Recent Advances in Homogeneous Catalysts for the Asymmetric Hydrogenation of Heteroarenes

Chiral bisphosphine Ph-BPE ligand: a rising star in asymmetric synthesis

Chiral 1,2-bis(2,5-diphenylphospholano)ethane (Ph-BPE) is a class of optimal organic bisphosphine ligands with C2-symmetry. Ph-BPE with its excellent catalytic performance in asymmetric synthesis has attracted much attention of chemists with increasing popularity and is growing into one of the most commonly used organophosphorus ligands, especially in asymmetric catalysis. Over two hundred examples have been reported since 2012. This review presents how Ph-BPE is utilized in asymmetric synthesis and how powerful it is as a chiral ligand or even a catalyst in a wide range of reactions including applications in the total synthesis of bioactive molecules.

Pd/C-Catalyzed Stereoselective Arene Hydrogenation of Benzocyclobutenes Enabled by π-Bond Localization

We developed here a Pd/C-catalyzed diastereoselective cis-hydrogenation of benzocyclobutene derivatives under mild conditions to deliver an array of bicyclo[4.2.0]octane scaffolds with up to five stereocenters. The π-bond localization enabled hydrogenation of the arene moiety to occur even at room temperature under 1 atm of a H2 atmosphere.

Nickel-catalyzed asymmetric hydrogenation for the preparation of α-substituted propionic acids

Transition metal-catalyzed asymmetric hydrogenation is one of the most efficient methods for the preparation of chiral α-substituted propionic acids. However, research on this method, employing cleaner earth-abundant metal catalysts, is still insufficient in both academic and industrial contexts. Herein, we report an efficient nickel-catalyzed asymmetric hydrogenation of α-substituted acrylic acids affording the corresponding chiral α-substituted propionic acids with up to 99.4% ee (enantiomeric excess) and 10,000 S/C (substrate/catalyst). In particular, this method can be used to obtain (R)-dihydroartemisinic acid with 99.8:0.2 dr (diastereomeric ratio) and 5000 S/C, which is an essential intermediate for the preparation of the antimalarial drug Artemisinin. The reaction mechanism has been investigated via experiments and DFT (Density Functional Theory) calculations, which indicate that the protonolysis of the C-Ni bond of the key intermediate via an intramolecular proton transfer from the carboxylic acid group of the substrate, is the rate-determining step.

Switchable and Chemoselective Arene Hydrogenation for Efficient Late Stage Applications

The incorporation of three-dimensional structures into drug molecules has demonstrated significant improvements in clinical success. Late-stage saturation of drug molecules provides a direct pathway for this transformation. However, achieving selective and controllable reduction of aromatic rings remains challenging, particularly when multiple aromatic rings coexist. Herein, we present the switchable and chemoselective hydrogenation of benzene and pyridine rings. The utility of the protocol has been comprehensively investigated in diversified substrates with the assistance of a fragment-screening technique. This approach provides convenient access to a diverse array of cyclohexane and piperidine compounds, prevalent in various bioactive molecules and drugs. Furthermore, it discloses promising avenues for applications in the late-stage switchable saturation of drugs, facilitating an increase in the fraction of sp3-carbons which holds the potential to enhance the medicinal properties of drugs.

Cyclometallated Imides as Templates for the H-Bond Directed Iridium-Catalyzed Asymmetric Hydrogenation of N-Methyl, N-Alkyl and N-Aryl Imines

A combined computational and experimental approach allowed us to develop overall the most selective catalyst for the direct hydrogenation of N-methyl, N-alkyl and N-aryl imines described to date. Iridium catalysts with a cyclometallated cyclic imide group provide selectivity of up to 99 % enantiomeric excess. Computational studies show that the selectivity results from the combined effect of H-bonding of the imide C=O with the substrate iminium ion and a stabilizing π-π interaction with the cyclometallated ligand. The cyclometallated ligand thus exhibits a unique mode of action, serving as a template for the H-bond directed approach of the substrate which results in enhanced selectivity. The catalyst (2) has been synthesized and isolated as a crystalline air-stable solid. X-ray analysis of 2 confirmed the structure of the catalyst and the correct position of the imide C=O groups to engage in an H-bond with the substrate. 19F NMR real-time monitoring showed the hydrogenation of N-methyl imines catalyzed by 2 is very fast, with a TOF of approx. 3500 h-1.

