Asymmetric catalysis of aldol reactions with chiral lewis bases

In an extension of studies both on the stereochemical course of the aldol addition and on Lewis-base-catalyzed allylation reactions, we have invented a new Lewis-base-catalyzed asymmetric aldol addition. This Account outlines the conceptual development, the identification of design criteria, and the underlying principles for such a process. The reduction of these elements to practice in the demonstration of enantioselective aldol additions of trichlorosilyl enolates catalyzed by chiral phosphoramides is also presented. From a combination of stereochemical, kinetic, and structural studies, an intruiging mechanistic hypothesis is forwarded that explains the origin of catalysis and diastereoselectivity.     

Chemical modification of preceramic polymers: Their reactions with transition metal complexes and transition metal powders

Conclusions This research has demonstrated that a variety of chemistries can be carried out with preceramic polymers that in general are characterized by the presence of an abundance of reactive functional groups. Such chemistries can serve to upgrade a given preceramic polymer by catalytic or stoichiometric processes; they can be used to from new and useful hybrid polymers from the original preceramic polymer, as shown in the present work and also in some of our previously published work; they can, by their pyrolysis in the presence of metal powders, act as chemical reagents that deliver the elements of interest for reaction with the metal to give useful ceramics. Thus the preparation of a preceramic polymer is not the end of the chemistry in the monomer-to-polymer-to-ceramic conversion, but rather it presents many possibilities for further chemistry.     

Polymer-supported transition metal clusters

Summary The synthesis of appropriate monomers bearing alkylidyne tricobaltnonacarbonyl cluster or sulfido tricobalt heptacarbonyl cluster is described. Organometallic copolymers are obtained by free-radical copolymerisation with other vinyl monomers. Direct metallation of macromolecules containing the dithioester or the thioamide functional groups was also studied. We show that the copolymerisation method is more convenient for the synthesis of pure materials containing a variable amount of definite cluster units. The potentiality of these new organometallic polymers with regard to homogeneous catalysis and for the modification of electrode surfaces is also discussed.     

过渡金属络合物催化合成联萘酚

使用CuCl 四甲基乙二胺络合物催化β 萘酚氧化偶联,联萘酚产率可达80%以上。考察了金属盐种类、配体用量对联萘酚产率的影响。当采用手性配体L 尼古丁或(1S,2R) 盐酸麻黄碱时,联萘酚产率中等,光学收率较低。     

Physico-chemical studies on the interaction of transition metal ions with Schiff bases: polarographic study of copper(II) complex of salicylaldehyde-serine

Copper(II) complex of Schiff base derived from salicylaldehyde-serine has been studied by polarographic method of analysis at 30° and at constant ionic strength in aqueous medium. The plots of i_dvs √h and i_dvsC were linear and passed through the origin, showing that the reduction process was diffusion controlled. The plots of log (i/i_d?ivsE_de were linear in all concentrations of Schiff base but the slope showed that the reduction process was irreversible. The kinetic parameters were determined by Koutecky's method. The composition and stability constant were determined by DeFord and Hume's method.     

