The scope and mechanism of phosphonium-mediated S(N)Ar reactions in heterocyclic amides and ureas

An efficient "one-step" synthesis of cyclic amidines and guanidines has been developed. Treatment of cyclic amides and ureas with benzotriazol-1-yloxytris(dimethylamino)phosphonium hexafluorophosphate (BOP), base, and nitrogen nucleophiles leads to the formation of the corresponding cyclic amidines and guanidines, typically in good to excellent yields. This method has also been used to prepare heteroaryl ethers and thioethers using phenol and thiophenol nucleophiles. Time course NMR and HPLC-MS studies have facilitated explicit characterization of the proposed intermediates (the phosphonium salt and HOBt adduct); the data reveal a stepwise reaction pathway.     

Preparation and characterization of polyaniline/manganese dioxide composites and their catalytic activity

This paper is devoted to the preparation of polyaniline/MnO2 (PANI/MnO2) composites via chemical oxidation of aniline in H2SO4 medium using beta-MnO2 as an oxidant. The parameters affecting the polymerization reaction are considered. These parameters are [aniline], amount of beta-MnO2, stirring time, and polymerization temperature. SEM, FT-IR, XRD, and TGA techniques are used to characterize the resulting composites. XRD measurements reveal the distortion of the crystal structure of beta-MnO2 after the polymerization reaction. Thus, the XRD pattern of PANI is predominating. The crystalline composites are obtained using higher molar ratio of [Ox]/[ANI] and at higher temperature. Increasing the amount of beta-MnO2 led to an increase in the acidic character of the obtained composites due to adsorption of excess H+ on the oxide surface. The thermal stability of the composites decreased with increasing both [aniline] and stirring time, while it increased with increasing amount of beta-MnO2. The applications of the composites in the oxidative degradation of Direct Red 81, Acid Blue 92, and Indigo Carmine dyes exhibited good catalytic activity in the presence of H2O2 as an oxidant. The reactions followed first-order kinetics and the rate constants were determined. The degradation reaction involved the catalytic action of the PANI counterpart of the composite toward H2O2 decomposition, which can lead to the generation of HO radicals as a highly efficient oxidant attacking the target dyes. The detailed kinetic studies and the mechanism of these catalytic reactions are under consideration in our group.     

A simple and rapid spectrophotometric method for the determination of nitrite by its decolorizing effect on peroxovanadate complex

A rapid, simple, and most economical spectrophotometric method was proposed for the determination of nitrite in various water samples, soil samples, and roots of leguminous plants. The method is based on decolorizing effect of nitrite on complex formed between hydrogen peroxide and vanadate in acidic medium. The decolorization of that complex by nitrite was exploited to monitor the reaction spectrophotometrically at 470 nm.The method was optimized for effect of concentrations of ammonium metavanadate, hydrogen peroxide, various acids, concentrations of sulphuric acid, order of reagents addition and color stability. The color of the complex was found to be stable for about 2 days, and the stability constant of the complex was also calculated by modified Job's method. The linearity range of the calibration graph was over 6.67-66.7 microg ml(-1) of nitrite with molar absorptivity, 0.276 x 10(3) mol(-1) l cm(-1) and Sandell's sensitivity, 0.1667 microg cm(-2). The method was applied successfully for the determination of nitrite in soil samples, various wastewater samples and roots of leguminous plants.     

Derivative spectrophotometric analysis of benzophenone (as an impurity) in phenytoin

Background

Biomass and municipal solid waste offer sustainable sources of energy; for example to meet heat and electricity demand in the form of combined cooling, heat and power. Combustion of biomass has a lesser impact than solid fossil fuels (e.g. coal) upon gas pollutant emissions, whilst energy recovery from municipal solid waste is a beneficial component of an integrated, sustainable waste management programme. Concurrent combustion of these fuels using a fluidised bed combustor may be a successful method of overcoming some of the disadvantages of biomass (high fuel supply and distribution costs, combustion characteristics) and characteristics of municipal solid waste (heterogeneous content, conflict with materials recycling). It should be considered that combustion of municipal solid waste may be a financially attractive disposal route if a 'gate fee' value exists for accepting waste for combustion, which will reduce the net cost of utilising relatively more expensive biomass fuels.

Results

Emissions of nitrogen monoxide and sulphur dioxide for combustion of biomass are suppressed after substitution of biomass for municipal solid waste materials as the input fuel mixture. Interactions between these and other pollutants such as hydrogen chloride, nitrous oxide and carbon monoxide indicate complex, competing reactions occur between intermediates of these compounds to determine final resultant emissions.

Conclusions

Fluidised bed concurrent combustion is an appropriate technique to exploit biomass and municipal solid waste resources, without the use of fossil fuels. The addition of municipal solid waste to biomass combustion has the effect of reducing emissions of some gaseous pollutants.     