Iridium-catalyzed reductive β-alkylation of (iso)quinoline derivatives by an in situ enone-trapping strategy

By employing [IrCp*Cl2]2/Mg(OMe)2/(CH2O)n as an applicable catalyst system, we report a reductive β-alkylation of (iso)quinolinium salts with cost-effective and readily available β-chloro ketones, proceeding with good chemoselectivity, mild reaction conditions, and without the need for introduction of a substituent at position-3 of the quinolyl skeleton. Mechanistic investigations suggest that the reaction proceeds via a sequence of hydride transfer-initiated dearomatization of (iso)quinolinium salts, in situ enamine-trapping of enone and a second round of hydride transfer to the coupling adducts. The present work offers an important complement to the synthesis of functionalized (iso)tetrahydroquinolines.

Iridium-Catalyzed Asymmetric Hydrogenation of Heteroaromatics with Multiple N Atoms via Substrate Activation: An Entry to 4,5,6,7-Tetrahydropyrazolo[1,5-a]pyrimidine-3-carbonitrile Core of a Potent BTK Inhibitor

The chiral 4,5,6,7-tetrahydropyrazolo[1,5-a]pyrimidine is the key core skeleton of potent Bruton's tyrosine kinase (BTK) inhibitor Zanubrutinib, and the catalyst-controlled asymmetric hydrogenation of planar multinuclear pyrimidine heteroarenes with multiple N atoms could provide an efficient route toward its synthesis. Owing to the strong aromaticity and poisoning effect toward chiral transition metal catalyst, asymmetric hydrogenation of pyrazolo[1,5-a]pyrimidines with multiple nitrogen atoms is still a challenge for synthesizing the chiral 4,5,6,7-tetrahydropyrazolo[1,5-a]-pyrimidine. Herein, an efficient iridium-catalyzed asymmetric hydrogenation of pyrazolo[1,5-a]pyrimidines has been developed using substrate activation strategy, with up to 99% ee. The decagram scale synthesis further demonstrated the potential and promise of this procedure in the synthesis of Zanubrutinib. In addition, a mechanistic study indicated that the hydrogenation starts with 1,2-hydrogenation.

Rhodium-Catalyzed Asymmetric Hydrogenation and Transfer Hydrogenation of 1,3-Dipolar Nitrones

Owing to their distinctive 1,3-dipolar structure, the catalytic asymmetric hydrogenation of nitrones to hydroxylamines has been a formidable and longstanding challenge, characterized by intricate enantiocontrol and susceptibility to N-O bond cleavage. In this study, the asymmetric hydrogenation and transfer hydrogenation of nitrones were accomplished with a tethered TsDPEN-derived cyclopentadienyl rhodium(III) catalyst (TsDPEN: p-toluenesulfonyl-1,2-diphenylethylene-1,2-diamine), the reaction proceeds via a novel 7-membered cyclic transition state, producing chiral hydroxylamines with up to 99 % yield and >99 % ee. The practical viability of this methodology was underscored by gram-scale catalytic reactions and subsequent transformations. Furthermore, mechanistic investigations and DFT calculations were also conducted to elucidate the origin of enantioselectivity.

Spiro-Josiphos Ligands for the Ir-Catalyzed Asymmetric Synthesis of Chiral Amines under Hydrogenation Conditions

Transition metal-catalyzed asymmetric hydrogenation possesses unparalleled advantages to prepare chiral amines. Here we reported a novel ligand that combined Josiphos and a spirobiindane scaffold and simultaneously investigated its application in Ir-catalyzed asymmetric hydrogenation for the synthesis of chiral amines. Excellent catalytic activity (5000 TON), high enantioselectivity (up to 99% ee), and broad substrate scope (different C═N substrates) make it highly promising for both academic research and industrial applications.

