bluuebunny/crossref_metadata_2025_split huggingface dataset:
Covalent Bonding of Crown Ethers to γ‐Zirconium Phosphate—New Layered Ion Exchangers Showing Selective Recognition
Authors: Brunet Ernesto , Huelva Manuel , Vázquez Raquel , Juanes Olga , Rodríguez‐Ubis Juan C. 
Published: July 2007

AbstractThe topotactic exchange reactions of γ‐zirconium phosphate and γ‐zirconium phosphate/methylphosphonate with phosphonic acids derived from azacrown ethers of different sizes was examined. Prior intercalation of hexylamine was necessary in most cases to attain the maximum exchange level predicted by molecular modeling. Materials with imbricated or nonimbricated layers were obtained depending on crown size and exchange level. Exchange of methylphosphonates in γ‐zirconium phosphate/methylphosphonate was only observed when hexylamine had previously been intercalated. The exchange with the bisphosphonic acid derived from 1,10‐diaza[18]‐crown‐6 gave a material with an interlayer distance compatible with pillaring of the γ‐phase. The materials obtained from the exchange reactions of γ‐zirconium phosphate with phosphonic acids derived from [12]crown‐4 and [18]crown‐6 showed thermodynamic selectivities towards Na+ and K+, respectively, that were much higher than those observed for the simple crowns in solution. This observation suggests that the inorganic molecular framework enhances molecular recognition by minimization of solvation effects.
Epoxidation and Oxygen Insertion Into Alkane Ch Bonds by Dioxirane Do Not Involve Detectable Radical Pathways
Authors: Adam Waldemar , Curci Ruggero , D'Accolti Lucia , Dinoi Anna , Fusco Caterina , Gasparrini Francesco , Kluge Ralph , Paredes Rodrigo , Schulz Manfred , Smerz Alexander K. , Angela Veloza L. , Weinkötz Stephan , Winde Roland 
Published: July 2007

AbstractThe dimethyldioxirane oxidation of α‐methylstyrene, trans‐cyclo‐octene, and 1‐vinyl‐2,2‐diphenylcyclopropane gave, under all reaction conditions employed, the corresponding epoxides in high yields. No radical products from allylic oxidation, from trans/cis isomerization, or from cyclopropylcarbinyl rearrangement (radical clock) were observed. Even for these alkenes, which are prone to radical reactions, the previously established electrophilic concerted mechanism applies, rather than the recently proposed radical mechanism. The selective hydroxylation of (–)‐2–phenylbutane by dimethyldioxirane gave only (– )‐2–phenylbutan‐2‐ol with complete retention of configuration and no loss of optical purity. Thus, a radical‐chain oxidation is also discounted in the oxygen insertion into hydrocarbon C‐H bonds for dioxiranes.
1′, 5′ ‐Anhydrohexitol Oligonucleotides: Synthesis, Base Pairing and Recognition by Regular Oligodeoxyribonucleotides and Oligoribonucleotides
Authors: Hendrix Chris , Rosemeyer Helmut , Verheggen Ilse , Van Aerschot Arthur , Seela Frank , Herdewijn Piet 
Published: July 2007

AbstractOligonucleotides constructed of 1′, 5′‐anhydrohexitol nucleoside building blocks (hexitol nucleic acids, HNA) are completely stable towards 3′‐exonuclease and form very stable self‐complementary duplexes as well as sequence‐selective stable duplexes with the natural DNA and RNA. Triple‐helix formation has also been observed. These hybridisation characteristics are highly dependent on the base sequence and the experimental conditions. When using a phosphate buffer containing 0.1M NaCl, a homopurine HNA dodecamer gives a δTm of +3.0 °C/ base pair with RNA as complement. These oligomers may therefore be of considerable interest as antisense constructs.
On the Mechanism of Stereospecific Polymerization—Development of a Universal Model to Demonstrate the Relationship Between Metallocene Structure and Polymer Microstructure
Authors: van der Leek Y. , Angermund K. , Reffke M. , Kleinschmidt R. , Goretzki R. , Fink G. 
Published: July 2007

