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901.
Eleven ferrocene derivatives bearing the MM (M = Si and/or Ge) substituent bonded directly to the ring have been found to undergo alcoholysis at the MM linkage in the presence of either ferric chloride or ferricenium tetrachloroferrate. It is suggested that the oxidation-reduction process between the ferrocene derivatives and ferric ions gives rise to the corresponding substituted ferricenium ions, which, in turn may readily undergo a nucleophilic attack by alcohol at the adjacent highly polarized MM bond. The reaction is thus very similar to the acid-catalyzed alcoholysis of these ferrocene derivatives. 相似文献
902.
Syndiospecific living propylene polymerization catalyzed by titanium complexes having fluorine-containing phenoxy-imine chelate ligands 总被引:3,自引:0,他引:3
Mitani M Furuyama R Mohri J Saito J Ishii S Terao H Nakano T Tanaka H Fujita T 《Journal of the American Chemical Society》2003,125(14):4293-4305
The propylene polymerization behavior of a series of Ti complexes featuring fluorine-containing phenoxy-imine chelate ligands is reported. The Ti complexes combined with methylalumoxane (MAO) can be catalysts for living and, at the same time, stereospecific polymerization of propylene at room temperature or above. DFT calculations suggest that the attractive interaction between a fluorine ortho to the imine nitrogen and a beta-hydrogen of a growing polymer chain is responsible for the achievement of room-temperature living propylene polymerization. Although the Ti complexes possess C(2) symmetry, they are capable of producing highly syndiotactic polypropylenes. (13)C NMR is used to demonstrate that the syndiotacticity is governed by a chain-end control mechanism and that the polymerization is initiated exclusively via 1,2-insertion followed by 2,1-insertion as the principal mode of polymerization. (13)C NMR spectroscopy also elucidated that the polypropylenes produced with the Ti complexes possess regio-block structures. Substitutions on the phenoxy-imine ligands have profound effects on catalytic behavior of the Ti complexes. The steric bulk of the substituent ortho to the phenoxy oxygen plays a decisive role in achieving high syndioselectivity for the chain-end controlled polymerization. Over a temperature range of 0-50 degrees C, Ti complex having a trimethylsilyl group ortho to the phenoxy oxygen forms highly syndiotactic, nearly monodisperse polypropylenes (94-90% rr) with extremely high peak melting temperatures (T(m) = 156-149 degrees C). The polymerization behavior of the Ti complexes can be explained well by the recently proposed site-inversion mechanism for the formation of syndiotactic polypropylene by a Ti complex having a pair of fluorine-containing phenoxy-imine ligands. 相似文献
903.
1-Ehtoxycarbonyl-1, 2-dihydroquinoline-2-phosphonates () were treated with n-butyllithium followed by alkyl halides to afford the corresponding 4-alkylated phosphonates () with complete regioselectivity in 67–97% yields. The phosphonates () were converted to 4-alkylquinolines () in ca. 50% yields by treatment with sodium iodide in HMPA or by alkaline hydrolysis in aqueous ethanol. 相似文献
904.
Ultraviolet-B radiation upregulates expression of dectin-2 on epidermal Langerhans cells by activating the gene promoter 总被引:1,自引:0,他引:1
Bonkobara M Yagihara H Yudate T Chung JS Washizu T Ariizumi K Cruz PD 《Photochemistry and photobiology》2005,81(4):944-948
Epidermal Langerhans cells (LC) belong to the antigen-presenting cell (APC) family of dendritic cells that can initiate antigen-specific immunogenic or tolerogenic responses. In mice, we have shown ultraviolet-B (UV-B) irradiation to induce long-lasting suppression (tolerance) of contact hypersensitivity responses by converting LC from immunogenic to tolerogenic APC. The C-type lectin receptor, dectin-2, expressed preferentially by LC and dendritic cells, has also been shown to be involved in inducing this form of UV-B-induced immunosuppression. These observations led us to question whether UV-B can modulate dectin-2 expression by LC. In ICR mice engineered to express the dectin-2 gene promoter linked to a luciferase reporter gene, we found broadband UV-B treatment in vivo to activate the promoter in LC. In wild-type C3H/HeN mice, we found such treatment in vivo to yield LC with increased dectin-2 expression at both mRNA and protein levels. Broadband UV-B treatment in vitro of bone marrow-derived dendritic cells from these mice also showed upregulated expression of dectin-2 mRNA. These findings lead us to conclude that broadband UV-B upregulates dectin-2 expression in LC by activating the dectin-2 gene promoter. Such amplification suggests that UV-B-induced immunosuppression may be due (at least in part) to augmented dectin-2 expression in LC. 相似文献
905.
