Synthesis of functionalized 2-alkylidenetetrahydrofurans by cyclization of 1,3-bis(trimethylsilyloxy)-1,3-butadienes with epoxides

The Lewis acid mediated cyclization of epoxides with 1,3-bis(trimethylsilyloxy)-1,3-butadienes, electroneutral equivalents of 1,3-dicarbonyl dianions, results in the formation of 2-alkylidenetetrahydrofurans with a great variety of substitution patterns and functional groups. This includes the synthesis of 2,3'-bifuranylidenes and 7-oxabicyclo[4.3.0]nonanes. The cyclization of dienes with functionalized epoxides containing base-labile groups proceeds with good chemoselectivity. In all reactions, good regio- and E diastereoselectivities are observed. Based on the stereoselectivities observed for reactions of 1,2-disubstituted epoxides, a working hypothesis for the mechanism of the reaction is suggested.     

Enediynes in 11-membered rings. Synthesis, structure, and reactivity of highly strained but unusually stable macrocycles.

The synthesis of two new aromatic cyclic enediynes 1a and 1b using a double Wittig condensation, followed by bromination and di-dehydrobromination, is reported. The structures were determined using X-ray crystallography. The observed C(1)-C(6) distances of the enediyne moiety in the oxygen- and sulfur-containing eleven-membered rings are quite short with 3.44 and 3.50 A, respectively, indicating the highly strained nature of these compounds. Isodesmic calculations at the B3LYP/6-31G level of theory indicate a strain energy of 8.2 and 5.8 kcal/mol, respectively. Despite the highly strained ring system, the compounds were found to be stable under thermal, photochemical, and electron-transfer conditions. Using BLYP/6-311+G/BLYP/6-31G calculations, the origin of this unusual stability was traced to the high energy of the 1,4 benzyne diradicals formed via Bergman cyclization. For the radical cation, both the C(1)-C(6) and the C(1)-C(5) cyclization mode previously reported for acyclic analogues of 1 were found to be energetically prohibitive.     

Negatively charged hyperbranched polyether-based polyelectrolytes

The preparation of carboxylated hyperbranched polyglycerols of narrow polydispersity was achieved by modification (78–90%) of the hydroxyl end groups via Michael addition of acrylonitrile, followed by hydrolysis. High conversion could only be achieved for low molecular weight starting materials (520 and 1,030 g mol⁻¹). The solution properties of the resulting materials were investigated by dynamic light scattering (DLS), showing the formation of large aggregates with size depending on the pH value. After deposition on a negatively charged mica surface, the structures observed by atomic force microscope (AFM) show the coexistence of aggregates and single macromolecules. Most interesting, in the case of the lower molecular weight sample (PG 520 g mol⁻¹), extended and ordered terrace structures were formed, which are unprecedented for hyperbranched polymers and are of interest for surface modification in general.     

Vesicle adsorption and lipid bilayer formation on glass studied by atomic force microscopy

The adsorption of phosphatidylcholine (PC) vesicles (30, 50, and 100 nm nominal diameters) and of dye-labeled PC vesicles (labeled with 6% Texas Red fluorophore (TR) and encapsulated carboxy fluorescein (CF)) to glass surfaces was studied by contact mode atomic force microscopy in aqueous buffer. These studies were performed in part to unravel details of the previously observed isolated rupture of dye-labeled PC vesicles on glass (Johnson, J. M.; Ha, T.; Chu, S.; Boxer, S. G. Biophys. J. 2002, 83, 3371-3379), specifically to differentiate partial rupture, that is, pore formation and leakage of entrapped dye, from full rupture to form bilayer disks. In addition, the adhesion potential of PC vesicles on glass was calculated based upon the adhesion-driven flattening of adsorbed vesicles and a newly developed theoretical model. The vesicles were found to flatten considerably upon adsorption to glass (width-to-height ratio of approximately 5), which leads to an estimate for the adhesion potential and for the critical rupture radius of 1.5 x 10(-4) J/m2 and 250 nm, respectively. Independent of vesicle size and loading with dye molecules, the adsorption of intact vesicles was observed at all concentrations below a threshold concentration, above which the formation of smooth lipid bilayers occurred. In conjunction with previous work (Johnson, J. M.; Ha, T.; Chu, S.; Boxer, S. G. Biophys. J. 2002, 83, 3371-3379), these data show that 6% TR 20 mM CF vesicles adsorb to the surface intact but undergo partial rupture in which they exchange content with the external buffer.     

