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.

A Novel Hydrated Iron Vanadate Cathode Material for Advanced Aqueous Nickel-Ion Batteries.

Aqueous nickel-ion batteries (ANIBs) as an emerging energy storage device attracted much attention owing to their multielectron redox reaction and dendrite-free Ni anode, yet their development is hindered by the divalent properties of Ni2+ and the lack of suitable cathode materials. Herein, a hydrated iron vanadate (Fe2V3O10.5∙1.5H2O, FOH) with a preferred orientation along the (200) plane is innovatively proposed and used as cathode material for ANIBs. The FOH cathode exhibits a remarkable capacity of 129.3 mAh g-1 at 50 mA g-1 and a super-high capacity retention of 95% at 500 mA g-1 after 700 cycles. The desirable Ni2+ storage capacity of FOH can be attributed to the preferentially oriented and tunnel structures, which offer abundant reaction active planes and a broad Ni2+ diffusion path, the abundant vacancies and high specific surface area further increase ion storage sites and accelerate ion diffusion in the FOH lattice. Furthermore, the Ni2+ storage mechanism and structural evolution in the FOH cathode are explored through ex situ XRD, ex situ Raman, ex situ XPS and other ex situ characteristics. This work opens a new way for designing novel cathode materials to promote the development of ANIBs.

Borane-Catalyzed Tandem Cyclization/Hydrosilylation Towards Enantio- and Diastereoselective Construction of trans-2,3-Disubstituted-1,2,3,4-Tetrahydroquinoxalines.

Recent years have witnessed marked progress in the efficient synthesis of various enantioenriched 1,2,3,4-tetrahydroquinoxalines. However, enantio- and diastereoselective access to trans-2,3-disubstituted 1,2,3,4-tetrahydroquinoxalines remains much less explored. Herein we report that a frustrated Lewis pair-based catalyst generated via in situ hydroboration of 2-vinylnaphthalene with HB(C6 F5 )2 allows for the one-pot tandem cyclization/hydrosilylation of 1,2-diaminobenzenes and 1,2-diketones with commercially available PhSiH3 to exclusively afford trans-2,3-disubstituted 1,2,3,4-tetrahydroquinoxalines in high yields with excellent diastereoselectivities (>20 : 1 dr). Furthermore, this reaction can be rendered asymmetric by using an enantioenriched borane-based catalyst derived from HB(C6 F5 )2 and a binaphthyl-based chiral diene to give rise to enantioenriched trans-2,3-disubstituted 1,2,3,4-tetrahydroquinoxalines in high yields with almost complete diastereo- and enantiocontrol (>20 : 1 dr, up to >99 % ee). A wide substrate scope, good tolerance of diverse functionality and up to 20-gram scale production are demonstrated. The enantio- and diastereocontrol are achieved by the judicious choice of borane catalyst and hydrosilane. The catalytic pathway and the origin of the excellent stereoselectivity are elucidated by mechanistic experiments and DFT calculations.

Manganese-Catalyzed Asymmetric Formal Hydroamination of Allylic Alcohols: A Remarkable Macrocyclic Ligand Effect.

A unique family of chiral peraza N6 -macrocyclic ligands, which are conformationally rigid and have a tunable saddle-shaped cavity, is described. Utilizing their manganese(I) complexes, the first example of earth-abundant transition metal-catalyzed asymmetric formal anti-Markovnikov hydroamination of allylic alcohols was realized, providing a practical access to synthetically important chiral γ-amino alcohols in excellent yields and enantioselectivities (up to 99 % yield and 98 % ee). The single-crystal structure of a MnI complex indicates that the manganese atom coordinates with the chiral dialkylamine moiety in a bidentate fashion. Further DFT calculations revealed that five of the six nitrogen atoms in the ligand were engaged in multiple noncovalent interactions with Mn, an isopropanol molecule, and a ß-amino ketone intermediate via coordination, hydrogen bonding, and/or CH⋅⋅⋅π interactions in the transition state, showing a remarkable role of the macrocyclic framework.

