Asymmetric catalysis by chiral lanthanide complexes in water

The development of catalytic, asymmetric transformations in water is a challenging task. The lanthanides are becoming reagents of choice for many Lewis acid-catalyzed reactions in aqueous media as they are water tolerant. However, enantioselective reactions catalyzed by lanthanides are difficult to achieve in water due to the instability of the reported catalysts. Herein we report the development of stable, well-defined chiral lanthanide complexes and their effectiveness in the asymmetric reduction of alpha-keto acids in aqueous solution. This is the first example of asymmetric reduction by a chiral lanthanide complex in water. Although modest ees are obtained (40-50%) the ytterbium complexes offer a unique advantage as they have the ability to monitor, direct from the reaction mixture, the % ee for the reaction, by 1H NMR, through a dipolar analysis of the observed paramagnetic shift.     

Upgrading of high-boiling fraction of bio-oil in supercritical methanol

In this work, the upgrading reactions of high-boiling fraction (HBF) of bio-oil were carried out over a series of supported mono- and bi-metallic catalysts under the supercritical methanol condition. During these reactions, esterification and cracking (alcoholysis and hydrocracking) were the two dominant processes. PtNi/MgO exhibited good performance, and gave a high yield (72.4 wt.%) of refined oil. The acid-base properties of the supports have an important effect on the coke deposition on the catalyst surface. The acidic catalysts gave the somewhat lower product yields, but tended to inhibit coking reaction. This would improve the life of the catalysts in the practical applications. The refined oil is believed to be a potential substitute or partial substitute for the fossil transportation fuel.     

Microstructure-stability relations studies of porous chitosan microspheres supported palladium catalysts

In this study, polyethylene glycol (PEG) with different molecular weight, polyvinyl pyrrolidone (PVP), and polyvinyl alcohol (PVA), are chosen as porogens for preparing chitosan base porous microsphere supported palladium catalyst for coupling reactions. The pore structure of the microspheres was controlled by the compatibility of chitosan and counterpart polymers. The prepared porous chitosan microspheres supported palladium heterogeneous catalysts have been evaluated using the well-established Ullmann reductive homocoupling and the Heck cross-coupling reactions. The activities, stabilities and recyclability of the porous chitosan microspheres supported palladium catalysts are not only highly dependent upon the surface areas of the solid supports, but also upon the chemical properties of the water-soluble polymers. The degradation of the prepared heterogeneous palladium catalysts is mainly caused by a combination of the palladium leaching and the morphological transformation of the palladium species from the amorphous into the crystals.     

Recyclable Merrifield resin‐supported organocatalysts containing pyrrolidine unit through A3‐coupling reaction linkage for asymmetric Michael addition

Merrifield resin‐supported pyrrolidine‐based chiral organocatalysts A ? D through A³‐coupling reaction linkage have been developed and found to be highly effective catalysts for the Michael addition reaction of ketones with nitrostyrenes. The reactions generated the corresponding products in good yields (up to 92%), excellent enantioselectivities (up to 98% ee), and high diastereoselectivities (up to 99:1 dr). In addition, the catalysts can be reused at least five times without a significant loss of catalytic activity and stereoselectivity. Chirality, 2010. © 2009 Wiley‐Liss, Inc.     

Unexpected efficiency of non-C(2) -symmetric bis(hydroxyamide)-based zinc-chelate catalysts

Asymmetric bis(hydroxyamide)-based zinc-chelate catalysts are able to promote the enantioselective addition of diethylzinc to benzaldehyde in the absence of titanium with yields and ees comparable, or inclusively superior, to their C(2) -symmetric analogues. This unexpected fact demonstrates that the previously established assumption on the necessity of using C(2) -symmetric bis(hydrdoxyamides) to generate C(2) -symmetric zinc-chelate catalysts can be discarded, which expand the possibilities for designing new ligands based on the interesting hydroxyl-amide functional grouping.     

