On the structure of tetracoordinated organocobalt complexes of the type [CoR2L2]

A series of square-planar organocobalt complexes of the type [CoR₂L₂] (R = 2,3,4,6-C₆,HCl₄ and 2,3,6-C₆H₂Cl₃, L = PEtPh₂, PEt₂Ph, and PEt₃; R = 2,3,5,6-C₆HCl₄, and 2,6-C₆H₃Cl₂, L = PEt₂Ph, PEt₃, and $\text{1}\text{2}$dpe) have been prepared in which the electronegativities of the ligand R vary progressively. The reaction of o-C₆H₄ClMgBr with [CoCl₂L₂] (L = PEtPh₂ PEt₂Ph, γ-pic or $\text{1}\text{2}$bipy) did not give air stable compounds at room temperature, but the solutions obtained at ?78°C appear to contain square-planar species for L = PEtPh₂, PEt₂Ph, and γ-pic, and tetrahedral for L₂ = bipy. The tendency towards square-planar or tetrahedral structures for the compounds [CoR₂L₂] depends on the following factors in order of importance: (i) when the neutral ligand is a phosphine a square-planar structure is adopted; (ii) when L is an aromatic amine, bulky ortho substituents on R favour a square-planar structure; and (iii) a tetrahedral geometry is favoured by bidentate amine ligands. The electronegativity of the organic group R seems to be less important.     

Synthese regioselective de bases de mannich de cetones dissymetriques

Methods for the regioselective synthesis of Mannich bases starting from unsymmetrical ketones are described. The Mannich base on the more substituted carbon is obtained by reaction with (CH₃)₂N⁺CH₂,CF₃CO₂^- in CF₃CO₂H and its isomer on the less substituted carbon with (iPr)₂N⁺CH₂,ClO₄^- in CH₃CN. In the first case, the orientation corresponds to a non-selective attack of the immonium salt on a mixture of enols in which the more-substituted isomer is predominant. In the second ease, use of a bulky immonium salt induces a selectivity favoring the less-substituted enol.     

Chlorotrimethylsilane, a reagent for the direct quantitative analysis of fats and oils present in vegetable and meat samples

Acylglycerides present in oil seeds and meat can be transformed into volatile fatty esters using chlorotrimethylsilane (CTMS) and 1-pentanol as reagents. The volatile esters can then be analysed by GC. The method is quantitative and involves only minor sample manipulation. It often permits major recoveries of the total saponifiable lipids present in solid samples. A 40 min reaction time is enough to ensure the total conversion of saponifiable lipids to the corresponding FAPEs.     

Réaction de réformatsky sur les azirnes: synthesés d'aziridines esters et de pyrrolinones

Addition of Reformatsky reagents 1 to two azirines is reported. From 2-phényl-3,3-dimethyl azirine 2, aziridines esters 3 are obtained. 2-Phenyl-3-methyl azirine 4 gives aziridines esters 5 and diazepinones 6. Δ-3-Pyrrolinones or 5-methylene pyrrolidones are obtained from Grignard reagents and aziridines esters 3 and 5. The reaction mechanism is discussed; compounds 7,8,9,11 and 12 result from an aza-1-oxo-2-bicyclo-pentane (2,1,0) intermediate.     

Nitrogen Sorption Studies of Silica Particles Obtained in Emulsion and Microemulsion Media

The internal surface structures of silica aerogel particles synthesized using different catalysts in emulsion and microemulsion media have been investigated by means of N(2) adsorption and desorption isotherms. Surface fractal dimensions have been computed using different methods: Frankel-Halsey-Hill plots of the adsorption isotherms, the thermodynamic fractal isotherm equation, and a modification of the thermodynamic fractal isotherm equation. Silica aerogels synthesized in emulsion media with an acidic catalyst have a high specific surface area without micropores and show two separate ranges of scales where the surface fractal dimensions are different and constant. Silica aerogels synthesized in emulsion media with a basic catalyst have a moderate specific surface area with a high percentage of micropores and show constant surface fractal dimensions over a larger range. Silica aerogels synthesized in microemulsion media with a basic catalyst have a low specific surface area with a moderate percentage of micropores and show a moderate range of scales over which the surface fractal dimension is constant. Analyses by both the thermodynamic and modified thermodynamic methods give similar ranges of the surface fractal dimensions of the silica particles. Copyright 2000 Academic Press.     