Coligand role in the NHC nickel catalyzed C–F bond activation: investigations on the insertion of bis(NHC) nickel into the C–F bond of hexafluorobenzene

The reaction of [Ni(Mes₂Im)₂] (1) (Mes₂Im = 1,3-dimesityl-imidazolin-2-ylidene) with polyfluorinated arenes as well as mechanistic investigations concerning the insertion of 1 and [Ni(ⁱPr₂Im)₂] (1ipr) (ⁱPr₂Im = 1,3-diisopropyl-imidazolin-2-ylidene) into the C–F bond of C₆F₆ is reported. The reaction of 1 with different fluoroaromatics leads to formation of the nickel fluoroaryl fluoride complexes trans-[Ni(Mes₂Im)₂(F)(Ar^F)] (Ar^F = 4-CF₃-C₆F₄2, C₆F₅3, 2,3,5,6-C₆F₄N 4, 2,3,5,6-C₆F₄H 5, 2,3,5-C₆F₃H₂6, 3,5-C₆F₂H₃7) in fair to good yields with the exception of the formation of the pentafluorophenyl complex 3 (less than 20%). Radical species and other diamagnetic side products were detected for the reaction of 1 with C₆F₆, in line with a radical pathway for the C–F bond activation step using 1. The difluoride complex trans-[Ni(Mes₂Im)₂(F)₂] (9), the bis(aryl) complex trans-[Ni(Mes₂Im)₂(C₆F₅)₂] (15), the structurally characterized nickel(i) complex trans-[Ni^I(Mes₂Im)₂(C₆F₅)] (11) and the metal radical trans-[Ni^I(Mes₂Im)₂(F)] (12) were identified. Complex 11, and related [Ni^I(Mes₂Im)₂(2,3,5,6-C₆F₄H)] (13) and [Ni^I(Mes₂Im)₂(2,3,5-C₆F₃H₂)] (14), were synthesized independently by reaction of trans-[Ni(Mes₂Im)₂(F)(Ar^F)] with PhSiH₃. Simple electron transfer from 1 to C₆F₆ was excluded, as the redox potentials of the reaction partners do not match and [Ni(Mes₂Im)₂]⁺, which was prepared independently, was not detected. DFT calculations were performed on the insertion of [Ni(ⁱPr₂Im)₂] (1ipr) and [Ni(Mes₂Im)₂] (1) into the C–F bond of C₆F₆. For 1ipr, concerted and NHC-assisted pathways were identified as having the lowest kinetic barriers, whereas for 1, a radical mechanism with fluoride abstraction and an NHC-assisted pathway are both associated with almost the same kinetic barrier.

A combined experimental and theoretical study on the mechanism of the C–F bond activation of C₆F₆ with [Ni(NHC)₂] is provided.     

Quecksilber(II)‐chlorid‐ und ‐iodid‐Komplexe von Dithia‐ und Tetrathiakronenethern

