RuPHOX-Ru Catalyzed Asymmetric Cascade Hydrogenation of 3-Substituted Chromones for the Synthesis of Corresponding Chiral Chromanols

An efficient RuPHOX-Ru catalyzed asymmetric cascade hydrogenation of 3-substituted chromones has been achieved under mild reaction conditions, affording the corresponding chiral 3-substituted chromanols in high yields with excellent enantio- and diastereoselectivities (up to 99 % yield, >99 % ee and >20 : 1 dr). Control reactions and deuterium labelling experiments revealed that a dynamic kinetic resolution process occurs during the subsequent hydrogenation of the C=O double bond, which is responsible for the high performance of the asymmetric cascade hydrogenation. The resulting products allow for several transformations and it was shown that the protocol provides a practical and alternative strategy for the synthesis of chiral 3-substituted chromanols and their derivatives.

P-Stereogenic Phosphorus Ligands in Asymmetric Catalysis

Chiral phosphorus ligands play a crucial role in asymmetric catalysis for the efficient synthesis of useful optically active compounds. They are largely categorized into two classes: backbone chirality ligands and P-stereogenic phosphorus ligands. Most of the reported ligands belong to the former class. Privileged ones such as BINAP and DuPhos are frequently employed in a wide range of catalytic asymmetric transformations. In contrast, the latter class of P-stereogenic phosphorus ligands has remained a small family for many years mainly because of their synthetic difficulty. The late 1990s saw the emergence of novel P-stereogenic phosphorus ligands with their superior enantioinduction ability in Rh-catalyzed asymmetric hydrogenation reactions. Since then, numerous P-stereogenic phosphorus ligands have been synthesized and used in catalytic asymmetric reactions. This Review summarizes P-stereogenic phosphorus ligands reported thus far, including their stereochemical and electronic properties that afford high to excellent enantioselectivities. Examples of reactions that use this class of ligands are described together with their applications in the construction of key intermediates for the synthesis of optically active natural products and therapeutic agents. The literature covered dates back to 1968 up until December 2023, centering on studies published in the late 1990s and later years.

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.

Fe/Thiol Cooperative Hydrogen Atom Transfer Olefin Hydrogenation: Mechanistic Insights That Inform Enantioselective Catalysis

Asymmetric hydrogenation of activated olefins using transition metal catalysis is a powerful tool for the synthesis of complex molecules, but traditional metal catalysts have difficulty with enantioselective reduction of electron-neutral, electron-rich, and minimally functionalized olefins. Hydrogenation based on radical, metal-catalyzed hydrogen atom transfer (mHAT) mechanisms offers an outstanding opportunity to overcome these difficulties, enabling the mild reduction of these challenging olefins with selectivity that is complementary to traditional hydrogenations with H2. Further, mHAT presents an opportunity for asymmetric induction through cooperative hydrogen atom transfer (cHAT) using chiral thiols. Here, we report insights from a mechanistic study of an iron-catalyzed achiral cHAT reaction and leverage these insights to deliver stereocontrol from chiral thiols. Kinetic analysis and variation of silane structure point to the transfer of hydride from silane to iron as the likely rate-limiting step. The data indicate that the selectivity-determining step is quenching of the alkyl radical by thiol, which becomes a more potent H atom donor when coordinated to iron(II). The resulting iron(III)-thiolate complex is in equilibrium with other iron species, including FeII(acac)2, which is shown to be the predominant off-cycle species. The enantiodetermining nature of the thiol trapping step enables enantioselective net hydrogenation of olefins through cHAT using a commercially available glucose-derived thiol catalyst with up to 80:20 enantiomeric ratio. To the best of our knowledge, this is the first demonstration of asymmetric hydrogenation via iron-catalyzed mHAT. These findings advance our understanding of cooperative radical catalysis and act as a proof of principle for the development of enantioselective iron-catalyzed mHAT reactions.