Chemoselective Heterogeneous Hydrogenation of Sulfur Containing Quinolines under Mild Conditions

Sulfur, alongside oxygen and nitrogen, holds a prominent position as one of the key heteroatoms in nature and medicinal chemistry. Its significance stems from its ability to adopt different oxidation states, rendering it valuable as both a polarity handle and a hydrogen bond donor/acceptor. Nevertheless, the poisonous nature of its free electron pairs makes sulfur containing substrates inaccessible for many catalytic protocols. Strong and (at low temperatures) irreversible chemisorption to the catalyst's surface is in particular detrimental for heterogeneous catalysts, possessing only few catalytically active sites. Herein, we present a novel heterogeneous Ru-S catalyst that tolerates multiple sulfur functionalities, including thioethers, thiophenes, sulfoxides, sulfones, sulfonamides, and sulfoximines, in the hydrogenation of quinolines. The utility of the products was further demonstrated by subsequent diversifications of the sulfur functionalities.

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.

Unlocking the Asymmetric Hydrogenation of Tetrasubstituted Acyclic Enones

Asymmetric hydrogenation (AH) of tetrasubstituted olefins generating two stereocenters is still an open topic. There are only a few reports on the AH of tetrasubstituted olefins with conjugated functional groups, while this process can create useful intermediates for the subsequent elaboration of relevant end products. Most of the tetrasubstituted olefins successfully submitted to AH belong to a small number of functional classes; remarkably, the AH of tetrasubstituted acyclic enones still represents an unsolved challenge. Herein, we disclose a class of air-stable Ir-P,N catalysts, prepared in three steps from commercially available amino alcohols, that can hydrogenate, in minutes, a wide range of electronically and sterically diverse acyclic tetrasubstituted enones (including exocyclic ones) with high yields and high enantioselectivities. The factors responsible for the excellent selectivities were elucidated by combining deuterogenation experiments and theoretical calculations. The calculations indicated that the reduction follows an IrI /IrIII mechanism, in which enantioselectivity is controlled in the first migratory insertion of the hydride to the most electrophilic olefinic Cβ and the formation of the hydrogenated product via reductive elimination takes place prior to the coordination of dihydrogen and the subsequent oxidative addition. The potential of the new catalytic systems is demonstrated by the derivatization of hydrogenation products.

Transfer Hydrogenation of N- and O-Containing Heterocycles Including Pyridines with H3N-BH3 Under the Catalysis of the Homogeneous Ruthenium Precatalyst

In this study, we report a transfer hydrogenation protocol that utilizes borane-ammonia (H3N-BH3) as the hydrogen source and a commercially available RuCl3·xH2O precatalyst for the selective aromatic reduction of quinolines, quinoxalines, pyridines, pyrazines, indoles, benzofurans, and furan derivatives to form the corresponding alicyclic heterocycles in good to excellent isolated yields. Applications of this straightforward protocol include the efficient preparation of useful key pharmaceutical intermediates, such as donepezil and flumequine, including a biologically active compound.

Oxidative Dearomatization of Pyridines

Dearomatization of pyridines is a well-established synthetic approach to access piperidines. Although remarkably powerful, existing dearomatization processes have been limited to the hydrogenation or addition of carbon-based nucleophiles to activated pyridiniums. Here, we show that arenophile-mediated dearomatizations can be applied to pyridines to directly introduce heteroatom functionalities without prior substrate activation. The arenophile platform in combination with olefin oxidation chemistry provides access to dihydropyridine cis-diols and epoxides. These previously elusive compounds are now readily accessible and can be used for the downstream preparation of diversely functionalized piperidines.

Hydrogenation of functionalised pyridines with a rhodium oxide catalyst under mild conditions

Piperidines are one of the most widely used building blocks in the synthesis of pharmaceutical and agrochemical compounds. The hydrogenation of pyridines is a convenient method to synthesise such compounds as it only requires reactant, catalyst, and a hydrogen source. However, this reaction still remains difficult for the reduction of functionalised and multi-substituted pyridines. Here we report the use of a stable, commercially available rhodium compound, Rh2O3, for the reduction of various unprotected pyridines. The reaction only requires mild conditions, and the substrate scope is broad, making it practically useful.

