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Showing papers on "Catalysis published in 2000"


Journal ArticleDOI
TL;DR: The finding that the amino acid proline is an effective asymmetric catalyst for the direct aldol reaction between unmodified acetone and a variety of aldehydes is reported.
Abstract: Most enzymatic transformations have a synthetic counterpart. Often though, the mechanisms by which natural and synthetic catalysts operate differ markedly. The catalytic asymmetric aldol reaction as a fundamental C-C bond forming reaction in chemistry and biology is an interesting case in this respect. Chemically, this reaction is dominated by approaches that utilize preformed enolate equivalents in combination with a chiral catalyst.1 Typically, a metal is involved in the reaction mechanism.1d Most enzymes, however, use a fundamentally different strategy and catalyze the direct aldolization of two unmodified carbonyl compounds. Class I aldolases utilize an enamine based mechanism,2 while Class II aldolases mediate this process by using a zinc cofactor.3 The development of aldolase antibodies that use an enamine mechanism and accept hydrophobic organic substrates has demonstrated the potential inherent in amine-catalyzed asymmetric aldol reactions.4 Recently, the first small-molecule asymmetric class II aldolase mimics have been described in the form of zinc, lanthanum, and barium complexes.5,6 However, amine-based asymmetric class I aldolase mimics have not been described in the literature.7 Here we report our finding that the amino acid proline is an effective asymmetric catalyst for the direct aldol reaction between unmodified acetone and a variety of aldehydes. Recently we developed broad scope aldolase antibodies that show very high enantioselectivities, have enzymatic rate accelerations, and use the enamine mechanism of class I aldolases.4 During the course of these studies, we found that one of our aldolase catalytic antibodies (Aldolase Antibody 38C2, Aldrich) is an efficient catalyst for enantiogroup-differentiating aldol cyclodehydrations of 2,6-heptanediones to give cyclohexenones, including the Wieland-Miescher ketone.8,9 These intramolecular reactions are also catalyzed by proline (Hajos-Eder-Sauer-Wiechert reaction)10 and it has been postulated that they proceed via an enamine mechanism.11 However, the proline-catalyzed direct intermolecular asymmetric aldol reaction has not been described. Further, there are no asymmetric small-molecule aldol catalysts that use an enamine mechanism.7 Based on our own results and Shibasaki’s work on lanthanum-based small-molecule aldol catalysts,4,6 we realized the great potential of catalysts for the direct asymmetric aldol reaction. We initially studied the reaction of acetone with 4-nitrobenzaldehyde. Reacting proline (30 mol %) in DMSO/acetone (4:1) with 4-nitrobenzaldehyde at room temperature for 4 h furnished aldol product (R)-1 in 68% yield and 76% ee (eq 1). This result

2,283 citations


Journal ArticleDOI
TL;DR: In this article, the crystal structure of Ru complex 5, bearing a 1,3dimesityl-4,5-dihydroimidazol-2-ylidene and styrenyl ether ligand is disclosed.
Abstract: Several highly active, recoverable and recyclable Ru-based metathesis catalysts are presented. The crystal structure of Ru complex 5, bearing a 1,3-dimesityl-4,5-dihydroimidazol-2-ylidene and styrenyl ether ligand is disclosed. The heterocyclic ligand significantly enhances the catalytic activity, and the styrenyl ether allows for the easy recovery of the Ru complex. Catalyst 5 promotes ring-closing metathesis (RCM) and the efficient formation of various trisubstituted olefins at ambient temperature in high yield within 2 h; the catalyst is obtained in >95% yield after silica gel chromatography and can be used directly in subsequent reactions. Tetrasubstituted olefins can also be synthesized by RCM reactions catalyzed by 5. In addition, the synthesis and catalytic activities of two dendritic and recyclable Ru-based complexes are disclosed (32 and 33). Examples involving catalytic ring-closing, ring-opening, and cross metatheses are presented where, unlike monomer 5, dendritic 33 can be readily recovered.

