Stereochemical Aspects in the Asymmetric Michael Addition of Chiral Imines to Substituted Electrophilic Alkenes

The Michael-type addition of chiral imines, derived from racemic alpha-substituted cyclanones and optically active 1-phenylethylamine, to electrophilic alkenes, in neutral conditions, constitutes one of the most efficient methods for the stereocontrolled construction of quaternary carbon centers. In order to create an additional stereogenic center at the alpha- or beta-position to the quaternary one, the behavior of a variety of alpha- and beta-substituted alkenyl acceptors was examined. In general, these additions are highly regioselective, the alkylation taking place predominantly, if not exclusively, at the more substituted alpha-side of the imine function; however, in some cases (electrophilic alkenes 28 and 49), significant amounts (10-15%) of regioisomeric adducts were obtained. With the exception of methyl propiolate 52, a remarkable control of the absolute configuration of the adducts were always observed with these Michael acceptors. According to the general rule we have previously proposed, the alkylation process takes place preferentially on the less hindered pi-face of the more substituted secondary enamine, in tautomeric equilibrium with the starting imine. An excellent diastereocontrol was always obtained by using the present alpha- and beta-substituted alkenes. These stereochemical outcomes can be interpreted by invoking that the reaction proceeds through a compact approach of the reactants, the hydrogen atom at the nitrogen center of the enamine being transferred to the alpha-vinylic carbon atom of the acceptor, concertedly with the creation of the C-C bond. In this respect the "endo-approach" 58, in which the electron-withdrawing group of the acceptor faced to the nitrogen atom of the enamine (case of acceptors 10, methyl methacrylate, 24, 28, 43, 47, and 49) largely prevails over the "exo-approach" 59 (case of acceptor 38). This predominant "endo-preference" can be reasonably interpreted in terms of a cooperative effect between steric and stereoelectronic factors.     

Preferred orientation in filtercakes of kaolinite

The orientational order parameter of dense colloidal dispersions of plate-like particles as a function of volume fraction is measured using neutron diffraction. This non-invasive experimental approach directly provides the full particle orientation distribution from which the order parameters can be calculated. The orientation parameters are shown to be linked to the solids fractions of the cakes and the macroscopic permeability of the samples. However, this study suggests that, although orientation can be relevant for a given system, other factors can have a stronger influence, for example, the degree of dispersion or colloidal stability of the clay and may be the principle factor that controls permeability. In addition, we report enhanced ordering of these materials under the influence of an external cross-flow field.     

Double domino Knoevenagel hetero Diels-Alder strategy towards bis-pyrano-1,4-benzoquinones

Several bis-pyrano-l,4-benzoquinones have been synthesized by a double domino Knoevenagel hetero Diels-Alder reaction. The synthetic approach is highly efficient allowing the construction of complex polycyclic scaffolds with six new σ-bonds. These reactions performed more efficiently and more rapidly using microwave irradiation. The resulting bis-pyrano-1,4-benzoquinones are the first examples of a double domino Knoevenagel hetero Diels-Alder reaction. Our approach represents a novel contribution to the chemistry of 2,5-dihydroxy-l,4-benzoquinones and the first general method for the synthesis of bis-pyranobenzoquinones.     

Multifunctionality in spin crossover materials

One of the most important trends in the spin crossover (SCO) field is focused on the synthesis of new molecule-based functional materials in which the SCO properties may be combined with other physical or chemical properties in a synergic fashion. The current stage of investigations regarding interplay and synergic effects between SCO, magnetic coupling, liquid crystalline properties, host-guest interactions, non-linear optical properties, electrical conductivity, and ligand isomerization is highlighted and discussed.     

Synthesis, crystal structure, spectroscopic and electrochemical characterization of the dinuclear complex {tetra-μ-[(±)-2-(p-methoxyphenoxy)-propionato-O,O′]bis(aqua)dicopper(II)}

Structural, electrochemical and spectroscopic data of a new dinuclear copper(II) complex with (±)-2-(p-methoxyphenoxy)propionic acid are reported. The complex {tetra-μ-[(±)-2-(p-methoxyphenoxy)propionato-O,O′]-bis(aqua)dicopper(II)} crystallizes in the monoclinic system, space group P2₁/n with a = 14.149(1) ?, b = 7.495(1) ?, c = 19.827(1) ?, β = 90.62(1) and Z = 4. X-ray diffraction data show that the two copper(II) ions are held together through four carboxylate bridges, coordinated as equatorial ligands in square pyramidal geometry. The coordination sphere around each copper ion is completed by two water molecules as axial ligands. Thermogravimetric data are consistent with such results. The ligand has an “L” type shape due to the angle formed by the β-carbon of the propionic chain and the linked p-methoxyphenoxy group. This conformation contributes to the occurrence of a peculiar structure of the complex. The complex retains its dinuclear nature when dissolved in acetonitrile, but it decomposes into the corresponding mononuclear species if dissolved in ethanol, according to the EPR measurements. Further, cyclic voltammograms of the complex in acetonitrile show that the dinuclear species maintains the same structure, in agreement with the EPR data in this solvent. The voltammogram shows two irreversible reduction waves at E _pc = −0.73 and −1.04 V vs. Ag/AgCl assigned to the Cu(II)/Cu(I) and Cu(I)/Cu° redox couples, respectively, and two successive oxidation waves at E _pa =− 0.01 and +1.41 V vs. Ag/AgCl, assigned to the Cu°/Cu(I) and Cu(I)/Cu(II) redox couples, respectively, in addition to the oxidation waves of the carboxylate ligand.     

