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Asymmetric
Kinds of Asymmetric Terms modified by Asymmetric Selected AbstractsSynthesis and Characterization of Radical Cations Derived from Mono- and Biferrocenyl-Substituted 2-Aza-1,3-butadienes: A Study of the Influence of an Asymmetric and Oxidizable Bridge on Intramolecular Electron TransferEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 12 2005Vega Lloveras Abstract The synthesis and study of structural and electronic properties of mono-ferrocenyl ,-conjugated complexes 5a,d, whose electronic characteristics have been systematically varied by introducing an electron-donating or electron-withdrawing substituent either at the 1-position or at the 4-position of the 2-aza-1,3-butadiene moiety linked to the ferrocenyl unit, are presented. The structural and electronic properties of the homobimetallic complex 5f, with two ferrocene units linked through the asymmetric and oxidizable 2-aza-1,3-butadiene bridge, is also reported. The crystal structures of complexes 5b, 5d, and 5f show a large degree of conjugation in this family of compounds. Complexes 5 show a rich electrochemical behavior due both to the oxidation of ferrocenyl units and the 2-aza-1,3-butadiene bridge, as revealed by cyclic voltammetry. Radical cations 5+· were prepared from 5 by coulometric oxidations following their generation by absorption spectroscopy. The electronic properties of all reported neutral and oxidized ,-conjugated complexes have been investigated by means of UV/Vis,near-IR, EPR and 57Fe Mössbauer spectroscopy. The detailed study of mono-oxidized species 5a+·,5f+· has permitted the determination of the influence of an asymmetric bridge with an electroactive character on the intramolecular electron transfer (IET) phenomenon, thus demonstrating that the 2-aza-1,3-butadiene bridge promotes the IET between the two metallic units of 5f+· through two different pathways. The experimental data and conclusions are supported by DFT computations (B3LYP/3-21G*) and time-dependent DFT methods. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005) [source] Formation of (,-Alkenyl)- and (,-Vinylidene)palladium and -platinum Complexes by Oxidative Addition of 4,4-Dichloro-1,1-diphenyl-2-azabuta-1,3-diene , The Molecular Structure of an Unusual Asymmetric (,-Vinylidene)Pd,Pd ComplexEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 3 2003Michael Knorr Abstract 4,4-Dichloro-1,1-diphenyl-2-azabuta-1,3-diene (1) oxidatively adds to [Pd(PPh3)4] and [Pt(C2H4)(PPh3)2] giving rise to the ,-alkenyl complexes trans -[MCl{[C(Cl)=C(H),N=CPh2]}(PPh3)2] (2a: M = Pd; 2b: M = Pt). When 1 is treated with [Pd(PPh3)4] in a 1:2 ratio in refluxing toluene, the dimetallic ,-vinylidene complex [(PPh3)ClPd{,-[C=C(H),N=CPh2]}PdCl(PPh3)2] (3) is formed. In this fluxional compound, a PPh3 ligands migrates in a reversible manner between the two Pd centers. Substitution of the PPh3 ligands of 3 by 2 equiv. of Ph2PCH2PPh2 affords the A-frame complex [ClPd(,-dppm)2{,-[C=C(H),N=CPh2]}PdCl] (4). (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003) [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] Copper-Catalyzed Asymmetric 1,4-Hydroboration of Coumarins with Pinacolborane: Asymmetric Synthesis of DihydrocoumarinsADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 11-12 2010Hyohyun Kim Abstract An efficient asymmetric addition of pinacolborane to 4-substituted coumarins proceeded with high enantioselectivity in the presence of a copper(I)-QuinoxP complex as a catalyst to produce the corresponding 1,4-hydroboration products. Treatment of the intermediates with electrophiles, without isolation, resulted in enantioenriched dihydrocoumarins. The utility of this protocol was demonstrated in the formal synthesis of biologically active molecules. [source] Ligand-Accelerated Asymmetric [1,2]-Stevens Rearrangment of Sulfur Ylides via Decomposition of Diazomalonates Catalyzed by Chiral Bisoxazoline/Copper ComplexADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 3 2009Jian-Ping Qu Abstract The first example of catalytic asymmetric [1,2]-Stevens rearrangement reaction of 1,3-oxathiolanes with