Derivative spectrophotometric analysis of benzophenone (as an impurity) in phenytoin

ABSTRACT: Three simple and rapid spectrophotometric methods were developed for detection and trace determination of benzophenone (the main impurity) in phenytoin bulk powder and pharmaceutical formulations. The first method, zero-crossing first derivative spectrophotometry, depends on measuring the first derivative trough values at 257.6 nm for benzophenone. The second method, zero-crossing third derivative spectrophotometry, depends on measuring the third derivative peak values at 263.2 nm. The third method, ratio first derivative spectrophotometry, depends on measuring the peak amplitudes of the first derivative of the ratio spectra (the spectra of benzophenone divided by the spectrum of 5.0 μg/mL phenytoin solution) at 272 nm. The calibration graphs were linear over the range of 1-10 μg/mL. The detection limits of the first and the third derivative methods were found to be 0.04 μg/mL and 0.11 μg/mL and the quantitation limits were 0.13 μg/mL and 0.34 μg/mL, respectively, while for the ratio derivative method, the detection limit was 0.06 μg/mL and the quantitation limit was 0.18 μg/mL. The proposed methods were applied successfully to the assay of the studied drug in phenytoin bulk powder and certain pharmaceutical preparations. The results were statistically compared to those obtained using a polarographic method and were found to be in good agreement.     

Natural convection cooling of a heat source embedded on the bottom of an enclosure filled with Cu-water nanofluid: effects of various thermal boundary conditions

Steady natural convection cooling of a localized heat source at the bottom wall of an enclosure filled with Cu-water nanofluid for a variety of thermal boundary conditions at the sidewalls has been studied in the present paper. Finite difference method was employed to solve the dimensionless governing equations of the problem. The effects of governing parameters, namely, solid volume fraction, the different values of the heat source length and the different locations of the heat source on the streamlines and isotherms contours as well as maximum temperature, Nusselt number and average Nusselt number along the heat source were considered. The present results are validated by favorable comparisons with previously published results. The results of the problem are presented in graphical and tabular forms and discussed.     

Theoretical study on the unimolecular decomposition of thiophenol

The potential energy surface for the unimolecular decomposition of thiophenol (C(6)H(5)SH) is mapped out at two theoretical levels; BB1K/GTlarge and QCISD(T)/6-311+G(2d,p)//MP2/6-31G(d,p). Calculated reaction rate constants at the high pressure limit indicate that the major initial channel is the formation of C(6)H(6)S at all temperatures. Above 1000 K, the contribution from direct fission of the S-H bond becomes important. Other decomposition channels, including expulsion of H(2) and H(2)S are of negligible importance. The formation of C(6)H(6)S is predicted to be strong-pressure dependent above 900 K. Further decomposition of C(6)H(6)S produces CS and C(5)H(6). Overall, despite the significant difference in bond dissociation, i.e., 8-9 kcal/mol between the S-H bond in thiophenol and the O-H bond in phenol, H migration at the ortho position in the two molecules represents the most accessible initial channel.     

Crude peroxidase from onion solid waste as a tool for organic synthesis. Part II: oxidative dimerization-cyclization of methyl p-coumarate, methyl caffeate and methyl ferulate

The ability of a crude onion peroxidase preparation to act as a biocatalyst for the oxidative dimerization-cyclization of methyl p-coumarate, methyl caffeate and methyl ferulate is presented. The products of the reaction have been fully characterized and were found to possess potent antioxidant activity in a ferric reducing antioxidant power (FRAP) assay.     

Mechanistic study of the deamination reaction of guanine: a computational study

The mechanism for the deamination of guanine with H(2)O, OH(-), H(2)O/OH(-) and for GuaH(+) with H(2)O has been investigated using ab initio calculations. Optimized geometries of the reactants, transition states, intermediates, and products were determined at RHF/6-31G(d), MP2/6-31G(d), B3LYP/6-31G(d), and B3LYP/6-31+G(d) levels of theory. Energies were also determined at G3MP2, G3MP2B3, G4MP2, and CBS-QB3 levels of theory. Intrinsic reaction coordinate (IRC) calculations were performed to characterize the transition states on the potential energy surface. Thermodynamic properties (ΔE, ΔH, and ΔG), activation energies, enthalpies, and Gibbs free energies of activation were also calculated for each reaction investigated. All pathways yield an initial tetrahedral intermediate and an intermediate in the last step that dissociates to products via a 1,3-proton shift. At the G3MP2 level of theory, deamination with OH(-) was found to have an activation energy barrier of 155 kJ mol(-1) compared to 187 kJ mol(-1) for the reaction with H(2)O and 243 kJ mol(-1) for GuaH(+) with H(2)O. The lowest overall activation energy, 144 kJ mol(-1) at the G3MP2 level, was obtained for the deamination of guanine with H(2)O/OH(-). Due to a lack of experimental results for guanine deamination, a comparison is made with those of cytosine, whose deamination reaction parallels that of guanine.     

2-[(1H-benzimidazol-2-ylmethyl)-amino]-benzoic acid methyl ester: crystal structure, DFT calculations and biological activity evaluation

In the present study, structural properties of 2-[(1H-benzimidazol-2-ylmethyl)-amino]-benzoic acid methyl ester have been studied extensively by spectral methods and X-ray crystallography. Quantum mechanical calculations of energies, geometries, vibrational wavenumbers, NMR and electronic transitions were carried out by DFT using B3LYP functional combined with 6-31G(d) basis set. Natural bond orbitals (NBO) analysis and frontier molecular orbitals were performed at the same level of theory. DFT calculations showed good agreement between the theoretical and experimental values of optimized and X-ray structure as well as between the vibrational and NMR spectroscopy. The title compound was screened for its antibacterial activity referring to Tetracycline as standard antibacterial agent.