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Ee Values (ee + value)
Kinds of Ee Values Selected AbstractsTransition States of the Asymmetric Michael Reactions of Aldehydes Catalyzed by Trimethylsilyl-Protected DiphenylprolinolEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 16 2009Jian-Qiang Zhao Abstract The asymmetric Michael reactions of aldehydes and nitroalkenes catalyzed by trimethylsilyl-protected diphenylprolinol were investigated by using density functional theory calculations. As a result of the stereospecific blockade of the bulky diphenylsiloxymethyl group on the pyrrolidine ring, the Re face of the enamine double bond is effectively shielded. For acetaldehyde, there are two different conformers of the enamine intermediate. On the basis of the two conformers of the enamine intermediate, four different reaction pathways were considered and four different transition states were searched for the enantioselective asymmetric Michael reaction of acetaldehyde and nitroalkene. The lowest- and second-lowest-energy transition states are both formed via the same intermediate IM2. The enantiomeric excess, calculated to be 96,%,ee, is in good agreement with the experimental value. For propanal, on the basis of the four different conformers of the prolinol,enamine intermediate, eight different reaction pathways were considered and eight transition states were searched for the enantioselective asymmetric Michael reaction. The calculated ee value is 99.5,%, which is in good agreement with the experimental ee value of 99,%. The lowest- and second-lowest-energy transition states are formed via different enamine intermediates, which is different from the case of acetaldehyde. The calculations also reveal that the intermediates play an important role in the reactions.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009) [source] Enantioselective reduction of 2-keto-3-haloalkane phosphonates by baker's yeastHETEROATOM CHEMISTRY, Issue 6 2001Cheng-ye Yuan Bioreduction of 3-substituted-2-oxoal-kanephosphonates by baker's yeast afforded 3-substituted-2-hydroxy-alkanephosphonates in moderate to good yields and ee value. These compounds could serve as useful chirons for the stereoselective synthesis of phosphorus analogs of biologically active molecules including R-(,)-3-trimethylammonium-2-hydroxypropanoic acid and R-(,)-3- trimethylammonium-2-hydroxypropanoic acid. © 2001 John Wiley & Sons, Inc. Heteroatom Chem 12:551,556, 2001 [source] Prolylprolinol-Catalyzed Asymmetric Michael Addition of Aliphatic Aldehydes to NitroalkenesADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 4 2010Dengfu Lu Abstract Several novel prolylprolinol catalysts have been designed and synthesized. This type of compound showed high catalytic efficiency on promoting the direct addition of unmodified aldehydes to nitroalkenes. Among the catalysts surveyed, the least bulky member (8d) exhibited the best performance on both efficiency and stereoselectivity, providing the products with up to 97% ee value with 1.5,5,mol% catalyst loading. Additionally, computational studies of the transition state have been conducted to explain the high diastereo- and enantioselectivity. [source] Rhodium-Catalyzed Asymmetric Aqueous Pauson,Khand-Type ReactionCHEMISTRY - A EUROPEAN JOURNAL, Issue 13 2005Fuk Yee Kwong Dr. Abstract An interesting rhodium-catalyzed asymmetric aqueous Pauson,Khand-type reaction was developed. A chiral atropisomeric dipyridyldiphosphane ligand was found to be highly effective in this system. This operationally simple protocol allows both catalyst and reactants to be handled under air without precautions. Various enynes were transformed to the corresponding bicyclic cyclopentenones in good yield and enantiomeric excess (up to 95,% ee). A study of the electronic effects of the enyne substrates revealed a correlation between the electronic properties of the substrates and the ee value obtained in the product of the Pauson,Khand-type reaction. A linear free-energy relationship was observed from a Hammett study. [source] Charge-Transfer Effect on Chiral Phosphoric Acid Catalyzed Asymmetric