Brønsted-Acid-Promoted Rh-Catalyzed Asymmetric Hydrogenation of N-Unprotected Indoles: A Cocatalysis of Transition Metal and Anion Binding

Highly enantioselective synthesis of both enantiomers of tetrahydroquinoxaline derivatives via Ir-catalyzed asymmetric hydrogenation

A novel Ir-catalyzed asymmetric hydrogenation protocol for the synthesis of chiral tetrahydroquinoxaline (THQ) derivatives has been developed. By simply adjusting the reaction solvent, both enantiomers of mono-substituted chiral THQs could be selectively obtained in high yields with excellent enantioselectivities (toluene/dioxane: up to 93% yield and 98% ee (R); EtOH: up to 83% yield and 93% ee (S)). For 2,3-disubstituted chiral THQs, the cis-hydrogenation products were obtained with up to 95% yield, 20 : 1 dr, and 94% ee. Remarkably, this methodology was also applicable under continuous flow conditions, yielding gram-scale products with comparable yields and enantioselectivities (dioxane: 91% yield and 93% ee (R); EtOH: 90% yield and 87% ee (S)). Unlike previously reported Ir-catalyzed asymmetric hydrogenation protocols, this system exhibited a significant improvement as it required no additional additives. Furthermore, comprehensive mechanistic studies including deuterium-labeling experiments, control experiments, kinetic studies, and density functional theory (DFT) calculations were conducted to reveal the underlying mechanism of enantioselectivities for both enantiomers.

Asymmetric Hydrogenation of Racemic 2-Substituted Indoles via Dynamic Kinetic Resolution: An Easy Access to Chiral Indolines Bearing Vicinal Stereogenic Centers

The asymmetric hydrogenation (AH) of N-unprotected indoles is a straightforward, yet challenging method to access biologically interesting NH chiral indolines. This method has for years been limited to 2/3-monosubstituted or 2,3-disubstituted indoles, which produce chiral indolines bearing endocyclic chiral centers. Herein, we have reported an innovative Pd-catalyzed AH of racemic α-alkyl or aryl-substituted indole-2-acetates using an acid-assisted dynamic kinetic resolution (DKR) process, affording a range of structurally fascinating chiral indolines that contain exocyclic stereocenters with excellent yields, diastereoselectivities, and enantioselectivities. Mechanistic studies support that the DKR process relies on a rapid interconversion of each enantiomer of racemic substrates, leveraged by an acid-promoted isomerization between the aromatic indole and nonaromatic exocyclic enamine intermediate. The reaction can be performed on a gram scale, and the products can be derivatized into non-natural β-amino acids via facile debenzylation and amino alcohol upon reduction.

Highly enantioselective synthesis of both tetrahydroquinoxalines and dihydroquinoxalinones via Rh-thiourea catalyzed asymmetric hydrogenation

Chiral tetrahydroquinoxalines and dihydroquinoxalinones represent the core structure of many bioactive molecules. Herein, a simple and efficient Rh-thiourea-catalyzed asymmetric hydrogenation for enantiopure tetrahydroquinoxalines and dihydroquinoxalinones was developed under 1 MPa H2 pressure at room temperature. The reaction was magnified to the gram scale furnishing the desired products with undamaged yield and enantioselectivity. Application of this methodology was also conducted successfully under continuous flow conditions. In addition, 1H NMR experiments revealed that the introduction of a strong Brønsted acid, HCl, not only activated the substrate but also established anion binding between the substrate and the ligand. More importantly, the chloride ion facilitated heterolytic cleavage of dihydrogen to regenerate the active dihydride species and HCl, which was computed to be the rate-determining step. Further deuterium labeling experiments and density functional theory (DFT) calculations demonstrated that this reaction underwent a plausible outer-sphere mechanism in this new catalytic transformation.

