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991.
Transition structures associated with the C-C bond-formation step of the proline-catalyzed intermolecular aldol reaction
between acetone and isobutyraldehyde have been studies using density functional theory methods at the B3LYP/6-31G** computational
level. A continuum model has been selected to represent solvent effects. For this step, which is the stereocontrolling and
rate-determining step, four reactive channels corresponding to the syn and anti arrangement of the active methylene of the
enamine relative to the carboxylic acid group of l-proline and the re and si attack modes to both faces of the aldehyde carbonyl group have been analyzed. The B3LYP/6-31G**
energies are in good agreement with experiment, allowing us to explain the origin of the catalysis and stereoselectivity for
these proline-catalyzed aldol reactions.
Received: 2 April 2002 / Accepted: 18 July 2002 / Published online: 11 October 2002
Acknowledgements. This work was supported by research funds provided by the Ministerio de Educación y Cultura of the Spanish Government by
DGICYT (project PB98–1429). All the calculations were performed on a Cray–Silicon Graphics Origin 2000 of the Servicio de
Informática de la Universidad de Valencia. We are most indebted to this center for providing us with computer capabilities.
Correspondence to: L. R. Domingo e-mail: domingo@utopia.uv.es 相似文献
992.
A new tetradentate dihydrogen perchlorate macrocyclic ligand (2,4,9,11-tetraphenyl-1,5,8,12-tetraazacyclotetradeca-1,4,8,11-tetraene dihydrogen perchlorate) was prepared and characterised. The macrocycle behaves as a selective chelating ion-exchanger for some metal ions. The polystyrene-based membrane electrode is found to exhibit quite promising selectivity for Cr3+ ions. It can be used to estimate chromium concentrations in the range 3.16x 10(-6)-1.00x10(-1) M with a near-Nernstian slope of 17.5 mV per decade of concentration between pH 3.0 to 6.5. The electrode is found to possess a fast response time of 15 s and was used over a period of three months with good reproducibility (s = +/- 0.3 mV). The selectivity coefficient values for mono-, di- and trivalent cations indicate excellent selectivity for Cr3+ ions over a large number of other cations. Anions such as Cl- and SO4(2-) do not interfere and the electrode also works satisfactorily in a mixed organic-water solution. The sensor has been used as an indicator electrode for the potentiometric titration of Cr3+ with EDTA. The practical utility of the membrane sensor has also been demonstrated in solutions contaminated with detergents (CTAB and SDS). Above all, the membrane sensor has been very successfully used to determine Cr3+ in some foods. 相似文献
993.
Sandip Halder Theodor Schneller Rainer Waser Florian Thomas 《Journal of Sol-Gel Science and Technology》2007,41(3):203-207
Nickel thin films have been sputtered on standard Si/SiO2 substrates with TiO2 as an adhesive layer. The thermal stability of these substrates was analyzed. SEM images show an increase in grain size with
annealing temperature. They were found to be stable till 800°C, beyond which the nickel layer disintegrated. These substrates
were used for deposition of BaTiO3 and (Ba,Sr)TiO3 dielectric thin films under a reducing atmosphere. The dielectric thin films were processed with various pyrolysis and annealing
temperatures in order to optimize the dielectric properties. Increased pyrolysis temperatures showed an increase in the grain
size. Results on these nickelised substrates were finally compared with dielectric films deposited on platinized silicon substrates
under identical conditions but crystallized in an oxygen atmosphere. 相似文献
994.
Summary The macrocycles 2,10-dimethyl-3,4,8,9,15-pentaazabicyclo-[9.3.1]-pentadeca-1 (15),2,9,11,13-penta-ene-5,7-dione (L) and 2,11-dimethyl-3,4,8,9,10,16-pentaazabicyclo[10.3.1]-hexadeca-1 (16),2,10,12,14-penta-ene-5,8-dione (L) were prepared and characterized by elemental, i.r. and mass spectral data. The macrocycles react with various metal(II) chlorides to yield complexes of the types [MLCl2H2O] (M = Mn, Co, Ni, Cu or Zn), [MLCl2H2O] (M = Mn, Ni, Cu or Zn) and [Co3L2Cl4]Cl2. The complexes were characterized by physico-chemical and spectroscopic methods. 相似文献
995.
