. In this work, the N-heterocyclic carbene (NHC)-catalyzed desymmetrization reaction of biaryl frameworks for the synthesis of axially chiral aldehydes has been selected and theoretically investigated by using density functional theory (DFT). The fundamental pathway involves several steps, i.e., desymmetrization, formation of Breslow oxidation, esterification, and NHC regeneration. The desymmetrization and formation of Breslow processes have been identified as stereoselectivity-determining and rate-determining steps. Further weak interaction analyses proved that the C-H···O hydrogen bond and C-H···π interactions are responsible for the stability of the key stereoselective desymmetrization transition states. This research contributes to understanding the nature of NHC-catalyzed desymmetrization reactions for the synthesis of axially chiral compounds.

A New Series of Sandwich-Type 5,5'-Biterphenylenes: Synthetic Challenge, Structural Uniqueness and Photodynamics

A new series of biaryls, bi-linear-terphenylenes (BLTPs), were prepared using the tert-butyllithium-mediated cyclization as the key synthetic step. The three-dimensional structures of the studied compounds were verified using X-ray crystallography and DFT calculations. Tetraaryl(ethynyl)-substituted BLTPs are highly crowded molecules, and the internal rotation around the central C-C bond is restricted due to a high barrier (>50 kcal/mol). These structures contain several aryl/terphenylenyl/aryl sandwiches, where the through-space π-π (TSPP) interactions are strongly reflected in the shielding of 1 H NMR chemical shifts, reduction of oxidation potentials, increasing aromaticity of the central six-membered ring and decreasing antiaromaticity of the four-membered rings in a terphenylenyl moiety based on NICS(0) and iso-chemical shielding surfaces. Despite the restricted C-C bond associated intramolecular TSPP interactions for BLTPs in the ground state, to our surprise, the electronic coupling between two linear terphenylenes (LTPs) in BLTPs in the excited state is weak, so that the excited-state behavior is dominated by the corresponding monomeric LTPs. In other words, all BLTPs undergo ultrafast relaxation dynamics via strong exciton-vibration coupling, acting as a blue-light absorber with essentially no emission.

NHC-Catalyzed Chemo- and Enantioselective Reaction between Aldehydes and Enals for Access to Axially Chiral Arylaldehydes

A chiral carbene-catalyzed chemo- and enantioselective reaction with racemic biaryl aldehydes and α-bromoenals is developed for access to axially chiral 2-arylbenzaldehydes through atroposelective dynamic kinetic resolution (DKR) processes. This atroposelective DKR strategy can tolerate a broad scope of substrates with diverse functionalities. The axially chiral 2-aryl benzaldehyde products generally afford moderate to good yields and enantioselectivities. The axially chiral molecules afforded from the current approach are variable through simple transformations to afford axially chiral functional molecules with excellent optical purities.

MATEO: intermolecular α-amidoalkylation theoretical enantioselectivity optimization. Online tool for selection and design of chiral catalysts and products

The enantioselective Brønsted acid-catalyzed α-amidoalkylation reaction is a useful procedure is for the production of new drugs and natural products. In this context, Chiral Phosphoric Acid (CPA) catalysts are versatile catalysts for this type of reactions. The selection and design of new CPA catalysts for different enantioselective reactions has a dual interest because new CPA catalysts (tools) and chiral drugs or materials (products) can be obtained. However, this process is difficult and time consuming if approached from an experimental trial and error perspective. In this work, an Heuristic Perturbation-Theory and Machine Learning (HPTML) algorithm was used to seek a predictive model for CPA catalysts performance in terms of enantioselectivity in α-amidoalkylation reactions with R2 = 0.96 overall for training and validation series. It involved a Monte Carlo sampling of > 100,000 pairs of query and reference reactions. In addition, the computational and experimental investigation of a new set of intermolecular α-amidoalkylation reactions using BINOL-derived N-triflylphosphoramides as CPA catalysts is reported as a case of study. The model was implemented in a web server called MATEO: InterMolecular Amidoalkylation Theoretical Enantioselectivity Optimization, available online at: https://cptmltool.rnasa-imedir.com/CPTMLTools-Web/mateo . This new user-friendly online computational tool would enable sustainable optimization of reaction conditions that could lead to the design of new CPA catalysts along with new organic synthesis products.

