Synthetic studies on bafilomycin A1: stereoselective synthesis of the C12-C17 fragment and its coupling with the C1-C11 subunit

Stereoselective addition of (E)-1-lithio-2-tributylstannylethylene on a chiral cyclic di-t-butylsilyleneketal C₁₄-C₁₇ aldehyde afforded the required Felkin-Anh adduct for the synthesis of the C₁₂-C₁₇ fragment of bafilomycin A₁, the configuration of which was assigned unambiguously. After appropriate coupling with the enantiopure C₁-C₁₁ fragment, the C₁₂-C₁₇ subunit obtained here can be used for the study of the 16-membered macrolide formation either by an acyl activation or an intramolecular Stille reaction. Intermolecular esterification of the 15-OH with an acyl activation of the carboxylic acid of the C₁-C₁₁ fragment, in modified Yamaguchi's conditions, affords here an intermediate for examining an intramolecular Stille coupling.     

Anisotropic Magnetite Nanorods for Enhanced Magnetic Hyperthermia

There is an increasing need for new strategies to improve the heating efficiency or the specific absorption rate (SAR) of magnetite (Fe₃O₄), which is the only FDA approved magnetic material. We propose a facile approach to obtain well‐dispersed highly crystalline Fe₃O₄ nanorods (NRs) by the reduction of β‐FeOOH in an organic solvent and demonstrate that the SAR of Fe₃O₄ NRs can be enhanced by tuning their aspect ratios. Fe₃O₄ NRs with an aspect ratio of 4.5 have a much higher SAR as compared with 15 nm Fe₃O₄ nanoparticles and Fe₃O₄ NRs counterparts with an aspect ratio of 10. The highest SAR is greatly increased up to 1072 W g^?1 for an ac field of 33 kA m^?1 and a concentration of 5 mg mL^?1, which is mostly attributed to hysteresis losses. These findings pave a new pathway for the design and synthesis of novel anisotropic iron oxide nanostructures with an optimal heating efficiency for advanced hyperthermia.     

Assembly of Na3V2(PO4)3 Nanoparticles Confined in a One‐Dimensional Carbon Sheath for Enhanced Sodium‐Ion Cathode Properties

Structural and morphological control is an effective approach for improvement of electrochemical properties in rechargeable batteries. One‐dimensionally assembled structure composed of NASICON‐type Na₃V₂(PO₄)₃ nanoparticles were fabricated through an electrospinning method to meet the requirements for the development of efficient electrode materials in Na‐ion batteries. High‐temperature treatment of electrospun precursor fibers under an argon flow provides a nonwoven fabric of nanowires comprising crystallographically oriented nanoparticles of NASICON‐type Na₃V₂(PO₄)₃ within a carbon sheath. The mesostructure comprising NASICON‐type Na₃V₂(PO₄)₃ and carbon give a short sodium‐ion transport pass and an efficient electron conduction pass. Electrochemical properties of NASICON‐type Na₃V₂(PO₄)₃ are improved on the basis of one‐dimensional nanostructures designed in the present study.     

On the minimization of the global variance in the 1-Reduced local-energy matrix

By minimizing the global variance in the 1-reduced local-energy matrix E₁( X ₁; X ₁′), subject to the normalization of the 1-reduced density matrix ρ₁( X ₁; X ₁′), one derives an integral matrix equation for E₁( X ₁; X ₁′) as a functional of ρ₁( X ₁; X ₁′) at the location ( X ₁; X ₁′) of an arbitrary member of an N (≥ 2)-particle system. The implications for the possible local improvement in the accuracy of approximate wave functions through the imposition of global constraints are briefly discussed.     

B(C6F5)3: an efficient catalyst for reductive alkylation of alkoxy benzenes and for synthesis of triarylmethanes using aldehydes

Tris(pentafluorophenyl)borane [B(C₆F₅)₃] has been used as an efficient catalyst for reductive alkylation of alkoxy benzenes using aldehydes as an alkylating agent in the presence of polymethylhydrosiloxane (PMHS). Various alkylated trimethoxybenzene derivatives have been prepared in good to high yields. In addition, B(C₆F₅)₃ was also used as a catalyst for the reaction of electron-rich arenes with aldehydes to obtain triarylmethanes. The use of reductive alkylation protocol for the synthesis of an isochroman and tetrahydroisoquinoline derivatives has also been demonstrated.     

