Nickel-catalysed asymmetric hydrogenation of oximes

Electricity-driven organic hydrogenation using water as the hydrogen source

Hydrogenation is a pivotal process in organic synthesis and various catalytic strategies have been developed in achieving effective hydrogenation of diverse substrates. Despite the competence of these methods, the predominant reliance on molecular hydrogen (H2) gas under high temperature and elevated pressure presents operational challenges. Other alternative hydrogen sources such as inorganic hydrides and organic acids are often prohibitively expensive, limiting their practical utility on a large scale. In contrast, employing water as a hydrogen source for organic hydrogenation presents an attractive and sustainable alternative, promising to overcome the drawbacks associated with traditional hydrogen sources. Integrated with electricity as the sole driving force under ambient conditions, hydrogenation using water as the sole hydrogen source aligns well with the environmental sustainability goals but also offers a safer and potentially more cost-effective solution. This article starts with the discussion on the inherent advantages and limitations of conventional hydrogen sources compared to water in hydrogenation reactions, followed by the introduction of representative electrocatalytic systems that successfully utilize water as the hydrogen source in realizing a large number of organic hydrogenation transformations, with a focus on heterogeneous electrocatalysts. In summary, transitioning to water as a hydrogen source in organic hydrogenation represents a promising direction for sustainable chemistry. In particular, by exploring and optimizing electrocatalytic hydrogenation systems, the chemical industry can reduce its reliance on hazardous and expensive hydrogen sources, paving the way for safer, greener, and less energy-intensive hydrogenation processes.

Catalytic Reduction of Aromatic Nitro Compounds to Phenylhydroxylamine and Its Derivatives

Phenylhydroxylamine and its derivates (PHAs) are important chemical intermediates. Phenylhydroxylamines are mainly produced via the catalytic reduction of aromatic nitro compounds. However, this catalytic reduction method prefers to generate thermodynamically stable aromatic amine. Thus, designing suitable catalytic systems, especially catalysts to selectively convert aromatic nitro compounds to PHAs, has received increasing attention but remains challenging. In this review, we initially provide a brief overview of the various strategies employed for the synthesis of PHAs, focusing on reducing aromatic nitro compounds. Subsequently, an in-depth analysis is presented on the catalytic reduction process, encompassing discussions on catalysts, reductants, hydrogen sources, and a comprehensive assessment of the merits and drawbacks of various catalytic systems. Furthermore, a concise overview is provided regarding the progress made in comprehending the mechanisms involved in this process of catalytic reduction of aromatic nitro compounds. Finally, the main challenges and prospects in PHAs' production via catalytic reduction are outlined.

Ru-Catalyzed Asymmetric Hydrogenation of α,β-Unsaturated γ-Lactams

A highly efficient Ru-catalyzed asymmetric hydrogenation of α,β-unsaturated γ-lactams has been developed by using a C2-symmetric ruthenocenyl phosphine-oxazoline as the chiral ligand. This method achieves the enantioselective synthesis of chiral β-substituted γ-lactams in high yields and with excellent enantioselectivities (up to 99% yield with 99% ee). Mechanistic studies based on detailed control experiments and computational investigation revealed that the cationic Ru-complex acts as the active catalytic species; the protonation process of the oxa-π-allyl-Ru complex, which is formed by the migratory insertion of the C=C double bond to the Ru-H bond (the stereocontrolling step) followed by an isomerization process, is the rate-determining step, and the existence of PPh3 is crucial for the highly efficient catalytic behavior. The protocol provides a straightforward and practical pathway for the synthesis of key intermediates for several chiral drugs and bioactive compounds, particularly for the 150 kg-scale industrial production of Brivaracetam, an antiepileptic drug that shows 13-fold more potent binding to the synaptic vesicle protein 2A compared with the well-known Levetiracetam.

