Asymmetric Synthesis of Axially Chiral C−N Atropisomers

Molecules with restricted rotation around a single bond or atropisomers are found in a wide number of natural products and bioactive molecules as well as in chiral ligands for asymmetric catalysis and smart materials. Although most of these compounds are biaryls and heterobiaryls displaying a C−C stereogenic axis, there is a growing interest in less common and more challenging axially chiral C−N atropisomers. This review offers an overview of the various methodologies available for their asymmetric synthesis. A brief introduction is initially given to contextualize these axially chiral skeletons, including a historical background and examples of natural products containing axially chiral C−N axes. The preparation of different families of C−N based atropisomers is then presented from anilides to chiral five- and six-membered ring heterocycles. Special emphasis has been given to modern catalytic asymmetric strategies over the past decade for the synthesis of these chiral scaffolds. Applications of these methods to the preparation of natural products and biologically active molecules will be highlighted along the text.     

The hydroboration-oxidation of the isomeric trimethylsilylcyclo- hexenes and the isomeric trimethylsilylcyclopentenes.

The isomeric trimethylsilyl substituted cyclohexenes, 1-trimethylsilylcyclohexene, 3-trimethylsilylcyclohexene, and 4-trimethylsilylcyclohexene and the analogous trimethylsilyl substituted cyclopentenes, 1-trimethylsilylcyclopentene, 3-trimethylsilylcyclopentene and 4-trimethylsilylcyclopentene have been hydroborated and oxidized under kinetic and thermodynamic conditions. The hydroborating agents used were borane and thexylborane in THF. The products of these reactions show a dependence on temperature and the hydroborating agent used and are a result of a rather facile migration of the boron atom around the ring. Some synthetically useful reactions are discussed as are the structures of various trimethylsilyl substituted cyclohexanols and cyclopentanols.     

Dielectric studies of a laterally-linked siloxane ester dimer

This paper reports measurements of the dielectric response over the frequency range 10² to 10⁹?Hz of a liquid crystal dimer consisting of two ester mesogens laterally linked by an alkoxy chain containing a siloxane group. The synthesis and phase behaviour of the siloxane dimer are also reported. Results show that there are two relaxations in the isotropic phase and four in the nematic phase of the material. The possible molecular origins for these modes are given. It is found that there is a coupling between internal and external modes which gives rise to a cooperative mode as the temperature in the nematic phase is lowered towards a glass transition.     

Ozonation of Thioamide Containing Heterocycles. A New General and Selective Procedure for the Synthesis of C-2 Substituted Heteroazole Derivatives

2-Mercaptoheterazoles readily react with ozone in the presence of nucleophiles and under mild experimental conditions to form several C-2 substituted heterazoles.     

Synthesis,Characterization, and Thermal Behavior of Carboranyl–Styrene Decorated Octasilsesquioxanes: Influence of the Carborane Clusters on Photoluminescence

Novel polyhedral oligomeric silsesquioxanes (POSS) or octasilsesquioxanes with carboranyl–styrene fragments attached to each corner are described. These compounds have been synthesized by olefin‐metathesis reactions between octavinylsilsesquioxane and carboranyl–styrene compounds that possess different substituents (Ph, Me, or H). In all cases, these reactions, which were catalyzed by the Grubbs catalyst, are highly regioselective and yield exclusively the E isomers. The existence of the carborane cage in the POSS structure induces a remarkable thermal stability in these compounds. After combustion at 1000 °C, these carboranyl–POSS compounds exhibit a mass loss lower than 10 %. The UV/Vis absorption data of these carboranyl–POSS compounds shows a slight bathochromic shift with respect to the carboranyl–styrene monomers, with an absorption maximum around 262 nm. Nevertheless, important differences in the emission spectra of the carboranyl–POSS compounds with regard to their carboranyl–styrene precursors are observed; the phenyl‐o‐carborane‐containing POSS compound exhibits the highest fluorescence intensity (Φ_F=44 %), whereas for the POSS compound bearing the methyl substituent, and for the unsubstituted o‐carborane clusters, the fluorescence intensity is much lower (Φ_F=9 and 2 %, respectively). This is precisely the reverse of what occurs with the monomers, in which the unsubstituted o‐carboranyl–styrene compound exhibits the highest Φ_F, and a quenching of the fluorescence is observed in the phenyl‐o‐carboranyl–styrene compound. In addition, a large red shift of around 100 nm is observed for the POSS compounds with respect to their precursors. These experimental results can only be accounted for by the spatial ordering induced by the POSS core that eases interactions, which otherwise would not occur. These results have been confirmed by time‐dependent density functional theory (TDDFT) calculations that exclude a photoinduced electron transfer (PET) process in the POSS compounds.     

Monitoring of Zn(II) and Cd(II) adsorption on activated carbon from aqueous multicomponent solutions by differential pulse polarography (DPP)

The adsorption process of Zn(II) and Cd(II) from aqueous solution has been investigated from both kinetic and equilibrium standpoints, using differential pulse polarography (DPP) on a mercury dropping electrode as the analytical technique. With such an aim, adsorption experiments were performed using not only a single metal ion–Zn(II) or Cd(II) solution but also a multi-component ion metal–Zn(II), Cd(II) and Hg(II) solution. The influence of the pH change in the multi-component ion metal solution on the adsorption of Zn(II) and Cd(II) was also studied. The adsorption processes is relatively fast for Zn(II) and Cd(II). The presence of two foreign ions in the solution slightly speeds up the adsorption process for Zn(II) and significantly slows it down for Cd(II). The adsorption isotherms are similarly shaped for Zn(II) and Cd(II). The addition of the foreign ions has a more unfavourable effect on the adsorption for Cd(II) than for Zn(II). At pH 2, neither Zn(II) nor Cd(II) is adsorbed practically on the carbon. The voltammetric approach has proved to be a fast and efficient method that, at the same time, enables one to monitor the adsorption of Zn(II) and Cd(II) with potential on-line application, which could be useful in waste-water treatment.