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101.
Mas-Ballesté R Aullón G Champkin PA Clegg W Mégret C González-Duarte P Lledós A 《Chemistry (Weinheim an der Bergstrasse, Germany)》2003,9(20):5023-5035
Given the nucleophilicity of the [Pt(2)S(2)] ring, the evolution of [Pt(2)(mu-S)(2)(P intersection P)(2)] (P intersection P=1,2-bis(diphenylphosphino)ethane (dppe), 1,3-bis(diphenylphosphino)propane (dppp)) metalloligands in the presence of the simplest electrophilic species, the proton, has been studied. Combined use of experimental and theoretical data has allowed the whole set of reactions ensuing the protonation of the [Pt(2)S(2)] core to be established. The titration of [Pt(2)(mu-S)(2)(P intersection P)(2)] with HCl or HClO(4) was monitored mainly by (31)P[(1)H] NMR and mass techniques. Characterization of all the species involved was completed with the determination of the crystal structure of [Pt(SH)(2)(P intersection P)], for dppe and dppp, and [Pt(3)(mu(3)-S)(2)(dppp)(3)](PF(6))(2). The first protonation step of the [Pt(2)S(2)] core leads to the stable [Pt(2)(mu-S)(mu-SH)(P intersection P)(2)](+) complex, but the second step implies disintegration of the ring, thus giving rise to various mononuclear species. The subsequent evolution of some of these species allows regeneration of [Pt(2)(mu-S)(mu-SH)(P intersection P)(2)](+), evidencing the cyclic nature of this process. Whereas the reaction pathway is essentially common for both phosphine ligands, dppe and dppp, the different coordinating ability of Cl(-) or ClO(4) (-) determines the nature of the final products, [PtCl(2)(P intersection P)], [Pt(3)(mu(3)-S)(2)(P intersection P)(3)]Cl(2) or [Pt(3)(mu(3)-S)(2)(P intersection P)(3)](ClO(4))(2). DFT calculations have corroborated the thermodynamic feasibility of the reactions proposed on the basis of experimental data. 相似文献
102.
Beatriz Hernández‐Carlos Miriam Carmona‐Pineda Claudia Villanueva‐Cañongo Jesús F. López‐Olguín Agustín Aragón‐García Pedro Joseph‐Nathan 《Magnetic resonance in chemistry : MRC》2009,47(11):994-1003
The chemical study of Sechium mexicanum roots led to the isolation of the two new saponins {3‐O‐β‐D ‐glucopyranosyl (1 → 3)‐β‐D ‐glucopyranosyl‐2β,3β,16α,23‐tetrahydroxyolean‐12‐en‐28‐oic acid 28‐O‐α‐L ‐rhamnopyranosyl‐(1 → 3)‐β‐D ‐xylopyranosyl‐(1 → 4)‐α‐L ‐rhamnopyranosyl‐(1 → 2)‐α‐L ‐arabinopyranoside} (1) and {3‐O‐β‐D ‐glucopyranosyl (1 → 3)‐β‐D ‐glucopyranosyl‐2β,3β,16α,23‐tetrahydroxyolean‐12‐en‐28‐oic acid 28‐O‐α‐L ‐rhamnopyranosyl‐(1 → 3)‐β‐D ‐xylopyranosyl‐(1 → 4)‐[β‐D ‐apiosyl‐(1 → 3)]‐α‐L ‐rhamnopyranosyl‐(1 → 2)‐α‐L ‐arabinopyranoside} (2), together with the known compounds {3‐O‐β‐D ‐glucopyranosyl‐(1 → 3)‐β‐D ‐glucopyranosyl‐2β,3β,6β,16α,23‐pentahydroxyolean‐12‐en‐28‐oic acid 28‐O‐α‐L ‐rhamnopyranosyl‐(1 → 3)‐β‐D ‐xylopyranosyl‐(1 → 4)‐α‐L ‐rhamnopyranosyl‐(1 → 2)‐α‐L ‐arabinopyranoside} (3), tacacosides A1 (4) and B3 (5). The structures of saponins 1 and 2 were elucidated using a combination of 1H and 13C 1D‐NMR, COSY, TOCSY, gHMBC and gHSQC 2D‐NMR, and FABMS of the natural compounds and their peracetylated derivates, as well as by chemical degradation. Compounds 1–3 are the first examples of saponins containing polygalacic and 16‐hydroxyprotobasic acids found in the genus Sechium, while 4 and 5, which had been characterized partially by NMR, are now characterized in detail. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
103.
104.
Barreiro E Casas JS Couce MD Sánchez A Sordo J Varela JM Vázquez-López EM 《Dalton transactions (Cambridge, England : 2003)》2005,(9):1707-1715
We investigated the reactions of 1.5 : 1 : 1 mole ratio mixtures of triphenylphosphine, silver nitrate and 3-(aryl)-2-sulfanylpropenoic acids H(2)xspa in chloroform/water, where in the acid nomenclature, spa = 2-sulfanylpropenoato and x = p, Clp, mp, diBr-o-hp or f with p = 3-phenyl-, Clp = 3-(2-chlorophenyl)-, mp = 3-methoxyphenyl-, diBr-o-hp = 3-(3,5-dibromo-2-hydroxyphenyl)- and f = 3-(2-furyl)-. The compounds [Ag(PPh(3))(Hpspa)](1), [(AgPPh3)2(xspa)][x = Clp (2), o-mp (3), p-mp (4), diBr-o-hp (5) and f (6)] and [Ag(PPh3)3(Hfspa)](7) were isolated and all except 7 were characterized by IR, Raman and FAB mass spectrometry and by 1H, 13C and 31P NMR spectroscopy. Compound 6 was also characterized by (13)C CP/MAS, and compounds 1 and 6 by (109)Ag NMR spectroscopy. The crystal structures of 1, 2, 3, 4.(CH3)2CO, 5, 6.(CH3)2CO and 7 were determined by X-ray diffraction. has a supramolecular structure based on hydrogen bonding between dinuclear units, and all the other complexes adopt discrete structures. 2, 3, 4.(CH3)2CO, 5, and 6.(CH3)2CO are tetranuclear, and 7 mononuclear. The tetranuclear complexes contain the eight-membered coordination ring Ag4S2O2 (2, 3, 4.(CH3)2CO, 6.(CH3)2CO) or the twelve-membered ring Ag4(CO2)2S2 (5). 相似文献
105.
