Enantioselective organocatalytic aza-ene-type domino reaction leading to 1,4-dihydropyridines

A new general methodology was developed to access highly enantiomerically enriched 1,4-dihydropyridines (DHPs) 3 via an organocatalytic asymmetric aza-ene-type cascade reaction, cocatalyzed by (S)-diarylprolinol-TMS ether V and benzoic acid (BA). Both aliphatic and aryl enals 1 reacted smoothly with enaminones and β-enamino esters 2, affording highly functionalized 1,4-DHPs 3 in high enantioselectivities and good yields.     

Protonation of CH3N3 and CF3N3 in Superacids: Isolation and Structural Characterization of Long‐Lived Methyl‐ and Trifluoromethylamino Diazonium Ions

The methylamino diazonium cations [CH₃N(H)N₂]⁺ and [CF₃N(H)N₂]⁺ were prepared as their low‐temperature stable [AsF₆]^? salts by protonation of azidomethane and azidotrifluoromethane in superacidic systems. They were characterized by NMR and Raman spectroscopy. Unequivocal proof of the protonation site was obtained by the crystal structures of both salts, confirming the formation of alkylamino diazonium ions. The Lewis adducts CH₃N₃?AsF₅ and CF₃N₃?AsF₅ were also prepared and characterized by low‐temperature NMR and Raman spectroscopy, and also by X‐ray structure determination for CH₃N₃?AsF₅. Electronic structure calculations were performed to provide additional insights. Attempted electrophilic amination of aromatics such as benzene and toluene with methyl‐ and trifluoromethylamino diazonium ions were unsuccessful.     

Electrochemical Modification of Gold Electrodes with Azobenzene Derivatives by Diazonium Reduction

An electrochemical study of Au electrodes electrografted with azobenzene (AB), Fast Garnet GBC (GBC) and Fast Black K (FBK) diazonium compounds is presented. Electrochemical quartz crystal microbalance, ellipsometry and atomic force microscopy investigations reveal the formation of multilayer films. The elemental composition of the aryl layers is examined by X‐ray photoelectron spectroscopy. The electrochemical measurements reveal a quasi‐reversible voltammogram of the Fe(CN)₆^3?/4? redox couple on bare Au and a sigmoidal shape for the GBC‐ and FBK‐modified Au electrodes, thus demonstrating that electron transfer is blocked due to the surface modification. The electrografted AB layer results in strongest inhibition of the Fe(CN)₆^3?/4? response compared with other aryl layers. The same tendencies are observed for oxygen reduction; however, the blocking effect is not as strong as in the Fe(CN)₆^3?/4? redox system. The electrochemical impedance spectroscopy measurements allowed the calculation of low charge‐transfer rates to the Fe(CN)₆^3? probe for the GBC‐ and FBK‐modified Au electrodes in relation to bare Au. From these measurements it can be concluded that the FBK film is less compact or presents more pinholes than the electrografted GBC layer.     

Intermolecular repulsive-dispersive potentials explain properties of impurity spectra in soft solids

Behavior of optical impurity spectra in van der Waals glasses is rationalized with the aid of a two-particle Lennard-Jones model of intermolecular interactions. Simple mathematical manipulations with the 6-12 potential yield inhomogeneous distribution functions (IDFs) of zero phonon lines (ZPLs) at different compressions, and the expressions for wavelength dependent pressure shift coefficients of ZPLs (or holes), local phonon frequencies, and linear and quadratic coupling constants. Experimentally, the ZPL to phonon wing intensity ratios (Huang-Rhys or Debye-Waller factors) are measured for bacteriochlorophyll a in glass-forming triethylamine 5 K. Enhancement of coupling strength with increasing transition wavelength is observed, in qualitative agreement with the model.     

Existence of analytic solutions for the classical Stefan problem

We prove that under mild regularity assumptions on the initial data the two-phase classical Stefan problem admits a (unique) solution that is analytic in space and time.     

