Coordination of cyclo-octasulfur and cyclo-heptaselenium to dinuclear rhenium(I) systems

By substitution reactions of the coordinated THF ligands of Re(2)(mu-X)(2)(CO)(6)(THF)(2) by elemental chalcogens (S(8) and red selenium), the complexes Re(2)(mu-X)(2)(CO)(6)(S(8)) (X = Br, 1; I, 2), and Re(2)(mu-X)(2)(CO)(6)(Se(7)), (X = I, 3; Br, 4) have been prepared. Binuclear compound 3 was crystallographically established to be a coordination compound of cyclo-heptaselenium, two adjacent selenium atoms of the Se(7) ligand [Se-Se distance, 2.558(3) A] being bonded to rhenium(I), at an average Re-Se distance of 2.586(3) A, and the nonbonding Re.Re distance being 4.077(3) A. Spectroscopic evidence of the existence of these chalcogen complexes in solution is reported. The Re(2)(mu-X)(2)(CO)(6)(S(8)) complexes undergo S(8) displacement by THF, while the coordinated Se(7) moiety is less readily displaced from 3.     

The mechanism of N-haloamide promoted electrophilic additions. high regio and stereoselectivity in the conversion of 2-tert-butyl-3,6-dihydro-2H-pyran into bromohydrins and in the opening of the corresponding epoxides

The reactions of cis- and trans-2-tert-butyl-4,5-epoxytetrahydropyran with HBr and with LAH have been examined as a model for the nucleophilic step of the reaction of the corresponding olefin with NBA in aqueous dioxane. A remarkable 90:10 preference for electrophilic attack syn to the tert -butyl group in the NBA reaction is found and shows that the two epoxides, as well as the intermediate epibromonium ions, undergo nucleophilic attack with high preference for diaxial opening, even when this requires reaction at carbon 5, which is more subject than carbon 4 to the unfavourable inductive effect of the pyran ring oxygen. These results constitute a further proof in favour of a mechanism of N-haloamide promoted electrophilic additions in which the electrophilic step is rapidly reversible and product composition is determined during the nucleophilic step.     

Synthesis and characterization of novel Cd(II), Zn(II) and Ni(II) complexes with 2-quinolinecarboxaldehyde selenosemicarbazone. Crystal structure of bis(2-quinolinecarboxaldehyde selenosemicarbazonato)nickel(II)

New complexes of Cd(II), Zn(II) and Ni(II) with 2-quinolinecarboxaldehyde selenosemicarbazone (Hqasesc) were synthesized and structurally characterized. The structure of the ligand, Cd(II) and Zn(II) complexes was determined by NMR and IR spectroscopy, elemental microanalysis and molar conductivity measurements. Both complexes occur in solution in two forms, the major tetrahedral and minor octahedral. In the major Cd(II) complex one qasesc⁻ ligand is coordinated as a tridentate, the fourth coordination site being occupied by acetate, while in the major Zn(II) complex two qasesc⁻ ligands are coordinated as bidentates. In both minor complexes two qasesc⁻ ligands are coordinated as tridentates forming the octahedral geometry around the central metal ion. The only paramagnetic complex in the series is Ni(II) complex for which X-ray structure analysis was performed. The complex has the angularly distorted octahedral geometry with two qasesc⁻ ligands coordinated as tridentates, in a similar way as in the minor complexes of Cd(II) and Zn(II).     

Physical Hydrogels of Poly(vinyl alcohol) with Different Syndiotacticity Prepared in the Presence of Lactosilated Chitosan Derivatives

Poly(vinyl alcohol) (PVA) physical hydrogels were prepared by repeated freeze–thawing cycles using aqueous solutions of two PVA samples having different degrees of syndiotacticity, a‐PVA and s‐PVA with 55% and 61% of syndiotactic diads, respectively. The hydrogels were prepared in the presence of different amounts of lactosilated chitosan derivatives (LC) of different molecular weight. The PVA stereoregularity was found to have a dramatic effect on the amount of PVA incorporated into the hydrogels, leading to remarkable differences in the swelling degree and porosity of a‐PVA and s‐PVA hydrogels. A significant amount of LC was retained in the hydrogels after equilibrium swelling. The swelling of the a‐PVA hydrogels was found to increase significantly by increasing the amount of LC while it was only slightly increased in the case of s‐PVA hydrogels. The amount of LC released after equilibrium swelling was lower when chitosan derivatives with higher molecular weights were used. Increased initial concentrations of LC resulted in much higher porosity of the hydrogels. TGA and DSC studies showed that LC is stabilized by the incorporation in the PVA hydrogels. The melting temperature of the crystalline regions of PVA was not significantly influenced by LC. Conversely, the extension of the crystalline domains increased in the presence of LC. The retention of a chitosan derivative bearing β‐D ‐galactose side chain residues makes these hydrogels potentially useful as scaffolds for hepatocytes culture.

