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51.
Dr. Gunther Hellmann Dr. Achim Hack Dr. Eric Thiemermann Dr. Olaf Luche Prof. Dr. Gerhard Raabe Prof. Dr. Hans‐Joachim Gais 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(12):3869-3897
Enantiomerically pure triflones R1CH(R2)SO2CF3 have been synthesized starting from the corresponding chiral alcohols via thiols and trifluoromethylsulfanes. Key steps of the syntheses of the sulfanes are the photochemical trifluoromethylation of the thiols with CF3Hal (Hal=halide) or substitution of alkoxyphosphinediamines with CF3SSCF3. The deprotonation of RCH(Me)SO2CF3 (R=CH2Ph, iHex) with nBuLi with the formation of salts [RC(Me)? SO2CF3]Li and their electrophilic capture both occurred with high enantioselectivities. Displacement of the SO2CF3 group of (S)‐MeOCH2C(Me)(CH2Ph)SO2CF3 (95 % ee) by an ethyl group through the reaction with AlEt3 gave alkane MeOCH2C(Me)(CH2Ph)Et of 96 % ee. Racemization of salts [R1C(R2)SO2CF3]Li follows first‐order kinetics and is mainly an enthalpic process with small negative activation entropy as revealed by polarimetry and dynamic NMR (DNMR) spectroscopy. This is in accordance with a Cα? S bond rotation as the rate‐determining step. Lithium α‐(S)‐trifluoromethyl‐ and α‐(S)‐nonafluorobutylsulfonyl carbanion salts have a much higher racemization barrier than the corresponding α‐(S)‐tert‐butylsulfonyl carbanion salts. Whereas [PhCH2C(Me)SO2tBu]Li/DMPU (DMPU = dimethylpropylurea) has a half‐life of racemization at ?105 °C of 2.4 h, that of [PhCH2C(Me)SO2CF3]Li at ?78 °C is 30 d. DNMR spectroscopy of amides (PhCH2)2NSO2CF3 and (PhCH2)N(Ph)SO2CF3 gave N? S rotational barriers that seem to be distinctly higher than those of nonfluorinated sulfonamides. NMR spectroscopy of [PhCH2C(Ph)SO2R]M (M=Li, K, NBu4; R=CF3, tBu) shows for both salts a confinement of the negative charge mainly to the Cα atom and a significant benzylic stabilization that is weaker in the trifluoromethylsulfonyl carbanion. According to crystal structure analyses, the carbanions of salts {[PhCH2C(Ph)SO2CF3]Li? L }2 ( L =2 THF, tetramethylethylenediamine (TMEDA)) and [PhCH2C(Ph)SO2CF3]NBu4 have the typical chiral Cα? S conformation of α‐sulfonyl carbanions, planar Cα atoms, and short Cα? S bonds. Ab initio calculations of [MeC(Ph)SO2tBu]? and [MeC(Ph)SO2CF3]? showed for the fluorinated carbanion stronger nC→σ* and nO→σ* interactions and a weaker benzylic stabilization. According to natural bond orbital (NBO) calculations of [R1C(R2)SO2R]? (R=tBu, CF3) the nC→σ*S? R interaction is much stronger for R=CF3. Ab initio calculations gave for [MeC(Ph)SO2tBu]Li ? 2 Me2O an O,Li,Cα contact ion pair (CIP) and for [MeC(Ph)SO2CF3]Li ? 2 Me2O an O,Li,O CIP. According to cryoscopy, [PhCH2C(Ph)SO2CF3]Li, [iHexC(Me)SO2CF3]Li, and [PhCH2C(Ph)SO2CF3]NBu4 predominantly form monomers in tetrahydrofuran (THF) at ?108 °C. The NMR spectroscopic data of salts [R1(R2)SO2R3]Li (R3=tBu, CF3) indicate that the dominating monomeric CIPs are devoid of Cα? Li bonds. 相似文献
52.
Ludden MJ Sinha JK Wittstock G Reinhoudt DN Huskens J 《Organic & biomolecular chemistry》2008,6(9):1553-1557
Here, the stepwise assembly of an electroactive bionanostructure on a molecular printboard is described. The system consists of a cyclodextrin receptor monolayer (molecular printboard) on glass, a divalent linker, streptavidin (SAv), and biotinylated cytochrome c (cyt c). The divalent linker consists of a biotin moiety for binding to SAv and two adamantyl moieties for supramolecular host-guest interaction at the cyclodextrin molecular printboard. The binding of biotinylated cyt c onto a SAv layer bound to preadsorbed linker appeared to be highly specific. The coverages of cyt c as assessed by UV-vis spectroscopy and scanning electrochemical microscopy (SECM) appeared to be identical indicating that all cyt c units remained active. Moreover, the coverage values corresponded well with an estimate based on steric requirements, and the binding stoichiometry was therefore found to be by two biotin moieties of cyt c per one SAv molecule. 相似文献
53.
