Lewis acid catalysed rearrangements of unsaturated bicyclic [2.2.n] endoperoxides in the presence of vinyl silanes; access to novel Fenozan BO-7 analogues

Reactions of a series of unsaturated bicyclic [2.2.n] endoperoxides with allyltrimethylsilane in the presence TMSOTf or SnCl₄ provides the cis-configured endoperoxides 9a-12. It is proposed that this novel reaction proceeds via attack of the allylsilane on the carbocation derived from heterolytic cleavage of the endoperoxide bridge. The reaction proceeds with a high degree of diastereoselectivity and we propose that the bulky -CH₂SiMe₃ substituent adopts an equatorial position in a product-like transition state. In contrast to Fenozan B0-7, these compounds displayed poor antimalarial activity versus chloroquine-resistant parasites in vitro.     

Effect of reaction conditions on the product distribution in the liquid-phase acid-catalyzed etherification of 2-naphthol with methanol

Summary Bi-doped TiO₂photocatalysts have been prepared by a sol-gel method. The samples were examined by the methods of XRD, HRTEM and XPS. The photocatalytic activity of the catalysts was evaluated through the photocatalytic oxidation of p-nitroaniline (NO₂C₆H₄NH₂) in air.     

The di-π-methane rearrangement : The effect of 1,4-diene structure and conformation on the reaction efficiency and stereochemistry

The photochemistry of the stereoisomeric 1,3-dimethyl-3(2-phenylethenyl)cyclohexenes has been explored Direct irradiation of the cis- and trans-β-styrylcyclohexenes leads to di-π-methane rearrangement, producing the endo- and exo-3,7-dimethyl-8-phenybicyclo[5.1.0]oct-2-enes, and cis -trans isomerization, interconverting the 1,4-diene containing substrates. Triplet sensitized photolysis of both substituted cyclohexenes leads exclusively to cis-trans isomerization. Results from low conversion direct irradiations of the cis- and trans-β-styrylcyclohexenes indicate that the singlet rearrangements are stereospecific, and lead to formation of the 8-exo and 8-endo-phenylbicyclic octenes, respectively. The relationship between di-π-methane structure and triplet reaction efficiency, and the effect of conformation on the rearrangement stereochemistry are discussed.     

Effects of cationic and anionic micelles on the redox reaction of Erythrosine B by H2O2 in presence of Cu-Fe nanoparticles

Kinetic investigation of the degradation of Erythrosine B (EB) by H₂O₂ and copper-iron bimetallic nanoparticles was carried out spectrophotometrically. The degradation was carried out under the condition of sonication at 40°C. The co-precipitation method was used for the fabrication of CuO/Fe₂O₃ (Cu-Fe) in an alkaline solution and characterized by SEM, XRD, FTIR, TEM, EDS, and BET methods. In the absence of the Cu-Fe, no degradation of EB by H₂O₂ was observed. The presence of Cu-Fe resulted into the degradation of EB by H₂O₂. The kinetics of the degradation was studied under the conditions of variation of the amount of nanoparticles and at different concentrations of EB, H₂O₂, H⁺, surfactants (sodium dodecyl sulphate; SDS and cetyltrimethylammonium bromide; CTABr). The rate of reaction depends on the amount of Cu-Fe and [H₂O₂]. The rate constant values gave the peaked like curve (with maximum value at pH 3) at different pH. The dye degradation decreased with the increase in presence of (SDS) and (CTABr).     