Iridium-Catalyzed Asymmetric Hydrogenation of Unfunctionalized Cycloalkenes to Access Chiral 2-Aryl Tetralins

The transition-metal catalyzed asymmetric hydrogenation of unfunctionalized alkenes is challenging. Herein, we report an efficient iridium-catalyzed asymmetric hydrogenation of unfunctionalized cycloalkenes, delivering chiral 2-aryl tetralins in excellent yields and with moderate to excellent enantioselectivities. The reaction can be performed on a gram-scale with a low catalyst loading (S/C = 1000), and the reduced product was obtained without erosion of the enantioselectivity. Deuterium experiments indicated that the C═C bond in the substrate is hydrogenated directly without isomerization.

Asymmetric hydrogenation of ketimines with minimally different alkyl groups

Asymmetric catalysis enables the synthesis of optically active compounds, often requiring the differentiation between two substituents on prochiral substrates1. Despite decades of development of mainly noble metal catalysts, achieving differentiation between substituents with similar steric and electronic properties remains a notable challenge2,3. Here we introduce a class of Earth-abundant manganese catalysts for the asymmetric hydrogenation of dialkyl ketimines to give a range of chiral amine products. These catalysts distinguish between pairs of minimally differentiated alkyl groups bound to the ketimine, such as methyl and ethyl, and even subtler distinctions, such as ethyl and n-propyl. The degree of enantioselectivity can be adjusted by modifying the components of the chiral manganese catalyst. This reaction demonstrates a wide substrate scope and achieves a turnover number of up to 107,800. Our mechanistic studies indicate that exceptional stereoselectivity arises from the modular assembly of confined chiral catalysts and cooperative non-covalent interactions between the catalyst and the substrate.

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.

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.

Catalytic Asymmetric Access to Structurally Diverse N-Alkoxy Amines via a Kinetic Resolution Strategy

Chiral N-alkoxy amines are increasingly vital substrates in bioscience. However, asymmetric synthetic strategies for these compounds remain scarce. Catalytic kinetic resolution represents an attractive approach to prepare structurally diverse enantiopure N-alkoxy amines, which has remained elusive due to the notably reduced nucleophilicity of the nitrogen atom together with the low bond dissociation energies of labile NO-C and N-O bonds. We here report a general kinetic resolution of N-alkoxy amines through chemo- and enantioselective oxygenation. The mild and green titanium-catalyzed approach features broad substrate scope (55 examples), noteworthy functional group compatibility, high catalyst turnover number (up to 5200), excellent selectivity factor (s > 150), and scalability.

Synthesis of Hydroxylamine via Ketone-Mediated Nitrate Electroreduction

Hydroxylamine (HA, NH2OH) is a critical feedstock in the production of various chemicals and materials, and its efficient and sustainable synthesis is of great importance. Electroreduction of nitrate on Cu-based catalysts has emerged as a promising approach for green ammonia (NH3) production, but the electrosynthesis of HA remains challenging due to overreduction of HA to NH3. Herein, we report the first work on ketone-mediated HA synthesis using nitrate in water. A metal-organic-framework-derived Cu catalyst was developed to catalyze the reaction. Cyclopentanone (CP) was used to capture HA in situ to form CP oxime (CP-O) with C═N bonds, which is prone to hydrolysis. HA could be released easily after electrolysis, and CP was regenerated. It was demonstrated that CP-O could be formed with an excellent Faradaic efficiency of 47.8%, a corresponding formation rate of 34.9 mg h-1 cm-2, and a remarkable carbon selectivity of >99.9%. The hydrolysis of CP-O to release HA and CP regeneration was also optimized, resulting in 96.1 mmol L-1 of HA stabilized in the solution, which was significantly higher than direct nitrate reduction. Detailed in situ characterizations, control experiments, and theoretical calculations revealed the catalyst surface reconstruction and reaction mechanism, which showed that the coexistence of Cu0 and Cu+ facilitated the protonation and reduction of *NO2 and *NH2OH desorption, leading to the enhancement for HA production.

