排序方式: 共有73条查询结果,搜索用时 15 毫秒
51.
J. Ballesta Claver 《Talanta》2009,79(2):499-506
This paper presents an application of chromatographic separation based on an ultra-short monolithic column and chemiluminescent detection in an FIA type instrument manifold for the determination of four paraben mixtures: methylparaben (MP), ethylparaben (EP), propylparaben (PP) and butylparaben (BP). The separation is achieved in 150 s using two consecutive carriers: first 12% ACN:water that changes 75 s after injection to 27% ACN:water. The detection is based on the oxidation of the hydrolysis product of parabens, p-hydroxybenzoic acid, with Ce(IV) in the presence of Rhodamine 6G which evokes chemiluminescence of sufficient intensity to enable a sensitive determination of these species. After optimization of the variables involved, the analytical method is characterized, displaying the following values for concentration ranges, detection limits and precision, as relative standard deviation at low concentration (0.15 mg l−1)—MP: from 9.9 × 10−7 to 3.3 × 10−4 M; 1.9 × 10−8; 5.6%; EP: from 9.0 × 10−7 to 3.3 × 10−4 M; 2.8 × 10−8; 3.5%; PP: from 8.3 × 10−7 to 9.9 × 10−5 M; 2.3 × 10−8; 4.2%; and BP: from 7.7 × 10−7 to 9.9 × 10−5 M; 4.2 × 10−8 M; 6.2%. The method was applied and validated satisfactorily for the determination of these parabens in cosmetic samples, comparing the results against a liquid chromatography reference method. 相似文献
52.
Remanufacturing processes such as refurbishing and reconditioning can extend the life of a product returned from the field. This provides financial opportunities and allows manufacturers to engage in sustainable practices. However, the inability to access a sufficient quantity of reconditioned components from end‐of‐life products can force the concurrent utilization of new components. This paper deals with the determination of an optimal warranty policy where a mixture of new and reconditioned components are used to carry out replacements upon failure for products under warranty. A mathematical optimization model is developed to maximize the manufacturer's expected total profit based on four decision variables: the warranty length, the sale price, the age of reconditioned components, and the proportion of reconditioned components to be used. A numerical procedure is used to compute the optimal solution. Numerical results are provided and discussed to demonstrate the validity and the added value of the proposed approach. Copyright © 2016 John Wiley & Sons, Ltd. 相似文献
53.
Diéguez M Pàmies O Ruiz A Castillón S Claver C 《Chemistry (Weinheim an der Bergstrasse, Germany)》2001,7(14):3086-3094
A series of novel diphosphite ligands derived from readily available D-(+)-glucose has been synthesized. These ligands have been applied to the Rh-catalyzed hydroformylation of vinyl arenes. Both excellent enantioselectivities (up to 91%) and regioselectivities (up to 98.8%) were achieved under mild conditions. The advantage of these ligands is that their modular natures allow facile, systematic variation in the configurations at the stereocenters [C(3), C(5)] at the ligand bridge and in the biphenyl substituents, enabling their effects on the stereoselectivity to be studied. Results show that the absolute configuration of the product is governed by the configuration at the stereogenic center C(3), while the level of the enantioselectivity is influenced by a cooperative effect between stereocenters C(3) and C(5). Replacement of the tert-butyl substituent by methoxy substituents at the para positions of the biphenyl moieties improved the enantioselectivities. We have characterized the rhodium complexes formed under CO/H2 by NMR techniques and in situ IR spectroscopy and have observed that there is a relationship between the structure of the [HRh(CO)2(PP)] species and their enantiodiscriminating performance in hydroformylation. Enantioselectivities were highest with ligands with a strong bis-equatorial coordination preference, while an equilibrium of species with bis-equatorial and equatorial-axial coordination modes considerably reduced the ee's. 相似文献
54.
Summary The reaction of previously reported RhI and IrI cationic complexes towards carbon monoxide and triphenylphosphine has been studied. Carbonyl rhodium(I) mixed complexes of the formulae [Rh(CO)L2(PPh3)]ClO4, (L=tetrahydrothiophene(tht), trimethylene sulfide(tms), SMe2, or SEt2), [(CO)(PPh3)Rh{-(L-L)}2Rh(PPh3)(CO)](ClO4)2 (L-L= 2,2,7,7-tetramethyl-3,6-dithiaoctane (tmdto), (MeS)2(CH2)3 (dth), or 1,4-dithiacyclohexane (dt), [Rh(CO)L(PPh3)2]ClO4 (L= tht, tms, SMe2, or SEt2), and carbonyl iridium(I) complexes of the formulae [Ir(CO)2(COD)(PPh3)]ClO4, [Ir(CO)(COD)(PPh3)2]ClO4, [(CO)(COD)(PPh3) Ir{-(L-L)} Ir(PPh3)(COD)(CO)](ClO4)2 (L-L = tmdto or dt), [(CO)2 (PPh3)Ir(-tmdto)Ir(PPh3)(CO)2](ClO4)2, [(CO)2(PPh3) Ir(-dt)2Ir(PPh3)(CO)2](ClO4)2, were prepared by different synthetic methods. 相似文献
55.