1,748 citations


Journal ArticleDOI
TL;DR: A search for a practical method for the kinetic resolution reaction led to the discovery of highly enantiomer-selective hydrolytic ring-opening using the corresponding (salen)Co(III) catalyst, which displays extraordinary substrate generality, and allows practical access to enantiopure terminal epoxides on both laboratory and industrial scales.
Abstract: The discovery of the metal salen-catalyzed asymmetric ring-opening (ARO) of epoxides is chronicled. A screening approach was adopted for the identification of catalysts for the addition of TMSN3 to meso-epoxides, and the chiral (salen)CrN3 complex was identified as optimal. Kinetic and structural studies served to elucidate the mechanism of catalysis, which involves cooperative activation of both epoxide and azide by two different metal centers. Covalently linked bimetallic complexes were constructed on the basis of this insight, and shown to catalyze the ARO with identical enantioselectivity but 1−2 orders of magnitude greater reactivity than the monomeric analogues. Extraordinarily high selectivity is observed in the kinetic resolution of terminal epoxides using the (salen)CrN3/TMSN3 system. A search for a practical method for the kinetic resolution reaction led to the discovery of highly enantiomer-selective hydrolytic ring-opening using the corresponding (salen)CoIII catalyst. This system displays ext...

1,064 citations


Journal ArticleDOI
03 Mar 2000-Science
TL;DR: A water-soluble palladium(II) bathophenanthroline complex is a stable recyclable catalyst for the selective aerobic oxidation of a wide range of alcohols to aldehydes, ketones, and carboxylic acids in a biphasic water-alcohol system.
Abstract: Alcohol oxidations are typically performed with stoichiometric reagents that generate heavy-metal waste and are usually run in chlorinated solvents. A water-soluble palladium(II) bathophenanthroline complex is a stable recyclable catalyst for the selective aerobic oxidation of a wide range of alcohols to aldehydes, ketones, and carboxylic acids in a biphasic water-alcohol system. The use of water as a solvent and air as the oxidant makes the reaction interesting from both an economic and environmental point of view.

1,024 citations



Journal ArticleDOI
01 Jan 2000-Carbon
TL;DR: A series of activated carbons with different degrees of activation were oxidized with H2O2, (NH4)2S2O8 and HNO3 in order to introduce different oxygen surface complexes as discussed by the authors.

833 citations


Journal ArticleDOI
25 Feb 2000-Science
TL;DR: The results provide atomic-scale verification of a general mechanism originally proposed by Mars and van Krevelen in 1954 and are likely to be of general relevance for the mechanism of catalytic reactions at oxide surfaces.
Abstract: The structure of RuO(2)(110) and the mechanism for catalytic carbon monoxide oxidation on this surface were studied by low-energy electron diffraction, scanning tunneling microscopy, and density-functional calculations. The RuO(2)(110) surface exposes bridging oxygen atoms and ruthenium atoms not capped by oxygen. The latter act as coordinatively unsaturated sites-a hypothesis introduced long ago to account for the catalytic activity of oxide surfaces-onto which carbon monoxide can chemisorb and from where it can react with neighboring lattice-oxygen to carbon dioxide. Under steady-state conditions, the consumed lattice-oxygen is continuously restored by oxygen uptake from the gas phase. The results provide atomic-scale verification of a general mechanism originally proposed by Mars and van Krevelen in 1954 and are likely to be of general relevance for the mechanism of catalytic reactions at oxide surfaces.

