Home About us Contact
|
Home About us Contact | ||
|
|
||
Enantiomeric Excess (enantiomeric + excess)
Kinds of Enantiomeric Excess Terms modified by Enantiomeric Excess Selected AbstractsDramatic Improvement of the Enantiomeric Excess in the Asymmetric Conjugate Addition Reaction Using New Experimental Conditions.CHEMINFORM, Issue 38 2002A. Alexakis Abstract For Abstract see ChemInform Abstract in Full Text. [source] ChemInform Abstract: Synthesis of Homochiral 5- and 8-Substituted 2-[((2-(2,6-Dimethoxyphenoxy)ethyl)amino)methyl]-1,4-benzodioxanes and Electrophoretic Determination of Their Enantiomeric Excess.CHEMINFORM, Issue 27 2001Ermanno Valoti Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a "Full Text" option. The original article is trackable via the "References" option. [source] Parallel SFC/MS-MUX screening to assess enantiomeric purityCHIRALITY, Issue 8 2008Derek B. Laskar Abstract Enantiomeric excess (ee) was evaluated for two internally synthesized compound libraries using a high-throughput automated, intelligent four-channel parallel supercritical fluid chromatography/mass spectrometry system equipped with a multiplexed ion source interface (SFC/MS-MUX). The two libraries contained compounds spanning a wide range of enantiomeric ratios with structurally diverse chemical scaffolds and stereogenic centers. The system analyzed each sample simultaneously against four chiral columns using up to six organic modifiers. Enhancements to our previously published parallel supercritical fluid chromatography/mass spectrometry system were implemented to address the challenges associated with automated trace enantiomer identification and quantitation. A reversal of enantiomer elution order was observed for several samples across multiple CSPs and modifiers. The relationship between elution order and % ee accuracy is presented for compounds exhibiting high, middle and low % ee values. Despite incidences in which the minor enantiomer eluted prior to the major enantiomer with less than baseline resolution, the overall % ee was in agreement with separations in which full baseline resolution was achieved. The methods presented here demonstrate the value and utility of high-throughput ee determinations to support drug discovery and development programs. Chirality, 2008. © 2008 Wiley-Liss, Inc. [source] NMR studies of chiral organic compounds in non-isotropic phasesCONCEPTS IN MAGNETIC RESONANCE, Issue 3 2008Marek J. Potrzebowski Abstract In this article, new applications and perspectives of one- and two-dimensional NMR spectroscopy for study of chiral organic compounds in the non-isotropic phases (solid state and liquid crystals) are presented. The review is organized into five sections. In the first part, theoretical background and short introduction to solid state NMR are shown. The second part presents how NMR isotropic chemical shift can be used for distinguishing of racemates and enantiomers. In the third section, the power of the ODESSA pulse sequence for investigation of racemates, enantiomers and establishing of enantiomeric excess are discussed. The fourth part shows the application of analysis of principal elements of chemical shift tensors obtained by means of 2D NMR techniques for searching of absolute configuration and conformational changes in the solid state. The final part presents recent achievements of chiral liquid crystals NMR methodology for study of chiral compounds. © 2008 Wiley Periodicals, Inc. Concepts Magn Reson Part A 32A:201,218, 2008. [source] Advances in the enantioseparation of second-generation antidepressant drugs by electrodriven methodsELECTROPHORESIS, Issue 1 2006Roberto Mandrioli Abstract Stereochemistry is steadily increasing in importance in the development of new drugs, and the availability of pure enantiomer drugs can make therapy safer and more efficacious. In particular, almost all second-generation antidepressant drugs possess one or more chiral centres; however, only some of them are administered as single enantiomers. A fundamental part of the quality control of pharmaceutical formulations is the determination of enantiomeric excess and enantiomeric purity; this is also important for the therapeutic drug monitoring of depressed patients. For this purpose, efficient and reliable analytical methods are needed and electrodriven techniques (most of all CE, CEC and MEKC) are