Computationally-assisted approach to the vibrational spectra of molecular crystals: study of hydrogen-bonding and pseudo-polymorphism.

A new computationally-assisted methodology (PiMM), which accounts for the effects of intermolecular interactions in the crystal, is applied to the complete assignment of the Raman and infrared vibrational spectra of room temperature forms of crystalline caffeine, theobromine, and theophylline. The vibrational shifts due to crystal packing interactions are evaluated from ab initio calculations for a set of suitable molecular pairs, using the B3LYP/6-31G* approach.The proposed methodology provides an answer to the current demand for a reliable assignment of the vibrational spectra of these methyl-xanthines, and clarifies several misleading assignments. The most relevant intermolecular interactions in each system and their effect on the vibrational spectra are considered and discussed. Based on these results, significant insights are obtained for the structure of caffeine in the anhydrous form (stable at room temperature), for which no X-ray structure has been reported. A possible structure based on C((8))--H...N((9)) and C((1,3))--H...O intermolecular interactions is suggested.     

Determination of manganese and zinc in powdered chocolate samples by slurry sampling using sequential multi-element flame atomic absorption spectrometry

In the present work, a slurry sampling flame atomic absorption spectrometric method to determine directly manganese and zinc in powdered chocolate samples is proposed. The optimization step was performed using univariate methodology involving the following factors: nature and concentration of the acid solution, sonication time, and particle size. The established conditions led to the use of a sample mass of 150 mg, 2.0 mol L^− 1 nitric acid solution, sonication time of 15 min, and a slurry volume of 50 mL. This method allows the determination of manganese and zinc with detection limit of 52 and 61 ng g^− 1, respectively, and a precision expressed as relative standard deviation (RSD) of 2.6% and 3.2% (both, n = 10) for contents of manganese and zinc of 52.4 and 100.0 μg g^− 1, respectively. The proposed method was applied for determination of manganese and zinc in five powdered chocolate samples. In these, the manganese content varied from 42.8 to 52.7 and from 88.6 to 102.4 μg g^− 1 of zinc. The analytical results were compared with the results obtained by analysis of these samples after digestion using open vessel and acid bomb digestion procedures and determination using FS-FAAS. The statistical comparison by t-test (95% confidence level) showed no significant difference between these results.     

Unprecedented base effect on the synthesis of chiral phosphinate esters: A new route to P-chiral phosphine oxides of high enantiomeric purity

The stereochemical outcome of the reaction of chiral secondary alcohols with a phosphinyl chloride was found to be highly dependent on the achiral base used. Thus, the reaction of the readily available sugar derived carbinols, 1 and 2, with methylphenylphosphinyl chloride in the presence of triethylamine yields stereoselectively the corresponding Snp-phosphinates 3Sp and 5Sp in 94 and 92% diastereomeric excess (de). Simply changing the base from triethylamine to pyridine affords Rp-phosphinates 4Rp and 6Rp epimers to 3Sp and 5Sp at the phosphinyl phosphorus in 50 and 40% de respectively. These phosphinate esters were found to be good P-chiral transferring intermediates, they react with Grignard reagents under very mild conditions to give the corresponding phosphine oxides. Both enantiomers Sp- and Rp-o-anisylmethylphenylphosphine oxide (PAMPO) as well Sp- and Rp- methylphenylpropyl phosphine oxide were obained enantiomerically pure in high yields     

Stereoselective synthesis of substituted exo-glycals from 1-exo-methylene pyranoses

Halo-exo-glycals of the gluco-, manno- and galacto- series, readily prepared by reaction of 1-exo-methylene pyranoses with iodonium dicollidinium triflate (IDCT), undergo Suzuki or B-alkyl Suzuki cross-coupling reactions with boronic acids or alkyl boranes to yield, in a stereoselective manner, functionalized exo-glycals.     

Self-selection in olefin cross-metathesis: the effect of remote functionality

Olefin cross-metathesis (CM) is potentially an attractive method for generating dynamic combinatorial libraries (DCLs). In order for the CM reaction to be useful for DCL production, the course of the reaction and product distribution must be relatively insensitive to functionality remote from the reacting centers. We report on the CM of a series of allyl- and homoallylamides that are strongly dependent on remote functionality. This includes an unusual example of a cis-selective CM. [Reaction: see text]