Baeyer-Villiger Oxidation of 3-Substituted Cyclobutanones Using 30% Aqueous H2O2 as the Oxidant,CHINESE JOURNAL OF CHEMISTRY, Issue 9 2010Senmiao Xu Abstract The intermolecular charge-transfer effect has been employed for the first time as a modulating approach to affect the enantioselectivity in asymmetric catalysis by taking the chiral phosphoric acid catalyzed asymmetric Baeyer-Villiger oxidation of 3-aryl cyclobutanones as the reaction prototype. It was found that the electron acceptor additives were able to effectively tune the enantioselectivity via donor-acceptor interaction with the catalyst and up to 9% enhancement of ee value was observed in a favorable case. [source] Monomeric Cu(II) Complex Containing Chiral Phase-transfer Catalyst as Ligand and Its Asymmetrically Catalytic ReactionCHINESE JOURNAL OF CHEMISTRY, Issue 2 2008Zhi-Rong QU Abstract The thermal treatment of CuCl2 with N -(4,-vinylbenzyl)cinchonidinium chloride (L1) afforded a monomeric discrete homochiral copper(II) complex N -4,-(vinylbenzyl)cinchonidinium trichlorocoprate(II) (1). Their applications to the enantioselectively catalytic alkylation reaction of N -(diphenylmethylidene)glycine tert -butyl ester (3) show that the higher ee value observed in catalyst 1 than that in the corresponding free ligand L1 is probably due to the rigidity enhancement after the coordination of N atom of quinoline ring to the copper ion. [source] Synthesis of Aminophosphine Ligands with Binaphthyl Backbones for Silver(I)-catalyzed Enantioselective Allylation of Benzaldehyde,CHINESE JOURNAL OF CHEMISTRY, Issue 11 2002Yi Wang Abstract A series of aminophosphine ligands was synthesized from 2-amino-2,-hydroxy-1,1,-binaphthyl (NOBIN). Their asymmetric induction efficiency was examined for silver(I) catalyzed enantioselective allylation reaction of benzaldehyde with allyltributyltin. Under the optimized reaction conditions, quantitative yield as well as moderate ee value (54.5% ee) of product was achieved by the catalysis with silver(I)/3 complex. The effects of the binaphthyl backbone and the substitutes situated at chelating N, P atoms on enantioselectivity of the reaction were also discussed. [source] Regio- and Stereo-selective Bioreduction of Diketo- n -butylphosphonate by Baker's YeastCHINESE JOURNAL OF CHEMISTRY, Issue 11 2002Ke Wang Abstract A regio- and stereo-selective reduction of diketo-n-butylphosphonates by baker's yeast was reported. The chemical yield and ee value of these reactions are highly dependent on the structure of substrates. The resulting optical active hydroxyalkanephosphonates can be used as chirons for the synthesis of polyfunctional organophosphorus compounds. As useful building block, a series of ,, ,-unsaturated ketones bearing chiral hydroxy group in addition to trifluoromethyl moiety was prepared via the Homer-Wadsworth-Emmons (HWE) reaction of the biotransformation products. [source] Asymmetric Aza-Morita,Baylis,Hillman Reactions of Alkyl Vinyl Ketones with N -Protected Imines or In Situ Generated N -Protected IminesEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 21 2010Xiao-Yang Guan Abstract DABCO-catalyzed aza-MBH reactions of N -Boc imines with MVK and EVK have been thoroughly investigated in the paper. The asymmetric version of this aza-MBH reaction was also systematically investigated by using a chiral amine or a chiral phosphane catalyst. It was found that most of the N -protected imines are suitable substrates under the mild reaction conditions and are able to give the corresponding adducts in moderate yields with high ee values. The TQO- or LB1-catalyzed aza-MBH reactions of N -protected ,-amidoalkyl phenyl sulfones or ,-amidoalkyl p -tolyl sulfones with MVK could be well conducted, which provides a facile and direct route to obtain highly enantioselective aza-MBH adducts. The Boc protecting group of the aza-MBH product could be easily removed under acidic conditions to give the corresponding ,-methylene-,-amino ketone or ,-methylene-,-amino alcohol derivatives in good yields. [source] Enantioselective Ag-Catalyzed Allylation of Aldimines,EUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 30 2009Marina Naodovic Abstract A highly enantioselective synthesis of homoallylic amines, using allyltrimethoxysilane under AgI catalytic conditions, has been developed. Among the chiral ligands investigated, a remarkable difference in the resulting AgI complexes was observed. Under mild conditions and low catalyst loadings, homoallylamines were produced in high ee values (up to 80,%) and good yields. The methodology can be further extended to a diastere- and enantioselective crotylation of aldimines.