very efficient and inexpensive candidates for the role. In this review, the enantioselective electrodriven methods available for the analysis of second-generation antidepressant are presented and discussed. In particular, the following pharmacological classes of antidepressants will be considered: selective serotonin reuptake inhibitors (fluoxetine, citalopram, paroxetine, sertraline); norepinephrine reuptake inhibitors (reboxetine); serotonin and norepinephrine reuptake inhibitors (venlafaxine, milnacipran, duloxetine); and noradrenergic and specific serotonergic antidepressants (mirtazapine). [source] Comparative Study of Cyanobacteria as Biocatalysts for the Asymmetric Synthesis of Chiral Building BlocksENGINEERING IN LIFE SCIENCES (ELECTRONIC), Issue 2 2006J. Havel Abstract The three representative cyanobacteria, Synechococcus PCC7942, Anabaena variabilis, and Nostoc muscorum, were studied for their ability to asymmetrically reduce the prochiral ketones 2,-3,-4,-5,-6,-pentafluoroacetophenone, ethyl 4-chloroacetate, 4-chloroacetophenone, and ethylbenzoylacetate to the corresponding chiral alcohols. Photosynthesis as well as respiration was applied for intracellular regeneration of the NAD(P)H cofactor. It was shown for the first time that all cyanobacteria were able to reduce the prochiral ketones asymmetrically without light for cofactor regeneration. By comparison of the cell specific product formation capacities of cyanobacteria with typical heterotrophic whole cell biocatalysts in batch processes, it is shown that comparable or, in some cases, better performances at high enantiomeric excess (ee > 99.8,%) are obtained. As a consequence of a generally strong product inhibition, in situ product removal must be applied in order to restore process efficiency when using cyanobacteria as biocatalysts. [source] Fine-Tuning Ligands for Catalysis Using Supramolecular StrategiesEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 29 2007Vincent F. Slagt Abstract Coordinative bonds have been used to prepare supramolecular ligands leading to well-defined catalysts formed by assembly. The construction of these ligands is based on selective metal,ligand interactions between nitrogen donor atoms of phosphorus-nitrogen building blocks and various zinc(II) porphyrins. The major advantage of this supramolecular approach of catalyst preparation is the simplification of ligand variation enabling straightforward modification of steric, electronic and chiral properties of the supramolecular ligand. A large number of new ligands becomes accessible by this modular variation of the building blocks. The ligand assembly based on pyridyl phosphites and zinc(II) porphyrin with electron-withdrawing substituents led to a twelve-fold increase in activity and an increase in enantioselectivity from 17 to 50,% in the rhodium-catalyzed hydrogenation of dimethyl itaconate. The first examples of assemblies based on non-chiral ligands and chiral zinc(II) porphyrin template molecules show, as proof of principle, an enantiomeric excess up to 18,% in the asymmetric palladium-catalyzed allylic alkylation. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007) [source] Water-Soluble Arene Ruthenium Complexes Containing a trans -1,2-Diaminocyclohexane Ligand as Enantioselective Transfer Hydrogenation Catalysts in Aqueous SolutionEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 22 2005Jérôme Canivet Abstract The cationic chloro complexes [(arene)Ru(H2N,NH2)Cl]+ (1: arene = C6H6; 2: arene = p -MeC6H4iPr; 3: arene = C6Me6) have been synthesised from the corresponding arene ruthenium dichloride dimers and enantiopure (R,R or S,S) trans -1,2-diaminocyclohexane (H2N,NH2) and isolated as the chloride salts. The compounds are all water-soluble and, in the case of the hexamethylbenzene derivative 3, the aqua complex formed upon hydrolysis [(C6Me6)Ru(H2N,NH2)OH2]2+ (4) could be isolated as the tetrafluoroborate salt. The molecular structures of 3 and 4 have been determined by single-crystal X-ray diffraction analyses of [(C6Me6)Ru(H2N,NH2)Cl]Cl and [(C6Me6)Ru(H2N,NH2)OH2][BF4]2. Treatment of [Ru2(arene)2Cl4] with the monotosylated trans -1,2-diaminocyclohexane derivative (TsHN,NH2) does not yield the expected cationic complexes, analogous to 1,3 but the neutral deprotonated complexes [(arene)Ru(TsN,NH2)Cl] (5: arene = C6H6; 6: arene = p -MeC6H4iPr; 7: arene = C6Me6; 8: arene = C6H5COOMe). Hydrolysis of the chloro complex 7 in aqueous solution gave, upon precipitation of silver chloride, the corresponding monocationic aqua complex [(C6Me6)Ru(TsHN,NH2)(OH2)]+ (9) which was isolated and characterised as its tetrafluoroborate salt. The enantiopure complexes 1,9 have been employed as catalysts for the transfer hydrogenation of acetophenone in aqueous solution using sodium formate and water as a hydrogen source. The best results were obtained (60 °C) with 7, giving a catalytic turnover frequency of 43 h,1 and an enantiomeric excess of 93,%. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005) [source] Enantioselective Binaphthophosphepine-Promoted [3+2] Annulations of N -Ts- and N -DPP-Imines with Allenoates and 2-ButynoatesEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 1 2009Nathalie Pinto Abstract The use of binaphthophosphepine 1a as a catalyst for the [3+2] cyclisation between allenoates or 2-butynoates andimines was investigated. The effects of the imine protecting group on both the catalytic activity and enantioselectivity were determined by comparing the behaviour of N -tosyl- and N -DPP-imines. The N -DPP-imines displayed lower reactivity, but afforded the desired pyrrolines in higher enantiomeric excess (73,92,% ee). The DPP protecting group was removed from the final pyrrolines under mild conditions to afford the corresponding secondary amines. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009) [source] Ferrocenyliminophosphites as Easy-to-Modify Ligands for Asymmetric CatalysisEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 29 2007Konstantin N. Gavrilov Abstract Several N,P bidentate phosphite-type ligands derived from readily available ferrocene-based iminoalcohols were successfully used in Rh-catalysed hydrogenations and Pd-catalysed allylic substitutions of a variety of substrates. Moderate-to-high catalytic activities under standard conditions were observed, and the enantiomeric excess of the products were up to 97,%. Results obtained under systematic variation of the ligand parameters indicate that the enantioselectivity is largely determined by the nature of the phosphocentre and also by the substituent in the C*HN-fragment. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007) [source] Stereoselective Hydrolysis of Quaternary Quinuclidinium Benzoates Catalyzed by ButyrylcholinesteraseEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 2 2003Ines Primoz Abstract Four chiral, quaternary, N -methyl and N -benzyl derivatives of (R)- and (S)-quinuclidin-3-yl benzoates were synthesized and studied as substrates of horse serum butyrylcholinesterase (BChE). The kcat for the substrates decreased in the order (R)- N -methyl > (R)- N -benzyl (2.3-fold slower) >> (S)- N -methyl (70.5-fold slower reaction), while for the (S)- N -benzyl ester inhibition of the enzyme was observed. The kinetics of inhibition (Ka = 3.3 ,M) indicated that binding to the catalytic site of BChE occurred. From the ratio of the kcat/KM values of both enantiomers an enantiomeric excess of 95% was calculated for N -methyl derivatives. Thus, BChE is suitable as a biocatalyst for the resolution of racemic quaternary quinuclidinium esters. In order to explain the experimental data, combined quantum chemical (HF/3,21G*) and semiempirical (PM3) calculations within the ONIOM scheme of the stable species in the acylation step were performed. Geometry optimizations were carried out for all benzoate esters for an assumed active site model of BChE. It was confirmed that hydrolysis is affected to an appreciable extent by a proper geometrical orientation of substrates at the choline subsite. The energies of the optimized systems were in good agreement with the experimental data. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003) [source] Chiral alcohol production by NADH-dependent phenylacetaldehyde reductase coupled with in situ regeneration of NADHFEBS JOURNAL, Issue 9 2002Nobuya Itoh Phenylacetaldehyde reductase (PAR) produced by styrene-assimilating Corynebacterium strain ST-10 was used to synthesize chiral alcohols. This enzyme with a broad substrate range reduced various prochiral aromatic ketones and ,-ketoesters to yield optically active secondary alcohols with an enantiomeric purity of more than 98% enantiomeric excess (e.e.). The Escherichia coli recombinant cells which expressed the par gene could efficiently produce important pharmaceutical intermediates; (R)-2-chloro-1-(3-chlorophenyl)ethanol (28 mg·mL,1) from m -chlorophenacyl chloride, ethyl (R)-4-chloro-3-hydroxy butanoate) (28 mg·mL,1) from ethyl 4-chloro-3-oxobutanoate and (S)- N-tert -butoxycarbonyl(Boc)-3-pyrrolidinol from N -Boc-3-pyrrolidinone (51 mg·mL,1), with more than 86% yields. The high yields were due to the fact that PAR could concomitantly reproduce NADH in the presence of 3,7% (v/v) 2-propanol in the reaction mixture. This biocatalytic process provided one of the best asymmetric reductions ever reported. [source] Analysis of the enantiomeric ratios of chiral components in absolute jasmineFLAVOUR AND FRAGRANCE JOURNAL, Issue 3 2001S. Tamogami Abstract Determination of the enantiomeric ratios of chiral components in absolute jasmine, an important raw fragrance material, was studied. Jasmine oils from Egypt, India and France were employed. It was remarkable that the enantiomeric excess of most of these chiral components was not 100% ee. There was a significant difference in the ratios of enantiomers of several characteristic aroma compounds among these products. In particular methyl jasmonate in Indian jasmine oil contains less of the major (1R, 2R)-isomer compared with those of France and Egypt. Copyright © 2001 John Wiley & Sons, Ltd. [source] Synthesis of the Antibiotic (R)-Reutericyclin via Dieckmann CondensationHELVETICA CHIMICA ACTA, Issue 11 2005Roswitha Böhme (R)-Reutericyclin ((R)- 1), a bactericidal, amphiphilic natural product with a trisubstituted tetramic acid moiety, was prepared in four steps from D -leucine in an overall yield of 24%. The chiral heterocyclic portion of 1 was synthesized by Dieckmann cyclization of ethyl N -(acetoacetyl)leucinate (7), and the resulting pyrrole derivative 8 was N -acylated with (E)-dec-2-enoyl chloride in the presence of BuLi at ,,70° (Scheme,2). This new procedure is straightforward and allows the synthesis of both antipodes of reutericyclin in an enantiomeric excess (ee) of ca. 80%. [source] Combining Designer Cells and Click Chemistry for a One-Pot Four-Step Preparation of Enantiopure ,-HydroxytriazolesADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 13 2010Wiktor Szymanski Abstract The multistep catalytic process using designer cells, either added as freshly prepared suspensions or as stable lyophilized powder, and click reaction can be performed in one pot. The sequence of four reactions allows the production of both enantiomers of ,-hydroxytriazoles with high enantiomeric excess. [source] One-Step Synthesis of Chiral Azamacrocycles via Palladium-Catalyzed Enantioselective Amination of 1,5-Dichloroanthraquinone and 1,5-DichloroanthraceneADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 13 2010Elena R. Ranyuk Abstract Asymmetric amination of 1,5-dichloroanthraquinone and 1,5-dichloroanthracene with di- and trioxadiamines catalyzed by palladium complexes with various chiral phosphine ligands gave chiral macrocycles with ee values of up to 60%. The dependence of the chemical yields and enantiomeric excess on the nature of the starting compounds and the phosphine ligands employed was demonstrated. An unexpected spontaneous resolution upon crystallization of the macrocycle comprising anthraquinone and dioxadiamine moieties was observed while in the case of the macrocycle with a trioxadiamine linker racemic monocrystals were obtained. Crystallization of the enantiomerically enriched mixtures afforded chiral macrocycles with 88,99% ee. [source] Cutting Long Syntheses Short: Access to Non-Natural Tyrosine Derivatives Employing an Engineered Tyrosine Phenol LyaseADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 4 2010Birgit Seisser Abstract The chemical synthesis of 3-substituted tyrosine derivatives requires a minimum of four steps to access optically enriched material starting from commercial precursors. Attempting to short-cut the cumbersome chemical synthesis of 3-substituted tyrosine derivatives, a single step biocatalytic approach was identified employing the tyrosine phenol lyase from Citrobacter freundii. The enzyme catalyses the hydrolysis of tyrosine to phenol, pyruvate and ammonium as well as the reverse reaction, thus the formation of tyrosine from phenol, pyruvate and ammonium. Since the wild-type enzyme possessed a very