Highly dispersed palladium nano-catalyst anchored on N-doped nanoporous carbon microspheres derived from chitosan for efficient and stable hydrogenation of quinoline

Under the background of green chemistry, the synthesis of N-heterocycles using efficient, stable and long-life catalysts has still faced great challenges. Herein, we used biomass resource chitosan to fabricate a nanoporous chitosan carbon microsphere (CCM), and successfully designed a stable and efficient Pd nano-catalyst (CCM/Pd). Various physicochemical characterizations provided convincible evidences that the palladium nanoparticles (NPs) were tightly and evenly dispersed on the CCM with a mean diameter of 2.28 nm based on the nanoporous structure and abundant functional N/O groups in CCM. Importantly, the graphitized constructure, the formed defects and larger surface area in CCM were able to promote the immobilization of Pd NPs and the electron transfer between Pd and CCM, thereby significantly improving the catalytic activity. The CCM/Pd catalyst was applied for hydrogenation of quinoline compounds, which showed excellent catalytic activity and durability, as well as good substrate applicability. The application of renewable biomass-based catalysts contributes to the progression of a green/sustainable society.

Colloidal ruthenium catalysts for selective quinaldine hydrogenation: Ligand and solvent effects

Colloidal Ru nanoparticles (NP) display interesting catalytic properties for the hydrogenation of (hetero)arenes as they proceed efficiently in mild reaction conditions. In this work, a series of Ru based materials was used in order to selectively hydrogenate quinaldine and assess the impact of the stabilizing agent on their catalytic performances. Ru nanoparticles stabilized with polyvinylpyrrolidone (PVP) and 1-adamantanecarboxylic acid (AdCOOH) allowed to obtain 5,6,7,8-tetrahydroquinaldine with a remarkable selectivity in mild reaction conditions by choosing the suitable solvent. The presence of a carboxylate ligand on the surface of the Ru NP led to an increase in the activity when compared to Ru/PVP catalyst. The stabilizing agent had also an impact on the selectivity, as carboxylate ligand modified catalysts promoted the selectivity towards 1,2,3,4-tetrahydroquinaldine, with bulky carboxylate displaying the highest ones.

Rhodium-Catalyzed Asymmetric Functionalization of Quinoxalinium Salts

We report a rhodium-catalyzed asymmetric addition of aryl and alkenyl boronic acids to quinoxalinium salts that generates dihydroquinoxalines with high enantioselectivity. Functionalization of the reaction products, dihydroquinoxaline, allows the preparation of tetrahydroquinoxalines with various substitution patterns.

Protocol for stereodivergent asymmetric hydrogenation of quinoxalines

Chiral 1,2,3,4-tetrahydroquinoxalines are ubiquitous in natural products and bioactive molecules. Herein, we disclose a protocol for stereodivergent asymmetric hydrogenation of disubstituted quinoxalines for the preparation of both cis- and trans-enantioenriched disubstituted tetrahydroquinoxalines (up to >20:1 d.r. and 99% ee). We describe steps for synthesis of ligands and substrate, setup of hydrogenation of disubstituted quinoxalines, and purification of products. Additionally, we provide detailed diagrams of the hydrogenation installation. For complete details on the use and execution of this protocol, please refer to Liu et al.1.

An Asymmetric Hydrogenation/N-Alkylation Sequence for a Step-Economical Route to Indolizidines and Quinolizidines

The direct catalytic asymmetric hydrogenation of pyridines for the synthesis of piperidines remains a challenge. Herein, we report a one-pot asymmetric hydrogenation of pyridines with subsequent N-alkylation using a traceless Brønsted acid activation strategy. Catalyzed by an iridium-BINAP complex, the substrates undergo ketone reduction, cyclization and pyridine hydrogenation in sequence to form indolizidines and quinolizidines. The absolute configuration of the stereocenter of the alcohol is retained and influences the formation of the second stereocenter. Experimental and theoretical mechanistic studies reveal that the chloride anion and certain noncovalent interactions govern the stereoselectivity of the cascade reaction throughout the catalytic process.

