bluuebunny/crossref_metadata_2025 huggingface dataset:
An Analytical Study of the Redox Behavior of 1,10‐Phenanthroline‐5,6‐dione, its Transition‐Metal Complexes, and its N‐Monomethylated Derivative with regard to their Efficiency as Mediators of NAD(P)+ Regeneration
Authors: Hilt Gerhard , Jarbawi Tafeeda , Heineman William R. , Steckhan Eberhard
Published: July 2007

The synthesis as well as the chemical and electrochemical properties of homoleptic and heteroleptic (trispyridyl‐methylamine as coligand) transition‐metal complexes (Ru and Co) of 1,10‐phenanthroline‐5,6‐dione (PD) and of its N‐monomethylated derivative (PDMe+) are described. In particular, their ability to abstract hydride ions was studied. Electrochemical investigations with cyclic voltammetry, rotating disk electrode experiments, and spectroelectrochemical methods at different pH values gave an insight into the complex electrochemistry of the compounds described, which is strongly influenced by a hydration pre‐equilibrium. The electrochemically active quinone form of the transition‐metal complexes can be reduced to the hydroquinone state in acidic solution and to their transition‐metal‐stabilized semi‐quinone states for neutral and basic solutions, whereas PDMe+ is reduced to the hydroquinone state in both acidic and neutral solutions. The compounds can also be reduced chemically, and are efficient catalysts for the indirect oxidation of the enzymatic cofactor NAD(P)H. For the indirect aerobic NAD(P)H oxidation, up to 900 turnovers per hour can be observed, an achievement yet to be reached by other catalyst systems.
Rhodium(I)‐Assisted Stereoselective Coupling of an Alkyl, Aryl or Vinyl Group with a Vinylidene Ligand: A Novel Synthetic Route to π‐Allyl and π‐Butadienyl Rhodium Complexes
Authors: Werner Helmut , Wiedemann Ralf , Steinert Paul , Wolf Justin
Published: July 2007

In the first part of this work, a general method for the preparation of aryl, methyl, vinyl and alkynyl(vinyl‐idene)rhodium(I) complexes trans‐[Rh(R')‐(CCHR)(PiPr3)2] (8–14, 18–22) and trans‐[Rh(R')(CCMe2)(PiPr3)2] (16, 17) from the corresponding chloro(vinyl‐idene) derivatives and Grignard reagents is described. Whilst compounds 8 and 10–13 react with pyridine to give trans‐[Rh(CCR)(py)(PiPr3)2] (23–25) by elimination of R'H, treatment of 8–11, 16, and 18 with carbon monoxide yields the square‐planar η1‐vinyl and η1‐butadienylrhodiumcarbonyl complexes trans‐[Rh{η1‐(Z)‐C(R')CHR}(CO)(PiPr3)2] (27–32). The reaction of 8 or 18 with methyl or tert‐butylisocyanide leads stereoselectively to the isocyaniderhodium(I) compounds trans‐ [Rh{η1‐(Z)‐C(R)CHPh}(CNR') (PiPr3)2] (33–35). Acid‐induced cleavage of the rhodium‐carbon σ bond of 27, 30, or 31 with CH3CO2H gives trans‐[Rh(η1‐O2CCH3)‐(CO)(PiPr3)2] (38) and the corresponding olefin or diene, respectively. In the absence of a Lewis base such as pyridine, CO, or CNR', compounds 18–20 rearrange in benzene at 40–50 °C to afford the isomeric π‐allyl complexes [Rh(η3‐1‐RC3H4)(PiPr3)2] (40–42) almost quantitatively. The vinyl(vinylidene) compounds 11 and 12 also undergo an intramolecular rearrangement that leads to the η3‐2,3,4‐butadienyl‐ or to the alkynyl(ethene)rhodium(I) isomers, depending on the reaction conditions. In an analogous manner to the η1‐vinyl‐and η1‐butadienyl(carbonyl) derivatives 27, 30, and 31, the π‐allyl and π‐butadienyl complexes also react with acetic acid to give [Rh(η1‐O2CCH3)(PiPr3)2] (47) and the respective olefin.
A Triangular Iron(III) Complex Potentially Relevant to Iron(III)‐Binding Sites in Ferreascidin
Authors: Bill Eckhard , Krebs Carsten , Winter Manuela , Gerdan Michael , Trautwein Alfred X. , Flörke Ulrich , Haupt Hans‐Jürgen , Chaudhuri Phalguni
Published: July 2007

An asymmetric triangular FeIII complex has been synthesized by an unusual FeII‐promoted activation of salicylaldoxime. Formation of the ligand 2‐(bis(salicylideneamino)methyl)phenol in situ is believed to occur through the reductive deoximation of salicylaldoxime by ferrous ions. The trinuclear ferric complex has been characterized on the basis of elemental analysis, IR, variable‐temperature magnetic susceptibility, and EPR and Mössbauer spectroscopies. The molecular structure established by X‐ray diffraction consists of a trinuclear structure with a [Fe3(μ3‐O)(μ2‐OPh)]6+ core. Two iron ions are in a distorted octahedral environment having FeN2O4 coordination spheres, and the five‐coordinated third iron ion, with an FeNO4 coordination sphere, is in a trigonal bipyramidal environment. The magnetic susceptibility measurements revealed an St = 5/2 ground state with the antiparallel exchange interactions J = – 34.3 cm−1, J' = – 4.7 cm−1, and D = ‐ 0.90 cm−1. The EPR results are consistent with a ground state of S = 5/2 together with a negative D5/2 value. The Mössbauer isomer shifts together with the quadrupole splitting also provide evidence for the high‐spin state of the three ferric sites. Magnetic Mössbauer spectra lead to the conclusion that the internal magnetic fields possibly lie in the plane of the three ferric ions.
Extended‐Core Discotic Liquid Crystals Based on the Intramolecular H‐Bonding in N‐Acylated 2,2′‐Bipyridine‐3,3′‐diamine Moieties
Authors: Palmans Anja R. A. , Vekemans Jozef A. J. M. , Fischer Hartmut , Hikmet Rifat A. , Meijer E. W.
Published: July 2007

