Effect of substituents at silicon on the oxidative addition of trisubstituted silanes to [RhCl(cod)PPh3]

Summary Trisubstituted silanes, HSiR_3–n X _n (R = Me, Et, Pr or Bu; X = Cl, OEt or Ph; and n = 0–3) readily undergo oxidative addition to complex [RhCl(cod)PPh₃] (where cod = cycloocta-1,5-diene).The quantitative correlation between rate constants, k ₁, of the reaction, followed spectrophotometrically at 20 °C in benzene solutions, and the structure of tri-substituted silanes represented by stereoelectronic parameters , and E of the substituents, was established: logk ₁ = a + b + c + dE. The reaction rate is accelerated by electron-withdrawing substituents at silicon and retarded by the bulk and pd donation of nonalkyl substituents.Author to whom all correspondence should be directed.     

A proton NMR study on aggregation of cationic surfactants in water: effects of the structure of the headgroup

 ¹H NMR chemical shifts of solutions of the following cationic surfactants in D₂O were determined as a function of their concentrations: cetyltrimethylammonium chloride, CTACl, a 1 : 1 molar mixture of CTACl and toluene, cetylpyridinium chloride, CPyCl, cetyldimethylphenylam-monium chloride, CDPhACl, cetyldimethylbenzylammonium chloride, CDBzACl, cetyldimethyl-2-phenylethylammonium chloride, CDPhEtACl, and cetyldimethyl-3-phenylpropylammonium chloride, CDPhPrACl. Plots of observed chemical shifts versus [surfactant] are sigmoidal, and were fitted to a model based on the mass-action law. Satisfactory fitting was obtained for the discrete protons of all surfactants. From these fits, we calculated the equilibrium constant for micelle formation, K, the critical micelle concentration, CMC and the chemical shifts of the monomer, δ_mon and the micelle δ_mic. ¹H NMR-based CMC values are in excellent agreement with those which we determined by surface tension measurements of surfactant solutions in H₂O, allowing for the difference in structure between D₂O and H₂O. Values of K increase as a function of increasing the size of the hydrophilic group, but the free energy of transfer per CH₂ group of the phenylalkyl moiety from bulk water to the micellar interface is approximately constant, 1.9±0.1 kJ mol^-1. Values of (δ_mic–δ_mon) for the surfactant groups at the interface, e.g., CH₃–(CH₂)₁₅–N+(CH₃)₂ and within the micellar core, e.g., CH₃–(CH₂)₁₅–N⁺ were used to probe the (average) conformation of the phenyl group in the interfacial region. The picture that emerges is that the aromatic ring is perpendicular to the interface in CDPhACl and is more or less parallel to it in CDBzACl, CDPhEtACl, and CDPhPrACl. Received: 23 February 1996 Accepted: 29 August 1996     

Solvatochromic equilibria of cobalt(II) chloride in binary mixtures of dipolar aprotic solvents with water

The visible absorption spectra of CoCl₂ in binary mixtures of acetone, dmf, dma and dmso with H₂O show the existence of configuration equilibria between tetrahedral (T) and octahedral (O) species. Linear correlations between the values of log K (K = [O]/[T]) and the mole fraction of H₂O, _w, have been observed in all the systems under study; their slopes decrease linearly with an increase of the donor number, DN, of the organic component of the mixture. The formation of octahedral species is enthalpy favoured but entropy disfavoured. It was found that both H and S values change linearly with _w and the slopes of these relationships depend clearly on the DN values.     

Preparation of methyl (1R,2S,5S)- and (1S,2R,5R)-2-amino-5-tert-butyl-cyclopentane-1-carboxylates by parallel kinetic resolution of methyl (RS)-5-tert-butyl-cyclopentene-1-carboxylate

Comparison of the kinetic and parallel kinetic resolutions of methyl (RS)-5-tert-butyl-cyclopentene-1-carboxylate allows for the efficient synthesis of both (1R,2S,5S)- and (1S,2R,5R)-enantiomers of methyl 2-amino-5-tert-butyl-cyclopentane-1-carboxylate.     