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009) [source] The Mechanism of the Stetter Reaction , A DFT StudyEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 33 2008Kirsty J. Hawkes Abstract On the basis of Breslow's mechanism for benzoin condensation, a model asymmetric Stetter reaction has been investigated using DFT methods. In contrast to the concerted benzoin condensation, after formation of the Breslow intermediate the Stetter reaction is found to be a two-step process in which the rate-determining C,C coupling of the Breslow intermediate and the Michael acceptor precedes final proton transfer. In addition, the enolamine is found to play a significant role in the stereochemistry of the product, with the energy difference between stereoisomers of this intermediate reflected throughout the remainder of the reaction sequence. Consequently, electronic and steric control of the stereochemistry of this intermediate should directly enhance the ee values of the product. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008) [source] Oxidative Amidation of Aldehydes and Alcohols with Primary Amines Catalyzed by KI-TBHPEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 21 2008K. Rajender Reddy Abstract Oxidative amidation of aldehydes and alcohols with amines to afford the corresponding amides in excellent yields and ee values over 98,% is achieved by using a catalytic amount of KI in combination with TBHP as the external oxidant. This method avoids the use of expensive and/or air-sensitive reagents. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008) [source] Novel Chiral (Salen)MnIII Complexes Containing a Calix[4]arene Unit as Catalysts for Enantioselective Epoxidation Reactions of (Z)-Aryl AlkenesEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 16 2005Maria E. Amato Abstract New asymmetric (salen)MnIII and UO2 complexes containing a calix[4]arene unit in the ligand framework were synthesized. The UO2 complexes were characterized by 1H-, 13C-, 2D TOCSY and T-ROESY NMR spectroscopy. Furthermore, the structure of one UO2 complex was determined by single-crystal X-ray analysis. The data showed that UO2 complexes, which can be considered in first approximation models of the Mn=O oxidant active species, possess a chiral pocket and adopt relevant conformations for the selectivity of the oxygen transfer process. Epoxidation data of model alkenes with the MnIII complexes showed moderate ee values and were not conclusive in indicating that the calix[4]arene unit might be able to influence the selectivity by a molecular recognition mechanism. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005) [source] Synergistic Effect of Pybox Substituents and Lanthanide Cations in Reversing the Asymmetric Induction in the Catalysed Diels,Alder Reaction between 3-Acryloyl-1,3-oxazolidin-2-one and CyclopentadieneEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 14 2004Giovanni Desimoni Abstract The enantioselectivity in the pybox/lanthanide(III) triflate catalysed Diels,Alder reaction between cyclopentadiene and 3-acryloyl-1,3-oxazolidin-2-one is profoundly influenced by the presence of a phenyl group in the 5,-position of the ligand, since it sometimes amplifies, and sometimes reverses the enantioselectivity induced by the 4,-substituent. The results can be summarised in three points. Of the catalysts derived from (4,R,5,R)-4-Me-5-Ph-pybox (6), the ScIII -based one is both strongly endo -selective and strongly enantioselective. Several catalysts derived from (4,R,5,R)- trans -diphenyl-pybox (9) give up to 50% yields of exo - 4 and, at least with PrIII and LaIII, induce excellent ee values in both endo and exo adducts. The (4,R,5,S)- cis -diphenyl-pybox (8) is a novel and valuable ligand, since opposite enantiomers can be obtained with good selectivity simply by changing the cation from ScIII to LaIII. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004) [source] Enders' SAMP-Hydrazone as Traceless Auxiliary in the Asymmetric 1,4-Addition of Cuprates to EnonesADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 13 2010Karsten Sammet Abstract Conjugate additions of Gilman cyanocuprates to (S)- N -amino-2-(methoxymethyl)pyrrolidine (SAMP)-hydrazones 4, 5 derived from cyclic and acyclic ,,,-unsaturated ketones were investigated. A protocol utilizing copper(II) sulfate/ammonium chloride was evolved, which allowed cleavage of SAMP (S)- 1 under the hydrolysis and work-up conditions, followed by recovery of the auxiliary with ethylenediaminetetraacetic acid (EDTA). The enantioselectivity of cuprate additions was dominated for cyclic SAMP-hydrazones 4 by the cuprate alkyl substituent and the ring size, in the case of acyclic arylidene SAMP-hydrazones 5, however, by the nature of the aryl substituent. Electron-donating substituents gave poor enantiomeric excesses, whereas electron-withdrawing groups provided excellent ee values of 98,99%. The configuration of the new stereocenter was determined to be (R). Moreover, a reaction sequence was developed which integrates a tandem 1,4-addition/methylation and traceless hydrolytic cleavage of the auxiliary (S)- 1 in a one-pot reaction, resulting in enantiomerically pure methyl ketones 11,13, each of them with>99% ee. [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] Catalytic Asymmetric Intramolecular Cascade Reaction for the Construction of Functionalized Benzobicyclo[4.3.0] Skeletons.ADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 11-12 2010Remote Control of Enantioselectivity Abstract A catalytic asymmetric version of the intramolecular ylide annulation has been developed which affords high ee values and diastereoselectivities and which further shows that spirobiindane-based chiral phosphines can be excellent organocatalysts. Both optically active benzobicyclo[4.3.0] compounds 2 and 2, with three continuous stereogenic centers could be obtained as major products selectively under neutral and mild conditions just by a choice of an additive. [source] Palladium(II) Complexes of C2 -Bridged Chiral Diphosphines: Application to Enantioselective Carbonyl-Ene ReactionsADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 8 2010He-Kuan Luo Abstract (11bR,11,bR)-4,4,-(1,2-Phenylene)bis[4,5-dihydro-3H -dinaphtho[2,1- c:1,,2,- e]phosphepin] [abbreviated as (R)-BINAPHANE], (3R,3,R,4S,4,S,11bS,11,bS)-4,4,-bis(1,1-dimethylethyl)-4,4,,5,5,-tetrahydro-3,3,-bi-3H -dinaphtho[2,1- c:1,,2,- e]phosphepin [(S)-BINAPINE], (1S,1,S,2R,2,R)-1,1,-bis(1,1-dimethylethyl)-2,2,-biphospholane [(S,S,R,R)-TANGPHOS] and (2R,2,R,5R,5,R)-1,1,-(1,2-phenylene)bis[2,5-bis(1-methylethyl)phospholane] [(R,R) -i- Pr-DUPHOS] are C2 -bridged chiral diphosphines that form stable complexes with palladium(II) and platinum(II) containing a five-membered chelate ring. The Pd(II)-BINAPHANE catalyst displayed good to excellent enantioselectivities with ee values as high as 99.0% albeit in low yields for the carbonyl-ene reaction between phenylglyoxal and alkenes. Its Pt(II) counterpart afforded improved yields while retaining satisfactory enantioselectivity. For the carbonyl-ene reaction between ethyl trifluoropyruvate and alkenes, the Pd(II)-BINAPHANE catalyst afforded both good yields and extremely high enantioselectivities with ees as high as 99.6%. A comparative study on the Pd(II) catalysts of the four C2 -bridged chiral diphosphines revealed that Pd(II)-BINAPHANE afforded the best enantioselectivity. The ee values derived from Pd(II)-BINAPHANE are much higher than those derived from the other three Pd(II) catalysts. A comparison of the catalyst structures shows that the Pd(II)-BINAPHANE catalyst is the only one that has two bulky (R)-binaphthyl groups close to the reaction site. Hence it creates a