Synergistic Pd/Cu-Catalyzed 1,5-Double Chiral Inductions

Much attention has been focused on the catalytic asymmetric creation of single chiral centers or two adjacent stereocenters. However, the asymmetric construction of two nonadjacent stereocenters is of significant importance but is challenging because of the lack of remote chiral induction models. Herein, based on a C═C bond relay strategy, we report a synergistic Pd/Cu-catalyzed 1,5-double chiral induction model. All four stereoisomers of the target products bearing 1,5-nonadjacent stereocenters involving both allenyl axial and central chirality could be obtained divergently by simply changing the combination of two chiral catalysts with different configurations. Control experiments and DFT calculations reveal a novel mechanism involving 1,5-oxidative addition, contra-thermodynamic η3-allyl palladium shift, and conjugate nucleophilic substitution, which play crucial roles in the control of reactivity, regio-, enantio-, and diastereoselectivity. It is expected that this C═C bond relay strategy may provide a general protocol for the asymmetric synthesis of structural motifs bearing two distant stereocenters.

Descriptor-Based Volcano Relations Predict Single Atoms for Hydroxylamine Electrosynthesis

Hydroxylamine (NH2OH) is an important feedstock in fuels, pharmaceuticals, and agrochemicals. Nanostructured electrocatalysts drive green electrosynthesis of hydroxylamine from nitrogen oxide species in water. However, current electrocatalysts still suffer from low selectivity and manpower-consuming trial-and-error modes, leaving unclear selectivity/activity origins and a lack of catalyst design principles. Herein, we theoretically analyze key determinants of selectivity/activity and propose the adsorption energy of NHO (Gad(*NHO)) as a performance descriptor. A weak *NH2OH binding affinity and a favorable reaction pathway (*NHO pathway) jointly enable single-atom catalysts (SACs) with superior NH2OH selectivity. Then, an activity volcano plot of Gad(*NHO) is established to predict a series of SACs and discover Mn SACs as optimal electrocatalysts that exhibit pH-dependent activity. These theoretical prediction results are also confirmed by experimental results, rationalizing our Gad(*NHO) descriptor. Furthermore, Mn-Co geminal-atom catalysts (GACs) are predicted to optimize Gad(*NHO) and are experimentally proved to enhance NH2OH formation.

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.

Challenging Task of Ni-Catalyzed Highly Regio-/Enantioselective Semihydrogenation of Racemic Tetrasubstituted Allenes via a Kinetic Resolution Process

The first earth-abundant transition metal Ni-catalyzed highly regio- and enantioselective semihydrogenation of racemic tetrasubstituted allenes via a kinetic resolution process as a challenging task was well established. This protocol furnishes expedient access to a diversity of structurally important enantioenriched tetrasubstituted allenes and chiral allylic molecules with high regio-, enantio-, and Z/E-selectivity. Remarkably, this semihydrogenation proceeded with one carbon-carbon double bond of allenes, which was regioselective complementary to the Rh-catalyzed asymmetric version. Deuterium labeling experiments and density functional theory (DFT) calculations were carried out to reveal the reasonable reaction mechanism and explain the regio-/stereoselectivity.

Catalytic Enantioselective Alkyne Addition to Nitrones Enabled by Tunable Axially Chiral Imidazole-Based P,N-Ligands

Although catalytic enantioselective alkyne addition is an established method for the synthesis of chiral propargylic alcohols and amines, addition to nitrones presents unique challenges, and no general chiral catalyst system has been developed. In this manuscript, we report the first Cu-catalyzed enantioselective alkyne addition to nitrones utilizing tunable axially chiral imidazole-based P,N-ligands. Our approach effectively overcomes difficulties in both reactivity and selectivity, resulting in a simple Cu-catalyzed protocol. The reaction accommodates a wide range of nitrones and alkynes, enabling the streamlined synthesis of chiral propargyl N-hydroxylamines via the enantioselective C-C bond formation. A diverse array of optically active nitrogen-containing compounds, including chiral hydroxylamines, can be accessed directly through facile transformations of the reaction products.