Bimetallic Ru/Ru-Catalyzed Asymmetric One-Pot Sequential Hydrogenations for the Stereodivergent Synthesis of Chiral Lactones

Asymmetric sequential hydrogenations of α-methylene γ- or δ-keto carboxylic acids are established in one-pot using a bimetallic Ru/Ru catalyst system, achieving the stereodivergent synthesis of all four stereoisomers of both chiral γ- and δ-lactones with two non-vicinal carbon stereocenters in high yields (up to 99%) and with excellent stereoselectivities (up to >99% ee and >20:1 dr). The compatibility of the two chiral Ru catalyst systems is investigated in detail, and it is found that the basicity of the reaction system plays a key role in the sequential hydrogenation processes. The protocol can be performed on a gram-scale with a low catalyst loading (up to 11000 S/C) and the resulting products allow for many transformations, particularly for the synthesis of several key intermediates useful for the preparation of chiral drugs and natural products.

X-type silyl ligands for transition-metal catalysis

Given the critical importance of novel ligand development for transition-metal (TM) catalysis, as well as the resurgence of the field of organosilicon chemistry and silyl ligands, to summarize the topic of X-type silyl ligands for TM catalysis is highly attractive and timely. This review particularly emphasizes the unique σ-donating characteristics and trans-effects of silyl ligands, highlighting their crucial roles in enhancing the reactivity and selectivity of various catalytic reactions, including small molecule activation, Kumada cross-coupling, hydrofunctionalization, C-H functionalization, and dehydrogenative Si-O coupling reactions. Additionally, future developments in this field are also provided, which would inspire new insights and applications in catalytic synthetic chemistry.

Mechanochemical Synthesis of Chromium(III) Complexes Containing Bidentate PN and Tridentate P-NH-P and P-NH-P' Ligands

Chromium(III) complexes bearing bidentate {NH2(CH2)2PPh2: PN, (S,S)-[NH2(CHPh)2PPh2]: P'N} and tridentate [Ph2P(CH2)2N(H)(CH2)2PPh2: P-NH-P, (S,S)-(iPr)2PCH2CH2N(H)CH(Ph)CH(Ph)PPh2: P-NH-P'] ligands have been synthesized using a mechanochemical approach. The complexes {cis-[Cr(PN)Cl2]Cl (1), cis-[Cr(P'N)Cl2]Cl (2), mer-Cr(P-NH-P)Cl3 (3), and mer-Cr(P-NH-P')Cl3 (4)} were obtained in high yield (95-97%) via the grinding of the respective ligands andthe solid Cr(III) ion precursor [CrCl3(THF)3] with the aid of a pestle and mortar, followed by recrystallization in acetonitrile. The isolated complexes are high spin. A single-crystal X-ray diffraction study of 2 revealed a cationic chromium complex with two P'N ligands in a cis configuration with P' trans to P' with chloride as the counteranion. The X-ray study of 4 shows a neutral Cr(III) complex with the P-NH-P' ligand in a mer configuration. The difference in molecular structures and bulkiness of the ligands influence the electronic, magnetic, and electrochemical properties of the complexes as exhibited by the bathochromic shifts in the electronic absorption peaks of the complexes and the relative increase in the magnetic moment of 3 (4.19 μβ) and 4 (4.15 μβ) above the spin only value (3.88 μβ) for a d3 electronic configuration. Complexes 1-4 were found to be inactive in the hydrogenation of an aldimine [(E)-1-(4-fluorophenyl)-N-phenylmethanimine] under a variety of activating conditions. The addition of magnesium and trimethylsilyl chloride in THF did cause hydrogenation at room temperature, but this occurred even in the absence of the chromium complex. The hydrogen in the amine product came from the THF solvent in this novel reaction, as determined by deuterium incorporation into the product when deuterated THF was used.

Synthesis, characterisation, and catalytic application of a soluble molecular carrier of sodium hydride activated by a substituted 4-(dimethylamino)pyridine

Recently main group compounds have stepped into the territory of precious transition metal compounds with respect to utility in the homogeneous catalysis of fundamentally important organic transformations. Inspired by the need to promote more sustainability in chemistry because of their greater abundance in nature, this change of direction is surprising since main group metals generally do not possess the same breadth of reactivity as precious transition metals. Here, we introduce the dihydropyridylsodium compound, Na-1,2-tBu-DH(DMAP), and its monomeric variant [Na-1,2-tBu-DH(DMAP)]·Me6TREN, and demonstrate their effectiveness in transfer hydrogenation catalysis of the representative alkene 1,1-diphenylethylene to the alkane 1,1-diphenylethane using 1,4-cyclohexadiene as hydrogen source [DMAP = 4-dimethylaminopyridine; Me6TREN = tris(N,N-dimethyl-2-aminoethyl)amine]. Sodium is appealing because of its high abundance in the earth's crust and oceans, but organosodium compounds have been rarely used in homogeneous catalysis. The success of the dihydropyridylsodium compounds can be attributed to their high solubility and reactivity in organic solvents.

