Enantioselective 1,4-Addition of Aliphatic Grignard Reagents to Enones Catalyzed by Readily Available Copper(I) thiolates derived from TADDOL. Preliminary communication

Two simple thiols derived from the parent TADDOL, α,α,α′,α′ tetraphenyl-2,2-dimethyl-1,3-clioxolan-4,5-dimethanol, are used to prepare Cu¹ complexes C and D to catalyze (0.05 equiv.) 1,4-additions of Grignard reagents RMgCl to cyclic enones with enantioselectivities which are comparable to or better than previously reported (enantiomer ratios up to 92:8).     

Hydrogen cycling of niobium and vanadium catalyzed nanostructured magnesium

The reaction of hydrogen gas with magnesium metal, which is important for hydrogen storage purposes, is enhanced significantly by the addition of catalysts such as Nb and V and by using nanostructured powders. In situ neutron diffraction on MgNb(0.05) and MgV(0.05) powders give a detailed insight on the magnesium and catalyst phases that exist during the various stages of hydrogen cycling. During the early stage of hydriding (and deuteriding), a MgH(1< x < 2) phase is observed, which does not occur in bulk MgH(2) and, thus, appears characteristic for the small particles. The abundant H vacancies will cause this phase to have a much larger hydrogen diffusion coefficient, partly explaining the enhanced kinetics of nanostructured magnesium. It is shown that under relevant experimental conditions, the niobium catalyst is present as NbH(1). Second, a hitherto unknown Mg-Nb perovskite phase could be identified that has to result from mechanical alloying of Nb and the MgO layer of the particles. Vanadium is not visible in the diffraction patterns, but electron micrographs show that the V particle size becomes very small, 2-20 nm. Nanostructuring and catalyzing the Mg enhance the adsorption speed that much that now temperature variations effectively limit the absorption speed and not, as for bulk, the slow kinetics through bulk MgH(2) layers.     

Using the diphosphanyl radical as a potential spin label: effect of motion on the EPR spectrum of an R1(R2)P--PR1 radical

The EPR spectrum of the novel radical Mes*(CH3)P--PMes* (Mes*=2,4,6-(tBu)3C6H2) was measured in the temperature range 100-300 K, and was found to be drastically temperature dependent as a result of the large anisotropy of the 31P hyperfine tensors. Below 180 K, a spectrum of the liquid solution is accurately simulated by calculating the spectral modifications due to slow tumbling of the radical. To achieve this simulation, an algorithm was developed by extending the well-known nitroxide slow-motion simulation technique for the coupling of one electron spin to two nuclear spins. An additional dynamic process responsible for the observed line broadening was found to occur between 180 K and room temperature; this broadening is consistent with an exchange between two conformations. The differences between the isotropic 31P couplings associated with the two conformers are shown to be probably due to an internal rotation about the P--P bond.     

Mechanochemical assembly of hydrogen bonded organic-organometallic solid compounds

Solvent-free reactions with molecular systems have been exploited to prepare hybrid organic-organometallic solids: grinding of the complex [Fe(eta 5-C5H4COOH)2] with solid bases B generates quantitatively the corresponding hydrogen bonded salts [Fe(eta 5-C5H4COOH)(eta 5-C5H4COO)][HB] (B = 1,4-diazabicyclo[2.2.2]octane, 1,4-phenylenediamine); gas-solid reactions are also possible with volatile bases.     

Une méthode simple de calcul des structures électroniques (σ + π) des borazines et des boroxines substitués de symétrie D3h: une estimation théorique de l'aromaticité comparée de ces molécules

Résumé Une utilisation conjointe des méthodes L.C.A.O.-C.U.V. et d'itération tournante permet d'atteindre des structures électroniques ( + ) cohérentes avec l'U.V.(et donc le gradient de population électronique de liaison qui conditionne l'aromaticité) de huit borazines et boroxines, substitués ou non, de symètrie D _3h . L'échelle d'aromaticité obtenue permet de rendre compte des propriétés magnétiques (RMN, RQ, effet Faraday, diamagnétisme) de ce type de molécules. On parvient en particulier aux deux conclusions essentielles suivantes: les borazines sont nettement plus aromatiques que les boroxines isologues et la greffe de groupements méthyles ou d'atomes de chlore, qu'elle se fasse sur le bore ou sur l'azote, confère toujours au dérivé substitué une aromaticité au moins égale à celle du borazine lui-même.

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A simple method to calculate the - and -electron structure of borazines and of substituted boroxines of symmetry D_3h: A theoretical estimation of the relative aromaticity of those molecules

( + ) electronic structures calculations are performed for eight D _3h borazines and boroxines, using in concert previously published U.V.-consistent H.M.O.-L.C.A.O. and bond by bond iteration methods. An estimation of bond electronic populations gradients is so obtained, allowing to set up a new aromaticity scale for such molecules. The main results are the following: Borazines are much more aromatic than isologous boroxines and B-or N-graft of alkyl groups or chlorine atoms never decreases (and generally increases) aromaticity.

