C. Christoph Tzschucke

Modern Separation Techniques for the Efficient Workup in Organic Synthesis

The shift of paradigm in combinatorial chemistry, from large compound libraries (of mixtures) on a small scale towards defined compound libraries where each compound is prepared in an individual well, has stimulated the search for alternative separation approaches. The key to a rapid and efficient synthesis is not only the parallel arrangement of reactions, but simple work-up procedures so as to circumvent time-consuming and laborious purification steps. During the initial development stages of combinatorial synthesis it was believed that rational synthesis of individual compounds could only be achieved by solid-phase strategies. However, there are a number of problems in solid-phase chemistry: most notably there is the need for a suitable linker unit, the limitation of the reaction conditions to certain solvents and reagents, and the heterogeneous reaction conditions. Further disadvantages are: the moderate loading capacities of the polymeric support and the limited stability of the solid support. In the last few years several new separation techniques have been developed. Depending on the chemical problem or the class of compounds to be prepared, one can choose from a whole array of different approaches. Most of these modern separation approaches rely on solution-phase chemistry, even though some of them use solid-phase resins as tools (for example, as scavengers). Several of these separation techniques are based on liquid-liquid phase separation, including ionic liquids, fluorous phases, and supercritical solvents. Besides being benign with respect to their environmental aspects, they also show a number of advantages with respect to the work-up procedures of organic reactions as well as simplicity in the isolation of products. Another set of separation strategies involves polymeric supports (for example, as scavengers or for cyclative cleavage), either as solid phases or as soluble polymeric supports. In contrast to solid-phase resins, soluble polymeric supports allow reactions to be performed under homogeneous conditions, which can be an important factor in catalysis. At the same time, a whole set of techniques has been developed for the separation of these soluble polymeric supports from small target molecules. Finally, miscellaneous separation techniques, such as phase-switchable tags for precipitation by chemical modification or magnetic beads, can accelerate the separation of compounds in a parallel format.

Non-Covalently Solid-Phase Bound Catalysts for Organic Synthesis

Catalysts immobilized on solid supports have become valuable tools for simplified product isolation and catalyst recycling. An alternative to covalent attachment to a solid support is to immobilize catalysts by non-covalent bonding through hydrogen bridges, or ionic, hydrophobic or fluorous interactions. Compared to covalent attachment, such non-covalent approaches increase the flexibility in the choice of the support material, reaction conditions and work-up strategies. Numerous catalytic reactions employing one of these non-covalent bonding strategies have meanwhile appeared in the literature.

Supramolecular Hydrogel Capsules Based on PEG: A Step Toward Degradable Biomaterials with Rational Design

Supramolecular microgel capsules based on polyethylene glycol (PEG) are a promising class of soft particulate scaffolds with tailored properties. An approach to fabricate such particles with exquisite control by droplet-based microfluidics is presented. Linear PEG precursor polymers that carry bipyridine moieties on both chain termini are gelled by complexation to iron(II) ions. To investigate the biocompatibility of the microgels, living mammalian cells are encapsulated within them. The microgel elasticity is controlled by using PEG precursors of different molecular weights at different concentrations and the influence of these parameters on the cell viabilities, which can be optimized to exceed 90% is studied. Reversion of the supramolecular polymer cross-linking allows the microcapsules to be degraded at mild conditions with no effect on the viability of the encapsulated and released cells.

Conversion of pyridine N-oxides to tetrazolopyridines.

An efficient and convenient procedure for the conversion of pyridine N-oxides to tetrazolopyridines by treatment with 4-toluene sulfonyl chloride and sodium azide in toluene at elevated temperature is described.

Synthesis of Trifluorostyrene Derivatives by Palladium‐Catalyzed Cross‐Coupling of Lithium Trimethoxy(trifluorovinyl)borate with Aryl Bromides

Trifluorostyrene (TFS) derivatives have been widely used as monomers for the preparation of polymers with fluorinated backbones. However, they have seen little use in other research fields, such as medicinal chemistry, probably due to the lack of a convenient preparative method. Most previous syntheses of TFS derivatives have involved multistep reaction sequences, often involving unstable reagents or intermediates and suffering from low overall yields. g,3] As a more direct synthetic route, the palladium-catalyzed crosscoupling of trifluorovinyl zinc reagents or trifluorovinyl tin reagents with aryl iodides has been described. 4] Recently, a palladium-catalyzed reaction of tetrafluoroethylene (TFE) with arylzinc compounds has been reported to give TFS derivatives in moderate yields. Although these catalytic methods are more efficient than other multistep approaches, their broad applicability suffers from the use of toxic and unstable reagents, such as organostannanes, organozinc reagents, or gaseous and explosive tetrafluoroethylene. Organoboron compounds have found widespread use in palladium-catalyzed Suzuki–Miyaura coupling reactions because they are easy to prepare, stable, and have relatively low toxicity. In the context of TFS derivatives only two examples for the coupling of potassium (trifluorovinyl)trifluoroborate have been described so far. Very recently, the coppermediated coupling of potassium (trifluoromethyl)trimethoxyborate with aryl iodides has been reported. Herein, we report an efficient and convenient method for the introduction of a trifluorovinyl group based on the palladium-catalyzed Suzuki–Miyaura cross-coupling reaction. This procedure employs stable borate 1 as the source of the trifluorovinylgroup and, thus, circumvents the inconvenient use of highly unstable TFE, toxic stannanes, or sensitive zinc reagents (Scheme 1).

