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.     

Raman Spectroscopic Study of Aluminum Silicate Complextion in Acidic Solutions from 25 to 150°C

Raman spectroscopic measurements were performed on aqueous acid to neutral silica-bearing solutions (0.005 ≤ m _Si ≤ 0.02, 0 ≤ pH ≤ 8) and Al–silica solutions at temperature from 20 to 150°C. At 20°C, the spectrum of silica-bearing solutions exhibits only the bands of water and a completely polarized band at 785 cm^?1. This band is attributed to the ν₁ band of the tetrahedral Si(OH)₄ molecule. In ${\text{Si(OH)}}_{\text{4}} {\kern 1pt} {\kern 1pt} - {\kern 1pt} {\text{AlCl}}_3 {\kern 1pt} - {\kern 1pt} {\text{HCl}}$ solutions, the intensity of this band decreases with increasing Al concentration, temperature, and pH. This decrease can be explained by the formation of an inner sphere complex between Al³⁺ and Si(OH)₄ according to the reaction: ${\text{Al}}^{{\text{3 + }}} {\text{ + H}}_{\text{4}} {\text{SiO}}_{\text{4}}^{\text{0}} ({\text{aq}}){\text{ }} \Leftrightarrow {\text{ AlH}}_{\text{3}} {\text{SiO}}_{\text{4}}^{{\text{2 + }}} {\text{ + H}}^{\text{ + }} $ The fraction of complexed silica deduced from raman spectroscopic measurements is in good agreement with that calculated for the similar solution compositions and temperatures using the complexation constant generated by Pokrovski et al. ⁽²³⁾ from potentiometric measurements. At ambient temperature, the formation of aluminum silicate complex is weak and does not account for more than ca. 5 % of the total Al in most natural waters. As temperature increases, this complex becomes more significant and can dominate Al speciation in acid (pH ≤ 2) hydrothermal solutions.     

Energy band structure of TiO2 and it's dependence on uniaxial stress

The electronic band structure of titanium dioxyde is calculated. The evolution of the levels and of the optical transitions under an uniaxial stress along the c axis has also been calculated.We confirm the existence, of a first order forbidden direct optical gap, and of a maximum along the Δ direction; the theoretical pressure coefficients ?E/?P for the Γ⁺₇ → Γ⁺₆ and Δ₅ (max) → Γ⁺₆ transitions are respectively 0.600 and 0.950 meV kbar^-1 while the corresponding experimental values are 0.625 and 0.985 meV kbar^-1.     

Aminosulf(ox)ides as ligands for Iridium(I)-catalyzed asymmetric transfer hydrogenation

A new class of efficient catalysts was developed for the asymmetric transfer hydrogenation of unsymmetrical ketones. A series of chiral N,S-chelates (6-22) was synthesized to serve as ligands in the iridium(I)-catalyzed reduction of ketones. Both formic acid and 2-propanol proved to be suitable as hydrogen donors. Sulfoxidation of an (R)-cysteine-based aminosulfide provided a diastereomeric ligand family containing a chiral sulfur atom. The two chiral centers of these ligands showed a clear effect of chiral cooperativity. In addition, aminosulfides containing two asymmetric carbon atoms in the backbone were synthesized. Both the sulfoxide-containing beta-amino alcohols and the aminosulfides derived from 1,2-disubstituted amino alcohols gave rise to high reaction rates and moderate to excellent enantioselectivities in the reduction of various ketones. The enantioselective outcome of the reaction was favorably affected by selecting the most appropriate hydrogen donor. Enantioselectivities of up to 97% were reached in the reduction of aryl-alkyl ketones.     

Approximation of surfaces with fault(s) and/or rapidly varying data, using a segmentation process, D m -splines and the finite element method

In many problems of geophysical interest, one has to deal with data that exhibit complex fault structures. This occurs, for instance, when describing the topography of seafloor surfaces, mountain ranges, volcanoes, islands, or the shape of geological entities, as well as when dealing with reservoir characterization and modelling. In all these circumstances, due to the presence of large and rapid variations in the data, attempting a fitting using conventional approximation methods necessarily leads to instability phenomena or undesirable oscillations which can locally and even globally hinder the approximation. As will be shown in this paper, the right approach to get a good approximant consists, in effect, in applying first a segmentation process to precisely define the locations of large variations and faults, and exploiting then a discrete approximation technique. To perform the segmentation step, we propose a quasi-automatic algorithm that uses a level set method to obtain from the given (gridded or scattered) Lagrange data several patches delimited by large gradients (or faults). Then, with the knowledge of the location of the discontinuities of the surface, we generate a triangular mesh (which takes into account the identified set of discontinuities) on which a D ^m -spline approximant is constructed. To show the efficiency of this technique, we will present the results obtained by its application to synthetic datasets as well as real gridded datasets in Oceanography and Geosciences.     