Enhanced electrocatalytic activity for H2O2 production by the oxygen reduction reaction: Rational control of the structure and composition of multi-walled carbon nanotubes

Hydrogen peroxide (H₂O₂) is a very useful chemical reagent, but the current industrial methods for its production suffer from serious energy consumption problems. Using high-activity and high-selectivity catalysts to electrocatalyze the oxygen reduction reaction (ORR) through a two-electron (2e^?) pathway is a very promising route to produce H₂O₂. In this work, we obtained partially oxidized multi-walled carbon nanotubes (MWCNTs) with controlled structure and composition by oxidation with concentrated sulfate and potassium permanganate at 40°C for 1 h (O-CNTs-40-1). The outer layers of O-CNTs-40-1 are damaged with defects and oxygen-containing functional groups, while the inner layers are maintained intact. The optimized structure and composition of the partially oxidized MWCNTs ensure that O-CNTs-40-1 possesses both a sufficient number of catalytic sites and good conductivity. The results of rotating ring disk electrode measurements reveal that, among all oxidized MWCNTs, O-CNTs-40-1 shows the greatest improvement in hydrogen peroxide selectivity (from ～ 30% to ～ 50%) and electron transfer number (from ～ 3.4 to ～ 3.0) compared to those of the raw MWCNTs. The results of electrochemical impedance spectroscopy measurements indicate that both the charge-transfer and intrinsic resistances of O-CNTs-40-1 are lower than those of the raw MWCNTs and of the other oxidized MWCNTs. Finally, direct tests of the H₂O₂ production confirm the greatly improved catalytic activity of O-CNTs-40-1 relative to that of the raw MWCNTs.     

SO2存在下钠、钙、铁及其复合物对NO-半焦反应的催化作用

采用程序升温还原技术,在SO2存在气氛下,对钠、钙、铁及其复合物的NO-半焦催化反应性能进行了研究。结果表明,在相同条件下三种金属对NO-半焦反应的催化活性顺序为：Fe>Ca>Na。SO2可能通过参与氧传递过程来影响金属对NO-半焦反应的催化活性。SO2的浓度对负载三种不同金属半焦催化活性的影响是不同的。SO2浓度对负载钠半焦的催化活性影响最为显著。在SO2存在气氛下,负载2％钠或铁和负载3％钙的半焦具有高的NO转化活性。在SO2存在气氛下,钠－钙和钠－铁二元金属复合催化剂的活性较高,这可能是由于这两种金属间存在协同作用的缘故。     

The effect of the protein environment on the structure and charge distribution of the retinal Schiff base in bacteriorhodopsin

The effects of the amino acid side chains of the binding pocket of bacteriorhodopsin (bR) and of a water molecule on the structure of the retinal Schiff base have been studied using Becke3LYP/6-31G* level of density functional theory. A model protonated Schiff base structure including six conjugated double bonds and methyl substituents was optimized in the presence of several amino acid side chains and of a water molecule, separately. The Schiff base structure was also calculated in the form of a neutral species. At each optimized complex geometry the atomic charges of the model Schiff base were calculated using Mulliken population analysis. In agreement with previously proposed counterion(s) of the protonated retinal Schiff base in bR, the results show that Asp₈₅ and Asp₂₁₂, which are present in the form of negatively charged groups, have significantly large effects on the structure and electronic configuration of both unprotonated and protonated model Schiff bases. The presence of a water molecule in the vicinity of the Schiff base demonstrates significant effects which are comparable to those of aspartate groups. Other side chains studied did not show any significant effect in this direction. Apart from the aspartate groups and the water molecule, in none of the other complexes studied are the atomic charges and the bond alternation of the model Schiff base significantly influenced by the presence of the neighboring amino acids. Received: 24 March 1998 / Accepted: 3 September 1998 / Published online: 10 December 1998     

The effect of glucose and temperature on the in vivo efficiency of photochemotherapy with meso-tetra-hydroxyphenyl-chlorin.

Balb/c athymic nude mice bearing WiDr human colon adenocarcinoma have been employed to investigate the effect of glucose administration, cooling or slight heating on the anti-tumor activity of photochemotherapy (PCT) with meso-tetra-hydroxyphenyl-chlorin (mTHPC). An apparent delay in the tumor growth is found by combining PCT with either single or multiple injections of glucose. The anti-tumor effect of PCT is slightly enhanced by cooling the tumor to 5°C. Cooling also enhances the efficiency of PCT and glucose injection combined. Heating the tumor to 37°C has no significant effect on either PCT alone or on the combination of PCT and glucose injection. Furthermore, the kinetics of the accumulation of mTHPC in tissue have been studied. Single or multiple injections of glucose have an enhancing effect on the accumulation of mTHPC in the tumor.     