Mercury(II) Chloride and Iodide Complexes of Dithia‐ and Tetrathiacrown Ethers The complexes [(HgCl₂)₂((ch)₂30S₄O₆)] ( 1 ), [HgCl₂(mn21S₂O₅)] ( 2 ), [HgCl₂(ch18S₂O₄)] ( 3 ) and [HgI(meb12S₂O₂)]₂[Hg₂I₆] ( 4 ) have been synthesized, characterized and their crystal structures were determined. In [(HgCl₂)₂((ch)₂30S₄O₆)] two HgCl₂ units are discretely bonded within the ligand cavity of the 30‐membered dichinoxaline‐tetrathia‐30‐crown‐10 ((ch)₂30S₄O₆) forming a binuclear complex. HgCl₂ forms 1 : 1 “in‐cavity” complexes with the 21‐membered maleonitrile‐dithia‐21‐crown‐7 (mn21S₂O₅) ligand and the 18‐membered chinoxaline‐dithia‐18‐crown‐6 (ch18S₂O₄) ligand, respectively. The 12‐membered 4‐methyl‐benzo‐dithia‐12‐crown‐4 (meb12S₂O₂) ligand gave with two equivalents HgI₂ the compound [HgI(meb12S₂O₂)]₂[Hg₂I₆]. In the cation [HgI(meb12S₂O₂)]⁺ meb12S₂O₂ forms with the cation HgI⁺ a half‐sandwich complex.     

Electron-transfer reduction of selected alcohols with alkalide K, K(15-crown-5)2 via organometallic intermediates

The course of the reaction of alkalide K⁻, K⁺(15-crown-5)₂1 with selected alcohols depends on the kind of alcohol and the mode of substrate delivery. In the case of methanol, potassium methoxide formed initially undergoes destruction at the excess of 1. It results in potassium oxide and methylpotassium. The latter opens the crown ether ring giving potassium tetraethylene glycoxide vinyl ether and methane. A similar course of the process is observed for propanol. Potassium glycidoxide is the main product formed in the reaction of 1 with glycidol. Its oxirane ring is opened at the excess of 1. Organopotassium alkoxides, i.e., potassium potassiomethoxide and dipotassium potassiopropane-1,2-dioxide are intermediate products of this reaction. They react then with the crown ether. Potassium methoxide, potassium enolate of acetaldehyde, dipotassium propane-1,2-dioxide and potassium tetraethylene glycoxide vinyl ether are the final products of this process.     

Insights on the origin of the structural phase transition in BaV10O15 from electronic structure calculations and the effect of Ti-doping on its structure and electrical transport properties

Band structure calculations at the level of LMTO-ASA provide insight into the electronic structure of BaV₁₀O₁₅ and the origin of the structural phase transition. A crystal orbital Hamiltonian population/integrated crystal orbital Hamiltonian population analysis provides evidence that the crystallographic phase transition is driven by V-V bond formation. As well, the energy bands near the Fermi level are very narrow, <1 eV, consistent with the fact that the observed insulating behavior can be due to electron localization via either Mott-Hubbard correlation and/or Anderson disorder. The partial solid solution, BaV_10−xTi_xO₁₅, was examined to study the effect of Ti-doping at the V sites on the structure and electronic transport properties. In spite of the non-existence of “BaTi₁₀O₁₅”, the limiting x=8, as indicated by a monotonic increase in the cell volume and systematic changes in properties. This limit may be due to the difficulty of stabilizing Ti²⁺ in this structure. For x=0.5 both the first order structural phase transition and the magnetic transition at 40 K are quenched. The samples obey the Curie-Weiss law to x=3 with nearly spin only effective moments along with θ values which range from −1090 K (x=0.5) to −1629 K (x=3). For x>3 a very large, ∼2×10⁻³ emu/mol, temperature independent (TIP) contribution dominates. Conductivity measurements on sintered, polycrystalline samples show semiconducting behavior for all compositions. Activation energies for Mott hopping derived from high temperature data range from ∼0.1 eV for x=0-1 and fall to a plateau of 0.06 eV for x=3-7. Low temperature data for x=3, 5 and 7 show evidence for Mott variable range hoping (VRH) with a T^1/4 law and in one case between 5 and 17 K, a Efros-Shklovskii correlated hopping, T^1/2 law, was seen, in sharp contrast to BaV₁₀O₁₅ where only the E-S law was observed up to 75 K. Seebeck coefficients are small (<35 μV/K), positive, roughly TIP and increase with increasing x up to x=5. This may point to a Heikes hopping of holes but a simple single carrier model is impossible. The compositions for x>3 are remarkable in that local moment behavior is lost, yet a metallic state is not reached. The failure of this system to be driven metallic even at such high doping levels is not fully understood but it seems clear that disorder induced carrier localization plays a major role.     

