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.     

The reactant state for substrate-activated turnover of acetylthiocholine by butyrylcholinesterase is a tetrahedral intermediate

Secondary beta-deuterium kinetic isotope effects have been measured as a function of substrate concentration for recombinant human butyrylcholinesterase-catalyzed hydrolysis of acetyl-L3-thiocholine (L = 1H or 2H). The isotope effect on V/K is inverse, D3V/K = 0.93 +/- 0.03, which is consistent with conversion of the sp2 hybridized carbonyl carbon of the scissile ester bond of the E + A reactant state to a quasi-tetrahedral structure in the acylation transition state. In contrast, the isotope effect on Vmax under conditions of substrate activation is markedly normal, D3(betaVmax) = 1.29 +/- 0.06, an observation that is consistent with accumulation of a tetrahedral intermediate as the reactant state for catalytic turnover. Generally, tetrahedral intermediates for nonenzymatic ester hydrolyses are high-energy steady-state intermediates. Apparently, butyrylcholinesterase displays an unusual ability to stabilize such intermediates. Hence, the catalytic power of cholinesterases can largely be understood in terms of their ability to stabilize tetrahedral intermediates in the multistep reaction mechanism.     

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).     

A generalization of regula falsi

Summary The method of nondiscrete mathematical induction is applied to a multistep variant of the secant method. Optimal conditions for convergence as well as error estimates, sharp in every step, are obtained.     

Two New Spermidine Alkaloids from Chisocheton weinlandii

The investigation of the MeOH extract of the leaves of Chisocheton weinlandii Harms (Meliaceae) revealed two new open‐chain spermidine alkaloids, chisitine 1 ( 1 ) and chisitine 2 ( 2 ). Their structures were elucidated by NMR spectroscopy, tandem‐mass spectrometry, and independant syntheses (Scheme 3). Detailed MS/MS fragmentation pathways are discussed for both compounds based on H/D exchange and ¹⁸O‐labeling experiments (Schemes 1 and 2).     

Radical amination with sulfonyl azides: a powerful method for the formation of C-N bonds

A novel reaction for the introduction of an azide moiety by means of a mild radical process is currently under development. Sulfonyl azides are suitable azidating agents for nucleophilic radicals, such as secondary and tertiary alkyl radicals. More electrophilic radicals, such as enolate radicals, do not react with sulfonyl azides. This feature allowed the development of efficient intra- and intermolecular carboazidations of olefins. Due to the versatility of the azido group, this reaction has an important synthetic potential, as already demonstrated by the preparation of the core of several alkaloids, particularly those containing an amino-substituted quaternary carbon center, such as FR901483.     

Binuclear rhodium(II) complexes with leucine and proline

The dimeric rhodium(II) complexes [Rh₂(leu)₄(H₂O)₂]- (ClO₄)₄ and [Rh₂(pro)₄(H₂O)₂](ClO₄)₄ have been prepared and characterized by elemental analyses, i.r., u.v.–vis. and ¹H-n.m.r. spectroscopy. The amino acid molecules are coordinated as bridging ligands via their carboxylato groups. Cyclic voltammetry in DMF has shown that the complexes undergo a quasi-reversible reduction to yield dimers containing a Rh ₂ ³⁺ core. Oxidation processes within the 0–1.5V range were not observed.     

Mixed-metal sandwich complexes [M(II)2(H2O)2Fe(III)2(P2W15O56)2]14- (M(II) = Co, Mn): synthesis and stability. The molecular structure of [M(II)2(H2O)2Fe(III)2(P2W15O56)2]14-

Two new mixed-metal sandwich complexes [M(II)2(H2O)2Fe(III)2(P2W15O56)2]14- (abbreviated [M2Fe2P4W30], M(II) = Co(II), Mn(II)) were obtained at pH 3 by addition of M2+ to [Na2(H2O)2Fe(III)2(P2W15O56)2]16- (abbreviated [Na2Fe2P4W30]) without substitution in the alpha-[P2W15O56]12- (abbreviated [P2W15]) units. Their X-ray structures are reported. At lower pH, back conversion to [Na2Fe2P4W30] was followed by 31P NMR, electrochemistry and UV-visible spectroscopy. The preparation and the characterization in solution of the lacunary intermediate [NaCo(II)(H2O)2Fe(III)2(P2W15O56)2]15- (abbreviated [NaCoFe2P4W30]) is also described.     

Heck reactions of aryl bromides with alk-1-en-3-ol derivatives 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 alk-1-en-3-ol with a variety of aryl bromides. In the presence of hex-1-en-3-ol or oct-1-en-3-ol, the β-arylated carbonyl compounds were selectively obtained. Linalool and 2-methylbut-3-en-2-ol led to the corresponding 1-arylalk-1-en-3-ol derivatives. Turnover numbers up to 69,000 can be obtained for this reaction. A minor electronic effect of the substituents of the aryl bromide was observed. Similar reaction rates were observed in the presence of activated aryl bromides such as bromoacetophenone and deactivated aryl bromides such as bromoanisole.