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991.
Takashi Tozawa Dr. Hitoshi Nagao Yoshinobu Yamane Teruaki Mukaiyama Prof. Dr. 《化学:亚洲杂志》2007,2(1):123-134
Chiral quaternary ammonium phenoxides were readily prepared from commercially available cinchona alkaloids and proved to be useful new asymmetric organocatalysts. Among various chiral quaternary ammonium phenoxides, a cinchonidine‐derived catalyst that bears both a sterically hindered N1‐9‐anthracenylmethyl group and a strongly electron withdrawing 9‐O‐3,5‐bis(trifluoromethyl)benzyl group were found to be highly effective for the Michael addition of ketene silyl acetals (derived from phenyl carboxylates) and α,β‐unsaturated ketones followed by lactonization. Optically active 3,4‐dihydropyran‐2‐one derivatives were obtained in high yields with excellent control of enantio‐ and diastereoselectivity. This catalyst can be handled in air and stored at room temperature in a sealed bottle without decomposition for at least one month. 相似文献
992.
María L. Dell'Arciprete Carlos J. Cobos Dr. Jorge P. Furlong Dr. Daniel O. Mártire Dr. Mónica C. Gonzalez Dr. 《Chemphyschem》2007,8(17):2498-2505
The kinetics of the oxidation of pyridine, 3‐chloropyridine, 3‐cyanopyridine, 3‐methoxypyridine and 3‐methylpyridine mediated by SO4 < M ‐> radicals are studied by flash photolysis of peroxodisulphate, S2O82?, at pH 2.5 and 9. The absolute rate constants for the reactions of both, the basic and acid forms of the pyridines, are determined and discussed in terms of the Hammett correlation. The monosubstituted pyridines react about 10 times faster with sulphate radicals than their protonated forms, the pyridine ions. The organic intermediates are identified as the corresponding hydroxypyridine radical adducts and their absorption spectra compared with those estimated employing the time‐dependent density functional theory with explicit account for bulk solvent effects. A reaction mechanism which accounts for the observed intermediates and the pyridinols formed as products is proposed. 相似文献
993.
994.
New theorems are established for cages (or polyhedra) with trivalent vertices. One theorem says that all such cages have at least three Kekulé structures (or perfect matchings). Thence, resonance generally appears as a possibility. Another theorem says that for every even vertex count >70 there is at least one cage of a preferable subclass, while for vertex count <70 the sole preferable cage is that of the truncated icosahedron. Thence, the unique role of the buckminsterfullerene structure for C60 is mathematically indicated.[/p]Work supported by the Welch Foundation of Houston, Texas. 相似文献
995.
Fullerenes-extracted soot (FES) is the by-product of fullerenes production. Retention characteristics at different temperatures for 17 volatile organic compounds (VOCs) on FES are measured. The adsorption and desorption efficiencies for VOCs on FES adsorbent tubes range from 40.8 to 117%, most of them being 100+/-20%. The values are compared with Tenax GR, an adsorbent commonly used in environmental analysis. FES can be used as an adsorbent of low cost to collect VOCs in environmental samples. 相似文献
996.
Using a modification of the Kharasch-Sosnovsky reaction, the oxidation of oxazolines and thiazolines bearing a variety of 2-alkyl substituents (chiral and achiral) were smoothly oxidized to their corresponding oxazoles and thiazoles, respectively. The key feature involved in the successful implementation of this important oxidation was the use of a mixture of Cu(I) and Cu(II) salts to enhance the oxidation of the intermediate captodative radical, 24. The main limitation of this method was shown when the oxidation failed with oxazolines/thiazolines lacking the carboalkoxy group at C-4. 相似文献
997.
J. Wang X. D. Zhang Y. Zhang W. G. Jia Zh. R. Liu 《Journal of Structural Chemistry》2004,45(1):114-123
The crystal and molecular structures of the [PrIII(nta)(H2O)2]·H2O (nta = nitrilotriacetic acids), K3[GdIII(nta)2(H2O)]·6H2O, and K3[YbIII(nta)2]·5H2O complexes have been determined by single-crystal X-ray structure analyses. In [PrIII(nta)(H2O)2]·H2O, the PrIIINO8 part forms a nine-coordinate pseudo-monocapped square antiprismatic structure in which one N and three O atoms are from one nta ligand in the same molecule, three O atoms from another nta ligand in the neighboring molecule and two O atoms from two coordinate water molecules. In K3[GdIII(nta)2(H2O)]·6H2O, the [GdIII(nta)2(H2O)3- complex anion has a nine-coordinate pseudo-monocapped square antiprismatic structure in which each nta acts as a tetradentate ligand with one N atom of the amino group and three O atoms of the carboxylic groups. In K3[YbIII(nta)2]·5H2O, each nta also acts as a tetradentate ligand with one N atom of amino group and three O atoms of the carboxylic groups, but the [YbIII(nta)2
3- complex anion has an eight-coordinate structure with a distorted square antiprismatic prism. All the results including those for [TmIII(nta)(H2O)2]·2H2O confirm the inferences on the coordinate structures and coordination numbers of rare earth metal complexes with the nta ligand. 相似文献
998.