A new type of disc‐shaped molecule (1 a‐c) has been synthesised and characterised. The molecules were built up by linking three lipophilic, N‐monoacylated 2,2′‐bipyridine‐3,3′‐diamine wedges to a central 1,3,5–benzenetricarbonyl unit. They show liquid crystalline behaviour, as shown by DSC, polarisation microscopy and X‐ray diffraction. In all cases the mesophase was characterised as a Dho phase. From 1H NMR results it was shown that the interior of compounds 1 a‐c preferentially adopts a C3 symmetrical conformation owing to strong intramolecular H‐bonding, which gives rise to an extended core. This large core induces strong interactions between molecules, leading to mesophases of enhanced thermal stability.
Investigations into the biosynthesis of porphyrins and corrins—calculations on 1,3‐allylic strain and [1,5]‐sigmatropic rearrangements in pyrroles, furans, and thiophenes
Authors: Tietze Lutz F. , Schulz Gerhard
Published: July 2007

The [1,5]‐sigmatropic rearrangements of hydrogen and other groups, such as methyl, tert‐butyl, allyl, benzyl, and azafulvenium, in pyrroles and, to some extent, in furans and thiophenes, has been studied by using semiempirical and ab initio methods. These systems are used as models to explain the ring D inversion in the biosynthesis of uroporphyrinogen III and the stereoselective shift of a methyl group in the biosynthesis of vitamin B12. The difference in energy between the competing pathways of hydrogen and methyl shifts is significantly lower than for cyclopentadiene. The rearrangements are usually concerted, except for in strongly resonance‐stabilized systems, such as azafulvenium cations. Furthermore, ab initio calculations of the 1,3‐allylic strain for a range of substituted pyrroles has been performed, and the results compared with semiempirical data.
Highly Stereoselective Synthesis of Bicyclo[n.3.0]alkanes by Titanium Tetrachloride Promoted [3 + 2] Cycloaddition of Allylsilanes and 1‐Acetylcycloalkenes
Authors: Knölker Hans‐Joachim , Foitzik Norbert , Goesmann Helmut , Graf Regina , Jones Peter G. , Wanzl Günter
Published: July 2007

The titanium tetrachloride promoted reaction of allylsilanes 1 with 1‐acetylcyclohexene is shown to afford the silylbicyclo[4.3.0]nonanes 9 ([3 + 2] cycloaddition products) along with the 1‐acetyl‐2‐allylcyclohexane 4 (Hosomi‐Sakurai product). Here we report that systematic variation of the substituents at the silicon atom of 1 allows suppression of the classical Hosomi‐Sakurai reaction in favor of the [3 + 2] cycloaddition. Cycloaddition of the allylsilanes 1 d, 1 i, and 1 k with 1‐acetylcycloalkenes 10, containing a 5‐, 6‐, 7‐, 8‐, or 12‐membered ring, gives rise to the corresponding silylbicyclo[n.3.0]alkanes 11 – 13. The cycloaddition of allyltriisopropylsilane (1 k) and 1‐acetyl‐2‐methylcycloalkenes 15 provides silylbicyclo[n.3.0]alkanes 16 with two contiguous quaternary carbon centers. The stereochemistry of the silylbicyclo[n.3.0]alkanes 11 a ‐ c and 14 is unambiguously determined by X‐ray analysis and 13C NMR spectroscopy.
The Reaction of a Triorganochloroindate with Dioxygen
Authors: Kräuter Thomas , Neumüller Bernhard
Published: July 2007

The reaction of CsCl with In(CH2Ph)3 yields the triorganochloroindate Cs[(PhCH2)3InCl] (1). Treatment of In(CH2Ph)3 with dry O2 gives the alkoxide [(PhCH2)2InOCH2Ph]2 (2), whereas the reaction of 1 with O2 leads to Cs2[O{PhCH2In(OCH2Ph)2}4] (3). According to X‐ray structural investigations, 1 contains Cs2Cl2 four‐membered rings as basic structural motif. The dimers are connected by Cs‐π‐electron interactions to form a layer structure. Compound 3 can be described as a metalla(crown ether) fragment coordinating two Cs+ ions. Additional shielding of the Cs centers by weak Cs‐π‐electron contacts was observed.
Molybdenum‐Catalyzed Olefin Epoxidation: Ligand Effects
Authors: Thiel Werner R. , Eppinger Jörg
Published: July 2007

We synthesized substituted pyrazolylpyridine ligands to examine their donor properties by spectroscopic (IR, NMR) and computational (AM 1) methods. The influence of the substitution patterns on spectroscopic and thermodynamic features of molybdenum oxobisperoxo complexes [(L–L)MoO(O2)2] (L–L=2‐(1‐alkyl‐3‐pyrazolyl)pyridine/pyrazine) correlates with the activities of the complexes in catalytic olefin epoxidation reactions. This further proof for the relation between the Lewis acidity and the catalytic activity of epoxidation catalysts supports a reaction mechanism in which the peroxo complex activates the oxidizing agent (H2O2, ROOH) instead of directly transferring an oxygen atom from a π2‐peroxo ligand to the olefin.
A Novel Type of Isomerism in [3]Catenanes
Authors: Ashton Peter R. , Boyd Sue E. , Claessens Christian G. , Gillard Richard E. , Menzer Stephan , Stoddart J. Fraser , Tolley Malcom S. , White Andrew J. P. , Williams David J.
Published: July 2007