Haoshuang Gu Yongming Hu Hao Wang Xiangrong Yang Zhenglong Hu Ying Yuan Jin You 《Journal of Sol-Gel Science and Technology》2007,42(3):293-297
Single crystalline nanowires of lead titanate (PbTiO3) were fabricated by hydrothermal method at 200°C using lead acetate and n-tetrabutyl titanate as starting materials, where sodium hydroxide was served as a mineralizer. Crystalline phases, microstructure
and optical properties of PbTiO3 nanowires were investigated. The PbTiO3 nanowires were uniform and continuous along the long axis, and were composed of single crystalline PbTiO3 with a tetragonal perovskite structure. The diameter of a single nanowire was around 12 nm and the length reached up to 3 μm.
The chemical composition of the samples and the valence states of elements were determined by X-ray photoelectron spectroscopy
(XPS). The ultraviolet/visible absorption spectroscopic investigation suggested that the absorption edge of optical transition
of the first excitonic state occurred at around 320 nm. A blue-green light emission peaking at about 471 nm (2.63 eV) is observed
at room temperature, and the intensity of this emission increased with increasing excitation wavelength. Oxygen vacancies
are responsible for the light emission of PbTiO3 nanowires. 相似文献
996.
J.S. Yadav B.V. Subba Reddy K.V. Raghavendra Rao G.G.K.S. Narayana Kumar 《Tetrahedron letters》2007,48(31):5573-5576
Indoles undergo smooth alkylation with propargylic alcohols in the presence of 10 mol % of scandium triflate under mild conditions to produce 3-propargylated indoles in excellent yields with high selectivity. 相似文献
997.
A simple, rapid, cost-effective and accurate high performance thin layer chromatographic method has been developed for quantification
of valerenic acid in Valeriana jatamansi and Valeriana officinalis which is one of the stable compounds of Valeriana officinalis and designated as a key marker compound. Valerenic acid makes substantial contribution to the sedative and spasmolytic activity
of the essential oil and extract of Valeriana officinalis. Separation and quantification was achieved by HPTLC using ternary mobile phase of hexane: ethyl acetate: acetic acid (80:20:0.5
v/v) on precoated silica gel 60F254 aluminium plates and densitometric determination was carried out after derivatization with anisaldehyde-sulphuric acid reagent
at 700 nm, in absorption-reflectance mode. The calibration curves were linear in the range of (500 ng–2.5 μg). This is the
first HPTLC report for the identification and quantification of valerenic acid in Valeriana jatamansi and Valeriana officinalis. 相似文献
998.
Mátyás Milen László Hazai Pál Kolonits György Kalaus Lajos Szabó Ágnes Gömöry Csaba Szántay 《Central European Journal of Chemistry》2005,3(1):118-136
Transformation of β-carboline derivatives into optically active entities were studied and the de and ee values of the resulted compounds were detected.
Dedicated to Professor Károly Lempert on his 80th birthday. 相似文献
999.
Total synthesis of (-)-lentiginosine was achieved from D-mannitol using highly stereoselective reactions. Similarly, (+)-lentiginosine was synthesized from L-tartaric acid. 相似文献
1000.
Thermodynamics and crystallization kinetics of the hydrothermal synthesis of Na4Ti4Si3O10 (NaTS) were systemically studied by both experiments and model simulation. Experimental results showed that the curve of
crystallinity with time was a characteristic signmoid in the shape that indicated the crystallization of Na4Ti4Si3O10 was a typical spontaneous nucleation process on the laboratory scale. Crystallization of NaTS belongs to the liquid-liquid
transformation mechanism and the reaction is endothermic (ΔH = 15.3 kJ/mol). A mathematic model of crystallization kinetics was developed to simulate the synthesis of NaTS. Runge-Kutta
and simplex methods were adopted to solve the partial differential equations. Model results fitted well with the experimental
data and showed that the synthesis process belongs to spontaneous nucleation and crystal growth. Moreover, the very small
crystal growth constant (5.6·10−7) and gel dissolution constant (7.0·10−7) indicate they are the rate-limiting steps of the whole synthesis process. 相似文献