Convenient, enantioselective hydrosilylation of imines in protic media catalyzed by a Zn-trianglamine complex

Chiral hexamine macrocycle derived from trans-1,2-diaminocyclohexane (DACH) in a complex with diethylzinc efficiently catalyzes the asymmetric hydrosilylation of N-phosphorylated aryl-alkyl or aryl-aryl ketimines in protic media with enantiomeric excess of the product approaching 100%. The cyclic structure of the trianglamine ligand increases the enantioselectivity and/or the yield of the reaction, in comparison to the catalysis by acyclic N,N'-dibenzyl-DACH ligands. Density functional theory (DFT) computations on the structure of the model ligand-zinc complex and on the structures of the pre-organized reactants together with the calculations of possible transition states allow rationalization of the direction of the asymmetric induction of the hydrosilylation reaction. This is the first example of asymmetric catalysis of the hydrosilylation reaction of ketimines with the use of a readily available and inexpensive macrocyclic trianglamine ligand.     

Cross metathesis approach to retinoids and other β-apocarotenoids

Cross metathesis (CM) reactions between polyenes, such as β-carotene, canthaxanthin or retinyl acetate, and various alkenes or dienes in the presence of second generation Hoveyda-Grubbs (H II) or Grubbs (G II) catalysts were investigated. Depending on the cross partner different apocarotenoids were obtained. Cross metathesis reactions of retinyl acetate proved to be fully regioselective. Carotenoid CM reactions afforded mixtures of two products due to competing cleavage of the C11-C12 and C15-C15′ double bonds. However, regioselectivity can be controlled by choice of appropriate reaction conditions. The reactions of polyenes with dienes worked better in respect of yields and diastereoselectivities than those with monounsaturated cross partners.     

Polarographic Behavior of Manganese(II) in the Presence of Oxalate Ions in Perchlorate and Sulfate Solutions

The dc polarographic method has been applied to study coordination equilibria between Mn(II) and oxalate ions in perchlorate and sulfate solutions. The stoichiometries of complexes formed in solution and those reduced at a dropping mercury electrode were established. The stability constants of the Mn(II) oxalate and sulfate complexes, as well as their diffusion coefficients, were determined at a constant ionic strength 0.5 mol⋅L⁻¹ and 25 °C. The stabilities of these Mn(II) complexes were compared with the corresponding complexes of other divalent metal ions. The polarographic method was able to identify complexes that have not been established by other methods and to determine their stability constants with high accuracy.     

Structure of self-implanted silicon annealed under enhanced hydrostatic pressure

Implantation of any ions at a sufficiently high dose and energy (E) into single-crystalline Si leads to the creation of amorphous Si (aSi), with damages peaking near the projected range (R _p) of implanted species. Enhanced hydrostatic pressure (HP) at a high temperature (HT) influences the recrystallization of aSi. The structure of self-implanted Czochralski silicon (Si⁺ dose, D=2×10¹⁶ cm^?2, E=150 keV, R _p=0.22 μm) processed for 5 h at 1400 or 1520 K under HPs up to 1.45 GPa was investigated by X-ray, secondary ion mass spectrometry and photoluminescence methods. The implantation of Si produces vacancies (V) and self-interstitials (Si_i). Vacancies and Si_is form complex defects at HT–HP, also with contaminants (e.g. oxygen, always present in Czochralski silicon). The mobility and recombination of V and Si_i as well as the kinetics of recrystallization are affected by HP, thus processing at HT–HP affects the recovery of aSi.     

Phenoxazine Based Units- Synthesis, Photophysics and Electrochemistry

A few new phenoxazine-based conjugated monomers were synthesized, characterized, and successfully used as semiconducting materials. The phenoxazine-based oligomers have low ionization potentials or high-lying HOMO levels (~4.7 eV), which were estimated from cyclic voltammetry. Conjugated oligomers offer good film??forming, mechanical and optical properties connected with their wide application. These results demonstrate that phenoxazine-based conjugated mers are a promising type of semiconducting and luminescent structures able to be used as thin films in organic electronics.     

Fine-tuning of properties of bismacrocyclic dinuclear cyclidene receptors by N-methylation

N-Methylated bismacrocyclic Cu and Ni complexes were synthesised and structurally characterised in the solid state. Their properties in solution were analysed by using NMR and ESR spectroscopies and electrochemical methods. Face-to-face biscyclidenes linked through polymethylene chains form rectangular boxlike cations. These moieties can host some small guest molecules (water, pi-electron donating compounds) and are stabilised by a shell of neighbouring counterions. For the bismacrocyclic dinuclear complexes containing two nickel or two copper ions, the intramolecular interactions between the metallic centres are strengthened through methylation of the macrocyclic components, as compared with the nonmethylated species. We report the electron coupling created by two unpaired electrons coming from two copper centres observed by ESR spectroscopy. Methylation weakens the electron-acceptor properties of the complexes, which leads to less effective binding of the pi-electron-donating guests. It also increases the stability of the lower oxidation states. In the case of the copper complexes, both Cu(II)/Cu(I) and Cu(II)/Cu(III) reversible one-electron transfers are seen in the voltammograms. These changes in properties are interpreted as the consequences of steric repulsion between the methyl substituents and the macrocyclic ring.     

Molecular equilibria investigations in 3-acetylpyridine using non-linear dielectric effect and dipole moment measurements

The dipole moment and non-linear dielectric effect for diluted solutions of 3-acetylpyridine were studied within the temperature range 288–323 K in six solvents of different polarity. Taking into account the trans-cis equilibrium and dimerization the enthalpy and entropy were calculated directly from the experimental data and under the assumption that the polarity of the environment is independent of temperature. Significant differences in the values of thermodynamical functions were found to be strongly dependent on the method used. The results obtained showed a higher stability of the polar cis conformer in more-polar environments. It was also shown that the relatively stable dimers of 3-acetylpyridine appearing in non-polar solvents when transferred to more-polar environments become unstable and their structure is no longer well defined.     

Application of the cap-pair effect in reducing the inhibition of electrode processes. The determination of zinc in the presence of tween-80

The “cap-pair” effect, which accelerates electrode processes, is shown to be useful in decreasing the effects of inhibitors on reduction processes. The determination of zinc in the presence of Tween-80 is used as an example. In the square-wave polarography of zinc at the 10^-5 M level, addition of thiourea (10 g l^-1) eliminates or greatly reduces the inhibitory action of Tween-80, thus allowing precise determinations.     

Further investigation of the mass fragmentation of dinucleotide analogues containing the 6-azauracil moiety

The mass fragmentation of four dinucleotide analogues in which uracil and 6-azauracil derivatives are connected by a trimethylene bridge is discussed. The fragmentation pathways have been proposed on the basis of high resolution data and metastable transitions. The distinct influence of alkyl substituents in the 5-position of the uracil moiety on the fragmentation pathway has been observed and a novel fragmentation of 6-azapyrimidine ring noted.