Promoting Catalytic Performance Involving Hydrogen Spillover by Ion Exchange of Pt@A Catalysts to Regulate Reactant Adsorption

Zeolite-encapsulated metal nanoparticle systems have exhibited interesting catalytic performances via the hydrogen spillover process, yet how to further utilize the function of zeolite supports to promote catalytic properties in such a process is still challenging and has rarely been investigated. Herein, to address this issue, the strategy to strengthen the adsorption energy of reactant onto the zeolite surface via a simple ion exchange method has been implemented. Ion-exchanged linde type A (LTA) zeolite-encapsulated platinum nanoclusters (Pt@NaA, Pt@HA, Pt@KA, and Pt@CaA) were prepared to study the influence of ion exchange on the catalytic performance in the model reaction of hydrogenation of acetophenone to 1-phenylethanol. The reaction results showed that the Pt@CaA catalyst exhibited the best catalytic activity in the series of encapsulated catalysts, and the selectivity of 1-phenylethanol approached 100%. As revealed by density functional theory (DFT) calculations and acetophenone temperature-programmed desorption (acetophenone-TPD) experiments, in comparison with introduced cations of Na+, H+, and K+, ion-exchanged Ca2+ on the zeolite maximumly enhanced the adsorption of carbonyl groups in acetophenone, playing a critical role in achieving the highest activity and excellent catalytic selectivity among the Pt@A catalysts.

Catalyst Selection over an Electrochemical Reductive Coupling Reaction toward Direct Electrosynthesis of Oxime from NOx and Aldehyde

Aqueous electrochemical coupling reactions, which enable the green synthesis of complex organic compounds, will be a crucial tool in synthetic chemistry. However, a lack of informed approaches for screening suitable catalysts is a major obstacle to its development. Here, we propose a pioneering electrochemical reductive coupling reaction toward direct electrosynthesis of oxime from NOx and aldehyde. Through integrating experimental and theoretical methods, we screen out the optimal catalyst, i.e., metal Fe catalyst, that facilitates the enrichment and C-N coupling of key reaction intermediates, all leading to high yields (e.g., ∼99% yield of benzaldoxime) for the direct electrosynthesis of oxime over Fe. With a divided flow reactor, we achieve a high benzaldoxime production of 22.8 g h-1 gcat-1 in ∼94% isolated yield. This work not only paves the way to the industrial mass production of oxime via electrosynthesis but also offers references for the catalyst selection of other electrochemical coupling reactions.

Cobalt-Catalyzed Enantioselective Reductive Amination of Ketones with Hydrazides

An efficient cobalt-catalyzed asymmetric reductive amination of ketones with hydrazides has been realized, directly producing valuable chiral hydrazines in high yields and enantioselectivities (up to 98% enantiomeric excess).

Hydrophobic Trinuclear Copper Cluster-Containing Organic Framework for Synergetic Electrocatalytic Synthesis of Amino Acids

Electrocatalytic synthesis of amino acids provides a promising green and efficient pathway to manufacture the basic substances of life. Herein, reaction of 2,5-perfluroalkyl-terepthalohydrazide and tris(4-µ2 -O-carboxaldehyde-pyrazolato-N, N')-tricopper affords a crystalline trinuclear copper cluster-containing organic framework, named F-Cu3 -OF. Incorporation of abundant hydrophobic perfluroalkyl groups inside the channels of F-Cu3 -OF is revealed to successfully suppress the hydrogen evolution reaction via preventing H+ cation with large polarity from the framework of F-Cu3 -OF and in turn increasing the adsorption of other substrates with relatively small polarity like NO3 - and keto acids on the active sites. The copper atoms with short distance in the trinuclear copper clusters of F-Cu3 -OF enable simultaneous activization of NO3 - and keto acids, facilitating the following synergistic and efficient C─N coupling on the basis of in situ spectroscopic investigations together with theoretical calculation. Combination of these effects leads to efficient electroproduction of various amino acids including glycine, alanine, leucine, valine, and phenylalanine from NO3 - and keto acids with a Faraday efficiency of 42%-71% and a yield of 187-957 µmol cm-2 h-1 , representing the thus far best performance. This work shall be helpful for developing economical, eco-friendly, and high-efficiency strategy for the production of amino acids and other life substances.

The Proton of Alcohols as Hydrogen Source in Diboron-Mediated Nickel-Catalyzed Asymmetric Transfer Hydrogenation of Cyclic N-Sulfonyl Imines

The proton of alcohols as the sole hydrogen source in diboron-mediated nickel-catalyzed asymmetric transfer hydrogenation of cyclic N-sulfonyl imines has been developed, providing the chiral cyclic sulfamidates in excellent enantioselectivities. The mechanistic investigations suggested that the proton of alcohols could be activated by tetrahydroxydiboron to form active nickel hydride species.

