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971.
Thomas J. Colacot 《Journal of Chemical Sciences》2000,112(3):197-207
The chiral dirhodium(II) carboxamidates are a unique class of chiral catalysts useful for asymmetric inter- and intramolecular
cyclopropanation, cyclopropenation and C-H insertion reactions with excellent enantioselectivities. The broad applications
of these catalysts in organic syntheses are briefly reviewed. 相似文献
972.
Kazuhiro Yamamoto Yoshiyuki Ishihama Kazuya Sakata 《Journal of polymer science. Part A, Polymer chemistry》2010,48(17):3722-3728
Previously described Cr‐montmorillonite (Cr‐MMT) was found to retain reactivity in the ethylene polymerization reaction regardless of which alkyl‐metal was used for workup in the preparation process, as long as alkylaluminium was used as a cocatalyst in the actual polymerization reaction. Introduction of hydrogen pressure was found to regulate the polymerization to give the product polymer with a narrower weight distribution, albeit with a somewhat smaller average molecular weight. Supporting metallocene onto Cr‐MMT produced a binuclear catalyst system which gave rise to bimodal polyethylene (PE). Polymer composition of the produced high density polyethylenes (HDPEs) could be controlled by changing factors such as the polymerization conditions and the identity of the metallocene compounds. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 3722–3728, 2010 相似文献
973.
Antonella Silvestri Fabia Grisi Stefano Milione 《Journal of polymer science. Part A, Polymer chemistry》2010,48(16):3632-3639
The alcoholysis of the heteroscorpionate methyl aluminum complex (bpzmp)AlMe2 ( 1 ) (bpzmp = 2,4‐di‐tert‐butyl‐6‐(bis‐(3,5‐dimethylpyrazol‐1‐yl)methyl)phenoxo), promoted both by phenol and isopropanol, has been investigated. The reaction of 1 with phenol afforded the dimeric mono(phenoxo) derivative 2 , whereas the alcoholysis of 1 with the less acidic isopropanol involved the coordinated heteroscorpionate ligand and led to the tetrahedral complex 3 in which the aluminum atom is surrounded by one κ2‐N,O? coordinated bpzmp ligand and one η1‐O? coordinated ppzmp ligand (ppzmp = 2,4‐di‐tert‐butyl‐6‐(i‐propoxy‐(3,5‐dimethylpyrazol‐1‐yl)methyl)phenoxo). Complexes 1 – 3 have been tested in the ring opening polymerization (ROP) of L ‐lactide. The dimeric mono(phenoxo) derivative 2 was inactive in the ROP of L ‐lactide. Quite surprisingly, complex 3 was found to be active in ROP of L ‐ and rac‐lactide, showing a good molar‐mass control. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 3632–3639, 2010 相似文献
974.
Backbone rearrangement during olefin capture as the rate limiting step in molecular olefin polymerization catalysis and its effect on comonomer affinity 下载免费PDF全文
Francesco Zaccaria Roberta Cipullo Peter H. M. Budzelaar Vincenzo Busico Christian Ehm 《Journal of polymer science. Part A, Polymer chemistry》2017,55(17):2807-2814
Available experimental data for several metallocenes indicate that the ethene/propene copolymerization ratio rc can be much more temperature dependent than would be expected if competing insertion transition states (TS) are rate limiting. Detailed exploration of the reaction paths reveals in several cases the existence of a “capture‐like” transition state before the actual insertion, with free energies close to the insertion TS. Movement around these transition states does not just involve monomer and chain, but also clear distortion of the ligand skeleton to allow entry of the monomer. Taking these additional TSs into account leads to much improved agreement with experiment for a series of metallocenes and a constrained geometry catalyst system. Depending on catalyst and temperature, selectivity is determined by competing insertion/insertion, capture/insertion or capture/capture. It seems likely that this is a common situation especially for highly efficient catalysts, complicating (but not preventing) prediction of copolymerization performance. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 2807–2814 相似文献
975.
Ludwig Maier 《Phosphorus, sulfur, and silicon and the related elements》2013,188(3-4):465-470
Abstract N,N-Disubstituted formamides such as dimethylformamide. N-formylpyrrolidine, N-formylpiperidine etc. and also pyridine and hexamethylphosphonic acid triamide catalyze the chlorination of phosphonates with thionyl chloride to give phosphonyl dichlorides in high yield. Thus RP(O)CI2, R ? CH3, C2H5, n-C12H25, C6H5, have been isolated in better than 90 per cent yield. The procedure is less satisfactory for the production of 2-chloroethylphosphonyl dichloride. Only a 34 per cent yield was realized. The chlorination proceeds very likely through the intermediate formation of the halfesters, RP(O)(OR)CI since these could be isolated under favorable circumstances. 相似文献
976.
