Mécanisme de la réaction de cyclisation en milieu aqueux de carbanilates de propargyle en méthylène-4 oxazolidinones-2

The alkaline hydrolysis of propargyl 3-chlorocarbanilates was investigated kinetically using uv spectropotometry. The cyclization to 4-methoylene-2-oxazolisinones proceeds via the numcleophilic attack of the substrate anion concomitantly with that of the hydroxide ion on the triple bond. The nucleophilic attack leads directly to the heterocycle whereas the hydroxide ion attack gives rise to a ketocarbamate which is then cyclized to the oxazolidione.     

Observation of the V2O5(001) surface using ambient atomic force microscopy

Constant force images of the V₂O₅(001) surface were recorded in ambient conditions with atomic force microscopy. All images exhibit the 11.5 Å × 3.5 Å. periodicity expected for a bulk terminated surface. However, images reveal differences from the ideal structure. The experimental results are interpreted in terms of preferential adsorption sites for water molecules. Because these sites are thought to influence the catalytic properties of the surface, their characterization is an important step towards understanding how the atomic-scale structure of a surface influences its properties.     

Palladium catalyzed Suzuki C-C couplings in an ionic liquid: nanoparticles responsible for the catalytic activity

A new family of functionalized ligands derived from norborn-5-ene-2,3-dicarboxylic anhydride has been used in Suzuki C-C cross-couplings between aryl boronic acids and aryl bromide derivatives in [BMI][PF(6)] (BMI=1-n-butyl-3-methyl-imidazolium), using palladium acetate as catalytic precursor. High conversions and yields are obtained when amine ligands containing hydroxy groups are involved. TEM analyses after catalysis show the formation of small nanoparticles, in contrast to the agglomerates observed when nanoparticles are intentionally preformed, with a consequent decrease in the catalytic activity in the latter case. Some tests, including the correlation effect between solvent and ligand, are carried out to try to identify the true nature of the catalyst. All the results obtained suggest that formation of nanoparticles is required to lead to a catalytically active system.     

The mechanism of nucleation and growth from CdGeAs2 glass

Distinctively different microstructures were obtained depending on the heating rate to which CdGeAs₂ glasses were exposed during the devitrification process. Differential scanning calorimetry showed that under very slow heating rates, the single crystallization peak splits into three superimposed exotherms. Quenching from the temperature at which selected features appear on the DSC trace and using X-ray diffraction and transmission electron microscopy interactively, demonstrated the initial formation of a metastable doped precursor germanium crystalline phase. This crystalline structure continued to grow until it could no longer accommodate the impurities, and the transformation terminated. These regions then acted as heterogeneous nucleation sites for the growth of chalcopyrite structured CdGeAs₂ with a spherulitic microstructure. The spherulites grew as stacked plates from the nucleation site. At higher temperatures, the metastable germanium structure converted, growing backwards toward the center of each precursor, into the stable chalcopyrite structure, permitted by appropriate ionic diffusion shifting composition toward stoichiometry. At faster heating rates, less of these germanium precursors formed, creating a visually bimodal grain size distribution in the microstructure, as compared to slower heating rates. Using the simplex algorithm, the mechanism and activation energy of each reaction are reported as well as the deconvoluted enthalpies of each transformation.     

An Efficient Strategy to Drive Nanoparticles into Carbon Nanotubes and the Remarkable Effect of Confinement on Their Catalytic Performance

Are you in? Bimetallic PtRu nanoparticles have been selectively confined inside or deposited outside carbon nanotubes (see picture). The confined nanoparticles display significantly higher selectivity and catalytic activity in hydrogenation reactions.

     

RNA PROTEIN CROSSLINKS INTRODUCED INTO E. coli RIBOSOMES BY USE OF THE INTRINSIC PROBE 4-THIOURIDINE

Abstract— 70S Ribosome substituted by the uridine photoactivable analogue 4-thiouridine has been prepared by an in vivo method (substitution level 4.5%). The r-proteins crosslinked to 16S and 23S rRNA before and after 366-nm photoactivation were identified. Proteins S2-S7-S9/11-S18 are found linked to 16S RNA in dark-prepared 30S subunits. Illumination increases uniformly their binding by a factor of 2.5. Similarly, proteins L5-L15-L18-L23-L28-L32 are found crosslinked to 23S RNA in dark-prepared 50S subunits. Photoactivation increases their binding but in addition promotes the covalent linking of proteins L1-L3-L4.     

Measuring the aspect ratios of ZnO nanobelts

Nanobelts are new materials that have a rectangular cross-section and are characterized by widths and width-to-thickness aspect ratios. In this paper, the thickness and aspect ratios of ZnO nanobelts are measured by a conjunction application of convergent beam electron diffraction (CBED) and electron energy-loss spectroscopy (EELS). The thicknesses of thicker nanobelts are first determined by CBED under two-beam diffracting condition, then they are used to determine the electron inelastic mean-free-path (MFP) length, which is 161±15 nm for ZnO at 200 kV. The thicknesses of the thinner nanobelts are then determined by EELS using the calibrated MFP. The results show that the aspect ratio depends on conditions under which the sample was synthesized.     

The free radical chemistry of carboxylic esters of 2-selenopyridine-N-oxide: a convenient synthesis of (L)-vinylglycine

Optically pure (L)-vinylglycine has been synthesised by two different methods. The first of these involves protected (L)-glutamate esters of $\text{N}$-hydroxy-2-seleno-pyridine. Such esters are shown to undergo the same decarboxylotive rearrangement as their thio-analogues. Oxidative elimination of the selenopyridine residue with ozone, and with the aid of hex-1-ene as sacrificial olefin for the work-up, gave the desired (L)-vinylglycine derivatives. Similarly, the modified Hunsdiecker reaction on the terminal carboxyl of suitably protected (L)-glutamic derivatives gave the nor-bromide which readily afforded the corresponding phenylselenides on treatment with phenylselenide anion. The sequence was then as above. Using the methyl ester with corbobenzyloxy protection for the amino-function an overall yield of crystalline optically pure (L)-vinylglycine of about 45% was obtained by either route.