Nouveaux exemples d'acétonides d'aldéhydosucres furanniques

Further examples of O-isopropylidenaldehydosugars in the furanose series Formyl-bearing furanose derivatives 1–3 (α-D -xylo or α-D -erythro configurations) and 5–8 (configurations α-D -ribo, α-D -lyxo, β-L -threo and D -arabino) have been prepared by classical synthetic steps and their properties mainly spectroscopic, reported. The coupling constant between the formyl and the vicinal proton is always small. Like other members of the series, these new aldehydosugars constitute very useful and flexible synthetic intermediates.     

Potassium addition to Fe/MgO, Ni/MgO and Ni/SiO2: Reducibility, dispersity and stabilization of the promoter by the support

The influence of potassium addition on the morphological properties of Fe/MgO and Ni/MgO has been investigated: KNO₃ addition to precursors results in a decrease of the reducibility and in a smaller dispersity of the metallic phase. A loss of potassium is observed at reduction temperatures in excess of 773 K. Silica-supported Ni behaves differently: silica inhibits the potassium volatilization and nickel reducibility is enhanced. This illustrates how promoter effects can be influenced by the nature of support.

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Fe/MgO Ni/MgO: KNO₃ . , 773 K. , : . .

     

An electron spin resonance study of radicals from chloramine-t-1 : Spin trapping of radicals produced in acid media

Under acid conditions aqueous solutions of chloramine-T form nitrogen-centred radicals via loss of the chlorine atom. The nitrogen radicals produced have been studied by ESR spectroscopy using the spintrapping method. Adducts of the spin trap phenyl-t-butyl nitrone are oxidized by chloramine-T in acid media to give a paramagnetic product in which the unpaired electron interacts with two inequivalent nitrogen atoms. The spin trap 5,5-dimethyl-2-pyrrolidine-1 -oxide is oxidized rapidly to 5,5-dimethyl-2-pyrrolidone-1-oxyl by chloramine-T under acid onditions. The water soluble trap α-4-pyridyl-1-oxide-N-t-butyl nitrone forms a stable nitroxide adduct with a nitrogen radical of chloramine-T in acid solution. Identical results were obtained with chloramine-B (sodium salt of N-chlorobenzene sulfonamide), indicating the involvement of the N-chloramine group in radical formation.     

Electrodynamic investigations of conduction processes in humid microcrystalline cellulose tablets

The conduction mechanism in microcrystalline cellulose (MCC) tablets at varying relative humidity (RH) has been investigated by using the techniques of low frequency dielectric spectroscopy and transient current analysis at room temperature. The dependence on RH on the measured conductivity and charge carrier density indicates that a high-power-law-exponent percolation process of cations being conducted on water molecules occupying available 6-OH units on the cellulose chains is the dominating dc conduction mechanism at RH below 3 wt % of moisture content. The experimentally observed decrease in charge carrier mobility with increasing moisture content shows that protons and H3O+ ions that are being blocked at empty 6-OH sites also contribute to the charge transport process in cellulose at low moisture contents.     

Synthesis,Structure and Reactivity of PH‐Phosphoraneiminato‐ and Iminophosphoraneiminato Group 4 Transition‐Metal Complexes

The versatile coordination chemistry of the well‐investigated phosphoraneiminato‐ligand R₃PN^— ( I ) was extended by the successive introduction of protons to the phosphorus atom. The position of the resulting equilibrium between the NH‐phosphanylamido‐ [R₂P‐NH^—] and the PH‐phosphoraneiminato‐form [R₂HP=N^—] is affected by the Lewis acidity of the coordinated metal fragment. Experimental studies on complexes with various substitution patterns at the group 4 metal center R₂HP=N[M] ( II ) were unambiguously confirmed by DFT‐calculations. The isolation of group 4 PH‐dihydrido‐phosphoraneiminato‐complexes RH₂P‐N[M] ( III ) is prevented by the low thermodynamic stability of the target molecules, also supported by the results of ab initio calculations. However, an access to the by then unknown transition‐metal substituted iminophosphanes RP=N[M] ( IV ) was verified for the first time. Within extensive studies on the coordination chemistry of bis(imino)phosphoranes RP(=NR′)(=NR″), several species of group 4 complexes R(R′N=)P=N[M] ( V ) were isolated and structurally characterized. In this case, investigations on the NH/PH‐tautomerism were performed exclusively on theoretical level, because the required educts are experimentally non‐accessible due to their kinetic instability.     

