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21.
Nanometer-sized metallic palladium particles can be synthesized by hydrogen reduction of Pd2+ ions dissolved in the water core of a water-in-CO2 microemulsion. The Pd nanoparticles, stabilized by the micromeulsion and uniformly dispersed in the supercritical fluid phase, are effective catalysts for hydrogenation of olefins. Examples of rapid and efficient hydrogenation of water-soluble and CO2-soluble olefins catalyzed by the Pd nanoparticles in supercritical CO2 are given. 相似文献
22.
Redox reactions of Co(edta)? with Ru(NH3)5L2+ (L = 3- and 4-aminopyridine (AmPy)) were found to follow an outer-sphere electron transfer mechanism. The specific rate constants are (3.26 ± 0.03) × 102 and (3.07 ± 0.04) × 103 M?1S?1, for L = 3- and 4-AmPy, respectively, at μ, = 0.10 M LiClO4, pH = 8.0 (tris) and T = 25 °C. The rate constants of oxidations for a series of Ru(NH3)5L2+ complexes are higher than those of the corresponding Fe(CN)5L3- complexes by factors of 4 to 15 even after corrections for differences in reduction potentials and in charges of the complexes. Nonadiabaticity in the reactions of Fe(CN)5L3 complexes may account for the difference in the relative reactivities. 相似文献
23.
Teng-Kuei Yang Dong-Sheng Lee Tsung-Fan Teng Ting-Ting Jong Mei-Yueh Chien 《中国化学会会志》1991,38(4):401-403
Introducing two chiral auxiliaries, sulfoxide and menthol, into an acrylate would have either matched or mismatched pair systems. The presence of chiral menthol in α-sulfinylacrylates can change the diastereoselectivity from 24% d.e. to a single diastereomer by manipulation of the double asymmetric strategy. 相似文献
24.
To facilitate the determination of the fatty acid composition of tissues and the investigation of fatty acid metabolism, we developed a method for the rapid separation by high-performance liquid chromatography and quantitation (by ultraviolet light absorption) of p-bromophenyl esters of fatty acids which vary in chain length from 10 to 22 carbon atoms. The utility of the method was demonstrated by evaluating the fatty acid composition of human uterine decidua vera tissue and human endometrial stromal cells that are maintained in monolayer culture. 相似文献
25.
You-Xian Chen Marvin D. Rausch James C. W. Chien 《Journal of polymer science. Part A, Polymer chemistry》1995,33(13):2093-2108
Bis(1-indenyl)-di[1′S, 2′R, 5′S)-methoxy]silane ( 1 ) was converted into a mixture of corresponding ansa-diastereomeric zirconocenes. Further purification afforded a single dia-stereomer, di[(1′S, 2′R, 5′S)-methoxy] silylene-bis[η5-1(R, R)-(+)-indenyl] dichlorozirconium ( 2 ), which is optically active and hydrocarbon soluble. Extremely rapid ethylene, propylene, and ethylene-hexene polymerizations were observed both in toluene and n-heptane solutions; for instance, at 50°C, activity for ethylene polymerization reaches ~ 1.5×1010 (g of PE/((mol of Zr) · [C2H4] · h). The “bare” zirconocenium ion generated from 2/TIBA/Ph3CB(C6F5)4 exhibits unusual polymerization behaviors; the polymerization activity increases monotonically with temperature of polymerization (Tp) up to a conventional polymerization condition (50–70°C), and the 13C NMR study shows that the isotactic poly-propylene obtained has fairly high [mmmm] methyl pentad distributions at high Tp (?25°C with [mmmm] ~ 0.93–0.75) and a perfect stereoregularity at low Tp (?0°C with [mmmm] > 0.99). The catalyst precursors 2 and Et(Ind)2ZrCl2 ( 3 ) supported on silica by different approaches produced poly(olefins) of different molecular weights and stereoregularities, and a methylaluminokane and Ph3CB(C6F5)4 free silica-supported zirconocene system was found to be activated by triisobutylaluminum. © 1995 John Wiley & Sons, Inc. 相似文献
26.
