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51.
Lewis acids such as ZrCl(4) or HfCl(4) catalyze the hydrostannation of acetylenes 1 by tributyltin hydride to produce the cis vinylstannanes 2 by regio- and stereoselective anti-hydrostannation. The hydrostannation of acetylenes using dibutyltin dihydride was also catalyzed by ZrCl(4) to give the stereodefined Z-Z divinyltin derivatives 4 by an anti-hydrostannation pathway. The use of nonpolar solvents such as toluene or hexane was essential for obtaining high stereoselectivity and chemical yield. Since ZrCl(4) and HfCl(4) are not soluble in such solvents, the hydrostannations were carried out in a heterogeneous system. The reactions of internal acetylenes with Bu(3)SnH proceeded smoothly, although the use of stoichiometric amounts of ZrCl(4) gave better results. The ZrCl(4)-catalyzed hydrostannation at 0 degrees C gave better yields and stereoselectivities than the reaction at room temperature. To help clalify the reason, the reaction of Bu(3)SnH with ZrCl(4) was monitored by (1)H and (119)Sn NMR spectroscopy, and it was found that Bu(3)SnH reacted with ZrCl(4) at room temperature to afford a mixture of tributyltin hydride, dibutyltin dihydride, and tetrabutyltin. 相似文献
52.
Tan L Yasuda N Yoshikawa N Hartner FW Eng KK Leonard WR Tsay FR Volante RP Tillyer RD 《The Journal of organic chemistry》2005,70(20):8027-8034
[Chemical reaction: See text] A Et3Al mediated intramolecular epoxide opening, cyclopropanation reaction is described. The transformation provided highly functionalized bicyclo[3.1.0]hexane systems in high efficiency and with perfect H or F endo selectivity. Application of this reaction to the synthesis of mGluR2/3 agonist 1 (43% overall yield) and a few intermediates suitable for the synthesis of other bicyclo[3.1.0]hexane mGluR2/3 agonists is discussed. 相似文献
53.
Yamamoto Y Terui N Tachiiri N Minakawa K Matsuo H Kameda T Hasegawa J Sambongi Y Uchiyama S Kobayashi Y Igarashi Y 《Journal of the American Chemical Society》2002,124(39):11574-11575
Paramagnetic NMR and optical studies of the oxidized forms of mesophile Pseudomonas aeruginosa cytochrome c(551) and its quintuple mutant (F7A/V13M/F34Y/E43Y/V78I), and thermophile Hydrogenobacter thermophilus cytochrome c(552) demonstrated that the amino acid side chain packings in the protein interior influence the coordination bond between the heme iron and the axial methionine in the proteins. The strength of heme axial coordinations was found to correlate with the overall protein thermostability. 相似文献
54.
In bacteriorhodopsin (bR), Arg-82bR has been proven to be a very important residue for functional role of this light-driven proton pump. The arginine residue at this position is a super-conserved residue among archaeal rhodopsins. pharaonis phoborhodopsin (ppR; or called as "pharaonis sensory rhodopsin II") has its absorption maximum at 498 nm and acts as a sensor in the membrane of Natronobacterium pharaonis, mediating the negative phototaxis from the light of wavelength shorter than 520 nm. To investigate the role of the arginine residue (Arg-72ppR) of ppR corresponding to Arg-82bR, mutants whose Arg-72ppR was replaced by alanine (R72A), lysine (R72K), glutamine (R72Q) and serine (R72S) were prepared. These mutants were unstable in low concentrations of NaCl and lost their color gradually when the proteins were solubilized with 0.1% n-dodecyl-beta-D-maltoside. The order of instability was R72S > R72A > R72K > R72Q > the wild type. The rates of denaturation were reduced in a solution of high concentrations of monovalent anions. 相似文献
55.
Di Li Naoki Ohashi Shunichi Hishita Taras Kolodiazhnyi Hajime Haneda 《Journal of solid state chemistry》2005,178(11):3293-3302
An overall comparative study was carried out on N-doped, F-doped, and N-F-codoped TiO2 powders (NTO, FTO, NFTO) synthesized by spray pyrolysis in order to elucidate the origin of their visible-light-driven photocatalysis. The comparisons in their experimentally obtained characteristics were based on the analysis of XPS, UV-Vis, PL, NH3-TPD and ESR spectra. The comparisons in their theoretically predicted properties were based on the analysis of the calculated electronic structures. As the results, N-doping into TiO2 resulted in not only the improvement in visible-light absorption but also the creation of surface oxygen vacancies. F-doping produced several beneficial effects including the creation of surface oxygen vacancies, the enhancement of surface acidity and the increase of Ti3+ ions. Doped N atoms formed a localized energy state above the valence band of TiO2, whereas doped F atoms themselves had no influence on the band structure. The photocatalytic tests indicated that the NFTO demonstrated the highest visible-light activity for decompositions of both acetaldehyde and trichloroethylene. This high activity was ascribed to a synergetic consequence of several beneficial effects induced by the N-F-codoping. 相似文献
56.
