Enhanced covariance spectroscopy from minimal datasets

A novel approach is described for the determination of reliable high-resolution homonuclear NMR covariance spectra from minimal datasets. It uses a sparse sampling scheme along the indirect dimension together with a comprehensive analysis of finite sampling effects that eliminates spurious correlations. The scheme, which is demonstrated for TOCSY and COSY, offers a substantial speed up over current methods, rendering it suitable for high-throughput screening applications.     

Magnetic properties of ferromagnetic quasi-1D copper-peptide compounds: exchange interactions and very low temperature phase transitions

The magnetic properties of the Cu(II)-peptide compounds (L-tyrosyl-L-leucinato)Cu(II) and (L-tryptophyl-glycinato)Cu(II), to be identified as Cu(II)Tyr-Leu and Cu(II)Trp-Gly, respectively, have been investigated by specific heat (0.08 < T < 28 K), dc magnetization (2 < T < 80 K, with B(0) = mu(o)H < or = 9 T), and ac magnetic susceptibility (with B(0) = 0 for 0.03 < T < 3 K and B(0) up to 9 T for 2 < T < 80 K) measurements. Above approximately 1 K, the specific heat and magnetization of both compounds display a ferromagnetic (FM) spin chain behavior sustained by syn-anti carboxylate bridges connecting equatorially Cu(II) ions at about 5 A. To model this behavior, we calculated the eigenvalues of Heisenberg chains with up to 20 spins 1/2 and used the method of Bonner and Fisher. A global fit of the model to the specific heat and magnetization data gives 2J(0)/k(B) = 3.60(5) K and 2.59(5) K for the intrachain exchange interactions in Cu(II)Tyr-Leu and Cu(II)Trp-Gly, respectively (H(ex)(i,j) = -2J(0) S(i).S(j)). These values of 2J(0) are discussed in terms of structural properties of the carboxylate bridges in the two compounds. Using the parameters obtained from the global fit, we calculated isothermal susceptibilities in agreement with the ac susceptibilities measured with small applied dc magnetic fields. However, the ac susceptibility measured with applied dc fields larger than 1 T lie between the values calculated for the isothermal and adiabatic susceptibilities. At 0.16 K for Cu(II)Tyr-Leu and 0.53 K for Cu(II)Trp-Gly, the observed specific heat and magnetic susceptibility display peaks associated to three-dimensional magnetic phase transitions. The interchain exchange couplings 2J(1) producing the 3D magnetic order are ferromagnetic and have magnitudes 2J(1)/k(B) approximately 0.015 and 0.073 K for Cu(II)Tyr-Leu and Cu(II)Trp-Gly, respectively.     

Efficient palladium catalyzed synthesis of heteroaromatic sulfoxides

The present Letter describes the efficient synthesis of novel heteroaromatic sulfoxides by means of a palladium catalyzed heteroarylation of sulfenate anions. Triazolopyridine, pyridine and thiophene sulfoxides can be obtained under mild conditions and in high yield from the corresponding heteroaryl bromides.     

Electrochemical Oxidation of Ibuprofen and Its Voltammetric Determination at a Boron‐Doped Diamond Electrode

The electrochemical oxidation of ibuprofen at a boron‐doped diamond electrode (BDDE) and its voltammetric determination is reported for the first time. A well‐defined oxidation peak was observed at around 1.6 V in 0.1 mol L^?1 H₂SO₄ solution with 10 % (v/v) ethanol at the BDDE surface activated by either cathodic or anodic pretreatments. A differential‐pulse voltammetric method for the determination of ibuprofen in pharmaceutical formulations was optimized with a detection limit of 5 µmol L^?1 and compared with the British Pharmacopeia method.     

Conformational flexibility of a synthetic glycosylaminoglycan bound to a fibroblast growth factor. FGF-1 recognizes both the (1)C(4) and (2)S(O) conformations of a bioactive heparin-like hexasaccharide

The first direct NMR determination of the conformation of a conformationally flexible heparin-like hexasaccharide bound to a key receptor, FGF-1, is described. The determination has been based on the use of a 13C-labeled protein and a regular 12C sugar. FGF-1 recognizes several conformations of the iduronic moieties of the hexasaccharide. Therefore, this case is different than that described for the controversial recognition of heparin-like saccharides by AT-III, which seems to recognize just one conformation of the iduronic acid residues.     

