Diastereodivergent asymmetric sulfa-Michael additions of α-branched enones using a single chiral organic catalyst

A significant limitation of modern asymmetric catalysis is that, when applied to processes that generate chiral molecules with multiple stereogenic centers in a single step, researchers cannot selectively access the full matrix of all possible stereoisomeric products. Mirror image products can be discretely provided by the enantiomeric pair of a chiral catalyst. But modulating the enforced sense of diastereoselectivity using a single catalyst is a largely unmet challenge. We document here the possibility of switching the catalytic functions of a chiral organic small molecule (a quinuclidine derivative with a pendant primary amine) by applying an external chemical stimulus, in order to induce diastereodivergent pathways. The strategy can fully control the stereochemistry of the asymmetric conjugate addition of alkyl thiols to α-substituted α,β-unsaturated ketones, a class of carbonyls that has never before succumbed to a catalytic approach. The judicious choice of acidic additives and reaction media switches the sense of the catalyst's diastereoselection, thereby affording either the syn or anti product with high enantioselectivity.     

New insights into the mechanism of activation of atom transfer radical polymerization by Cu(I) complexes

The kinetics of activation of RX by a Cu(I) complex has been investigated in MeCN both in the absence and presence of halide ions. The system Cu(I)/L/X(-) (L = Me(6)TREN) is mainly composed of Cu(I)L(+), XCu(I)L and Cu(I)X(2)(-), but only Cu(I)L(+) is found to be an active catalyst reacting with RX.     

Short-term electricity procurement: A rolling horizon stochastic programming approach

This paper addresses the problem faced by a large electricity consumer in determining the optimal procurement plan over a short-term time horizon. The inherent complexity of the problem, due to its dynamic and stochastic nature, is dealt by means of the stochastic programming modeling framework. In particular, a two-stage problem is formulated with the aim of establishing the optimal amount of electricity to be purchased through bilateral contracts and in the Day-Ahead Electricity Market. Recourse actions are used to hedge against uncertainty related to future electricity prices and consumer’s needs. The optimal plan is defined so to minimize the overall cost and to control risk, which is measured in the form of violation of budget constraints. The stochastic model is dynamically solved in a rolling horizon fashion by iteratively considering more and more recent information and a planning horizon of decreasing length. Extensive numerical experiments have been carried out to assess the performance of the proposed dynamic decision approach. The results collected considering a real test case are very encouraging and provide evidence of the superiority of the approach also in comparison with other alternative procurement strategies.     

Steric and Electronic Effects on the Configurational Stability of Residual Chiral Phosphorus‐Centered Three‐Bladed Propellers: Tris‐aryl Phosphanes

A series of tris‐aryl phosphanes, structurally designed to exist as residual enantiomers or diastereoisomers, bearing substituents differing in size and electronic properties on the aryl rings, were synthesized and characterized. Their electronic properties were evaluated on the basis of their electrochemical oxidation potential determined by voltammetry. The configurational stability of residual phosphanes, evaluated by dynamic HPLC on a chiral stationary phase or/and by dynamic ¹H and ³¹P NMR spectroscopy, was found to be rather modest (barriers of about 18–20 kcal mol^?1), much lower than that shown by the corresponding phosphane oxides (barriers of about 25–29 kcal mol^?1). For the first time, the residual antipodes of a tris‐aryl phosphane were isolated in enantiopure state and the absolute configuration assigned to them by single‐crystal anomalous X‐ray diffraction analysis. In this case, the racemization barrier could be calculated also by CD signal decay kinetics. A detailed computational investigation was carried out to clarify the helix reversal mechanism. Calculations indicated that the low configurational stability of tris‐aryl phosphanes can be attributed to an unexpectedly easy phosphorus pyramidal inversion which, depending upon the substituents present on the blades, can occur even on the most stable of the four conformers constituting a single residual stereoisomer.     

Internal quality control as a tool for planning a robustness study regarding a multiresidue method for pesticides found in olive oil

The implementation of the internal quality assurance program allows for demonstration of the performance characteristics of a method, as well as the avoidance of erroneous results. There is increasing concern in testing laboratories to ensure that the analytical process remains stable, giving reliable results under statistical control. Robustness is tested by introducing variations in experimental conditions and examining the effects on the results. The European document SANCO/12495/2011, “Method validation and quality control procedures for pesticide residue analysis in food and feed”, defines robustness as a parameter that can be derived from ongoing method verification. Internal quality control tools, such as the recovery control chart, were employed in this study to enhance the stability of the recovery rates and to investigate the experimental conditions that have a major influence on the quantification of recovery rates. The method investigated in this study permitted us to investigate eighteen pesticides in olive oil by using a gas chromatography–mass spectrometry technique. This method has been accredited to ISO/IEC 17025:2005 standards and was applied for 1 year in routine conditions for pesticide residues at the Italian National Reference Laboratory. The recovery control chart has showed that all recovery rates for 1 year were close to the maximum limit (120 %) of recovery performance criteria. Consequently, a “positive” bias has affected all data over longer periods of time. A robustness test was planned in order to investigate the grounds that most influenced the variability of the results. The robustness test involved the following three parameters: solvent used to dissolve the final extract, internal standards, and type of olive oil.     