Two-Dimensional Perovskite Oxynitride K2LaTa2O6N with an H+/K+ Exchangeability in Aqueous Solution Forming a Stable Photocatalyst for Visible-Light H2 Evolution

Undoped layered oxynitrides have not been considered as promising H₂-evolution photocatalysts because of the low chemical stability of oxynitrides in aqueous solution. Here, we demonstrate the synthesis of a new layered perovskite oxynitride, K₂LaTa₂O₆N, as an exceptional example of a water-tolerant photocatalyst for H₂ evolution under visible light. The material underwent in-situ H⁺/K⁺ exchange in aqueous solution while keeping its visible-light-absorption capability. Protonated K₂LaTa₂O₆N, modified with an Ir cocatalyst, exhibited excellent catalytic activity toward H₂ evolution in the presence of I⁻ as an electron donor and under visible light; the activity was six times higher than Pt/ZrO₂/TaON, one of the best-performing oxynitride photocatalysts for H₂ evolution. Overall water splitting was also achieved using the Ir-loaded, protonated K₂LaTa₂O₆N in combination with Cs-modified Pt/WO₃ as an O₂ evolution photocatalyst in the presence of an I₃⁻/I⁻ shuttle redox couple.     

Alkene and Alkyne Oxidative Cleavage Catalyzed by RuO4 in Environmentally Acceptable Solvents

Abstract

The application of CCl₄, C₆H₁₂, EtOAc, and Me₂CO as solvents for biphasic systems has been compared for oxidative cleavage of alkenes and alkynes by RuO₄ to carboxylic acids, using the RuCl₃ · nH₂O–IO(OH)₅ reagent for which an improved procedure is described. Cyclohexane is an effective and economic replacement for the environmentally unfriendly CCl₄; acetone and ethyl acetate are less effective.     

Superior gas separation performance of dual-layer hollow fiber membranes with an ultrathin dense-selective layer

A concept demonstration has been made to simultaneously enhance both O₂ and CO₂ gas permeance and O₂/N₂ and CO₂/CH₄ selectivity via intelligently decoupling the effects of elongational and shear rates on dense-selective layer and optimizing spinning conditions in dual-layer hollow fiber fabrication. The dual-layer polyethersulfone hollow fiber membranes developed in this work exhibit an O₂/N₂ selectivity of 6.96 and an O₂ permeance of 4.79 GPU which corresponds to an ultrathin dense-selective layer of 918 Å at room temperature. These hollow fibers also show an impressive CO₂/CH₄ selectivity of 49.8 in the mixed gas system considering the intrinsic value of only 32 for polyethersulfone dense films. To our best knowledge, this is the first time to achieve such a high CO₂/CH₄ selectivity without incorporating any material modification. The above gas separation performance demonstrates that the optimization of dual-layer spinning conditions with balanced elongational and shear rates is an effective approach to produce superior hollow fiber membranes for oxygen enrichment and natural gas separation.     

A convenient solution method for conversion of a W(II) octahedral cluster to W(IV) triangular cluster: synthesis and characterization of Cs3Na2[W3Se4(CN)9] · 0.5Et4NBr · 5H2O

A new solution method for the preparation of a triangular W(IV) cluster starting from an octahedral W(II) cluster is reported. The product Cs₃Na₂[W₃Se₄(CN)₉]?·?0.5Et₄NBr?·?5H₂O (1) has been synthesized by boiling W₆Br₁₂ in an aqueous solution of Na₂Se _x , followed by filtering off the dark solid which was then heated with an aqueous solution of NaCN and crystallized by the addition of CsBr and Et₄NBr. The compound was characterized by IR, UV-Vis, elemental analysis, and single crystal X-ray diffraction. The described W(II)-to-W(IV) conversion in solution is reported for the first time.     

Metal Ion Recognition. Interaction of New Oxygen-Nitrogen Donor Macrocycles with Selected Transition and Post-Transition Metal Ions

As part of an ongoing investigation of the factors influencing metal ionrecognition, we have investigated structure/function relationships involvingthe metal-ion binding by three new N-benzyl-substituted, 15- and 16-membered,macrocyclic ligands incorporating N₂O ₃- and N ₃O ₃-donor sets (withthe N ₃O ₃-system consisting of a N ₂O ₃-macrocyclic ring with an attachedCH ₂CH ₂NCH ₂C ₆H ₅ pendant arm). Selected solid complexes of thelatter ligand were isolated and the X-ray structures of individual Ni(II) and Ag(I) complexeswere obtained.Where solubility permitted, potentiometric titration studies in 95% methanolwere employed to investigate the binding affinities of all three ligand derivativestowards Co(II), Ni(II), Cu(II), Zn(II), Cd(II), Ag(I) and Pb(II). The 15-memberedN ₂O ₃-ring was found to be selective for Ag(I) over the other six metalsinvestigated, including Cu(II). However, the presence of a further nitrogen donorin the form of the pendant benzylamine functionality in the N ₃O ₃-donorsystem results in an increase in its binding affinity for Ag(I) but an even greaterincrease occurs for Cu(II). As a consequence, the latter ion is now more stronglybound than Ag(I). The factors influencing these respective selectivities are discussed.     