narrow substrate spectrum, structure-guided, site-directed mutagenesis was required to change the substrate specificity of this CC bond forming enzyme. The best variant M379V transformed, for example, o -cresol, o -methoxyphenol and o -chlorophenol efficiently to the corresponding tyrosine derivatives without any detectable side-product. In contrast, all three phenol compounds were non-substrates for the wild-type enzyme. Employing the mutant, various L -tyrosine derivatives (3-Me, 3-OMe, 3-F, 3-Cl) were obtained with complete conversion and excellent enantiomeric excess (>97%) in just a single ,green' step starting from pyruvate and commercially available phenol derivatives. [source] Tandem Enzyme/Gold-Catalysis: From Racemic ,-Allenic Acetates to Enantiomerically Enriched 2,5-Dihydrofurans in One PotADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 14-15 2009Martta Asikainen Abstract We report the first example of a tandem kinetic resolution/cycloisomerization of racemic allenic acetates in the presence of Burkholderia cepacia lipase (PS Amano SD) and catalytic amounts of chloroauric acid (HAuCl4) which affords 2,5-dihydrofurans, as well as unreacted starting material, in one pot with high enantiomeric excess and moderate to good yield. [source] A Novel Proline-Valinol Thioamide Small Organic Molecule for a Highly Enantioselective Direct Aldol ReactionADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 14-15 2009Bing Wang Abstract A new prolinethioamide compound 4, prepared from readily available natural amino acid L -proline and amino alcohol L -valinol, has been found to be an active catalyst for the direct aldol reaction of various aldehydes with acetone, cyclohexanone or cyclopentanone at 0,°C. Using only 2,mol% loading of this organocatalyst, the reaction could give high enantioselectivity with up to 96% enantiomeric excess for the reaction of 2-nitrobenzaldehyde with acetone. And as for the cyclohexanone, the excellent diastereoselectivity (anti/syn: 99/1) and enantioselectivity (99% ee) could be achieved when reacted with 3-nitrobenzaldehyde in water in the presence of this thioamide 4. This structurally simple catalyst is a highly efficient prolinethioamide derivative, and the terminal hydroxy group in this catalyst is a primary alcohol which is different from the previously reported prerequisite secondary or tertiary alcohol of prolinamides. Our results suggest a new strategy in the design of diversiform organic catalysts for direct asymmetric aldol reactions and related transformations. [source] Productive Asymmetric Styrene Epoxidation Based on a Next Generation Electroenzymatic MethodologyADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 14-15 2009Reto Ruinatscha Abstract We have established a novel and scalable methodology for the productive coupling of redox enzymes to reductive electrochemical cofactor regeneration relying on efficient mass transfer of the cofactor to the electron-delivering cathode. Proof of concept is provided by styrene monooxygenase (StyA) catalyzing the asymmetric (S)-epoxidation of styrene with high enantiomeric excess, space-time yields, and current efficiencies. Highly porous reticulated vitreous carbon electrodes, maximized in volumetric surface area, were employed in a flow-through mode to rapidly regenerate the consumed FADH2 cofactor required for StyA activity. A systematic investigation of the parameters determining cofactor mass transfer revealed that low FAD concentrations and high flow rates enabled the continuous synthesis of the product (S)-styrene oxide at high rates, while at the same time the accumulation of the side-products acetophenone and phenylacetaldehyde was minimized. At 10,,M FAD and a flow rate of 150,mL,min,1, an average space-time yield of 0.35,g,L,1,h,1 could be achieved during 2,h with a final (S)-styrene oxide yield of 75.2%. At two-fold lower aeration rates, the electroenzymatic reaction could be sustained for 12,h, albeit at the expense of lower (59%) overall space-time yields. Under these conditions, as much as 20.5% of the utilized current could be channeled into (S)-styrene oxide formation. In comparison with state-of-the-art electroenzymatic methodologies for the same conversion, (S)-styrene oxide synthesis could be improved up to 150-fold with respect to both reaction time and space-time yield. These productivities constitute