Efficient regioselective synthesis of 6-amino-5-benzoyl-1-substituted 2(1H)-pyridinones

[Reaction: see text]. A regioselective and efficient approach toward 6-amino-5-benzoyl-1-substituted 2(1H)-pyridinones by reaction of acyclic ketene aminals with propiolic acid ester was developed. The effect of the solvent and temperature on the regioselectivity of the reaction and the compatibility of the target compounds to functional group manipulations was examined. Substrates with an ortho substituent build atropisomers due to the restricted rotation around the C-N bond. The enantiomers were separated, and the barrier of rotation was determined experimentally. Quantum chemical calculations allowed a ranking of the barrier heights, and a new mechanism of rotation by deformation of the central pyridinone moiety is proposed.     

Phase behavior and nano-emulsion formation by the phase inversion temperature method

Formation of oil-in-water nano-emulsions has been studied in the water/C12E4/isohexadecane system by the phase inversion temperature emulsification method. Emulsification started at the corresponding hydrophilic-lipophilic balance temperature, and then the samples were quickly cooled to 25 degrees C. The influence of phase behavior on nano-emulsion droplet size and stability has been studied. Droplet size was determined by dynamic light scattering, and nano-emulsion stability was assessed, measuring the variation of droplet size as a function of time. The results obtained showed that the smallest droplet sizes were produced in samples where the emulsification started in a bicontinuous microemulsion (D) phase region or in a two-phase region consisting of a microemulsion (D) and a liquid crystalline phase (L(alpha)). Although the breakdown process of nano-emulsions could be attributed to the oil transference from the smaller to the bigger droplets, the increase in instability found with the increase in surfactant concentration may be related to the higher surfactant excess, favoring the oil micellar transport between the emulsion droplets.     

Nonequilibrium dynamics in lattice ecosystems: Chaotic stability and dissipative structures

A generalized coupled map lattice (CML) model of ecosystem dynamics is presented. We consider the spatiotemporal behavior of a prey-predator map, a model of host-parasitoid interactions, and two-species competition. The latter model can show phase separation of domains (Turing-like structures) even when chaos is present. We also use this CML model to explore the time evolution and structural properties of ecological networks built with a set of N competing species. The May-Wigner criterion is applied as a measure of stability, and some regularities in the stable networks observed are discussed.     

Etude de la structure de carbanions en α du soufre

Carbanions α to sulfoxides and sulfones C₆H₅S(O)nCH₂M (n  1, 2; M  Li or K) have been studied by infrared and Raman spectroscopy. A strong increase of the force constant of the SCH₂M bond, and a similar decrease of that of the SO bond(s) are observed. This implies that the negative charge is delocalized over the carbon and oxygen atoms. ¹H NMR shows that carbanions α to sulfoxides are configurationally stable at low temperature. The interconversion energy depends on the polarizing ability of the counter-ion (Li⁺: ΔG

13.6 kcal mol^-1; K⁺: ΔG

10.7 kcal mol^-1). The coupling constant of the methylenic hydrogens (J_AB 2 Hz) confirms the nearly planar configuration of the anionic carbon in the α metallated sulfoxides.     

Design, synthesis, conformational analysis and nucleic acid hybridisation properties of thymidyl pyrrolidine-amide oligonucleotide mimics (POM)

Pyrrolidine-amide oligonucleotide mimics (POM) 1 were designed to be stereochemically and conformationally similar to natural nucleic acids, but with an oppositely charged, cationic backbone. Molecular modelling reveals that the lowest energy conformation of a thymidyl-POM monomer is similar to the conformation adopted by ribonucleosides. An efficient solution phase synthesis of the thymidyl POM oligomers has been developed, using both N-alkylation and acylation coupling strategies. 1H NMR spectroscopy confirmed that the highly water soluble thymidyl-dimer, T2-POM, preferentially adopts both a configuration about the pyrrolidine N-atom and an overall conformation in D2O that are very similar to a typical C3'-endo nucleotide in RNA. In addition the nucleic acid hybridisation properties of a thymidyl-pentamer, T5-POM, with an N-terminal phthalimide group were evaluated using both UV spectroscopy and surface plasmon resonance (SPR). It was found that T5-POM exhibits very high affinity for complementary ssDNA and RNA, similar to that of a T5-PNA oligomer. SPR experiments also showed that T5-POM binds with high sequence fidelity to ssDNA under near physiological conditions. In addition, it was found possible to attenuate the binding affinity of T5-POM to ssDNA and RNA by varying both the ionic strength and pH. However, the most striking feature exhibited by T5-POM is an unprecedented kinetic binding selectivity for ssRNA over DNA.