The self‐assembly of three [3]‐catenanes based on a single tetracationic cyclophane—cyclobis(bipyridinium‐1,4‐di‐ethoxybenzene)—with pairs of identical interlocked crown ethers—bis‐p‐phenyl‐ene[34]crown‐10, bis‐1,5‐dioxynaphthal‐ene[38]crown‐10, or tetrafluoro‐p‐phenyl‐ene‐p‐phenylene[34]crown‐10—has been achieved in yields of 34, 31, and 33%, respectively. The solid‐state structures of these [3]catenanes, determined by X‐ray crystallography, are consistent with molecules having approximately C2h symmetry. In the solution state, the π‐electron rich aromatic ring systems of the crown ether components, residing within the cavity of the tetracationic cyclophane, can adopt two different relative orientations, thus giving rise to two distinct isomeric forms possessing either C2h or D2 symmetries. Two dynamic processes have been characterized in the [3]catenane incorporating bis‐1,5‐dioxynaphthalene[38]‐crown‐10 macrocycles, by means of variable‐temperature 1H NMR spectroscopy and subsequent lineshape analyses. The slower process is the exchange between C2h and D2 isomers, and the faster process is the rotation of the bipyridinium units of the cyclophane around their NN axes. The evaluation of the free energies of activation at 298 K, which are, for the slower process, ca. 16 kcal mol−1, and for the faster one, ca. 14.5 kcal mol−1, has enabled us to propose a scenario in which the two included 1,5‐dioxynaphthalene ring systems move continuously about the center of the cavity of the tetracationic cyclophane. A partial dissociation of one 1,5‐dioxynaphthalene ring system allows the adjacent bipyridinium units to rotate about their long axes, whereas a complete dissociation is presumably necessary to allow a 1,5‐dioxynaphthalene ring system to rotate about its OO axis. The isomerism between the C2h and the D2 isomers of the [3]catenane incorporating the two bis‐1,5‐dioxynaphthalene[38]crown‐10 rings is dependent upon 1) translational motions between the two crown ether rings and the central tetracationic cyclophane and 2) conformational changes within at least one of the two crown ether rings.
A Surprising Switch from the Myers–Saito Cyclization to a Novel Biradical Cyclization in Enyne–Allenes: Formal Diels–Alder and Ene Reactions with High Synthetic Potential
Authors: Schmittel Michael , Keller Manfred , Kiau Susanne , Strittmatter Marc
Published: July 2007

If there is an aryl substituent on the acetylene terminus of enyne‐allenes, then its reaction mode may be changed from the Myers‐Saito cyclization to a novel C2–C6 cyclization resulting in a net intramolecular Diels‐Alder or ene reaction. As a consequence, the thermal cyclization of readily accessible acyclic enyne‐allenes can be utilized for the synthesis of complex benzofulvene and benzofluorene derivatives. Kinetic results of the C2–C6 cyclization reaction indicate a two‐step reaction pathway with a benzofulvene biradical intermediate.
Achiral Cyclodextrin Analogues
Authors: Ashton Peter R. , Cantrill Stuart J. , Gattuso Giuseppe , Menzer Stephan , Nepogodiev Sergey A. , Shipway Andrew N. , Stoddart J. Fraser , Williams David J.
Published: July 2007

The synthesis of a new family of cyclodextrin (CD) analogues is described. This family consists of novel cyclic oligosaccharides built from monosaccharides that possess the same relative but opposite absolute (D‐ and L‐) configurations. The alternation of such D‐ and L‐ residues—specifically, D‐ and L‐rhamnose or D‐ and L‐mannose—in a macrocyclic structure results in Sn‐type symmetry and, consequently, optical inactivity. The synthesis of these cyclic oligosaccharides was achieved by an economical polycondensation/cycloglycosylation approach that relies on an appropriately‐derivatized disac‐charide monomer and that avoids the time‐consuming, and often low‐yielding, stepwise growth of long linear oligosaccharide precursors. In the cases reported, the key precursors are the disaccharide monomers 1‐RR and 1‐MM, which bear both a glycosyl donor (cyanoethylidene function) and a glycosyl acceptor (trity‐loxy group). These compounds are able to undergo Tr+‐catalyzed polycondensation which, under appropriate dilution conditions, can be terminated by cycloglycosyl‐ation. Thus, compound 1‐RR was converted into a range of protected cyclic rhamnooligosaccharides 15–19 in 64% overall yield. All these products, including the unique cyclic dodeca‐ and tetradecasaccharides 18 and 19, have been isolated by preparative HPLC. Unexpectedly, treatment of the manno analogue of the disaccharide 1‐RR (compound 1‐MM) under the same conditions produced only the cyclic hexasaccharide 28 and numerous apparently linear oligomers. Removal of the protecting groups from 16–19 afforded the free cyclic oligosaccharides 21–24, which exhibited the predicted zero optical rotation and very simple NMR spectra, indicating highly symmetrical structures. X‐ray crystallography reveals that in the solid state the cyclooctaoside 21 possesses a C2 symmetric structure, on account of a slight deformation of its cylindrical shape. The channel‐type crystal packing of molecules of 21 forms nanotubes with an internal diameter of around 1 nm. Conversely, the cyclic hexasaccharide 29 possesses a Ci symmetric solid‐state structure and its molecules pack to form a parquet‐like superstructure.
BIPNOR: A New, Efficient Bisphosphine Having Two Chiral, Nonracemizable, Bridgehead Phosphorus Centers for Use in Asymmetric Catalysis
Authors: Robin Frédéric , Mercier François , Ricard Louis , Mathey François , Spagnol Michel
Published: July 2007

Optically active phosphorus ligands are widely used in homogeneous asymmetric catalysis. However, among the numerous available structures of this type, the subclass of optically active bisphosphines with at least one chiral phosphorus atom is rather underdeveloped. A bisphosphine with two chiral, nonracemizable bridgehead phosphorus centers, (meso,d/l)‐2,2′,3,3′‐tetraphenyl‐4,4′,5,5′‐tetramethyl‐6,6′‐bis‐1‐phosphanorborna‐2,5‐dienyl (BIPNOR), can be obtained by thermolysis of 1,1′‐bisphospholyl with diphenylacetylene. Here, we report the resolution of the d/l isomer by means of a chiral palladium complex to give the two optically active forms of BIPNOR. We then investigate the catalytic properties of BIPNOR, incorporated in RhI and RuII catalysts for the hydrogenation of olefins and ketones and in a PdII catalyst for asymmetric alkylation reactions. BIPNOR is shown to give good results in these catalytic reactions.
Studying the Stability of a Helical β‐Heptapeptide by Molecular Dynamics Simulations
Authors: Daura Xavier , van Gunsteren Wilfred F. , Rigo Dario , Jaun Bernhard , Seebach Dieter
Published: July 2007