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Zusammenfassung Es wird für acht Borazol- und Boroxolverbindungen mit D _3h -Symmetrie unter Verwendung einer am UV-Spektrum geeichten iterativen HMO-LCAO-Theorie die - und -Elektronenstruktur bestimmt. Hiermit kann eine Abschätzung des Gradienten der Bindungselektronendichten gewonnen werden, aus der eine Einteilung der Moleküle nach ihrem aromatischen Charakter hervorgeht. Es ergibt sich, daß die Borazole aromatischer sind als die isoelektronischen Boroxole und daß eine Substitution der H-Atome am Bor oder Stickstoff durch Chlor oder Methylgruppen die Aromatizität fast immer erhöht, jedenfalls aber nie erniedrigt.

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Recherche effectuée dans le cadre de la R.C.P. n du C.N.R.S.     

Characterization of low-molecular weight peptides in champagne wine by liquid chromatography/tandem mass spectrometry

This paper reports the use of an on-line LC–ESI–MS/MS method for the identification and quantification of di- and tripeptides in champagne wine without laborious sample pretreatment. The identification of these compounds, in their underivatised form, is based on identical retention times and ESI–MS spectra to those of reference standards. The presence of nine dipeptides (Arg–Ile, Ile–Arg, Ile–Val, Lys–Phe, Lys–Tyr, Phe–Lys, Tyr–Gln, Tyr–Lys, Val–Ile) and the absence of two tripeptides (Phe–Arg–Arg and Lys–Met–Asn) have been evidenced in the matrix. Calibration curves for each analyte were established using Phe–Arg as internal standard. The calibration curves were linear in the concentration range 0.1–10 mg L⁻¹ with a determination coefficient, r², better than 0.992. The accuracy for the calibration standard was estimated at between 92 and 102%. This method allows high recovery and satisfies the necessary requirements with respect to accuracy, repeatability and sensitivity. The first application of this analytical method to the measurement of di- and tri-peptides in different vintages of champagne wine is reported. Compositional changes in the peptides occurred depending on the vintage.     

An alternative model for partly diffusion controlled cage recombination

We proposed a simple kinetc model in which a diffusional physical process and a chemical process of reaction in a cage occur simultaneously. Good agreement between experimental results and the kinetic model are found.

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Immunosorbents: A new tool for pesticide sample handling in environmental analysis

 Immunoaffinity techniques have been widely used for the determination of different analytes in the medical field. However the use of antibodies immobilized in an appropriate support material to preconcentrate pesticides from environmental samples is only recent. The production of antibodies, election of supports, antibody immobilization procedures, elution of analytes from immunosorbents and the more recent applications in the field of pesticide analysis are reviewed. The present review concludes that immunosorbents have great potential and discusses the present limitations and expected future trends. Received: 29 July 1996 / Accepted: 14 August 1996     

Improvement and simplification of synthesis of 3-aryloxy-1,2-epoxypropanes using solvent-free conditions and microwave irradiations. Relation with medium effects and reaction mechanism

Some 3-aryloxy-1,2-epoxypropanes, interesting as potential synthons in β-adrenergic receptor antagonists preparation, were obtained in excellent yields (65-96% within 2-17 min) by microwave activation (monomode system) using solid-liquid solvent-free phase transfer catalysis (PTC). The best results for the O-alkylation of some phenols with epichlorohydrin were obtained using TBAB and NaOH/K₂CO₃ (1:4 mol/mol) as phase transfer catalyst and more acceptable basic system, respectively. These new procedure is compared with classical methods. Significant specific microwave effect (non-purely thermal) was evidenced in all cases. They were discussed in terms of reaction medium and mechanism, taking into account the variations in polarity of the systems.     

Characterization of the pores in hydrous ferric oxide aggregates formed by freezing and thawing

Hydrous ferric oxides (HFO) are efficient sorbents for inorganic and organic pollutants and therefore have great potentials in environmental science and engineering applications. Freezing and thawing of HFO suspensions leads to the formation of dense HFO aggregates. It facilitates the handling and increases the drying rate of HFO. In this study, we used a combination of pycnometry, gas adsorption (N(2) gas, water vapor), and small-angle neutron scattering (SANS) to characterize the porosity and pore size distribution of dense HFO aggregates formed by freezing dialyzed HFO suspensions at -25 degrees C and thawing them at room temperature. The crystallinity of the HFO, which was a 2-line ferrihydrite, was not affected by this treatment. Wet sieving and laser diffraction analysis showed that the dense HFO aggregates had a unimodal size distribution with an average diameter of 235+/-35 microm. Increasing the freezing rate by cooling with liquid N(2) (-196 degrees C) resulted in much smaller aggregates with an average diameter of 20 microm. Adding NaNO(3) electrolyte to the HFO suspensions prior to freezing also resulted in the formation of smaller aggregates. The dense HFO aggregates formed at -25 degrees C had a porosity of 0.73+/-0.02 ll(-1). SANS revealed a unimodal size distribution of pores, with an average pore diameter of 2.0 nm. The diameter of the HFO crystallites was estimated by transmission electron microscopy to be 1.9+/-0.5 nm. Geometrical considerations taking into account the unit particle and average pore size suggest that the crystallites retain 1-2 layers of hydration water during the coagulation induced by freezing. Analysis by N(2) gas adsorption showed that drying the dense HFO aggregates induced a reduction in porosity by about 25% and shifted the pore size distribution to smaller diameters. Rewetting during water vapor adsorption did not induce significant changes of the aggregate structure. The specific surface area of the dry HFO aggregates was between 320 and 380 m(2)g(-1).