Photolysis of organopolysilanes. The reaction of photochemically generated methylphenylsilylene with conjugated dienes

Photolysis of 2-phenylheptamethyltrisilane (I) in the presence of acyclic and cyclic conjugated dienes has been investigated using both a high-pressure mercury lamp with a quartz filter and a low-pressure mercury lamp with a Vycor filter. Irradiation of I in the presence of 1,3-butadiene, isoprene or 2,3-dimethylbutadiene with a high-pressure mercury lamp gave a product arising from photochemical isomerization of a silacyclopropane derivative and a compound apparently formed by 1,4-silylene addition, along with a 1/1 “ene” adduct of the diene to a photo-rearranged intermediate containing the silicon—carbon double bond. Irradiation of I in the presence of the conjugated diene with a low-pressure mercury lamp, followed by treatment of the product with methanol, afforded a methoxysilane arising from methanolysis of the corresponding silacyclopropane, together with the isomerization product, silacyclopentene and rearranged addition product. Irradiation of I in the presence of cyclopentadiene with a high-pressure mercury lamp produced methylphenylsilylcyclopentadiene, while irradiation of a similar mixture with a low-pressure mercury lamp followed by treatment with methanol gave 4-(methoxymethylphenylsilyl)-1-cyclopentene. With 1,3-cyclooctadiene, the photochemically generated methylphenylsilylene afforded many types of addition product. Photolysis of I in the presence of 1,3-cyclohexadiene, however, afforded none of the silylene addition products.     

四氟乙烯齐聚体的反应 四氟乙烯四、五聚体和芳香胺的反应

前已报道四氟乙烯四聚体(全氟-3,4-甲基己烯-3)(1)、五聚体(全氟-3,4-二甲基-4-乙基己烯-2)(2)和脂肪烷氧以及脂肪胺的亲核反应.本文报道化合物1,2和芳香胺如苯胺、β-萘胺的反应.由于烯烃1、2双键处于分子中间,因而当亲核试剂进攻时,双键容易发生重排,生成的末端基烯烃更具反应性,故导致一取代、二取代、三取代以及环化降解等复杂产物.     

Heats of Mixing of Mixed Solvent Solutions of Alkali Metal Salts of Substituted Benzenesulfonic Acids

The enthalpy change on mixing solutions of methyl-substituted benzenesulfonic acids and their salts, with salts having a common cation or anion, at constant ionic strength in mixtures of 1,4-dioxane with water, were measured at 25°C. The heat effects of mixing solutions having a common anion increase almost linearly with the reciprocal value of the dielectric constant of the solvent. The heat effects of mixing solutions having a common cation, which are all negative in water-rich solutions, become endothermic after a certain value of the dielectric constant is passed. The results are discussed in terms of the solute-solvent structural properties. This revised version was published online in July 2006 with corrections to the Cover Date.     

Correction of pH of diluted solutions of electrolytes by electrodialysis with bipolar membranes

The current efficiencies of the water dissociation water and the voltage-current characteristics of the bipolar (asymmetric bipolar) membranes were measured in a two-chamber electrochemical cell. The cell was formed of an MB-3 bipolar membrane or an asymmetric bipolar membrane, which is an MA-40 heterogeneous membrane with a thin surface layer in the form of a cation-selective homogeneous film and MA-40 and MA-41 heterogeneous monopolar membranes. The dissociation of water on MA-40 in 0.01 M sodium chloride decreased the current efficiency of the acid and alkali both in the channel with a bipolar membrane and in the channel with an asymmetric bipolar membrane. The effective ion transport numbers across MA-40 and MA-41 at different pH values were determined. The water dissociation rate on MA-40 decreased at pH > 9.5. A kinetic model of the electrodialysis of a dilute solution of sodium chloride in a two-chamber unit cell with a bipolar and anionite membranes was suggested.     

