The importance of alkali cations in the [{RuCl2(p-cymene)}2]-pseudo-dipeptide-catalyzed enantioselective transfer hydrogenation of ketones

We studied the role of alkali cations in the [{RuCl2(p-cymene)}2]-pseudo-dipeptide-catalyzed enantioselective transfer hydrogenation of ketones with isopropanol. Lithium salts were shown to increase the enantioselectivity of the reaction when iPrONa or iPrOK was used as the base. Similar transfer-hydrogenation systems that employ chiral amino alcohol or monotosylated diamine ligands are not affected by the addition of lithium salts. These observations have led us to propose that an alternative reaction mechanism operates in pseudo-dipeptide-based systems, in which the alkali cation is an important player in the ligand-assisted hydrogen-transfer step. DFT calculations of the proposed transition-state (TS) models involving different cations (Li+, Na+, and K+) confirm a considerable loosening of the TS with larger cations. This loosening may be responsible for the fewer interactions between the substrate and the catalytic complex, leading to lower enantiodifferentiation. This mechanistic hypothesis has found additional experimental support; the low ee obtained with [BnNMe3]OH (a large cation) as base can be dramatically improved by introducing lithium cations into the system. Also, the complexation of Na+, K+, and Li+ cations by the addition of [15]crown-5 and [18]crown-6 ethers and cryptand 2.1.1 (which selectively bind to these cations and, thus, increase their bulkiness), respectively, to the reaction mixture led to a significant drop in the enantioselectivity of the reaction. The lithium effect has proved useful for enhancing the reduction of different aromatic and heteroaromatic ketones.     

Study of synthesis of 2‐(2‐alkoxyphenyl)‐1H‐imidazoles. Comparison of oxidative aromatization reactions of imidazolines

The reaction of methyl salicylate with ethane‐1,2‐diamine has been used to prepare 2‐(2‐hydroxyphenyl)‐1H‐imidazoline. This compound was alkylated with alkyl halides to give five new 2‐(2‐alkoxyphenyl)‐1H‐imidazolines (alkyl = propyl, isopropyl, isobutyl, sec‐butyl, benzyl). Seven types of transformation reactions of imidazolines into the respective imidazoles were tested. Out of them successful were the dehydrogenation on palladium in toluene (several‐day refluxing), oxidation with activated manganese dioxide in toluene (several‐hour heating at 60 °C), and the oxidation with potassium nitrosodisulfonate (Fremy's salt) at room temperature. Seven new 2‐(2‐alkoxyphenyl)‐1H‐imidazoles were synthesized (alkyl = ethyl, propyl, isopropyl, butyl, isobutyl, sec‐butyl, benzyl) via mentioned methods. Comparison of individual oxidative aromatization reactions is discussed from the point of view of experimental arrangement, reaction time and conditions, purity of the products obtained, and yields.     

Validation of the extended Tanaka column characterization protocol by multivariate analysis of chromatographic retention of low‐molecular‐weight analytes on reversed phase columns using methanol and acetonitrile as organic modifiers

The validity of the extended Tanaka column characterization procedure against the retention behavior of 101 analytes of widely differing properties chromatographed on five differing stationary phase chemistries has been established using a chemometric technique called principal component analysis (PCA). It was concluded that the simple and conveniently determined column characterization parameters covered the same space in the PCA loading plot as the retention times for the 101 differing analytes. This confirms that the ten column characterization parameters of the extended Tanaka protocol encode the same information as the retention times of the 101 analytes. Significant selectivity differences were observed between stationary phases and the mobile‐phase modifiers – MeOH and MeCN. PCA contribution plots served as a convenient way to highlight specific selectivity differences between stationary phases. logD values exhibited a poor correlation with retention indicating that retention in RP‐LC is not solely dictated by the analyte's hydrophobicity. The use of MeOH was found to generate greater selectivity differences with the five stationary phases than when MeCN is used.     

EPR Studies on Branched High‐Spin Arylnitrenes

The UV (λ>305 nm) photolysis of triazide 3 in 2‐methyl‐tetrahydrofuran glass at 7 K selectively produces triplet mononitrene 4 (g=2.003, D_T=0.92 cm^?1, E_T=0 cm^?1), quintet dinitrene 6 (g=2.003, D_Q=0.204 cm^?1, E_Q=0.035 cm^?1), and septet trinitrene 8 (g=2.003, D_S=?0.0904 cm^?1, E_S=?0.0102 cm^?1). After 45 min of irradiation, the major products are dinitrene 6 and trinitrene 8 in a ratio of ～1:2, respectively. These nitrenes are formed as mixtures of rotational isomers each of which has slightly different magnetic parameters D and E. The best agreement between the line‐shape spectral simulations and the experimental electron paramagnetic resonance (EPR) spectrum is obtained with the line‐broadening parameters Γ(E_Q)=180 MHz for dinitrene 6 and Γ(E_S)=330 MHz for trinitrene 8 . According to these line‐broadening parameters, the variations of the angles Θ in rotational isomers of 6 and 8 are expected to be about ±1 and ±3°, respectively. Theoretical estimations of the magnetic parameters obtained from PBE/DZ(COSMO)//UB3LYP/6‐311+G(d,p) calculations overestimate the E and D values by 1 and 8 %, respectively. Despite the large distances between the nitrene units and the extended π systems, the zero field splitting (zfs) parameters D are found to be close to those in quintet dinitrenes and septet trinitrenes, where the nitrene centers are attached to the same aryl ring. The large D values of branched septet nitrenes are due to strong negative one‐center spin–spin interactions in combination with weak positive two‐center spin–spin interactions, as predicted by theoretical considerations.     

