New chiral phosphine-phosphite ligands in the enantioselective palladium-catalyzed allylic alkylation

A series of chiral phosphine-phosphite ligands 1-6 have been synthesized and used in the enantioselective palladium-catalyzed reaction of rac-1,3-diphenyl-2-propenyl acetate with dimethyl malonate as nucleophile. Ligands 1a, 2, 3, 5a, 6a, and 6b have been synthesized starting from racemic tert-butylphenylphosphinoborane. The use of dynamically resolved Li phosphide (-)-sparteine provided the optically pure ligands. Crystals of the allylpalladium (6a) complex were obtained, suitable for X-ray crystal structure determination. The X-ray crystal structure of the allylpalladium (6a) complex revealed a longer palladium-carbon bond distance trans to the phosphine moiety indicating that the attack of the nucleophile takes place at the carbon trans to the phosphine moiety. This was confirmed by the fact that the phosphine moiety did not affect the enantioselectivity directly. Under mild reaction conditions, enantioselectivities up to 83% were obtained (25 degrees C) with ligand 1e. Systematic variation of the ligand bridge and the phosphite moiety showed that the configuration of the product is controlled by the atropisomerism of the biphenyl substituent at the phosphite moiety. The conformation of the biphenyl group, in turn, is controlled by the substituent at the chiral carbon in the bridge. Ligands with large bite angles yielded higher enantioselectivities.     

Intercalation of Benzamide into Kaolinite

Well-crystallized kaolinite (K) was initially reacted at 60 degrees C with a water/dimethylsulfoxide (DMSO) mixture and the resulting intercalation derivative (K-DMSO) was characterized by powder X-ray diffractometry (PXRD), thermal analysis (simultaneous TG and DSC), and Fourier-transformed infrared spectroscopy (FTIR). Benzamide crystals were then melted with the K-DMSO derivative at 140 degrees C for 4 days, when a gradual displacement of DMSO by benzamide was observed within the interlayer spacing of the modified kaolinite. The resulting material, after extensive washing with acetone, was characterized and compared to the results obtained previously for the K-DMSO composite. Benzamide intercalation proceeded by gradual displacement of DMSO molecules until completion. The structural stabilization of the K-BZ derivative was explained through the establishment of hydrogen bonds between the carbonyl oxygen atoms of the intercalated benzamide and aluminol groups present at the surface of the kaolinite layer. The interlamellar spacing of K-BZ was shown to be possibly occupied by benzamide molecules that were located at a 68 degrees orientation in relation to the layer surface. Unlike most intercalation molecules such as DMSO, variations in the interplanar spacing of kaolinite were consistent with the nonkeying of any other part of the molecule between the aluminosilicate interlayers. Copyright 2000 Academic Press.     

Structure of zirconium dioxide based porous glasses

This study is devoted to investigation of the micro- and mesostructure (including fractal properties) of porous zirconia glasses synthesized by precipitation from zirconium n-propoxide solutions in the presence of different hydrolyzing-agent (H₂O) quantities at different temperatures. Analysis of small-angle neutron, ultra-small-angle neutron and X-ray scattering, scanning electron microscopy allows concluding that the synthesized glasses are complex systems with a three-level hierarchical fractal structure. It is revealed that both the temperature of synthesis and the H₂O concentration in the initial solution significantly affect the structural characteristics of the glasses.     

Study of the heavy-fermion compound CeRu<Subscript>2</Subscript>Si<Subscript>2</Subscript> by the small-angle neutron scattering method

In order to directly observe neutron scattering by heavy fermion quasiparticles at low temperatures, a CeRu₂Si₂ single crystal has been studied by the small-angle neutron scattering method. In the experiment, neutron scattering is observed at T = 0.85 K for momentum transfers q ≤ 0.04 Å^?1, which is treated as the orbital component of magnetic scattering by heavy fermion quasiparticles. It has been found that the application of a magnetic field H = 1 T leads to both an increase in the observed scattering and its anisotropy with respect to the field direction. Moreover, measurements in the magnetic field reveal additional scattering for q > 0.04 Å^?1, which is well described by a Lorentzian and is interpreted as neutron magnetic scattering by spin-density fluctuations with a correlation radius R_c ≈ 30 Å.     