AbstractWith the discovery of stereorigid bridged metallocenes, soluble catalysts became available for the stereospecific polymerization of α‐olefins. A relatively simple mechanism was used to explain the stereospecificity, primarily in terms of the catalyst symmetry. In this paper we demonstrate that the simple rule of thumb that C2‐symmetric catalysts produce isotactic and Cs‐symmetric catalysts syndiotactic polypropylene is too narrow. The introduction of one methyl group at the Cp ring in the [{iPr(CpFlu)}ZrCl2]/ MAO system (Flu = fluorenyl, MAO = methylalumoxan) reduces the Cs symmetry to C1, and the resulting catalyst produces hemiisotactic polypropylene. The analogous catalyst with a bulkier tert‐butyl group at the Cp ring gives isotactic polypropylene. When the C2 symmetry of [{Me2Si(Ind)2}ZrCl2] (Ind = indenyl) is reduced to C1, a metallocene can be obtained that produces atactic polypropylene. We have broken away from the symmetry‐based model and developed a universal model, which accurately describes the experimental microstructures of the polymers by considering the four lowestenergy conformers of the metallocene species coordinating to prochiral propene (Rre, Sre, Ssi, and Rsi) and the positional changes that the polymer chain undergoes during insertion. The relative energy levels of the four diastereomers can be determined by molecular modeling calculations; these energy gradations, in particular the size of the energy gaps, are decisive in determining the stereospecificity. Also, the model permits the stereoerrors to be classified and explained. Through this model the stereosequence of a polymer chain can be calculated and predicted.
Homoleptic Cuprates(II) with Multiply Deprotonated α‐Cyclodextrin Ligands
Authors: Klüfers Peter , Piotrowski Holger , Uhlendorf Jürgen 
Published: July 2007

AbstractIn the presence of cupric ions, α‐cyclodextrin (α‐CD) is multiply deprotonated in alkaline aqueous solution. With lithium or sodium as the counterions, trinuclear sandwich‐type cuprate ions of the formula [Cu3(α‐CDH_6)2]6‐ are formed (1 and 2), whereas dinuclear [Cu2(α‐CDH_4)2]4‐ ions are isolated in the form of the potassium (3a) and the rubidium salts (3b). The structures of the cyclodextrinatocuprate ions resemble cylinders, since the usual conical shape of a cyclodextrin is altered due to the formation of shorter hydrogen bonds on the O2/O3 rim of the α‐cyclodextrin torus. Despite anion formation, the cavity of the cyclodextrin is still hydrophobic, giving rise to the intercalation of acetone guests in 2 and 3.
Biosynthesis of Nitric Oxide—Quantum Chemical Modelling of Nω‐Hydroxy‐l‐arginine Formation
Authors: Wasielewska Ewa , Witko Malgorzata , Stochel Grażyna , Stasicka Zofia 
Published: July 2007

AbstractThe electronic structure (charge distribution, bond indices, character of the frontier orbitals) and geometry (bond distances and angles) of L‐arginine and N‐methyl‐L‐arginine were determined by means of the INDO procedure. The method was also adopted to model the conversion of L‐arginine into N‐hydroxy‐L‐arginine in biological systems. This revealed that the approach of diatomic O species does not result in reaction, whereas the approach of either an O atom or an O2‐ ion leads to insertion of oxygen and formation of hydroxy‐L‐arginine. The insertion of oxygen between the nitrogen and hydrogen atoms leads to more stable products than insertion into the C–H bond. The same results were obtained for N‐methyl‐L‐arginine, and are consistent with the hypothesis that the inhibitive effect of N‐substitution in L‐arginine is of no importance for the first step in the biosynthesis of NO (hydroxylation process). The mechanistic considerations based on the charge distribution and frontier orbital characteristics led to the conclusion that the most probable mechanism of L‐arginine hydroxylation consists in electrophilic attack of [FeO]3+ at the Nω‐H bond, initiated by the reduction of L‐arginine+, followed by insertion of oxygen and product oxidation.
Dimethyl Peroxide Radical Cation: A New Theoretical and Experimental Approach to the C2H6O•+2 Potential Energy Surface
Authors: Schalley Christoph A. , Fiedler Andreas , Hornung Georg , Wesendrup Ralf , Schröder Detlef , Schwarz Helmut 
Published: July 2007