Development of a General and Selective Nanostructured Cobalt Catalyst for the Hydrogenation of Benzofurans, Indoles and Benzothiophenes

The selective hydrogenation of benzofurans in the presence of a heterogeneous non-noble metal catalyst is reported. The developed optimal catalytic material consists of cobalt-cobalt oxide core-shell nanoparticles supported on silica, which has been prepared by the immobilization and pyrolysis of cobalt-DABCO-citric acid complex on silica under argon at 800 °C. This novel catalyst allows for the selective hydrogenation of simple and functionalized benzofurans to 2,3-dihydrobenzofurans as well as related heterocycles. The versatility of the reported protocol is showcased by the reduction of selected drugs and deuteration of heterocycles. Further, the stability, recycling, and reusability of the Co-nanocatalyst are demonstrated.

Chiral oxalamide phosphine (COAP)-Pd-catalyzed enantioselective cascade formal [4 + 1] annulation for enantioenriched 2,3-disubstituted indolines and further DFT study on regio- and stereocontrol

COAP-Pd-catalyzed asymmetric cascade formal [4 + 1] annulation was developed between racemic vinyl benzoxazinones and N-tosylhydrazone sodium salts, affording trans-2,3-disubstituted indolines in good yields with high stereoselectivity.

Palladium-Catalyzed Asymmetric Hydrogenation of Unprotected 3-Substituted Indoles

A palladium-catalyzed asymmetric hydrogenation of unprotected 3-substituted indoles was developed, providing a series of 3-substituted indolines in excellent yields with ≤94.4:5.6 er. The large sterically hindered bisphosphine ligand played a crucial role in the enantioselective control. In addition, the gram-scale hydrogenation experiment and product derivatizations were performed successfully.

Manganese-Catalyzed Asymmetric Hydrogenation of 3H-Indoles

The asymmetric hydrogenation (AH) of 3H-indoles represents an ideal approach to the synthesis of useful chiral indoline scaffolds. However, very few catalytic systems based on precious metals have been developed to realize this challenging reaction. Herein, we report a Mn-catalyzed AH of 3H-indoles with excellent yields and enantioselectivities. The kinetic resolution of racemic 3H-indoles by AH was also achieved with high s-factors to construct quaternary stereocenters. Many acid-sensitive functional groups, which cannot be tolerated when using a state-of-the-art ruthenium catalyst, were compatible with manganese catalysis. This new process expands the scope of this transformation and highlights the uniqueness of earth-abundant metal catalysis. The reaction could proceed with catalyst loadings at the parts per million (ppm) level with an exceptional turnover number of 72 350. This is the highest value yet reported for an earth-abundant metal-catalyzed AH reaction.

Activation Modes in Asymmetric Anion-Binding Catalysis

Over the past two decades, enantioselective anion-binding catalysis has emerged as a powerful strategy for the induction of chirality in organic transformations. The stereoselectivity is achieved in a range of different reactions by using non-covalent interactions between a chiral catalyst and an ionic substrate or intermediate, and subsequent formation of a chiral contact ion-pair upon anion-binding. This strategy offers vast possibilities in catalysis and the constant development of new reactions has led to various substrate activation approaches. This review provides an overview on the different activation modes in asymmetric anion-binding catalysis by looking at representative examples and recent advances made in this field.

Stereoselective intermolecular radical cascade reactions of tryptophans or ɤ-alkenyl-α-amino acids with acrylamides via photoredox catalysis

The radical cascade reaction is considered as one of the most powerful methods to build molecular complexity. However, highly stereoselective intermolecular radical cascade reactions that can produce complex cyclic compounds bearing multiple stereocenters via visible-light-induced photocatalysis have been challenging yet desirable. Herein we report a facile and efficient synthesis of multi-substituted trans-fused hexahydrocarbazoles via a stereoselective intermolecular radical cascade reaction of readily available tryptophans and acrylamides enabled by visible-light-induced photoredox catalysis. The trans-fused hexahydrocarbazoles with up to five stereocenters including two quaternary ones can be accessed in up to 82% yield, >20/1 diastereoselectivity, and 96% ee. Interestingly, the tetrahydrocarbazoles are favorably formed when the reaction is performed under air. Moreover, by simply switching the starting material from tryptophans to ɤ-alkenyl substituted α-amino acids, this protocol can be further applied to the stereoselective syntheses of 1,3,5-trisubstituted cyclohexanes which are otherwise challenging to access. Preliminary mechanistic studies suggest that the reaction goes through radical addition cascade and radical-polar crossover processes.