Synthesis of annelated derivatives of tetrafluoro-10<Emphasis Type="Italic">H</Emphasis>-imidazo[1,2-<Emphasis Type="Italic">b</Emphasis>][1,2]benzooxazin-10-one and 2-pentafluorophenylpyrazine 1,4-dioxide by condensation of alicyclic 2-hydroxyamino oximes with pentafluorophenylglyoxal

Reactions of alicyclic 2-hydroxyamino oximes with pentafluorophenylglyoxal afford mixtures of annelated derivatives of 2-pentafluorophenylpyrazine 1,4-dioxide and tetrafluoro- 10H-imidazo[1,2-b][1,2]benzooxazin-10-one. The structures of the latter were established by X-ray diffraction analysis.Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1636–1639, August, 2004.     

Etude par RMN du 13C d'un heterocycle derive du dibora-2,4 diazaro-1,3 benzene et du borazine correspondant

¹³C NMR spectra and complete assignment by means of coupled spectra and off resonance experiments, are reported for a new kind of organoboron heterocycle and the related borazine (o-CH₃C₆H₄NBCH₃)₃; π-charge densities, obtained from LCAO-MO Hückel calculations, are in reasonable agreement with experimental chemical shifts for the former compound.     

Synthesis and Biological Activity of Some New Pyrazolo[3,4‐b]Pyrazines

New derivatives of pyrazolo[3,4‐b]pyrazine and related heterocycles were prepared starting from 6‐amino‐3‐methyl‐1‐phenyl‐1H‐pyrazolo[3,4‐b]pyrazine‐5‐carbonitrile ( 2 ) and using the key intermediates 4 , 5 , 6 , 14 , 15 and 16 . Some of the prepared compounds were evaluated for their antifungal and antibacterial activities.     

Kinetics of calcium phosphate reduction by carbon

A fundamental study was made on the reduction of calcium phosphate by carbon. The mechanism of reduction was presented on applying different additions. Both silica and alumina increased the extent of reduction but with variable rates. The activation energies were calculated on the basis of first-order reactions. The phases formed during reduction were investigated by X-ray analysis.     

Interplay of Electronic Structure and Bulk Properties in 2D and 3D Ternary Carbonitrides from First Principles

The changes in electronic structure and hardness as inferred from the bulk modulus are investigated for model structures of ternary compounds XC₃N₃ (X = B, Al, P, As, Ga) within the framework of density functional theory (DFT). The optimisations of the proposed two‐ (2D) and three‐dimensional (3D) structures and the calculations of the bulk moduli are performed by a pseudo potential method. The electronic structures are calculated with the augmented sphere wave method (ASW). The obtained hardness for 2D BC₃N₃ system (B₀ ～ 220 GPa) points to a magnitude close to that of graphitic C₃N₄. For heavier X atoms it decreases rapidly. This is equally observed for the 3D systems examined in the β‐C₃N₄ structure for which B₀(β‐BC₃N₃) amounts to ～330 GPa. Within the magnitude of the well known hard material cubic BN, the BC₃N₃ phases can be predicted as candidates for ultra hard materials. The electronic effect induced by the chemical nature of the X substitutional was examined according to its position in the periodic table i.e. X_III or X_V. Both, band structures and the electron localisation function (ELF) were used for this analysis. The ELF plots show a decreasing covalency with heavier X‐atoms. Potential applications of the devised systems are proposed such as dopings with atoms (Li, rare gas) and molecules (N₂).     

Hygrometric Determination of Water Activities, Osmotic and Activity Coefficients, and Excess Gibbs Energy of the System MgSO4–K2SO4–H2O

Ternary aqueous solutions of MgSO₄ and K₂SO₄ have been studied by the hygrometric method at 25°C. The relative humidity of this system is measured at total molalities from 0.35 mol-kg^–1 to about saturation for three ionic-strength fractions (y = 0.25, 0.50, and 0.80 of MgSO₄. The data allow calculation of water activities and osmotic coefficients. From these measurements, the Pitzer ionic mixing parameters are determined and used to predict the solute activity coefficients in the mixture. The results are used to calculate the excess Gibbs energy at total molalities for ionic-strength fraction y.