F-doped TiO2 (FTO) powders were synthesized by spray pyrolysis (SP) from an aqueous solution of H2TiF6. The resulting FTO powders possessed spherical particles with a rough surface morphology and a strong surface acidity. The fluorine concentrations in the FTO powders calculated from XPS spectra significantly depended on SP temperature and ranged from 2.76 to 9.40 at.%. The FTO powder prepared at SP temperature of 1173 K demonstrated the highest photocatalytic activity for the decomposition of gas-phase acetaldehyde under both ultraviolet (UV) and visible light (vis) irradiations, and it was higher than that of commercial P 25. This high photocatalytic activity was ascribed to several beneficial effects produced by F-doping: enhancement of surface acidity, creation of oxygen vacancies, and increase of active sites. It was interesting to point out that the vis photocatalytic activity of FTO powders was achieved by the creation of surface oxygen vacancies rather than the improvement of optical absorption property of bulk TiO2 in vis region. 相似文献
57.
A flexible and stereodivergent synthesis of the cis- and trans-fused 2,5-disubstituted octahydroquinolinone ring systems bearing all four stereogenic centers for the synthesis of the decahydroquinoline type of dendrobatid alkaloids has been achieved. The strategy involves stereoselective and stereodivergent construction of 2,3,6-trisubstituted piperidine ring systems using the Michael type of conjugate addition reaction to the enaminoesters 1 and 3, the intramolecular aldol type of cyclization reaction of keto aldehydes 11 and 12, and ring-closing metathesis of 21 as key steps. 相似文献
58.
Tsuneyuki Sato Naoki Morita Hitoshi Tanaka Tadatoshi Ota 《Journal of polymer science. Part A, Polymer chemistry》1989,27(8):2497-2508
Solvent effect on the polymerization of di-n-butyl itaconate (DBI) with dimethyl azobisisobutyrate (MAIB) was investigated at 50 and 61°C. The solvents used were found to affect significantly the polymerization. The polymerization rate (Rp) and the molecular weight of the resulting polymer are lower in more polar solvents. The initiation rate (Ri) by MAIB, however, shows a trend of being rather higher in polar solvents. The stationary state concentration of propagating poly(DBI) radical was determined by ESR in seven solvents. The rate constants of propagation (kp) and termination (kt) were evaluated by using Rp, Ri, and the polymer radical concentration observed. The kp value decreases fairly with increasing polarity of the solvent used, whereas kt is not so influenced by the solvents. The solvent effect on kp is explained in terms of a difference in the environment around the terminal radical center of the growing chain. Copolymerization of DBI with styrene (St) was also examined in three solvents with different physical properties. The poly(DBI) radical shows a lower reactivity toward St in a more polar solvent. 相似文献
59.
Fukushima S Miyata K Nishiyama N Kanayama N Yamasaki Y Kataoka K 《Journal of the American Chemical Society》2005,127(9):2810-2811
An A-B-C type triblock copolymer, tandemly aligning two types of polycations with different pKa values in a single polymer strand, was developed for the construction of novel polyplex micelles, satisfying a high DNA condensing ability as well as a proton buffering activity directed to elevating gene transfection. The micelle might feature the distinctive three-layered structure, where an inner polyplex layer of condensed pDNA with poly(l-lysine) (pKa approximately 9.4) as the C segment is successively wrapped with an intermediate layer of poly[(3-morpholinopropyl)aspartamide] (B segment) with a comparatively low pKa of approximately 6.2, to provide a buffering effect, and an outer PEG layer (A segment) as a biocompatible palisade. 相似文献
60.
Takashi Murashima Kazuyuki Hayata Jun Matsui Daisuke Miyoshi Takashi Yamada Naoki Sugimoto 《Tetrahedron letters》2007,48(48):8514-8517
The DNAs modified with tetraphenyl porphyrin at the center of 13mer oligonucleotide were synthesized using phosphoramidite chemistry and automated DNA synthesis. When the porphyrin modified oligonucleotide was annealed with its complementary strand, they formed a standard B-form duplex. The porphyrin moiety intercalated in the duplex, and moderately lowered the thermal stability. 相似文献