1,720,969 research outputs found
Catalytic Asymmetric Ring Opening of Cyclopentadiene Heteronucleophiles Cycloadducts with Organometallic Reagents.
No full textEquilibria and UV-spectral characteristics of BrCl, BrCl2- and Br2Cl- species in 1,2-dichloroethane - Stereoselectivity and kinetics of the electrophilic addition of these species to alkenes
No full textUV/Vis spectrophotometric measurements have been used to determine the equilibrium constants and the absorption spectra of BU4N+Br2Cl- and BU4N+BrCl2- species in 1,2-dichloroethane (DCE) at 25 degreesC. The formation constants and the absorption spectra of BrCl from Br-2 and Cl-2 and of BU4N+Cl3- species from Bu4N+Cl- and Cl-2 were also determined in this solvent at 25 degreesC. The kinetics and the products of bromochlorination of several aryl-substituted olefins with BrCl, with Br-2 in the presence of an excess of Bu4N+Cl- (Br-2/ Cl-), and with BrCl in the presence of an excess of BU4N+Cl- (BrCl/Cl-) were investigated in DCE at different temperatures. The kinetics and product distribution data for the reactions with Br-2/Cl- or BrCl/Cl- are interpreted on the basis of a mechanism involving a product- and rate-determining nucleophilic attack by chloride on the olefin-halogen (Br-2 or BrCl) pi complex. The data relating to the electrophilic addition of BrCl are interpreted in terms of the initial formation of a 1:1 olefin-BrCl pi complex, the ionization of which, catalyzed by a second halogen molecule, gives a bromonium. (or beta -bromocarbenium) BrCl2- ion pair able to collapse to the corresponding bromochloro adducts. A significant proportion of the ion pair intermediates may, however, actually be present as bromonium (or beta -bromocarbenium) Br2Cl- and Br-3-ion pairs, and the collapse of these should be responsible for the formation of the dibromides in BrCl addition
Highly EnantioselctiveCoper-Phosporamidite-Catalyzed Coniugate Addition of Dialkylzinc Reagent to Acyclic alfa-beta-Unsaturated Imides
No full textA new and practical way to introduce an alkyl fragment in the β-position of aliphatic carboxylic acid derivatives with high enantioselectivities by the use of a commercially available chiral ligand is reported. N-Acyl pyrrolidinones, as simple derivatives of an α,β-unsaturated carboxylic acid, were found to be the substrates of choice featuring good reactivity and high enantioselectivities (up to >99% ee)
Regio- and Enationselective Copper-Catalyzed Addition of Dialkylzinc Reagents to Cyclic 2-Alkenyl Aziridines
No full textThe reactions of some organocopper reagents with cyclic 2-alkenyl aziridines were examined. The stereoselectivity of the addition reaction of dialkylzinc can be strongly influenced by the use of phosphoramidites as ligands for copper. Chiral copper complexes of 2,2′-binaphthyl-based phosphoramidites were shown to be highly effective catalysts for the regio- (SN2′) and anti-stereoselective addition of dialkylzinc reagents to these compounds according to a kinetic resolution protocol. The corresponding new cyclic allylic amine reaction products were obtained with a good enantioselectivity (83% ee)
Catalytic Asymmetric Ring Opening of 2,3-Substituted Norbornenes with Organometallic Reagents: A New Formal Aza Functionalization of Cyclopentadiene
No full textAn unprecedented regio- and anti-stereoselective asymmetric ring opening of 1,3-cyclopentadiene-heterodienophile cycloadducts, including also 2,3-diazabicyclo[2.2.1]heptenes, with hard alkyl metals and copper-phosphoramidite catalysts, is reported. The induced ring opening, in conjunction with C-C bond formation, gives a catalytic and practical access to new hetero-functionalized cyclopentenes in an enantioenriched form (up to 86% ee)
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Unprecedented copper-catalyzed asymmetric conjugate addition of organometallic reagents to a,ß-unsaturated lactams
No full textFor the first time, an excellent enantioselectivity has been obtained in the conjugate addition of hard organometallic reagents to α,β-unsaturated lactams bearing appropriate protecting-activating groups on the nitrogen
Reazioni Catalitiche Asimmetriche di Composto Ossabiciclici e Ossidi di Arene con Reagenti di dialchilzinco
No full textCatalytic Regiodivergent Kinetic Resolution of Allylic Epoxides: a New Entry to Allylic and Homoallylic Alcohols with High Optical Puriy
No full textA novel regiodivergent kinetic resoln. of a series of allylic epoxides with alkylzinc reagents is described. Results demonstrate the potential of chiral copper-phosphoramidite catalysts for enantiomer differentiation of allylic epoxides, allowing a chiral catalyst-stereoregulated synthesis of cyclic allylic and homoallylic alcs. with high optical purities
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