β‐Peptides consisting entirely of homochiral β‐amino acids R‐CH(NH2)‐CH2CO2H form 31‐helices in solution, as shown previously by NMR analysis of pyridine and methanol solutions. The stability of the helical secondary structure of one such β‐peptide (H‐β‐HVal‐β‐HAla‐β‐HLeu‐(S,S)‐β‐HAla(αMe)‐β‐HVal‐β‐HAla‐β‐HLeu‐OH, 1) has been investigated by molecular dynamics simulations using the GROMOS 96 molecular model and force field (962 methanol molecules; T = 298, 350, 400 K; with and without NOE distance restraints). The restraints derived from the NMR studies were equally well satisfied by both the restrained and the unrestrained room‐temperature molecular dynamics simulations. The 31‐helical conformation of 1 was shown to be so stable that it was restored spontaneously within 400 ps after unfolding had been induced by a sudden increase of the temperature from 298 to 350 K.
Relaxometric Determination of the Exchange Rate of the Coordinated Water Protons in a Neutral GdIII Chelate
Authors: Aime Silvio , Botta Mauro , Fasano Mauro , Paoletti Silvia , Terreno Enzo
Published: July 2007

The exchange rate of the coordinated water molecule in the neutral complex [Gd(DTPA‐BBA)(H2O)] (DTPA‐BBA = 1,7‐bis[(N‐benzylcarbamoyl)methyl]‐1,4,7‐triazaheptane‐1,4,7‐triacetate or diethylenetriaminopentaacetate N,N′‐bis(benzylamide)) is slower than in the parent complex [Gd(DTPA)(H2O)]2‐. From the analysis of the temperature dependence of the solvent 17O NMR transverse relaxation time in an aqueous solution of the paramagnetic complex, a value of 4.5 × 105 s‐1 (at 298 K) is obtained for the exchange rate of the coordinated water molecule. This rate constant does not vary in the pH range 7–12. Conversely, over the same pH range and at 298 K and 20 MHz, the longitudinal water proton relaxivity increases from 4.8 to 6.5 s‐1 mM‐1. The analysis of the dependence of the longitudinal water proton relaxation rate on magnetic field and temperature at pH 7 and pH 12 shows that the increase in relaxivity at basic pH has to be assigned to the contribution of the prototropic exchange at the water molecule in the inner coordination sphere of the metal ion. This exchange process is catalyzed by OH‐ ions (kP = 1.7 × 109 M‐1 s‐1 at 298 K) and causes an increase in the observed relaxivity when it occurs at a rate larger than the exchange rate of the entire water molecule. At pH 12 the limiting effect of the slow exchange rate for the coordinated water molecule is removed, and the longitudinal water proton relaxivity measured at this pH then represents the maximum value attainable for this complex.
The Ineluctable Need for in Situ Methods of Characterising Solid Catalysts as a Prerequisite to Engineering Active Sites
Authors: Thomas John Meurig
Published: July 2007

Ultimate success in the design of solid oxide catalysts as well as other covalently bonded or heterogenised organometallic catalysts predicates knowledge of precisely what structure it is that has to be targetted. This, in turn, demands the greatest possible precision in determining, under operating conditions, the structure of the catalyst in general and of the active site in particular. Combined X‐ray absorption spectroscopy and X‐ray diffraction are ideal tools for such in situ investigations. Examples of such studies and of engineered catalysts, the structure of which have been determined in atomic detail, are given.
A Pyrrole‐Imidazole Polyamide Motif for Recognition of Eleven Base Pair Sequences in the Minor Groove of DNA
Authors: Swalley Susanne E. , Baird Eldon E. , Dervan Peter B.
Published: July 2007

A new upper limit of binding site size is defined for the 2:1 overlapped polyamide: DNA motif. Eight‐ring polyamides composed of four‐ring subunits containing pyrrole (Py) and imidazole (Im) amino acids linked by a central β‐alanine (β) spacer („4‐β‐4 ligands”︁) were designed for recognition of eleven base pair sequences as antiparallel dimer (4‐β‐4)2.DNA complexes in the minor groove. The DNA binding properties of three polyamides, ImPyPyPy‐β‐PyPyPyPy‐β‐Dp, ImImPyPy‐β‐PyPyPyPy‐β‐Dp, and ImImImPy‐β‐PyPyPyPy‐β‐Dp, were analyzed by footprinting experiments on DNA fragments containing the respective match sites 5'‐AGTAATTTACT‐3', 5'‐AGGTATTACCT‐3', and 5'‐AGGGATTCCCT‐3' (Dp = dimethylaminopropylamide). Quantitative footprint titrations reveal that each polyamide binds its respective target site with subnanomolar affinity and 7‐fold to over 30‐fold specificity over double‐base‐pair mismatch sites. A 20‐fold decrease in binding affinity is observed for placement of a side‐by‐side β‐β pairing opposite G.C/C.G relative to placement opposite a A.T/T.A base pair. The use of side‐by‐side antiparallel β‐alanine residues as an A.T/T.A‐specific DNA binding element provides a new pairing rule for polyamide design. Expanding the DNA binding site size targeted by pyrrole‐imidazole polyamides represents an important step in the development of cell‐permeable synthetic ligands for the control of gene‐specific regulation.
In Search for Mononuclear Helical Lanthanide Building Blocks with Predetermined Properties: Triple‐stranded Helical Complexes with N,N,N',N'‐tetraethylpyridine‐2,6‐dicarboxamide
Authors: Renaud Fabien , Piguet Claude , Bernardinelli Gérald , Bünzli Jean‐Claude G. , Hopfgartner Gérard
Published: July 2007

