'Non-covalent synthesis' of a chiral host of calix[6]arene and enantiomeric discrimination

'Non-covalent synthesis' of novel chiral hosts (calix[6]arene-chiral amine complexes) and its application to enantiomeric discrimination was investigated by (1)H NMR spectroscopy. The topology of a ternary complex was proposed for the calix[6]arene-amine-sulfoxide to rationalize the chiral recognition.     

Die potentiometrische Mikrobestimmung von Kupfer in organischen Verbindungen nach Verbrennung in einem modifizierten Schöniger-Kolben

Zusammenfassung Nach Verbrennung der Substanz in einem modifizierten Sauerstoffkolben werden die Verbrennungsprodukte in 2,5 ml 8M HNO₃ absorbiert; das gebildete CuO wird vom Pt-Netz durch eine spezielle Technik vollständig gelöst. Nach dem Erkalten wird die Lösung in die Titrationszelle übertragen, mit NaOH in Gegenwart von Phenolphthalein neutralisiert und das Cu(II) schließlich in gepufferter Lösung mit 0,01N Cyanoferrat(II) potentiometrisch titriert. Eine kupferselektive Elektrode und eine Single-Junction-Referenzelektrode dienen in Kombination mit einem pH-Meter mit gedehnter Skala zur Erkennung des Titrationsendpunktes. Die Resultate sind innerhalb ±0,10% genau: die Wiederfindungsrate von Kupfer liegt zwischen 99,65 und 100,10%; die Standardabweichung beträgt 0,04%. Die Bedingungen und die Charakteristiken der Titration von Cu(II) mit Cyanoferrat(II) ebenso wie die Störung einiger Anionen werden diskutiert. Der beschriebene modifizierte Verbrennungskolben eignet sich zur Mikrobestimmung auch anderer Elemente in metallorganischen Verbindungen.

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Potentiometric microdetermination of copper in organic compounds after combustion in a modified schöniger's flask

Summary After combustion of the sample in a modified oxygen flask, the combustion products are absorbed in 2,5 ml 8M HNO₃ and the CuO formed is completely dissolved from the Pt-basket by a special technique. After cooling, the solution is transferred to the titration cell and neutralized with NaOH in the presence of phenolphthalein; the copper(II) is finally titrated potentiometrically in a buffered medium with 0,01N hexacyanoferrate (II). A copper-selective electrode and a single junction reference electrode are used in combination with an expanded-scale pH-meter to detect the endpoint. The results obtained are very accurate and reproducible: the max. error does not exceed 0.10% (abs.), the recoveries of copper range from 99.65 to 100.10% and the standard deviation is 0.04%. Conditions and features of the titration of copper(II) with hexacyanoferrate(II) as well as the effect of some interfering anions are discussed. The modified oxygen flask described is also useful for the microdetermination of other elements in organometallic compounds.

     

Synthesis of fluorinated drimanes. Preparation of 9αF-drimenin

A stereoselective approach to the 9α-fluorinated analogue of the natural drimane sesquiterpene drimenin starting from β-ionone is described. β-Ionone is transformed into a bicyclic β-cetoester from which 9αF-drimenin is prepared through stereoselective electrophilic fluorination at the C-9 with N-fluorobenzenesulfonimide followed by Wittig methylenation, allylic bromination, bromine-hydroxyl exchange and γ-lactonization.     

Tungsten(VI)-gluconic acid complexes: Polarimetric and13C-N.m.r. Studies in an excess of tungsten(VI)

Summary A polarimetric study of the tungsten (VI)-gluconic acid system in an excess of metal reveals the formation of four stable complexes: two monomers with 1 : 2 and 2: 1 stoichiometries and two dimers of 2:2 composition. The pH ranges of these species, the amount of acid equivalents their formation requires, and their conditional stability constants have also been calculated.The probable coordination of the organic ligand to the metallic centre in solutions containing different metal : ligand ratios has been investigated by¹³C-n.m.r. spectroscopy. The results confirm the formation of different complexes depending on the reagent which is in excess. The behaviour of the system when the metal is in excess is related to that of other polyhydroxylic ligands such as mannitol and sorbitol.     