Catalytic Enantioselective [4+2] Cycloadditions of Salicylaldehyde Acetals with Enol Ethers

A readily accessible conjugate-base-stabilized carboxylic acid (CBSCA) catalyst facilitates highly enantioselective [4+2] cycloaddition reactions of salicylaldehyde-derived acetals and cyclic enol ethers, resulting in the formation of polycyclic chromanes with oxygenation in the 2- and 4-positions. Stereochemically more complex products can be obtained from racemic enol ethers. Spirocyclic products are also accessible.

Access to Axially Chiral Aryl Aldehydes via Carbene-Catalyzed Nitrile Formation and Desymmetrization Reaction

An approach utilizing N-heterocyclic carbene for nitrile formation and desymmetrization reaction is developed. The process involves kinetic resolution, with the axially chiral aryl monoaldehydes obtained in moderate yields with excellent optical purities. These axially chiral aryl monoaldehydes can be conveniently transformed into functionalized molecules, showing great potential as catalysts in organic chemistry.

Organocatalytic Enantioselective Nucleophilic Addition of Indole Imine 5-Methides

Despite the enormous developments in the asymmetric transformations of indole imine methides (IIMs), the remote asymmetric induction involving IIMs remains challenging due to the spatial interaction requirement between the substrate and catalyst. Herein we report the first catalytic asymmetric nucleophilic addition to indole imine 5-methide (5-IIM), the only topological isomer of IIMs whose asymmetric addition remains unknown. Despite the challenging remote stereocontrol, high efficiency and respectable enantioselectivity were achieved to provide access to a range of enantioenriched indole-containing triaryl alkanes.

Heterodifunctionalization of Electron-Rich Alkynes Catalyzed by in Situ Generated Silylium Ions

Silylium ions are versatile Lewis acids in organic synthesis. While they have been well-known for the activation of σ donors, catalysis initiated by the activation of π donors remains underdeveloped, particularly for alkynes. Herein, we demonstrate an example of silylium-catalyzed alkyne heterodifunctionalization. The silylium ion generated in situ from HNTf2 and the silyl reagent serve as superior catalysts in the efficient silylphosphination and silylcyanation of electron-rich alkynes with excellent regio- and stereoselectivity. The compatibility of this protocol with strongly coordinating ligands (Ph2P and CN) not only complements the metal-catalyzed systems but also expands the scope of silylium-catalyzed reactions.

Decarboxylative Iodination and Suzuki-Miyaura Coupling Reactions to Access Chiral 3,3'-Diaryl-1,1'-bi-2-naphthols

An efficient synthesis of the enantiomerically pure 3,3'-bis-arylated BINOLs is accomplished through decarboxylative iodination of the dimethyl ether derivative of BINOL-3,3'-dicarboxylic acid followed by Suzuki-Miyaura coupling using a one-pot protocol. The decarboxylative iodination is effected with the dimethyl ether derivative of BINOL-3,3'-dicarboxylic acid using iodine as a terminal oxidant and the cheaply available K3PO4 as a base under neat conditions. This protocol facilitated the introduction of the aryl group at the 3,3'-position on the binaphthyl system using aryl boronic acid through a palladium-catalyzed Suzuki-Miyaura coupling reaction.

Primary activation of para-quinone methides by chiral phosphoric acid for enantioselective construction of tetraarylmethanes

Demonstrated here is an asymmetric nucleophilic addition via primary activation of para-quinone methides (p-QMs) based on a chiral phosphoric acid catalytic system. In sharp contrast to previous CPA-based bifunctional activation processes that all required the nucleophiles to have an effective hydrogen bond donor unit (e.g., OH, NH), here no such unit is required in the nucleophile. N-protected indole nucleophiles were successfully utilized for the synthesis of chiral tetraarylmethanes with high efficiency and enantioselectivity under mild conditions. Therefore, this protocol significantly expanded the scope of asymmetric transformations of p-QMs.

Discovery and synthesis of atropisomerically chiral acyl-substituted stable vinyl sulfoxonium ylides

Atropisomerism is a type of conformational chirality that plays a critical role in various fields of chemistry, including synthetic, medicinal and material chemistry, and its impact has been widely recognized. Although chiral atropisomerism in rotationally restricted aryl-aryl bonds has garnered substantial interest and led to important discoveries in chiral catalysts and drug development, the exploration of non-aryl atropisomers has fallen behind. Here we reveal a previously unexplored form of non-aryl atropisomerism by linking a sterically congested olefin to a sulfoxonium ylide. A streamlined synthetic approach to these novel molecules was developed through the hydrofunctionalization of alkynyl sulfoxonium ylides. Notably, an enantioselective organocatalytic strategy was developed to prepare these non-aryl atropisomers in high optical purity. This form of atropisomerism offers new routes for investigating the functional properties of axially chiral molecules.