The ligand N,N,N',N'‐tetra‐ethylpyridine‐2,6‐dicarboxamide (L9) reacts with trivalent lanthanide ions (LnIII) to give stable mononuclear triple‐stranded helical complexes [Ln(L9)3]3+ (Ln = La to Lu). The crystal and molecular structures of [La(L9)3](ClO4)3.2.5C2H5CN (8) and [Eu(L9)3](TfO)3.2THF (9) show that the three ligand strands are each meridionally tricoordinated to produce a pseudo‐tricapped trigonal prismatic arrangement of the nine donor atoms (six amide O and three pyridine N) around LnIII. The distortions in the LaIII coordination sphere of 8 are more significant than for EuIII in 9, and the photophysical studies on Eu‐doped (2%) La, Gd, and Lu complexes confirm a better structural match of L9 for the heavy LnIII ions. The separation of contact and pseudo‐contact contributions to the induced lanthanide paramagnetic NMR shifts in [Ln(L9)3]3+ shows that the triple‐helical structure is maintained in acetonitrile, but a minor structural change relative to that observed in the solid state occurs between TbIII and ErIII leading to two distinct isostructural series for Ln = La‐Tb and Ln = Er‐Lu. The origin of this effect together with its consequences for the dynamic helical (P→M) interconversion and stability of [Ln(L9)3]3+ in solution are discussed. A detailed investigation of the emission properties of [Ln(L9)3]3+ (Ln = Eu, Tb) shows that mixed pyridine‐carboxamide ligands can be used to simultaneously optimize the structural and photophysical properties in mononuclear triple‐helical lanthanide building blocks.
Nondipolar Structures With Threefold Symmetry For Nonlinear Optics
Authors: Wortmann Rüdiger , Glania Christoph , Krämer Peter , Matschiner Ralf , Jens Wolff J. , Kraft Stefan , Treptow Björn , Barbu Eugen , Längle Daniela , Görlitz Gunter
Published: July 2007

The second‐order polarizabilities β(— 2ω; ω, ω) of six nondipolar NLO chromophores—1,3,5‐tris(isopropyl‐amino)‐2,4,6‐trinitrobenzene (1), 1,3,5‐tris(isopropylamino)‐2,4,6‐tris(trifluoro‐methylsulfonyl)benzene (2), tris(dimethyl‐imoniomethyl)methide diperchlorate (3), 2,4,6‐tris[4‐(N,N‐diethylamino)phenyl‐ethynyl]‐1,3,5‐triazine (4), 2,4,6‐tris[4‐(N,N‐diethylamino)phenyl]‐1,3,5‐triazine (5), and tris(2‐dimethylaminoethenyl)‐methinium perchlorate (6)—were studied by polarized hyper‐Rayleigh scattering (HRS). The NLO activity of these two‐dimensional NLO chromophores is attributed to three‐level contributions to the β values, for which different optimization strategies are investigated. Exceptionally large second‐order polarizabilities were observed for the conjugationally elongated structures: the β value of triazine 4 is 15 times larger than that of the one‐dimensional two‐level chromophore 4‐nitroaniline. Results obtained with the polarized HRS experiment are discussed with respect to local field corrections, calibration, and systematic sources of error.
Calix[4]Arene Podands and Barrelands Incorporating 2,2″‐Bipyridine Moieties and Their Lanthanide Complexes: Luminescence Properties
Authors: Ulrich Gilles , Ziessel Raymond , Manet Ilse , Guardigli Massimo , Sabbatini Nanda , Fraternali Franca , Wipff Georges
Published: July 2007

A new family of cone‐shaped podands and barrel‐shaped cryptands based on calix[4]arenes incorporating 5,5′‐substituted 2,2′‐bipyridine subunits were prepared and characterized. The Eu3+ and Tb3+ complexes of the podands bearing two, three, or four bipyridine chromophores could be isolated. High molar absorption coefficients (εmax = 39 600M−1 cm−1 for Eu4 and 26 700M−1 cm−1 for Eu3) and high metal luminescence quantum yields (16% for Eu4 and 15% for Eu3) were obtained. Molecular dynamics simulations on Eu4 showed that the bipyridine arms wrap around the lanthanide cation, efficiently shielding the cation from solvent molecules. In the presence of chloride counter‐ions the fourth bipyridine arms does not coordinate the lanthanide ion. Ligands bearing two bipyridine units and two additional functional groups—ethyl butyrate or N‐propylpyrrole—did not give stable lanthanide complexes. The barre‐land containing two calix[4]arene moieties and four bipyridine groups did not form complexes with lanthanide ions, most probably because of the rigidity of the ligand.
Noncovalent Assembly of Functional Groups on Calix[4]Arene Molecular Boxes
Authors: Timmerman Peter , Vreekamp Remko H. , Hulst Ron , Verboom Willem , Reinhoudt David N. , Rissanen Kari , Udachin Konstantin A. , Ripmeester John
Published: July 2007

Calix[4]arenes diametrically substituted at the upper rim with two melamine units spontaneously form well‐defined box‐like assemblies in the presence of two equivalents of 5,5‐diethylbarbituric acid. These assemblies, consisting of nine different components, are held together by 36 hydrogen bonds and are stable in apolar solvents at concentrations of up to 10−4M. This paper reports the first X‐ray crystal structure, and the MALDI TOF mass spectra together with the complete 1H NMR spectroscopic characterization of these hydrogen‐bonded assemblies. The crystal structure clearly shows that the assemblies are stereogenic, as a result of the antiparallel orientation of the two rosette motifs. Furthermore, the synthesis of twelve new 1,3‐bis(melamine)calix[4]arenes carrying different numbers and types of functionalities at the upper rim is described. Detailed 1H NMR spectroscopic studies on the assembly behavior of these functionalized calix[4]arenes shows that 1) polar substituents (e.g. nitro, cyano) hardly affect the stability of the hydrogen‐bonded assembly; 2) hydrogen bond donating or accepting groups, like amino and acetamido, can disturb assembly of the boxes under certain conditions by destabilizing the calix[4]arene pinched cone conformation as a result of intramolecular hydrogen bond formation; and 3) sterically bulky groups (e.g. tBu) can significantly inhibit the formation of the hydrogen‐bonded assembly, but this effect very much depends on the exact positions of the groups.
Opposite Stereochemical Effects Exerted by CeCl3 and TiCl4 on the Lewis Acid Mediated Reduction of α‐Alkyl‐β‐ketophosphine Oxides with Metallic Hydrides: A Highly Stereoselective Protocol for the Synthesis of syn and anti α‐alkyl‐β‐Hydroxyphosphine Oxides
Authors: Bartoli Giuseppe , Bosco Marcella , Sambri Letizia , Dalpozzo Renato , Marcantoni Enrico
Published: July 2007