Zero-field optical detected magnetic resonance of [70]-fullerene

The phosphorescence spectrum of [70]-fullerene in a glassy toluene matrix has been investigated at 77, 4.2 and 1.2 K. At 77 K, only one decay time (47.0 ms) has been observed while at 4.2 and 1.2 K two decay times (46.7, 5.6 ms and 46.6, 24.2 ms, respectively) were recorded. The zero field splitting parameters |D|=0.00489(25) cm^?1 and |E|=0.00135(7) cm^?1 were measured at 1.2 K using a pulsed ODMR technique.     

Stability in a population model without random deaths by the Verhulst factor

A large amount of population models use the concept of a carrying capacity. Simulated populations are bounded by invoking finite resources through a survival probability, commonly referred to as the Verhulst factor. The fact, however, that resources are not easily accounted for in actual biological systems makes the carrying capacity parameter ill-defined. Henceforth, we deem it essential to consider cases for which the parameter is unnecessary. This work demonstrates the possibility of Verhulst-free steady states using the Penna aging model, with one semelparous birth per adult. Stable populations are obtained by setting a mutation threshold that is higher than the reproduction age.     

Surface modification of gold electrodes with anthraquinone diazonium cations

Electrochemical grafting of anthraquinone (AQ) groups to gold electrodes was carried out in acetonitrile (ACN) and in aqueous acidic media (0.05 M H₂SO₄). For the first time, the covalent attachment of AQ to gold is demonstrated. Electrochemical quartz crystal microbalance (EQCM), atomic force microscopy (AFM) and cyclic voltammetry (CV) were used to characterise the AQ-modified Au electrodes. Electrografting from both solutions containing the corresponding diazonium salt yielded a strongly attached AQ layer. AFM examination showed that a multilayer AQ film was formed. The CV behaviour of the modified electrodes was tested in 0.1 M KOH and a quasi-reversible response was observed.     

Comparison of two implementations of the incremental construction algorithm in flexible docking of thrombin inhibitors

A set of 32 known thrombin inhibitors representing different chemical classes has been used to evaluate the performance of two implementations of incremental construction algorithms for flexible molecular docking: DOCK 4.0 and FlexX 1.5. Both docking tools are able to dock 10–35% of our test set within 2 Å of their known, bound conformations using default sampling and scoring parameters. Although flexible docking with DOCK or FlexX is not able to reconstruct all native complexes, it does offer a significant improvement over rigid body docking of single, rule-based conformations, which is still often used for docking of large databases. Docking of sets of multiple conformers of each inhibitor, obtained with a novel protocol for diverse conformer generation and selection, yielded results comparable to those obtained by flexible docking. Chemical scoring, which is an empirically modified force field scoring method implemented in DOCK 4.0, outperforms both interaction energy scoring by DOCK and the Böhm scoring function used by FlexX in rigid and flexible docking of thrombin inhibitors. Our results indicate that for reliable docking of flexible ligands the selection of anchor fragments, conformational sampling and currently available scoring methods still require improvement.     

Study of the Influence of the Nature and Position of the Halogen Atom in the N-Halogenophenyl-2,5-dimethylpyrrole Upon the Vibrational (Hyper)polarizability: Theoretical Study

The two contributions, vibrational and electronic, to the electrical properties polarizability and first hyperpolarizability of the N-[(2, 3, or 4)-fluorophenyl]-2,5-dimethylpyrrole are evaluated theoretically at the HF/6-31G level within the double harmonic oscillator approximation. The calculations demonstrate that, with the exception of the second harmonic generation, the vibrational contribution to the first hyperpolarizability is important. However, the vibrational polarizability, contributes at most, 10% to the total electric polarizability. The analysis upon the sum-over-states expressions shows that there are only few modes which contribute strongly and, generally, have small vibrational energies. The effect of the fluorine substitution by an other substituent is also addressed.