Enantioselective and Regiodivergent Synthesis of Dihydro-1,2-oxazines from Triene-Carbamates via Chiral Phosphoric Acid-Catalysis

Conjugated trienes are fascinating building blocks for the rapid construction of complex polycyclic compounds. However, limited success has been achieved due to the challenging regioselectivity control. Herein, we report an enantio- and diastereoselective process allowing to regioselectively control the functionalization of NH-triene-carbamates. Synthesis of chiral cis-3,6-dihydro-2H-1,2-oxazines is achieved by a chiral phosphoric acid catalyzed Nitroso-Diels-Alder cycloaddition involving [(1E,3E,5E)-hexa-1,3,5-trien-1-yl]carbamates. Moreover, modular access to three different regioisomers with excellent diastereoselectivities and high to excellent enantioselectivities is obtained by a careful choice of the reaction conditions. A computational study reveals that the regioselectivity is influenced by the steric demand of the substituents at the 6-position of the triene, as well as noncovalent interactions between the two cycloaddition partners. Utility of each regioisomeric cycloadduct is highlighted by a variety of synthetic transformations.

Modular and diverse synthesis of amino acids via asymmetric decarboxylative protonation of aminomalonic acids

Stereoselective protonation is a challenge in asymmetric catalysis. The small size and high rate of transfer of protons mean that face-selective delivery to planar intermediates is hard to control, but it can unlock previously obscure asymmetric transformations. Particularly, when coupled with a preceding decarboxylation, enantioselective protonation can convert the abundant acid feedstocks into structurally diverse chiral molecules. Here an anchoring group strategy is demonstrated as a potential alternative and supplement to the conventional structural modification of catalysts by creating additional catalyst-substrate interactions. We show that a tailored benzamide group in aminomalonic acids can help build a coordinated network of non-covalent interactions, including hydrogen bonds, π-π interactions and dispersion forces, with a chiral acid catalyst. This allows enantioselective decarboxylative protonation to give α-amino acids. The malonate-based synthesis introduces side chains via a facile substitution of aminomalonic esters and thus can access structurally and functionally diverse amino acids.

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.

Influence of Coordination to Silver(I) Centers on the Activity of Heterocyclic Iodonium Salts Serving as Halogen-Bond-Donating Catalysts

Kinetic data based on 1 H NMR monitoring and computational studies indicate that in solution, pyrazole-containing iodonium triflates and silver(I) triflate bind to each other, and such an interplay results in the decrease of the total catalytic activity of the mixture of these Lewis acids compared to the separate catalysis of the Schiff condensation, the imine-isocyanide coupling, or the nucleophilic attack on a triple carbon-carbon bond. Moreover, the kinetic data indicate that such a cooperation with the silver(I) triflate results in prevention of decomposition of the iodonium salts during the reaction progress. XRD study confirms that the pyrazole-containing iodonium triflate coordinates to the silver(I) center via the pyrazole N atom to produce a rare example of a pentacoordinated trigonal bipyramidal dinuclear silver(I) complex featuring cationic ligands.

Chiral Phosphoric Acid Catalyzed Asymmetric Hydrolysis of Biaryl Oxazepines for the Synthesis of Axially Chiral Biaryl Amino Phenol Derivatives

The development of catalytic asymmetric reaction with water as the reactant is challenging due to the reactivity- and stereoselectivity-control issues resulted from the low nucleophilicity and the small size of water. We disclose herein a chiral phosphoric acid (CPA) catalyzed atroposelective ring-opening reaction of biaryl oxazepines with water. A series of biaryl oxazepines undergo the CPA catalyzed asymmetric hydrolysis in a highly enantioselective manner. The key for the success of this reaction is the use of a new SPINOL-derived CPA catalyst and the high reactivity of biaryl oxazepine substrates towards water under acidic conditions. Density functional theory calculations suggest that the reaction proceeds via a dynamic kinetic resolution pathway and the CPA catalyzed addition of water to the imine group is both enantio- and rate-determining.