Reactions of late transition metal complexes with molecular oxygen

Limited natural resources, high energy consumption, economic considerations, and environmental concerns demand that we develop new technologies for the sustainable production of chemicals and fuels. New methods that combine the selective activation of C-H bonds of hydrocarbons with oxidation by a green oxidant such as molecular oxygen would represent huge advances toward this goal. The spectacular selectivity of transition metals in cleaving C-H bonds offers the potential for the direct use of hydrocarbons in the production of value-added organics such as alcohols. However, the use of oxygen, which is abundant, environmentally benign, and inexpensive (particularly from air), has proven challenging, and more expensive and less green oxidants are often employed in transition-metal-catalyzed reactions. Advances in the use of oxygen as an oxidant in transition-metal-catalyzed transformations of hydrocarbons will require a better understanding of how oxygen reacts with transition metal alkyl and hydride complexes. For alkane oxidations, researchers will need to comprehend and predict how metals that have shown particularly high activity and selectivity in C-H bond activation (e.g. Pt, Pd, Rh, Ir) will react with oxygen. In this Account, we present our studies of reactions of late metal alkyls and hydrides with molecular oxygen, emphasizing the mechanistic insights that have emerged from this work. Our studies have unraveled some of the general mechanistic features of how molecular oxygen inserts into late metal hydride and alkyl bonds along with a nascent understanding of the scope and limitations of these reactions. We present examples of the formation of metal hydroperoxide species M-OOH by insertion of dioxygen into Pt(IV)-H and Pd(II)-H bonds and show evidence that these reactions proceed by radical chain and hydrogen abstraction pathways, respectively. Comparisons with recent reports of insertion of oxygen into other Pd(II)-H complexes, and also into Ir(III)-H and Rh(III)-H complexes, point to potentially general mechanisms for this type of reaction. Additionally, we observed oxygen-promoted C-H and H-H reductive elimination reactions from five-coordinate Ir(III) alkyl hydride and dihydride complexes, respectively. Further, when Pd(II)Me(2) and Pt(II)Me(2) complexes were exposed to oxygen, insertion processes generated M-OOMe complexes. Mechanistic studies for these reactions are consistent with radical chain homolytic substitution pathways involving five-coordinate M(III) intermediates. Due to the remarkable ability of Pt(II) and Pd(II) to activate the C-H bonds of hydrocarbons (RH) and form M-R species, this reactivity is especially exciting for the development of partial alkane-oxidation processes that utilize molecular oxygen. Our understanding of how late transition metal alkyls and hydrides react with molecular oxygen is growing rapidly and will soon approach our knowledge of how other small molecules such as olefins and carbon monoxide react with these species. Just as advances in understanding olefin and CO insertion reactions have shaped important industrial processes, key insight into oxygen insertion should lead to significant gains in sustainable commercial selective oxidation catalysis.     

Polymerization of N-substituted 2-propynamides with transition metal catalysts

Summary Polymerization of N-substituted 2-propynamides proceeded in the presence of Pd(II) catalysts such as [Pd(CH₃CN)₄](BF₄)₂, PdCl₂(PhCN)₂-n-BuLi, and PdCl₂(nbd)-n-BuLi. The molecular weights of the obtained polymers ranged 8,500 to 14,000, depending on the catalysts. From the ¹H NMR spectra, these polymers were found to have alternating double bonds in the main chain. Received: 25 December 2000/Revised version: 17 January 2001/Accepted: 19 January 2001     

Sintered metal fibers coated with transition metal oxides as structured catalysts for hydrogen peroxide decomposition

This study aimed at the development of active and stable structured catalyst for the decomposition of hydrogen peroxide under industrial conditions. Sintered metal fibers of stainless steel (SMF_SS) in the form of flat panels were surface coated by different catalytic metal oxides. The oxide coating has been carried out by incipient wetness impregnation of SMF_SS with suitable precursors followed by calcination in air at temperatures between 500 and 600 °C. The coating of SMF_SS renders a very thin homogeneous oxide layer of less than 1 μm covering the individual fibers and ensures an optimal accessibility of the treated H₂O₂ solution to the catalytically active phase without any mass-transfer limitations. In the decomposition of hydrogen peroxide, the 5 wt.% MnO_x/5 wt.% (Al₂O₃ + MgO)/SMF_SS was found to be the most active and stable catalyst in a range of pH = 4–7 at 25 °C exhibiting a significant first order rate constant of 5.2 min⁻¹ at pH 7. The same catalyst was tested in the back end of the epoxidation of soybean oil process demonstrating its suitability for industrial application.     