Zhengtian Yu James C. W. Chien 《Journal of polymer science. Part A, Polymer chemistry》1995,33(1):125-135
Stereocontrol energy (ΔE0) is investigated as a measure of enantioselectivity of ansa-zircoocenium catalyst in propylene polymerization; it was calculated with MM2 (molecular mechanics) force field using π complex (°C) and transition state (TS) geometries obtained by ab initio molecular orbital methods. Both rac-ethylenebis (1-η5-indenyl) - ( 1 ) and rac-ethylenebis (1-η5-4,5,7,8-tetrahydroindenyl) ( 2 ) zirconocenium species are isospecific in either the π-complexes or the transition states. The stereoselectivity is greater if there is α-agostic interaction; it is lowered in the cases of β and γ agostic interactions. The 13C-NMR steric pentad distribution indicates the poly(propylene) to be much less stereoregular than that predicted by ΔE0. Following the occurrence of a regiochemical insertion error, the addition of another monomer via any mode is prohibitively unfavorable. The catalyst suffers loss of stereospecificity as temperature of polymerization increases. Insertion via transition states involving different agostic interactions could be one explanation for the observed loss. © 1995 John Wiley & Sons, Inc. 相似文献
27.
James C. W. Chien 《Journal of polymer science. Part A, Polymer chemistry》1979,17(8):2555-2565
The following criteria are proposed to judge whether a coordination polymerization may be diffusion controlled or not: (1) If the number-average molecular weight and polydispersity of the polymer calculated from kinetic rate constants as a function of time agree with the experimental values, the polymerization is not diffusion controlled. (2) The polymerization may be diffusion controlled if the Thiele modulus, the ratio of the characteristic diffusion time to the characteristic reaction time, is much greater than unity; if it is much smaller than unity, the polymerization is reaction controlled. (3) If an initial linear dependence of rate of polymerization on catalyst concentration changes over to a square-root dependence, the polymerization may be diffusion limited. (4) The polymerization is likely to be diffusion limited if the instantaneous rate of polymerization is proportional to the rate of particle growth when the proportionality coefficient is the surface area of the particle. Criterion (1) is a necessary and sufficient condition as stated, as its converse is not true. All the other criteria are merely necessary but not sufficient conditions. The established Ziegler–Natta catalysts have activities too low to cause diffusion limitation; the Phillips catalyst system is likely to be diffusion limited. The polydispersity of polyolefins produced with Ziegler–Natta catalysts are not the consequence of diffusion control but are the characteristics of the catalysts in their kinetics of initiation, propagation, chain transfer, and termination. 相似文献
28.
29.
Ya‐Ching Shen Yun‐Sheng Lin Shaw‐Man Hsu Ashraf Taha Khalil Shih‐Sheng Wang Ching‐Te Chien Yao‐Haur Kuo Chang‐Hung Chou 《Helvetica chimica acta》2007,90(7):1319-1329
The phytochemical investigation of the more polar fractions from the leaves and twigs of Taxus sumatrana (Taxaceae) afforded five new taxane diterpene esters, tasumatrols P–T ( 1 – 5 ) possessing an 11(15→1),11(10→9)‐diabeotaxane skeleton. Compounds 1, 4 , and 5 contain an α‐hydroxy group at C(14), while 3 has no OH group at either C(13) or C(14). Compound 2 is a natural 4,5‐acetonide derivative, while 4 has an unusual spiro‐connected 2‐hydroxy‐2‐phenyl‐1,3‐dioxolane ring. Ten known taxoids, were also isolated in the course of the chromatographic fractionation. Five additional new O‐acetyl derivatives 3a, 4a, 4b, 5a , and 5b were prepared from the taxanes 3 – 5 . The structures of all new compounds were established on the basis of their spectroscopic analyses. Compound 1 showed mild cytotoxic activity against human Hela and Daoy tumor cells. 相似文献
30.
Aihara H Alston-Garnjost M Bakken JA Barbaro-Galtieri A Barnes AV Barnett BA Bengtsson H Blumenfeld BJ Bross AD Buchanan CD Chamberlain O Chien C Clark AR Cordier A Dahl OI Day CT Derby KA Eberhard PH Fancher DL Fujii H Fujii T Gabioud B Gary JW Gorn W Hadley NJ Hauptman JM Hofmann W Huth JE Hylen J Kamae T Kaye HS Kenney RW Kerth LT Koda RI Kofler RR Kwong KK Layter JG Lindsey CS Loken SC Lu X Lynch GR Madansky L Madaras RJ Maruyama K Marx JN Matthews JA Melnikoff SO Moses W Nemethy P 《Physical review letters》1985,54(4):270-273