Gasphasen-Reaktionen. 92. Thermische HCl-Abspaltung aus Alkyldichlorphosphanen (H3C/H)3C?PCl2 zu Phosphaalkenen (H3C/H)2CPCl und Phosphaalkinen (H3C/H)C≡P

Gasphase Reactions. 92 Thermal Elimination of HCl from Alkyldichlorophosphanes (H₃C/H)₃C? PCl₂ to Phosphaalkenes (H₃C/H)₂C?PCl and Phosphaalkines (H₃C/H)C≡P The alkyldichlorophosphanes H₃C? PCl₂, ClH₂? PCl₂, (H₃C)H₂C? PCl₂ and (H₃C)₂HC? PCl₂ split off HCl on heating in a gasflow under reduced pressure. PE spectroscopic gas analysis proves that under these conditions the short-lived phosphaalkenes H₂C?PCl, (H₃C)H₂C?PCl and – catalyzed by [MgCl₂? MgO/SiO₂]_∞ – (H₃C)₂C?PCl as well as the phosphaalkines HC≡P and (H₃C)C≡P are formed, all of which can be isolated by low temperature condensation. Based on the PES ionization patterns recorded and on the MNDO calculations for their assignment, the π_CP multiple bonds are discussed. The presumable pathway of the HCl elimination is rationalized for (H₃C)H₂C? PCl₂ by an approximate MNDO energy hypersurface.     

Coligand role in the NHC nickel catalyzed C–F bond activation: investigations on the insertion of bis(NHC) nickel into the C–F bond of hexafluorobenzene

The reaction of [Ni(Mes₂Im)₂] (1) (Mes₂Im = 1,3-dimesityl-imidazolin-2-ylidene) with polyfluorinated arenes as well as mechanistic investigations concerning the insertion of 1 and [Ni(ⁱPr₂Im)₂] (1ipr) (ⁱPr₂Im = 1,3-diisopropyl-imidazolin-2-ylidene) into the C–F bond of C₆F₆ is reported. The reaction of 1 with different fluoroaromatics leads to formation of the nickel fluoroaryl fluoride complexes trans-[Ni(Mes₂Im)₂(F)(Ar^F)] (Ar^F = 4-CF₃-C₆F₄2, C₆F₅3, 2,3,5,6-C₆F₄N 4, 2,3,5,6-C₆F₄H 5, 2,3,5-C₆F₃H₂6, 3,5-C₆F₂H₃7) in fair to good yields with the exception of the formation of the pentafluorophenyl complex 3 (less than 20%). Radical species and other diamagnetic side products were detected for the reaction of 1 with C₆F₆, in line with a radical pathway for the C–F bond activation step using 1. The difluoride complex trans-[Ni(Mes₂Im)₂(F)₂] (9), the bis(aryl) complex trans-[Ni(Mes₂Im)₂(C₆F₅)₂] (15), the structurally characterized nickel(i) complex trans-[Ni^I(Mes₂Im)₂(C₆F₅)] (11) and the metal radical trans-[Ni^I(Mes₂Im)₂(F)] (12) were identified. Complex 11, and related [Ni^I(Mes₂Im)₂(2,3,5,6-C₆F₄H)] (13) and [Ni^I(Mes₂Im)₂(2,3,5-C₆F₃H₂)] (14), were synthesized independently by reaction of trans-[Ni(Mes₂Im)₂(F)(Ar^F)] with PhSiH₃. Simple electron transfer from 1 to C₆F₆ was excluded, as the redox potentials of the reaction partners do not match and [Ni(Mes₂Im)₂]⁺, which was prepared independently, was not detected. DFT calculations were performed on the insertion of [Ni(ⁱPr₂Im)₂] (1ipr) and [Ni(Mes₂Im)₂] (1) into the C–F bond of C₆F₆. For 1ipr, concerted and NHC-assisted pathways were identified as having the lowest kinetic barriers, whereas for 1, a radical mechanism with fluoride abstraction and an NHC-assisted pathway are both associated with almost the same kinetic barrier.