Kinetics study of Ziegler-Natta catalyst based on TiCl3 modified by Lewis bases

Summary The kinetic in propylene polymerization of catalysts containing di-n-butyl ether (DBE) - Cat.A or di-n-butyl ether and ethyl benzoate (EB) - Cat. B used as internal electron-donor compounds was investigated. Comparing catalyst compositions with performances, one could conclude that the different kinetics observed were due to the different Lewis bases employed during catalyst synthesis. It seems that the electron-donors have modified the catalyst sites. It has found that added ethyl benzoate modified the structure of the active sites and increased the polymerization rate constant.     

过渡金属磷化物催化剂综述

过渡金属磷化物催化剂具有特殊的晶体结构,在催化加氢脱硫、加氢脱氮及电化学制氢反应中具有优异的催化活性。主要简述过渡金属磷化物催化剂的结构、制备方法和应用。多金属、复合多功能型过渡金属磷化物催化剂将在催化制氢反应中受到更多的关注。     

Zeolite-supported transition metal catalysts by design

Zeolite supported metal Catalysts are prepared by ion exchange or incipient wetness impregnation, or by deposition of metal complexes, followed by calcination and reduction. Hydrogen reduction of encaged transition metal ions yields metal dusters or isolated atoms and protons of high Brønsted acidity. The elementary steps in the genesis of mono- and bimetallic metal dusters have been unravelled by a combination of dynamic and spectroscopic methods. Small metal clusters can be anchored to zeolite cage walls by transition metal ions or protons; however, adsorption of a CO which can displace the protons from the metal, initiates migration of the primary carbonyl dusters leading to their coalescence. At low temperature, this process is limited by the geometry of the metal core vis-à-vis the cage window. Protons of high Brensted acidity can reoxidize metals at elevated temperature, even selectively “leach” the less noble metal atoms out of bimetal clusters. Large metal particles, present in zeolite voids or at the external surface, can be transformed into small particles, exposing virtually all their metal atoms, by either of two novel in situ techniques which are based on oxidative dispersion with oxygen or chlorine gas.     

Thermoelectric properties of layered oxyselenides with 3d transition metal ions

Layered oxychalcogenides have received extensive attention in the fields of magnetism, superconductivity, lithium battery, and luminescence due to their unique electronic, magnetic properties, and layered structure, among which layered oxyselenides have excellent and promising thermoelectric performance, such as BiCuSeO and Bi₂LnO₄Cu₂Se₂. Here, we successfully synthesized Sr₂MO₂Cu₂Se₂ (M = Co, Ni, Zn) and Sr₂FeO₃CuSe and investigated the thermoelectric properties at a wide temperature range (298–923 K). They have a relatively high Seebeck coefficient (>300 μV K⁻¹) in medium to high temperature range and possess a low thermal conductivity. The power factor and ZT reach 65 μW m⁻¹ K⁻¹ and 0.07 at 923 K for intrinsic Sr₂NiO₂Cu₂Se₂, and a higher performance is expected to be achieved by strategies like carrier concentration optimization and band structure engineering.     

Recent transition metal catalysts for syndiotactic polystyrene

Syndiotactic polystyrene has attracted much interest in scientific and industrial research after its first synthesis in 1985 and has led to a fast commercialization of this polymer. The catalyst systems used for this coordination polymerization of styrene are a key point in this development to provide high polymerization activities and syndiotacticities of the polymers obtained.This literature review gives a comprehensive overview on the recent transition metal catalysts comprising the literature since about 2000 and especially on the transition metal complexes investigated in the syndiospecific homopolymerization of styrene. It includes the polymerization activity of the catalysts, the syndiotacticity of the polymers received as well as the discussion of the relationships between catalyst structure and polymerization activity. The complex-coordination mechanism of the syndiospecific polymerization of styrene is summarized in general at the beginning.The review of the recent transition metal catalysts for the syndiospecific styrene polymerization includes transition metal complexes, cocatalysts (methylaluminoxanes and boron compounds), activators and chain transfer agents, and supported and heterogenized catalysts. Transition metal complexes contain group 4 transition metal complexes (mono- and bis-cyclopentadienyl complexes, metal complexes of other ring systems such as indenyl, fluorenyl and other complexes, di- and multi-nuclear complexes, and non-metallocene complexes), and metal complexes of other transition metals (groups 8–10, rare earth metals, and others). The chapter on mono-cyclopentadienyl complexes demonstrates an overview on the investigations using unsubstituted cyclopentadienyl and pentamethylcyclopentadienyl complexes, on the influence of the variation of the structure of the cyclopentadienyl ligand, and on the effect of the variation of ancillary complex ligands besides cyclopentadienyl.This summary also considers recent developments in the preparation of new transition metal complexes based on the synthesis of completely novel π-ligands of the half-metallocenes, the success in attaining high syndiospecificities with transition metal complexes based on rare earth metals, the coordination polymerization in aqueous systems, the syndiospecific living polymerization, and new activators for the catalysts, with regard to syndiotactic polystyrenes.     