Integrated biodiesel production: a comparison of different homogeneous catalysts systems

The most common catalysts for biodiesel production are homogeneous basic catalysts. In the present paper, a comparison is made of different basic catalysts (sodium methoxide, potassium methoxide, sodium hydroxide and potassium hydroxide) for methanolysis of sunflower oil. All the reactions were carried out under the same experimental conditions in a batch stirred reactor and the subsequent separation and purification stages in a decanter. The analytical methods included gas chromatography and the determination of fat and oil conventional parameters. The biodiesel purity was near 100 wt.% for all catalysts. However, near 100 wt.% biodiesel yields were only obtained with the methoxide catalysts. According to the material balance of the process, yield losses were due to triglyceride saponification and methyl ester dissolution in glycerol. Obtained biodiesel met the measured specifications, except for the iodine value, according to the German and EU draft standards. Although all the transesterification reactions were quite rapid and the biodiesel layers achieved nearly 100% methyl ester concentrations, the reactions using sodium hydroxide turned out the fastest.     

Synthesis of xylal- and arabinal-based crown ethers and their application as asymmetric phase transfer catalysts

New xylal- and arabinal-based monoaza-15-crown-5 ethers were synthesized starting from l - and d -xylose, and l - and d -arabinose, respectively. These monosaccharide-based chiral macrocycles were tested as phase transfer catalysts in a few asymmetric reactions. The xylal-based crown compounds proved to be efficient catalysts in a few liquid-liquid phase reactions. The epoxidation of trans-chalcone and the Darzens condensation of α-chloroacetophenone with benzaldehyde took place with complete diastereoselectivity and up to 77% ee and 58% ee, respectively. It was found that the substituents in the aromatic ring of the chalcone and the α-chloroacetophenone had an influence on the enantioselectivity. The highest ee values were obtained in the epoxidation of 4-chlorochalcone (81% ee) and in the reaction of a 2-naphthyl analogue (96% ee), while in the Darzens condensation of 4-phenyl-α-chloroacetophenone with benzaldehyde, a maximum ee of 91% was detected. The configuration of the monosaccharide unit in the crown ring influenced the absolute configuration of the epoxyketones synthesized.     

Asymmetric aldol reactions catalyzed by efficient and recyclable silica-supported proline-based peptides

A series of silica-supported proline-based peptides were synthesized and applied as catalysts for direct asymmetric intermolecular aldol reactions. Among these, a peptide with two L-proline units was found to be the most efficient one for the asymmetric aldol reactions between acetone and aromatic aldehydes. The reactions generated the corresponding products with satisfactory isolated yields (up to 97%) and enantiomeric excesses (up to 96%) in the presence of this catalyst (5 mol %). Furthermore, the silica-supported organocatalyst could be recovered and recycled by a simple filtration of the reaction solution and used for five consecutive trials without loss of its reactivity.     

Post Iron Decoration of Mesoporous Nitrogen‐Doped Carbon Spheres for Efficient Electrochemical Oxygen Reduction

Iron–nitrogen–carbon (Fe–N–C) catalysts are considered as the most promising nonprecious metal catalysts for oxygen reduction reactions (ORRs). Their synthesis generally involves complex pyrolysis reactions at high temperature, making it difficult to optimize their composition, pore structure, and active sites. This study reports a simple synthesis strategy by reacting preformed nitrogen‐doped carbon scaffolds with iron pentacarbonyl, a liquid precursor that can effectively form active sites with the nitrogen sites, enabling more effective control of the catalyst. The resultant catalyst possesses a well‐defined mesoporous structure, a high surface area, and optimized active sites. The catalysts exhibit high ORR activity comparable to that of Pt/C catalyst (40% Pt loading) in alkaline media, with excellent stability and methanol tolerance. The synthetic strategy can be extended to synthesize other metal–N–C catalysts.     

