The 1,2,4-triazolyl cation: thermolytic and photolytic studies

The generation of the 1,2,4-triazolyl cation (1) has been attempted by the thermolysis and photolysis of 1-(1,2,4-triazol-4-yl)-2,4,6-trimethylpyridinium tetrafluoroborate (2) and the thermolysis of 1- and 4-diazonium-1,2,4-triazoles, using mainly mesitylene as the trapping agent. Thermolysis of 2 gave mostly 1,2,4-triazole, together with 3-(1,2,4-triazol-4-yl)-2,4,6-trimethylpyridine, 4-(1,2,4-triazol-4-ylmethyl)-2,6-dimethylpyridine, and 4-(2,4,6-trimethylbenzyl)-2,6-dimethylpyridine. Thermolysis of each of the diazonium salts in the presence of mesitylene again gave mainly triazole together with very low yields of 1-(1,2,4-triazol-1-yl)-2,4,6-trimethylbenzene and the corresponding -4-yl isomer in about the same ratio. On the other hand, photolysis of 2 in mesitylene gave mainly 1-(1,2,4-triazol-1-yl)-2,4,6-trimethylbenzene. A photoinduced electron transfer from mesitylene to 2 has been observed and preliminary laser flash photolyses of 2 and the corresponding 2,4,6-triphenylpyridinium salt have been carried out. The observed transients are explained as arising from the first excited states of the pyridinium salts rather than from 1. Ab initio MO calculations are reported and indicate that the predicted electronic ground-state of the triazolyl cation is a triplet state of B1 symmetry with five pi electrons, which corresponds to a diradical cation (1c). Possible mechanisms for the formation of the various products are proposed.     

FLCs with a five-membered ring in the mesogenic core

A study has been undertaken of the structure-activity relationship of eight new chiral compounds having either a pyrazole (series P) or an isoxazole (series I) ring as a central bridge in the mesogenic core. The presence of dimers in the pyrazole compounds accounts for their lower P s values in comparison with the isoxazole analogues. The corresponding four beta-diketone precursors have also been studied and these, as expected given their bent molecular shape, show much worse mesomorphic and ferroelectric behaviour. In order to complete the study, the molecular dipoles of the three types of derivative have been determined using AM1 calculations. Two types of chiral tail have been incorporated into the compounds: alkoxy and alkanoyloxy. The latter tail gives rise to the best mesomorphic and ferroelectric properties. A study of the tail conformations by MM2 calculations provides an explanation of these results. The highest P s value (137 nC cm ) has been obtained for the isoxazole derivative with the (2S)-2-butyloxypropanoyloxy chiral tail (compound I4). The potential of all twelve compounds as chiral dopants for FLC mixtures has been evaluated by a study of 10 mol% binary mixtures in a standard host system.     

Biosynthesis of the Spermine Alkaloid Aphelandrine

Aphelandrine ( 1 ) is shown to be biosynthesized in the root cells of Aphelandra tetragona (VAHL ) NEES from labelled putrescine ( 4 ), spermidine ( 5 ), and cinnamic acid ( 3 ). Whether spermine ( 6 ) and the (p-hydroxycinnamoyl)spermidine 8 are precursors of 1 is uncertain, since the latter is hydrolysed to a large extent before incorporation, and the former is metabolized to 4 and 5 . Methionine ( 7 ) is the source of the 3-aminopropyl unit of 5 and 6 .     

Micro-composites based on polylactic acid with kaolinite or rice husk particles and their performance on water vapor permeability

In this work, the effect of addition of kaolinite and rice husk microparticles on the water vapor permeability of polylactic acid (PLA) was evaluated. The PLA-composites materials were characterized by X-ray diffraction, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and optical microscopy. For the composite preparation, an experimental design was established using two factors: additive type and concentration. When evaluating the water vapor permeability of the composites, it was found that the incorporation of kaolinite at 1% and 5% reduced the permeability coefficient of PLA. Furthermore, the inclusion of 5% kaolinite showed an increase on the tensile strength of the material. By contrast, the addition of rice husk produced an increase on water vapor permeability of the material and a decrease of both the tensile strength and ductility (percent of elongation at fracture). The higher dispersion and more regular morphology of the kaolinite particles could explain the enhancement of the gas barrier. A similar thermal behavior was observed for both PLA and composite materials, confirming that no damage of these properties was caused with the incorporation of the filler particles.     