Ruthenium-Catalyzed Enantioselective Hydrogenation of 9-Phenanthrols.

The enantioselective hydrogenation of arenols to corresponding chiral cyclic alcohols remains a challenge because of their aromaticity and the difficulty in controlling the regio-, chemo-, and stereoselectivity. In this work, the first highly efficient ruthenium-catalyzed enantioselective hydrogenation of 9-phenanthrols has been successfully realized under mild conditions via trapping the unstable keto tautomers. The method provides a facile access to a range of chiral 9,10-dihydrophenanthren-9-ols with up to 98 % yield and >99 % ee. The hydrogenation pathway includes base-promoted tautomerization of 9-phenanthrols and Ru-catalyzed asymmetric hydrogenation of the in situ generated unstable keto tautomers.

Rhodium-Catalyzed ON-OFF Switchable Hydrogenation Using a Molecular Shuttle Based on a [2]Rotaxane with a Phosphine Ligand.

A novel pH-responsive molecular shuttle based on a [2]rotaxane with a phosphine ligand has been designed and synthesized. In the rhodium-catalyzed hydrogenation of α,ß-dehydroamino acid esters and aryl enamides, ON/OFF-switchable catalysis was accomplished with high ON/OFF ratios by adjusting the movements of the rotaxane wheels located at the catalyst terminals with acid/base. Mechanistic studies using NMR spectroscopy and quasi in situ X-ray photoelectron spectroscopy revealed that RhIII -hydride species are possibly formed in a H2 atmosphere when the catalyst is in the OFF state. During the reaction, a heterolytic activation of dihydrogen occurs by the interlocked rotaxane dibenzylamine and RhI catalytic center acting as a frustrated Lewis pair. Subsequent homolytic splitting of dihydrogen with the newly formed RhI -hydride species generates RhIII -hydride species. These findings show that a substrate-selective hydrogenation can be achieved by using the OFF-state catalyst.

Diastereodivergent Synthesis of Chiral 4-Fluoropyrrolidines (exo and exo') Based on the Cu(II)-Catalyzed Asymmetric 1,3-Dipolar Cycloaddition.

1,3-Dipolar cycloaddition of azomethine ylides and electron deficient alkenes is widely studied for rapid installation of pyrrolidine frameworks. Despite significant advances, the major limitations of this process are creating chiral pyrrolidines bearing a quaternary stereogenic center and controlling the diastereoselectivity. Herein, we present an exo-selective asymmetric 1,3-dipolar cycloaddition to access chiral pyrrolidines with four contiguous stereogenic centers, including a fluorinated quaternary stereogenic center at C4, wherein a Cu(OAc)2/(S)-tol-BINAP catalyst and α-fluoro-α,ß-unsaturated arylketone dipolarophiles are used. Epimerization promoted by 5.0 equiv of DBU at 90 °C results in the formation of chiral 4-fluoropyrrolidines (exo') while maintaining the optical purity.

Diaza-Crown Ether-Bridged Chiral Diphosphoramidite Ligands: Synthesis and Applications in Asymmetric Catalysis.

A small library of diaza-crown ether-bridged chiral diphosphoramidite ligands was prepared. In the rhodium-catalyzed asymmetric hydrogenation and hydroformylation reactions, these ligands exhibited distinct properties in catalytic activity and/or enantioselectivity. Hydrogenated products with opposite absolute configurations could be obtained in high yields with excellent ee values by utilizing (S,S)-L1 and (S,S)-L3, respectively. Meanwhile, the addition of alkali metal cations caused variations in catalytic outcomes, showing the supramolecular tunability of these Rh/diphosphoramidite catalytic systems.

Enantioselective synthesis of tunable chiral pyridine-aminophosphine ligands and their applications in asymmetric hydrogenation.