801 citations


Journal ArticleDOI
TL;DR: In this paper, the authors report on the activity of Cu-and Ni-containing cerium oxide catalysts for low-temperature water-gas shift (WGS) in nanocrystalline form by urea co-precipitation-gelation method.
Abstract: In this paper we report on the activity of Cu- and Ni-containing cerium oxide catalysts for low-temperature water-gas shift (WGS). Bulk catalysts were prepared in nanocrystalline form by the urea co-precipitation–gelation method. Lanthanum dopant (10 at.%) was used as a structural stabilizer of ceria, while the content of Cu or Ni was in the range of 5–15 at.% (2–8 wt.%). At low metal loadings, Cu or Ni were present in the form of highly dispersed oxide clusters, while at high loadings, clusters as well as particles of CuO or NiO (>10 nm in size) were present on ceria. Both Cu and Ni increased the reducibility of ceria, as evidenced by H2-TPR experiments. The WGS reaction activity of Ce(La)Ox was increased significantly by addition of a small amount (2 wt.%) of Cu or Ni. The catalysts were not activated prior to testing. Steady-state WGS kinetics were measured over the temperature range of 175–300 and 250–300°C, respectively, for Cu- and Ni–Ce(La)Ox. The activation energy of the reaction over Ce(La)Ox was 58.5 kJ/mol, while it was 38.2 and 30.4 kJ/mol, respectively, over the 5 at.% Ni–Ce(La)Ox and 5 at.% Cu–Ce(La)Ox catalysts in CO-rich conditions. A co-operative redox reaction mechanism, involving oxidation of CO adsorbed on the metal cluster by oxygen supplied to the metal interface by ceria, followed by H2O capping the oxygen vacancy on ceria, was used to fit the kinetics. Parametric studies were mainly performed with the 5 at.% Cu–(La)Ox catalyst. Notably, this material requires no activation and retains high WGS activity and stability at temperatures up to 600°C.

800 citations


Journal ArticleDOI
TL;DR: In this paper, the authors showed that the conversion of triglycerides (TG), diglycerides (DG), and monoglycerides (MG) appeared to be second order up to 30 min of reaction time.
Abstract: Methyl esters were produced by transesterification of palm oil with methanol in the presence of a catalyst (KOH). The rate of transesterification in a batch reactor increased with temperature up to 60°C. Higher temperatures did not reduce the time to reach maximal conversion. The conversion of triglycerides (TG), diglycerides (DG), and monoglycerides (MG) appeared to be second order up to 30 min of reaction time. Reaction rate constants for TG, DG, and MG hydrolysis reactions were 0.018–0.191 (wt%·min)−1, and were higher at higher temperatures and higher for the MG reaction than for TG hydrolysis. Activation energies were 14.7, 14.2, and 6.4 kcal/mol for the TG, DG, and MG hydrolysis reactions, respectively. The optimal catalyst concentration was 1% KOH.

746 citations


Journal ArticleDOI
TL;DR: Nanosized palladium colloids are involved in the catalysis of phosphane-free Heck and Suzuki reactions with simple palladium salts such as PdCl(2) or Pd(OAc)(2), as demonstrated by transmission electron microscopic investigations.
Abstract: Nanosized palladium colloids, generated in situ by reduction of PdII to Pd0 [Eq. (a)], are involved in the catalysis of phosphane-free Heck and Suzuki reactions with simple palladium salts such as PdCl2 or Pd(OAc)2, as demonstrated by transmission electron microscopic investigations.

738 citations


Journal ArticleDOI
17 Mar 2000-Science
TL;DR: Efficient electrophilic metalation of aromatic C-H bonds leading to new C-C bond formation through regio- and stereoselective addition to alkynes and alkenes has been realized by a catalytic amount of palladium( II) or platinum(II) compounds in a mixed solvent containing trifluoroacetic acid at room temperature.
Abstract: Efficient electrophilic metalation of aromatic C-H bonds leading to new C-C bond formation through regio- and stereoselective addition to alkynes and alkenes has been realized by a catalytic amount (0.02 to 5 mole percent) of palladium(II) or platinum(II) compounds in a mixed solvent containing trifluoroacetic acid at room temperature. Various arenes undergo unexpected selective trans hydroarylation to terminal or internal CcC bonds inter- and intramolecularly with high efficiency (up to a turnover number of 4500 for palladium), especially for electron-rich arenes, giving thermodynamically unfavorable cis-alkenes, and the oxygen- and nitrogen-containing heterocycles. The simplicity, generality, and efficiency of this process should be very attractive to the possible industrial application for the functionalization of arenes.