On the size effect in nanocrystalline cerium dioxide: Is the Tsunekawa model correct?

Nanocrystalline cerium-dioxide samples with different coherent-scattering region sizes are analyzed by advanced techniques, specifically, small-angle neutron scattering, X-ray and neutron diffraction, and spectrometry of the chemical shift in the characteristic X-ray lines. The obtained data confirm the presence of the size effect, i.e., the dependence of lattice constants on the size of coherent-scattering regions. Meanwhile, no traces of oxygen nonstoichiometry and a related deviation in the effective degree of cerium oxidation from the standard value are found. The results obtained call into question the Tsunekawa model traditionally used to explain the size effect in this compound.     

Mesostructure of xerogels of hydrated zirconium dioxide

The fractal properties of xerogels of hydrated zirconium dioxide ZrO₂, which are synthesized by precipitation from a zirconyl nitrate solution at various pH values, are studied. Small-angle neutron scattering data and low-temperature nitrogen adsorption data indicate the strong effect of the acidity of the medium on the fractal dimension of xerogels and the size of constituent monomer particles. It is shown that this effect is also characteristic of xerogels of hydrated hafnium dioxide HfO₂, which is a chemical analogue of hydrated zirconium dioxide.     

Structural Analysis of Aluminum Oxyhydroxide Aerogel by Small Angle X-Ray Scattering

The work presents studies on the microstructure and mesostructure of nanostructured aluminum oxyhydroxide formed as a high porous monolithic material through the surface oxidation of aluminum liquidmetal solution in mercury in a temperature- and humidity-controlled air atmosphere. The methods of X-ray diffraction analysis, thermal analysis, the low temperature adsorption of nitrogen vapors, transmission electron microscopy, small-angle and very small-angle neutron scattering, and small-angle X-ray scattering are used for comprehensive investigation of the samples synthesized at 25°С as well as that annealed at temperatures up to 1150°C. It is found that the structure of the monolithic samples can be described within the framework of a three-level model involving primary heterogeneities (typical length scale of rc ≈ 9–19 Å), forming fibrils (cross-sectional radius R ≈ 36–43 Å and length L ≈ 3200–3300 Å) or lamellae (thickness T ≈ 110 Å and width W ≈ 3050 Å) which, in turn, are integrated into large-scale aggregates (typical size R _c ≈ 1.25–1.4 μm) with an insignificant surface roughness. It is shown that a high specific surface (~200 m²/g) typical for the initial sample is maintained upon its thermal annealing up to 900°С, and it decreases to 100 m²/g after heat treatment at 1150°С due to fibrillary agglomeration.     

Specific features of the mesostructure of amorphous iron(III) hydroxide xerogels synthesized in an ultrasonic field

The influence of high-power ultrasonic treatment on the structure of amorphous iron(III) hydroxide xerogels prepared by precipitation from iron(III) nitrate solutions of different concentrations has been investigated. An analysis of the small-angle neutron scattering data allowed us to reveal for the first time that ultrasound substantially affects the mesostructure of amorphous xerogels, including the homogeneity, the fractal dimension, and the size of constituting monomer particles.     

Oxygen nonstoichiometry of nanocrystalline ceria

Correlation relations between oxygen nonstoichiometry and particle size in nanocrystalline CeO_{2 − x} are revised. The ceria unit cell parameter is shown to increase from 0.5410 to 0.5453 nm as the particle size decreases from 23 to 2.3 nm. The CeO_{2 − x} critical particle size where cerium(IV) is completely reduced to cerium(III) is calculated as 1.1–1.3 nm.     

Evolution of composition and fractal structure of hydrous zirconia xerogels during thermal annealing

The mesostructure of amorphous hydrous zirconia xerogels and the products of their heat treatment was studied for the first time using powder X-ray diffraction and small-angle neutron scattering (SANS). The samples prepared at low and high pH values have fundamentally different phase compositions and structures. The high-temperature annealing of hy drous zirconia xerogels is useful for manufacturing materials with controlled surface fractal dimensions.