A density functional theory study of alkyl group migration in RMn(CO)5 complexes

The kinetics of alkyl group migration in RMn(CO)₅ complexes ( R=CH₃, C₂H₅ and C₃H₇) were studied. Isomers of CH₃Mn(CO)₅ with an agostic structure, an η¹ structure, and an η² structure were found to be local minima on the system's potential energy surface. Transition states for the inter-conversion of these species were also located. The activation free energy for this migration reaction was compared with experimental data and provides insights into the important steps in the overall reaction mechanism.     

Ligand substitution in the heterometallic cluster Cp*IrOs3(μ-H)2(CO)10

The reactions of the heterometallic cluster Cp*IrOs₃(μ-H)₂(CO)₁₀ with phosphines, isonitriles and pyridine under TMNO activation afforded the substitution products Cp*IrOs₃(μ-H)₂(CO)_10−nL_n (n = 1, 2; L = PPh₃, P(OMe)₃, ^tBuNC, CyNC or py) in good yields. For the monosubstituted derivatives, the substitution site was exclusively at an osmium atom in an axial position for L = phosphine or phosphite. Spectroscopic evidence suggested the presence of isomers in solution for the PPh₃ derivative. In contrast, for L = isonitrile, the ligand occupied an equatorial site. In the disubstituted derivatives, the group 15 ligands were coordinated to two different osmium atoms, one each at an axial and an equatorial site. The isomerism and fluxional behaviour of some of these clusters have also been examined.     

Extremal electron pairs

Extremal pair functions for an n-electron wave function of a closed-shell state are defined as linear combinations of spin-orbital-product pair functions that make some functionals (e.g., r²₁₂ or r⁻¹₁₂) extremal. They are related to the natural spin geminals in the uncorrelated limit and are useful both for an analysis of wave functions in view of an understanding of the chemical bond and for the treatment of electron correlation. Numerical examples are shown and discussed for He₂ as well as the 10-electron systems Ne, HF, H₂O, NH₃, and CH₄. © 1996 John Wiley & Sons, Inc.     

The synthesis,electrochemistry and molecular structure of [Fe(η5-C5H4S)2Mo(NO){HB(3,5-Me2C3N2H)3}]

The new bimetallic complex [Fe(η⁵-C₅H₄S)₂Mo(NO){HB(3,5-Me₂C₃N₂H)₃)}] has been obtained from the reaction between [Fe(η⁵-C₅H₄SH)₂] and [Mo(NO){HB(3,5-Me₂C₃N₂H)₃}I₂]. Electrochemical studies reveal an anomalously cathodic oxidation potential for the metallocene redox centre. An X-ray diffraction study has revealed an FMo distance of 4.147(2) Å, with the ferrocenyl moiety oriented towards the nitrosyl ligand on the molybdenum atoms (Fe---O 3.976(6) Å), but provides no evidence for an interaction between the iron atom and the molybdenum-bound nitrosyl which might account for the electrochemical findings.     

Defect pyrochlores as catalyst supports

The possibility of designing an active support for dispersed Pt for an automobile exhaust converter was demonstrated. Complete reduction of NO to N₂ is accomplished above 250°C by the defect-pyrochlore support Pb₂Pb_xRu_2?xO_6+δ, and oxidation of hydrocarbons or CO to CO₂ is achieved by the dispersed Pt.     

Statistical experimental design-driven discovery of room-temperature conditions for palladium-catalyzed cyanation of aryl bromides

A combination of Pd₂(dba)₃·CHCl₃ (0.5 mol %) and commercially available, air-stable phosphonium salt [(t-Bu)₃PH]BF₄ (1.4 mol %) in a presence of Zn powder and Zn(CN)₂ as the cyanide source comprises an extremely efficient catalyst system for the cyanation of a diverse array of aryl bromides, at room temperature. This result emerged from an experimental strategy that combines the advantages of parallel, automated experimentation with the design of experiments (DOE) for the effective definition of an optimal set of reaction conditions.     

A Redox‐Mediator‐Free Solar‐Driven Z‐Scheme Water‐Splitting System Consisting of Modified Ta3N5 as an Oxygen‐Evolution Photocatalyst

Tantalum nitride (Ta₃N₅) modified with various O₂‐evolution cocatalysts was employed as a photocatalyst for water oxidation under visible light (λ>420 nm) in an attempt to construct a redox‐mediator‐free Z‐scheme water‐splitting system. Ta₃N₅ was prepared by nitriding Ta₂O₅ powder under a flow of NH₃ at 1023–1223 K. The activity of Ta₃N₅ for water oxidation from an aqueous AgNO₃ solution as an electron acceptor without cocatalyst was dependent on the generation of a well‐crystallized Ta₃N₅ phase with a low density of anionic defects. Modification of Ta₃N₅ with nanoparticulate metal oxides as cocatalysts for O₂ evolution improved water‐oxidation activity. Of the cocatalysts examined, cobalt oxide (CoO_x) was found to be the most effective, improving the water‐oxidation efficiency of Ta₃N₅ by six to seven times. Further modification of CoO_x/Ta₃N₅ with metallic Ir as an electron sink allowed one to achieve Z‐scheme water splitting under simulated sunlight through interparticle electron transfer without the need for a shuttle redox mediator in combination with Ru‐loaded SrTiO₃ doped with Rh as a H₂‐evolution photocatalyst.     