Axially chiral styrene-based organocatalysts and their application in asymmetric cascade Michael/cyclization reaction

An axially chiral styrene-based organocatalyst, featuring a combination of axially chiral styrene-based structure and a pyrrole ring, has been designed and synthesized. This catalyst demonstrates remarkable capabilities in producing a wide range of densely substituted spirooxindoles that feature an alkyne-substituted quaternary stereogenic center. These spirooxindoles are generated through mild cascade Michael/cyclization reactions, resulting in high conversion rates and exceptional enantioselectivity. Our catalytic model, based on experiments, X-ray structure analysis and DFT calculations suggests that chiral matched π-π interactions and multiple H-bonds between the organocatalyst and substrates play significant roles in controlling the stereoselectivity of the reaction.

Diverse Synthesis of 2H-Isoindole-Based Polycyclic Aromatic Compounds

1,3-Disubstituted N-aryl-2H-isoindoles have been synthesized by a cascade reaction of divinyl ethers, which are derived from easily available 4-bromoisocoumarins, with substituted anilines in HFIP. This cascade reaction consists of a ring-opening step through addition-elimination mechanism and the following 5-exo-tet type ring-closing step via the intramolecular nucleophilic substitution reaction. Thus obtained 2H-isoindoles have been derivatized to high-order nitrogen-containing polycycles including less accessible benzo[a]ullazines.

Development of Stable Carbanionic Substituents

During the past decade, carbon (C-H) acids depicted as 'Tf2 CHR' (Tf=CF3 SO2 ) have attracted considerable attention as a new class of superacidic molecules, which show stronger acidity than sulfuric acid molecules. In recent years, the author has developed a synthetic methodology for such strong acids and has opened the door to chemistry of highly stabilised carbanions [Tf2 CR]- , which are the conjugate bases of the carbon acids. These carbanion-containing salts are stable and easy-to-handle species. Our efforts have revealed that the ionic but lipophilic characters of this type of carbanion can be used as a unique 'substituent' for increasing both the water solubility and the lipophilicity of organic compounds. This Personal Account provides an overview of our [Tf2 CR]- chemistry, including its synthesis, structure, reactivity, and applications.

Mild and selective hydrogen-deuterium exchange for aromatic hydrogen of amines

The direct electrophilic deuteration of the aromatic moiety in aromatic and aralkyl amines is reported. The acid-catalyzed deuteration is facilitated by deuterated trifluoromethanesulfonic acid, [D]triflic acid, CF3 SO3 D, TfOD, which acts as both the reaction solvent and the source of the deuterium label. The mild conditions enable room temperature hydrogen/deuterium exchange for most of the para-substituted aromatic amine derivatives studied. In addition, short reaction times and a high degree of aromatic deuteration are achieved and isolation of the product is simple. The optical activity of the chiral aralkyl amines studied was preserved.

Organocatalytic Atroposelective Construction of Axially Chiral Compounds Containing Benzimidazole and Quinoline Rings

An organocatalytic atroposelective strategy for the construction of axially chiral compounds containing benzimidazole and quinoline rings is described. The enantioselective heteroannulation reaction of 2-alkynylbenzimidazoles with ortho-aminophenylketones proceeded smoothly in the presence of chiral phosphoric acid to provide axially chiral heterobiaryls with good yields and enantioselectivities. This is the first example of the combination of benzimidazole and quinoline rings at the 2- and 3-positions, respectively, into axially chiral heterobiaryls by this new strategy.

Dual-Hydrogen-Bond Donor and Brønsted Acid Cocatalysis Enables Highly Enantioselective Protio-Semipinacol Rearrangement Reactions

A catalytic protio-semipinacol ring-expansion reaction has been developed for the highly enantioselective conversion of tertiary vinylic cyclopropyl alcohols into cyclobutanone products bearing α-quaternary stereogenic centers. The method relies on the cocatalytic effect of a chiral dual-hydrogen-bond donor (HBD) with hydrogen chloride. Experimental evidence is provided for a stepwise mechanism where protonation of the alkene generates a short-lived, high-energy carbocation, which is followed by C-C bond migration to deliver the enantioenriched product. This research applies strong acid/chiral HBD cocatalysis to weakly basic olefinic substrates and lays the foundation for further investigations of enantioselective reactions involving high-energy cationic intermediates.

Asymmetric Acyclic 1,3-Difunctionalization of Vinyl Carbenes via Site-Selective Vinylogous Mannich-Type Interception of Oxonium Ylides

A novel and highly stereoselective acyclic 1,3-difunctionalization of vinyl metal carbene species has been developed via Rh(II)/chiral phosphoric acid co-catalyzed three-component reactions of vinyldiazoacetates with alcohols and imines. This innovative approach features excellent regio-, diastereo-, and enantioselectivities, demonstrating a broad scope and functional group compatibility. Notably, this is the first example of three-component asymmetric acyclic 1,3-difunctionalization with in situ-formed vinyl metal carbenes.