Significantly enhanced catalytic performance of Pd nanocatalyst on AlOOH featuring abundant solid surface frustrated Lewis pair for improved hydrogen activation

The catalytic performance of a catalyst is significantly influenced by its ability to activate hydrogen. Constructing frustrated Lewis pairs (FLPs) with the capacity for hydrogen dissociation on non-reducible supports remains a formidable challenge. Herein, we employed a straightforward method to synthesize a layered AlOOH featuring abundant OH defects suitable for constructing solid surface frustrated Lewis pair (ssFLP). The results indicated that the AlOOH-80 (synthesized at 80 °C) possessed an appropriate crystalline structure conducive to generating numerous OH defects, which facilitated the formation of ssFLP. This was further evidenced by the minimal water adsorption in the AlOOH-80, inversely correlated with the quantity of defects in the catalyst. As expected, the Pd loaded onto AlOOH (Pd/AlOOH-80) exhibited excellent catalytic activity in hydrogenation reactions, attributed to abundant defects available for constructing ssFLP. Remarkably, the Pd/AlOOH-80 catalyst, with larger-sized Pd nanoparticles, displayed notably superior activity compared to commercial Pd/Al2O3 and Pd/C, both featuring smaller-sized Pd nanoparticles. Evidently, under the influence of ssFLP, the size effect of Pd nanoparticles did not dominate, highlighting the pivotal role of ssFLP in enhancing catalytic performance. This catalyst also exhibited exceptionally high stability, indicating its potential for industrial applications.

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.

Iridium-Catalyzed Enantioselective Transfer Hydrogenation of 1,1-Dialkylethenes with Ethanol: Scope and Mechanism

Despite half a century's advance in the field of transition-metal-catalyzed asymmetric alkene hydrogenation, the enantioselective hydrogenation of purely alkyl-substituted 1,1-dialkylethenes has remained an unmet challenge. Herein, we describe a chiral PCNOx-pincer iridium complex for asymmetric transfer hydrogenation of this alkene class with ethanol, furnishing all-alkyl-substituted tertiary stereocenters. High levels of enantioselectivity can be achieved in the reactions of substrates with secondary/primary and primary/primary alkyl combinations. The catalyst is further applied to the redox isomerization of disubstituted alkenols, producing a tertiary stereocenter remote to the resulting carbonyl group. Mechanistic studies reveal a dihydride species, (PCNOx)Ir(H)2, as the catalytically active intermediate, which can decay to a dimeric species (κ3-PCNOx)IrH(μ-H)2IrH(κ2-PCNOx) via a ligand-remetalation pathway. The catalyst deactivation under the hydrogenation conditions with H2 is much faster than that under the transfer hydrogenation conditions with EtOH, which explains why the (PCNOx)Ir catalyst is effective for the transfer hydrogenation but ineffective for the hydrogenation. The suppression of di-to-trisubstituted alkene isomerization by regioselective 1,2-insertion is partly responsible for the success of this system, underscoring the critical role played by the pincer ligand in enantioselective transfer hydrogenation of 1,1-dialkylethenes. Moreover, computational studies elucidate the significant influence of the London dispersion interaction between the ligand and the substrate on enantioselectivity control, as illustrated by the complete reversal of stereochemistry through cyclohexyl-to-cyclopropyl group substitution in the alkene substrates.