Monitoring the Transmembrane Proton Gradient Generated by Cytochrome bo3 in Tethered Bilayer Lipid Membranes Using SEIRA Spectroscopy.

Membrane proteins act as biocatalysts or ion/proton pumps to convert and store energy from ubiquitous environmental sources. Interfacing these proteins to electrodes allows utilizing the energy for enzymatic biofuel cells or other auspicious biotechnological applications. To optimize the efficiency of these devices, appropriate membrane models are required that ensure structural and functional integrity of the embedded enzymes and provide structural insight. We present a spectroelectrochemical surface-enhanced infrared absorption (SEIRA) and electrical impedance spectroscopy (EIS) study of the bacterial respiratory ubiquinol/cytochrome bo3 (cyt bo3) couple incorporated into a tethered bilayer lipid membrane (tBLM). Here, we employed a new lipid tether (WK3SH, dihydrocholesteryl (2-(2-(2-ethoxy)ethoxy)ethanethiol), which was synthesized using a three-step procedure with very good yield and allowed measuring IR spectra without significant spectral interference of the tBLM. The functional integrity of the incorporated cyt bo3 was demonstrated by monitoring the enzymatic O2 reduction current and the formation of the transmembrane proton gradient. Based on a SEIRA-spectroscopic redox titration, a shift of the pH-dependent redox potential of the ubiquinones under turnover conditions was correlated with an alkalinization of the submembrane reservoir by +0.8 pH units. This study demonstrates the high potential of tBLMs and the SEIRA spectroscopic approach to study bioenergetic processes.

Polyglycerol-Based Copper Chelators for the Transport and Release of Copper Ions in Biological Environments.

Here, the synthesis and characterization of three improved nanosystems is presented based on amino functionalized hyperbranched polyglycerol (hPG; M(w) = 16.8 kDa) as potential copper(II) chelators. The ligands, N-methyl-N-picolylglycine amide, 2,6-pyridine dicarboxylic acid monoamide, and cyclam tetraacetic acid (TETA) monoamide, are covalently attached to the polymer with amide bonds. In this paper, the Cu(II) loading capacity, the stability of the Cu(II)-loaded carriers at different pHs, with competing ligands and in human serum, as well as the transport of Cu(II) in biological systems are investigated. For the first time, a different cytotoxicity of functionalized polymer nanoparticles with and without Cu(II) is observed. The cyclam-based carrier combines the highest loading capacity (29 Cu ions/nanoparticle), best stability with respect to pH and EDTA (45% remaining Cu after 24 h), lowest cytotoxicity (IC50 > 100 × 10(-6) M (unloaded), 1500 × 10(-6) M Cu(II); Cu:carrier 29:1), and the highest stability in human serum.

Electrocatalytic Carbon Dioxide Reduction by Using Cationic Pentamethylcyclopentadienyl–Iridium Complexes with Unsymmetrically Substituted Bipyridine Ligands

Eight [Ir(bpy)Cp*Cl](+) -type complexes (bpy= bipyridine, Cp*=1,2,3,4,5-pentamethylcyclopentadienyl) containing differently substituted bipyridine ligands were synthesized and characterized. Cyclic voltammetry (CV) of the complexes in Ar-saturated acetonitrile solutions showed that the redox behavior of the complexes could be fine tuned by the electronic properties of the substituted bipyridine ligands. Further CV in CO2 -saturated MeCN/H2 O (9:1, v/v) solutions showed catalytic currents for CO2 reduction. In controlled potential electrolysis experiments (MeCN/MeOH (1:1, v/v), Eapp =-1.80 V vs Ag/AgCl), all of the complexes showed moderate activity in the electrocatalytic reduction of CO2 with good stability over at least 15 hours. This electrocatalytic process was selective toward formic acid, with only traces of dihydrogen or carbon monoxide and occasionally formaldehyde as byproducts. However, the turnover frequencies and current efficiencies were quite low. No direct correlation between the redox potentials of the complexes and their catalytic activity was observed.

Enantiopure 2-substituted glyceraldehyde derivatives by aza-Claisen rearrangement or C-alkylation of enamines.

2-Alkyl derivatives of butane-2,3-diacetal (BDA) protected glyceraldehyde were stereoselectively prepared by aza-Claisen rearrangement of N-allyl-enammonium ions or C-alkylation of enamines. This allows rapid and convenient access to densely functionalized chiral building blocks.

Application of Non-Covalently Solid-Phase Bound Catalysts

Supported catalysts have become valuable tools for simplified product isolation and catalyst recycling. The common method is covalent attachment to a solid support. An alternative strategy is to immobilize catalysts by non-covalent bonding through hydrogen bridges, ionic, hydrophobic or fluorous interactions. Compared to covalent attachment, such non-covalent approaches increase the flexibility in the choice of the support-material, reaction conditions and work-up strategies. Numerous catalytic reactions employing one of these non-covalent fixation strategies have meanwhile appeared in the literature.