A small library of tunable chiral pyridine-aminophosphine ligands were enantioselectively synthesized based on chiral 2-(pyridin-2-yl)-substituted 1,2,3,4-tetrahydroquinoline scaffolds, which were obtained in high yields and with excellent enantioselectivities via ruthenium-catalyzed asymmetric hydrogenation of 2-(pyridin-2-yl)quinolines. The protocol features a wide substrate scope and mild reaction conditions, enabling scalable synthesis. These chiral P,N ligands were successfully applied in the Ir-catalyzed asymmetric hydrogenation of benchmark olefins and challenging seven-membered cyclic imines including benzazepines and benzodiazepines. Excellent enantio- and diastereoselectivity (up to 99% ee and >20 : 1 dr), and/or unprecedented chemoselectivity were obtained in the asymmetric hydrogenation of 2,4-diaryl-3H-benzo[b]azepines and 2,4-diaryl-3H-benzo[b][1,4]diazepines.

Consecutive Intermolecular Reductive Amination/Asymmetric Hydrogenation: Facile Access to Sterically Tunable Chiral Vicinal Diamines and N-Heterocyclic Carbenes.

A highly enantioselective iridium- or ruthenium-catalyzed intermolecular reductive amination/asymmetric hydrogenation relay with 2-quinoline aldehydes and aromatic amines has been developed. A broad range of sterically tunable chiral N,N'-diaryl vicinal diamines were obtained in high yields (up to 95 %) with excellent enantioselectivity (up to >99 % ee). The resulting chiral diamines could be readily transformed into sterically hindered chiral N-heterocyclic carbene (NHC) precursors, which are otherwise difficult to access. The usefulness of this synthetic approach was further demonstrated by the successful application of one of the chiral vicinal diamines and chiral NHC ligands in a transition-metal-catalyzed asymmetric Suzuki-Miyaura cross-coupling reaction and asymmetric ring-opening cross-metathesis, respectively.

Rapid Construction of Structurally Diverse Quinolizidines, Indolizidines, and Their Analogues via Ruthenium-Catalyzed Asymmetric Cascade Hydrogenation/Reductive Amination.

A rapid construction of enantioenriched benzo-fused quinolizidines, indolizidines, and their analogues by ruthenium-catalyzed asymmetric cascade hydrogenation/reductive amination of quinolinyl- and quinoxalinyl-containing ketones has been developed. This reaction proceeds under mild reaction conditions, affording chiral benzo-fused aliphatic N-heterocyclic compounds with structural diversity in good yields (up to 95 %) with excellent diastereoselectivity (up to >20:1 dr) and enantioselectivity (up to >99 % ee). Furthermore, this catalytic protocol is applicable to the formal synthesis of (+)-gephyrotoxin.

Self-Collapsing of Single Molecular Poly-Propylene Oxide (PPO) in a 3D DNA Network.

On the basis of DNA self-assembly, a thermal responsive polymer polypropylene oxide (PPO) is evenly inserted into a rigid 3D DNA network for the study of single molecular self-collapsing process. At low temperature, PPO is hydrophilic and dispersed uniformly in the network; when elevating temperature, PPO becomes hydrophobic but can only collapse on itself because of the fixation and separation of DNA rigid network. The process has been characterized by rheological test and Small Angle X-Ray Scattering test. It is also demonstrated that this self-collapsing process is reversible and it is believed that this strategy could provide a new tool to study the nucleation-growing process of block copolymers.

Asymmetric Hydrogenation of In†Situ Generated Isochromenylium Intermediates by Copper/Ruthenium Tandem Catalysis.

The first asymmetric hydrogenation of in situ generated isochromenylium derivatives is enabled by tandem catalysis with a binary system consisting of Cu(OTf)2 and a chiral cationic ruthenium-diamine complex. A range of chiral 1H-isochromenes were obtained in high yields with good to excellent enantioselectivity. These chiral 1H-isochromenes could be easily transformed into isochromanes, which represent an important structural motif in natural products and biologically active compounds. The chiral induction was rationalized by density functional theory calculations.