Zeolite-encapsulated copper(II) complexes with NNO-tridentate Schiff base ligands: catalytic activity for methylene blue (MB) degradation under near neutral conditions

Three novel copper Schiff base complexes, L1Cu(OAc)-L3Cu(OAc), bearing NNO tridentate ligands were synthesized and successfully entrapped in zeolite. All free and encapsulated complexes were fully characterized through experiments combined with theoretical calculations, and were subsequently employed as catalysts to activate H2O2 for degradation of methylene blue (MB). The catalytic activity of free complexes was tunable by substitution effects. The complex L3Cu(OAc) displayed enhanced efficiency by adopting bulky and donor substitutions due to the lower oxidation states. However, the free complexes exhibited modified structural and catalytic properties upon encapsulation into the zeolite. The constraint from the zeolite holes and coordination geometry caused the alteration of electronic structures and subsequently modified the reactivity. This study revealed that upon encapsulation, the larger molecular dimension of L3Cu(OAc) resulted in additional distorted geometry, leading to higher catalytic efficiency for MB degradation with more blue shifts in the UV-Vis spectrum. There was high catalytic activity by LnCu(OAc)-Y compared to that of the free complex, and high recyclability under near neutral conditions. In addition, the catalytic efficiency of L3Cu(OAc)-Y was higher or equivalent compared to other catalysts. This work provides new complexes with NNO tridentate ligands encapsulated inside zeolite and explains the relationship between the modified structure and functionality.

Metal-Free Heterogeneous Asymmetric Hydrogenation of Olefins Promoted by Chiral Frustrated Lewis Pair Framework

The development of metal-free and recyclable catalysts for significant yet challenging transformations of naturally abundant feedstocks has long been sought after. In this work, we contribute a general strategy of combining the rationally designed crystalline covalent organic framework (COF) with a newly developed chiral frustrated Lewis pair (CFLP) to afford chiral frustrated Lewis pair framework (CFLPF), which can efficiently promote the asymmetric olefin hydrogenation in a heterogeneous manner, outperforming the homogeneous CFLP counterpart. Notably, the metal-free CFLPF exhibits superior activity/enantioselectivity in addition to excellent stability/recyclability. A series of in situ spectroscopic studies, kinetic isotope effect measurements, and density-functional theory computational calculations were also performed to gain an insightful understanding of the superior asymmetric hydrogenation catalysis performances of CFLPF. Our work not only increases the versatility of catalysts for asymmetric catalysis but also broadens the reactivity of porous organic materials with the addition of frustrated Lewis pair (FLP) chemistry, thereby suggesting a new approach for practical and substantial transformations through the advancement of novel catalysts from both concept and design perspectives.

PPM Ir-f-phamidol-Catalyzed Asymmetric Hydrogenation of γ-Amino Ketones Followed by Stereoselective Cyclization for Construction of Chiral 2-Aryl-pyrrolidine Pharmacophores

Transition metal catalysts with a million turnovers and excellent selectivity are rarely reported but are crucial for the industrial manufacture of optical pure pharmaceuticals, natural products, and fine chemicals. In this paper, we report an unprecedented aninoic Ir-f-phamidol catalyst for asymmetric hydrogenation of γ-amino ketones followed by stereoselective cyclization for construction of valuable chiral 2-aryl-pyrrolidine pharmacophores. The Ir-f-phamidol catalyst showed up to 1,000,000 TON and >99% ee, as well as excellent tolerance of substrates and protecting groups, providing various chiral amino alcohol intermediates. Upon optimization of the conditions, the stereoselective cyclization reaction was highly smooth and efficient (quantitative conversions, 92 to >99% ee). Finally, this solution was applied in the preparation of high-value chiral entities containing such chiral 2-aryl-pyrrolidine pharmacophores.

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.

Non-Noble Metal Incorporated Transition Metal Dichalcogenide Monolayers for Electrochemical CO2 Reduction: A First-Principles Study

Using non-noble metal atoms as catalysts is attractive for decreasing the cost of the CO2 reduction reaction (CO2RR). By screening first-row transition metals and noble metals through extensive first-principles calculations, non-noble Sc and Ti single atoms binding on vacancy-defected transition metal dichalcogenide (TMD) monolayers exhibit better catalytic performance and selectivity for electrochemical CO2RR than noble metal single atoms. The overpotentials of Sc and Ti atoms for the CO2RR can be reduced lower than 0.09 V after applying suitable biaxial tensile strains on vacancy-defected TMDs, which are approximately 1 order of magnitude lower than that of most reported metal atom catalysts. The vacancy defects of TMDs and charge transfer to metal atoms induced by tensile strain play a key role in improving the catalytic activity of non-noble metal single atoms. These results highlight a possible way to design new single atom catalysts for electrochemical CO2RR by utilizing the combination of non-noble metal atoms, defected TMDs, and strain engineering.