Photocatalytic radical cascade reactions enable the facile construction of diverse cyclic compounds, though they rely on templated precursors. In this paper, the authors report on stereoselective intermolecular radical cascade reaction between tryptophan or ɤ-alkenyl substituted amino acids and acrylamides to synthesise multi-substituted trans-fused hexahydrocarbazoles or 1,3,5-trisubstituted cyclohexanes.

Enantioselective synthesis of cis-hexahydro-γ-carboline derivatives via Ir-catalyzed asymmetric hydrogenation

A novel synthetic route was developed for the construction of a chiral cis-hexahydro-γ-carboline derivative through Ir/ZhaoPhos-catalyzed asymmetric hydrogenation of corresponding tetrahydro-γ-carboline with high yields (up to 99% yield), excellent diastereoselectivities (up to >99 : 1 dr) and enantioselectivities (up to 99% ee), and high substrate-to-catalyst ratios (up to 5000).

Iridium-Catalyzed Hydroiodination and Formal Hydroamination of Olefins with N-Iodo Reagents and Molecular Hydrogen: An Umpolung Strategy

We herein report a convenient method to convert olefins to organic iodides and amines using an Ir/ZhaoPhos catalyst, molecular hydrogen, and an electrophilic iodine(I) reagent. High yields and regioselectivities were obtained under mild conditions. In addition, basic workup with potassium carbonate leads to C-N products. Control experiments and DFT calculations tentatively excluded the pathway involving the in situ formation of HI. Instead, a catalytic cycle involving the hydrogenation of the haliranium ion intermediate was proposed.

Recent Advances in the Enantioselective Synthesis of Chiral Amines via Transition Metal-Catalyzed Asymmetric Hydrogenation

Chiral amines are key structural motifs present in a wide variety of natural products, drugs, and other biologically active compounds. During the past decade, significant advances have been made with respect to the enantioselective synthesis of chiral amines, many of them based on catalytic asymmetric hydrogenation (AH). The present review covers the use of AH in the synthesis of chiral amines bearing a stereogenic center either in the α, β, or γ position with respect to the nitrogen atom, reported from 2010 to 2020. Therefore, we provide an overview of the recent advances in the AH of imines, enamides, enamines, allyl amines, and N-heteroaromatic compounds.

Recent Developments in Enantio- and Diastereoselective Hydrogenation of N-Heteroaromatic Compounds

The enantioselective and diastereoselective hydrogenation of N-heteroaromatic compounds is an efficient strategy to access chirally enriched cyclic heterocycles, which often possess highly bio-active properties. This strategy, however, has only been...

Access to Polycyclic Thienoindolines via Formal [2+2+1] Cyclization of Alkynyl Indoles with S8 and K2S

The syntheses of polycyclic thienoindolines bearing a dihydrothiophene or tetrahydrothiophene subunit have not been reported, despite the fact that such compounds may have interesting medicinal properties. Herein, we report a protocol for accessing polycyclic dihydrothiophenes by means of formal [2+2+1] intramolecular dearomatizing cyclization of alkynyl indoles with K₂S and S₈ as the sources of sulfide. In addition, tetrahydrothienoindolines were stereoselectively synthesized via a one-pot, two-step protocol involving AgNO₃-catalyzed alkenyl dearomatization followed by two nucleophilic addition reactions involving K₂S.

