Stereocontrolled alkylation of unsaturated compounds with alkoxytitanacyclopropane reagents

Interaction of titanium(IV) isopropoxide with ethylmagnesium bromide and its higher homologs leads to the generation of alkoxytitanacyclopropane species, which are able to act in the reactions with unsaturated compounds as 1,2‐dicarbanionic alkylating agents. The present review is focused on the processes of intermolecular regio‐ and stereoselective alkylation of carboxylic esters and allylic alcohol derivatives with alkoxytitanacyclopropane reagents. The oxophilicity of the titanium atom and its tendency to form sterically crowded ate complexes are probably the main factors providing a high level of selectivity of these transformations. © 2008 The Japan Chemical Journal Forum and Wiley Periodicals, Inc. Chem Rec 8: 269–278; 2008: Published online in Wiley InterScience ( www.interscience.wiley.com ) DOI 10.1002/tcr.20155     

Badly approximable points on planar curves and a problem of Davenport

Let \({\mathcal {C}}\) be two times continuously differentiable curve in \({\mathbb {R}}^2\) with at least one point at which the curvature is non-zero. For any \(i,j \geqslant 0\) with \(i+j =1\) , let \({\mathbf {Bad}}(i,j)\) denote the set of points \((x,y) \in {\mathbb {R}}^2\) for which \( \max \{ \Vert qx\Vert ^{1/i}, \, \Vert qy\Vert ^{1/j} \} > c/q \) for all \( q \in {\mathbb {N}}\) . Here \(c = c(x,y)\) is a positive constant. Our main result implies that any finite intersection of such sets with \({\mathcal {C}}\) has full Hausdorff dimension. This provides a solution to a problem of Davenport dating back to the sixties.     

Mechanochemistry-Amended Barbier Reaction as an Expedient Alternative to Grignard Synthesis**

Organomagnesium halides (Grignard reagents) are essential carbanionic building blocks widely used in carbon-carbon and carbon-heteroatom bond-forming reactions with various electrophiles. In the Barbier variant of the Grignard synthesis, the generation of air- and moisture-sensitive Grignard reagents occurs concurrently with their reaction with an electrophile. Although operationally simpler, the classic Barbier approach suffers from low yields due to multiple side reactions, thereby limiting the scope of its application. Here, we report a mechanochemical adaptation of the Mg-mediated Barbier reaction, which overcomes these limitations and facilitates the coupling of versatile organic halides (e.g., allylic, vinylic, aromatic, aliphatic) with a diverse range of electrophilic substrates (e.g., aromatic aldehydes, ketones, esters, amides, O-benzoyl hydroxylamine, chlorosilane, borate ester) to assemble C−C, C−N, C−Si, and C−B bonds. The mechanochemical approach has the advantage of being essentially solvent-free, operationally simple, immune to air, and surprisingly tolerant to water and some weak Brønsted acids. Notably, solid ammonium chloride was found to improve yields in the reactions of ketones. Mechanistic studies have clarified the role of mechanochemistry in the process, indicating the generation of transient organometallics facilitated by improved mass transfer and activation of the surface of magnesium metal.     

In the footsteps of August Michaelis: Syntheses and Thermodynamics of Extremely Low-Volatile Ionic Liquids

A series of nine different known ionic liquids or low melting salts was synthesised and purified. They are composed of the [NTf₂]^– (bis(trifluoromethane)sulfonimide), [OTf]^– (trifluoro-methane-sulfonate), or [B(CN)₄]^– (tetracyanidoborate) anion and [Ph₄P]⁺ (tetraphenylphosphonium), [Ph₃BzP]⁺ (triphenylbenzyl phosphonium), [ⁿBu₄P]⁺ (tetra-ⁿbutylphosphonium), [ⁿBuPh₃P]⁺ (tri-phenyl-ⁿbutylphosphonium), [ⁿBu₄N]⁺ (tetra-ⁿbutylammonium), or the [PPN]⁺ (bis(triphenylphosphine)iminium) cation. Precise vapour pressure data and enthalpies of vaporisation were measured using the Quartz Crystal Microbalance (QCM) method and evaluated. Structure-property relations are established using the obtained data as well as literature known data of ILs with alkyl-substituted imidazolium cations. It turns out that ILs with the tetracyanidoborate anion have even higher values of the enthalpy of vaporisation than those with the common [NTf₂]⁻ or [OTf]⁻ anion and therefore are even less volatile.     