the most efficient reaction reported for asymmetric in vitro epoxidations of styrene. [source] Enantioselective Silylcyanation of Aldehydes and Ketones by a Titanium Catalyst Prepared from a Partially Hydrolyzed Titanium Alkoxide and a Schiff Base LigandADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 10 2009Kazuhiko Yoshinaga Abstract In the presence of small amount (0.2,1.0 mol%) of a titanium complex catalyst prepared from a partially hydrolyzed titanium alkoxide and an optically active tridentate Schiff base ligand, the enantioselective silylcyanation of aldehydes and ketones proceeded in a short reaction time at room temperature to afford the corresponding optically active cyanohydrin derivatives in excellent chemical yield with high enantiomeric excess (86,97% ee). The results indicate that partially hydrolyzed titanium alkoxides are a promising titanium source for the preparation of efficient catalysts for asymmetric synthesis. [source] Convenient Enantioselective Hydrosilylation of Ketones Catalyzed by Zinc-Macrocyclic Oligoamine ComplexesADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 7-8 2009Jadwiga Gajewy Abstract Chiral macrocyclic tetra- and hexamine macrocycles derived from trans -1,2-diaminocyclohexane (DACH) in complexes with diethylzinc efficiently catalyze the asymmetric hydrosilylation of aryl alkyl ketones with enantiomeric excess of the product up to 89%. The cyclic structure of the trianglamine ligand increases the enantioselectivity of the reaction, in comparison to the catalysis with the acyclic N,N, -dibenzyl-DACH ligand. Density functional theory (DFT) computations on the structures of ligand-zinc complexes and on the structures of these complexes with a coordinated acetophenone molecule allow us to rationalize the direction of the asymmetric induction of the hydrosilylation reaction as well as the superiority of the cyclic ligand compared to the acyclic one. This is the first example of asymmetric catalysis for the hydrosilylation reaction of ketones with the use of a readily available, inexpensive, and reusable macrocyclic trianglamine ligand. [source] Chirality Transfer in Imidazolium Camphorsulfonate Ionic Liquids through Ion Pairing EffectsADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 3 2009Karola Schneiders Abstract The paper describes our studies on ion pair interactions in ionic liquids (IL) using an asymmetric hydrogenation reaction as probe. Three different ionic liquids carrying prochiral keto-functionalized cations were hydrogenated in the presence of their chiral, enantiomerically pure counter-ion using an achiral heterogeneous ruthenium catalyst. For the hydrogenation of N -(3,-oxobutyl)- N -methylimidazolium camphorsulfonate (2), N -(3,-oxobutyl)imidazolium camphorsulfonate (4) and N -(5,-oxohexyl)- N -methylimidazolium camphorsulfonate (6) we found a strong dependency of the enantiomeric excess (ee in the cation) on the polarity of the solvent, the concentration of the IL and the structure of the IL. The highest ee values of up to 94% were found for the hydrogenation of 2 in ethanol. Interestingly, we observed that the ee (and consequently the strength of ion pair interaction) had a pronounced maximum for a certain concentration of the IL in the solvent depending on the nature of the solvent and on the substrate. Remarkably, the concentration leading to the maximum ee could be rationalized by independent determination of the degree of dissociation which was obtained by a combination of diffusion-ordered NMR spectroscopy and conductivity measurements. [source] Kinetic Study of the Asymmetric Hydrogenation of Methyl Acetoacetate in the Presence of a Ruthenium Binaphthophosphepine ComplexADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 1-2 2009Eva Öchsner Abstract The asymmetric hydrogenation of methyl acetoacetate (MAA) in methanol using dibromobis{(S)-4-phenyl-4,5-dihydro-3H -dinaphtho[2,1- c: 1,,2,- e]phosphepine}-ruthenium was studied in detail. For the determination of the reaction network, data from kinetic experiments were compared to different possible reaction networks using the kinetic software Presto Kinetics. The simulation was optimised to describe the reaction accurately with a minimal set of process parameters and reaction equations. For the best model the reaction orders, collision factors and activation energy of all reaction