A Bifunctional Copper Catalyst Enables Ester Reduction with H2: Expanding the Reactivity Space of Nucleophilic Copper Hydrides

Employing a bifunctional catalyst based on a copper(I)/NHC complex and a guanidine organocatalyst, catalytic ester reductions to alcohols with H₂ as terminal reducing agent are facilitated. The approach taken here enables the simultaneous activation of esters through hydrogen bonding and formation of nucleophilic copper(I) hydrides from H₂, resulting in a catalytic hydride transfer to esters. The reduction step is further facilitated by a proton shuttle mediated by the guanidinium subunit. This bifunctional approach to ester reductions for the first time shifts the reactivity of generally considered "soft" copper(I) hydrides to previously unreactive "hard" ester electrophiles and paves the way for a replacement of stoichiometric reducing agents by a catalyst and H₂.

Beyond hydrogen bonding: recent trends of outer sphere interactions in transition metal catalysis

The implementation of interactions beyond hydrogen bonding in the 2^nd coordination sphere of transition metal catalysts is rare. However, it has already shown great promise in last 5 years, providing new tools to control the activity and selectivity as here reviewed.

Iridium-Catalyzed Enantioselective Hydrogenation of Indole and Benzofuran Derivatives

Enantioselective hydrogenation of a broad spectrum of N-, O-, and S-containing aromatic benzoheterocycles or nonaromatic unsaturated heterocycles has been realized by using an Ir/SpinPHOX (SpinPHOX=spiro[4,4]-1,6-nonadiene-based phosphine-oxazoline) complex as the catalyst, affording an array of the corresponding chiral benzoheterocycles (30 examples) with excellent enantioselectivities (>99 % ee in most cases) and turnover numbers up to 500.

Asymmetric Hydrogenation of 2-Aryl-3-phthalimidopyridinium Salts: Synthesis of Piperidine Derivatives with Two Contiguous Stereocenters

Asymmetric hydrogenation of 2-aryl-3-phthalimidopyridinium salts catalyzed by the Ir/SegPhos catalytic system was described, leading to the corresponding chiral piperidine derivatives bearing two contiguous chiral centers, with high levels of enantioselectivities and diastereoselectivities. A gram-scale experiment has demonstrated the utility of this approach. The phthaloyl group could be easily removed and then smoothly converted to key intermediate (+)-CP-99994 as one of the neurokinin 1 receptor antagonists.

Recent Advances in Homogeneous Catalysts for the Asymmetric Hydrogenation of Heteroarenes

The asymmetric hydrogenation of heteroarenes has recently emerged as an effective strategy for the direct access to enantioenriched, saturated heterocycles. Although several homogeneous catalyst systems have been extensively developed for the hydrogenation of heteroarenes with high levels of chemo- and stereoselectivity, the development of mild conditions that allow for efficient and stereoselective hydrogenation of a broad range of substrates remains a challenge. This Perspective highlights recent advances in homogeneous catalysis of heteroarene hydrogenation as inspiration for the further development of asymmetric hydrogenation catalysts, and addresses underdeveloped areas and limitations of the current technology.

Ruthenium-Catalyzed Electrochemical Synthesis of Indolines through Dehydrogenative [3 + 2] Annulation with H2 Evolution

A dehydrogenative [3 + 2] annulation reaction of aniline derivatives and alkenes has been developed via the ruthenium-electron catalytic systems for the synthesis of versatile indolines. Electricity is used as a sustainable oxidant to regenerate the active Ru(II) catalyst and promote H₂ evolution. This protocol is ecofriendly and easy to handle as it uses a simple undivided cell in mild conditions without the employment of metal oxidants.