Uri Yermiyahu Shlomo Nir Gozal Ben-Hayyim Uzi Kafkafi Gunther F. E. Scherer Thomas B. Kinraide 《Colloids and surfaces. B, Biointerfaces》1999,14(1-4):237-249
The hypotheses that genotypic differences in salinity tolerance may result from (i) differences in global surface charge density or (ii) from differences in global Ca2+ binding were tested. An attempt was made to correlate the differing salinity tolerance of four melon cultivars with surface properties of vesicles extracted from the plasma membrane (PM) of their root cells. Surface characterization involved measurements of electrophoretic mobility and sorption of 45Ca2+ to the vesicles in the presence of varying concentrations of Ca2+, Na+ and Mg2+. Irrespective of salinity tolerance, vesicles from the four cultivars yielded similar ζ potentials under similar conditions, indicating similar global surface charge densities. Sorption studies with vesicles from two cultivars differing in salinity tolerance predicted independently this result of equal surface charge density. The estimated global binding affinities of Ca2+, Na+ and Mg2+ to the PM of both cultivars were the same with binding coefficients of 50, 0.8 and 9 M−1, respectively. Consequently, the hypotheses enumerated above to interpret genotypic differences in salinity toxicity are rejected. However, vesicles from the salt-resistant strain sorbed 19% more Ca2+ per given amount of protein in the membrane, indicating the existence of a larger number of negatively charged surface sites per given amount of protein and a smaller amount of protein per given area of membrane. Genotypic differences in site-specific Ca2+-binding affinity (e.g. at ion channels) remain a viable hypothesis for genotypic differences in salinity tolerance. 相似文献
54.
55.
Binary mixtures of ethylene glycols HO(CH2CH2O)xH (x = 3–5) with glycerol were investigated by fast atom bombardment mass Spectrometry. The relative intensities in the ethylene glycol partial mass spectra as a function of concentration exhibit a steep slope in the low concentration range. With increasing concentration the curves asymptotically approach the 100% line. This effect increases from triethylene to pentaethylene glycol. Comparison of these results with those of surface tension measurements leads to the conclusion that the high sensitivity of fast atom bombardment mass Spectrometry for ethylene glycols is caused by the surface-active properties of these compounds, which lead to an enrichment of the surface investigated with the substance molecules. 相似文献
56.
57.
Damm M Holzer M Radspieler G Marsche G Kappe CO 《Journal of chromatography. A》2010,1217(50):7826-7832
An efficient microwave-assisted high-throughput protein hydrolysis protocol was developed utilizing strongly microwave absorbing silicon carbide-based microtiter platforms. The plates are equipped with 20 bore holes having the proper dimensions for holding standard screw-capped HPLC/GC vials. Due to the possibility of heating up to four heating platforms simultaneously (80 vials), parallel microwave-assisted acid hydrolyses can be performed under carefully controlled conditions significantly reducing the overall time required for protein hydrolysis and the subsequent evaporation step required for larger volumes of acid. An extensive optimization of the hydrolysis conditions has demonstrated that 5min irradiation at 160°C with 6N HCl leads to comparable results in terms of total and individual amino acid recovery as the traditional method requiring 24h heating at 110°C. Complete hydrolysis of several proteins and synthetic peptides was performed using 25μg of sample material and 100μL of 6N HCl in a dedicated low-volume HPLC/GC vial. Since the hydrolysis and subsequent analysis can be performed from the same vial, errors caused by sample transfer can be minimized. Control experiments have demonstrated that the observed rate enhancements are the result of a purely thermal/kinetic effect as a consequence of the considerable higher reaction temperatures. 相似文献
58.
Kumar R Spuesens T Mechet P Kumar P Raz O Olivier N Fedeli JM Roelkens G Baets R Van Thourhout D Morthier G 《Optics letters》2011,36(13):2450-2452
Using a 7.5 μm diameter disk fabricated with III-V-on-silicon fabrication technology, we demonstrate bias-free all-optical wavelength conversion for non-return-to-zero on-off keyed pseudorandom bit sequence (PRBS) data at the speed of 10 Gbits/s with an extinction ratio of more than 12 dB. The working principle of such a wavelength converter is based on free-carrier-induced refractive index modulation in a pump-probe configuration. We believe it to be the first bias-free on-chip demonstration of all-optical wavelength conversion using PRBS data. All-optical gating measurements in the pump-probe configuration with the same device have revealed that it is possible to achieve wavelength conversion beyond 20 Gbits/s. 相似文献
59.
60.
M. Fischer F. Stefani G. Gerbeth 《The European Physical Journal B - Condensed Matter and Complex Systems》2008,65(4):547-554
Using a spherically symmetric mean-field α2-dynamomodel for Earth’s magnetic field reversals, we show thecoexistence of the noise-induced phenomena coherence resonance
and stochastic resonance. Stochastic resonance was recentlyinvoked to explain the 100 kyr periodicity in the distribution
ofresidence times between reversals.The comparison of the resulting residence time distribution withthe paleomagnetic one
allows for some estimate ofthe effective diffusion time of the Earth’s core which may be100 kyr or slightly below rather than200 kyr
as it would result from the molecular resistivity. 相似文献