Iodine-promoted transfer of dihydrogen from ketones to alkenes, triphenylmethyl, and diphenylmethyl derivatives

Herein, a novel class of transfer hydrogenation agent, cycloheptanone, was successfully employed in metal-free hydrogenation facilitated by iodine. A series of alkenes, triphenylmethyl derivatives, and diphenylmethyl derivatives were reduced to the desired compounds in moderate to excellent yields. The transfer hydrodeuteration of alkenes using α-deuterated cyclododecanone exhibited high regioselectivity. Preliminary mechanism studies confirmed the origins of the two hydrogen atoms involved in the reduction of alkenes. The current study paves the way for the use of ketones as unique transfer hydrogenation agents in chemical synthesis.

Borrowing Hydrogen β-Phosphinomethylation of Alcohols Using Methanol as C1 Source by Pincer Manganese Complex

Herein, we report a manganese-catalyzed three-component coupling of β-H containing alcohols, methanol, and phosphines for the synthesis of γ-hydroxy phosphines via a borrowing hydrogen strategy. In this development, methanol serves as a sustainable C1 source. A variety of aromatic and aliphatic substituted alcohols and phosphines could undergo the dehydrogenative cross-coupling process efficiently and deliver the corresponding β-phosphinomethylated alcohol products in moderate to good yields. Mechanistic studies suggest that this transformation proceeds in a sequential manner including catalytic dehydrogenation, aldol condensation, Michael addition, and catalytic hydrogenation.

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.

Virus-Like Magnetic Heterostructure: an Outstanding Metal-Complex Active Platform Enables High-Efficiency Separation and Catalysis

Assembled heterostructure systems, as emerging functional materials, have broad applications ranging from enzyme and drug payload to catalysis and purification. However, these require trial- and -error design process and complex experimental environment to generate heterostructure materials. Here, this study describes an easy-to-execute strategy to fabricate magnetic heterostructure as multifunctional delivery system. We utilize first-row transition metal copper and nitroso/amino ligand as modules to assemble around Fe3 O4 magnetic nanoparticles by excessed mild stimuli and fabricate the magnetic heterostructure materials (Fe3 O4 @ TACN NPs (tetraamminecopper (II) nitrate)). Notably, the Fe3 O4 @ TACN NPs present with cat's-whisker structure containing ligand and metal center. The nitroso-group ligands exhibit strong binding affinity to heme-structure enzyme, ensuring effective capture and isolate of cytochrome C (Cyt-c), resulting in their excellent isolation property. The copper complex-powered magnetic heterostructure materials can effectively isolation Cyt-c from complex biological sample (pork heart). Importantly, the Fe3 O4 @ TACN NPs coordinated with heme-structure, induced methionine 80 (Met80) disassociates from heme prosthetic group, and contributed to peroxidase-like (POD-like) activities increasing. These results exhibit that copper complex-powered magnetic heterostructure materials can not only satisfy the Cyt-c isolation and immobilization in an alkaline medium, but also be of the potential for improving the immobilization enzyme reactor performance.

Transition Metal-Catalyzed Enantioselective Synthesis of Chiral Five- and Six-Membered Benzo O-heterocycles

Enantiomerically enriched five- and six-membered benzo oxygen heterocycles are privileged architectures in functional organic molecules. Over the last several years, many effective methods have been established to access these compounds. However, comprehensive documents cover updated methodologies still in highly demand. In this review, recent transition metal catalyzed transformations lead to chiral five- and six-membered benzo oxygen heterocycles are presented. The mechanism and chirality transfer or control processes are also discussed in details.

A Tetradentate Ligand Enables Iron-Catalyzed Asymmetric Hydrogenation of Ketones in a CO- or Isocyanide-Free Fashion

We herein reported the design and synthesis of a ferrocene-based tetradentate ligand that is featured with modular synthesis and rigid skeleton. Its iron(II) complex facilitates asymmetric direct hydrogenation of ketones without the participation of extra strong-field ligand such as CO and isocyanide. Hydride donor lithium aluminum hydride (LAH) converted non-reactive Fe(II) species to reactive Fe(II) hydride species. With this catalyst, various chiral alcohols including the intermediate for montelukast could be prepared with satisfactory yields and enantioinduction.