Enantioselective conjugate addition of nitro compounds to α,β-unsaturated ketones: an experimental and computational study

A series of chiral thioureas derived from easily available diamines, prepared from α-amino acids, have been tested as catalysts in the enantioselective Michael additions of nitroalkanes to α,β-unsaturated ketones. The best results are obtained with the bifunctional catalyst prepared from L-valine. This thiourea promotes the reaction with high enantioselectivities and chemical yields for aryl/vinyl ketones, but the enantiomeric ratio for alkyl/vinyl derivatives is very modest. The addition of substituted nitromethanes led to the corresponding adducts with excellent enantioselectivity but very poor diastereoselectivity. Evidence for the isomerization of the addition products has been obtained from the reaction of chalcone with [D(3)]nitromethane, which shows that the final addition products epimerize under the reaction conditions. The epimerization explains the low diastereoselectivity observed in the formation of adducts with two adjacent tertiary stereocenters. Density functional studies of the transition structures corresponding to two alternative activation modes of the nitroalkanes and α,β-unsaturated ketones by the bifunctional organocatalyst have been carried out at the B3LYP/3-21G* level. The computations are consistent with a reaction model involving the Michael addition of the thiourea-activated nitronate to the ketone activated by the protonated amine of the organocatalyst. The enantioselectivities predicted by the computations are consistent with the experimental values obtained for aryl- and alkyl-substituted α,β-unsaturated ketones.     

An analysis of two local measures of the electronic localization: a comparison with the ELF and the exchange-correlation density results

In this work we have explored the performance of two functions, recently proposed by Ayers [J. Chem. Sci., 2005, 117, 441], with the purpose of quantifying local electron localization. The first function, ζ(h), measures the total fluctuation per electron in the number of electrons at a given position r(1), while the second one, ζ(R), is a local representation of the minimum fluctuation criterion for electron localization. The study is carried out through a set of diatomic molecules that covers a wide range of covalent/polar character. Additionally, we have also calculated the electron localization function and the exchange-correlation hole along the internuclear axis. We have found that, for all the studied molecules, the numerical integration involved in computing ζ(h) did not converge. We think that this is so because the hole correlation calculations are not able to yield its correct asymptotic decaying behavior for large absolute values of the internuclear distances. On the other hand, the calculation of ζ(R) has proved to be feasible, and the information obtained from it has been concluded to be compatible to that rendered by the electron localization function (ELF) and the exchange-correlation density. Moreover, it has been also found that the results for ζ(R) allow to quantify the relative degree of electron localization within different molecular regions.     

Systematic structural studies on cobalt(II) complexes of tricyclohexylphosphine oxide and related ligands

The new cobalt(II) phosphine oxide complexes Co(Cy₃PO)₂Cl₂ (1), Co(Cy₃PO)₂Br₂ (2), Co(Cy₃PO)₂I₂ (3), Co(Ph₂CyPO)₂Cl₂ (4), Co(Ph₂CyPO)₂Br₂ (5), Co(Ph₂CyPO)₂I₂ (6), Co(Ph₂EtPO)₂Br₂ (7), Co(Cy₃PO)₂(NCS)₂ (8) and Co(Cy₃PO)₂(NO₃)₂ (9) have been prepared mainly by the reaction of anhydrous CoX₂ (X = Cl, Br, I, NCS, NO₃) with the appropriate phosphine oxide. The complexes were characterised by single-crystal X-ray crystallography supported by IR and UV-Vis absorption spectroscopy. The structural analyses show that the cobalt(II) centre adopts a distorted tetrahedral coordination geometry except for 9 which displays an octahedral geometry. Systematic structural features of these complexes are explained within this paper.     

Highly diastereo- and enantioselective direct Barbas-List aldol reactions promoted by novel benzamidoethyl and benzamidopropyl prolinamides in water

Four novel benzamido-functionalized prolinamides have been prepared and tested as organocatalysts for enantioselective aldol reaction of aldehydes and cyclic ketones in water. In particular, prolinamide derived from achiral ethylene diamine was the best catalyst leading to anti aldols in excellent diastereomeric (up to 98/2) and enantiomeric (up to 99/1) ratios, thereby showing that lateral amide functionalities might be a key issue for facilitating "in water" chemistry. These catalysts are cheaper and easier to prepare than those previously described.     

Computational study of the thermal decomposition and the thermochemistry of allyl ethers and allyl sulfides

In spite of the several experimental and computational studies on the thermal decomposition of allyl ethers and allyl sulfides, there are still disagreements on aspects of the reaction mechanism, such as the true nature of the transition states and the grade of synchronicity of the reactions. This work presents a computational study of the gas-phase thermolysis reaction of allyl ethers and allyl sulfides substituted at α-carbon, at the M05-2X/6-31+G(d,p) level of theory and a temperature range from 586.15 to 673.15 K. The substituent groups were methyl, ethyl, n-propyl, i-propyl, allyl, benzyl and acetonyl. It was found that the sulfides react faster than the homologous ethers and that the substituent groups with the capacity of delocalize charge increase the reaction rate. Through natural bond orbital calculations, the transition states were characterized. The synchronicities and atomic charges of the studied reactions were determined. A computational study at the G3 level of theory on the thermochemistry of allyl ethers and sulfides was also carried out.