Determination of betamethasone and triamcinolone acetonide by GC-NCI-MS in excreta of treated animals and development of a fast oxidation procedure for derivatisation of corticosteroids.

The use of corticosteroids in combination with other hormonal substances has long been known to result in increased mass gain with bovines. Practice has demonstrated, however, that even the single use of a glucocorticoid may result in growth promoting effects. In addition to the popular dexamethasone, more recently other corticosteroids have also been misused for fattening purposes. The first part of this study deals with the detection of two of them, namely betamethasone and triamcinolone acetonide. Betamethasone was administered orally to a cow at a dose of 50 mg d-1 for 5 d, then later the same cow was injected intramuscularly with a dose of 50 mg of betamethasone dipropionate. Excretion in urine and faeces was followed with both HPLC-enzyme immunoassay and a previously described method based on negative chemical ionization mass spectrometry (NCI-MS) after oxidation. For the triamcinolone acetonide study a cow was treated with 50 mg d-1 of the drug during a 7 d period. Excretion in faeces was followed with GC-NCI-MS. As triamcinolone acetonide is resistant to the previously described oxidation procedure, however, a hydrolysis step had to be introduced prior to oxidation. In addition to this specific modification necessary for triamcinolone acetonide, in a subsequent part of this study the original oxidation procedure with pyridinium chlorochromate was re-investigated especially to shorten the procedure. With the introduction of potassium dichromate the reaction time could be decreased from 3 h to 10 min.     

Nickel-catalyzed trans-alkynylboration of alkynes via activation of a boron-chlorine bond

Reaction of (diisopropylamino)chloroboryl ethers of alkynols with alkynylstannanes in the presence of nickel catalysts afforded formal trans-alkynylboration products in good yields. The products were isolated as pinacol boronates after treatment of the crude reaction mixture with acetic anhydride and pinacol in the presence of a base. The trans-alkynylboration products underwent Suzuki-Miyaura coupling reaction with organic halides. A 2-borylalkenylnickel(II) complex, in which the boryl and nickel groups are located in a trans fashion, was isolated in a reaction of the chloroboryl ether with a single equivalent of a nickel(0)-phosphine complex.     

Evaluation of antidiabetic and antioxidant potential of lupeol in experimental hyperglycaemia

Oxidative stress, produced under diabetic conditions, is a possible cause of various forms of tissue damage. The concentrations of antioxidant enzymes in cases of diabetes are significantly decreased, with a concomitant increase in lipid peroxidation. In this study, lupeol, a phytoconstituent from Solanum xanthocarpum, is shown to suppress the progression of diabetes after 21 days. Lupeol treatment caused decreases in glycated haemoglobin, serum glucose and nitric oxide, with a concomitant increase in serum insulin level. Furthermore, treatment with lupeol also increased antioxidant levels, with a decrease in the level of thiobarbituric acid-reactive oxygen species.     

FT-IR observation of covalent labelling of lysozyme crystals by organometallic complexes of transition metals

Electrophilic complexes of iron and chromium which have been reported to react with proteins in solution have been reacted with hen-egg white lysozyme (HEWL) in both the solution and crystal phases under similar pH and buffer conditions. This work was carried out with a view to developing novel side-chain selective heavy metal derivatives for protein X-ray crystallographic studies. Reaction of HEWL with a tricarbonyldienyliron cation (1) in aqueous solution led to modification of the sole histidine residue with concurrent reversible modification of other protein residues. Reaction of (1) with crystalline HEWL showed no covalent binding and only a build up of a hydrolysis product in the water channels of the crystal was observed. Reactions with a series of tricarbonylarylchromium pyrylium salts (2) led to the formation of stable covalent HEWL derivatives in solution. Chromatographic and IR spectroscopic studies showed that binding took place specifically at the epsilon-amino group of lysine residues to give a series of mono- and di-substituted products. When crystals of HEWL were soaked with the chromium reagents covalent binding to some of the lysine residues was also observed. In contrast, HEWL crystals which had their lysine side chains disabled did not bind any of the chromium reagents.     