Iridium-Catalyzed 1,3-Rearrangement of Allylic Ethers

The 1,3-rearrangement of allylic derivatives has rarely been reported, except for allylic alcohols. Herein, we describe an iridium-catalyzed 1,3-rearrangement of readily available allylic ethers to access the difficultly prepared allylic ethers with a large steric hindrance. The developed method shows a broad substrate scope and could be used in the late-stage modification of several natural products. In addition, a possible reaction pathway is also provided on the basis of the control experiments.

Cobalt-Catalyzed Efficient Asymmetric Hydrogenation of α-Primary Amino Ketones

Based on an amino-group-assisted coordination strategy and a proton-shuttle-activated outer-sphere mode, the cobalt-catalyzed asymmetric hydrogenation of α-primary amino ketones has been developed, resulting in the efficient synthesis of chiral vicinal amino alcohols bearing functionalized aryl rings in high yields and enantioselectivities (up to 99% enantiomeric excess (ee)) within 0.5 h.

High-Grade Ferronickel Concentrates Prepared from Laterite Nickel Ore by a Carbothermal Reduction and Magnetic Separation Method

Nickel is widely used in industrial processes and plays a crucial role in many applications. However, most of the nickel resource mainly exists as nickel oxide in laterite nickel ore with complex composition, resulting in difficulty in upgrading the nickel content using physical separation methods. In this study, high-grade ferronickel concentrates were obtained through a carbothermal reduction and magnetic separation using laterite nickel ore and anthracite as raw materials. The effects of different types of additives (CaF2, Na2SO4, and H3BO3), carbon ratio (the molar ratio of oxygen atoms in the laterite nickel ore to carbon atoms in anthracite), and grinding time on the recoveries and grades of ferronickel concentrates were experimentally investigated, along with the microstructure and chemical composition of the products. CaF2 was proved to be the primary active additive in the aggregation and growth of the ferronickel particles and the improvement of the grade of the product. Under the optimal conditions of CaF2 addition of 9.85 wt%, carbon ratio of 1.4, and grinding time of 240 s, high-grade magnetically separable ferronickel concentrate with nickel grade 8.93 wt% and iron grade 63.96 wt% was successfully prepared. This work presents a practical method for the highly efficient recovery and utilization of iron and nickel from low-grade laterite nickel ore, contributing to the development of strategies for the sustainable extraction and utilization of nickel resources.

Catalytic Asymmetric Oxidation of Amines to Hydroxylamines

Chiral hydroxylamines are increasingly common structural elements in pharmaceuticals and agrochemicals, but their asymmetric synthesis remains challenging. Although enantioselective oxidation is the most straightforward method to prepare chiral oxides with a higher oxidation state, asymmetric and even nonasymmetric amine oxidation to hydroxylamines has been poorly addressed. We report a titanium-catalyzed asymmetric oxidation of racemic amines providing a broad range of structurally diverse chiral hydroxylamines with excellent chemo- and enantioselectivity. Notably, hydroxylamines bearing diverse substituent patterns on the stereocenters, including α,α-ester-alkyl, α,α-amide-alkyl, α,α-aryl-alkyl, α,α-alkynyl-alkyl, and α,α-dialkyl, are well tolerated with good functional group compatibility. Catalyst turnover numbers up to 5000 and selectivity factors up to 278 are observed. This finding offers a democratized platform to chiral hydroxylamines as design elements for drug discovery and provides insights into metal-catalyzed asymmetric oxidation of challenging substrates.

Iridium-Catalyzed Remote Site-Switchable Hydroarylation of Alkenes Controlled by Ligands

An iridium-catalyzed remote site-switchable hydroarylation of alkenes was reported, delivering the products functionalized at the subterminal methylene and terminal methyl positions on an alkyl chain controlled by two different ligands, respectively, in good yields and with good to excellent site-selectivities. The catalytic system showed good functional group tolerance and a broad substrate scope, including unactivated and activated alkenes. More importantly, the regioconvergent transformations of mixtures of isomeric alkenes were also successfully realized. The results of the mechanistic studies demonstrate that the reaction undergoes a chain-walking process to give an [Ar-Ir-H] complex of terminal alkene. The subsequent processes proceed through the modified Chalk-Harrod-type mechanism via the migratory insertion of terminal alkene into the Ir-C bond followed by C-H reductive elimination to afford the hydrofunctionalization products site-selectively.