Journal ArticleDOI
TL;DR: The ternary system consisting of [RuCl2(η6-benzene)]2, N-tosylethylenediamine or ethanolamine, and KOH (Ru:amine:KOH = 1:1:2 molar ratio) catalyzes reversible hydrogen transfer between alcohols and carbonyl compounds.
Abstract: The ternary system consisting of [RuCl2(η6-benzene)]2, N-tosylethylenediamine or ethanolamine, and KOH (Ru:amine:KOH = 1:1:2 molar ratio) catalyzes reversible hydrogen transfer between alcohols and carbonyl compounds. The use of chiral amine auxiliaries effects asymmetric transformation. The theoretical calculations using methanol/formaldehyde transformation as the model indicates the operation of a novel metal−ligand bifunctional catalysis, which is contrary to currently accepted putative pathways. The results reveal that: (1) KOH is necessary for the generation of a formal 16-electron Ru complex, Ru(NHCH2CH2Y)(η6-benzene) (Y = O or NH) (catalyst), from an 18-electron Ru chloride, RuCl(NH2CH2CH2Y)(η6-benzene) (precatalyst), by a Dcb elimination of HCl, and not for increasing alkoxide concentration; (2) Ru alkoxides do not intervene in transfer hydrogenation; (3) the Ru alkoxide, even if formed, serves merely as a reservoir of the 16-electron catalyst; (4) the key 18-electron Ru hydride, RuH(NH2CH2CH2Y)(...

Journal ArticleDOI
TL;DR: These ligands are effective for almost all substrate combinations that have been previously reported with various other ligands, and they represent the most generally effective catalyst system reported to date.
Abstract: Palladium complexes supported by (o-biphenyl)P(t-Bu)2 (3) or (o-biphenyl)PCy2 (4) are efficient catalysts for the catalytic amination of a wide variety of aryl halides and triflates. Use of ligand 3 allows for the room-temperature catalytic amination of many aryl chloride, bromide, and triflate substrates, while ligand 4 is effective for the amination of functionalized substrates or reactions of acyclic secondary amines. The catalysts perform well for a large number of different substrate combinations at 80−110 °C, including chloropyridines and functionalized aryl halides and triflates using 0.5−1.0 mol % Pd; some reactions proceed efficiently at low catalyst levels (0.05 mol % Pd). These ligands are effective for almost all substrate combinations that have been previously reported with various other ligands, and they represent the most generally effective catalyst system reported to date. Ligands 3 and 4 are air-stable, crystalline solids that are commercially available. Their effectiveness is believed t...

Journal ArticleDOI
TL;DR: In this paper, a safe, simple, compact process generates high-purity hydrogen gas on demand from base-stabilized, aqueous solutions of NaBH4, by using a ruthenium, Ru, catalyst.

Journal ArticleDOI
TL;DR: The concept of atom efficiency is a useful tool for rapid evaluation of the amount of waste generated by alternative processes as mentioned in this paper, and the general theme of atom-efficient, catalytic processes is illustrated with industrially relevant examples.
Abstract: The key to waste minimization in fine chemicals manufacture is the widespread substitution of classical organic syntheses employing stoichiometric amounts of inorganic reagents with cleaner, catalytic alternatives. The E factors (by waste per kg product) of chemical processes increase dramatically on going downstream from bulk to fine chemicals and pharmaceuticals, mainly owing to the use of "stoichiometric" methods. The concept of atom efficiency is a useful tool for rapid evaluation of the amount of waste generated by alternative processes. The general theme of atom-efficient, catalytic processes is illustrated with industrially relevant examples. These include catalysis by solid acids and bases, catalytic reductions and oxidations, catalytic C-C bond formation, asymmetric catalysis, biocatalysis, and catalysis in novel media (aqueous and fluorous biphasic systems, supercritical fluids, and ionic liquids).