A general, highly efficient methodology for obtaining both syn and anti β‐hydroxyphosphine oxides by reduction of the corresponding β‐ketophosphine oxides is described. The nature of the Lewis acid was found to be pivotal in determining the outcome of these reactions. Strongly chelating TiCl4 led to the anti isomer in high diastereoisomeric excess in noncoordinating solvents (CH2Cl2) at ‐78°C with BH3/py as reducing agent, while nonchelating CeCl3 gave a high excess of the syn isomer in coordinating solvents (THF) at the same temperature with LiBH4 as reducing agent. In the latter case, CeCl3 is essential in achieving high yields and stereoselectivity, since it allows the reaction to be performed at low temperatures. Otherwise, higher temperatures (0°C) are required, which lower both yields and selectivities. Moreover, each step of the protocol for the synthesis of stereodefined disubstituted olefins from alkylphosphine oxides (Warren's modification of the Horner procedure) has been optimized, and the optimized procedure has been applied to the synthesis of muscalure, the pheromone of the domestic fly.
Structure, Stability, Dynamics, High‐Field Relaxivity and Ternary‐Complex Formation of a New Tris(aquo) Gadolinium Complex
Authors: Nonat Aline , Fries Pascal H. , Pécaut Jacques , Mazzanti Marinella
Published: July 2007

The tripodal hexadentate picolinate ligand dpaa3− (H3dpaa=N,N′‐bis[(6‐carboxypyridin‐2‐yl)methyl]glycine) has been synthesised. It can form 1:1 and 1:2 lanthanide/ligand complexes. The crystal structure of the bis(aquo) lutetium complex [Lu(dpaa)(H2O)2] has been determined by X‐ray diffraction studies. The number of water molecules was determined by luminescence lifetime studies of the terbium and europium complexes. The tris(aquo) terbium complex shows a fairly high luminescence quantum yield (22 %). The [Gd(dpaa)(H2O)3] complex displays a high water solubility and an increased stability (pGd=12.3) with respect to the analogous bis(aquo) complex [Gd(tpaa)(H2O)2] (pGd=11.2). Potentiometric and relaxometric studies show the formation of a soluble GdIII hydroxo complex at high pH values. A unique aquohydroxo gadolinium complex has been isolated and its crystal structure determined. This complex crystallises as a 1D polymeric chain consisting of square‐shaped tetrameric units. In heavy water, the [Gd(dpaa)(D2O)3] complex shows a quite high HOD proton relaxivity at high field (11.93 s−1 mM−1 at 200 MHz and 298 K) because of the three inner‐sphere water molecules. The formation of ternary complexes with physiological anions has been monitored by relaxometric studies, which indicate that even under conditions favourable to the formation of adducts with oxyanions, the mean relaxivity remains higher than those of most of the currently used commercial contrast agents except for the citrate. However, the measured relaxivity (r̄1=7.9 s−1 mM−1) in a solution containing equimolar concentrations of [Gd(dpaa)(D2O)3] and citrate is still high. The interaction with albumin has been investigated by relaxometric and luminescence studies. Finally, a new versatile method to unravel the geometric and dynamic molecular factors that explain the high‐field relaxivities has been developed. This approach uses a small, uncharged non‐coordinating probe solute, the outer‐sphere relaxivity of which mimics that of the water proton. Only a routine NMR spectrometer and simple mathematical analysis are required.
The Nature of the AgI⋅⋅⋅AgIInteraction in Different Ag(NH3)2Dimers Embedded in Supramolecular Solids
Authors: Zheng Shao‐Liang , Volkov Anatoliy , Nygren Cara L. , Coppens Philip
Published: July 2007

An isolated silver(I) ammonia monomer, a dimer, and a novel dimer containing an intercalated water molecule have been embedded as guests in supramolecular frameworks, [Ag(NH3)2][(H2thpe)(H3thpe)]⋅MeCN (1), [{Ag(NH3)2}2][(H2thpe)2]⋅4.25 H2O (2), and [{Ag(NH3)2}‐H2O‐{Ag(NH3)2}][(H2thpe)2]⋅benzene (3) (H3THPE=tris(hydroxyphenyl)ethane). The [{Ag(NH3)2}2]2+dimer is not stable as an isolated entity, but is stabilized by hydrogen bonding in the supramolecular framework. The water‐intercalated silver(I) ammonia dimer, which constitutes a novel species, is also subject by hydrogen bonding in concentrated solutions. The destabilization energy of the dimer relative to isolated monomers is calculated to be ≈300 kJ mol−1by both perturbation methods and DFT theory. For the water‐intercalated dimer it is calculated to be ≈200 kJ mol−1according to the BSSE‐corrected MP2 calculation. The different aggregate states show a dramatic variation of absorption and emission properties, in accordance with the concentration dependent red‐shift observed in solutions. Natural‐bond‐orbital analysis shows that the disilver‐ammonium‐aquo “sandwich” cation in3is stabilized by interaction between the π lone pair orbital on the oxygen atom of the water molecule and AgIN σ antibonding molecular orbital.
Iterative Tandem Catalysis of Secondary Diols and Diesters to Chiral Polyesters
Authors: van As Bart A. C. , van Buijtenen Jeroen , Mes Tristan , Palmans Anja R. A. , Meijer E. W.
Published: July 2007

The well‐known dynamic kinetic resolution of secondary alcohols and esters was extended to secondary diols and diesters to afford chiral polyesters. This process is an example of iterative tandem catalysis (ITC), a polymerization method where the concurrent action of two fundamentally different catalysts is required to achieve chain growth. In order to procure chiral polyesters of high enantiomeric excess value (ee) and good molecular weight, the catalysts employed need to be complementary and compatible during the polymerization reaction. We here show that Shvo's catalyst and Novozym 435 fulfil these requirements. The optimal polymerization conditions of 1,1′‐(1,3‐phenylene) diethanol (1,3‐diol) and diisopropyl adipate required 2 mol % Shvo's catalyst and 12 mg Novozym 435 per mmol alcohol group in the presence of 0.5 M 2,4‐dimethyl‐3‐pentanol as the hydrogen donor. With these conditions, chiral polyesters were obtained with peak molecular weights up to 15 kDa, an ee value up to 99 % and with 1–3 % ketone end groups. Also with the structural isomer, 1,4‐diol, a chiral polyester was obtained, albeit with lower molecular weight (8.3 kDa) and slightly lower ee (94 %). Aliphatic secondary diols also resulted in enantio‐enriched polymers but at most an ee of 46 % was obtained with molecular weights in the range of 3.3–3.7 kDa. This low ee originates from the intrinsic low enantioselectivity of Novozym 435 for this type of secondary aliphatic diols. The results presented here show that ITC can be applied to procure chiral polyesters with good molecular weight and high ee from optically inactive AA–BB type monomers.
Neonatal maternal separation alters adult eyeblink conditioning and glucocorticoid receptor expression in the interpositus nucleus of the cerebellum
Authors: Wilber Aaron A. , Southwood Christopher J. , Sokoloff Greta , Steinmetz Joseph E. , Wellman Cara L.
Published: July 2007