Recent advances of Cp*Ir complexes for transfer hydrogenation: focus on formic acid/formate as hydrogen donors

Transfer hydrogenation reactions offer synthetically powerful strategies to deliver various hydrogenated compounds with the advantages of efficiency, atom economy, and practicability. On one hand, formic acid/formate function as promising hydrogen sources owing to their readily obtainable, inexpensive, and easy to handle nature. On the other hand, Cp*Ir complexes show high activities in transfer hydrogenation. This review highlights progress achieved for transfer hydrogenation of CO, CC, and CN bonds of a variety of unsaturated substrates, as well as amides focusing on Cp*Ir complexes as catalysts and formic acid/formate as hydrogen sources.

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.

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.

A new binuclear Ni(II) complex, an effective A3-coupling catalyst in solvent-free condition

A newly binuclear nickel(II) complex, [Ni2(en)4(ox)](ClO4)2 (1) (where en = ethylenediamine, and ox = oxalate), has been isolated from a reaction of NiCl2·6H2O, ethylenediamine, ammonium oxalate and sodium perchlorate in water and its crystal structure has been determined by X-ray crystallography and infra-red techniques. Compound 1 was successfully employed to promote the one-pot reaction of aldehydes, amines and acetylenes for the construction of corresponding propargylamines under solvent-free media with fine yields. Further studies reveal this catalytic system can be refreshed and used again in five runs.

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.

Origins of Regioselectivity in CuH-Catalyzed Hydrofunctionalization of Alkenes

Factors controlling the regioselectivity in alkene hydrocupration were computationally investigated using energy decomposition analysis. The results demonstrate that the Markovnikov-selective hydrocupration with electronically activated mono-substituted olefins is mostly affected by the destabilizing Pauli repulsion, which is due to the electron delocalization effect. The anti-Markovnikov-selective hydrocupration with 1,1-dialkyl-substituted terminal olefins is dominated by the repulsive electrostatic interactions, which is because of the unequal π electron distribution caused by the induction effect of alkyl substituents.

Manganese-Catalyzed Chemoselective Coupling of Secondary Alcohols, Primary Alcohols and Methanol

Herein, we report a manganese-catalyzed three-component coupling of secondary alcohols, primary alcohols and methanol for the synthesis of β,β-methylated/alkylated secondary alcohols. Using our method, a series of 1-arylethanol, benzyl alcohol derivatives, and methanol undergo sequential coupling efficiently to construct assembled alcohols with high chemoselectivity in moderate to good yields. Mechanistic studies suggest that the reaction proceeds via methylation of a benzylated secondary alcohol intermediate to generate the final product.

IrI(η4-diene) precatalyst activation by strong bases: formation of an anionic IrIII tetrahydride

The reaction between [IrCl(COD)]₂ and dppe in a 1 : 2 ratio was investigated in detail under three different conditions. [IrCl(COD)(dppe)], 1, is formed at room temperature in the absence of base. In the presence of a strong base at room temperature, hydride complexes that retain the carbocyclic ligand in the coordination sphere are generated. In isopropanol, 1 is converted into [IrH(1,2,5,6-η²:η²-COD)(dppe)] (2) on addition of KO^tBu, with k₁₂ = (1.11 ± 0.02) × 10^-4 s^-1, followed by reversible isomerisation to [IrH(1-κ-4,5,6-η³-C₈H₁₂)(dppe)] (3) with k₂₃ = (3.4 ± 0.2) × 10^-4 s^-1 and k₃₂ = (1.1 ± 0.3) × 10^-5 s^-1 to yield an equilibrium 5 : 95 mixture of 2 and 3. However, when no hydride source is present in the strong base (KO^tBu in benzene or toluene), the COD ligand in 1 is deprotonated, followed by β-H elimination of an Ir^I-C₈H₁₁ intermediate, which leads to complex [IrH(1-κ-4,5,6-η³-C₈H₁₀)(dppe)] (4) selectively. This is followed by its reversible isomerisation to 5, which features a different relative orientation of the same ligands (k₄₅ = (3.92 ± 0.11) × 10^-4 s^-1; k_5-4 = (1.39 ± 0.12) × 10^-4 s^-1 in C₆D₆), to yield an equilibrated 32 : 68 mixture of 4 and 5. DFT calculations assisted in the full rationalization of the selectivity and mechanism of the reactions, yielding thermodynamic (equilibrium) and kinetic (isomerization barriers) parameters in excellent agreement with the experimental values. Finally, in the presence of KO^tBu and isopropanol at 80 °C, 1 is transformed selectively to K[IrH₄(dppe)] (6), a salt of an anionic tetrahydride complex of Ir^III. This product is also selectively generated from 2, 3, 4 and 5 and H₂ at room temperature, but only when a strong base is present. These results provide an insight into the catalytic action of [IrCl(COD)(LL)] complexes in the hydrogenation of polar substrates in the presence of a base.