deep chiral space that can efficiently control the reaction behavior in the carbonyl-ene reactions resulting in excellent enantioselectivity. [source] Enantioselective Hydrogenation of ,-Dehydroamino Acid Esters Catalyzed by Rhodium Complexes with Chiral Bisaminophosphine LigandsADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 7 2010Xianfeng Sun Abstract A highly efficient strategy for the synthesis of a series of chiral bisaminophosphine ligands was well established with several remarkable features. The synthetic utility of these ligands was explored for rhodium-catalyzed asymmetric hydrogenations of ,-dehydroamino acid esters. Up to 98% ee values were achieved for the enantioselective synthesis of aminocarboxylic acids and their derivatives, which are very important chiral building blocks for the synthesis of a variety of natural products and biologically active molecules. [source] Highly Efficient Rhodium Catalysts for the Asymmetric Hydroformylation of Vinyl and Allyl Ethers using C1 -Symmetrical Diphosphite LigandsADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 2-3 2010Aitor Gual Abstract Here, we describe the successful application of novel glucofuranose-derived 1,3-diphosphites in the rhodium-catalysed asymmetric hydroformylation of vinyl acetate, 2,5-dihydrofuran and 2,3-dihydrofuran. In the hydroformylation of vinyl acetate, total regioselectivity and high ee (up to 73%) were obtained. When 2,3- and 2,5-dihydrofuran were the substrates, total chemo- and regioselectivities were achieved together with ees up to 88%. These results correspond to the highest ee values reported to date in the asymmetric hydroformylation of these substrates. The HP-NMR studies of the [RhH(CO)2(L)] species (L=15 and 17) demonstrated that both ligands coordinate to the Rh centre in an eq-eq fashion. The complex [RhH(CO)2(15)] was detected as a single isomer with characteristic features of eq-eq coordination. However, the broadening of the corresponding signals indicated that this species is rapidly interchanging in solution. In contrast, complex [RhH(CO)2(17)] was detected as a mixture of two conformational isomers at low temperature due to the greater flexibility of the monocyclic backbone of this ligand. [source] Highly Enantioselective Epoxidation of Unfunctionalized Olefins Catalyzed by Chiral Jacobsen's Catalyst Immobilized on Phenoxy-Modified Zirconium Poly(syrene-phenylvinylphos- phonate)phosphateADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 1 2010Xiaochuan Zou Abstract Chiral Jacobsen's catalyst was axially immobilized onto phenoxy-modified zirconium poly(styrene-phenylvinylphosphonate)phosphate (ZPS-PVPA). The immobilized catalysts show comparable ee values for asymmetric epoxidation of styrene and much higher ee values for ,-methylstyrene (73.7% vs. 54.0%) and indene (99.9% vs. 65.0%) than the homogeneous Jacobsen's catalyst. Moreover, the as-synthesized catalysts are relatively stable and can be recycled at least five times without significant loss of activity and enantioselectivity. A point worth emphasizing is that the heterogeneous catalysts afforded remarkable increases of conversion and ee values in the absence of expensive O-coordinating axial bases for the asymmetric epoxidation of olefins, especially for the epoxidation of ,-methylstyrene (conversion: from 24.3% to 99.9%; ee: from 29.4% to 73.7%), which may overcome the last obstacle for the potential industry application of chiral Jacobsen's catalyst. [source] Axial Chirality Control by 2,4-Pentanediol for the Alternative Synthesis of C3*-TunePhos Chiral Diphosphine Ligands and Their Applications in Highly Enantioselective Ruthenium-Catalyzed Hydrogenation of ,-Keto EstersADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 16 2009Xianfeng Sun Abstract A highly efficient strategy for the synthesis of a series of C3*-TunePhos chiral diphosphine ligands was well established with several remarkable features. The synthetic utility of these ligands was explored for the ruthenium-catalyzed asymmetric hydrogenation of ,-keto esters. Up to 99% ee values were achieved for the enantioselective synthesis of ,-hydroxy acid derivatives, which are very important chiral building blocks for the synthesis of a variety of natural products and biologically active molecules. [source] An Improved Protocol for the Direct Asymmetric Aldol Reaction in Ionic Liquids, Catalysed by Onium Ion-Tagged ProlinesADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 11-12 2007Marco Lombardo Abstract Two onium ion-tagged prolines, imidazolium bis(trifluoromethylsulfonyl)imide-substituted proline 6 and butyldimethylammonium bis(trifluoromethylsulfonyl)imide-substituted proline 7, were synthesised and their catalytic activity in the direct asymmetric aldol condensation was studied in ionic liquids. For the reaction of acetone with various aldehydes, using 5,% of the catalyst, the yields of the aldols varied between 50,85,% while the ee values were in the 80,85,% range. Other ketones were studied too, the yields obtained in those cases being in the 35,78,% range while the enantioselectivities varied between 75,94,%. [source] Do Trinuclear Triplesalen Complexes Exhibit Cooperative Effects?CHEMISTRY - A EUROPEAN JOURNAL, Issue 33 2010Characterization, Enantioselective Catalytic Sulfoxidation by Chiral Trinuclear FeIII Triplesalen Complexes, Synthesis Abstract The chiral triplesalen ligand H6chand provides three chiral salen ligand compartments in a meta -phenylene arrangement by a phloroglucinol backbone. The two diastereomeric versions H6chandRR and H6chandrac have been used to synthesize the enantiomerically pure chiral complex [(FeCl)3(chandRR)] (3RR) and the racemic complex [(FeCl)3(chandrac)] (3rac). The molecular structure of the free ligand H6chandrac exhibits at the terminal donor sides the O-protonated phenol,imine tautomer and at the central donor sides the N-protonated keto,enamine tautomer. The trinuclear complexes are comprised of five-coordinate square-pyramidal FeIII ions with a chloride at the axial positions. The crystal structure of 3rac exhibits collinear chiral channels of ,11,Å in diameter making up 33.6,% of the volume of the crystals, whereas the crystal structure of 3RR exhibits voids of 560,Å3. Mössbauer spectroscopy demonstrates the presence of FeIII high-spin ions. UV/Vis spectroscopy is in accordance with a large delocalized system in the central backbone evidenced by strong low-energy shifts of the imine ,,,* transitions relative to that of the terminal units. Magnetic measurements reveal weak intramolecular exchange interactions but strong magnetic anisotropies of the FeIII ions. Complexes 3rac and 3RR are good catalysts for the sulfoxidation of sulfides providing very good yields and high selectivities with 3RR being enantioselective. A comparison of 3RR and [FeCl(salen,)] provides higher yields and selectivities but lower enantiomeric excess values (ee values) for 3RR relative to [FeCl(salen,)]. The low ee values of 3RR appeared to be connected to a strong ligand folding in 3RR, opening access to the catalytically active high-valent Fe,O species. The higher selectivity is assigned to a cooperative stabilization of the catalytically active high-valent Fe,O species through the phloroglucinol backbone in the trinuclear complexes. [source] Asymmetric Epoxidation of Terminal Olefins with Binaphthyl Strapped Porphyrin Catalysts: (-( Stacking Interaction and Steric Effects on the EnantioselectivitiesCHINESE JOURNAL OF CHEMISTRY, Issue 5 2009Qizhi REN Abstract Two binaphthyl strapped porphyrins with similar chiral auxiliaries 1b and 2b were used as efficient catalysts for asymmetric epoxidation of both styrene derivatives and non-aromatic olefin substrates. Theoretical calculation of styrene approach to both catalysts has been performed. The subtle difference of the chiral cavities between two porphyrins has been analyzed by 1H NMR. The ,-, stacking interaction between aromatic substrates and catalysts might be one factor for the dramatic different enantioselectivities. Besides, the steric effect of the binaphthyl handle of 1b and 2b also causes the high ee values for non-aromatic olefin epoxidations. [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] | ||||||||||||||||||||||