分子动力学模拟研究纳米碳管中甲醇-水混合溶液的结构与输运性质

利用分子动力学模拟方法,对比考察了平衡条件、外压作用、梯度电场作用下,摩尔比为1:1的甲醇-水混合溶液在纳米碳管(CNT)中的静态结构以及输运行为.研究发现:在平衡体系与外压作用下,纳米碳管内甲醇与水呈现出明显的不混溶现象,甲醇主要分布于管壁附近,水分子主要分布于纳米碳管轴心附近;而在梯度电场作用下,纳米碳管由疏水性向亲水性转变,更多的水分子分布于管壁,导致纳米碳管内甲醇-水的不混溶现象消失.另一方面,在外压作用下,纳米碳管内甲醇与水呈现单向移动;而在梯度电场下,甲醇与水呈现快速的双向移动,其流通量较相应外压作用体系高出近一个数量级,但由于双向的流通量大小相近,导致净流通量与外压作用下的净流通量差异不大.     

Studies of the circular dichroism spectra of dissymmetric Schiff-bases by means of the exciton chirality method

The circular dichroism spectra of some dissymmetrical Schiff-bases derived from condensation of 2 mol salicylaldehyde with 1 mol of a chiral diamine are analyzed in terms of a simple conformational model taking the angle between the aromatic chromophores as a variable. The rotatory strengths of the π→π* transitions are calculated with AM1 molecular orbitals in combination with the dipole-coupling approximation. Calculated CD curves are found to be in reasonable agreement with the experimental CD spectra. Differences in the CD spectra of the Schiff-bases are discussed in relation to variation of the theoretical CD curve of ethylenebis(salicylideneimine) as a function of conformation.     

Chemical modification of the surface of a sulfonated membrane by formation of a sulfonamide bond

This paper describes a novel approach for the surface modification of a cation-exchange membrane, bearing sulfonate groups, by a cationic layer. The modification procedure involved the chlorosulfonation of the sulfonate groups of the base membrane with thionyl chloride, followed by a reaction with a diamine to yield a sulfonamide bond and a terminal amine. The latter could be quaternized by reaction with methyl iodide or protonated by soaking in acidic media. The membranes were characterized in detail by attenuated total reflectance Fourier transform infrared and X-ray photoelectron spectroscopies as well as elemental analysis to confirm that the above reactions occurred. The selectivity of these membranes toward the electrochemically assisted transport of protons versus Zn2+ metallic cations was determined during an electrodialysis in a two-compartment electrochemical cell. The data indicate a significant decrease of the transport of the metallic cations following modification of the membrane with the cationic layer. The later allows for the transport of protons from the catholyte to the anolyte compartment with much improved selectivity since the divalent cations are excluded from the membrane due to the electrostatic barrier of the cationic layer.     

分子动力学模拟研究纳米碳管中甲醇-水混合溶液的结构与输运性质

利用分子动力学模拟方法,对比考察了平衡条件、外压作用、梯度电场作用下,摩尔比为1：1 的甲醇-水混合溶液在纳米碳管（CNT）中的静态结构以及输运行为. 研究发现：在平衡体系与外压作用下,纳米碳管内甲醇与水呈现出明显的不混溶现象,甲醇主要分布于管壁附近,水分子主要分布于纳米碳管轴心附近;而在梯度电场作用下,纳米碳管由疏水性向亲水性转变,更多的水分子分布于管壁,导致纳米碳管内甲醇-水的不混溶现象消失. 另一方面,在外压作用下,纳米碳管内甲醇与水呈现单向移动;而在梯度电场下,甲醇与水呈现快速的双向移动,其流通量较相应外压作用体系高出近一个数量级,但由于双向的流通量大小相近,导致净流通量与外压作用下的净流通量差异不大.     