Carbon monoxide induced decomposition of the active site [Ni-4Fe-5S] cluster of CO dehydrogenase

During the past two years, crystal structures of Cu- and Mo-containing carbon monoxide dehydrogenases (CODHs) and Ni- and Fe-containing CODHs have been reported. The active site of CODHs from anaerobic bacteria (cluster C) is composed of Ni, Fe, and S for which crystallographic studies of the enzymes from Carboxydothermus hydrogenoformans, Rhodospirillum rubrum, and Moorella thermoaceticarevealed structural similarities in the overall protein fold but showed substantial differences in the essential Ni coordination environment. The [Ni-4Fe-5S] cluster C in the fully catalytically competent dithionite-reduced CODH II from C. hydrogenoformans (CODHII(Ch)) at 1.6 A resolution contains a characteristic mu(2)-sulfido ligand between Ni and Fe1, resulting in a square-planar ligand arrangement with four S-ligands at the Ni ion. In contrast, the [Ni-4Fe-4S] clusters C in CO-treated CODH from R. rubrum resolved at 2.8 A and in CO-treated acetyl-CoA synthase/CODH complex from M. thermoacetica at 2.2 and 1.9 A resolution, respectively, do not contain the mu(2)-sulfido ligand between Ni and Fe1 and display dissimilar geometries at the Ni ion. The [Ni-4Fe-4S] cluster is composed of a cubane [Ni-3Fe-4S] cluster linked to a mononuclear Fe site. The described coordination geometries of the Ni ion in the [Ni-4Fe-4S] cluster of R. rubrum and M. thermoacetica deviate from the square-planar ligand geometry in the [Ni-4Fe-5S] cluster C of CODHII(Ch). In addition, the latter was converted into a [Ni-4Fe-4S] cluster under specific conditions. The objective of this study was to elucidate the relationship between the structure of cluster C in CODHII(Ch) and the functionality of the protein. We have determined the CO oxidation activity of CODHII(Ch) under different conditions of crystallization, prepared crystals of the enzyme in the presence of dithiothreitol or dithionite as reducing agents under an atmosphere of N(2) or CO, and solved the corresponding structures at 1.1 to 1.6 A resolutions. Fully active CODHII(Ch) obtained after incubation of the enzyme with dithionite under N(2) revealed the [Ni-4Fe-5S] cluster. Short treatment of the enzyme with CO in the presence of dithiothreitol resulted in a catalytically competent CODHII(Ch) with a CO-reduced [Ni-4Fe-5S] cluster, but a prolonged treatment with CO caused the loss of CO-oxidizing activity and revealed a [Ni-4Fe-4S] cluster, which did not contain a mu(2)-S. These data suggest that the [Ni-4Fe-4S] cluster of CODHII(Ch) is an inactivated decomposition product originating from the [Ni-4Fe-5S] cluster.     

Quantitative on-line high-resolution NMR spectroscopy in process engineering applications

In many technical processes, complex multicomponent mixtures have to be handled, for example, in reaction or separation equipment. High-resolution NMR spectroscopy is an excellent tool to study these mixtures and gain insight in their behavior in the processes. For on-line studies under process conditions, flow NMR probes can be used in a wide range of temperature and pressure. A major challenge in engineering applications of NMR spectroscopy is the need for quantitative evaluation. Flow rates, recovery times, and other parameters of the on-line NMR experiments have to be optimized for this purpose. Since it is generally prohibitive to use deuterated solvents in engineering applications, suitable techniques for field homogenization and solvent signal suppression are needed. Two examples for the application of on-line NMR spectroscopic experiments in process engineering are presented, studies on chemical equilibria and reaction kinetics of the technically important system formaldehyde-water-methanol and investigations on reactive gas absorption of CO(2) in aqueous solutions of monoethanolamine.