Axially Chiral Sulfonic Acids for Brønsted Acid Catalysis: 8-Benzoimidazolylnaphthalene-1-sulfonic Acids and Their Derivatives

A new type of axially chiral sulfonic acid was developed. The synthesis is based on cheap commercially available materials and a practical method for optical resolution via diastereomeric salt formation, which can provide both enantiomers. Eleven benzoimidazolylnaphthalenesulfonic acids were prepared and four of them were isolated as pure and stable atropisomers. Moreover, several of these sulfonic acids were transformed into triflyl imides to further expand the range of dissociation constants.

Asymmetric Synthesis of Remotely Chiral Naphthols and Naphthylamines via Naphthoquinone Methides

Quinone methides are well-established intermediates in asymmetric synthesis. In contrast, their extended analogues with the carbonyl and methide units distributed across two different rings have not been exploited in asymmetric synthesis. Herein, we achieved the first asymmetric process involving such intermediates. Specifically, the use of suitable chiral phosphoric acids enabled in situ generation of 2-naphthoquinone 8-methides and the corresponding aza counterparts for mild one-pot asymmetric nucleophilic addition. These processes provided rapid access to a wide range of previously less accessible remotely chiral naphthols and naphthylamines with both high efficiency and excellent enantioselectivity. Control experiment and DFT calculations provided important insights into the reaction mechanism, which likely involves two phosphoric acid molecules in the enantiodetermining transition states. This work serves as a proof of concept for the exploitation of new types of extended quinone methides as versatile intermediates for asymmetric synthesis, thus providing a new platform for the efficient construction of remote benzylic stereogenic centers of aromatic compounds.

Computational molecular refinement to enhance enantioselectivity by reinforcing hydrogen bonding interactions in major reaction pathway

Computational analyses have revealed that the distortion of a catalyst and the substrates and their interactions are key to determining the stability of the transition state. Hence, two strategies "distortion strategy" and "interaction strategy" can be proposed for improving enantiomeric excess in enantioselective reactions. The "distortion strategy" is used as a conventional approach that destabilizes the TS (transition state) of the minor pathway. On the other hand, the "interaction strategy" focuses on the stabilization of the TS of the major pathway in which an enhancement of the reaction rate is expected. To realize this strategy, we envisioned the TS stabilization of the major reaction pathway by reinforcing hydrogen bonding and adopted the chiral phosphoric acid-catalysed enantioselective Diels-Alder reaction of 2-vinylquinolines with dienylcarbamates. The intended "interaction strategy" led to remarkable improvements in the enantioselectivity and reaction rate.

Carbene-Catalyzed Atroposelective Annulation for Quick Access to Axially Chiral Thiazine Derivatives

An N-heterocyclic carbene (NHC)-catalyzed atroposelective annulation reaction is disclosed for quick and efficient access to thiazine derivatives. A series of axially chiral thiazine derivatives bearing various substituents and substitution patterns were produced in moderate to high yields with moderate to excellent optical purities. Preliminary studies revealed that some of our products exhibit promising antibacterial activities against Xanthomonas oryzae pv. oryzae (Xoo) that causes rice bacterial blight.

Nature-inspired catalytic asymmetric rearrangement of cyclopropylcarbinyl cation

In nature, cyclopropylcarbinyl cation is often involved in cationic cascade reactions catalyzed by natural enzymes to produce a great number of structurally diverse natural substances. However, mimicking this natural process with artificial organic catalysts remains a daunting challenge in synthetic chemistry. We report a small molecule-catalyzed asymmetric rearrangement of cyclopropylcarbinyl cations, leading to a series of chiral homoallylic sulfide products with good to excellent yields and enantioselectivities (up to 99% enantiomeric excess). In the presence of a chiral SPINOL-derived N-triflyl phosphoramide catalyst, the dehydration of prochiral cyclopropylcarbinols occurs rapidly to generate symmetrical cyclopropylcarbinyl cations, which are subsequently trapped by thione-containing nucleophiles. A subgram-scale experiment and multiple downstream transformations of the sulfide products are further pursued to demonstrate the synthetic utility. Notably, a few heteroaromatic sulfone derivatives could serve as "covalent warhead" in the enzymatic inhibition of severe acute respiratory syndrome coronavirus 2 main protease.