Catalytic Asymmetric Conjugate Reduction

Enantioselective reduction reactions are privileged transformations for the construction of trisubstituted stereogenic centers. While these include established synthetic strategies, such as asymmetric hydrogenation, methods based on the enantioselective addition of hydridic reagents to electrophilic prochiral substrates have also gained importance. In this context, the asymmetric conjugate reduction (ACR) of α,β-unsaturated compounds has become a convenient approach for the synthesis of chiral compounds with trisubstituted stereocenters in α-, β-, or γ-position to electron-withdrawing functional groups. Because such activating groups are diverse and amenable of further derivatizations, ACRs provide a general and powerful synthetic entry towards a variety of valuable chiral building blocks. This Review provides a comprehensive collection of catalytic ACR methods involving transition-metal, organic, and enzymatic catalysis since its first versions dating back to the late 1970s.

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.

Photosensitized Radical-Anion-Driven Metal-Free Selective Reduction of Aldehydes Using Graphene Oxide as an Electron Relay Mediator under Visible Light

Despite the modern boost, developing a new photocatalytic system for the reduction of aldehydes is still challenging due to their high negative reduction potential. Herein, we have used a metal-free photoinduced electron-transfer system based on a cheap and readily available organic dye eosin Y (EY), graphene oxide (GO), and ammonium oxalate (AO) for photocatalytic reduction of structurally diverse aldehydes under sustainable conditions. The protocol shows remarkable selectivity for the photocatalytic reduction of aldehydes over ketones. The decisive interaction of GO and AO with the various states of EY (ground, singlet, triplet, and radical anions), which are responsible for the commencement of the reaction, was examined by various theoretical, optical, electrochemical, and photo-electrochemical studies. The synergetic system of GO, EY, and AO is appropriate for enhancing the separation efficiency of visible-light-induced charge carriers. GO nanosheets act as an electron reservoir to accept and transport photogenerated electrons from the photocatalytic system to the reactant. The reduction of the GO during the process ruled out the back transfer of photoexcited charges. Control experiments explained that the reaction involves two stages: electron transfer and protonation. This process eliminates the necessity of precious-metal-based photocatalysts or detrimental sacrificial agents and overcomes the redox potential limitations for the photoreduction of aldehydes.

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.

Rational Design of a Facially Coordinating P,N,N Ligand for Manganese-Catalysed Enantioselective Hydrogenation of Cyclic Ketones

DFT calculations on the full catalytic cycle for manganese catalysed enantioselective hydrogenation of a selection of ketones have been carried out at the PBE0-D3PCM //RI-BP86PCM level. Mn complexes of an enantiomerically pure chiral P,N,N ligand have been found to be most reactive when adopting a facial coordination mode. The use of a new ligand with an ortho-substituted dimethylamino-pyridine motif has been calculated to completely transform the levels of enantioselectivity possible for the hydrogenation of cyclic ketones relative to the first-generation Mn catalysts. In silico evaluation of substrates has been used to identify those likely to be reduced with high enantiomer ratios (er), and others that would exhibit less selectivity; good agreements were then found in experiments. Various cyclic ketones and some acetophenone derivatives were hydrogenated with er's up to 99 : 1.