Oxygen evolution reaction on transition metal borides

The oxygen evolution reaction on the transition metal borides (mainly Co_xB and Ni_xB) was investigated with various composition ratio of BMetal and sintering temperature. The activity of oxygen evolution reaction became maximum at the composition range of BCo =$\text{1}\text{3}$ and BNi ? $\text{1}\text{3}$, respectively. The sintering temperature also affected largely the activity of oxygen evolution reaction and the metal borides with the composition ratio BMetal = $\text{1}\text{3}$ gave the highest activity at the sintering temperature lower than 300°C for nickel borides and 400 ～ 500°C for cobalt borides. The oxygen evolution reaction on iron boride and lanthanum hexaboride was also briefly discussed, and the order of the activity of oxygen evolution reaction was as follows:

The Tafel slope of oxygen evolution reaction on metal borides was varied from ca. 40 mV for Ni (ca 70 mV for Co) to ca. 120 mV with the increase in the activity of oxygen evolution reaction. The oxide formation became easier with the increase in the activity of oxygen evolution reaction and the differential capacity of the oxide formation in the range more cathodic than the oxygen evolution increased with the decrease in the apparent activation energy of oxygen evolution reaction at 0.60 V (HgHgO).     

Nanocrystalline multicomponent entropy stabilised transition metal oxides

Multicomponent entropy stabilised oxides containing four and five metal elements in equiatomic amounts were successfully synthesised in nanocrystalline form by nebulised spray pyrolysis (NSP), flame spray pyrolysis (FSP) and reverse co-precipitation (RCP) techniques, demonstrating that entropy stabilisation of these recently discovered materials is independent of the synthesis method. Both 4- and 5-cationic systems, (Co,Mg,Ni,Zn)O and (Co,Cu,Mg,Ni,Zn)O, can be stabilised into a single rocksalt structure directly only using NSP, while in FSP and RCP, stabilisation can be achieved after thermal treatment. This result indicates, that in 5-cationic NSP system configurational entropy is high enough to directly stabilise single rocksalt phase at lower temperature, while higher synthesis temperature is required to compensate the lower configurational entropy in 4-cationic system. Retention of single-phase at room temperature indicates sluggish diffusion kinetics, making entropy stabilised phases quenchable.     

Protein‐transition metal ion networks

Proteins obtained from agricultural sources were blended with divalent metal ions to see if binding reactions occurred between protein chains. Feather keratin, egg albumin, and wheat gluten showed elastic modulus increases of 2–3 times with addition of divalent transition metal ions Cu²⁺ and Zn²⁺. Increasing concentrations of ions resulted in increased stiffness. Birefringence experiments performed concurrently with tensile experiments showed refractive index changes indicative of network formation. Binding divalent alkaline earth metal Ca²⁺ ions did not result in an elastic modulus increase. Addition of Zn²⁺ to egg albumin resulted in a 34% decrease in water permeability but no change in oxygen permeability. FTIR spectroscopy showed that the directed valence of the transition metals was primarily binding glycerol and amide sites on the protein and secondarily carbonyl sites on the protein. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007