Intramolecular homolytic substitution at selenium: synthesis of novel selenium-containing vitamin E analogues

Treatment of 1-(benzylselenenyl)-5-butyl-5-nonanol (10) with oxalyl chloride followed by the sodium salt of N-hydroxypyridine-2-thione afforded the corresponding pyridine-2-thione-N-oxycarbonyl (PTOC) oxalate ester which was not isolated but immediately heated to provide 2,2-dibutylselenane (7). This transformation presumably involves a tertiary alkyl radical that undergoes intramolecular homolytic substitution at selenium with loss of the benzyl radical to provide the selenium-containing ring system (7). A similar protocol, when applied to 1-(2-benzylselenenyl-5-methoxyphenyl)-3-methyl-3-heptanol (18) and 1-(2-benzylselenenyl-5-methoxyphenyl)-3,7,11,15-tetramethyl-3-hexadecanol (19), followed by deprotection, afforded the selenium-containing alpha-tocopherol analogues 4 and 1f, respectively, in moderate yields. To the best of our knowledge, these transformations represent the first examples of tertiary radicals involved in homolytic substitution chemistry at selenium.     

Rapid intramolecular interactions between organic isocyanates and hexafluorobut-2-yne on a dirhodium centre; X-ray crystal structure of (η-C5H5)2Rh2 {μ2(η3-C(CF3)C(CF3)C(O)N(Ph)}

Complexes of formula (η-C₅H₅₂Rh₂{CF₃C₂CF₃ · RNCO} have been prepared by three methods, from reactions between organic isocyanates and (η-C₅H₅)₂Rh₂(CO)(CF₃C₂CF₃) or (η-C₅H₅)₂Rh₂(CO)₂(CF₃C₂CF₃); by treatment of (η-C₅H₅)₂Rh₂(CO)(CF₃C₂CF₃) with organic azides; and by oxidation with Me₃NO of the organic isocyanide in (η-C₅H₅)₂Rh₂(CO)(CNR)(CF₃C₂CF₃). The crystal and molecular structure of the complex (η-C₅H₅)₂Rh₂{CF₃C₂CF₃ · RNCO} with R = Ph has been determined from single crystal X-ray diffraction data. This reveals that the isocyanate has condensed with the hexafluorobut-2-yne to form an amide ligand of the form C(CF₃)C(CF₃)C(=O)N(R); this bridges the two rhodium atoms in a μ₂η³-manner.     

On the internally contracted multireference CI method with full contraction

The configuration interaction (CI ) method where the efficiency of the generators of the unitary group is most fully exploited is the internally contracted multireference CI method. In the most recent version of this method the semi-internal configurations were kept uncontracted, which means that the number of configurations can still be quite large. In the present study the necessary formulas are derived for the case where the semi-internal states are also contracted. The highest density matrix that appears in these formulas is of order 5, and the computational treatment of this large matrix is discussed in detail.     

Influence of nanoporous carbon electrode thickness on the electrochemical characteristics of a nanoporous carbon|tetraethylammonium tetrafluoroborate in acetonitrile solution interface

Electrochemical characteristics for the nanoporous carbon|Et₄NBF₄+acetonitrile interface have been studied by cyclic voltammetry and impedance spectroscopy methods. The influence of the electrolyte concentration and thickness of the nanoporous electrode material on the shape of the cyclic voltammetry and impedance curves has been established and the reasons for these phenomena are discussed. A value of zero charge potential, depending slightly on the structure and concentration of the electrolyte, the region of ideal polarizability and other characteristics have been established. The nanoporous nature of the carbon electrodes introduces a distribution of resistive and capacitive elements, giving rise to complicated electrochemical behaviour. Analysis of the complex plane plots shows that the nanoporous carbon|Et₄NBF₄+acetonitrile electrolyte interface can be simulated by an equivalent circuit, in which two parallel conduction paths in the solid and liquid phases are interconnected by the double-layer capacitance in parallel with the complex admittance of the hindered reaction of the charge transfer or of the partial charge transfer (i.e. adsorption stage limited) process. The values of the characteristic frequency depend on the electrolyte concentration and electrode potential, i.e. on the nature of the ions adsorbed at the surface of the nanoporous carbon electrode. The value of the solid state phase resistance established is independent of the thickness of the electrode material.     

