Highly chemoselective hydrogenolysis of iodoarenes

The catalytic hydrodehalogenation reaction using molecular hydrogen and Pd/C has been revisited. It is shown that the speed of removal of halogen increases with increasing electronegativity I < Br < Cl. Nevertheless, selective dehydrohalogenation in compounds containing other reducible functions can be achieved only with iodine and not with bromine or chlorine. Selective deiodination of iodobenzophenone could be accomplished without reducing the carbonyl group. Hydrogenolysis of azidoiodoaromatic compounds to the corresponding azido compounds is high yielding. This selectivity was exploited for the labeling of benzophenone- and azido-containing compounds by deuterium and tritium.     

Determination of arsenic species in marine organisms by HPLC-ICP-OES and HPLC-HG-QFAAS

Separation and quantification of six arsenic species have been performed in cod, tuna and mussel samples by high performance liquid chromatography (HPLC) using inductively coupled plasma-optical emission spectrometry (ICP-OES) and hydride generation-quartz furnace atomic absorption spectrometry (HG-QFAAS) as detection techniques. It has been shown that arsenic extraction with a water-methanol (11) mixture is sufficiently quantitative for the cod and tuna, in which arsenic is mainly present as arsenobetaine (about 90% of total As extracted). In contrast, only 60% of the element is extracted from the mussels and the chromatograms obtained reveal the presence of an unknown compound. Detection limits are in the g ml^–1 range for the HPLC-ICP-OES technique (quantification of arsenobetaine and arsenocholine) and in the ng ml^–1 range for the HPLC-HG-QFAAS system (quantification of arsenite, arsenate, monomethylarsonic and dimethylarsinic acids).     

NMR Studies on Hydrolysis and Condensation Reactions of Alkoxysilanes Containing Si—H Bonds

Hydrolysis and condensation reactions of four organically modified alkoxides, used for the preparation of silicon oxycarbide gel precursors, have been followed by 29Si NMR. Triethoxysilane (HSi(OEt)3) and methyldiethoxysilane (MeHSi(OEt)2) react extremely fast compared to methyltriethoxysilane (MeSi(OEt)3) and tetraethoxysilane (Si(OEt)4). Co-hydrolysis reactions between different pairs of precursors—MeSi(OEt)3/Si(OEt)4; MeSi(OEt)3/HSi(OEt)3; MeHSi(OEt)2/Si(OEt)4; and MeHSi(OEt)2/HSi(OEt)3—were investigated by solution state 29Si and 17O NMR. Despite significantly different reactivities between precursors, evidence for co-condensation reactions has been found for each system. Finally, two-dimensional 29Si-1H heteronuclear correlation MAS-NMR spectroscopy was used to probe the local environments of the various Si sites in the product hybrid networks.     

Speciation analysis of antimony in marine biota by HPLC-(UV)-HG-AFS: Extraction procedures and stability of antimony species

Speciation analysis of antimony in marine biota is not well documented, and no specific extraction procedure of antimony species from algae and mollusk samples can be found in the literature. This work presents a suitable methodology for the speciation of antimony in marine biota (algae and mollusk samples). The extraction efficiency of total antimony and the stability of Sb(III), Sb(V) and trimethylantimony(V) in different extraction media (water at 25 and 90 °C, methanol, EDTA and citric acid) were evaluated by analyzing the algae Macrosystis integrifolia (0.55 ± 0.04 μg Sb g⁻¹) and the mollusk Mytilus edulis (0.23 ± 0.01 μg Sb g⁻¹). The speciation analysis was performed by anion exchange liquid chromatography (post-column photo-oxidation) and hydride generation atomic fluorescence spectrometry as detection system (HPLC-(UV)-HG-AFS). Results demonstrated that, based on the extraction yield and the stability, EDTA proved to be the best extracting solution for the speciation analysis of antimony in these matrices. The selected procedure was applied to antimony speciation in different algae samples collected from the Chilean coast. Only the inorganic Sb(V) and Sb(III) species were detected in the extracts. In all analyzed algae the sum of total antimony extracted (determined in the extracts after digestion) and the antimony present in the residue was in good agreement with the total antimony concentration determined by HG-AFS. However, in some extracts the sum of antimony species detected was lower than the total extracted, revealing the presence of unknown antimony species, possibly retained on the column or not detected by HPLC-(UV)-HG-AFS. Further work must be carried out to elucidate the identity of these unknown species of antimony.     