A robust Zintl cluster for the catalytic reduction of pyridines, imines and nitriles

Despite p-block clusters being known for over a century, their application as catalysts to mediate organic transformations is underexplored. Here, the boron functionalized [P7] cluster [(BBN)P7]2- ([1]2-; BBN = 9-borabicyclo[3.3.1]nonane) is applied in the dearomatized reduction of pyridines, as well as the hydroboration of imines and nitriles. These transformations afford amine products, which are important precursors to pharmaceuticals, agrochemicals, and polymers. Catalyst [1]2- has high stability in these reductions: recycling nine times in quinoline hydroboration led to virtually no loss in catalyst performance. The catalyst can also be recycled between two different organic transformations, again with no loss in catalyst competency. The mechanism for pyridine reduction was probed experimentally using variable time normalization analysis, and computationally using density functional theory. This work demonstrates that Zintl clusters can mediate the reduction of nitrogen containing substrates in a transition metal-free manner.

Highly enantioselective Rh-catalyzed asymmetric reductive dearomatization of multi-nitrogen polycyclic pyrazolo[1,5-a]pyrimidines

A highly enantioselective rhodium-catalyzed reductive dearomatization of 7-substituted pyrazolo[1,5-a]pyrimidines has been realized for the first time by two strategies to afford chiral 4,5,6,7-tetrahydropyrazolo[1,5-a]pyrimidines with excellent enantioselectivities of up to 98% ee. This method also provides an efficient approach for the synthesis of the powerful BTK inhibitor, zanubrutinib.

Copper-Catalyzed Chemoselective Asymmetric Hydrogenation of C=O Bonds of Exocyclic α,β-Unsaturated Pentanones

A highly chemoselective earth-abundant transition metal copper catalyzed asymmetric hydrogenation of C=O bonds of exocyclic α,β-unsaturated pentanones was realized using H2 . The desired products were obtained with up to 99 % yield and 96 % ee (enantiomeric excess) (99 % ee, after recrystallization). The corresponding chiral exocyclic allylic pentanol products can be converted into several bioactive molecules. The hydrogenation mechanism was investigated via deuterium-labelling experiments and control experiments, which indicate that the keto-enol isomerization rate of the substrate is faster than that of the hydrogenation and also show that the Cu-H complex can only catalyze chemoselectively the asymmetric reduction of the carbonyl group. Computational results indicate that the multiple attractive dispersion interactions (MADI effect) between the catalyst with bulky substituents and substrate play important roles which stabilize the transition states and reduce the generation of by-products.

Gold-Catalyzed Intramolecular Hydroarylation and Transfer Hydrogenation of N-Aryl Propargylamines to Construct Tetrahydroquinolines and 5,6-Dihydro-4H-pyrrolo[3,2,1-ij]quinolines

A novel protocol of gold-catalyzed N-aryl propargylamines to construct tetrahydroquinolines and 5,6-dihydro-4H-pyrrolo[3,2,1-ij]quinolines in moderate to good yields has been developed through the tandem reactions of intramolecular hydroarylation and transfer hydrogenation. The strategy has the advantages of easy access to raw materials, simple reaction conditions, good substrate compatibility, high efficiency, and excellent regioselectivity.

Metallated dihydropyridinates: prospects in hydride transfer and (electro)catalysis

Hydride transfer (HT) is a fundamental step in a wide range of reaction pathways, including those mediated by dihydropyridinates (DHP-s). Coordination of ions directly to the pyridine ring or functional groups stemming therefrom, provides a powerful approach for influencing the electronic structure and in turn HT chemistry. Much of the work in this area is inspired by the chemistry of bioinorganic systems including NADH. Coordination of metal ions to pyridines lowers the electron density in the pyridine ring and lowers the reduction potential: lower-energy reactions and enhanced selectivity are two outcomes from these modifications. Herein, we discuss approaches for the preparation of DHP-metal complexes and selected examples of their reactivity. We suggest further areas in which these metallated DHP-s could be developed and applied in synthesis and catalysis.