Scanning electron micrographs of PVA‐LC hydrogels: (a) a‐PVA; (b) a‐PVA/LC₁₅₀ 80:20; (c) a‐PVA/LC₁₅₀ 50:50.     

Electroanalytical Determination of Lidocaine in Pharmaceutical Preparations Using Boron‐Doped Diamond Electrodes

A new electroanalytical procedure was developed for the determination of lidocaine in commercial local anesthetics products containing lidocaine as the active ingredient. The procedure is based on the use of electrochemical methods as cyclic and square‐wave voltammetry, with boron‐doped diamond electrodes. The oxidation of lidocaine in Britton–Robinson buffer (0.1 mol L^?1) using this type of electrode gives rise to one irreversible peak in 1.68 V (versus Ag/AgCl). The detection and quantification limits obtained from pure water were 10.0 and 34.4 μg/L, respectively. The proposed electrochemical method was also successfully applied to the analysis of commercially available pharmaceutical preparations. The electrochemical responses of pharmaceutical preparations (gels) were identical to those of standard lidocaine. No influence of propyleneglycol present in the gels on the voltammetric responses was observed. Lidocaine recoveries ranged from 97.6% to 99.2%.     

Oxidative Intramolecular Coupling of Amidocuprates as a Novel Route to Amines and Hydrazines

Amidocuprates, derived from organocopper reagents and lithium amides upon exposure to oxygen at low temperature, provide new amine derivatives in satisfactory yields. Details of this flexible and simple methodology are given. The reaction mechanism is analyzed in terms of an oxidative intramolecular coupling of aminyl radicals with the ligands on Cu in the intermediate amidocuprate. This reaction is a mild and efficient method for N-alkylation, -vinylation, and -arylation by which a number of amines, not easily accessible by normal routes, can be synthesized. Once applied to lithium hydrazides, it also provides a new and straightforward entry to N-substituted hydrazines.     

Quadratic estimates for the number of integer points in convex bodies

We give bounds for the mean square deviation with respect to arbitrary probability measures of the number of integer points in translated or dilated convex bodies. The proofs are based on Fourier analytic methods.     

Contrasting chemoselectivities in the ultrasound and microwave assisted bromination reactions of substituted alkylaromatics with N-bromosuccinimide

Ultrasound and microwave assisted bromination reactions of various alkylaryls with N-bromosuccinimide, either neat or in water, shows diverse chemoselectivity. Thus, ring substitution occurs in water with ultrasound, whereas with microwaves both side-chain α-bromination and ring substitution occur. With neat reactants, side-chain α-bromination predominates for microwave assisted reactions. In the presence of water the chemoselectivity with microwave-promoted bromination is similar to that observed using classical methods.     

Synthesis and spectroscopic characterization of new triorganotin(IV) complexes with the bis(1‐methyl‐1H‐imidazol‐2‐ylthio)acetate ligand: effects on trout erythrocyte components

Triorganotin(IV) derivatives containing the anionic ligand bis(1‐methyl‐1H‐imidazol‐2‐ylthio)acetate [(S‐tim)₂CHCO₂]⁻ were synthesized from the reaction between R₃SnCl acceptors (R = Me and Ph) and the sodium salt of the ligand. Mono‐nuclear complexes of the type [(S‐tim)₂CHCO₂]SnR₃ were obtained, which were fully characterized by elemental analyses and FT‐IR in the solid state, and by NMR (¹H, ¹³C and ¹¹⁹Sn) spectroscopy and electrospray ionization mass in solution. The toxic effects shown by these compounds on trout erythrocyte components showed that the toxicity of the organotin(IV) complexes depends on the nature and on the lipophilicity of the substituents on the metal centre. Copyright © 2007 John Wiley & Sons, Ltd.     

Exploring the reaction of iodine with α‐diazo esters

In this paper we describe the unprecedented reaction between α‐diazo esters 1 and iodine. The reaction, carried out in the presence of aqueous NaHCO₃, afforded the Z‐isomer of the corresponding unsaturated‐2‐iodo ester 8 . The configuration of compounds 8 was determined using the ³J_{C? H} coupling between carbonyl carbon atom and alkene proton. Mechanistic considerations accounting for the observed phenomena and including quantum chemical calculations are proposed. Copyright © 2008 John Wiley & Sons, Ltd.