New Complexes of Ditopic Ligands with “d” and/or “s” Metal Ions

Abstract

The asymmetric compartmental macrocycles containing one N₂O₂ or N₃O₂ Schiff base and one O₂O₃ or O₂O₄ crown-ether like chamber, have been obtained by condensation reaction of the formyl precursors 3,3′-(3-oxapentane-1, 5-diyldioxy) bis (2-hydroxybenzaldehyde) or 3,3′-(3,6-dioxaoctane-1,8-diyldioxy)-bis(2-hydroxybenzaldehyde) with ethyl ethylenediamine (H₂L_A, H₂L_C), 1,5-diamino-3-azamethylpentane(H₂L_B, H₂L_D), also in the presence of metal ions as templating agents. These ditopic ligands, with dissimilar coordination sites, have been designed and used for the selective complexation of “d” and/or “s” metal ions, respectively into the Schiff base and the crown ether chamber. The selectivity of these processes strongly depends on the size and on the donor atom sets of the sites. The possibility to obtain mononuclear M(L)·nH₂O (M = Ni²⁺, Cu²⁺, Co²⁺), Mn(L)(CH₃COO)·nH₂O or Na(L) and hetero-dinuclear MNa(L)(CH₃COO) (M = Ni²⁺, Cu²⁺, Co²⁺) and MnNa(L)(CH₃COO)₂·nH₂O complexes has been successfully tested. The ligands and complexes have been characterized by ir, nmr, mass spectrometry and magnetic susceptibility measurements.

Two of the ligands used for the preparation of the solid samples, i.e., to H₂L_A and H₂L_B, have been employed to study complexation reactions of Co(II) and Na(I) in solution. In order to obtain information on the ligand preorganization effect toward the complex stabilities, a simpler open chain parent compound of H₂L_B (H₂L_E) has been also prepared and studied. FT-IR spectra show that H₂L_A is unable to complex Na⁺ in DMSO while the complexation reactions of Na⁺ by H₂L_B and of Co²⁺ by H₂L_A take place with slow kinetics. Therefore, thermodynamic data have been obtained only for the systems Co²⁺/H₂L_B and Co²⁺/H₂L_E. The thermodynamic parameters obtained for the complexation reactions show that the pre-organization of the donor atoms in H₂L_B does not add a significant contribution to the stabilities of the complexes. Both H₂L_B and H₂L_E form in DMSO 1:1, 1:2 and 1:3 = M:L complexes with very similar stabilities and almost equal enthalpies of formation. Physico-chemical studies suggest besides that the slow reaction of Na⁺ with H₂L_B is probably due to the formation of a 1:1 complex where the metal cation, initially occupying the O₃ cage of the ligand, slowly binds also the oxygens of the phenolic moieties. Spectral and calorimetric data on solutions containing H₂L_B and different Co²⁺: Na⁺ ratios evidence that in DMSO no stable heterodinuclear complexes form when the neutral ligand is considered.     

A new class of luminescent tricarbonyl rhenium(I) complexes containing bridging 1,2-diazine ligands: electrochemical, photophysical, and computational characterization

A novel class of luminescent tricarbonyl rhenium(I) complexes of general formula [Re2(mu-X)2(CO)6(mu-diaz)] (X=halogen and diaz=1,2-diazine) was prepared by reacting [ReX(CO)5] with 0.5 equiv of diazine (seven different ligands were used). The bridging coordination of the diazine in these dinuclear complexes was confirmed by single-crystal X-ray analysis. Cyclic voltammetry in acetonitrile showed for all the complexes (but the phthalazine derivative) a chemically and electrochemically reversible ligand-centered reduction, as well as a reversible metal-centered bielectronic oxidation. With respect to the prototypical luminescent [ReCl(CO)3(bpy)] complex, the oxidation is more difficult and the reduction easier (about +0.3 V), so that a similar highest occupied molecular orbital-lowest unoccupied molecular orbital gap is observed. All of the complexes exhibit photoluminescence at room temperature in solution, with broad unstructured emission from metal-to-ligand charge-transfer states, at lambda in the range 579-620 nm. Lifetimes (tau=20-2200 ns) and quantum yields (Phi up to 0.12) dramatically change upon varying the bridging ligand X and the diazine substituents: in particular, quantum yields decrease in the series Cl, Br, and I and in the presence of substituents at the alpha positions of the pyridazine ring. A combined density functional and time-dependent density functional study of the geometry, relative stability, electronic structure, and photophysical properties of all the pyridazine derivatives was performed. The nature of the excited states involved in the electronic absorption spectra was ascertained, and trends in the energy of the highest occupied and lowest unoccupied molecular orbitals upon changing the pyridazine substituents and the bridging halogen ligands were discussed. The observed emission properties of these complexes were shown to be related to a combination of steric and electronic factors affecting their ground-state geometry and their stability.