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91.
Imidazol-2-ylidenes, a family of N-heterocyclic carbenes (NHC), are efficient catalysts in the transesterification involving numerous esters and alcohols. Low catalyst loadings of aryl- or alkyl-substituted NHC catalysts mediate the acylation of alcohols with enol acetates in short reaction times at room temperature. Commercially available and more difficult to cleave methyl esters react with primary alcohols in the presence of alkyl-substituted NHC to efficiently form the corresponding esters. While primary alcohols are selectively acylated over secondary alcohols with use of enol esters as acylating agents, methyl and ethyl esters can be employed as protective agents for secondary alcohols in the presence of the more active alkyl-substituted NHC catalysts. The NHC-catalyzed transesterification protocol was simplified by generating the imidazol-2-ylidene catalysts in situ. 相似文献
92.
2-(2-Bromophenyl)ethyl groups have been used as building blocks in radical cyclisation reactions onto azoles to synthesise tri- and tetra-cyclic heterocycles. 2-(2-Bromophenyl)ethyl methanesulfonate was used to alkylate azoles (imidazoles, pyrroles, indoles and pyrazoles) for the synthesis of the radical precursors. Cyclisations of the intermediate aryl radicals yield new 6-membered rings attached to the azoles. The aryl radicals undergo intramolecular homolytic aromatic substitution onto the azole rings. Tributylgermanium hydride has been used with success to replace the toxic and troublesome tributyltin hydride. Initial studies show that the protocol can be used on solid phase resins. The molecular and crystal structures of methyl 5,6-dihydroimidazo[5,1-a]iso-quinoline-1-carboxylate and methyl 5,6-dihydroimidazo[2,1-a]isoquinoline-3-carboxylate were determined by X-ray crystallography. 相似文献
93.
Amyes TL Diver ST Richard JP Rivas FM Toth K 《Journal of the American Chemical Society》2004,126(13):4366-4374
We report second-order rate constants kDO (M-1 s-1) for exchange for deuterium of the C(2)-proton of a series of simple imidazolium cations to give the corresponding singlet imidazol-2-yl carbenes in D2O at 25 degrees C and I = 1.0 (KCl). Evidence is presented that the reverse protonation of imidazol-2-yl carbenes by solvent water is limited by solvent reorganization and occurs with a rate constant of kHOH = kreorg = 10(11) s-1. The data were used to calculate reliable carbon acid pK(a)s for ionization of imidazolium cations at C(2) to give the corresponding singlet imidazol-2-yl carbenes in water: pKa = 23.8 for the imidazolium cation, pKa = 23.0 for the 1,3-dimethylimidazolium cation, pKa = 21.6 for the 1,3-dimethylbenzimidazolium cation, and pKa = 21.2 for the 1,3-bis-((S)-1-phenylethyl)benzimidazolium cation. The data also provide the thermodynamic driving force for a 1,2-hydrogen shift at a singlet carbene: K12 = 5 x 10(16) for rearrangement of the parent imidazol-2-yl carbene to give neutral imidazole in water at 298 K, which corresponds to a favorable Gibbs free energy change of 23 kcal/mol. We present a simple rationale for the observed substituent effects on the thermodynamic stability of N-heterocyclic carbenes relative to a variety of neutral and cationic derivatives that emphasizes the importance of the choice of reference reaction when assessing the stability of N-heterocyclic carbenes. 相似文献
94.
Millea KM Kass IJ Cohen SA Krull IS Gebler JC Berger SJ 《Journal of chromatography. A》2005,1079(1-2):287-298
The performance characteristics of multidimensional liquid chromatographic protein separations were evaluated using on-line electrospray mass detection, and a novel workflow for automated LC/MS data processing. Two-dimensional ion exchange/reversed-phase LC separations of Escherichia coli cytosol were conducted using either a continuous linear or discontinuous step gradient in the first dimension. Chromatographic profiles of the top 100 most abundant components were characterized to assess overall separation reproducibility within each mode, and to characterize differences in component distribution between the two modes of operation. Analysis of the resulting data indicates that multidimensional separations of complex protein mixtures can be done reproducibly. Furthermore, under the conditions employed within this study, a linear first dimension gradient was more effective at fractionating the protein mixture, distributing fewer major components to multiple second dimension cycles than an equivalent step gradient. The application of on line mass spectrometry, and automated processing of the resulting data, proved valuable for producing component level analysis of multidimensional protein separations. 相似文献
95.