Performance of heterogeneous ZrO2 supported metaloxide catalysts for brown grease esterification and sulfur removal

In order to achieve a viable biodiesel industry, new catalyst technology is needed which can process a variety of less expensive waste oils, such as yellow grease and brown grease. However, for these catalysts to be effective for biodiesel production using these feedstocks, they must be able to tolerate higher concentrations of free fatty acids (FFA), water, and sulfur. We have developed a class of zirconia supported metaloxide catalysts that achieve high FAME yields through esterification of FFAs while simultaneously performing desulfurization and de-metallization functions. In fact, methanolysis, with the zirconia supported catalysts, was more effective for desulfurization than an acid washing process. In addition, using zirconia supported catalysts to convert waste grease, high in sulfur content, resulted in a FAME product that could meet the in-use ASTM diesel fuel sulfur specification (<500 ppm). Possible mechanisms of desulfurization and de-metallization by methanolysis were proposed to explain this activity.     

QSAR study of the enantiomeric excess in asymmetric catalytic reactions with topological indices and an artificial neural network

The relationships between the enantiomer excess of product in catalytic asymmetric reactions and the structures of the catalysts or reagents in several asymmetric reactions were studied using a backpropagation (BP) neural network with topological indices and their chiral expansions. The trained network can be used to screen new asymmetric catalysts, estimate catalytic effects, design reaction environments, and prove or improve the proposed reaction mechanism.     

Multi-catalysis reactions: new prospects and challenges of biotechnology to valorize lignin

Considerable effort has been dedicated to the chemical depolymerization of lignin, a biopolymer constituting a possible renewable source for aromatic value-added chemicals. However, these efforts yielded limited success up until now. Efficient lignin conversion might necessitate novel catalysts enabling new types of reactions. The use of multiple catalysts, including a combination of biocatalysts, might be necessary. New perspectives for the combination of bio- and inorganic catalysts in one-pot reactions are emerging, thanks to green chemistry-driven advances in enzyme engineering and immobilization and new chemical catalyst design. Such combinations could offer several advantages, especially by reducing time and yield losses associated with the isolation and purification of the reaction products, but also represent a big challenge since the optimal reaction conditions of bio- and chemical catalysis reactions are often different. This mini-review gives an overview of bio- and inorganic catalysts having the potential to be used in combination for lignin depolymerization. We also discuss key aspects to consider when combining these catalysts in one-pot reactions.     

Enzymatic resolution of alkyn-3-ols in non-aqueous medium

Summary Resolution of alkyn-3-ols has been achieved using a lipase from Candida rugosa to esterify the alcohols with trifluoroethyl butyrate in hexane to give the (S)-alcohols and the (R)-esters. Subsequent reacylation of the product alcohols and alcoholysis of the esters with 1-butanol furnished the (S)-alcohols with good (86–91% ees) and the (R)-butyrate with moderate enantiomeric purities (62–64% ees).     

Protein-assisted pericyclic reactions: an alternate hypothesis for the action of quantal receptors

The rules for allowable pericyclic reactions indicate that the photoisomerizations of retinals in rhodopsins can be formally analogous to thermally promoted Diels-Alder condensations of monoenes with retinols. With little change in the seven-transmembrane helical environment these latter reactions could mimic the retinal isomerization while providing highly sensitive chemical reception. In this way archaic progenitors of G-protein-coupled chemical quantal receptors such as those for pheromones might have been evolutionarily plagiarized from the photon quantal receptor, rhodopsin, or vice versa. We investigated whether the known structure of bacteriorhodopsin exhibited any similarity in its active site with those of the two known antibody catalysts of Diels-Alder reactions and that of the photoactive yellow protein. A remarkable three-dimensional motif of aromatic side chains emerged in all four proteins despite the drastic differences in backbone structure. Molecular orbital calculations supported the possibility of transient pericyclic reactions as part of the isomerization-signal transduction mechanisms in both bacteriorhodopsin and the photoactive yellow protein. It appears that reactions in all four of the proteins investigated may be biological analogs of the organic chemists' chiral auxiliary-aided Diels-Alder reactions. Thus the light receptor and the chemical receptor subfamilies of the heptahelical receptor family may have been unified at one time by underlying pericyclic chemistry.     