Dynamic electrochemical impedance spectroscopy measurements of passive layer cracking under static tensile stresses

Electrochemical impedance spectroscopy allows the examination of corrosion susceptibility and resistance for different construction materials, in particular the determination of the properties of their passive films. This technique makes possible the analysis of electrochemical processes in time domain, including rapid phenomena such as changes in the properties of passive films, but it has never been used for passive layer cracking examination. In many cases, fracture of the passive film under tensile stresses leads to stress corrosion cracking. Therefore, investigations of passive layer cracking on austenitic stainless steels under tensile stresses facilitate the understanding of the mechanism of stress corrosion cracking in these common engineering materials. The effect of static tensile stresses on the passive film cracking behaviour of type 304L stainless steel immersed in 0.5 M NaCl solution at room temperature has been investigated. This paper presents the impedance spectra obtained for 304L stainless steel samples at different potential values.Contribution to the 3rd Baltic Conference on Electrochemistry, Gdansk-Sobieszewo, Poland, 23–26 April 2003Dedicated to the memory of Harry B. Mark, Jr. (28 February 1934–3 March 2003)     

A study of the chemiluminescence behavior of myoglobin with luminol and its analytical applications

A chemiluminescence signal at 425 nm was observed when ferric state myoglobin was mixed with luminol in alkaline medium. Because the signal was remarkably enhanced in the presence of Fe(CN)₆ ^4–, analytical applications were investigated in a flow-injection system. The increase in chemiluminescence was linearly dependent on myoglobin concentration in the range 0.1 to 100 nmol L^–1, and the limit of detection was 0.04 nmol L^–1 with relative standard deviation 3.2% (3). It was also found that binding of Mb with the ligands CN^–, SCN^–, and F^– significantly inhibited the chemiluminescence reaction. The linear dynamic ranges for the ligands were 1.0–300.0, 0.1–3.0, and 0.5–100.0 nmol L^–1, and the limits of detection (S/N=3) 0.4, 0.04, and 0.2 nmol L^–1, for F^–, CN^–, and SCN^–, respectively. The relative standard deviations were 5.32%, 6.13%, and 3.38% for 0.1 nmol L^–1 CN^–, 0.5 nmol L^–1 SCN^–, and 1.0 nmol L^–1 F^–, respectively. At a flow rate of 2.0 mL min^–1 the assay could be accomplished in 1 min, including sampling and washing. The method has been successfully applied to the determination of myoglobin in human urine and F^– in water samples. A possible mechanism of chemiluminescence production by myoglobin and luminol is presented.     

One-pot Three-component Condensation Reactions in Water. An Efficient and Improved Procedure for the Synthesis of Furan Annulated Heterocycles

Summary. The environment-friendly one-pot three-component condensation reactions of 4-hydroxycoumarin or 4-hydroxy-6-methylpyrone, p-substituted benzaldehyde, and alkyl or aryl isocyanides to afford furocoumarines or furopyranones in water in good yields after about one hour at 75°C are reported.     

The first example of Diels-Alder cycloaddition of ortho-xylylenes to meso-tetraarylporphyrins containing electron-deficient β,β-double bonds

β-Nitro-5,10,15,20-tetraphenylporphyrin and its zinc complex, or 2,7-dinitro-5,10,15,20-tetraphenylporphyrin, react with 1,3-dihydrobenzo[c]thiophene 2,2-dioxide in refluxing 1,2,4-trichlorobenzene, giving rise to chlorins, bacteriochlorins or isobacteriochlorins. The products obtained are attractive intermediates for further functionalization of porphyrins and may be of potential use as sensitizers in photodynamic therapy.     

Hydrothermal Synthesis,Conversion Reaction,and Crystal Structure of 2-D 2-bromo-1,4-benzenedicarboxylate Copper(II) Complex with 2,2′-bipyridine

A fully deprotonated 2-bromo-1,4-benzenedicarboxylate copper(II) complex, [Cu(bbdc)(2,2′-bipy)](H₂O) (bbdc= 2-bromo-1,4-benzenedicarboxylate dianion, 2,2′-bipy=2,2′-bipyridine), was synthesized under hydrothermal conditions. This compound also can be prepared from the partly deprotonated [Cu(Hbbdc)₂(2,2′-bipy)] under basic and hydrothermal conditions. The crystal structure consists of infinite chains of [Cu₂(μ₄-bbdc)]²⁺ building blocks connected by bis-monodentate bbdc ligands and exhibits an extended 2-D architecture. Electronic supplementary material Electronic supplementary material is available for this article at and accessible for authorised users.     

Determination of affinity constants and response factors of the noncovalent dimer of gramicidin by electrospray ionization mass spectrometry and mathematical modeling

The dimerization of gramicidin, a 15-residue membrane peptide, in solution can be viewed as a model for protein-protein interactions. We reported previously that the dimer can be observed when electrosprayed from organic solvents and that the abundances of the dimer depends on the dielectric constant of the solvent. Here, we report an effort to determine an affinity constant for the dimerization of gramicidin by using gas-phase abundance. Two issues affecting the determination are the electrospray-induced dissociation of the dimer and discrimination in the electrospray of the dimer compared with the monomer. Other methods developed for the purpose of determining affinity from mass spectral abundance do not address the dissociation of the complex in the gas phase or can not be applied for cases of low affinity constant, K(a). We present a mathematical model that uses the ratio of the signal intensities of the dimer and the monomer during a titration. The model also incorporates the dissociation and an electrospray ionization-response factor of the dimer for extracting the affinity constant for the dimerization of gramicidin. The dimerization constants from the new method agree within a factor of two with values reported in the literature.