The structural models of chalcogenide glasses AsxS1?x for x = 0.2 and 0.5 have been constructed by the three-step procedure and the characteristic dependence of atomic scale structure on the As content has been analysed. The AsS bonds are most favorable and the distributions of bond lengths and bond angles are very narrow around their minimum energy configurations. The glass of x = 0.2 consists of cross-linked linear chains and the number of molecular units increases drastically in the range of x = 0.4 ≈ 0.5. Furthermore, the two-dimensional network cannot be seen for x = 0.2 and 0.5, which is the essential difference from the glass with x = 0.4. 相似文献
906.
We have investigated the surface kinetics during metalorganic vapor-phase epitaxy (MOVPE), using high-vacuum scanning tunneling microscopy (STM) observation of two-dimensional (2D) nuclei and denuded zones. Using Monte Carlo simulations based on the solid-on-solid model, from 2D nucleus densities we estimated the surface diffusion coefficients of GaAs and AlAs to be 2 × 10−6 and 1.5 × 10−7 cm2/s at 530°C, and the energy barriers for migration to be 0.62 and 0.8 eV, respectively. The 2D nucleus size in the [110] direction was about two times larger than that in the [
10] direction. The size anisotropy is caused primarily by a difference in the lateral sticking probability (Ps) between steps along the [
10] direction (A steps) and steps along the [110] direction (B steps). The Ps ratio was estimated to be more than 3:1. Denuded zone widths on upper terraces were 2 ± 0.5 times wider than those on lower terraces. This showed that Ps at descending steps was 10 to 3 × 102 times larger than Ps at ascending steps. 相似文献
907.
Satoru Amou Kazuhiko Takeuchi Michihiko Asai Kouichi Niizeki Takashi Okada Makoto Seino Osamu Haba Mitsuru Ueda 《Journal of polymer science. Part A, Polymer chemistry》1999,37(19):3702-3709
Regiocontrolled polymer (2) having 2-naphthol unit was prepared by oxidative coupling polymerization of bis(2-naphthol) (1). Polymerizations were conducted in dichloromethane in the presence of [di-μ-hydroxo-bis(N,N,N′,N′-tetramethylethylenediamine)copper(II)] chloride [CuCl(OH)TMEDA] under air at room temperature, producing polymers with number-average molecular weights up to 12,000. The structure of polymer 2 was characterized by 270 MHz 1H–NMR and 68.5 MHz 13C–NMR spectroscopies and was estimated to consist almost completely of 1,1′-linkage. The polymer was readily soluble in polar aprotic solvents and tetrahydrofuran at room temperature. Thermogravimetric analysis of polymer 2 showed 10% weight loss at 450°C in nitrogen. The model reactions were studied to clarify the applicability of CuCl(OH)TMEDA for coupling of naphthol derivatives. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 3702–3709, 1999 相似文献
908.
Makoto Fujita Shu-Yan Yu Takahiro Kusukawa Hidenori Funaki Katsuyuki Ogura Kentaro Yamaguchi 《Angewandte Chemie (International ed. in English)》1998,37(15):2082-2085
Nanometer-sized bowls: six metal centers and four tridentate ligands self-assemble in aqueous solution to give the macrotricycle 1 , which has approximate dimensions of 3×2×2 nm. Another macrocycle (not shown), obtained with a different ligand, is of similar size and topology but differs considerably from 1 in its host–guest behavior. 相似文献
909.
910.
Shigeo Matsuda Akira Ishikubo Akinori Kuzuya Morio Yashiro Makoto Komiyama 《Angewandte Chemie (International ed. in English)》1998,37(23):3284-3286
Even in the presence of a large excess of Zn II ions , sequence-selective RNA hydrolysis is achieved by DNA conjugates involving a dinuclear ZnII complex (shown schematically). This is because the cooperation of two ZnII ions is essential for the RNA scission. 相似文献