Kinetic study of the crosslinking reaction of 1,2‐polybutadiene with dicumyl peroxide in the absence and presence of vinyl acetate

The crosslinking reaction of 1,2-polybutadiene (1,2-PB) with dicumyl peroxide (DCPO) in dioxane was kinetically studied by means of Fourier transform near-infrared spectroscopy (FTNIR). The crosslinking reaction was followed in situ by the monitoring of the disappearance of the pendant vinyl group of 1,2-PB with FTNIR. The initial disappearance rate (R₀) of the vinyl group was expressed by R₀ = k[DCPO]^0.8[vinyl group]^−0.2 (120 °C). The overall activation energy of the reaction was estimated to be 38.3 kcal/mol. The unusual rate equation was explained in terms of the polymerization of the pendant vinyl group as an allyl monomer involving degradative chain transfer to the monomer. The reaction mixture involved electron spin resonance (ESR)-observable polymer radicals, of which the concentration rapidly increased with time owing to a progress of crosslinking after an induction period of 200 min. The crosslinking reaction of 1,2-PB with DCPO was also examined in the presence of vinyl acetate (VAc), which was regarded as a copolymerization of the vinyl group with VAc. The vinyl group of 1,2-PB was found to show a reactivity much higher than 1-octene and 3-methyl-1-hexene as model compounds in the copolymerization with VAc. This unexpectedly high reactivity of the vinyl group suggested that an intramolecular polymerization process proceeds between the pendant vinyl groups located on the same polymer chain, possibly leading to the formation of block-like polymer. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 4437–4447, 2004     

The effect of β-CD polymers structure on the efficiency of copper(II) ion flotation

β-Cyclodextrin (β-CD) polymers were prepared by cross-linking of β-CD with phtalic and 3-nitrophtalic anhydride in anhydrous N,N-dimethylformamide (DMF) in the presence of NaH. The weight-average molecular weight (M _W) and the chemical structure of the polymers were determined using high performance size exclusion chromatography (HPSEC) with refractive index (RI) detector, and ¹H NMR spectroscopy. The molecular weight of the polymer increased with molar ratio of substrates and reaction temperature. ¹H NMR spectra revealed that the β-CD polymers contained both mono- and diesters of phtalic and 3-nitrophtalic acids. In the case of phtalic moieties about four or five diester moieties groups and for 3-nitrophtalic moieties about to or three diester moieties are linkages for β-CD molecule, respectively. Results of copper(II) flotation obtained with the use of nonylphenol polyoxyethyl glycol ether as an non-anionic surfactant and β-CD polymers as complexation collector agent, show␣that the removal of Cu²⁺ decreases with increase of molecular mass of β-CD polymers linked by phtalic or␣3-nitrophtalic anhydrides. For both derivatives with pH increase the copper(II) removal increase. The highest flotation removal, i.e. 93%, was found for β-CD polymers synthesized at 100 °C with molar ratio CD:NaH:3-nitrophtalic anhydride equal to 1:7:7.     

The thiol-olefin co-oxidation (TOCO) Reaction—IV : Temperature effects on product distribution in the toco reaction of indene and aromatic thiols

The stereochemistry of the TOCO reaction is found to be temperature dependent: increasing amounts of cis addition products (approaching 50%) are formed as the temperature is lowered to ?23° and also when the temperature is raised to ca 60°. The sensitivity of this temperature effect depends on the electronic character of the substituent present in the benzenethiol. The product distribution under a variety of experimental conditions and the kinetics of the TOCO reaction were examined in order to arrive at a self-consistent mechanism.     