Neonatal maternal separation alters learning and memory. Glucocorticoids also modulate adult learning and memory, and neonatal maternal separation alters forebrain glucocorticoid receptor (GR) concentrations. We used eyeblink classical conditioning to assess the effect of neonatal maternal separation on associative learning. We assessed delay eyeblink conditioning, GR expression, and total neuron number in the interpositus nucleus, a critical site of plasticity in eyeblink conditioning, in adult rats that had undergone either standard animal facilities rearing, handling for 15 min, or maternal separation for either 15 or 60 min per day on postnatal days 2–14. At 2–3 months of age, delay eyeblink classical conditioning was assessed. Brains were processed for GR immunohistochemistry, and GR expression in the interpositus nucleus was assessed using a computer‐based densitometry system. Neuron counts and nuclear volumes were obtained from an alternate series of thionin‐stained sections. Maternal separation significantly impaired eyeblink conditioning in male but not female rats. Handling and maternal separation did not significantly affect interpositus neuron number and volume. However, prolonged maternal separation significantly increased GR expression in the posterior interpositus in males, and increases were correlated with eyeblink conditioning. In female rats, maternal separation and handling did not significantly alter interpositus neuron number, volume, or GR protein expression, and GR expression did not correlate with eyeblink conditioning. Thus, neonatal maternal separation produces adult deficits in eyeblink conditioning and alterations in GR expression in its neural substrate in a sex‐dependent manner. © 2007 Wiley Periodicals, Inc. Develop Neurobiol, 2007.
Ion‐Selective Electrode Potentiometry in Environmental Analysis
Authors: De Marco Roland , Clarke Graeme , Pejcic Bobby
Published: July 2007

This review will illustrate how it is possible to develop ion‐selective electrode (ISE) methodologies that meet the stringent requirements (i.e., high selectivities and very low detection limits) for the analysis of important analytes in the environment, and will present a variety of examples on the application of ISEs in environmental analysis. Despite the experimental biases that have limited the analytical performance of ISEs through apparently high detection limits and modest selectivities, there has been a plethora of research in the application of ISEs in the monitoring of environmentally important trace metals and anions in natural waters and soils. Most popular has been the analysis of free metals in natural waters, as this parameter is known to be a master variable in the uptake and toxicology of trace metals on aquatic biota reflecting the bioavailability of trace metals in the environment. Furthermore, as copper is a major trace metal in coastal waters due to its extensive use in antifouling paints on sea vessels and structures, there are many reports in the literature on the use of the copper ISE in assays of either free copper or the copper complexing capacity of natural waters and soil peats. Moreover, there have been a variety of studies showing a strong correlation between free copper levels and the toxicity of copper on a variety of marine and fresh water organisms. Nevertheless, there are numerous reports in the literature that have used ISEs to monitor important anions such as fluoride, phosphate, sulfate, nitrate, nitrite, chloride, cyanide, etc., as well as other important cations such as ammonium and chromium(VI) in waste and natural waters. In conclusion, this review will present new and interesting perspectives on the application of ISEs in environmental analysis using approaches such as real‐time remote monitoring of water quality, shipboard monitoring of environmentally important analytes using flow analysis instrumentation, the use of robust all‐solid‐state ISEs in submersible instruments for long‐term deployment in the field, and innovative analytical approaches such as backside calibration and switchtrodes that avoid standard addition analysis and the concomitant perturbation in analyte speciation in natural samples.
Fenton oxidation of TCE vapors in a foam reactor
Authors: Kan Eunsung , Kim Seongyup , Deshusses Marc A.
Published: July 2007

Oxidation of dilute TCE vapors in a foam reactor using Fenton's reagent was investigated. The new process relies on rapid mass transfer from the gas undergoing treatment to a fine aqueous foam and oxidation by Fenton's reagents. Laboratory investigations demonstrated that TCE elimination capacity was as high as 56–61 g TCE m−3reactorh−1 with a removal efficiency of 62% and TCE mineralization of 83% at gas retention time of 30 s and TCE inlet concentration of 0.6 g m−3. Compared to oxidation in wet scrubbers packed with commercially available plastic packings and using the same composition of Fenton's reagents, the foam reactor configuration provided a higher rate absorption and greater elimination capacity of TCE vapors. The treatment performance far exceeded those of current bioreactors and was comparable to those of advanced oxidation techniques such as TiO2/UV and ozone/UV, suggesting that Fenton oxidation in foam reactors may be a promising technique for treating TCE and other recalcitrant vapors. © 2007 American Institute of Chemical Engineers Environ Prog, 2007
Astrocyte glycogen and brain energy metabolism
Authors: Brown Angus M. , Ransom Bruce R.
Published: July 2007

The brain contains glycogen but at low concentration compared with liver and muscle. In the adult brain, glycogen is found predominately in astrocytes. Astrocyte glycogen content is modulated by a number of factors including some neurotransmitters and ambient glucose concentration. Compelling evidence indicates that astrocyte glycogen breaks down during hypoglycemia to lactate that is transferred to adjacent neurons or axons where it is used aerobically as fuel. In the case of CNS white matter, this source of energy can extend axon function for 20 min or longer. Likewise, during periods of intense neural activity when energy demand exceeds glucose supply, astrocyte glycogen is degraded to lactate, a portion of which is transferred to axons for fuel. Astrocyte glycogen, therefore, offers some protection against hypoglycemic neural injury and ensures that neurons and axons can maintain their function during very intense periods of activation. These emerging principles about the roles of astrocyte glycogen contradict the long held belief that this metabolic pool has little or no functional significance. © 2007 Wiley‐Liss, Inc.
Metabolic correlates of nonalcoholic fatty liver in women and men
Authors: Vega Gloria Lena , Chandalia Manisha , Szczepaniak Lidia S. , Grundy Scott M.
Published: July 2007

