The benzoylation of uracil and thymine

Uracil and thymine react with benzoyl chloride in acetonitrile-pyridine solution at room temperature to give first their 1-$\text{N}$-benzoyl (2b and 3b) and then their 1-$\text{N}$, 3-$\text{N}$-dibenzoyl derivatives (4a and 4b, respectively); the latter compounds are converted into the corresponding 3-$\text{N}$-benzoyl derivatives (5a and 6a) under mild conditions of basic hydrolysis.     

Validation of the density-functional based tight-binding approximation method for the calculation of reaction energies and other data

We investigated the performance of the approximative density functional method DFTB versus BLYP and G2 with respect to zero-point corrected reaction energies, vibrational frequencies, and geometry parameters for a set of 28 reactions and 22 representative molecules containing C, H, N, and O (DFTB--density-functional based tight-binding approximation). The DFTB reaction energies show a mean absolute deviation versus the G2 reference of 4.3 kcalmol only. The corresponding value for the vibrational frequencies amounts to 75 cm(-1) versus BLYP/cc-pVTZ. With very few exceptions bond lengths and angles are in excellent agreement with the results of higher-level methods.     

The structure of a model intermediate for the nickel-catalyzed cyclodimerization of allene

The stoichiometric reaction between allene, zerovalent nickel and a chelating diphosphane leads to the formation of a bis-methylenenickelacyclopentane derivative whose structure has been established by X-ray crystallography.     

Die Bildungswärmen von MnSi1,73, MnSi,Mn5Si3 und Mn3Si

By means of vapor pressure measurements using theTorker method (Torsion-Knudsen Effusion Recoil) as well as theKnudsen tanspiration technique in connection with a mass spectrometer, the heats of formation of the following compounds were found to be –6.5±0.7 kcal/g atom (Mn_0.366Si_0.634), –7.8±0.6 kcal/g atom (Mn_0.50Si_0.50), –7.3±0.6 kcal/g atom (Mn_0.625Si_0.375), and –6.6±0.6 kcal/g atom (Mn_0.75Si_0.25).

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Mit 7 Abbildungen     

Asymmetric synthesis of [7]helicene-like molecules

[reaction: see text] A new approach to nonracemic [7]helicene-like molecules has been developed. Stereoselective Co(I)-mediated [2 + 2 + 2] cycloisomerization of aromatic triynes containing an asymmetric carbon atom produces [7]helicene-like scaffolds in diastereomeric ratios up to 100:0. This central-to-helical chirality transfer can be controlled by the absolute configuration at the asymmetric center and by the presence of carbon substituents.     

Reactions of substituted furo[3,2-<Emphasis Type="Italic">b</Emphasis>]pyrrole-5-carboxhydrazides and their biological activity

The reactions of substituted furo[3,2-b]pyrrole-5-carboxhydrazides 1 with 5-arylfuran-2-carboxaldehydes 2, 4,5-disubstituted furan-2-carboxaldehydes 3 and thiophene-2-carboxaldehyde 4 has been studied. The advantage of microwave irradiation on some of these reactions was reflected in the reduced reaction time and increased yields. Reactions of 1 with 4-substituted 1,3-oxazol-5(4H)-ones 11 led to diacylhydrazines 13 or to imidazole derivatives 14 depending on the temperature. 1,2,4-Triazole-3-thione 17 was synthesized by two-step reaction of 1 with phenylisothiocyanate and subsequent base-catalyzed cyclization of thiosemicarbazide 16. The effects of hydrazones 5–10 on inhibition of photosynthetic electron transport in spinach chloroplasts and chlorophyll content in the antialgal suspensions of Chlorella vulgaris were investigated.     

Preparation of polyfluorinated cycloalk-1-enyl-, alk-1-enyl-, and alkyliodine tetrafluorides using XeF2 in the presence of appropriate Lewis acids as fluorooxidant