Journal ArticleDOI
TL;DR: In this paper, the deactivation of catalysts has been investigated in the presence of palladium and platinum catalysts under mild conditions (293 −353 K and atmospheric pressure) and several causes of deactivation have been put forward: oxidation of metal, blocking of active sites by strong adsorption of sideproducts, metal leaching and growth of platinum crystallites.

Journal ArticleDOI
01 Jun 2000-Energy
TL;DR: In this paper, rice husks were pyrolysed at 550°C with zeolite ZSM-5 catalyst upgrading of the pyrolysis vapours at catalyst temperatures of 400, 450, 500, 550, and 600°C.

Journal ArticleDOI
TL;DR: A review of the development of catalytic asymmetric hetero-Diels-Alder reactions of carbonyl compounds and imines can be found in this paper, where a number of different chiral catalysts have been used.
Abstract: Asymmetric catalysis is a challenge for chemists: How can we design catalysts to achieve the goal of forming optically active compounds? This review provides the reader with an overview of the development of catalytic asymmetric hetero-Diels–Alder reactions of carbonyl compounds and imines. Since its discovery, the Diels–Alder reaction has undergone intensive development and is of fundamental importance for synthetic, physical, and theoretical chemists. The Diels–Alder reaction has been through different stages of development, and at the beginning of the 21st century catalytic Diels–Alder reactions are one of the main areas of focus. The preparation of numerous compounds of importance for our society is based on cycloaddition reactions to carbonyl compounds and imines. There are several parallels between the reactions of carbonyl compounds and those of imines, which, however, begin to vanish on entering the field of catalytic reactions. Why? From a mechanistic point of view some similarities can be drawn, but the synthetic development of catalytic enantioselective hetero-Diels–Alder reactions of imines are several years behind those of the carbonyl compounds. For hetero-Diels–Alder reactions of carbonyl compounds there a number of different chiral catalysts, and great progress has been achieved in developing enantioselective reactions for unactivated and activated carbonyl compounds. In contrast the development of catalytic enantioselective hetero-Diels–Alder reactions of imines is in its infancy and only few catalytic reactions have been published. This review will focus on the most important developments, and discuss the synthetic and mechanistic aspects of enantioselective hetero-Diels–Alder reactions of carbonyl compounds catalyzed by chiral Lewis acids. For the hetero-Diels–Alder reactions of imines, the diastereoselective reactions of optically substrates catalyzed by Lewis acids will be presented first, followed by the catalytic enantioselective reactions.

Journal ArticleDOI
TL;DR: In this article, carbon-supported Pt-based binary alloy electrocatalysts (Pt-Co, Pt-Cr and Pt-Ni) were prepared by incipient wetness method to investigate the origin of the enhanced activity of the oxygen reduction reaction in fuel cells.

Journal ArticleDOI
TL;DR: In this paper, the authors showed that the formation of filamentous carbon was significantly influenced by the metal particle size and proceeded mostly over the metal particles larger than 7-nm, and the loss of catalytic activity at 973 K was mainly caused by coke deposition and sintering.
Abstract: CO2 reforming of CH4 was carried out over Ni–alumina aerogel catalysts prepared with various Ni loadings. The preparation of alumina supported Ni catalysts via sol–gel synthesis and subsequent supercritical drying led to the formation of very small metal particles, which are evenly distributed over the alumina support. The activity of the aerogel catalysts increased along with increasing metal loading, and eventually, the SAA25 (0.25 in Ni/Al mole ratio) catalyst exhibited the high activity comparable to that of a 5 wt.% Ru/alumina catalyst (ESCAT44, Engelhard). Compared to the alumina-supported Ni catalyst prepared by conventional impregnation method, Ni–alumina aerogel catalysts showed a remarkably low coking rate due to highly dispersed metal particles. From TEM micrograph studies, it was observed that the formation of filamentous carbon was significantly influenced by the metal particle size and proceeded mostly over the metal particles larger than 7 nm. The loss of catalytic activity at 973 K was mainly caused by coke deposition and sintering.

Journal ArticleDOI
TL;DR: In this paper, a simple, convenient, and safe chemical process generates high purity hydrogen gas on demand from stable, aqueous solutions of sodium borohydride, NaBH4, and ruthenium-based (Ru), catalyst.