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.

Water compatible silica supported iron trifluoroacetate and trichloroacetate: as prominent and recyclable Lewis acid catalysts for solvent-free green synthesis of hexahydroquinoline-3-carboxamides

Silica supported iron trifluoroacetate and iron trichloroacetate green Lewis acid catalysts were developed by a novel, cheap, environment-friendly approach and utilized in the synthesis of hexahydroquinoline-3-carboxamide derivatives. The structure and morphology of the prepared Lewis acid catalysts were studied by FTIR, PXRD, FE-SEM, HR-TEM, EDX, BET, TGA and NH3-TPD techniques. The present catalysts shows maximum conversion efficiency in hexahydroquinoline-3-carboxamide derivatives synthesis at 70 °C in solvent free reaction condition with best product yield in a short reaction time. Both catalysts are reusable and simple to recover, and perform meritoriously in water as well as in a variety of organic solvents. The key advantages of the current synthetic route are permitting of a variety of functional groups, quick reaction time, high product yield, mild reaction condition, recyclability of catalyst and solvent-free green synthesis. This makes it more convenient, economic and environmentally beneficial.

A 13-million turnover-number anionic Ir-catalyst for a selective industrial route to chiral nicotine

Developing catalysts with both useful enantioselectivities and million turnover numbers (TONs) for asymmetric hydrogenation of ketones is attractive for industrial production of high-value bioactive chiral entities but remains a challenging. Herein, we report an ultra-efficient anionic Ir-catalyst integrated with the concept of multidentate ligation for asymmetric hydrogenation of ketones. Biocatalysis-like efficacy of up to 99% ee (enantiomeric excess), 13,425,000 TON (turnover number) and 224 s^-1 TOF (turnover frequency) were documented for benchmark acetophenone. Up to 1,000,000 TON and 99% ee were achieved for challenging pyridyl alkyl ketone where at most 10,000 TONs are previously reported. The anionic Ir-catalyst showed a novel preferred ONa/MH instead of NNa/MH bifunctional mechanism. A selective industrial route to enantiopure nicotine has been established using this anionic Ir-catalyst for the key asymmetric hydrogenation step at 500 kg batch scale, providing 40 tons scale of product.

Enantioselective Synthesis of Chiral Cyclic Hydrazines by Ni-Catalyzed Asymmetric Hydrogenation

An efficient Ni-(S,S)-Ph-BPE complex that catalyzed asymmetric hydrogenation of cyclic N-acyl hydrazones has been developed to produce various chiral cyclic hydrazines in high yields with excellent enantioselectivities of up to >99% enantiomeric excess. Moreover, the hydrogenation can not only proceed smoothly on a gram scale under lower catalyst loading (S/C = 3000) without any decrease of enantioselectivity but can also be applied to the asymmetric synthesis of a RIP-1 kinase inhibitor.

Recent Advances in the Synthesis of Di- and Trisubstituted Hydroxylamines

As an underrepresented functional group in bioorganic and medicinal chemistry, the hydroxylamine unit has historically received little attention from the synthetic community. Recent developments, however, suggest that hydroxylamines may have broader applications such that a review covering recent developments in the synthesis of this functional group is timely. With this in mind, this review primarily covers developments in the past 15 years in the preparation of di- and trisubstituted hydroxylamines. The mechanism of the reactions and key features and shortcomings are discussed throughout the review.

