bluuebunny/crossref_metadata_2025_split huggingface dataset:
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

AbstractAn 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

AbstractA 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

AbstractThe [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

AbstractThe 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

AbstractThe 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

AbstractWe 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

AbstractThe 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

AbstractIf 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

AbstractThe 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

AbstractOptically 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

Abstractβ‐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

AbstractThe 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

AbstractUltimate 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

AbstractA 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

AbstractThe 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

AbstractThe 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

AbstractA 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.