Heck reactions of 2-substituted enol ethers with aryl bromides catalysed by a tetraphosphine/palladium complex

cis,cis,cis-1,2,3,4-Tetrakis(diphenylphosphinomethyl)cyclopentane/[PdCl(C₃H₅)]₂ efficiently catalyses the Heck reaction of β-substituted enol ethers with aryl bromides. Employing β-methoxystyrene, 3-ethoxyacrylonitrile or methyl 3-methoxyacrylate, the regioselective α-arylation of these enol ethers was observed in all cases, and mixtures of Z and E isomers were generally obtained, which in many cases yielded a single ketone product after acid treatment. The stereoselectivity of this reaction depends on steric and electronic factors, and better stereoselectivities in favour of Z isomers were observed with electron-rich or sterically congested aryl bromides. Better yields were obtained for this reaction with electron-rich or sterically congested aryl bromides than with electron-poor aryl bromides. This observation suggests that with these β-substituted enol ethers the rate-limiting step of the catalytic cycle is not the oxidative addition of the aryl bromide to the palladium complex.     

Size fractionation of aquatic colloids and particles by cross-flow filtration: analysis by scanning electron and atomic force microscopy

The suitability of the combined application of environmental scanning electron microscopy (ESEM), scanning electron microscopy (SEM) and atomic force microscopy (AFM) for the evaluation of the ability of cross-flow filtration (CFF) to perform adequate size fractionation of freshwater colloids and particles was examined. ESEM and SEM imaging provided reference images of the CFF-generated fractions and, in estimating the experimental cut-off diameter of the membrane, provided evidence that separation was not consistent with nominal pore sizes of the membranes. However, analysis of the images showed that size distribution of CFF-generated fractions and the estimated cut-off diameter of the membranes were dependent on the advantages and limitations of the two imaging techniques. With both ESEM and SEM, best estimates of size cut-offs were lower than the nominal pore size of the membrane in the case of 0.45 μm membranes, but roughly accurate in the case of 0.1 μm pore size membranes. The results also suggested that the effectiveness of CFF may benefit from a pre-separation step using a minimally perturbing technique such as split thin-flow fractionation. AFM demonstrated the presence of colloids smaller than 50 nm in all fractions including the retentates, showing that CFF fractionation is not fully quantitative and not based on size alone. The results indicate that previous studies investigating trace element partitioning using CFF may need re-evaluation as the importance of particles and large colloids may be over-estimated.     

Palladium‐catalyzed selective decarboxylative coupling reaction versus direct C―H arylation for arylation of heteroaromatics

Conditions for selective palladium‐catalyzed decarboxylative 2‐arylation of 3‐substituted thiophene and furan derivatives bearing an ester at C2 position have been established. By using 2 mol% phosphine‐free Pd(OAc)₂ as the catalyst and a mixture of KOH and K₂CO₃ as the bases, in dimethylacetamide, moderate to good yields of the desired 2‐arylated products were obtained. A range of functional groups such as nitrile, nitro, formyl or acetyl on the aryl bromides was tolerated. This method allows us to employ in some cases more convenient reactants in terms of cost or physical properties (boiling point) for arylations. Copyright © 2013 John Wiley & Sons, Ltd.     

Preliminary study of the thermally stimulated blue luminescence of ulexite

In this paper, novel results on the blue thermally stimulated luminescence (TSL) emission of ulexite (NaCaB₅O₆(OH)₆·5H₂O) have been studied. The four maxima appearing at 60, 110, 200 and 240°C on the TSL glow curves of this borate could be respectively associated to: (i) the first dehydration (NaCaB₅O₆(OH)₆·5H₂O→NaCaB₅O₆(OH)₆·3H₂O), (ii) the creation-annihilation of the three-hydrated phase, (iii) the Na-coordinated chains dehydroxylation and the starting point of the alkali self-diffusion through the lattice and (iv) the amorphisation of the lattice. These results are fairly well correlated with the differential thermal analyses (DTA), in situ thermal observations under environmental scanning electron microscope (TESEM) and thermal X-ray diffraction (TXRD) techniques.     

Palladium-tetraphosphine complex: an efficient catalyst for the coupling of aryl halides with alkynes

The cis,cis,cis-1,2,3,4-tetrakis(diphenylphosphinomethyl)-cyclopentane-[PdCl(eta3-C3H5)]2 system catalyses the coupling of aryl halides with alkynes with very high ratios of substrates-catalyst in good yields; a turnover number of 2600000 can be obtained for the reaction of 4-trifluoromethylbromobenzene with phenylacetylene in the presence of this catalyst.     

The solubility of an hydroxyaluminosilicate

Hydroxyaluminosilicates (HAS) are critical secondary mineral phases in the biogeochemical cycle of aluminium. They are formed from the reaction of silicic acid (Si(OH)₄) with an aluminium hydroxide template and act as a geochemical control of the biological availability of Al. There are two main forms of HAS which we have called HAS_A and HAS_B and which of these will predominate will depend upon the Si(OH)₄ to Al ratio in any one environment. In all but the most heavily weathered environments or those undergoing a progressive acidification Si(OH)₄ will be present in significant excess to Al and HAS_B will be the dominant secondary mineral phase. We have tried to determine the solubility of HAS_B(s) so that its contribution to Al solubility control might be compared with other secondary minerals such as Al(OH)_3(gibbsite). In preliminary experiments, the dissolution of HAS_B(s) was found to be non-congruent with almost no Al being released during 18 months ageing. We then demonstrated that HAS_B(s) was significantly less soluble than Al(OH)_3(s) prepared under identical experimental conditions. We have used this information to describe a solubility expression for HAS_B(s) at a predefined quasi-equibrium and to calculate a solubility constant.

K^*Al₂Si₂O₅(OH)₄=[Al₂O⁴⁺][SiO₂]²[OH^-]⁴