Zhang Y Furyk S Bergbreiter DE Cremer PS 《Journal of the American Chemical Society》2005,127(41):14505-14510
Aqueous processes ranging from protein folding and enzyme turnover to colloidal ordering and macromolecular precipitation are sensitive to the nature and concentration of the ions present in solution. Herein, the effect of a series of sodium salts on the lower critical solution temperature (LCST) of poly(N-isopropylacrylamide), PNIPAM, was investigated with a temperature gradient microfluidic device under a dark-field microscope. While the ability of a particular anion to lower the LCST generally followed the Hofmeister series, analysis of solvent isotope effects and of the changes in LCST with ion concentration and identity showed multiple mechanisms were at work. In solutions containing sufficient concentrations of strongly hydrated anions, the phase transition of PNIPAM was directly correlated with the hydration entropy of the anion. On the other hand, weakly hydrated anions were salted-out through surface tension effects and displayed improved hydration by direct ion binding. 相似文献
96.
Reactions of CrO2F2 with MF or MF2 gave the corresponding M2CrO2F4 and MCrO2F4 fluorochromates. With the Lewis Acids (SO3, TaF5, SbF5) and (CF3CO)2O known and new chromyl compounds [CrO2(CF3COO)2, CrO2(SO3F)2, CrO2FTaF6, CrO2FSbF6, CrO2FSb2F11] were produced. Chromyl fluoride and inorganic salts (CF3COONa and NaNO3) produced the following complexes - Na2CrO2F2(CF3COO)2 and Na2CrO2F2(NO3)2. Unusual solid products were obtained with CrO2F2 and NO, NO2, SO2.A new method of preparing CrO2F2 is also presented. 相似文献
97.
On-chip electrochromatography using sol-gel immobilized stationary phase with UV absorbance detection 总被引:1,自引:0,他引:1
A chromatography column on a chip was fabricated by immobilizing reversed-phase stationary phase particles (5 microm, C4) using sol-gel technology. Channels were fabricated in quartz using photolithography and wet etching. Localization of the stationary phase was achieved by immobilizing the stationary phase at the desired location in the separation channel prior to bonding of the cover plate. Cross channel design was employed for gated injection. An optical fiber setup was developed for carrying out on-chip UV absorbance detection. The effective optical path length was theoretically determined for the trapezoidal shaped channel and the result was shown to match closely with the experimentally determined value. The effect of applied voltage on velocity was evaluated using thiourea as an unretained marker. Separation performance of the stationary phase was demonstrated by separation of three peptides (Trp-Ala, Leu-Trp and Trp-Trp) under isocratic chromatographic conditions. 相似文献
98.
99.
The polymer-assisted solution-phase (PASP) synthesis of a 192-member 2-D array of 1,5-biaryl pyrazoles 4[1-12,1-16] is reported. The synthesis was performed in a fully automated manner using a multiprobe top-filtration robot and incorporates a "catch and release" step to afford library compounds directly in high yield and purity. 相似文献
100.
Heidari AH Braun RJ Hirsa AH Snow SA Naire S 《Journal of colloid and interface science》2002,253(2):295-307
The drainage of a thin liquid film with an insoluble monolayer down a vertical wall is studied. Lubrication theory is used to develop a model where the film is pinned at the top with a given thickness and the film drains into a bath at the bottom. A nonlinear equation of state is used for the surface tension and the surface viscosity is a nonlinear function of the surfactant concentration; these are appropriate for some aqueous systems. The three partial differential equations are solved via discretization in space and then the resulting differential algebraic system is solved. Results are described for a wide range of parameters, and the conditions under which the free surface is immobilized are discussed. 相似文献