Copper-catalysed synthesis of 3,5-disubstituted isoxazoles enabled by pyridinyl benzimidazol (PBI) as a bidentate <Emphasis Type="Italic">N</Emphasis>-chelating ligand under mild conditions

In this paper, we introduced pyridinyl benzimidazol (PBI) as an easy-to-handle and bidentate N-chelating ligand that promote clean synthesis of 3,5-disubstituted isoxazoles in the presence of copper acetate as catalyst. This catalytic approach initiates with the hydroxyamination of aldehydes followed by chlorination and then generation of nitrile oxide which subsequently undergoes click-type [3 + 2]-dipolar cycloaddition with alkynes to give isoxazoles. This method provides an alternative green process to construct isoxazole derivatives.     

The visible absorbance maximum of 2-hydroxy-1,4-naphthoquinone as a novel probe for the hydrogen bond donor abilities of solvents and solvent mixtures

Summary The electronic absorption spectra of 2-hydroxy-1,4-naphthoquinone (HNQ) in one- and two-component solvents are discussed. The visible absorption of this reagent has been shown to arise from a charge-transfer transition of the zwitterionic tautomer stabilized by a hydrogen bond donating solvent. The formation constant of the 1:1 hydrogen bonded complex with methanol, ethanol, andn-propanol has been determined at 25 °C from the spectral behaviour in mixed solvents. The tautomerization equilibrium ofHNQ in aqueous ethanol is demonstrated and characterized. The dependence of the absorbance maximum ofHNQ on the hydrogen bond donor ability of the solvent in the visible spectrum, as measured by theTaft-Kamlet parameter, has been established. Experimentally derived evidence is supplied to justify the recommendation ofHNQ as a novel probe for hydrogen bond donor ability in pure solvents and in mixed aqueous solvents.

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Das Absorptionsmaximum von 2-Hydroxy-1,4-naphthochinon im sichtbaren Bereich als neues Maß für die Wasserstoffbrückenbindungsbildungsfähigkeit von Lösungsmitteln und Lösungsmittelgemischen

Zusammenfassung Die Absorptionsspektren von 2-Hydroxy-1,4-naphthochion (HNQ) in Ein- und Zweikomponentensystemen werden diskutiert. Die Absorption im sichtbaren Bereich stammt von einemcharge-transfer — Übergang des zwitterionischen Tautomers, das über Wasserstoffbrückenbindungen zum Lösungsmittel stabilisiert wird. Die Bildungskonstanten der binären Komplexe mit Methanol, Ethanol undn-Propanol wurden aus spektroskopischen Daten in Lösungsmittelgemischen bei 25 °C emittelt. Das Tautomeriegleichgewicht vonHNQ in wäßrigem Ethanol wird iskutiert. Die Abhängigkeit des Absorptionsmaximums vonHNQ vond der Wasserstoffbrückenbindungsbildungsfähigkeit des Lösungsmittels wird mittels desT aft-Kamlet — Parameters beschrieben. Experimentelle Ergebnisse ermutigen zur Verwendung vonHNQ als neue Testsubstanzfür die Wasserstoffbrückenbindungsbildungsfähigkeit von reinen und gemischten Lösungsmitteln.

     

Structure-property relationships for novel wholly aromatic polyamide-hydrazides containing various proportions of para-phenylene and meta-phenylene units: I. Synthesis and characterization