Journal ArticleDOI
TL;DR: In this paper, a perovskite type La1−−xSrxMnO3 (x = 0-0.5) oxides were prepared by the amorphous citrate process, characterised by X-ray diffraction, oxygen desorption, temperature-programmed reduction, infrared and Xray photoelectron spectroscopic techniques, and tested for methane combustion within the 473 −1073 −K temperature range.
Abstract: Perovskite type La1 − xSrxMnO3 (x = 0–0.5) oxides were prepared by the amorphous citrate process, characterised by X-ray diffraction, oxygen desorption, temperature-programmed reduction, infrared and X-ray photoelectron spectroscopic techniques, and tested for methane combustion within the 473–1073 K temperature range. Since catalyst activity was found to depend strongly on BET areas and to a lesser extent, on the degree of substitution (x), intrinsic activities were computed for La1 − xSrxMnO3 catalyst series. Among the compositions investigated, the degree of substitution x = 0.2 showed the highest intrinsic activity within the temperatures explored. Characterisation techniques made possible to correlate catalytic performance with the structural characteristics of the oxides. The stability of Mn4+ is probably the most important parameter, but excess of oxygen and atomic surface composition should also be taken into account.

Journal ArticleDOI
TL;DR: In this paper, a fraction of bio-oil, generated from fast pyrolysis, was catalytically steam reformed at 825 and 875°C, high space velocity (up to 126,000h−1) and low residence time (26 ms) using a fixed-bed micro-reactor interfaced with a molecular beam mass spectrometer.
Abstract: Catalytic steam reforming of condensable vapors (i.e. bio-oils) derived from pyrolysis of biomass is a technically viable process for hydrogen production. In this study the aqueous fraction of bio-oil, generated from fast pyrolysis, was catalytically steam reformed at 825 and 875°C, high space velocity (up to 126,000 h−1) and low residence time (26 ms). Using a fixed-bed micro-reactor interfaced with a molecular beam mass spectrometer (MBMS), a variety of research and commercial nickel-based catalysts were tested. The catalysts were prepared by impregnation of an α-Al2O3 support with nickel and additives. Since the main constraint in reforming bio-oils is catalyst deactivation caused by carbon deposition, two strategies were applied to improve the performance of the catalysts. The first approach aimed at enhancing steam adsorption to facilitate the partial oxidation, i.e. gasification of coke precursors. The second one attempted to slow down the surface reactions leading to the formation of the coke precursors due to cracking, deoxygenation, and dehydration of adsorbed intermediates. Magnesium and lanthanum were used as support modifiers to enhance steam adsorption while cobalt and chromium additives were applied to reduce coke formation reactions. The cobalt-promoted nickel and chromium-promoted nickel supported on MgO-La2O3-α-Al2O3 catalysts showed the best results in the laboratory tests. At the reaction conditions progressive catalyst deactivation was observed leading to a decrease in the yields of hydrogen and carbon dioxide and an increase in carbon monoxide. The loss of activity also resulted in the formation of higher amounts of methane, benzene and other aromatic compounds. Commercial catalysts that were developed for steam reforming of natural gas and crude oil fractions proved to be more efficient for hydrogen production from bio-oil than most of the research catalysts mainly due to the higher water–gas shift activity.


Journal ArticleDOI
24 Mar 2000-Langmuir
TL;DR: Carbon aerogels and Cr-, Fe-, Co-, and Ni-containing carbon aerogel were obtained by pyrolysis, at temperatures between 500 and 1800 °C, of the corresponding carbon-airogels prepared by the sol−gel metho...
Abstract: Carbon aerogels and Cr-, Fe-, Co-, and Ni-containing carbon aerogels were obtained by pyrolysis, at temperatures between 500 and 1800 °C, of the corresponding aerogels prepared by the sol−gel metho...