The preparation of polymer microcapsules of well defined size in the range of 10–50 μm with different shell thickness to
core diameter ratios is described. An aerosol of monodisperse droplets of a homogeneous ternary liquid system which contained
a hydrophobic component and a hydrophilic component dissolved in a high-volatile mutual solvent, was produced by dispersing
with a vibrating-orifice aerosol generator. After the evaporation of the solvent in a nitrogen atmosphere the particles demix
and form a two-phase droplet of core-shell type. These droplets were illuminated with UV light and polymerized to highly monodisperse
microcapsules with a solid polymer shell and a liquid core. The properties of the resulting particles (size, size distribution,
shell thickness, shape and surface characteristics) were investigated by scanning electron microscopy, Raman spectroscopy
on single optically levitated particles, and confocal Raman micro spectroscopy. The microcapsules were highly monodisperse
and have spherical shape.
Received: 24 July 1996 Accepted: 29 August 1996 相似文献
999.
Twelve new germanium substituted diphenyltin dipropionates with the general formula (R1GeCHR2‐CHR3COO)2SnPh2 where R1 = N(CH2CH2O)3, (C6H5)3 and (CH3C6H4)3, R2 = H, CH3, C6H5, p‐CH3C6H4, p‐CH3OC6H4, p‐ClC6H4, and R3 = H, CH3 have been synthesized by the reaction of diphenyltin oxide with a germanium substituted propionic acid. All the compounds were characterized by elemental analysis, IR, multi‐nuclear (1H, 13C, 119Sn) NMR and Mössbauer spectroscopies as well as mass spectrometry. The in vitro antibacterial activity of selected compounds is also reported. 相似文献
1000.
Iterative extended Hükel calculations for all valence electrons and iterative PPP calculations in the variable electronegativity formalism for -electrons were performed on benzene, pyridine, fluorobenzene, and the pyrylium ion. The charge distributions for all compounds were found more uniform and plausible with the iteration procedures than without. Polarization effects from the-electrons were found to be of importance for the -electrons. The lone-pair picture of the highest occupied MO in pyridine is preserved in the iterative extended Hückel method, and two lone-pairs were obtained on the fluorine atom of fluorobenzene. The results indicate that this atom is not hybridized.
Sponsored in part by King Gustaf VI Adolfs 70-Years Fund for Swedish Culture, Knut and Alice Wallenberg's Foundation, and in part by the Swedish National Research Council. 相似文献
Zusammenfassung Benzol, Pyridin, Fluorbenzol und Pyriliumion wurden mit einer iterativen EH-Methode und einer iterativen PPP-VE-Methode berechnet. Es zeigte sich, daß Ladungsverteilungen bei Benutzung des Iterationsverfahrens besser beschrieben werden. Polarisationseffekte der-Elektronen auf die -Elektronen stellen sich als wichtig heraus. Auch in der IEH-Methode bleibt das oberste besetzte MO des Pyridins ein einsames Elektronenpaar. Am Fluoratom im Fluorbenzol werden zwei einsame-Elektronenpaare erhalten. Die Resultate führen zu der Annahme, daß dieses Atom nicht hybridisiert ist.
Resumé Des calculs par les méthodes itératives de Hückel étendu pour tous les électrons de valence et de PPP avec électronégativité variable pour les électrons ont été effectués pour le benzène, la pyridine, le fluorobenzène et l'ion pyrylium. Les distributions de charge pour tons ces composés sont plus uniformes et plus plausibles avec les procédés itératis que sans. Les électrons ont un effet de polarisation important sur les électrons . La plus haute orbitale occupée dans la pyridine reste identifiable comme la paire libre dans la méthode de Hückel étendue, et l'on obtient deux paires libres sur l'atome de fluor du fluorobenzène. Cet atome n'est pas hybride.
Sponsored in part by King Gustaf VI Adolfs 70-Years Fund for Swedish Culture, Knut and Alice Wallenberg's Foundation, and in part by the Swedish National Research Council. 相似文献