A series of novel wholly aromatic polyamide-hydrazides was synthesized by a low temperature solution polycondensation reaction of either 4-amino-3-hydroxybenzhydrazide or 3-amino-4-hydroxybenzhydrazide with an equimolar amount of either terephthaloyl chloride (TCl), isophthaloyl chloride (ICl), or mixtures of various molar ratios of TCl and ICl in anhydrous N,N-dimethylacetamide (DMAc) as a solvent. Polymer structures were identified by elemental analysis and infrared spectroscopy. All the polymers have the same structural formula except the way of linking phenylene units inside the polymer chain. The content of para- and meta-phenylene moieties was varied within this series so that the changes in the latter were 10 mol % from polymer to polymer, starting from an overall content of 0-100 mol %. The prepared polymers were characterized for their properties in order to acquire clear understanding of the influence exerted by controlled structural variations in these polymers upon some of important properties, such as solubility, intrinsic viscosity, moisture regain, mechanical properties and thermal as well as thermo-oxidative stability. The polymers were readily soluble in several organic polar solvents such as DMAc, N,N-dimethylformamide, dimethyl sulphoxide, N-methyl-2-pyrrolidone and hexamethyl phosphoramide and could be cast into flexible films. Their solubilities were found to increase remarkably with introduction of meta-phenylene moieties into the polymer chains. Their intrinsic viscosities ranged from 0.73 to 4.83 dl g⁻¹ in DMAc at 30 °C and increased with the increase of para-phenylene units content. Mechanical properties of the films produced from these polymers are improved markedly by substitution of para-phenylene units for meta-phenylene units. Thermogravimetric studies revealed that the completely para-oriented type of polymer has better thermal and thermo-oxidative stability relative to that of the other polymers. Moreover, the results reveal that the prepared polymers have a great affinity to water sorption. The hydrophilic character increases as a function of meta-oriented phenylene rings incorporated into the polymer chains.     

Simultaneous determination of phenol and mononitrophenol isomers using PLS regression and conventional and derivative spectrophotometry

The partial least squares regression method (PLS) was tested as a calibration procedure for the simultaneous determination of phenol, o-nitrophenol, m-nitrophenol and p-nitrophenol by both conventional and first derivative UV/Vis spectrophotometry. The experiments were conducted in the acidic, neutral and basic media. The results obtained by the application of the PLS procedure on the conventional and first derivative spectra in two solvent media were compared. It was found that the results obtained in the basic medium have better performance characteristics than those obtained in the acidic or neutral media. Comparable results were obtained in the case of both conventional and first derivative absorbance data. The proposed method was applied to the determination of the four phenol derivatives in natural spiked water samples at concentration levels between 1.0 and 10.0 microg ml(-1) with average recoveries in the range 96% - 99%.     

Synthesis and spectroscopic studies of complexes containing the Schiff base 1-acetylferrocene(thio)semicarbazone

Summary [RuCl₂(CO)₂] _n reacts with the Schiff base 1-acetylferrocenethiosemicarbazone, [Fe(-Cp)(-C₅H₄MeC=NN-HCSNH₂)] to give [Fe(-Cp)(-C₅H₄MeC=NN-HCSNH₂)RuCl₂(CO)₂] and with 1-acetylferrocenesemicarbazone [Fe(-Cp)(-C₅H₄MeC=NN-HCSNH₂)] to give [Fe(-Cp)(-C₅H₄MeC=NN-HCSNH₂)RuCl₂-(CO) ₂]. Spectroscopic data indicate that the Schiff bases act as bidentate ligands and coordinate to ruthenium via the hydrazinic N and either the S or O atoms, respectively, giving stable heterobimetallic complexes, which have been characterized by i.r. and ¹H-n.m.r. spectroscopies, and elemental analyses.Part of this work was presented at the First International Conference in Chemistry and its applications in Doha, Qatar, 1993.     

Novel synthesis of meso-tetraarylporphyrins using CF3SO2Cl under aerobic oxidation

meso-Tetraarylporphyrins are synthesized from pyrrole and aryl aldehydes cleanly and efficiently in one pot at room temperature using equimolar amount of CF₃SO₂Cl in the presence of air as oxidant. By this novel method 5,10,15,20-tetraarylporphyrins can be prepared in excellent yields.     

Existence results for coupled systems of quadratic integral equations of fractional orders

We are concerning here with the existence of at least one positive continuous solution of coupled systems of quadratic integral equations of fractional orders. Then an existence theorem for a coupled system of Cauchy problems will be proved.