Some critical factors for photocatalysis on self-organized TiO2 nanotubes

In the present work, different intrinsic and extrinsic parameters are investigated that affect the photocatalytic activity of self-organized TiO₂ nanotube layers. Particularly, the influence of annealing temperature and annealing atmosphere, the influence of different gas purging in the electrolyte, and the effect of applied voltage on the photocatalytic degradation rates of acid orange (AO7) are discussed. We find that the effect of the reducing gas atmosphere dominates over the anatase/rutile ratio in activating the nanotube layers. Moreover, we show that the effect of different gas purging (Ar and O₂) of the electrolyte affects the reaction rate twofold: (1) by providing electron acceptor states and also by (2) a different change in the red–ox potential, i.e., the band bending in TiO₂. By an external anodic voltage, the reaction rates can be increased drastically due to increased band bending. Nevertheless, the magnitude of the effect is also affected by the presence or absence of O₂ in the electrolyte.     

Syntheses and modifications of bisdiazonium salts of 3,8-benzo[c]cinnoline and 3,8-benzo[c]cinnoline 5-oxide onto glassy carbon electrode and the characterization of the modified surfaces

The goal of this study was to prepare novel glassy carbon electrode surfaces using two similar bis-diazonium salts, 3,8-benzo[c]cinnoline (3,8-BCC-BDAS) and 3,8-benzo[c]cinnoline 5-oxide (3,8-BCCNO-BDAS) at the glassy carbon (GC) surface. These diazonium salts were reduced electrochemically and covalently electrografted onto the glassy carbon electrode surface to form modified electrodes. Electrochemical reduction of 3,8-BCC-BDAS and 3,8-BCCNO-BDAS salts on the electrode surface yielded a compact and stable film. The existence of BCC moieties on the GC surface was characterized by X-ray photoelectron spectroscopy, reflectance-adsorption infrared spectroscopy, cyclic voltammetry, ellipsometry, and electrochemical impedance spectroscopy. The stability and working potential range of the novel modified electrodes were also studied. The possibility of analytical application of these novel surfaces for inorganic cations and especially selectivity to copper ions was investigated. 3,8-diaminobenzo[c]cinnoline (3,8-DABCC) and its 5-oxide derivative (3,8-DABCCNO) were synthesized from the reductive cyclization of 2,2′-dinitrobenzidine and prepared their bisdiazonium salts via the tetrazotization reactions of the diamines with NaNO₂. The structures of 3,8-DABCC and 3,8-DABCCNO and their corresponding bisdiazonium salts are confirmed by spectral analysis.     

Novel pseudo-delocalized anions for lithium battery electrolytes

A novel anion concept of pseudo-delocalized anions, anions with distinct positive and negative charge regions, has been studied by a computer aided synthesis using DFT calculations. With the aim to find safer and better performing lithium salts for lithium battery electrolytes two factors have been evaluated: the cation-anion interaction strength via the dissociation reaction LiAn ? Li(+) + An(-) and the anion oxidative stability via a vertical ionisation from anion to radical. Based on our computational results some of these anions have shown promise to perform well as lithium salts for modern lithium batteries and should be interesting synthetic targets for future research.     

The Aharonov–Bohm effect in graphene rings

This is a review of electronic quantum interference in mesoscopic ring structures based on graphene, with a focus on the interplay between the Aharonov–Bohm effect and the peculiar electronic and transport properties of this material. We first present an overview on recent developments of this topic, both from the experimental as well as the theoretical side. We then review our recent work on signatures of two prominent graphene-specific features in the Aharonov–Bohm conductance oscillations, namely Klein tunneling and specular Andreev reflection. We close with an assessment of experimental and theoretical development in the field and highlight open questions as well as potential directions of the developments in future work.     

Large area single-mode parity-time-symmetric laser amplifiers

By exploiting recent developments associated with parity-time (PT) symmetry in optics, we here propose a new avenue in realizing single-mode large area laser amplifiers. This can be accomplished by utilizing the abrupt symmetry breaking transition that allows the fundamental mode to experience gain while keeping all the higher order modes neutral. Such PT-symmetric structures can be realized by judiciously coupling two multimode waveguides, one exhibiting gain while the other exhibits an equal amount of loss. Pertinent examples are provided for both semiconductor and fiber laser amplifiers.