Amorphous RuCoP Ultrafine Nanoparticles Supported on Carbon as Efficient Catalysts for Hydrogenation of Adipic Acid to 1,6-Hexanediol

As an important raw material for organic synthesis, the 1,6-hexanediol (HDOL) is synthesized by the complicated two-step process traditionally. The hydrogenation of adipic acid (AA) is a potential way to prepare 1,6-hexanediol. At present, amorphous RuMP (M: Co, Ni, Fe, etc.)-based alloys with low Ru content were developed by co-precipitation as the efficient catalysts for converting AA to HDOL via hydrogenation. Among these RuMP catalysts, RuCoP alloys exhibited the highest selectivity and yield to HDOL owing to the electronic effect. The selectivity and yield of HDOL for the optimized RuCoP/C sample was achieved to 80% and 64%, respectively, at 65 bar and 220 °C. A series of RuCoP alloys with different degrees of crystallinity and particle sizes were prepared to investigate the effect of morphology and structure on its catalytic performance. The results indicated that the high catalytic activity of RuCoP/C resulted from its rich active sites due to its amorphous phase and small particle size.

Homogeneous Manganese-Catalyzed Hydrofunctionalizations of Alkenes and Alkynes: Catalytic and Mechanistic Tendencies

In recent years, many manganese-based homogeneous catalytic precursors have been developed as powerful alternatives in organic synthesis. Among these, the hydrofunctionalizations of unsaturated C-C bonds correspond to outstanding ways to afford compounds with more versatile functional groups, which are commonly used as building blocks in the production of fine chemicals and feedstock for the industrial field. Herein, we present an account of the Mn-catalyzed homogeneous hydrofunctionalizations of alkenes and alkynes with the main objective of finding catalytic and mechanistic tendencies that could serve as a platform for the works to come.

Synergy of metal nanoparticles and organometallic complex in NAD(P)H regeneration via relay hydrogenation

Most, if not all, of the hydrogenation reactions are catalyzed by organometallic complexes (M) or heterogeneous metal catalysts, but to improve both the activity and selectivity simultaneously in one reaction via a rational combination of the two types of catalysts remains largely unexplored. In this work, we report a hydrogenation mode though H species relay from supported metal nanoparticles (NPs) to M, where the former is responsible for H₂ dissociation, and M is for further hydride transferring to reactants. The synergy between metal NPs and M yields an efficient NAD(P)H regeneration system with >99% selectivity and a magnitude higher activity than the corresponding metal NPs and M. The modularizing of hydrogenation reaction into hydrogen activation with metal NPs and substrate activation with metal complex paves a new way to rationally address the challenging hydrogenation reactions.

Catalyst-free photo-reductions of aromatic olefins and carbonyl compounds

We describe herein a catalyst-free, traditional reductant-free strategy for the direct photoinduced hydrogenation or deuteration of aromatic olefins, ketones, and aldehydes with simple bases as the only additives. A broad range of substrates were demonstrated with high yields and deuterium incorporations. Mechanistic experiments indicate a radical mechanism.

Cobalt-Catalyzed Hydrogenation Reactions Enabled by Ligand-Based Storage of Dihydrogen

The use of supporting ligands that can store either protons or electrons has emerged as a powerful strategy in catalysis. While these strategies are potent individually, natural systems mediate remarkable transformations by combining the storage of both protons and electrons in the secondary coordination sphere. As such, there has been recent interest in using this strategy to enable fundamentally different transformations. Furthermore, outsourcing H-atom or hydrogen storage to ancillary ligands can also enable alternative mechanistic pathways and thereby selectivity. Here, we describe the application of this strategy to facilitate radical reactivity in Co-based hydrogenation catalysis. Metalation of previously reported dihydrazonopyrrole ligands with Co results in paramagnetic complexes, which are best described as having Co(II) oxidation states. These complexes catalytically hydrogenate olefins with low catalyst loadings under mild conditions (1 atm H₂, 23 °C). Mechanistic, spectroscopic, and computational investigations indicate that this system goes through a radical hydrogen-atom transfer (HAT) type pathway that is distinct from classic organometallic mechanisms and is supported by the ability of the ligand to store H₂. These results show how ancillary ligands can facilitate efficient catalysis, and furthermore how classic organometallic mechanisms for catalysis can be altered by the secondary coordination sphere.

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.