Comprehensive study of the methyl effect on the solvolysis rates of bridgehead derivatives

The effect of a bridgehead methyl group on the hydride ion affinity in the gas phase of bicyclo[1.1.1]pent-1-yl (1+), 1-norbornyl (3+), cubyl (5+), 1-adamantyl (7+), bicyclo[2.2.2]oct-1-yl (9+),and bicyclo[3.1.1]hept-1-yl (11+) cations has been studied using density functional theory and ab initio methods. It is concluded that the methyl group always increases the stability of the substituted cations. The effect of the solvent on the stability of methyl-substituted cations in relation to the unsubstituted cations has been studied using the polarizable continuum model of the self-consistent reaction field theory. In the case of rearranging cations, the nucleophilic assistance of the solvent is determined by means of the interaction energy of the corresponding water complexes. It is concluded that the solvent causes the relative stabilization of the parent cations. As a consequence, most of the methyl-substituted bridgehead derivatives show a lower solvolysis rate than the corresponding unsubstituted compounds. A nonqualitative explanation of the methyl effect on the relative stability of bridgehead cations in both gas phase and solution is given for the first time. The ratios of solvolysis products in the case of rearranging bridgehead cations have also been computed from the relative stability of the intermediate water complexes.     

Kinetics and Mechanism of Aminolysis of Phenyl Acetates in Aqueous Solutions of Poly(ethylenimine)

Second-order rate constants (k(n)) for the aminolysis of some phenyl acetates with poly(ethylenimine) (PEI) were obtained in a pH range 4.36-11.20 at 25 degrees C in 1 M KCl. Linear Bronsted-type plots (log k(n) vs pK(N) of PEI) were found for less reactive esters 2-nitrophenyl acetate, 4-acetoxy-3-chlorobenzoic acid, and 4-acetoxybenzenesulfonate with slopes of 0.92, 0.99, and 0.82, respectively. Curved plots were obtained for 3-acetoxy-2,6-dinitrobenzoic acid and 4-acetoxy-3-nitrobenzenesulfonate, which are consistent with a stepwise reaction. The most likely mechanism involves the existence of a tetrahedral intermediate (T(+/-)) and a change in the rate-determining step from its breakdown to its formation when the basicity of the polyamine increases. A semiempirical equation was used to calculate the values of limiting slopes of the plots (0.9 and 0.1 for both esters) and pK(N) at the center of the curvature of the plots (pK(N degrees ) = 7.94 and 9.02, respectively). The values of pK(N degrees ) are lower than those estimated for the aminolysis of the same esters with simple monomeric amines (pK(n degrees ) > 11) because of a better leaving ability of the aryl oxide ion from the tetrahedral intermediate when amino groups of PEI instead of simple amines are involved. Estimation of the pK's of the reactive intermediates and of the microscopic rate constants for the proton transfer from T(+/-) to PEI or from PEIH(+) to T(+/-) indicates that either base or acid catalysis is unimportant in the aminolysis of these esters by PEI.     

Multistep adsorption of anionic dyes on silica/chitosan hybrid. 1. Comparative kinetic data from liquid- and solid-phase models

In this work, a hybrid silica/chitosan was synthesized and characterized by nitrogen elemental analysis and thermal analysis (TG, DTG, DTA, and DSC) and BET surface area. The hybrid was used in adsorption studies of two anionic dyes from aqueous solutions. A rise of temperature accelerates mass transfer of dyes into the hybrid. However, the maximum adsorption capacities reach similar values from 25 to 55 degrees C. The kinetic data were first evaluated in relation to the decrease of the time-related residual concentration of the dyes in solution, where the second-order model has presented the best fitting. The solid-phase interaction of dye data presents a rough fitting to the traditional first-order Lagergren kinetic model. However, a modified Avrami kinetic equation was successfully fitted to the kinetic quantities, where from five to seven kinetic regions were found. A pore-diffusion model has also demonstrated that the diffusion is the rate-controlling interaction mechanism. However, the experimental-calculated comparative values are the best way to evaluate a specific aqueous- or solid-phase kinetic model.     

Fused pyrazolopyrimidines. I. Pyrazolo[4,3-e]-v-triazolo[1,5-a]pyrimidine. A new heterocyclic system

5-Azido-4-cyano-1-phenylpyrazole reacts with benzylcyanide in the presence of sodium methoxide to give the title ring system.     

Binuclear Cu2+ complex mediated discrimination between L-glutamate and L-aspartate in water

L-glutamate and L-aspartate selectivity is achieved by the action of two Cu2+ metal ions rightly disposed in a cyclophane-type macrocyclic framework; electrochemical sensing of glutamate has been achieved by adsorption of the copper complexes on graphite electrodes.     

Copper bronze catalyzed Heck reaction in ionic liquids

Heck reaction of aryl iodides and activated aryl bromides catalyzed by copper bronze in tetrabutylammonium bromide as solvent and tetrabutylammonium acetate as base was developed. The effective catalysts are Cu nanoparticles deriving from the reaction of iodobenzene with copper bronze. These nanoparticles are very stable in tetraalkylammonium salts, are easily recycled, and can be stored for months without a loss of catalytic efficiency. [reaction: see text]