Previously unknown polyfluorocyclohexenyl, and acyclic perfluoroalkenyliodine tetrafluorides were prepared in high yields. Perfluorocyclohex-1-enyliodine tetrafluoride was obtained from pentafluoroiodobenzene using XeF₂-NbF₅ in aHF. The reaction of C₆F₅I with the weaker fluorooxidant XeF₂-BF₃ in 1,1,1,3,3-pentafluorobutane (PFB) yielded C₆F₅IF₂, perfluorocyclohexa-1,4-dienyliodine difluoride, C₆F₅IF₄, perfluorocyclohexa-1,4, and 1,3-dienyliodine tetrafluoride as intermediate products on parallel reaction routes. Both perfluoroalkenyl iodides, cis- and trans-(CF₃)₂CFCFCFI, reacted with XeF₂-BF₃ in PFB to give the corresponding perfluoroalkenyliodine tetrafluorides, cis- and trans-(CF₃)₂CFCFCFIF₄. Even perfluoroalkyl iodides can be fluorinated by this reagent as was demonstrated by the preparation of C₆F₁₃IF₄ from C₆F₁₃I. Generally, the CFCIF_n fragment (n = 0, 2, or 4) in cyclic or acyclic perfluoroalkenyliodine compounds R_FIF_n did not undergo a transformation to the corresponding perfluoroalkyliodine compound. Furthermore, no perfluoroorganoiodine hexafluorides were detected in reactions with the fluorooxidant XeF₂-aHF or BF₃ or NbF₅.     

Quantification of analytes in overlapping peaks from capillary electrophoresis using multivariate curve resolution-alternating least squares methods

Application of multivariate curve resolution with alternating least squares (ALS) methods to second-order data from capillary electrophoresis diode array detector (CE-DAD) is shown. Second-order data are easily obtained by setting individual data matrix of CE run one in top of the other. Initial qualitative solutions obtained by evolving factor analysis can be further optimized by simultaneous analysis of multiple electrophoresis run data with ALS regression. Quantification is achieved by the comparison of the analyte peak areas with that of pure standards. During the ALS regression procedure, the following constraints were applied: (i) both concentrations and unit pure spectra of the resolved components must be positive; (ii) elution profiles have an unimodal shape; (iii) correspondence exists between common species in the different data matrices; (iv) the pure spectrum of each species is the same in all runs where it is present. The above methods were applied for the determination of dinitrotoluene (DNT) isomeric compounds in overlapping peaks from CE.     

Polyaniline-coated silica gel

Silica gel microspheres 7 and 15 μm in diameter were coated with an overlayer of polyaniline camphorsulfonate or hydrochloride during the oxidative polymerization of aniline. Coated silica gel and polyaniline precipitate were separated using a difference in sedimentation rate. In an alternative approach, the microspheres were modified with polyaniline in the presence of 35 nm colloidal silica. This technique prevented the macroscopic precipitation of polyaniline. Coatings of neat, 3-aminopropyl- and octadecyl-modified silica gel with polyaniline hydrochloride were compared. The surface composition of coated microspheres was characterized by X-ray photoelectron spectroscopy. Potential applications of particles in electrorheology, organic catalysis, and in modeling of conductivity behavior in composites are demonstrated.     

Solvation and Ion-Pairing Effects of Choline Acetate Electrolyte in Protic and Aprotic Solvents Studied by NMR Titrations

Choline-based electrolytes have been proposed as environmentally friendly and low-cost alternatives for secondary zinc air batteries. Choline acetate [Ch]⁺[OAc]⁻ in protic (D₂O) and aprotic (DMSO-d₆) solvents has been studied by means of concentration-dependent ¹H NMR, viscosity, and density measurements. The viscosities have been calculated on the basis of the Jones-Dole equation and showed that the dominant contribution originates from short-range ion-solvent interactions. Site-specific association affinities were assigned from NMR chemical shift titrations. In DMSO-d₆, the hydroxyl group of choline was found to have the smallest dissociation constant followed by the methyl group of acetate. The corresponding Gibbs energies at low concentration were found to be in agreement with a solvent-separated ion pair (2SIP) configuration, whereas at concentrations above 300 mM, a solvent-shared ion pair (SIP) configuration was assigned. For [Ch]⁺[OAc]⁻ in D₂O, association effects were found to be weaker, attributed to the high dielectric constant of the solvent. On time scales on the order of 100 ms, NMR linewidth perturbations indicated a change in the local rotational dynamics of the ions, attributed to short-range cation-solvent interactions and not to solvent viscosity. At 184 mM, 40 % of the cations in DMSO-d₆ and 10 % in D₂O were found to exhibit short-range interactions, as indicated by the linewidth perturbations. It was found that at about 300 mM, the ions in DMSO-d₆ exhibit a transition from free to collective translational dynamics on time scales on the order of 400 ms. In DMSO-d₆, both ions were found to be almost equally solvated, whereas in D₂O solvation of acetate was stronger, as indicated by the obtained effective hydrodynamic radii. For [Ch]⁺[OAc]⁻ in DMSO-d₆, the results suggest a solvent-shared ion association with weak H-bonding interactions for concentrations between 0.3–1 M. Overall, the extent of ion association in solvents such as DMSO is not expected to significantly limit charge transport and hinder the performance of choline-based electrolytes.