Translating Planar Heterocycles into Three-Dimensional Analogs by Photoinduced Hydrocarboxylation

The rapid preparation of complex three-dimensional (3D) heterocyclic scaffolds is a key challenge in modern medicinal chemistry. Despite the increased probability of clinical success for small molecule therapeutic candidates with increased 3D complexity, new drug targets remain dominated by flat molecules due to the abundance of coupling reactions available for their construction. In principle, heteroarene hydrofunctionalization reactions offer an opportunity to transform readily accessible planar molecules into more three-dimensionally complex analogs through the introduction of a single molecular vector. Unfortunately, dearomative hydrofunctionalization reactions remain limited. Herein, we report a new strategy to enable the dearomative hydrocarboxylation of indoles and related heterocycles. This reaction represents a rare example of a heteroarene hydrofunctionalization that meets the numerous requirements for broad implementation in drug discovery. The transformation is highly chemoselective, broad in scope, operationally simple, and readily amenable to high-throughput experimentation (HTE). Accordingly, this process will allow existing libraries of heteroaromatic compounds to be translated into diverse 3D analogs and enable exploration of new classes of medicinally relevant molecules.

Migraine drugs

According to recent studies, migraine affects more than 1 billion people worldwide, making it one of the world’s most prevalent diseases. Although this highly debilitating illness has been known since ancient times, the first therapeutic drugs to treat migraine, ergotamine (Gynergen) and dihydroergotamine (Dihydergot), did not appear on the market until 1921 and 1946, respectively. Both drugs originated from Sandoz, the world’s leading pharmaceutical company in ergot alkaloid research at the time. Historically, ergot alkaloids had been primarily used in obstetrics, but with methysergide (1-methyl-lysergic acid 1′-hydroxy-butyl-(2S)-amide), it became apparent that they also held some potential in migraine treatment. Methysergide was the first effective prophylactic drug developed specifically to prevent migraine attacks in 1959. On the basis of significantly improved knowledge of migraine pathophysiology and the discovery of serotonin and its receptors, Glaxo was able to launch sumatriptan in 1992. It was the first member from the class of triptans, which are selective 5-HT1B/1D receptor agonists. Recent innovations in acute and preventive migraine therapy include lasmiditan, a selective 5-HT1F receptor agonist from Eli Lilly, the gepants, which are calcitonin gene-related peptide (CGRP) receptor antagonists discovered at Merck & Co and BMS, and anti-CGRP/receptor monoclonal antibodies from Amgen, Pfizer, Eli Lilly, and others.

Graphical abstract

P-Stereogenic Ir-MaxPHOX: A Step toward Privileged Catalysts for Asymmetric Hydrogenation of Nonchelating Olefins

The Ir-MaxPHOX-type catalysts demonstrated high catalytic performance in the hydrogenation of a wide range of nonchelating olefins with different geometries, substitution patterns, and degrees of functionalization. These air-stable and readily available catalysts have been successfully applied in the asymmetric hydrogenation of di-, tri-, and tetrasubstituted olefins (ee's up to 99%). The combination of theoretical calculations and deuterium labeling experiments led to the uncovering of the factors responsible for the enantioselectivity observed in the reaction, allowing the rationalization of the most suitable substrates for these Ir-catalysts.

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.

cis-Selective Hydrogenation of Aryl Germanes: A Direct Approach to Access Saturated Carbo- and Heterocyclic Germanes

A catalytic approach of synthesizing the cis-selective saturated carbo- and heterocyclic germanium compounds (3D framework) is reported via the hydrogenation of readily accessible aromatic germanes (2D framework). Among the numerous catalysts tested, Nishimura's catalyst (Rh2O3/PtO2·H2O) exhibited the best hydrogenation reactivity with an isolated yield of up to 96%. A broad range of substrates including the synthesis of unprecedented saturated heterocyclic germanes was explored. This selective hydrogenation strategy could tolerate several functional groups such as -CF3, -OR, -F, -Bpin, and -SiR3 groups. The synthesized products demonstrated the applications in coupling reactions including the newly developed strategy of aza-Giese-type addition reaction (C-N bond formation) from the saturated cyclic germane product. These versatile motifs can have a substantial value in organic synthesis and medicinal chemistry as they show orthogonal reactivity in coupling reactions while competing with other coupling partners such as boranes or silanes, acquiring a three-dimensional structure with high stability and robustness.