steps were determined. Additionally, the influence of reaction temperature and hydrogen pressure on the enantiomeric excess (ee) of the reaction was studied. It was found that high reaction temperatures and high hydrogen pressures result in increasing enantioselectivities. [source] DIOS-MSEED: A chip-based method for measurement of enantiomeric excess by kinetic resolution/mass spectrometryISRAEL JOURNAL OF CHEMISTRY, Issue 4 2001Zhouxin Shen The determination of enantiomeric excess by kinetic resolution mass spectrometry has been implemented with the Desorption/Ionization On Silicon (DIOS) MS technique. Measurements can thereby be made much more rapidly than was previously possible, bringing this general methodology for screening asymmetric catalysts closer to true high-throughput status. [source] Nitrilase-Catalyzed Enantioselective Synthesis of Pyrrolidine- and Piperidinecarboxylic AcidsADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 8-9 2007Margit Winkler Abstract The enantioselective synthesis of the non-proteinogenic amino acids ,-proline and nipecotic acids from their readily available nitriles is achieved in high enantiomeric excess by commercially available nitrilases. The presented procedure comprises not more than 4 steps, thus considerably reducing the multiple steps generally required. Amide formation is also observed for specific heterocyclic nitriles. [source] Highly Efficient Threonine-Derived Organocatalysts for Direct Asymmetric Aldol Reactions in WaterADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 6 2007Xiaoyu Wu Abstract The introduction of siloxy groups at the hydroxy function of natural threonine resulted in efficient hydrophobic organocatalysts, which could efficiently catalyze the direct aldol reactions of both cyclic and acyclic ketones with aromatic aldehydes in water with excellent enantiomeric excess. [source] Enantioselective Hydrogenation of N -Acetyldehydroamino Acids over Supported Palladium CatalystsADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 3 2007György Szöll Abstract The enantioselective hydrogenation of two N -acetyldehydroamino acids over Cinchona alkaloid-modified, supported palladium catalysts has been studied. Moderate enantioselectivities, up to 36,%, were obtained in the hydrogenation of 2-acetamidocinnamic acid over cinchonidine-modified Pd/TiO2 under low hydrogen pressure. Increase in the pressure or use of benzylamine as additive led to a gradual decrease in the enantiomeric excess and eventually inversion of the sense of the enantioselectivity. On the contrary, the optical purity of the product resulting from the hydrogenation of 2-acetamidoacrylic acid was significantly increased by addition of benzylamine to the reaction mixture. Enantiomeric excess values up to 58,% and 60,% were obtained over Pd/Al2O3 modified by cinchonidine and cinchonine, respectively. These optical purities are the best obtained in the hydrogenation of dehydroamino acid derivatives over chirally modified heterogeneous metal catalysts. [source] Asymmetric Epoxidation of Olefins by Manganese(III) Complexes Stabilised on Nanocrystalline Magnesium OxideADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 9 2006M. Choudary Abstract The asymmetric epoxidation of unfunctionalised olefins to epoxides is realised by using manganese(III) complexes stabilised on nanocrystalline magnesium oxide in the presence (1R,2R)-(,)-diaminocyclohexane as a chiral ligand in good yields and up to 91,% enantiomeric excess. [source] Planar- and Central-Chiral N,O- [2.2]Paracyclophane Ligands: Non-Linear-Like Effects and ActivityADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 4-5 2006Frank Lauterwasser Abstract The non-linear-like effect (NLLE), activity, temperature dependence, and kinetics of hydroxy[2.2]paracyclophane ketimine ligands have been investigated with the 1,2-addition reaction of diethylzinc to cyclohexanecarbaldehyde. A linear correlation between the enantiomeric excess of AHPC ketimine ligands bearing a phenylethyl side group and the product was observed with 0.5,mol,% of catalyst loading. On increasing the catalyst loading to 4,mol,%, a precipitate of the inactive heterochiral species was formed and resulted in a positive non-linear-like effect. The enantiomeric ratio was found to have linear temperature dependence. [source] | ||||||||||||||||||||||||||||||||||