Co-Catalyzed Asymmetric Hydrogenation. The Same Enantioselection Pattern for Different Mechanisms

The mechanism of the recently reported catalyzed asymmetric hydrogenation of enyne 1 catalyzed by the Co-(R,R)-QuinoxP* complex was studied by DFT. Conceivable pathways for the Co(I)-Co(III) mechanism were computed together with a Co(0)-Co(II) catalytic cycle. It is commonly assumed that the exact nature of the chemical transformations taking place along the actually operating catalytic pathway determine the sense and level of enantioselection of the catalytic reaction. In this work, two chemically different mechanisms reproduced the experimentally observed perfect stereoselection of the same handedness. Moreover, the relative stabilities of the transition states of the stereo induction stages were controlled via exactly the same weak disperse interactions between the catalyst and the substrate.

Catalytic Asymmetric Hydroalkoxylation and Formal Hydration and Hydroaminoxylation of Conjugated Dienes

The straightforward construction of stereogenic centers bearing unprotected functional groups, as in nature, has been a persistent pursuit in synthetic chemistry. Abundant applications of free enantioenriched allyl alcohol and allyl hydroxylamine motifs have made the asymmetric hydration and hydroaminoxylation of conjugated dienes from water and hydroxylamine, respectively, intriguing and efficient routes that have, however, been unachievable thus far. A fundamental challenge is the failure to realize transition-metal-catalyzed enantioselective C-O bond constructions via hydrofunctionalization of conjugated dienes. Here, we perform a comprehensive study toward the stereoselective formal hydration and hydroaminoxylation of conjugated dienes by synthesizing a set of new P,N-ligands and identifying an aryl-derived oxime as a surrogate for both water and hydroxylamine. Asymmetric hydroalkoxylation with new P,N-ligands is also elucidated. Furthermore, versatile derivatizations following hydration provide indirect but concise routes to formal hydrophenoxylation, hydrofluoroalkoxylation, and hydrocarboxylation of conjugated dienes that have been unreported thus far. Finally, a ligand-to-ligand hydrogen transfer process is proposed based on the results of preliminary mechanistic experiments.

Ni-Catalyzed Asymmetric Hydrogenation of α-Substituted α,β-Unsaturated Phosphine Oxides/Phosphonates/Phosphoric Acids

Efficient Ni/(S,S)-Ph-BPE-catalyzed asymmetric hydrogenation of α-substituted α,β-unsaturated phosphine oxides/phosphonates/phosphoric acids has been successfully developed, and a wide range of chiral α-substituted phosphines hydrogenation products were obtained in generally high yields with excellent enantioselective control (92%-99% yields, 84%->99% ee). This method features a cheap transition metal nickel catalytic system, high functional group tolerance, wide substrate scope generality, and excellent enantioselectivity. A plausible catalytic cycle was proposed for this asymmetric hydrogenation according to the results of deuterium-labeling experiments.

Precise Synthesis of Chiral Z-Allylamides by Cobalt-Catalyzed Asymmetric Sequential Hydrogenations

Asymmetric sequential hydrogenations of conjugated enynes have been developed using a Ph-BPE-Co^I catalyst for the precise synthesis of chiral Z-allylamides in high activity (up to 1000 substrate/catalyst (S/C)) and with excellent enantioselectivity (up to >99 % enantiomeric excess (ee)). Mechanism experiments and theoretical calculations support a cationic Co^I /Co^III redox catalytic cycle. The catalytic activity difference between cobalt complexes of Ph-BPE and QuinoxP* was explained by the process decomposition of rate-determining step in the second hydrogenation.

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.

The Origin of Stereoselectivity in the Hydrogenation of Oximes Catalyzed by Iridium Complexes: A DFT Mechanistic Study