Nickel-Catalyzed Asymmetric Hydrogenation of α-Substituted Vinylphosphonates and Diarylvinylphosphine Oxides

Chiral α-substituted ethylphosphonate and ethylphosphine oxide compounds are widely used in drugs, pesticides, and ligands. However, their catalytic asymmetric synthesis is still rare. Of the only asymmetric hydrogenation methods available at present, all cases use rare metal catalysts. Herein, we report an efficient earth-abundant transition-metal nickel catalyzed asymmetric hydrogenation affording the corresponding chiral ethylphosphine products with up to 99 % yield, 96 % ee (enantiomeric excess) (99 % ee, after recrystallization) and 1000 S/C (substrate/catalyst); this is also the first study on the asymmetric hydrogenation of terminal olefins using a nickel catalyst under a hydrogen atmosphere. The catalytic mechanism was investigated via deuterium-labelling experiments and calculations which indicate that the two added hydrogen atoms of the products come from hydrogen gas. Additionally, it is believed that the reaction involves a Ni^II rather than Ni⁰ cyclic process based on the weak attractive interactions between the Ni catalyst and terminal olefin substrate.

Visible Light-Mediated, Diastereoselective Epimerization of Morpholines and Piperazines to More Stable Isomers

We report a photocatalyzed epimerization of morpholines and piperazines that proceeds by reversible hydrogen atom transfer (HAT) and provides an efficient strategy for editing the stereochemical configurations of these saturated nitrogen heterocycles, which are prevalent in drugs. The more stable morpholine and piperazine isomers are obtained from the more synthetically accessible but less stable stereoisomers, and a broad scope is demonstrated in terms of substitution patterns and functional group compatibility. The observed distributions of diastereomers correlate well with the relative energies of the diastereomer pairs as determined by density functional theory (DFT) calculations. Mechanistic studies, including luminescence quenching, deuterium labeling reactions, and determination of reversibility support a thiyl radical mediated HAT pathway for the epimerization of morpholines. Investigation of piperazine epimerization established that the mechanism is more complex and led to the development of thiol free conditions for the highly stereoselective epimerization of N,N'-dialkyl piperazines for which a previously unrecognized radical chain HAT mechanism is proposed.

Synthesis of Saturated N-Heterocycles via a Catalytic Hydrogenation Cascade

Saturated N-heterocycles are prominent motifs found in various natural products and pharmaceuticals. Despite the increasing interest in this class of compounds, the synthesis of saturated bicyclic azacycles requires tedious multi-step syntheses. Herein, we present a one-pot protocol for the synthesis of octahydroindoles, decahydroquinolines, and octahydroindolizines through a cascade reaction.

Ru/Pd Complex and Its Monometallic Fragments as Catalysts for Norbornene Polymerization via ROMP and Addition

The [Ru(PPh3)2Cl-piperidine(4-aminomethyl)] complex (mono-Ru) was synthesized from [Ru(PPh3)3Cl2] and 4-(aminomethyl)piperidine, whereas the [(PPh3)PdCl(Shiff-pip)] complex (mono-Pd) was obtained by reacting [Pd(PPh3)2Cl2] with its respective Schiff base ligand, both at a 1:1 molar ratio. The heterobimetallic [RuCl2(PPh3)2](μ-Schiff)Pd(PPh3)Cl] complex (Ru/Pd) was synthesized via a one-pot, three-component reaction of mono-Ru, [(Pd(PPh3)2Cl2] and salicylaldehyde. All complexes were fully characterized by FTIR, UV-Vis, and NMR spectroscopy, as well as elemental analysis, MALDI-TOF mass spectrometry, cyclic voltammetry, and computational studies. Ru/Pd was able to polymerize norbornene (NBE) by two different mechanisms: ROMP and vinyl polymerization. The Ru fragment was active for ROMP of NBE, reaching yields of 68 and 31% for mono-Ru and Ru/Pd, respectively, when the [NBE]/[Ru] = 3000 molar ratio and 5 μL EDA addition were employed at 50 °C. The poly(norbornene) (polyNBE) obtained presented an order of magnitude of 104 g mol−1 and Ð values between 1.48 and 1.79. For the vinyl polymerization of NBE, the Pd fragment was active using MAO as a cocatalyst, reaching a yield of 47.0% for Ru/Pd and quantitative yields for mono-Pd when [Al]/[Pd] = 2500 and [NBE]/[Pd] = 20,000 molar ratios were employed, both at 60 °C.