A Low‐Temperature Molecular Precursor Approach to Copper‐Based Nano‐Sized Digenite Mineral for Efficient Electrocatalytic Oxygen Evolution Reaction

In the urge of designing noble metal‐free and sustainable electrocatalysts for oxygen evolution reaction (OER), herein, a mineral Digenite Cu₉S₅ has been prepared from a molecular copper(I) precursor, [{(PyHS)₂Cu^I(PyHS)}₂](OTf)₂ ( 1 ), and utilized as an anode material in electrocatalytic OER for the first time. A hot injection of 1 yielded a pure phase and highly crystalline Cu₉S₅, which was then electrophoretically deposited (EPD) on a highly conducting nickel foam (NF) substrate. When assessed as an electrode for OER, the Cu₉S₅/NF displayed an overpotential of merely 298±3 mV at a current density of 10 mA cm^?2 in alkaline media. The overpotential recorded here supersedes the value obtained for the best reported Cu‐based as well as the benchmark precious‐metal‐based RuO₂ and IrO₂ electrocatalysts. In addition, the choronoamperometric OER indicated the superior stability of Cu₉S₅/NF, rendering its suitability as the sustainable anode material for practical feasibility. The excellent catalytic activity of Cu₉S₅ can be attributed to the formation of a crystalline CuO overlayer on the conductive Cu₉S₅ that behaves as active species to facilitate OER. This study delivers a distinct molecular precursor approach to produce highly active copper‐based catalysts that could be used as an efficient and durable OER electro(pre)catalysts relying on non‐precious metals.     

Studying catalytic activity of ternary mixed-metal Keggin H7SiV3W9O40 as a versatile acid catalyst for the synthesis of β-acetamido ketones

One-pot four-component condensation of an aromatic aldehyde with an enolisable ketone and acetyl chloride is investigated in the presence of H₇SiV₃W₉O₄₀ in acetonitrile. The prepared catalyst was characterized by standard techniques such as XRD, SEM, and FT-IR to verify the Keggin structure of nanocatalyst. Furthermore, findings revealed a very good catalytic activity for the applied vanadium substituted heteropolyacid in this acid catalyzed condensation reaction. A series of Keggin Si and V-substituted heteropolyacids were compared for the synthesis of β-acetamido-β-(4-chlorophenyl)propiophenone, which showed a better catalytic activity for H₇SiV₃W₉O₄₀ and H₄SiW₁₂O₄₀ than the lacunary and mixed metal Keggin H₄SiW₉Mo₃O₄₀ and H₅SiW₉Mo₂VO₄₀. In addition, effect of the nitrilating agent was also demonstrated in this catalytic system and findings show less proficiency for PhCN compared to acetonitrile. Moreover, propiophenone as an α-substituted enolisable ketone was reacted with some aromatic aldehydes and the anti isomer is detected as the major diastereomer. Therefore, H₇SiV₃W₉O₄₀ can be introduced as a new effective, inexpensive, and eco-friendly catalyst for the introduced four-component condensation reaction.     

Hydrogen generation from H<Subscript>2</Subscript>O/H<Subscript>2</Subscript>O<Subscript>2</Subscript>/MnWO<Subscript>4</Subscript> system

Hydrogen gas as a clear energy resource was found to be largely bubbled from a H₂O/H₂O₂/MnWO₄ system. MnWO₄ powder was fabricated by an aqueous reaction method. The powder was characterized with X-ray diffraction (XRD), transmission electron microscope (TEM), and X-ray photoelectron spectrometry (XPS). The efficiency of the hydrogen generation increases with an increase in initial pH in the appropriate range, H₂O₂ proportion, MnWO₄ proportion, and intensity of light resource. Calcining at 400 °C for 1 h can make the MnWO₄ powder synthesized by an aqueous reaction more effective for H₂ generation and more stable in higher initial pH. The MnWO₄ catalyst shows a long-term stability for photocatalytic H₂ generation. A mechanism was suggested for the hydrogen generation from the H₂O/H₂O₂/MnWO₄ system together with XPS analysis.