Herein the reaction mechanism and the origin of stereoselectivity of asymmetric hydrogenation of oximes to hydroxylamines catalyzed by the cyclometalated iridium (III) complexes with chiral substituted single cyclopentadienyl ligands (Ir catalysts A1 and B1) under acidic condition were unveiled using DFT calculations. The catalytic cycle for this reaction consists of the dihydrogen activation step and the hydride transfer step. The calculated results indicate that the hydride transfer step is the chirality-determining step and the involvement of methanesulfonate anion (MsO^-) in this reaction is of importance in the asymmetric hydrogenation of oximes catalyzed by A1 and B1. The calculated energy barriers for the hydride transfer steps without an MsO^- anion are higher than those with an MsO^- anion. The differences in Gibbs free energies between TSA5-1fR/TSA5-1fS and TSB5-1fR/TSB5-1fS are 13.8/13.2 (ΔΔG^‡ = 0.6 kcal/mol) and 7.5/5.6 (ΔΔG^‡ = 1.9 kcal/mol) kcal/mol for the hydride transfer step of substrate protonated oximes with E configuration (E-2a-H⁺) with MsO^- anion to chiral hydroxylamines product R-3a/S-3a catalyzed by A1 and B1, respectively. According to the Curtin-Hammet principle, the major products are hydroxylamines S-3a for the reaction catalyzed by A1 and B1, which agrees well with the experimental results. This is due to the non-covalent interactions among the protonated substrate, MsO^- anion and catalytic species. The hydrogen bond could not only stabilize the catalytic species, but also change the preference of stereoselectivity of this reaction.

Iridium-Catalyzed Double Asymmetric Hydrogenation of 2,5-Dialkylienecyclopentanones for the Synthesis of Chiral Cyclopentanones

Herein, we report an efficient iridium-catalyzed double asymmetric hydrogenation of 2,5-dialkylienecyclopentanones, delivering the chiral 2,5-disubstituted cyclopentanones in excellent yields and stereoselectivities. The results of the kinetic experiments and control experiments indicated that the two C═C bonds were hydrogenated in a stepwise manner and the second stereocenter was synergistically controlled by the chiral catalyst and the chirality of monohydrogenated product. The hydrogenated products can be prepared on a gram-scale and are easily derivatized.

Rh-Catalyzed Sequential Asymmetric Hydrogenations of 3-Amino-4-Chromones Via an Unusual Dynamic Kinetic Resolution Process

Rh-catalyzed sequential asymmetric hydrogenations of 3-amino-4-chromones have been achieved for the first time via an unprecedented dynamic kinetic resolution under neutral conditions, providing (S,R)-3-amino-4-chromanols in high yields (up to 98%) with excellent enantio- and diastereoselectivities (up to 99.9% ee and 20:1 dr). The mechanistic studies based on control experiments and density functional theory (DFT) calculations suggest that the dynamic kinetic resolution process for the intermediate enantiomers generated in the first hydrogenation step proceeded via a stereomutation (or called chiral assimilation) pathway from an undesired enantiomer to the desired enantiomer rather than via traditional racemization of the undesired enantiomer. The protocol can be performed on a gram scale with a relatively low catalyst loading and offers a practical and convenient pathway for synthesizing a series of bioactive chromanols and their derivatives.

Asymmetric Hydrogenation of Oximes Synergistically Assisted by Lewis and Brønsted Acids

Due to their low reactivity, difficult enantiocontrol, and proneness to N-O bond cleavage, the catalytic asymmetric hydrogenation of oximes to hydroxylamines has remained a significant challenge. Herein, a Lewis and Brønsted acid cooperation strategy was established for the asymmetric hydrogenation of oximes, providing the corresponding hydroxylamines with up to 95% yield and up to 96% ee. Addition of Lewis and Brønsted acid was crucial to obtain high conversion and enantioselectivity. Mechanistic investigations indicates that the thiourea fragment of the ligand, Lewis acid (In(OTf)₃ or Zn(OAc)₂), as well as the Brønsted acid (l-CSA) played vital roles in the control of reactivity and enantioselectivity of the reaction. In addition, the synthetic elaboration of this transformation was demonstrated by gram scale experiment with retention of the yield and enantioselectivity.

Synthesis of N-alkoxy amines and hydroxylamines via the iridium-catalyzed transfer hydrogenation of oximes

Cationic iridium (Ir) complexes were found to catalyze the transfer hydrogenation of oximes to access N-alkoxy amines and hydroxylamines, and the reaction was accelerated by trifluoroacetic acid. The practical application of this protocol was demonstrated by a gram-scale transformation and two-step synthesis of the fungicide furmecyclox (BAS 389F) in overall yields of 92 and 85%, respectively. An asymmetric protocol using chiral Ir complexes to afford chiral N-alkoxy amines was demonstrated, but the low yields/ee obtained indicated that further development was required.