Nonalcoholic hepatic steatosis associates with a clustering of metabolic risk factors and steatohepatitis. One risk factor for hepatic steatosis is obesity, but other factors likely play a role. We examined metabolic concomitants of hepatic steatosis in nonobese and obese men and women. Sixty-one obese women and 35 obese men were studied; both those with and without hepatic steatosis were compared against each other and against nonobese controls (17 women and 32 men) without hepatic steatosis. Obesity (defined as ≥25% body fat in men and ≥35% in women), was identified by x-ray absorptiometry, whereas hepatic steatosis (≥5.5% liver fat) was detected by magnetic resonance spectroscopy. The primary endpoint was a difference in insulin sensitivity. Obese groups with and without steatosis had similar body fat percentages. Compared with obese women without hepatic steatosis, those with steatosis were more insulin resistant; the same was true for men, although differences were less striking. Obese subjects with hepatic steatosis had higher ratios of truncal-to-lower body fat and other indicators of adipose tissue dysfunction compared with obese subjects without steatosis.

Conclusion

: These results support the concept that obesity predisposes to hepatic steatosis; but in addition, insulin resistance beyond that induced by obesity alone and a relatively high ratio of truncal-to-lower body fat usually combined with obesity to produce an elevated liver fat content. (Hepatology 2007.)
Genetic polymorphisms in the methylenetetrahydrofolate reductase and thymidylate synthase genes and risk of hepatocellular carcinoma
Authors: Yuan Jian-Min , Lu Shelly C. , Van Den Berg David , Govindarajan Sugantha , Zhang Zhen-Quan , Mato Jose M. , Yu Mimi C.
Published: July 2007

Methylenetetrahydrofolate reductase (MTHFR) and thymidylate synthase (TYMS) are known to play a role in DNA methylation, synthesis, and repair. The genetic mutations in

MTHFR

and

TYMS

genes may have influences on their respective enzyme activities. Data on the association studies of the

MTHFR

and

TYMS

genetic polymorphisms and risk of hepatocellular carcinoma (HCC) are sparse.

MTHFR

and

TYMS

genotypes were determined on 365 HCC cases and 457 healthy control subjects among Hispanic and non-Hispanic whites and African-Americans in Los Angeles County, California, and among Chinese in the city of Nanning, Guangxi, China. Relative to the high-activity genotype, each low-activity genotype of

MTHFR

was associated with a statistically nonsignificant 30% to 50% reduction in risk of HCC. Relative to the

TYMS3′UTR +6/+6

genotype, individuals with 1 or 2 copies of the deletion allele had a statistically significant 50% reduction in risk of HCC. When we examined HCC risk by the total number of mutant alleles in the 3 polymorphic loci of

MTHFR/TYMS

(range, 0-4), there was a monotonic decrease in risk with increasing number of mutant alleles (

P

for trend = 0.003). Individuals possessing the maximum number of mutant alleles (

i.e.,

4) had an odds ratio of 0.46 (95% confidence interval = 0.23-0.93) for HCC compared with those with no or only 1 mutant allele.

Conclusion:

This study supports the hypothesis that reduced MTHFR activity and enhanced TYMS activity, both of which are essential elements in minimizing uracil misincorporation into DNA, may protect against the development of HCC. (Hepatology 2007.)
On the feasibility of the microscale approach for a multistep biotransformation: sitosterol side chain cleavage
Authors: Marques Marco PC , de Carvalho Carla CCR , Claudino Mauro JC , Cabral Joaquim MS , Fernandes Pedro
Published: July 2007

BACKGROUND: The millilitre/microlitre‐scale approach is widely used for screening purposes and one‐pot biotransformations, but it has seldom been applied to whole cell multistep biotransformations and two‐liquid biphasic processes. The present study aims to contribute to filling this gap by using 24‐well microtitre plates as reactors for the side chain cleavage of sitosterol with resting cells of Mycobacterium sp. NRRL B‐3805 and comparing results with other set‐ups.

RESULTS: High catalytic activity in microtitre plates and aqueous medium was observed at 250 rpm shaking frequency and 25 mm shaking diameter with 80% headspace and a mixture of Mg2+, Zn2+, Mn2+ and Co2+ salts. Data obtained with microtitre plates were reproduced in other set‐ups when aqueous medium was used, whereas higher biotransformation rates were recorded in stirred reactors when biphasic systems were employed.

CONCLUSION: Results suggest that microtitre plates may be used for the characterisation of complex bioconversion systems to establish representative screening systems, particularly if based in aqueous phase systems. Probing experiments also demonstrated the feasibility of using microlitre/millilitre shaken bioreactors to evaluate organic/aqueous two‐liquid bioconversion systems, although the enhanced mass transfer patterns resulting from the biphasic system hampered the translation of the data to the more common stirred reactor configuration. Copyright © 2007 Society of Chemical Industry
Inter‐ and intramolecular hetero Diels‐Alder reactions, 52. Effect of high pressure on the kinetics and selectivity of the intramolecular hetero Diels‐Alder reaction of an 1‐oxa‐1,3‐butadiene
Authors: Buback Michael , Abeln Johannes , Hübsch Thomas , Ott Christian , Tietze Lutz F.
Published: July 2007

The intramolecular hetero Diels‐Alder reaction of the benzylidene‐isoxazolone 3 giving the adducts 4 and 5 is studied in dichlormethane under high pressure up to 5 kbar. The kinetics is measured by on‐line FT‐IR spectroscopy up to 3 kbar. The cycloaddition shows a clear pressure‐induced increase in diastereoselectivity. For the difference in activation volume ΔΔV≠ of the reactions affording the two diastereomers, a value of −(1.6 ± 0.2) cm3 · mol−1 is obtained. The activation volume ΔV is determined to be ‐(19.4 ± 0.5) cm3 · mol−1 at 343 K. The activation enthalpy and entropy, as derived from the overall rate coefficient k measured at 1000 bar, are (62.9 ± 1.7) kJ · mol−1 (at 343 K) and ‐(135 ± 11) J · mol−1 · K−1.