Journal ArticleDOI
TL;DR: Mixtures of Pd(2)(dba)(3) or PD(OAc)(2) and BINAP catalyze the cross-coupling of amines with a variety of aryl bromides, and certain classes of secondary amines are also effectively transformed.
Abstract: Mixtures of Pd2(dba)3 or Pd(OAc)2 and BINAP catalyze the cross-coupling of amines with a variety of aryl bromides. Primary amines are arylated in high yield, and certain classes of secondary amines are also effectively transformed. The process tolerates the presence of several functional groups including methyl and ethyl esters, enolizable ketones, and nitro groups provided that cesium carbonate is employed as the base. Most reactions proceed to completion with 0.5−1.0 mol % of the palladium catalyst; in some cases, catalyst levels as low as 0.05 mol % Pd may be employed. Reactions are considerably faster if Pd(OAc)2 is employed as the precatalyst, and the order in which reagents are added to the reaction has a substantial effect on reaction rate. It is likely that the catalytic process proceeds via bis(phosphine)palladium complexes as intermediates. These complexes are less prone to undergo undesirable side reactions which lead to diminished yields or catalyst deactivation than complexes of the correspon...

Journal ArticleDOI
TL;DR: In this paper, achiral and chiral C3-symmetric complexes LMOR have been synthesized and employed in the ring-opening polymerization of l-, rac-, and meso-lactide in CH2Cl2 at 25 °C and below.
Abstract: Single-site achiral and chiral C3-symmetric complexes LMOR, where M = Mg and Zn, L = an η3-trispyrazolyl- or η3-trisindazolyl-borate ligand and R = Et, tBu, Ph, or SiMe3, have been synthesized and employed in the ring-opening polymerization of l-, rac-, and meso-lactide in CH2Cl2 at 25 °C and below. The polymerization occurs by acyl cleavage and gives rise to polylactide, PLA, with PDI of 1.1−1.25 up to 90% conversion. Studies of the kinetics of polymerization reveal first order behavior in both lactide and metal catalyst. For L = tris(3-tert-butylpyrazolyl)borate, (tBupz)3BH, polymerization of ∼500 equiv of l-lactide proceeds to 90% conversion within 1 h and 6 d for the magnesium and zinc catalysts, respectively. The zinc complexes are, however, more tolerant to air and moisture and solid samples where R = SiMe3 are persistent in air for several days. The rate of polymerization is also significantly influenced by the nature of the η3-L spectator ligand. Chiral C3-symmetric catalysts, where L = tris(indaz...

Journal ArticleDOI
TL;DR: Palladium nanoparticles stabilized by poly(N-vinyl-2-pyrrolidone) (PVP) are efficient catalysts for the Suzuki reactions in aqueous medium, suggesting that the catalytic reaction occurs on the surface of the Pd nanoparticles.


Journal ArticleDOI
TL;DR: In this paper, the second-order rate constants for sulfide oxidations by peroxymonocarbonate ion (HCO4-) are ∼300-fold greater than those for H2O2, and this increase is consistent with expectations based on expectations.
Abstract: Bicarbonate ion is an effective activator for hydrogen peroxide in the oxidation of sulfides. Kinetic and spectroscopic results support the formation of peroxymonocarbonate ion (HCO4-) as the oxidant in the catalytic reactions. The reaction of hydrogen peroxide and bicarbonate to form HCO4- occurs rapidly at 25 °C (t1/2 ≈ 300 s) near neutral pH in aqueous solution and alcohol/water mixtures, and an equilibrium analysis of the reaction by 13C NMR leads to an estimate of the electrode potential for the HCO4-/HCO3- couple (1.8 V vs NHE). Solubility of the bicarbonate catalyst is enhanced by the use of NH4HCO3 rather than by the use of group 1 salts, which tend to have lower solubility in the mixed solvents and can lead to phase separation. Rate laws and mechanistic analyses are presented for the oxidation of ethylphenylsulfide and related sulfides. The second-order rate constants for sulfide oxidations by HCO4- are ∼300-fold greater than those for H2O2, and this increase is consistent with expectations based...