diazomalonates has been developed and up to 90% ee is achieved by the bisoxazoline 4e/copper(II) triflate [Cu(OTf)2] complex. [source] Asymmetric 1,4-Addition of Organoboron Reagents to Quinone Monoketals Catalyzed by a Chiral Diene/Rhodium Complex: A New Synthetic Route to Enantioenriched 2-AryltetralonesADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 4-5 2007Norihito Tokunaga Abstract A novel synthetic approach to 2-aryltetralones with high ee has been developed through asymmetric 1,4-addition of arylboronic acids to naphthoquinone monoketals catalyzed by a rhodium complex with the (R,R)-Ph-bod* ligand. The asymmetric addition proceeded in high yields with excellent enantioselectivity. [source] Seven characteristics of medical evidenceJOURNAL OF EVALUATION IN CLINICAL PRACTICE, Issue 2 2000FRCPC, Ross E. G. Upshur BA (Hons) Abstract This paper outlines seven essential characteristics of medical evidence and describes the implications of these for both the theory of evidence-based medicine and clinical practice. The seven characteristics are: (1) Provisional; (2) Defeasible; (3) Emergent; (4) Incomplete; (5) Constrained; (6) Collective and (7) Asymmetric. It is argued that the epistemological theory that best fits medical evidence is that of fallibilism. [source] Regio- and stereoselective synthesis of isoxazolidine derivatives by asymmetric 1,3-dipolar cycloaddition reaction of chiral nitrones with 1-propene-1,3-sultoneJOURNAL OF HETEROCYCLIC CHEMISTRY, Issue 4 2008Hong-Kui Zhang Asymmetric 1,3-dipolar cycloadditions of chiral nitrones to 1-propene-1,3-sultone (1) were investigated. Chiral nitrones 6a-e reacted with sultone 1 in toluene at 90 °C for 24-36 h to give the corresponding isoxazolidines in moderate yields with high regioselectivities and stereoselectivities. The diastereoselectivity of this reaction varied with the choice of dipolarophile and the steric demands of nitrones. When sultone 1 was allowed to react chiral nitrone 6e, a much better diastereoselectivity of up to 5.1:1 was observed. [source] Bank Interest Rate Adjustments: Are They Asymmetric?THE ECONOMIC RECORD, Issue 237 2001G.C. Lim This paper is concerned with the asymmetric adjustments between three Australian bank interest rates: a bank bill rate, a loan rate and a deposit rate. A multivariate asymmetric error-correction model is applied to capture the interplay of long-run relationships between the levels of the rates and short-run relationships between the changes in the rates. The empirical analysis, for the sample period 1990:01,2000:04, shows that interest rate adjustments, in response to positive and negative shocks, are asymmetric in the short run, but not in the long run. In particular, the results suggest that banks adjust their loan and deposit rates, in response to a change in the bank-bill rate, at a faster rate during periods of monetary easings (negative changes) than during periods of monetary tightenings (positive changes). [source] Entropy-Controlled Catalytic Asymmetric 1,4-Type Friedel,Crafts Reaction of Phenols Using Conformationally Flexible Guanidine/Bisthiourea Organocatalyst,ANGEWANDTE CHEMIE, Issue 40 2010Dr. Yoshihiro Sohtome Soft Skills: Organische Verbindungen mit flexibler Konformation (,weiche" Organokatalysatoren) vermitteln die Titelreaktion. Sie sind in der Lage, Prozesse durch die unterschiedlichen Aktivierungsentropien (,,S,S,R) der reaktiven Intermediate zu steuern, und ergeben hohe Stereoselektivitäten, ohne dass eine Abstimmung der Reaktionstemperaturen erforderlich wäre (siehe Schema). Ar=3,5-(CF3)2C6H3. [source] Rhodium-Catalyzed Asymmetric Conjugate Addition of Organoboronic Acids to Nitroalkenes Using Chiral Bicyclo[3.3.0] Diene Ligands,ANGEWANDTE CHEMIE, Issue 33 2010Zhi-Qian Wang Nichts geht über Diene: In der Titelreaktion mit anspruchsvollen Nitroalkenen ohne ,-Substituenten als Substraten und unter ArB(OH)2/KHF2 -Bedingungen waren chirale Bicyclo[3.3.0]-Diene die besten Liganden. Np = Naphthyl. [source] Asymmetric [3+2] Cycloadditions of Allenoates and Dual Activated Olefins Catalyzed by Simple Bifunctional N -Acyl Aminophosphines,ANGEWANDTE CHEMIE, Issue 26 2010Hua Xiao Einfach