Electroorganic synthesis. Reductive alkylation of functionalized 1,2-dithiole-3-thiones

Two-electron reduction of some substituted 1,2-dithiole-3-thiones 1 followed by alkylation of the dianionic intermediates leads through electrosynthesis to mixture of Z and E isomers of the corresponding substituted alkyl-(3-thioalkyl)-2-propenedithioates 2, 3 in satisfactory yield. The structure of those products was established by ¹³C and ¹H nmr and mass spectroscopy. The isomers ratios were determined by nmr spectroscopy.     

Kinetics of thermal decomposition of Co3O4 powder and single crystals

Kinetics of thermal decomposition in vacuum of Co₃O₄ powder as well as single crystals has been investigated. Discrepancies with the results of previous authors have been discussed. Decomposition of Co₃O₄ proceeds through formation of a compact layer of CoO and hence diffusion is the rate-limiting factor. The experimental curves α(t) be described for 0.05 < α < 0.85 using a modified Ginstling-Brounshtein equation: 1 ? 2α/3 ? (1 ? α)^2/3 = ktⁿ where the activation energy varies with the degree of decomposition.     

Stereochimie de la des benzoates d'argent sur des diesters iododesoxy ns: I- Action des benzoates d'argent sur des di-o-aroyl-1,2 iododesoxy-3 ns-glycerols

The stereochemistry of the reaction of silver benzoates on chiral di-O-aroyl-1,2 iododeoxy-3-sn-glycerols has been reinvestigated. The reaction proceeds either with or without neighbouring group participation, depending on : - possible electrophilic assistance by the silver ion to the ionisation of the substrate (occuring only with dissociated silver salts), - ability of substituants R¹ and R² on the substrates to stabilize a positive charge. Rearrangements are at minimum with R¹ and R² electronreleasing groups in the substrate, silver salts un-or weakly dissociated and bearing a good nucleophile. For one case, a regio- and stereo specific reaction is observed.     

The principle of imperfect synchronization: I. Ionization of carbon acids

The Principle of Imperfect Synchronization (PIS) states that a product stabilizing factor will lower the intrinsic rate of a reaction if it develops late, but increase the intrinsic rate if the factor develops early. It is shown in this paper that much of the structure-reactivity behavior of proton transfers involving carbon acids can be understood in the context of this principle. The factors discussed include the effect of resonance near the reaction site as well as in remote substituents, the polar effect of mote substituents, and the effects of solvation of reactants and products A sample matheatical formalism is developed to describe the contribution of each factor to the intrinsic rate constant. The possible reasons why there is a lack of synchronization between the various events which occur during the reaction are also discussed     

catena‐Poly[[aqua­barium(II)]‐μ‐aqua‐bis­(μ‐2′‐carboxy­biphenyl‐2‐carboxyl­ato)]

In the title compound, [Ba{HOOC(C₆H₄)₂CO₂}₂(H₂O)₂] or [Ba(C₁₄H₉O₄)₂(H₂O)₂], the Ba atoms are coordinated by nine O atoms, six from two 2′‐carboxy­biphenyl‐2‐carboxyl­ate (Hbpdc⁻) ligands and three from three coordinated water mol­ecules, resulting in the formation of face‐sharing distorted monocapped square anti­prisms. The Hbpdc⁻ ligands bridge the Ba atoms to form a one‐dimensional helical polymer, with a Ba⋯Ba distance across the chain of 4.1386 (17) Å. Adjacent chains are parallel to each other. The two independent ligands are tetra­dentate and have the same coordination mode, exhibiting μ‐oxo bridges and η⁸‐chelation. The crystal structure is further stabilized by hydrogen bonds within each chain.     

Preconcentration of lead,cadmium, copper and zinc in water at the pg g−1 level by non-boiling evaporation

An evaluation of the non-boiling evaporation technique for the preconcentration of Pb, Cd, Cu, Zn at the pg g^?1 level in water samples is presented. Various improvements were made to allow efficient control of contamination problems at these extremely low concentrations. They include the choice of FEP Teflon for the evaporation containers and the use of sophisticated cleaning, ageing and pre-conditionning procedures. Detailed calibration graphs were obtained down to the sub-pg g^?1 level by processing ultra-low concentration standards. This technique was then applied to the determination of these four metals in snow samples collected in Greenland and Antarctica.     

Synthesis of Unsubstituted and 4,4′-Substituted Oligobipyridines as Ligand Strands for Helicate Self-Assembly

A general strategy for the synthesis of oligobipyridine ligands 2 – 5 containing from two to five 2,2′-bipyridine subunits, for helical metal complexes is described (sec Scheme). Both the unsubstituted parent strands ( a series) as well as their derivatives bearing fester or amide functions in the 4,4′-positions of the bipyridine moieties ( b – d series) have been obtained.     

Identification,synthesis and conformational study of a CN(R,S) synthon by-product: (5R, 10R)-5,10-diphenyl-1,6-diaza-3,8-dioxabicyclo[4.4.1]undecane

The title compound, containing a new heterocyclic skeleton, was identified by X-ray crystallography as the product of condensation of (R)-phenylglycinol with an excess of formaldehyde. The molecule adopts a rigid double twist-chair conformation in both solid and solution states.     

Thermodynamics of complexation of monovalent metal cations by the ionophore monensin free acid in acetonitrile

The standard Gibbs functions, enthalpies, and entropies for the formation of the complexes of monensin acid with monovalent metal cations in acetonitrile were obtained using various methods (conductometry, potentiometry, and calorimetry). Complexation is appreciable (from 2.0 to 4.5 in terms of association constant logarithm) and controlled mainly by the enthalpic effect. Comparison with formation of the corresponding neutral complexes permits a comparison to the process of monensin mediated transport of cation in membranes. A two-step process involving first the formation of the acidic complex is suggested.     

Characterization of Two Light-Harvesting Subunits Isolated from the Brown Alga Pelvetia canaliculata: Heterogeneity of Xanthophyll Distribution

The main light-harvesting fraction from Pelvetia canaliculata was isolated on a sucrose density gradient from digitonin-solubilized chloroplasts. After further solubilization by dodecyl maltoside, the bulk fraction was separated into two subunits by preparative isoelectric focusing. The more acidic brown fraction was mainly composed of 22 kDa polypeptides having an apparent pI of 4.55. Its pigment composition was very simple, containing chlorophyll (Chi) a, Chi c and fucoxanthin. The in vivo spectral properties of fucoxanthin, namely a shift in light absorption to the green and efficient energy transmission to Chi a, were conserved in this subunit. No xanthophyll associated with photoprotection was found in this band, even when obtained from photoinhibited thalli. The less acidic green band contained predominantly 22 kDa polypeptides that were resolved into numerous components by denaturing isoelectric focusing. Its pigment composition was more complex, containing, in addition, pigments of the so-called xanthophyll cycle. In photoinhibited thalli, about half of the violaxanthin was converted into antheraxanthin and zeaxanthin. All the pigments of the xanthophyll cycle were specifically associated with this subunit, and it may thus have a central role in the thermal dissipation of the absorbed light energy as postulated for light-harvesting complex II isolated from green plants.