[reaction: see text] The Pd-catalyzed cycloaddition of didehydrobiphenylenes 2a,b, generated from the corresponding 3-(trimethylsilyl)-2-biphenylenyl triflates with fluoride, furnishes the C3-symmetric trimers 1a,b in which the embedded triphenylene unit is distorted to increase the aromaticity of the central benzene ring. Cocyclization of 2a,b with dimethyl acetylenedicarboxylate provides the phenanthrene- and naphthalenecarboxylic ester analogues, depending on the catalyst used. 相似文献
106.
A theoretical study of the formation of X-X bonds in complexes with the general formula [M(2)(mu-X)(2)L(4)] (M = group 10 and X = group 16 elements) having d(8) transition-metal atoms is presented. The existence of two energy minima for some complexes, with short and long X-X distances, is shown by density functional theory calculations, and the factors responsible for it are analyzed, including a strong influence of the nature of the metals and ligands on the relative stability of the two isomers. The influence of the bite angle of chelating terminal ligands and the nature of the donor atom on the relative stabilities of the two isomers are also discussed. 相似文献
107.
Dr. Sergi Montserrat Hélio Faustino Prof. Dr. Agustí Lledós Prof. Dr. Jose L. Mascareñas Dr. Fernando López Dr. Gregori Ujaque 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(45):15248-15260
The mechanism of the gold‐catalyzed intermolecular cycloaddition between allenamides and 1,3‐dienes has been explored by means of a combined experimental and computational approach. The formation of the major [4+2] cycloaddition products can be explained by invoking different pathways, the preferred ones being determined by the nature of the diene (electron neutral vs. electron rich) and the type of the gold catalyst (AuCl vs. [IPrAu]+, IPr=1,3‐bis(2,6‐diisopropylphenyl)imidazole‐2‐ylidene). Therefore, in reactions catalyzed by AuCl, electron‐neutral dienes favor a concerted [4+3] cycloaddition followed by a ring contraction event, whereas electron‐rich dienes prefer a stepwise cationic pathway to give the same type of formal [4+2] products. On the other hand, the theoretical data suggest that by using a cationic gold catalyst, such as [IPrAuCl]/AgSbF6, the mechanism involves a direct [4+2] cycloaddition between the diene and the gold‐activated allenamide. The theoretical data are also consistent with the observed regioselectivity as well as with the high selectivity towards the formation of the enamide products with a Z configuration. Finally, our data also explain the formation of the minor [2+2] products that are obtained in certain cases. 相似文献
108.
Foundations of Chemistry - 相似文献
109.
María Díaz-González César Fernández-Sánchez Agustín Costa-García 《Analytical sciences》2002,18(11):1209-1213
Amperometric analysis of indigo carmine at a bare screen-printed electrode placed in an FIA system is reported. This compound is easily detected at a potential of -0.3 V (vs. Ag pseudo-reference electrode) without observing any fouling of the electrode surface, thus allowing the repetitive use of the same electrode in a reproducible manner (coefficients of variation down to 7% for more than 20 consecutive determinations). A linear range of three orders of magnitude and a limit of detection in the sub-micromolar range were attained for this molecule. Based on these studies, indirect amperometric measurements of alkaline phosphatase (ALP) activity in solution were easily carried out using 3-indoxyl phosphate substrate. Its hydrolysis catalyzed by ALP gave rise to indigo product. This product is insoluble in aqueous solutions but it was easily converted into its soluble parent compound, indigo carmine, by addition of fuming sulfuric acid to the reaction media. Using this approach, we achieved a linear range of more than one order of magnitude and a limit of detection of 1 U/l ALP, for an enzymatic reaction time of 60 min. 相似文献
110.
A. Gustavo Gonzalez M. Angeles Herrador Agustín G. Asuero 《Accreditation and quality assurance》2005,10(4):149-154
The first Supplemental Guide to the current edition of the Guide to the expression of uncertainty measurement (GUM) deals with the propagation of distributions, and emphasizes the use of Monte-Carlo Simulation (MCS) for estimating the uncertainty of measurands. In order to carry out MCS we need computer programs for generating pseudo-random numbers and for solving numerically the integral equations arising when simulating the values of the specification variables with a given probability density function (PDF). Crystal-Ball commercial software is very suitable for performing MCS and to estimate the expanded uncertainty for the measurand. The use of Crystal-Ball is illustrated with two working examples dealing with specification models of non-linear features and with correlated variables (such as the slope and intercept of calibration straight lines), respectively.An erratum to this article can be found at 相似文献