und difunktionell sind N -Acylaminophosphan-Katalysatoren wie 1 für die ersten asymmetrischen [3+2]-Cycloadditionen zwischen Allenoaten und doppelt aktivierten Olefinen. Die Cycloadditionen ergeben hoch funktionalisierte chirale Cyclopentene mit strikter Regioselektivität sowie guten bis hervorragenden Enantioselektivitäten und Ausbeuten. [source] Chiral Tetrafluorobenzobarrelenes as Effective Ligands for Rhodium-Catalyzed Asymmetric 1,4-Addition of Arylboroxines to ,,,-Disubstituted ,,,-Unsaturated Ketones,ANGEWANDTE CHEMIE, Issue 23 2010Ryo Shintani Dr. Quartäre Kohlenstoffstereozentren werden durch die rhodiumkatalysierte 1,4-Addition leicht zugänglicher Arylboronsäureanhydride an ,,,-disubstituierte ,,,-ungesättigte Ketone in Gegenwart eines chiralen Tetrafluorbenzobarrelen-Liganden hoch enantioselektiv aufgebaut. [source] Asymmetric, Protecting-Group-Free Total Synthesis of (,)-Englerin,A,ANGEWANDTE CHEMIE, Issue 20 2010Qianghui Zhou Eine goldkatalysierte Cyclisierung (siehe Schema) ist der Schlüsselschritt einer Totalsynthese des Naturstoffs (,)-Englerin,A aus (R)-Citronellal, die über alle Stufen ohne Schutzgruppenmanipulationen auskommt. [source] Chemoenzymatic Synthesis and Application of Bicyclo[2.2.2]octadiene Ligands: Increased Efficiency in Rhodium-Catalyzed Asymmetric Conjugate Additions by Electronic Tuning,ANGEWANDTE CHEMIE, Issue 15 2010Yunfei Luo Liganden nach Wunsch: Eine Serie von bicyclischen 1,4-Dimethyl[2.2.2]dien-Liganden mit flexibler Substitution in der Brücke ist durch Lipase-katalysierte Racematspaltung mit anschließender sechsstufiger Reaktionssequenz zugänglich. Die Methylgruppen am Brückenkopf führen zu einer erhöhten Reaktionseffizienz, wodurch in der Titelreaktion weniger Arylboronsäure nötig ist. [source] Multinuclear Catalyst for Copper-Catalyzed Asymmetric Conjugate Addition of Organozinc Reagents,ANGEWANDTE CHEMIE, Issue 13 2010Kohei Endo Prof. Jede Menge Metall: Eine kupferkatalysierte asymmetrische konjugierte Addition mit einer Umsatzzahl von 2000 gelang mithilfe eines von Binol abgeleiteten Liganden. Die ausgezeichnete Leistungsfähigkeit des Katalysators lässt sich mit der Bildung eines mehrkernigen Komplexes wie 1 erklären. Binol=2,2,-Dihydroxy-1,1,-binaphthyl. [source] Catalytic Asymmetric 6, Electrocyclization: Enantioselective Synthesis of Functionalized Indolines,ANGEWANDTE CHEMIE, Issue 52 2009Eleanor Wie man einen Ring schließt: Ein neuer Ansatz für die katalytische asymmetrische 6,-Elektrocyclisierung führt zu einem hoch enantioselektiven Prozess, der für die Synthese von chiralen Indolinen genutzt wurde (siehe Schema). Die Behandlung von N -Aryliminen unter Phasentransferbedingungen in Gegenwart von N -Benzylcinchonidiniumchlorid erzeugt ein delokalisiertes 2-Azapentadienyl-Anion, das mit bis zu 99,% Ausbeute und 98,%,ee cyclisiert. [source] Catalytic Asymmetric Conjugate Addition of Nitroalkanes to 4-Nitro-5-styrylisoxazoles,ANGEWANDTE CHEMIE, Issue 49 2009Andrea Baschieri Zweimal Nitro: 4-Nitro-5-styrylisoxazole dienten als maskierte ,,,-ungesättigte Carbonsäuren in der katalytischen asymmetrischen Titelreaktion. Der 4-Nitroisoxazolkern aktiviert das konjugierte Alken und eine latente Carboxylatfunktion. Die Reaktion läuft mit 5,Mol-% eines einfach zugänglichen Phasentransferkatalysators bei Raumtemperatur mit bemerkenswerter Diastereo- und Enantioselektivität ab (siehe Schema). [source] Asymmetric ruthenium-catalysed [2+2] cycloadditions between bicyclic alkenes and chiral aryl-substituted acetylenic acyl camphorsultam alkynesACTA CRYSTALLOGRAPHICA SECTION C, Issue 12 2009Jordan D. Goodreid The regio- and absolute stereochemistry of (7S)- N -[4-(3-thienyl)tricyclo[4.2.1.02,5]non-3-en-3-ylcarbonyl]-2,10-camphorsultam tetrahydrofuran hemisolvate, C24H29NO3S2·0.5C4H8O, and (7S)- N -[4-(4-tolyl)tricyclo[4.2.1.02,5]non-3-en-3-ylcarbonyl]-2,10-camphorsultam, C27H33NO3S, have been established. One contains a half-occupancy tetrahydrofuran solvent molecule located on a twofold axis and the other contains two crystallographically unique molecules which are nearly identical. The extended structures of both complexes can be explained via weak C,H...O interactions, which