A Pronounced Halogen Effect on the Organogelation Properties of Peripherally Halogen Functionalized Poly(benzyl ether) Dendrons.

An interesting halogen-substituent effect on the organogelation properties of poly(benzyl ether) dendrons is reported. A new class of poly(benzyl ether) dendrons with halo substituents decorating their periphery was synthesized and fully characterized. A systematic study on the gelation abilities, thermotropic behaviors, aggregated microstructures, and mechanical properties of self-assembled organogels was performed to elucidate the halogen-substituent effects on their organogelation propensity. It was found that the exact halogen substitutions on the periphery of dendrons exert a profound effect on the organogelation propensity, and dendrons Gn -Cl (n=2, 3) and G2 -I proved to be highly efficient organogelators. The cooperation of multiple π-π, dispersive halogen, CH-π, and weak C-H⋅⋅⋅X hydrogen-bonding interactions were found to be the key contributor to forming the self-assembled gels. Dendritic organogels formed from Gn -Cl (n=2, 3) in 1,2-dichloroethane exhibited thixotropic-responsive properties, and such thixotropic organogels are promising materials for future research and applications.

Synthesis of the Carbon-Coated Nanoparticle Co9S8 and Its Electrochemical Performance as an Anode Material for Sodium-Ion Batteries.

A Co9S8/C nanocomposite is prepared using a solid-state reaction followed by a facile mechanical ball-milling treatment, with sucrose as the carbon source. The phases, morphologies, and detailed structures of the Co9S8/C nanocomposite are well-characterized using X-ray diffraction (XRD), X-ray photoelectron spectroscopy, and high-resolution transmission electron microscopy. When evaluated as an anode material for sodium-ion batteries, the Co9S8/C nanocomposite electrode displays a reversible capacity of ∼567 mA h g-1 in the initial cycle and maintains a reversible capacity of ∼320 mA h g-1 after 30 cycles, indicating a larger capacity and a stable cycling performance. For comparison, the electrochemical performances of Co9S8 and Co9S8/C samples synthesized using the solid-state reaction are also displayed. The ex situ XRD and transmission electron microscopy tests demonstrate that Co9S8 undergoes a conversion-type sodium storage mechanism.

From Weakness to Strength: C-H/π-Interaction-Guided Self-Assembly and Gelation of Poly(benzyl ether) Dendrimers.

The C-H/π interactions as the key driving force for the construction of supramolecular gels remain a great challenge because of their weak nature. We hereby employed for the first time weak C-H/π interactions for the construction of supramolecular dendritic gels based on peripherally methyl-functionalized poly(benzyl ether) dendrimers. Their gelation property is highly dependent on the nature of the peripheral methyl groups. Furthermore, single-crystal X-ray analysis and NMR spectroscopy revealed that multiple C-H/π interactions between the proton of the methyl group and the electron-rich peripheral methyl-substituted aryl ring played significant roles in the formation of supramolecular nanofibers and organogels. This study uncovers the critical role of weak noncovalent interactions and provides new insights into the further design of self-assembled nanomaterials.

Asymmetric Hydrogenation of Quinoline Derivatives Catalyzed by Cationic Transition Metal Complexes of Chiral Diamine Ligands: Scope, Mechanism and Catalyst Recycling.