Solid phase extraction of Pb(II) in water samples using a new hybrid inorganic-organic mesoporous silica prior to its determination by FAAS

A mesoporous silica has been chemically modified with 5-mercapto-1-methyltetrazole. The newly synthesized material was characterized by powder X-ray diffraction, N₂ adsorption, FT-IR, ¹³C-NMR spectroscopy and elemental analysis, and used to preconcentrate Pb(II) from aqueous solutions. The effect of several variables on the adsorption capacity (i.e. stirring time, pH, interfering ions, presence of other heavy metals in the medium, etc.) has been studied using batch and column techniques. The adsorption capacity of the material followed the order: Pb(II) >> Cu(II) > Cd(II) >>Mn(II) > Ni(II) > Co(II). In column experiments a pre-concentration factor of 200 was obtained for Pb(II). Spiked tap water, mineral water and river water were used for the preconcentration and determination of Pb(II) by flame atomic absorption spectrometry, and a 101–103% recovery was obtained. The limit of detection and quantification values of the method were found to be 2.22·10^?6 mM and 8.20·10^?6 mM, respectively. The relative standard deviation for four preconcentration experiments was found to be ≤9% in all cases.     

Voltammetric analysis of Pb(II) in natural waters using a carbon paste electrode modified with 5-mercapto-1-methyltetrazol grafted on hexagonal mesoporous silica

A hexagonal mesoporous silica (HMS) functionalized with a 5-mercapto-1-methyltetrazole derivative was employed to prepare a chemically modified carbon paste electrode for Pb(II) detection in aqueous solution by square wave adsorptive stripping voltammetry. The optimal operating conditions were 5 min preconcentration time at pH 6.5, and 120 s electrolysis time in 0.2 mol L^?1 HCl. Under these conditions, the voltammetric signal increased linearly with the preconcentration time in the range 1 to 10 min and with the Pb(II) concentration in the range 1 to 100?µg L^?1. The electrode was reproducible and sensitive. Simultaneous determination of Pb, Cd and Cu was also carried out with the electrode. The accuracy of the method was validated by analysing Pb(II) in tap water and groundwater samples.     

C-branched chiral (racemic) macrocyclic amino acids: structure of their Ni(II), Zn(II) and Cu(II) complexes

A new procedure for the synthesis of macrocyclic embedded bis-α-amino acids and their use as cation-ligands is described. These compounds are able to form stable Cu(II), Zn(II) and Ni(II) complexes as long as they have a flexible tether between the two nitrogen atoms. For a given macrocycle, the X-ray diffraction studies revealed diastereomerically pure complexes having different geometries depending on the metal ion.     

Regioselective and Stepwise [8 + 2] Cycloaddition Reaction between Alkynyl-Fischer Carbene Complexes and Tropothione

The formal [8 + 2] cycloaddition reaction between alkynyl Fischer carbene complexes and tropothione leads to the regioselective formation of novel 3aH-cyclohepta[b]thiophene carbene complexes. Computational DFT calculations indicate that the process proceeds stepwise via antiaromatic zwitterionic intermediates.     

Fluoration de telomeres polyhalogenes par le fluorure de potassium (2ème partie). Synthese d'olefines perfluorees de hauts poids moleculaires

Fluorination of chlorofluorinated telomers of general formula Cl(CFClCF₂)_nCCl₃ (n ? 7) was studied using potassium fluoride in dimethylsulfoxide as fluorinating agent. The reaction leads to a mixture of perfluorinated linear olefins having 15 to 30 carbon atoms : CF₃(CF₂)_pCFCF(CF₂)_mCF₃ (p + m = 2n?3).We perfected a method to determine molecular weights of the telomers by gel permeation chromatography (G.P.C.). The olefins obtained by fluorination are analyzed both by vapor phase chromatography (V.P.C.) and by ¹⁹F nuclear magnetic resonance (N.M.R.).