Stereochemical consequences of threefold symmetry in asymmetric catalysis: distorting C3 Chiral 1,1,1-tris(oxazolinyl)ethanes ("trisox") in CuII Lewis acid catalysts

The underlying conceptual differences in exploiting two- and threefold rotational symmetry in the design of chiral ligands for asymmetric catalysis have been addressed in a comparative study of the catalytic performance of bisoxazoline (BOX) and tris(oxazolinyl)ethanes (trisox) containing copper(II) Lewis acid catalysts. The differences become apparent in constructing new catalysts by systematically "deforming" the stereodirecting ligand by inverting chiral centres or replacing chiral by achiral oxazolines. The catalytic alpha-amination of ethyl 2-methylacetoacetate with dibenzyl azodicaboxylate, which occurs with high enantioselectivity for both Ph(2)-BOX and Ph(3)-trisox copper catalysts, has been employed as the test reaction. In the trisox-copper complex [Cu(II)(iPr(3)-trisox)(kappa(2)-O,O'-MeCOCHCOOEt)](+)[BF(4)](-) (1), which was characterised by X-ray diffraction, two of the oxazoline groups are coordinated to the central copper atom, whilst the third oxazoline unit is dangling with the N-donor pointing away from the metal centre. A similar arrangement is found for the stereochemically "mixed" C(1)-trisox complex [Cu(II){(Ph(3)-trisox(R,S,S)}(kappa(2)-O,O'-MeCOCHCOOEt)(H(2)O)](+)[ClO(4)](-) (2), in which the phenyl substituents adopt a first coordination sphere meso arrangement. The almost identical selectivity of the Ph(3)-trisox(R,R,R)- and Ph(2)-BOX(R,R)-derived catalysts is as expected from the proposed model of the active catalyst based on a partially decoordinated podand. The behaviour of the "desymmetrised" trisox-Cu catalysts may be rationalised in terms of a general steady-state kinetic model for the three possible active bisoxazoline-copper species, which are expected to be in rapid exchange with each other in solution. This applies to both the trisox derivatives with stereochemically inverted and achiral oxazoline rings. The study underscores previous observations of superior performance of the catalysts bearing C(3)-chiral stereodirecting ligands as compared to systems of lower symmetry.     

pH-dependence of the aqueous electrochemistry of the two-electron reduced alpha-[Mo18O54(SO3)] sulfite Dawson-like polyoxometalate anion derived from its triethanolammonium salt

The electrochemistry of the Dawson-like sulfite polyoxometalate anion alpha-[Mo18O54(SO3)2]6-, derived from the TEAH6{alpha-[Mo18O54(SO3)2]} salt (TEAH+ is the triethanolammonium cation; pKa=7.8), has been investigated in aqueous media using cyclic and rotated disk voltammetry at glassy carbon electrodes and bulk electrolysis, with a focus on the pH-dependence for oxidation to alpha-[Mo18O54(SO3)2]4-. In buffered media at pH>or=4, the cyclic voltammetric response for alpha-[Mo18O54(SO3)2]6- reveals two partially resolved one-electron oxidation processes corresponding to the sequential generation of alpha-[Mo18O54(SO3)2]5- and alpha-[Mo18O54(SO3)2]4-. At lower pH, using electrolytes containing sulfuric acid, the two waves coalesce but the individual apparent E0' reversible formal potential values for the two processes can be extracted down to pH 2 by assuming that reversible protonation accompanies fast electron transfer. The results for 2相似文献   

Harmonics-to-noise ratios in normally speaking prepubescent girls and boys

This study examined harmonics-to-noise ratios (HNR) in 4 groups of normally speaking children. HNRs were calculated for the vowels /ai/ and /inverted v/, selected from conversational speech samples of 80 children aged 4, 6, 8, and 10 years (10 boys and 10 girls at each age level). HNR values for /inverted v/ were significantly higher than those for /ai/. Significant age differences emerged for /ai/ between ages 4 and 8, and ages 8 and 10. Girls obtained significantly higher HNRs than boys for the /ai/ vowel. Overall, the HNR values for these normally speaking children were lower than those reported for normally speaking adults. These findings suggest that acoustic values for children cannot be validly compared to those for adults, and that the child's gender and age should be taken into account when applying spectral analyses to research and/or clinical situations.     