The effect of P/N/A distribution on the parachors of petroleum fractions

This study has investigated the influence of varying the ratio of paraffin, naphthene and aromatic (P/N/A) components in narrow, wide and real petroleum fractions on the parachors of the fractions. Surface tension and density measurements have been made on synthetic hydrocarbon mixtures simulating real SC8, SC10, SC13 groups, their mixtures and a kerosene sample covering the range SC4–SC14 in order to determine their experimental parachors. Two methods to compute the parachors of fractions are proposed based on the equation of Broseta, the correlations of Riazi and Al-Sahhaf and the pseudocompound approach of Daubert. The first, denoted method I, is given by

and the second, denoted method II, by

P_a(cut)=z_PP_a(P)+z_AP_a(A)+z_NP_a(N)

where P_a is the parachor of the cut, ω the acentric factor, P, N and A the P/N/A components, z_P/N/A the mole fractions of the P/N/A components and the other terms have their usual meaning. Both of these methods were found to compute the parachors of mixtures representing the SC8, SC10 and SC13 groups, mixtures of these groups and the kerosene sample with an absolute deviation of 2%. Comparison of the experimental parachors with calculated ones show that the parachor of a petroleum fraction depends not only on its molecular weight but also on the ratio of the P/N/A components. It was also found that the parachor of each of the families of P, N and A components could be expressed as a linear function of molecular weight leading to the following expression for a mixture containing P, N and A components

P_a(cut)=(z_PA_P+z_NA_N+z_AA_A)MW+(z_PB_P+z_NB_N+z_AB_A)

where the A and B terms were those found in the linear regression fit for each family. The approach proposed gives quantitatively better results than methods such as that of Fawcett, where the P/N/A distribution is not taken into account in the computation of petroleum fraction parachors.     

Mechanistic aspects of the Wolff-Kishner reaction. V. The cation effect on the reaction of benzophenone hydrazone in butyl carbitol and decyl alcohol

The rate of the Wolff-Kishner reaction of benzophenone hydrazone in butyl carbitol increases as the cation of the alkoxide base is varied in the order K>Na>Li>Mg. The replacement of butyl carbitol by 1-decanol also accelerates the reaction, and an additional increase is caused by the presence of crown ether. On the basis of changes in the activation parameters, it is concluded that the reactivity of the hydrazone anion increases as the contact ion pair becomes more fully solvated and dissociated.     

The silicon effect on the regioselectivity of the Tsuji-Trost reaction. Experimental and theoretical approaches

Regioselectivity of the Tsuji-Trost reaction on allyl acetates and carbonates substituted with silyl groups at the olefinic moiety has been analyzed. Silicon atom in the central carbon atom increases the stability of the intermediate π-allylpalladium cation with respect to the isomeric π-allylpalladium cation featuring the silicon in a terminal carbon atom.     

Studies on reactivities of molybdenum clusters — The additive reaction of SbCl3 to trimolybdenum clusters with loose coordination sites and the crystal structure of the reaction product

The additive reaction of SbCl₃ to trimolybdenum clusters with loose coordination sites, Mo₃(μ₃-S) (μ-S)₃ [S₂P (OEt)₂]₄ · L' (1 L' ? H₂O; 2, L' ? C₃H₃ON), in a C₂H₅OH-HCl medium yields complexes {Mo₃ (μ₃-S) [(μ-S)₃ · SbCl₃] [S₂P (OEt)₂]₄ · (C₂H₅OH)} (C₂H₅OH) (3) and {Mo₃ (μ₃-S) [(μ-S)₃ · SbCl₃] [S₂P (OEt)₂]₃ [SXP (OEt)₂] · (C₃H₃ON)} (X ? S,O) (4), respectively. The crystal and molecular structures of these two compounds have been determined by single crystal X-ray crystallography: crystal 3 belongs to the monoclinic system, P2₁,/n, while crystal 4 belongs to the triclinic system, P 1, The intensity data were collected on an Enraf-Nonius CAD-4 diffractometer with graphite-monochromatized MoKα radiation by using the ω-2θ technique. The structures were solved by the Patterson method as well as direct methods and refined by full matrix least-squares method. Final R values were 0.078 for crystal 3 and 0.068 for crystal 4, respectively. The result of the structure determination showed that the two molecular structures are practically formed by connecting a trimolybdenum clusters with loose coordination sites, {Mo₂(μ₃-S)- (μ-S)₃[S₂P(OEt)₂]₄ · L (L ? C₂H₅OH, C₃H₃ON), with a SbCl₃ molecule through three (μ-S) atoms to form the {Mo SbS₄} core of a cubane-like type. There are relatively weak coordination bonds between the Sb and the three (μ-S) atoms, while no bonding interaction between the Sb and the three Mo atoms is found. In the additive compounds the structural character of the trimolybdenum clusters with loose coordination sites has significantly been changed. In addition, 3 still has the reactivity for the loosely coordinated EtOH to be replaced by an oxazole ring to form crystal 4. However, the three (μ-S) atoms may be regarded as triple bridging S atoms each connecting two Mo atoms and a Sb atom. The information obtained serves as an evidence that the tetranuclear clusters might be formed by a [3 + 1] reaction mode.     