Poloxamer association in aqueous solution

Aqueous solutions of the surface active poly(oxyethylene)-poly(oxypropylene)-poly(oxyethylene) block copolymers (poloxamers) were studied using photon correlation spectroscopy (quasi-elastic light scattering) and viscosity measurements. Poloxamers 184 and 237 showed detectable aggregates at 25° only at concentrations above about 6% with size increasing with concentration and with significant polydispersity, probably indicating a multiple association process. At 35°, however, essentially invariant values for the hydrodynamic radius were found over a wide concentration range and the systems were essentially monodisperse: these systems are more likely to be represented by a closed association model. The more hydrophilic poloxamer 188, however, retained its concentration dependence of aggregate size up to 55°. The variation with temperature of both the hydrodynamic radius of aggregates and the intrinsic viscosity of several poloxamers was rationalized by relating the temperature-dependency curves to the cloud point of the poloxamer. In some cases only certain sections of the curve are observable when the cloud point is high, e.g., >100°, or low, e.g., <40°.     

Impact of the Nature of the Organic Spacer on the Crystallization Kinetics of UiO‐66(Zr)‐Type MOFs

The influence of the constitutive dicarboxylate linkers (size, functional group) over the crystallization kinetics of a series of porous Zr metal–organic frameworks with the UiO‐66 topology has been investigated by in situ time‐resolved energy dispersive X‐ray diffraction (EDXRD). Both large aromatic spacers (2,6‐naphthalene‐, 4,4′‐biphenyl‐ and 3,3′‐dichloro‐4,4′‐azobenzene‐dicarboxylates) and a series of X‐functionalized terephthalates (X=NH₂, NO₂, Br, CH₃) were investigated in dimethylformamide (DMF) at different temperatures and compared with the parent UiO‐66. Using different crystallization models, rate constants and further kinetic parameters (such as activation energy) have been extracted. Finally, the impact of the replacement of the toxic DMF by water on the crystallization kinetics was studied through the synthesis of the functionalized UiO‐66‐NO₂ solid.     

Catalytic stereoselective alkene aziridination with sulfonimidamides

Diastereoselective copper-catalyzed alkene aziridination has been investigated using chiral nitrenes generated from sulfonimidamides in the presence of an iodine(III) oxidant. Starting from a stoichiometric amount of the substrates, the corresponding aziridines were isolated with excellent yields of up to 96%. Good levels of asymmetric induction were obtained in the case of electron-poor olefins, with an optimal de of 94% being reached starting from tert-butyl acrylate. Matching and mismatching effects were also observed upon the use of chiral copper catalysts for the aziridination of styrene.     

Straightforward access to enantioenriched pyrazyl alcohols using chiral organomagnesiates

The clean iodine-metal exchange of iodopyrazine and subsequent enantioselective addition of the intermediate pyrazyl magnesiate to aldehydes using ((R,R)-TADDOLate)Bu₂MgLi₂ is reported. New chiral pyrazylalcohols were obtained in enantiomeric excesses of up to 90%.     

Rheological and thermal behaviours of a hyperbranched polycarbosilane

A hyperbranched polymer, a precursor of silicon carbide (SiC), was successfully synthesized using a hydrosilylation reaction with Karstedt's catalyst. This reaction was optimized with the use of a rheometer coupled with an infrared spectrometer. The polymeric precursor was characterized using NMR and Fourier transform infrared spectroscopies, and dynamic rheology. The polymerization reaction was followed in situ by combined rheological and infrared measurements, indicating a gel‐like behaviour for alkene conversions higher than 0.55. Overall second‐order kinetics was determined for the hydrosilylation reaction. Pyrolysis at 1400 °C led to porous materials with β‐SiC and free carbon.