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51.
Nimitsiriwat N Marshall EL Gibson VC Elsegood MR Dale SH 《Journal of the American Chemical Society》2004,126(42):13598-13599
In an unprecedented transformation, amide ligands are found to attack the imine carbon centers of tridentate Schiff base ligands attached to tin. The process is reversible, and the resultant (masked) amide species can be exploited as latent single-site initiators for the controlled polymerization of rac-lactide. 相似文献
52.
This paper compares the kinetics of exchanges of phenylethanethiolate ligands (PhC2S-) of the monolayer-protected clusters (MPCs) Au(38)(SC2Ph)(24) and Au(140)(SC2Ph)(53) with p-substituted arylthiols (p-X-PhSH), where X = NO(2), Br, CH(3), OCH(3), and OH. First-order rate constants at 293 K for exchange of the first ca. 25% of the ligands on the molecule-like Au(38)(SC2Ph)(24) MPC, measured using (1)H NMR, vary linearly with the in-coming arythiol concentration; ligand exchange is an overall second-order reaction. Remarkably, the second-order rate constants for ligand exchange on Au(38)(SC2Ph)(24) are very close to those of corresponding exchange reactions on the larger nanoparticle Au(140)(SC2Ph)(53) MPCs. These are the first results that quantitatively show that the chemical reactivity of different sized nanocrystals is almost independent of size; presumably, this is because the locus of the initial ligand exchanges is a common kind of site, thought to be the nanocrystal vertexes. The rates of later stages of exchange (beyond ca. 25%) differ for Au(38) and Au(140) cores, the latter being much slower presumably due to its larger terrace-like surface atom content. The reverse exchange reaction was studied for Au(38)(p-X-arylthiolate)(24) MPCs (X = NO(2), Br, and CH(3)), where the in-coming ligand is now phenylethanethiol. Remarkably, the rate constants of both forward and reverse exchanges display identical substituent effects, which implies a concurrent bonding of both in-coming and leaving ligands to the Au core in the rate-determining step, as in an associative mechanism. X = NO(2) gives the fastest rates, and the ratio of forward and reverse rate constants gives an equilibrium constant of K(EQ,PE) = 4.0 that is independent of X. 相似文献
53.
This paper describes a rapid, quantitative liquid chromatographic analysis and extraction of methadone and its two major metabolites from rat plasma, using difenoxin as the internal standard. Using a C18 column, resolution of all sample components and the internal standard is achieved with a mobile phase of 25:75 acetonitrile-0.08% diethylamine in 1000 mL water, pH 2.3, at a flow rate of 1.5 mL/min. The injection volume is 100 microL. Standards are linear over the range 25-100 ng, with a lower limit of detection for methadone of 0.25 ng. Within- and between-run coefficients of variation (CV) are 1.24% and 2.94%, respectively. Extraction of methadone and its metabolites from rat plasma uses a solid-phase extraction technique that is highly efficient. Extraction efficiencies of 90.3%, 99.6%, 85.9% and 93.8% were achieved for methadone, its primary and secondary metabolites, and difenoxin, respectively. 相似文献
54.
A synthesis of four Annonaceous acetogenins, asiminocin, asimicin, asimin, and bullanin, by a modular approach from seven fundamental subunits, A-G, is described. The approach employs a central core aldehyde segment, C, to which are appended an aliphatic terminus, A or B, a spacer subunit, D or E, and a butenolide terminus, F or G. Coupling of the A, B, D, and E segments to the core aldehyde unit is effected by highly diastereoselective additions of enantiopure allylic indium or tin reagents. The butenolide termini are attached to the ACD, BCE, or BCD intermediates by means of a Sonogashira coupling. The design of the core, spacer, and termini subunits is such that any of the C30, C10, or C4 natural acetogenins or stereoisomers thereof could be prepared. IC50 values for the four aforementioned acetogenins against H-116 human colon cancer cells were found to be in the 10(-3) to 10(-4) microM range. The IC90 activities were ca. 10(-3) microM for asimicin and asimin but only 0.1-1 microM for bullanin and asiminocin. 相似文献
55.
Nicola M Howarth W.Edward LindsellEuan Murray Peter N Preston 《Tetrahedron letters》2003,44(44):8089-8092
The adenine, cytosine and thymine peptide nucleic acid (PNA) monomers and PNA T10 oligomers bearing either a diacetylenic or stearoyl moiety at the N- or C-terminus have been successfully prepared. The resulting thymine monomeric and T10-mer derivatives have been subsequently incorporated into polydiacetylene-containing liposomes. 相似文献
56.
When the surface of a Pt electrode is oxidized in aqueous 1 M H2SO4 at +0.8 to +2.0 V vs. SSCE for a few seconds to 10 min, disconnected, washed and dried, and placed in CH3CN solvent, a negative potential scan shows that the surface oxide is reduced in two waves at potentials between?1.2 and?1.6 V vs. SSCE. The combined charge of the two waves amounts to 0.3–2.5 layers of oxide, depending on anodization potential and time. The more easily reduced oxide becomes non-reducible after reaction of the electrode with methyltrichlorosilane. The two oxide waves are interpreted as surface and subsurface oxide layers. 相似文献
57.
Wang G Huang T Murray RW Menard L Nuzzo RG 《Journal of the American Chemical Society》2005,127(3):812-813
Visible-near-IR luminescence spectra of gold MPCs that are similar, irrespective of the number of core atoms (all <2 nm diameter) and different monolayers, are reported. The luminescence can be quantitatively invoked by introducing polar ligands into nonpolar MPC monolayers and by galvanic exchange of metal atoms on the MPC core surface with different metals. The observed emissions are believed to result from surface-localized states that depend on both the core metal of the nanoparticle and the ligands attached to the metal surface. 相似文献
58.
Marshall Ledbetter John Cuculo Paul Tucker 《Journal of polymer science. Part A, Polymer chemistry》1984,22(6):1435-1459
The structure and properties of a poly(ethylene terephthalate) extrudate, prepared by converging flow and high pressure, were examined. Relatively high levels of crystallinity and orientation were produced by the combination of an orienting flow and high-pressure quench. The fine structure was examined in detail by x-ray diffraction, negative staining–transmission electron microscopy, and amine etching–gel permeation chromatography. The morphology was directly relatable to this unique preparative technique. Modulus was determined by a compressive Rheovibron technique. Low shrinkage and the compressive modulus were related to the structure in the development of a structural model. 相似文献
59.
Non-conjugated -dialkylated alkenes were oxidized to aldehydes in the presence of α,β-unsaturated carbonyl functional groups, providing a new synthesis of and a synthesis of that led to a revised structure for gastrolactone. 相似文献
60.
A study has been made of the principal rearrangement products resulting from formic acid treatment of 19-hydroxy-ent-beyerene epoxide (3). In concentrated solutions 3 has been found to undergo a deep seated rearrangement to the allylic alcohol (14). A mechanism for the formation of 14 is proposed involving a novel 1,4-hydride shift in the bicyclo[3:2:1]octane C/D ring system following cleavage of the C15O bond. Supporting evidence has been obtained from a study of the specifically labelled epoxide (4), the deuterium in 14 appearing exclusively at C12. Four products (7 and 11–13) emanating from the known beyerane → kaurane interconversion have been identified. 相似文献