Recent Developments in Enantioselective Transition Metal Catalysis Featuring Attractive Noncovalent Interactions between Ligand and Substrate

Enantioselective transition metal catalysis is an area very much at the forefront of contemporary synthetic research. The development of processes that enable the efficient synthesis of enantiopure compounds is of unquestionable importance to chemists working within the many diverse fields of the central science. Traditional approaches to solving this challenge have typically relied on leveraging repulsive steric interactions between chiral ligands and substrates in order to raise the energy of one of the diastereomeric transition states over the other. By contrast, this Review examines an alternative tactic in which a set of attractive noncovalent interactions operating between transition metal ligands and substrates are used to control enantioselectivity. Examples where this creative approach has been successfully applied to render fundamental synthetic processes enantioselective are presented and discussed. In many of the cases examined, the ligand scaffold has been carefully designed to accommodate these attractive interactions, while in others, the importance of the critical interactions was only elucidated in subsequent computational and mechanistic studies. Through an exploration and discussion of recent reports encompassing a wide range of reaction classes, we hope to inspire synthetic chemists to continue to develop asymmetric transformations based on this powerful concept.

Noncovalent Interaction-Assisted Ferrocenyl Phosphine Ligands in Asymmetric Catalysis

Noncovalent interactions are ubiquitous in nature and are responsible for the precision control in enzyme catalysis via the cooperation of multiple active sites. Inspired by this principle, noncovalent interaction-assisted transition metal catalysis has emerged recently as a powerful tool and has attracted intense interest. However, it is still highly desirable to develop efficient and operationally convenient ligands along this line with new structural motifs. Based on the specific nature of hydrogen bonding and ion pairing interactions, we developed a series of noncovalent interaction-assisted chiral ferrocenyl phosphine ligands, including Zhaophos, Wudaphos, and miscellaneous SPO-Wudaphos. Due to the assistance of noncovalent interactions, this catalytic mode is capable of achieving transition metal catalyzed asymmetric hydrogenation and other transformations with remarkable improvement of reactivity and selectivity. In some specific challenging cases, this probably represents one of the most productive methods. Moreover, these ligands are easily prepared, air stable, and highly tunable, meeting the requirements of industrial application.In this Account, we give a concise review of recent advances in asymmetric catalysis. By means of hydrogen bonding interactions, Rh- and Ir-Zhaophos complexes exhibited excellent activities and enantioselectivities in asymmetric hydrogenation of a wide range of substrates: C═C bonds of substituted conjugate alkenes with neutral hydrogen bond acceptors, including nitro groups, carbonyl groups (ketones, esters, amides, maleinimides, and anhydrides), ethers, and sulfones; C═N bonds of substituted iminium salts with chloride as an anionic hydrogen bond acceptor, including N-H imines and cyclic imines; N-heteroaromatic compounds with HCl as an additive, including unprotected quinolines, isoquinolines, and indoles; carbocation of substituted oxocarbenium ions. By means of ion pairing interactions, Rh-Wudaphos complexes enabled the catalytic asymmetric hydrogenation of α-substituted unsaturated carboxylic acids, carboxy-directed α,α-disubstituted terminal olefins, and sodium α-arylethenylsulfonates. Rh-SPO-Wudaphos utilized both hydrogen bonding and ion pairing interactions in asymmetric hydrogenation of α-substituted unsaturated carboxylic acids and phosphonic acids. In addition, Zhaophos has achieved highly selective intramolecular reductive amination and inter- and intramolecular asymmetric decarboxylative allylation. Investigations into mechanism implied that noncovalent interactions were involved in the catalytic cycle and played a critical role for both high reactivity and selectivity. Notably, a rare ionic hydrogenation pathway has been proposed in some cases. Furthermore, these catalytic systems have been used in the gram-scale synthesis of natural products and pharmaceuticals.

Asymmetric Transfer Hydrogenation of N-Unprotected Indoles with Ammonia Borane

A metal-free asymmetric transfer hydrogenation of unprotected indoles was successfully realized using a catalyst derived from HB(C₆F₅)₂ and (S)-tert-butylsulfinamide with ammonia borane as a hydrogen source. A variety of indolines were achieved in 40-78% yields with up to 90% ee.