A two-stage packing of a 30 wt.% Ga2O3/Al2O3 catalyst and a mixture catalyst of 30 wt.% Ga2O3/Al2O3 with 10 wt.% of Mn2O3 is shown to be quite efficient to NO selective reduction by a lower hydrocarbon under lean conditions. A high NO reduction conversion was observed in the temperature range of 200-600°C. 相似文献
977.
Nitr0alkenesareversatilesyntheticintermediatesaspowerfulelectron-deficientreactantsincycloadditionandconjugatadditionreacti0ntoproducealargevarietyfunctionalizedcarbocyclicrings'.Purthermore,thenitrogrouPcanbeconvertedintoawiderangeoffiinctionalities2-Al0ughDiels-Alderreaction3andcyclopropanationoffunctionalizedcarbonnucleophiles4toY-chiralnitr0alkenehavebeenreportedrecently.Sofarthedetailedresearch0ftheconjugateadditionsoffimctionalizedalcoholst0chira1nitf0keneshasn0tbeenreported.Herewewish… 相似文献
978.
Maria Luisa Cubeiro Hernan Morales Mireya R. Goldwasser Maria Josefina Pérez-Zurita Fernando González-Jiménez 《Reaction Kinetics and Catalysis Letters》2000,69(2):259-264
Catalysts containing copper, zinc and aluminium oxides are used extensively in hydrogenation reactions, low temperature water-gas shift and low pressure methanol synthesis processes. In the present study, the effects of certain pertinent preparation parameters on catalysts' activity have been studied, applying the saturated fractional factorial experimental design method. The influence of time and temperature of calcination, aging time, the method of mixing of reactant solutions, the nature of the precipitant and the molar ratio of the latter to metal nitrates have been investigated. The catalysts prepared, using sodium carbonate showed higher activities relative to those synthesized by applying sodium hydrogen carbonate. Higher temperatures and longer time of calcination have profound effects on the activity of catalysts prepared, using sodium hydrogen carbonate. Shorter aging times and higher molar ratios of precipitant to metal nitrates promoted the activity of catalysts prepared by sodium carbonate. In case of catalysts synthesized, using sodium hydrogen carbonate as the precipitant, the aging time should be maintained as short as possible in order the catalysts to acquire higher activities. 相似文献
979.
Afzal M. Yasmeen G. Saleem M. Afzal J. 《Journal of Thermal Analysis and Calorimetry》2000,62(1):277-284
The reduction of metal (Co, Ni, and Cu)-exchangedzeolite-A was studied by a temperature programmed reduction (TPR) technique.
The TPR profiles indicate that the metals are in a dispersed form. The hydrogen consumption in the reduction process demonstrates
that the metals are present in monovalent and divalent forms. High-temperature reduction peaks are also observed in the cases
of CoA and NiA. Nitrogen adsorption reveals that, on heating at high temperature, the pore capacity of zeolite-A increases
when exchanged with transition metals because more space is occupied by water molecules. This is confirmed by thermal analysis.
After dehydration, the cations are in changed positions; they are often located in hidden sites (hexagonal prism and sodalite
cages). The activation energy for the reduction process is calculated.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
980.
Binyuan Liu Yanhao Gao Xin Zhao Weidong Yan Xianhong Wang 《Journal of polymer science. Part A, Polymer chemistry》2010,48(2):359-365
Alternating copolymerization of propylene oxide (PO) and carbon dioxide (CO2) was realized under mild conditions with a moderate turnover frequency (TOF), employing sole bifunctional cobalt salen complexes containing Lewis acid metal center and covalent bonded Lewis base on the ligand. Variation of the covalent bonded Lewis base substituents on the salen ligands could tailor the catalytic activity with TOF changing from 19.3 to 34.9 h?1, polymeric/cyclic carbonate selectivity from 95.3 to 72.8%, and the head‐to‐tail structure in the polymer from 72.2 to 86.0%. The IR analysis confirmed that the Lewis base moiety on one molecule could coordinate with cobalt center of adjacent molecule, playing similar role to the Salen metal complex/Lewis base binary catalytic system. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 359–365, 2010 相似文献