Access to Unexplored 3D Chemical Space: cis ‐Selective Arene Hydrogenation for the Synthesis of Saturated Cyclic Boronic Acids

A new class of saturated boron‐incorporated cyclic molecules has been synthesized employing an arene‐hydrogenation methodology. cis‐Selective hydrogenation of easily accessible, and biologically important molecules comprising benzoxaborole, benzoxaborinin, and benzoxaboripin derivatives is reported. Among the various catalysts tested, rhodium cyclic(alkyl)(amino)carbene [Rh‐CAAC] (1) pre‐catalyst revealed the best hydrogenation activity confirming turnover number up to 1400 with good to high diastereoselectivity. A broad range of functional groups was tolerated including sensitive substituents such as −F, −CF₃, and −silyl groups. The utility of the synthesized products was demonstrated by the recognition of diols and sugars under physiological conditions. These motifs can have a substantial importance in medicinal chemistry as they possess a three‐dimensional structure, are highly stable, soluble in water, form hydrogen bonds, and interact with diols and sugars.

Azole-Directed Cobalt-Catalyzed Asymmetric Hydrogenation of Alkenes

The azole-directed cobalt-catalyzed asymmetric hydrogenation of alkenes has been developed with high efficiency. With this approach, chiral pyrazole compounds were obtained in quantitative yields and excellent enantioselectivities (up to 99 % ee) under mild conditions, and the hydrogenation was conducted on a gram scale with up to 2000 TON. Several useful applications were demonstrated including the convenient introduction of β-chirality to a drug intermediate containing an azole ring.

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.

Iridium‐Catalyzed Asymmetric Hydrogenation of 2,3‐Diarylallyl Amines with a Threonine‐Derived P‐Stereogenic Ligand for the Synthesis of Tetrahydroquinolines and Tetrahydroisoquinolines

Chiral compounds containing nitrogen heteroatoms are fundamental substances for the chemical, pharmaceutical and agrochemical industries. However, the preparation of some of these interesting scaffolds is still underdeveloped. Herein we present the synthesis of a family of P‐stereogenic phosphinooxazoline iridium catalysts from L‐threonine methyl ester and their use in the asymmetric hydrogenation of N‐Boc‐2,3‐diarylallyl amines, achieving very high enantioselectivity. Furthermore, the synthetic utility of the 2,3‐diarylpropyl amines obtained is demonstrated by their transformation to 3‐aryl‐tetrahydroquinolines and 4‐benzyl‐tetrahydroisoquinolines, which have not yet been obtained in an enantioselective manner by direct reduction of the corresponding aromatic heterocycles. This strategy allows the preparation of these types of alkaloids with the highest enantioselectivity reported up to date.

Molybdenum-Catalyzed Asymmetric Hydrogenation of Fused Arenes and Heteroarenes

The synthesis of enantioenriched molybdenum precatalysts for the asymmetric hydrogenation of substituted quinolines and naphthalenes is described. Three classes of pincer ligands with chiral substituents were evaluated as supporting ligands in the molybdenum-catalyzed hydrogenation reactions, where oxazoline imino(pyridine) chelates were identified as optimal. A series of 2,6-disubstituted quinolines was hydrogenated to enantioenriched decahydroquinolines with high diastereo- and enantioselectivities. For quinoline derivatives, selective hydrogenation of both the carbocycle and heterocycle was observed depending on the ring substitution. Spectroscopic and mechanistic studies established molybdenum η⁶-arene complexes as the catalyst resting state and that partial hydrogenation arises from dissociation of the substrate from the coordination sphere of molybdenum prior to complete reduction. A stereochemical model is proposed based on the relative energies of the respective coordination of the prochiral faces of the arene determined by steric interactions between the substrate and the chiral ligand, rather than through precoordination by a heteroatom.