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841.
Tae Hee Kim Hyun Mo Lee Hee Soo Park Sung Dong Kim Su Jin Kwon Atsushi Tahara Hideo Nagashima Bun Yeoul Lee 《应用有机金属化学》2019,33(4)
The original Sasol catalytic system for ethylene tetramerization is composed of a Cr source, a PNP ligand, and MAO (methylaluminoxane). The use of expensive MAO in excess has been a critical concern in commercial operation. Many efforts have been made to replace MAO with non‐coordinating anions (e.g., [B(C6F5)4]?); however, most of such attempts were unsuccessful. Herein, an extremely active catalytic system that avoids the use of MAO is presented. The successive addition of two equivalent [H(OEt2)2]+[B(C6F5)4]? and one equivalent CrCl3(THF)3 to (acac)AlEt2 and subsequent treatment with a PNP ligand [CH3(CH2)16]2C(H)N(PPh2)2 ( 1 ) yielded a complex presumably formulated as [ 1 ‐CrAl (acac)Cl3(THF)]2+[B(C6F5)4]?2, which exhibited high activity when combined with iBu3Al (1120 kg/g‐Cr/h; ~4 times that of the original Sasol system composed of Cr (acac)3, iPrN(PPh2)2, and MAO). Via the introduction of bulky trialkylsilyl substituents such as –SiMe3, –Si(nBu)3, or –SiMe2(CH2)7CH3 at the para‐position of phenyl groups in 1 (i.e., by using [CH3(CH2)16]2C(H)N[P(C6H4‐p‐SiR3)2]2 instead of 1 ), the activities were dramatically improved, i.e., tripled (2960–3340 kg/g‐Cr/h; more than 10 times that of the original Sasol system). The generation of significantly less PE (<0.2 wt%) even at a high temperature is another advantage achieved by the introduction of bulky trialkylsilyl substituents. NMR studies and DFT calculations suggest that increase of the steric bulkiness on the alkyl‐N and P‐aryl moieties restrict the free rotation around (alkyl)N–P (aryl) bonds, which may cause the generation of more robust active species in higher proportion, leading to extremely high activity along with the generation of a smaller amount of PE. 相似文献
842.
An amphiphilic tetrathiafulvalene molecule can gelate a variety of organic solvents in view of multiple intermolecular interactions, especially in polar solvent with the formation of highly-ordered columnar structures. The formation of mixed-valence states shows the semiconductive behaviors with the conductivity of 10-4 S/cm, as promising candidates for organic electronics. 相似文献
843.
The γ‐brass type phase Pt2Zn11—δ (0.2 < δ < 0.3) was prepared by reaction of the elements in evacuated silica ampoules. The structures of crystals grown in the presence of excess zinc or alternatively excess platinum were determined from single crystal X‐ray diffraction intensities and confirmed by Rietveld profile fits. Pt2Zn10.72(1) crystallizes in the space group I4¯3m, a = 908.55(4) pm, Z = 4. The structure refinement converged at RF = 0.0302 for Io > 2σ (Io) for 293 symmetrically independent intensi ties and 19 variables. The structure consists of a 26 atom cluster which is comprised of four crystallographically distinct atoms. The atoms Zn(1), Pt(1), Zn(2) and Zn(3) form an inner tetrahedron IT, an outer tetrahedron OT, an octahedron OH, and a distorted cuboctahedron CO respectively. About 14 % of the Zn(1) sites are unoccupied. Pt2Zn10.73 melts at 1136(2) K. It is a moderate metallic conductor (ρ298 = 0.2—0.9 mΩ cm) whose magnetic properties (χmol = —4.6 10—10 to —5.4 10—10 m3 mol—1) are dominated by the core diamagnetism of its components. 相似文献
844.
845.
Kim HG Lee CW Yun S Hong BH Kim YO Kim D Ihm H Lee JW Lee EC Tarakeshwar P Park SM Kim KS 《Organic letters》2002,4(22):3971-3974
[formula: see text] A new molecular system, 2,11-dithio[4,4]metametaquinocyclophane containing a quinone moiety, was designed and synthesized. As the quinone moiety can readily be converted into an aromatic pi-system (hydroquinone) upon reduction, the nanomechanical molecular cyclophane system exhibits a large flapping motion like a molecular flipper from the electrochemical redox process. The conformational changes upon reduction and oxidation are caused by changes of nonbonding interaction forces (devoid of bond formation/breaking) from the edge-to-face to face-to-face aromatic interactions and vice versa, respectively. 相似文献
846.