link the molecules together into two-dimensional sheets in the ab plane for the thienyl complex and ultimately into a three-dimensional structure for the tolyl derivative. The stereochemistry of both structures confirms that [2+2] cycloadditions of bicyclic alkenes and alkynes catalysed by ruthenium are exclusively exo. [source] Copper-catalyzed asymmetric 1,4-conjugate addition of dialkylzinc to enonesAPPLIED ORGANOMETALLIC CHEMISTRY, Issue 7 2010Shaohua Gou Abstract Asymmetric 1,4-conjugation addition of dialkylzinc (diethylzinc and dimethylzinc) to cyclic enones, chalcone and nitroalkenes was achieved by a 25 mol% (R)-6,6,-Br2 -BINOL(1f), 25 mol% CuSPh and 100 mol% dicyclohexylmethylamin(Cy2NMe) catalyst system. The Cu(I) catalyst system enables the cyclic enone, chalcone and nitroalkene generality with high enantioselectivity (up to 84% ee) and isolated yield (up to 94%) under mild reaction conditions. Copyright © 2010 John Wiley & Sons, Ltd. [source] Asymmetric 1,4-conjugation addition of diethylzinc to cyclic enones catalyzed by a self-assembled copper complexAPPLIED ORGANOMETALLIC CHEMISTRY, Issue 12 2009Shaohua Gou Abstract A new self-assembled catalyst system based on copper complex was developed for the 1,4-conjugation addition of diethyl zinc to cyclic enones. The self-assembled catalyst was readily prepared from (R)-[1,1,]binaphthalenyl-2,2,-diamine (1a), (L)-tryptophan (3f) and copper(I) chloride (CuCl). Several enones were found to be suitable substrates in the presence of the self-assembled copper catalyst [10 mol% (R)-1a, 10 mol% (L)-3f and 10 mol% CuCl] under optimized conditions. The desired products were afforded with high isolated yields (up to 98%) and moderate to good enantioselectivities (up to 80% ee) in mild conditions (at 0 °C). Copyright © 2009 John Wiley & Sons, Ltd. [source] ChemInform Abstract: Iridium/Chiral Diene-Catalyzed Asymmetric 1,6-Addition of Arylboroxines to ,,,,,,,-Unsaturated Carbonyl Compounds.CHEMINFORM, Issue 42 2010Takahiro Nishimura Abstract The title reaction of dienones, dienamides, or a dienoate affords ,-arylated carbonyl compounds with high enantioselectivities by use of the C2 -symmetric tetrafluorobenzobarrelene-derived iridium complex IRB as catalyst. [source] ChemInform Abstract: Catalytic Asymmetric 1,3-Dipolar Cycloaddition of ,-Iminonitriles.CHEMINFORM, Issue 38 2010Rocio Robles-Machin Abstract The catalytic asymmetric 1,3-cycloaddition of ,-iminonitriles is developed. [source] ChemInform Abstract: Chemoenzymatic Synthesis and Application of Bicyclo[2.2.2]octadiene Ligands: Increased Efficiency in Rhodium-Catalyzed Asymmetric Conjugate Additions by Electronic Tuning.CHEMINFORM, Issue 34 2010Yunfei Luo Abstract A new series of 1,4-disubstituted 2,5-diaryl bicyclo[2.2.2]octadienes are prepared. [source] ChemInform Abstract: exo-Selective Asymmetric 1,3-Dipolar Cycloaddition of Azomethine Ylides with Alkylidene Malonates Catalyzed by AgOAc/TF-BiphamPhos.CHEMINFORM, Issue 28 2010Zhi-Yong Xue 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] ChemInform Abstract: Copper-Catalyzed Asymmetric Conjugate Addition with Grignard Reagents and SimplePhos Ligands.CHEMINFORM, Issue 24 2010Laetitia Palais 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] ChemInform Abstract: Catalytic Asymmetric 6, Electrocyclization: Enantioselective Synthesis of Functionalized Indolines.CHEMINFORM, Issue 20 2010Eleanor E. Maciver 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] ChemInform Abstract: A Catalytic Asymmetric 6, Electrocyclization: Enantioselective Synthesis of 2-Pyrazolines.CHEMINFORM, Issue 20 2010Steffen Mueller 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] ChemInform Abstract: Asymmetric 1,4-Conjugate Addition of Diethylzinc to Cyclic Enones Catalyzed by a Self-Assembled Copper Complex.CHEMINFORM, Issue 15 2010Shaohua Gou 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] ChemInform Abstract: Catalytic Asymmetric Conjugate Addition of Nitroalkanes to 4-Nitro-5-styrylisoxazoles.CHEMINFORM, Issue 13 2010Andrea Baschieri 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] | ||||||||||||||||||||||||||||||||||||||||||||||||||||