Acceleration effect of poly(vinylpyrrolidone) on williamson reaction. II. Studies on the dissociation of sodium phenoxide in the presence of poly(vinylpyrrolidone)

In connection with the accelerating effect of poly(N-vinylpyrrolidone) (PVP) on a Williamson reaction, PhONa + NC₄H₉Br → PhO-(n-C₄H₉) + NaBr, the ionic dissociation of sodium phenoxide (PhONa) was studied by means of conductance measurements and ultraviolet spectroscopy. Results suggest that the degree of dissociation of PhONa increased with the amount of PVP as well as N-methylpyrrolidone (NMP), the monomeric analog, the effect of PVP being much larger than that of NMP at the same concentration. It is assumed that free phenolate anion produced by solvation of sodium cation with NMP or the pyrrolidone residues of PVP plays an important role in the acceleration of the reaction and that the higher reaction rates in PVP solution are due to the greater dissociation of PhONa.     

The effect of temperature on the efficiency parameters in adsorptive liquid chromatography

The effect of temperature on the efficiency of column systems in adsorptive liquid chromatography is reported. The influence of temperature changes on the values of the terms Cm2 and Ck in the HETP equation, which characterizessystems involving silica gel and benzene, ethylene chloride, butyl ether or cyclo- hexanone + heptane, is discussed for 9 substances. The results of the study have led to the following conclusions:

• 1 Temperature increase will always improve the efficiency of typical chromatographic systems.
• 2 As expected, an essential role is played by mass transfer resistance in the mobile phase occupying the pores of the packing material (parameter Cm2).
• 3 Mass transfer resistance resulting from sorption-desorption kinetics (C_k) does not play as essential a role in the systems investigated as does Cm2, but its value is significantly higher than that reported in the literature.

     

The effect of inert salts on the structure of the transition state in the SN2 reaction between thiophenoxide ion and butyl chloride

The effect of inert salts on the structure of the transition state has been determined by measuring the secondary alpha deuterium and the chlorine leaving group kinetic isotope effects for the S(N)2 reaction between n-butyl chloride and thiophenoxide ion in both methanol and DMSO. The smaller secondary alpha deuterium isotope effects and very slightly larger chlorine isotope effects found in both solvents when the inert salt is present suggests that the S(N)2 transition state is tighter and more product-like, with a shorter S-C(alpha) and very a slightly longer C(alpha)-Cl bond when the added salt is present. The salt effect on the reaction in methanol where the reacting nucleophile is the solvent-separated ion-pair complex is much greater than the salt effect on the reaction in DMSO where the reacting nucleophile is the free ion. This greater change in transition-state structure found when the inert salt is present in methanol is consistent with the solvation rule for S(N)2 reactions. The greater change in the S-C(alpha) bond is predicted by the bond strength hypothesis. A rationale for the changes found in transition-state structure when the inert salt is present is suggested for both the free-ion and the ion-pair reactions.