Scope, limitations and mechanistic aspects in the selective homogeneous palladium-catalyzed reduction of alkenes under transfer hydrogen conditions

A new and efficient mild Pd/P(t-Bu)₃ catalyst for selective reduction of various alkenes under transfer hydrogen conditions has been developed leading to the corresponding saturated derivatives in chemical yields varying from 65 to 98%. Mechanistic rationale of this reaction has been also demonstrated.     

Bicyclic aziridines as intermediates in the photolysis of polycyclic aromatic azides.

According to the subsequent treatment, azepines $\text{16}$, $\text{17}$, $\text{19}$ or 1,2-dihydro derivatives $\text{18}$ and $\text{20}$ are obtained in the photolysis in basic medium of β-anthracenic azides $\text{7b}$ and $\text{7c}$ and provide strong evidence for the intermediate formation of bicyclic aziridines.     

Vapor Pressure and Solid Phases of Methanol below Its Triple Point Temperature

We present an experimental work devoted to study of the thermodynamical properties of solid methanol. We combine Fourier transform infrared spectroscopy (FTIR) and mass spectrometry (MS) to measure, for the first time, the vapor pressure of various methanol solid phases and determine their Clausius-Clapeyron equations. We perform our experiments between T = 130 K and the triple point temperature T(t) = 175.61 K. When methanol is condensed from its vapor below T(t), we observe three different solid phases depending on temperature. A condensation at T = 130 K forms a metastable phase with an enthalpy of sublimation deltaH(metastable-vapor) = 42.9 +/- 0.5 kJ.mol(-1). Upon heating, this phase transforms itself at T approximately 145 K to the alpha-phase that has an enthalpy of sublimation deltaH(alpha-vapor) = 46.9 +/- 0.2 kJ.mol(-1). Cooling the alpha-phase does not lead back to the metastable phase, whereas heating this alpha-phase leads to the beta-phase occurrence at T(alpha-beta) = 157.36 K. This latter one is stable until T(t) and has an enthalpy of sublimation deltaH(beta-vapor) = 44.2 +/- 0.5 kJ.mol(-1).     

Probing electrochemical properties of pi-conjugated thienylenevinylenes/fullerene C(60) adducts by ESI/MS: evidence for dimerized cation-radicals

Oligothienylenevinylenes/C(60) dyads n-C and triads n-C(2) are studied by electrospray mass spectrometry. A clear correlation is observed between the nature of the charged species detected by mass spectrometry, i.e. protonated molecule [M + H], (+) cation radical M(+.) and dication M(++), and the oxidation potentials of the molecules. Moreover, under defined solubility conditions, mass spectrometry provides conclusive evidences for the reversible dimerization of cation-radicals of n-C(2) compounds.     

Nachweis von C (15)-epimeren Coleonen C und D in der Natur und ihre Bedeutung für den sterischen Verlauf der Bildung des Spirocyclopropanrings

Natural Occurrence of C(15)-Epimeric Coleons C and D and its Significance to the Stereochemistry of the Formation of a Spirocyclopropanring In a previous paper some evidence was presented for a predominant occurrence of epimeric coleons ((15S)-coleon C ((15 S)- 1a ), and (15S)-coleon D ((15S)- 2a )) in Plectranthus lanuginosus [1], The proposed structures have now been proven by careful comparison of their physical data with those of the already described C(15)-epimers whose structures were deduced by X-ray analysis of a derived cis-diketone 3 [5], Examination of the epimeric purity of coleons possessing a 2-hydroxy-1-methylethyl side-chain (coleons C , D , H , I , I ′ and derivatives) isolated from seven different species (Coleus, Plectranthus and Solenostemon) has shown that these coleons always occur as a mixture of C(15)-epimers, although in most cases with one predominant epimer. The consequences for the stereochemistry of the postulated in vivo formation of the methyl-substituted spirocyclopropane substructure is discussed.     

Gibbs functions for transfer of divalent metal cations from water to water + methanol mixtures using potentiometry, polarography and solubility measurements

Gibbs functions for transfer from water to methanol and to a full range of water + methanol mixtures were obtained for Cu²⁺, Cd²⁺, Pb²⁺, Hg²⁺, Zn²⁺, Mg²⁺, Ca²⁺, Sr²⁺ and Ba²⁺ using potentiometric or polarographic measurements in these solvents. In addition, data were obtained from the solubility products of the alkaline-earth iodates. From values obtained by the different methods and literature data, a table of selected values is given.     

Optimization of a GFAAS method for determination of total inorganic arsenic in drinking water

The new 10 μg l⁻¹ arsenic standard in drinking water has been a spur to the search for reliable routine analytical methods with a limit of detection at the μg l⁻¹ level. These methods also need to be easy to handle due to the routine analyses that are required in drinking water monitoring. Graphite furnace atomic absorption spectrometry (GFAAS) meets these requirements, but the limit of detection is generally too high except for methods using a pre-concentration or separation step. The use of a high-intensity boosted discharge hollow-cathode lamp decreases the baseline noise level and therefore allows a lower limit of detection. The temperature program, chemical matrix modifier and thermal stabilizer additives were optimized for total inorganic arsenic determination with GFAAS, without preliminary treatment. The optimal furnace program was validated with a proprietary software. The limit of detection was 0.26 μg As l⁻¹ for a sample volume of 16 μl corresponding to 4.2 pg As. This attractive technique is rapid as 20 samples can be analysed per hour. This method was validated with arsenic reference solutions. Its applicability was verified with artificial and natural groundwaters. Recoveries from 91 to 105% with relative standard deviation <5% can be easily achieved. The effect of interfering anions and cations commonly found in groundwater was studied. Only phosphates and silicates (respectively at 4 and 20 mg l⁻¹) lead to significant interferences in the determination of total inorganic arsenic at 4 μg l⁻¹.     

Free-energy calculations of protein-ligand cation-pi and amino-pi interactions: from vacuum to proteinlike environments

To probe the role of cation-pi and amino-pi interactions in the context of protein-ligand interactions, the stability of 55 X-ray cation/amino-pi motifs involving the Ade moieties of cofactor molecules and Arg, Lys, Asn, or Gln side chains of their host protein was evaluated using quantum chemistry calculations. The conjunction of vacuum interaction energies, vibrational entropy, and solvation contributions led to identify Arg-Ade as the most favorable cation/amino-pi complex in the solvents considered, followed by Asn/Gln-Ade and Lys-Ade: their minimum interaction free energies are approximately equal to -7, -4, and -2 kcal/mol, respectively, in the solvents of dielectric constant similar to that estimated for proteins (i.e., acetone, THF, and CCl(4)). Remarkably, these free-energy values of cation/amino-pi interactions correlate well with their frequency of occurrences in protein-ligand structures, which corroborates our approach in the absence of experimental data.