Kang KR Kim JS Chung SI Park MH Kim YW Lim D Lee SY 《Experimental & molecular medicine》2002,34(6):489-495
Deoxyhypusine synthase catalyzes the first step in the posttranslational synthesis of an unusual amino acid, hypusine, in the eukaryotic translation initiation factor 5A (eIF-5A) precursor protein. We earlier observed that yeast recombinant deoxyhypusine synthase was phosphorylated by protein kinase C (PKC) in vitro (Kang and Chung, 1999) and the phosphorylation rate was synergistically increased to a 3.5-fold following treatment with phosphatidylserine (P.Ser)/diacylglycerol (DAG)/ Ca(2+), suggesting a possible involvement of PKC. We have extended study on the phosphorylation of deoxyhypusine synthase in vivo in different cell lines in order to define its role on the regulation of eIF5A in the cell. Deoxyhypusine synthase was found to be phosphorylated by endogenous kinases in CHO, NIH3T3, and chicken embryonic cells. The highest degree of phosphorylation was found in CHO cells. Moreover, phosphorylation of deoxyhypusine synthase in intact CHO cells was revealed and the expression of phosphorylated deoxyhypusine synthase was significantly diminished by diacyl ethylene glycol (DAEG), a PKC inhibitor, and enhanced by phorbol 12-myristate 13-acetate (PMA) or Ca(2+)/DAG. Endogenous PKC in CHO cell and cell lysate was able to phosphorylate deoxyhypusine synthase and this modification is enhanced by PMA or Ca(2+) plus DAG. Close association of PKC with deoxyhypusine synthase in the CHO cells was evident in the immune coprecipitation and was PMA-, and Ca(2+)/phospholipid dependent. These results suggest that phosphorylation of deoxyhypusine synthase was PKC-dependent cellular event and open a path for possible regulation in the interaction with eIF5A precursor for hypusine synthesis. 相似文献
847.
Novel silicon-based alternating copolymers: synthesis,photophysical properties,and tunable EL colors
Hwan Kyu Kim Kyung Lim Paik Nam Seob Baek Youngil Lee Katsumi Yoshino 《Macromolecular Symposia》2003,192(1):135-150
We synthesized novel silicon-based alternating copolymers for tunable electroluminescent (EL) colors by Heck synthetic method. Their thermal, photophysical and electroluminescent properties were studied. Most of them exhibited a blue-green EL color at the operating voltage of lower than 12 V. Unusually, we observed the white EL color from a EL device based on SiPhPVK. From photophysical studies and the time-resolved PL spectroscopies, it might be attributed to the formation of stabilized excited state in SiPhPVK. Furthermore, in order to reduce the operating voltage of their LED with increasing the electron affinity of the main chain in silicon-based alternating copolymers, we synthesized the silicon-based copolymers containing electron transporting oxadiazole units in main chain. We also studied their photophysical and electroluminescent properties. 相似文献
848.
Huang JH Luci JJ Lee TY Swenson DC Jensen JH Messerle L 《Journal of the American Chemical Society》2003,125(7):1688-1689
Reduction of the organoditantalum allene complex (eta-C5Me4R)2Ta2(mu-X)X3(mu-eta1,eta3-C3H4) (R = Me (Cp*), Et; X = Cl, Br) with sodium amalgam leads to the propynylidene complex (eta-C5Me4R)2Ta2(mu-H)2X2(mu-HCCCH) by a formal double 1,3-C-H activation of the allene ligand. The solid-state molecular structure contains a planar HCCCH ligand bridging, in parallel coordination mode, the two tantalum atoms, with the HCCCH and Ta atoms coplanar. Key structural features are a Ta-Ta distance of 2.8817(7) A, propynylidene C-C-C angle of 153.7(13) degrees , C-C distance of 1.370(8) A, Ta-C(central) distance of 2.194(9) A, and Ta-C(terminal) distance of 1.970(9) A. Molecular orbital calculations on the complex at the RHF/SBK(d) and B3LYP/LanL2dz levels of theory demonstrate that the propynylidene ligand is best viewed formally as an allenediylidene(4-) ligand bonded to two d0 tantalum atoms via two Ta=C(terminal) double bonds and an unusual three-center, two-electron bridge bond involving both tantalum atoms and a lone pair on the planar, tetracoordinate central carbon. There is no net Ta-Ta bonding based on the orbital analysis. 相似文献
849.
Phytochemical investigation of the methanol extract of Euchresta formosana resulted in the isolation of thirty-four compounds. Compounds 1, 3-12, 15, 27, 29 and 32-24 were isolated from this species for the first time. These compounds were identified by spectral analyses and tested for antiplatelet aggregation and anti-HIV activities. Among these compounds, tectorigenin (1), 3',4',5-trihydroxyisoflavone (3), and euchretin F (19) were the most effective antiplatelet aggregation compounds; they inhibited both AA- (arachidonic acid) and collagen-induced platelet aggregation. Meanwhile, flemiphyllin (B), quercetin (13), euchretin M (23), and formosanatin C (26) inhibited HIV replication in H9 lymphocyte cells. 相似文献
850.
A series of 43 stilbene derivatives that showed cytotoxicity against human lung carcinoma (A549) was analyzed using comparative molecular field analysis (CoMFA) for defining the hypothetic pharmacophore model. The polyoxylated stilbenes were found to be active inhibitors of tubulin polymerization. Several cis-stilbenes are structurally similar to combretastatins. However, the trans-stilbenes are assumed to be close to resveratrol found in grapes and have been reported to be potential cancer chemopreventive agents by modulating the initiation, promotion, and progression of the carcinogenic process. With several synthesized compounds that were evaluated for antitumor cytotoxicity against human lung tumor cells (A549), the stilbene derivatives were subjected to CoMFA. To perform systematic molecular modeling of these compounds, a conformational search was carried out based on the precise dihedral angle analysis of the lead compound (1p). The X-ray crystallographic structure of combretastatin A-1 was also used for defining the active conformers of the compounds. After determining the energy-minimized conformers of the lead compound (1p), CoMFA was performed using five different alignments. The three dimensional (3D)-quantitative structure-activity relationship study resulted in reasonable cross-validated, conventional r(2) values equal to 0.640 and 0.958, respectively. 相似文献