The Interaction between Polycationic Poly-Lysine Dendrimers and Charged and Neutral Fluorescent Probes

The interactions between polycationic poly-lysine dendrimers and hydrophobic fluorescent probes (anionic ANS and neutral Prodan) were studied. R121 and R131 dendrimers were not able to interact with anionic and neutral hydrophobic groups. R124 was able to interact with neutral and anionic hydrophobic fluorescent probes, however mainly through hydrophobic forces. Dendrimers R155 and R169 showed the maximal effects. The strongest interactions observed for R169 can be explained by intramolecular folding (stacking) of its two L-proline residues. Using double fluorescence titration technique for ANS probe allowed to receive such constant of binding and the number of binding centers: for R121, 1.8·10³ (mol/l)⁻¹ and 1.07; for R124, 12.1·10³ (mol/l)⁻¹ and 0.48; for R131, 4.7·10³ (mol/l)⁻¹ and 0.48; for R155, 9.2·10³ (mol/l)⁻¹ and 1.36; for R169, 39.6·10³ (mol/l)⁻¹ and 0.97. Thus, neutral and anionic hydrophobic probes can be used for the fast preliminary screening of binding properties of newly synthesized polycationic dendrimers.     

Vaporization enthalpies of imidazolium based ionic liquids: dependence on alkyl chain length

Vaporization enthalpies of a series of ten 1-alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ionic liquids (ILs) [C(n) mim][NTf(2) ] with alkyl chain lengths of n=2, 3, 4, 6, 8, 10, 12, 14, 16, and 18 are determined by using a recently developed quartz crystal microbalance method. Due to the high sensitivity of the microbalance vapor studies can be extended to temperatures 60-100 K lower than those available with other methods. The results reveal a remarkably linear dependence of the vaporization enthalpies on the chain length at the reference temperature of 298 K.     

The influence of hydrogen bonding on the physical properties of ionic liquids

Potential applications of ionic liquids depend on the properties of this class of liquid material. To a large extent the structure and properties of these Coulomb systems are determined by the intermolecular interactions among anions and cations. In particular the subtle balance between Coulomb forces, hydrogen bonds and dispersion forces is of great importance for the understanding of ionic liquids. The purpose of the present paper is to answer three questions: Do hydrogen bonds exist in these Coulomb fluids? To what extent do hydrogen bonds contribute to the overall interaction between anions and cations? And finally, are hydrogen bonds important for the physical properties of ionic liquids? All these questions are addressed by using a suitable combination of experimental and theoretical methods including newly synthesized imidazolium-based ionic liquids, far infrared spectroscopy, terahertz spectroscopy, DFT calculations, differential scanning calorimetry (DSC), viscometry and quartz-crystal-microbalance measurements. The key statement is that although ionic liquids consist solely of anions and cations and Coulomb forces are the dominating interaction, local and directional interaction such as hydrogen bonding has significant influence on the structure and properties of ionic liquids. This is demonstrated for the case of melting points, viscosities and enthalpies of vaporization. As a consequence, a variety of important properties can be tuned towards a larger working temperature range, finally expanding the range of potential applications.     

Unusual Photoinduced Response of mTHPC Liposomal Formulation (Foslip)

Liposomal formulations of meso-tetra(hydroxyphenyl)chlorin (mTHPC) have already been proposed with the aim to optimize photodynamic therapy. Spectral modifications of these compounds upon irradiation have not yet been investigated. The objective of this study was to evaluate photobleaching properties of mTHPC encapsulated into dipalmitoylphosphatidylcholine (DPPC) liposomes, Foslip. Fluorescence measurements in DPPC liposomes with different DPPC:mTHPC ratios demonstrated a dramatic decrease in fluorescence anisotropy with increasing local mTHPC concentration, thus suggesting strong interactions between mTHPC molecules in lipid bulk medium. Exposure of Foslip suspensions to small light doses (<50 mJ/cm²) resulted in a substantial drop in fluorescence, which, however, was restored after addition to the sample of a non-ionic surfactant Triton X-100. We attributed this behavior to photoinduced fluorescence quenching. This effect depended strongly on the molar DPPC:mTHPC ratio and was revealed only for high local mTHPC concentrations. The results were interpreted supposing energy migration between closely located mTHPC molecules with its subsequent dissipation by the molecules of photoproduct acting as excitation energy traps. We further assessed the effect of photoinduced quenching in plasma protein solution. Relatively slow kinetics of photoinduced Foslip response during incubation in the presence of proteins was attributed to mTHPC redistribution from liposomal formulations to proteins. Therefore, changes in mTHPC distribution pattern in biological systems would be consistent with changes in photoinduced quenching and would provide valuable information on mTHPC interactions with a biological environment.     

Efficient implementation of periodic boundary conditions in Monte Carlo simulation

Determination of the shortest distances between particles is one of the most time-consuming parts of molecular simulation by the Monte Carlo method. In this work, we demonstrate that the use of signed-integer storage of coordinates in a scaled box allows one to skip multiple conditional statements in realization of periodic boundary conditions in cubic and rectangular boxes, which, in turn, increases the performance. Performance of the improved procedure was tested in NVT Monte Carlo simulations for liquid krypton and water. © 2018 Wiley Periodicals, Inc.