Identification and Preliminary Structure-Activity Relationship Studies of 1,5-Dihydrobenzo[e][1,4]oxazepin-2(3H)-ones That Induce Differentiation of Acute Myeloid Leukemia Cells In Vitro

Acute myeloid leukemia (AML) is the most aggressive type of blood cancer, and there is a continued need for new treatments that are well tolerated and improve long-term survival rates in patients. Induction of differentiation has emerged as a promising alternative to conventional cytotoxic chemotherapy, but known agents lack efficacy in genetically distinct patient populations. Previously, we established a phenotypic screen to identify small molecules that could stimulate differentiation in a range of AML cell lines. Utilising this strategy, a 1,5-dihydrobenzo[e][1,4]oxazepin-2(3H)-one hit compound was identified. Herein, we report the hit validation in vitro, structure-activity relationship (SAR) studies and the pharmacokinetic profiles for selected compounds.     

Silylated Melamine and Cyanuric Acid as Precursors for Imprinted and Hybrid Silica Materials with Molecular Recognition Properties

Imprinted materials : Monosilylated derivatives of melamine and cyanuric acid are used in the preparation of hybrid silica. The assembly of the melamine and cyanuric acid moieties through molecular recognition properties is the key factor in building a bridged silsesquioxane and an imprinted hybrid silica (see picture).

     

Null controllability of the complex Ginzburg–Landau equation

The paper investigates the boundary controllability, as well as the internal controllability, of the complex Ginzburg–Landau equation. Zero-controllability results are derived from a new Carleman estimate and an analysis based upon the theory of sectorial operators.     

Synthesis, crystal structure, and porosity estimation of hydrated erbium terephthalate coordination polymers

The reaction of the Er3+ ion with polycarboxylate ligands in gel media leads to coordination polymers exhibiting various structural types and dimensionalities. Five Er3+/1,4-benzenedicarboxylate-based coordination polymers have been obtained in such conditions. Four out of the five are new. Their crystal structures are reported and compared herein. Compound 1, namely, Er2Ter3(H2O)6, where H2Ter symbolizes the terephthalic acid, crystallizes in the space group P1 (No. 2) with a = 7.8373(10) A, b = 9.5854(2) A, c = 10.6931(2) A, alpha = 68.7770(8) degrees, beta = 70.8710(8) degrees, and gamma = 75.3330(12) degrees. It has already been reported elsewhere. The last four compounds are new. Compound 2, namely, Er2Ter3(H2O)6 x 2 H2O, crystallizes in the space group P121/a1 (No. 14) with a = 6.7429(2) A, b = 22.4913(7) A, c = 9.6575(3) A, and beta = 91.6400(18) degrees. Compound 3, namely Er2Ter3(H2O)8 x 2 H2O crystallizes in the space group P1 (No. 2) with a = 7.5391(2) A, b = 10.0533(3) A, c = 10.4578(3) A, alpha = 87.7870(10) degrees, beta = 82.5510(11) degrees, and gamma = 86.2800(16) degrees. Compound 4, namely, Er2Ter3(H2O)6 x 2 H2O crystallizes in the space group C2/c (No. 15) with a = 38.5123(13) A, b = 11.1241(4) A, c = 7.0122(2) A, and beta = 98.634(2) degrees. Compound 5, namely, Er2Ter3(H2O)6 x H2O, crystallizes in the space group P1 (No. 2) with a = 6.8776(10) A, b = 11.0420(2) A, c = 18.5675(3) A, alpha = 84.7240(6) degrees, beta = 81.8380(6) degrees, and gamma = 84.1770(8) degrees. A computational method has also been developed to evaluate the potential porosity of the coordination polymers. This method is described and then applied to the different Er2Ter3(H2O)n coordination polymers previously described.