Rafael Uson Luis A. Oro M. Angeles Garralda M. Carmen Claver Pascual Lahuerta 《Transition Metal Chemistry》1979,4(1):55-58
Summary The syntheses of [Rh(NBD)L2 ]ClO4 complexes (L=nitrogen donor ligand), obtained from [Rh(NBD)2]ClO4, and their reactions with triphenylphosphine and carbon monoxide are described.The catalytic activity of some of these and related complexes is considered. 相似文献
56.
Gual A Axet MR Philippot K Chaudret B Denicourt-Nowicki A Roucoux A Castillon S Claver C 《Chemical communications (Cambridge, England)》2008,(24):2759-2761
Ruthenium nanoparticles (RuNPs) were prepared through the hydrogenation of [Ru(COD)(COT)] (COD = 1,5-cyclooctadiene, COT = 1,3,5-cyclooctatriene) in the presence of diphosphites derived from carbohydrates as stabilizing agents, and interestingly, structural modifications of the diphosphite backbone were found to influence nanoparticle size and dispersity, as well as their catalytic activity in arene hydrogenation. 相似文献
57.
Antonio F. Bella Carmen Claver Francisco Sepúlveda Blanca R. Manzano 《Journal of organometallic chemistry》2008,693(7):1269-1275
Cationic palladium(II) complexes of the type containing bisnitrogen ligands with a pyrazole moiety were synthesized from the corresponding neutral derivatives [PdClMe(N-N′)]. Their characterization by 1H and 13C NMR spectroscopy in solution evidences the presence of the Pd-Me group cis to the pyrazole ring. The catalytic behaviour of the cationic complexes in CO/4-tert-butylstyrene copolymerization and CO/ethylene/4-tert-butylstyrene terpolymerization was investigated. Productivity was greatly enhanced when the reaction was carried out in 2,2,2-trifluoroethanol (TFE). Molecular weights and polydispersity (Mw/Mn) of the obtained polyketones resulted among the best reported for Cs-bisnitrogen planar ligands. 相似文献
58.
A new class of efficient catalysts was developed for the asymmetric hydrogenation of alpha,beta-unsaturated carboxylic acid derivatives by synthesizing a series of novel phosphine-phosphite ligands (4a-d) derived from readily available D-(+)-xylose. Excellent enantioselectivities (> 99%) were achieved under very mild reaction conditions (1 bar H(2) and 20 degrees C). Varying the biphenyl substituents in the phosphite moiety greatly affected the enantioselectivity in the hydrogenation reactions. The results also indicate that the sense of enantioselectivity is mainly controlled by the configuration of the phosphite moiety. (31)P[(1)H] NMR and kinetic studies on intermediates of the catalytic cycle show that the [Rh(P(1)-P(2))(enamide)]BF(4) species is the resting state and that the rate dependence is first order in rhodium and hydrogen pressure and zero order in enamide concentration. 相似文献
59.
Delphine Sanhes Aitor Gual Sergio Castillón Carmen Claver Montserrat Gómez Emmanuelle Teuma 《Tetrahedron: Asymmetry》2009,20(9):1009-1014
Two novel diphosphite ligands derived from 9,10-dihydroanthracene originating from a 1,3-diol chiral fragment, have been prepared in high yields from readily available starting materials. Rhodium and palladium catalytic systems containing these new P-donor ligands led to 51% and 90% ee in asymmetric hydroformylation and allylic alkylation processes, respectively. 相似文献
60.
Blanco C Godard C Zangrando E Ruiz A Claver C 《Dalton transactions (Cambridge, England : 2003)》2012,41(23):6980-6991
Palladium complexes bearing monodentate and bidentate phosphine ligands (1-7) were synthesised and used as catalyst precursors in the methoxycarbonylation of norbornene. The catalytic systems bearing ligands 1, 3 and 4 afforded excellent conversions (>99%) and selectivity of the ester (>99%). NMR investigations showed that using complex 1a as the precursor resulted in the protonated phosphine, 1-H(+), being formed under catalytic conditions and thus the addition of acid is not required for the activation of this system since the reaction involving the precursor with methanol under CO pressure produces 2 equivalents of HCl and leads to the formation of the active species. The protonation of ligand 4 under methoxycarbonylation conditions was also observed and the diprotonated diphosphine was isolated and characterised. This compound was tested as a ligand and acid source in a catalysis and provided excellent conversion and high selectivity to the ester. 相似文献