A combined experimental and theoretical study on the mechanism of the C–F bond activation of C₆F₆ with [Ni(NHC)₂] is provided.     

Preparation,Solid‐state Characteristics,and Catalytic Properties of Promoted Vanadium Phosphate Materials

The syntheses of transition metal promoted (M = Co, Cr, Fe, Mo) supported vanadium phosphate (VPO) catalysts (TiO₂ (anatase), γ‐Al₂O₃) and their characterization by N₂‐adsorption, X‐ray diffractometry (XRD), FTIR‐spectroscopy and determination of V‐valence state is reported. The catalytic properties were checked in the heterogeneous catalytic ammoxidation of 2, 6‐dichlorotoluene to the corresponding nitrile. The catalyst samples were prepared by synthesis of the precursor compound VOHPO₄ · 0.5 H₂O, impregnation using various metal salt solutions and mixing with the support materials. The characterization revealed increased surface areas for all the promoted samples in comparison to the basic materials. XRD showed the formation of (VO)₂P₂O₇ after calcinations as well as patterns of support materials (anatase, γ‐Al₂O₃). The formation of crystalline proportions of mixed oxides were not observed. The catalytic ammoxidation runs revealed a significant effect of the promoter metals on the catalytic properties by an increase of yield by ca. 20 % compared to bulk VPO. Almost complete conversion of 2, 6‐dichlorotoluene and 81 % yield of nitrile were observed using a 25 %VPCoO/γ‐Al₂O₃ catalyst.     

MENTHOR,a database system for the storage and retrieval of three-dimensional molecular structures and associated data searchable by substructural,biologic, physical,or geometric properties

Summary MENTHOR is a database system for the storage and retrieval of three-dimensional coordinate and charge information on molecules as well as of traditional biological and physical properties. Our molecular graphics system retrieves from MENTHOR structural information in individual molecules and receptor map/macromolecular binding site hypotheses. Substructural searches of MENTHOR are used to find starting coordinates for molecular modeling and traditional database searches of MENTHOR identify compounds for which modeling is needed. It also forms the data to be searched with ALLADDIN, our substructure/geometric search program. MENTHOR expedites molecular modeling by organizing previous work and facilitating transmission of information between individuals. Examples from modeling of D-2 receptor agonists are shown.     

Synthetic thermally reversible gel systems. V

Differential thermal analysis has been used to study the fusion of aqueous thermally reversible gels of gelatin and polyacrylylglycinamide (PAG). In the case of gelatin gels, endotherms close to the melting point are readily observed and these are sometimes preceeded by a small exothermic heat of gel reorganization. Calculations are presented to show that breaking of the gelatin gel network requires only a small fraction of the observed endothermic heat of fusion and that most of the heat is required for melting larger crystallites within gelatin aggregates and for perhaps a helix → coil transition. Failure to observe endotherms by DTA over the known temperature range of fusion of PAG gels is consistent with prior measurements and conclusions. The noncrystallinity of PAG gels and soluble aggregates together with a heat of crosslinking of only ?5 to ?10 kcal/mole of crosslinks places the heat of fusion of PAG gels outside the lower limits of DTA sensitivity.