Molecular modeling simulations of the morphology of polyphenylene dendrimers

Three polyphenylene dendrimers were studied by molecular modeling techniques with the goal of better defining the notion of shape persistence often associated with these molecules. We show that despite the rigidity of the monomers, a large variety of dendrimer morphologies is accessible, in large part due to the asymmetrical branching pattern of the monomers (they have nodes in meta and para positions with respect to their anchor point). The folding ability of a branch depends on the number and sequence of the meta and para nodes it contains: while some of the branches are always straight, others can fold back, and the amplitude of that folding increases with generation. As a result, the range of accessible morphologies increases with the generation, from a dense-shell model in low generation to a model intermediate between a dense shell and a dense core in high generation. When the typical A2B monomer is replaced by an A4B monomer, the dense packing limit is reached as early as the second generation because of a higher density and the presence of nodes in the ortho position, which are oriented backward.     

Spatial self-organization of comb macromolecules

The properties and the self-assembly of single comb macromolecules in solution were studied. The elastic properties of a polymer chain with a high density of side chains forming a cylindrical brush were discussed, in particular, its persistence length was calculated. The cases of brushes with flexible and rigid side chains, as well as brushes with two types of incompatible side segments, were considered. It was shown that brushes with rodlike dangling chains have a higher rigidity. In addition, a comb macromolecule with the hydrophobic main chain and hydrophilic side chains was considered. Such a macromolecules in a selective solvent forms a globule with the hydrophobic core and a soluble shell. The specific feature of the globule is its ability to acquire nonspherical spatial forms. Problems related to the stability and transformation of globule shape are discussed in detail.     

Mechanistic investigation of the iridium-catalysed alkylation of amines with alcohols

The [Cp*IrCl(2)](2)-catalysed alkylation of amines with alcohols was investigated using a combination of experimental and theoretical methods. A Hammett study involving a series of para-substituted benzyl alcohols resulted in a line with a negative slope. This clearly documents that a positive charge is built up in the transition state, which in combination with the measurement of a significant kinetic isotope effect determines hydride abstraction as being the selectivity-determining step under these conditions. A complementary Hammett study using para-substituted anilines was also carried out. Again, a line with a negative slope was obtained suggesting that nucleophilic attack on the aldehyde is selectivity-determining. A computational investigation of the entire catalytic cycle with full-sized ligands and substrates was performed using density functional theory. The results suggest a catalytic cycle where the intermediate aldehyde stays coordinated to the iridium catalyst and reacts with the amine to give a hemiaminal which is also bound to the catalyst. Dehydration to the imine and reduction to the product amine also takes place without breaking the coordination to the catalyst. The fact that the entire catalytic cycle takes place with all the intermediates bound to the catalyst is important for the further development of this synthetic transformation.     

Features of the occurrence of electrochemical processes in contact of sodium chloride solutions with the surface of superhydrophobic coatings on titanium

A joint analysis of the results of electrochemical studies and the evolution of the parameters of a sodium chloride solution droplet in contact with the coating under test reveals the pattern of changes in the surface state which result from the electrochemical reactions and adsorption-desorption processes at the coating/electrolyte interface. Features of the corrosion process are studied on titanium samples with different protective layers on the surface: (1) a natural oxide, (2) a coating prepared via plasma electrolytic oxidation (PEO coating), (3) a PEO coating with a hydrophobic layer, and (4) a PEO coating with a superhydrophobic nanocomposite layer. The best protective properties in a chloride-containing electrolyte are exhibited by the superhydrophobic nanocomposite coating. The mechanism of corrosion protection of this coating is formulated.     