Thio acid-mediated conversion of azides to amides – Exploratory studies en route to oroidin alkaloids

The utility of the thio acid-azide coupling reaction to afford amides is explored in imidazole-containing substrates for application in the total synthesis of examples of oroidin alkaloids. Good yields of the expected amides are obtained in both monomeric and dimeric substrates. Bis azides react preferentially at the 2-azido position but hydrosulfenylation and reduction interfere. 2-Thiophenyl and 2-oxo groups were evaluated as 2-amino surrogates, the thioether delivered the expected amide, whereas 2-imidazolone gave a mixture of the expected amide and the hydrosulfenylation product.     

Segmentation under geometrical conditions using geodesic active contours and interpolation using level set methods

Let I: be a given bounded image function, where is an open and bounded domain which belongs to ⁿ. Let us consider n=2 for the purpose of illustration. Also, let S={x_i}_i be a finite set of given points. We would like to find a contour , such that is an object boundary interpolating the points from S. We combine the ideas of the geodesic active contour (cf. Caselles et al. [7,8]) and of interpolation of points (cf. Zhao et al. [40]) in a level set approach developed by Osher and Sethian [33]. We present modelling of the proposed method, both theoretical results (viscosity solution) and numerical results are given. AMS subject classification 49L25, 74G65, 68U10     

Generalized fast marching method: applications to image segmentation

In this paper, we propose a segmentation method based on the generalized fast marching method (GFMM) developed by Carlini et al. (submitted). The classical fast marching method (FMM) is a very efficient method for front evolution problems with normal velocity (see also Epstein and Gage, The curve shortening flow. In: Chorin, A., Majda, A. (eds.) Wave Motion: Theory, Modelling and Computation, 1997) of constant sign. The GFMM is an extension of the FMM and removes this sign constraint by authorizing time-dependent velocity with no restriction on the sign. In our modelling, the velocity is borrowed from the Chan–Vese model for segmentation (Chan and Vese, IEEE Trans Image Process 10(2):266–277, 2001). The algorithm is presented and analyzed and some numerical experiments are given, showing in particular that the constraints in the initialization stage can be weakened and that the GFMM offers a powerful and computationally efficient algorithm.     

Sur les formules de quadrature numérique à nombre minimal de noeuds d'intégration

Résumè Cet article a pour objet la recherche, à partir de la théorie des polynômes orthogonaux, de conditions permettant l'obtention de formules de quadrature numérique sur des domaines de ⁿ, avec fonction poids, à nombre minimal de noeuds et exactes sur les espacesQ _k de polynômes de degré k par rapport à chacune de leurn variables. Ces résultats, complétés par des exemples numériques originaux dans ², adaptent à ces espacesQ _k ceux démontréq par H.J. Schmid [14] dans le cadre des espacesP _k de polynômes.

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About Cubature formulas with a minimal number of knots

Summary In this paper we search, from the orthogonal polynomial theory, for conditions which allow to obtain cubature formulas on sets of ⁿ, with weight function. which have a minimal number of knots and which are exact on the spaceQ _k of all polynomials of degree k with respect to each variablex _i, 1in.These results, completed by original numerical examples in ², adapt to the spacesQ _k those proved by H.J. Schmid [14] in the case of polynomial spacesP _k.

     

A Robust, Environmentally Benign Catalyst for Highly Selective Hydroformylation

By a sol-gel process a rhodium complex containing a diphosphane with a large natural P-Rh-P bite angle is covalently anchored in a silica matrix (see picture). The immobilized catalyst is a very selective hydroformylation catalyst that is completely and conveniently separated from the product and can be reused in numerous cycles.