Graphite oxide-supported CaO catalysts for transesterification of soybean oil with methanol

Graphite oxide (GO) supported CaO catalysts were prepared and successfully applied to the transesterification of soybean oil with methanol. The supports and resultant catalysts were characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM), N(2) adsorption, thermogravimetry (TG), X-ray photoelectron spectroscopy (XPS), temperature-programed desorption (TPD) and Fourier-transform infrared spectroscopy (FT-IR). The GO supported CaO catalysts exhibited excellent catalytic activity and were easily regenerated by simple heat-treatment. The oxygen-containing groups (i.e., hydroxyl, epoxide groups and carboxyl groups) present on the surface of GO likely act as anchoring centers for CaO. This work demonstrates that graphite oxide is an effective host material of catalytically active CaO nanoparticles for the transesterification of soybean oil with methanol to produce biodiesel.     

Development of a supported tri-metallic catalyst and evaluation of the catalytic activity in biomass steam gasification

A supported tri-metallic catalyst (nano-Ni–La–Fe/γ-Al₂O₃) was developed for tar reduction and enhanced hydrogen production in biomass steam gasification, with focuses on preventing coke deposition and sintering effects to lengthen the lifetime of developed catalysts. The catalyst was prepared by deposition–precipitation method and characterized by various analytical approaches. Following that, the activity of catalysts in biomass steam gasification was investigated in a bench-scale combined fixed bed reactor. With presence of the catalyst, the content of hydrogen in gas products was increased to over 10 vol.%, the tar removal efficiency reached 99% at 1073 K, and more importantly the coke deposition on the catalyst surfaces and sintering effects were avoided, leading to a long lifetime of catalysts.     

Radical-driven Fenton reactions: studies with paraquat, adriamycin, and anthraquinone 6-sulfonate and citrate, ATP, ADP, and pyrophosphate iron chelates

Using paraquat, adriamycin, and anthraquinone 6-sulfonate, we have investigated the ability of radical-driven Fenton reactions to oxidize formate or deoxyribose when catalyzed by iron complexed with citrate, ADP, ATP, or pyrophosphate. Radicals were generated either radiolytically or enzymatically with xanthine oxidase or ferredoxin reductase. With each radical source, the citrate, ADP, and ATP complexes were at least 50% as active as Fe(EDTA) at catalyzing deoxyribose oxidation, and slightly less active as catalysts of CO2 formation from formate. Fe(pyrophosphate) was less efficient and in some cases inactive. Although it is not possible to definitively identify the oxidant involved, it behaved more like the hydroxyl radical than the proposed ferryl or peroxoferrous species formed in equivalent reactions catalyzed by nonchelated iron, which can oxidize deoxyribose but not formate. Chelator concentrations of 1-2 mM were required for maximum effect, which implies that the major effect of the chelators is on the reactivity of Fe2+ in the Fenton reaction with H2O2. This also suggests that any iron available physiologically could participate in the Fenton reaction in a nonchelated form, and produce a ferryl species rather than the hydroxyl radical. Reactions of the organic radicals contrast with the equivalent reactions of superoxide (Haber-Weiss reaction) for which the same iron chelates are all very inefficient catalysts. Fenton reactions driven by organic reducing radicals may therefore contribute more to the toxicity of redox cycling compounds than equivalent reactions of superoxide.     

Polymer-supported catalysts: enantioselective hydrogenation and hydrogen transfer reduction

Different types of heterogenized catalysts were involved in asymmetric reactions. Hydrogen transfer reduction was performed with amino alcohols derived from poly((S)-(GMA-co-EGDMA or DVB)) and hydrogenation with BINAP grafted onto PEG or copolymerized with isocyanates as ligands. Attempts of catalysts recycling are reported.     

l-Prolinol as a highly enantioselective catalyst for Michael addition of cyclohexanone to nitroolefins

Though many chiral amines such as l-proline and its derivatives have proven to be versatile catalysts in many reactions, l-prolinol was seldom used as organocatalyst for reactions. Herein, we report the first l-prolinol catalyzed asymmetric Michael addition of cyclohexanone to nitroolefins in the presence of benzoic acid to afford Michael adducts with high diastereoselectivities (87:13–>99:1) and enantioselectivities (82–96%).