Multifunctional chiral aminophosphines for enantiodivergent catalysis in a palladium-catalyzed allylic alkylation reaction

Trifunctional MAP-based chiral phosphines were tested as new ligands in a Pd-catalyzed asymmetric allylic alkylation, demonstrating fast and enantiodivergent catalysis. The palladium complexes of representative ligands by X-ray analysis revealed a novel mode of P,N-coordination of the ligand to the palladium center, which may contribute to switching the sense of the asymmetric induction via combined steric and tunable H-bonding interactions between the metal complex and the substrates.

Site-selective nitrenoid insertions utilizing postfunctionalized bifunctional rhodium(ii) catalysts

We report a new strategy for the preparation of dirhodium(ii) complexes with the general formula Rh2(A)4 that allows the isolation of a dirhodium tetracarboxylate complex with a free amino group available for postfunctionalization. The postfunctionalization of this complex enables the incorporation of a variety of functional groups, including double and triple bonds as well as nucleophilic moieties, thus paving the way to new classes of polymeric as well as bifunctional catalysts, and polymetallic complexes. Furthermore, we demonstrate that a urea containing dirhodium(ii) complex enables site-selective nitrenoid insertions by remote hydrogen bonding control.

Highly efficient Ir-catalyzed asymmetric hydrogenation of benzoxazinones and derivatives with a Brønsted acid cocatalyst

The Ir-catalyzed highly efficient asymmetric hydrogenation of benzoxazinones and derivatives was successfully developed with N-methylated ZhaoPhos L5 as the ligand, affording various chiral dihydrobenzoxazinones and derivatives with excellent results.

The Ir-catalyzed highly efficient asymmetric hydrogenation of benzoxazinones and derivatives was successfully developed with N-methylated ZhaoPhos L5 as the ligand, which may display a new activation mode with a single anion-binding interaction among the substrate, cocatalyst Brønsted acid and ligand. This synthetic approach afforded a series of chiral dihydrobenzoxazinones and derivatives with excellent results (>99% conversion, 88–96% yields, 91–>99% ee, up to 40 500 TON). A key to success is the utilization of a strong Brønsted acid as the cocatalyst, such as hydrochloric acid, to form a possible single anion-binding interaction with the substrate and catalyst, which greatly contributed to the improvement of reactivity and enantioselectivity. Importantly, a creative and efficient synthetic route was developed to construct the important intermediate for the potential IgE/IgG receptor modulator through our catalytic methodology system.

Asymmetric hydrogenation of α,β-unsaturated sulfones by a rhodium/thiourea–bisphosphine complex

We herein report a method to synthesize chiral sulfone compounds.

Stereogenic cis-2-substituted-N-acetyl-3-hydroxy-indolines via ruthenium(ii)-catalyzed dynamic kinetic resolution-asymmetric transfer hydrogenation

Ruthenium(ii)-catalyzed dynamic kinetic resolution-asymmetric transfer hydrogenation of racemic 2-substituted-N-acetyl-3-oxoindolines to cis-2-substituted-N-acetyl-3-hydroxyindolines is reported. Using the homochiral {Ru[TfDPEN](p-cymene)} catalyst with S/C = 400 in a HCO2H/Et3N mixture, up to >99.9% ee and >99 : 1 dr are obtained with high yields (79-98%). This method provides the first example of preparing enantiomerically pure indolines through asymmetric transfer hydrogenation (ATH).

A one-pot process for the enantioselective synthesis of tetrahydroquinolines and tetrahydroisoquinolines via asymmetric reductive amination (ARA)

Asymmetric reductive amination for the synthesis of both chiral tetrahydroquinolines and tetrahydroisoquinolines has been realized with an Ir/ZhaoPhos catalytic system via a one-pot N-Boc deprotection/intramolecular asymmetric reductive amination (ARA) sequence.