Straightforward Synthesis of Bis[(trifluoromethyl)sulfonyl]ethylated Isocoumarins from 2-Ethynylbenzoates

Herein, we report a facile isocoumarin and isoquinolone preparation by taking advantage of an initial bis(triflyl)ethylation [triflyl = (trifluoromethyl)sulfonyl] reaction, followed by heterocyclization, which contrasts with our previous results on cyclobutene formation. The efficiency of the catalyst- and irradiation-free heterocyclization/bis(triflyl)ethylation sequence showed exquisite dependence on the electronic nature of the substituents at the 2-ethynylbenzoate(benzamide) precursors. Molecular docking of model bis(triflyl)ethylated isocoumarins on human acetylcholinesterase (hAChE) revealed promising biological activities through selective coordination on both the catalytic active site and peripheral active site.

Enantioselective Synthesis of Fluorinated Indolizidinone Derivatives

The enantioselective synthesis of fluorinated indolizidinone derivatives has been developed. The process involved an enantioselective intramolecular aza-Michael reaction of conjugated amides bearing a pendant α,β-unsaturated ketone moiety, catalyzed by the (S)-TRIP-derived phosphoric acid, followed by dimethyltitanocene methylenation and ring closing metathesis (RCM). Final indolizidine-derived products comprise a fluorine-containing tetrasubstituted double bond generated by the RCM reaction, which is a challenging task. The whole synthetic sequence took place in acceptable overall yields with excellent enantioselectivities.

A Catalytic Asymmetric Hydrolactonization

Despite recent advancements in the development of catalytic asymmetric electrophile induced lactonization reactions of olefinic carboxylic acids, the archetypical hydrolactonization has long remained an unsolved and well-recognized challenge. Here, we report the realization of a catalytic asymmetric hydrolactonization using a confined imidodiphosphorimidate (IDPi) Brønsted acid catalyst. The method is operationally simple, scalable, and compatible with a wide variety of substrates. Its potential is showcased with concise syntheses of the sesquiterpenes (-)-boivinianin A and (+)-gossonorol. Through in-depth physicochemical and DFT analyses, we derive a nuanced picture of the mechanism and enantioselectivity of this reaction.

Catalytic Asymmetric α-Functionalization of α-Branched Aldehydes

Aldehydes constitute a main class of organic compounds widely applied in synthesis. As such, catalyst-controlled enantioselective α-functionalization of aldehydes has attracted great interest over the years. In this context, α-branched aldehydes are especially challenging substrates because of reactivity and selectivity issues. Firstly, the transient trisubstituted enamines and enolates resulting upon treatment with an aminocatalyst or a base, respectively, would exhibit attenuated reactivity; secondly, mixtures of E- and Z-configured enamines/enolates may be formed; and third, effective face-discrimination on such trisubstituted sp2 carbon intermediates by the incoming electrophilic reagent is not trivial. Despite these issues, in the last 15 years, several catalytic approaches for the α-functionalization of prostereogenic α-branched aldehydes that proceed in useful yields and diastereo- and enantioselectivity have been uncovered. Developments include both organocatalytic and metal-catalyzed approaches as well as dual catalysis strategies for forging new carbon–carbon and carbon–heteroatom (C-O, N, S, F, Cl, Br, …) bond formation at Cα of the starting aldehyde. In this review, some key early contributions to the field are presented, but focus is on the most recent methods, mainly covering the literature from year 2014 onward.

Brønsted Acid-Catalyzed Reaction of N-arylnaphthalen-2-amines with Quinone Esters for the Construction of Carbazole and C-N Axially Chiral Carbazole Derivatives

We demonstrated here an efficient synthetic method of carbazole derivatives from readily available N-arylnaphthalen-2-amines and quinone esters catalyzed by Brønsted acid. With this strategy, a series of carbazole derivatives were obtained in good to excellent yields (76 to >99) under mild conditions. Large scale reaction illustrated the synthetic utility of this protocol. Meanwhile, a series of C-N axially chiral carbazole derivatives were also constructed in moderate to good yields (36-89% yield) with moderate to excellent atroposelectivities (44-94% ee) by using chiral phosphoric acid as a catalyst, which provides a novel strategy for the atroposelective construction of C-N axially chiral compounds and a new member of the C-N atropisomers.