Proton magnetic resonance studies of the phase structure of bulk-crystallized linear polyethylene

The broad-line proton NMR spectra of melt-crystallized samples of polyethylene covering a very wide range in molecular weight have been analyzed in terms of contributions from three components: (1) a crystalline region with crystals of orthorhombic form; (2) a noncrystalline region with liquid-like character which produces a Lorentzian contribution to the spectrum; and (3) an intermediate region in which the rotation of methylene groups about C—C bonds is partly hindered. The relative mass fractions as well as the character of these components depend greatly on the molecular weight. Samples of low molecular weight (e.g., ?30,000) are predominantly composed of lamellar crystalline regions with a minor amount of interfacial regions and no liquid-like interzonal regions. As the molecular weight increases beyond 45,000, an interzonal region with a liquid-like character associated with a higher molecular mobility is produced. Above a molecular weight of 100,000, this liquid-like character becomes pronounced with an increase in molecular mobility in the interfacial region.     

Structure of water in microscopic fractures of a silver iodide crystal

The results of a computer simulation of flat fractures with widths of 0.63, 1.25, and 2.5 nm filled with vapor molecules in a silver iodide crystals at 260 K were presented. The two-dimensional gas of molecules adsorbed on the walls was found to be strongly clustered. Before the pore was filled, its walls had been covered with a monomolecular water film with a characteristic hexagonal structure. The perpendicular growth of the film was hindered by the hydrophobicity of its surface; the adsorbed molecules were bonded with the walls by interactions with the ions of the second crystal layer to form a specific orientational molecular order in the region of contact with the wall. On the wall with silver cations, the molecular energy was lower and the entropy higher than on the wall with iodide anions; on the wall with a lower energy, the adsorption started earlier and was more active. In an extremely narrow pore having room for only one molecular layer, the monomolecular film consists of spots held on opposite walls; in each spot, the orientational molecular order is the one characteristic of the wall with which the spot is in contact.     

On the mechanism of reaction of radicals with tirapazamine

Ketyl radicals produced by photolysis of ketones or di-tert-butyl peroxide (DTBP) in alcohol solvents react rapidly with tirapazamine (TPZ). The acetone ketyl radical (ACOH) reacts with TPZ with an absolute second-order rate constant of (9.7 +/- 0.4) x 108 M-1 s-1. The reaction kinetics can be followed by monitoring the bleaching of TPZ absorption at 475 nm or the formation of a reaction product which absorbs at 320 and 410 nm. The ACOD radical reacts with TPZ in 2-propanol-OD with an absolute rate constant of (6.7 +/- 0.5) x 108 M-1 s-1, corresponding to a kinetic isotope effect (KIE) of 1.4. Deuteration of the radical on carbon (ACOH-d6) retards the reaction of the radical with TPZ even further (absolute rate constant = (4.8 +/- 0.04) x 108 M-1 s-1). This result corresponds to a KIE of 2.0. Radicals derived from dioxane and diisopropyl ether by flash photolysis of DTBP in ethereal solvent react with TPZ more slowly than do ketyl radicals. It is concluded that ketyl radicals react, in part, with TPZ in organic solvents by transfer of a hydrogen atom from the OH and CH3 groups of the ketyl radical to the oxygen atom at the N4 position of TPZ to form acetone or acetone enol and a radical derivative of TPZ (TPZH). The latter species absorbs at 320 and 405 nm, has a lifetime of hundreds of microseconds in alcohol solvents, and decays by disproportionation to form TPZ and a reduced heterocycle. The reduced heterocycle eventually forms a desoxytirapazamine by a polar mechanism. The results are supported by density functional theory calculations. It is proposed that dioxanyl radical will also react, in part, with TPZ by transfer of a hydrogen atom from the carbon adjacent to the radical center to the oxygen atom at the N4 position of TPZ. This produces the enol ether and the previously mentioned TPZH radical. It is further posited that ether radicals react a bit more slowly than ketyl radicals because they lack the second mode of hydrogen transfer (from the OH group) that is present in the ACOH radical. Our data are permissive of the possibility that ether radicals add to TPZ at a rate that is competitive with beta-hydrogen atom transfer.