This personal account is focused on the asymmetric hydrogenation of quinolines and their analogues recently developed by using phosphorus-free chiral cationic ruthenium(II)/η6 -arene-N-monosulfonylated diamine complexes. In our initial study, the chiral Ru-diamine complexes were found to be highly effective catalysts for the asymmetric hydrogenation of difficult quinoline substrates in room temperature ionic liquids (RTILs) with unprecedentedly excellent enantioselectivity. Our further systematic study revealed that a wide range of quinoline derivatives could be efficiently hydrogenated in alcoholic solvents, or under solvent-free and concentrated conditions with good to excellent stereoselectivity. Complexes of iridium analogues could also efficiently catalyze the asymmetric hydrogenation of quinolines in undegassed solvent. Asymmetric tandem reduction of various 2-(aroylmethyl)quinolines was achieved in high yield with excellent enantioselectivity and good diastereoselectivity. More challenging substrates, alkyl- and aryl-substituted 1,5- and 1,8-naphthyridine derivatives were successfully hydrogenated with these chiral ruthenium catalysts to give 1,2,3,4-tetrahydronaphthyridines with good to excellent enantioselectivity. Unlike the asymmetric hydrogenation of ketones, quinoline is reduced via a stepwise H+ /H- transfer process outside the coordination sphere rather than a concerted mechanism. The enantioselectivity originates from the CH/π attraction between the η6 -arene ligand in the Ru-complex and the fused phenyl ring of dihydroquinoline via a 10-membered ring transition state with the participation of TfO- anion. In addition, the Ru-catalyzed asymmetric hydrogenation of quinolines could be carried out in some environmentally benign reaction media, such as undegassed water, RTILs and oligo(ethylene glycol)s (OEGs). In the latter two cases, unique chemoselectivity and/or reactivity were observed. Catalyst recycling could also be realized by using [BMIM]PF6 and OEGs as solvents, as well as via magnetic nanoparticles. Applications of this catalytic protocol were also exemplified by the employments of the reduced products for the syntheses of some important natural alkaloids, pharmaceutical intermediates, as well as chiral diamine ligands.

Superstructure ZrV2O7 nanofibres: thermal expansion, electronic and lithium storage properties.

ZrV2O7 has attracted much attention as a negative thermal expansion (NTE) material due to its isotropic negative structure. However, rarely has investigation of the lithium storage behaviors been carried out except our first report on it. Meanwhile, the electrochemical behaviors and energy storage characteristics have not been studied in depth and will be explored in this article. Herein, we report on the synthesis, characterization and lithium intercalation mechanism of superstructure ZrV2O7 nanofibres that were prepared through a facile solution-based method with a subsequent annealing process. The thermal in situ XRD technique combined with the Rietveld refinement method is adopted to analyze the change in the temperature-dependent crystal structure. Benefiting from the nanostructured morphology and relatively high electronic conductivity, it presents acceptable cyclic stability and rate capability. According to the operando evolution of the XRD patterns obtained from electrochemical in situ measurements, the Li intercalation mechanism of the solid solution process with a subsequent conversion reaction can be concluded. Finally, the amorphous state of the electrodes after the initial fully discharged state can effectively enhance the electrochemical performances.

Highly Enantioselective Synthesis of Indolines: Asymmetric Hydrogenation at Ambient Temperature and Pressure with Cationic Ruthenium Diamine Catalysts.

A highly enantioselective synthesis of indolines by asymmetric hydrogenation of 1H-indoles and 3H-indoles at ambient temperature and pressure, catalyzed by chiral phosphine-free cationic ruthenium complexes, has been developed. Excellent enantio- and diastereoselectivities (up to >99 % ee, >20:1 d.r.) were obtained for a wide range of indole derivatives, including unprotected 2-substituted and 2,3-disubstituted 1H-indoles, as well as 2-alkyl- and 2-aryl-substituted 3H-indoles.

Highly Enantioselective Direct Synthesis of Endocyclic Vicinal Diamines through Chiral Ru(diamine)-Catalyzed Hydrogenation of 2,2'-Bisquinoline Derivatives.

An asymmetric hydrogenation of 2,2'-bisquinoline and bisquinoxaline derivatives, catalyzed by chiral cationic ruthenium diamine complexes, was developed. A broad range of chiral endocyclic vicinal diamines were obtained in high yields with excellent diastereo- and enantioselectivity (up to 93:7 dl/meso and >99 % ee). These chiral diamines could be easily transformed into a new class of chiral N-heterocyclic carbenes (NHCs), which are important but difficult to access.