AbstractThe structure and the unimolecular fragmentations of the metastable dimethyl peroxide radical cation have been investigated by mass spectrometric and isotopic labeling methods as well as high‐level ab initio calculations. In line with the theoretical results, neutralization‐reionization and charge reversal experiments suggest that ionized dimethyl peroxide bears a CH3OOCH•3 connectivity. In the cation the O‐O bond dissociation energy is larger than that of the neutral counterpart; in contrast, the C‐O bond strength is slightly and that of the C‐H bond significantly reduced upon ionization. These energetic changes upon one‐electron oxidation of CH3OOCH3 are also reflected in the NR and CR mass spectra of CH3OOCH•+3. Further, for metastable CH3OOCH•3 two major fragmentation pathways are observed: 1) Loss of a hydrogen atom by cleavage of a C‐H bond is associated with a skeletal reorganization, which gives rise to a proton‐bound formaldehyde dimer. 2) The expulsion of a CH3O• radical leads to protonated formaldehyde in a surprisingly specific double hydrogen transfer involving a [CH3OH/CH2O]• ion/dipole complex as central intermediate; this complex also accounts for other minor fragmentation channels. The structures of intermediates and transition states are calculated with the BECKE 3LYP density‐functional method employing a 6‐311++G** basis.
Calix[4]arene‐Based (Hemi)carcerands and Carceplexes: Synthesis, Functionalization, and Molecular Modeling Study
Authors: van Wageningen André M. A. , Timmerman Peter , van Duynhoven John P. M. , Verboom Willem , van Veggel Frank C. J. M. , Reinhoudt David N. 
Published: July 2007

AbstractThe synthesis of 11 calix[4]arene‐based carceplexes obtained by solvent or doped inclusion is reported. Carceplexes with amides, for example, DMF, NMP, and 1,5‐dimethyl‐2‐pyrrolidinone, and sulfoxides, for example, DMSO and thiolane‐1‐oxide, were obtained by solvent inclusion. In these cases the yield of the carceplex decreases with increasing guest size. Potential guests that do not form carceplexes by solvent inclusion, such as 2‐butanone and 3‐sulfolene, could be incarcerated by doped inclusion with 1,5‐dimethyl‐2‐pyrrolidinone as a solvent “doped” with 5–15 vol% of potential guest. The amide bridges of the carceplexes were converted into thioamide bridges in essentially quantitative yield by means of Lawesson's reagent in refluxing xylene. The dynamic properties of the incarcerated guests were examined by 2D NMR spectroscopy. Whereas for most guests a preference for one orientation inside the calix[4]arene‐based (thia)carcerands was observed, for DMA, NMP, and ethyl methyl sulfoxide inside calix[4]arene‐based (thia)carcerandstwo different orientationswere present. The energy barriers for interconversion between the various orientations of DMA, NMP, and ethyl methyl sulfoxide inside calix[4]arene‐based (thia)‐carcerands were determined with 2D EXSY NMR. The energy barriers are higher for the thiacarcerands than for the corresponding carcerands with amide bridges. This may be due to the stronger hydrogen‐bond‐donating character of the thioamide group. Furthermore, molecular modeling simulations indicate that in case of the thiacarcerand the cavity is smaller as a result of a smaller diametrical distance between the NHatoms. Our results demonstrate that molecular modeling can be used to estimate the energy barriers for interconversion; the calculated activation energies showed good quantitative agreement with the experimental values.
The Crystal Polymorphism of Tetrolic Acid (CH3CCCOOH): A Molecular Dynamics Study of Precursors in Solution, and a Crystal Structure Generation
Authors: Gavezzotti A. , Filippini G. , Kroon J. , van Eijck B. P. , Klewinghaus P. 
Published: July 2007

AbstractThe possible configurations of two molecules of tetrolic acid in a solvent box containing 226 carbon tetrachloride molecules are studied by molecular dynamics with the GROMOS package and force field over periods of up to 2000 picoseconds. The cyclic hydrogen‐bonded dimer was the most persistent configuration, but events leading to the cleavage of one hydrogen bond and thus the formation of a precursor to the crystal catemer motif were found to occupy up to 10% of the simulation times. The experimental bond‐breaking enthalpy was correctly reproduced. Two different crystal structure generation procedures were employed to reproduce the two observed polymorphic crystal structures and to predict other possible polymorphs; in all cases, some unobserved structures had more cohesive packing energies than the observed ones. The possible application of molecular dynamics in the study of the preliminary steps in crystal nucleation is discussed.
Towards 2‐Silaallenes: Synthesis of Spirocyclic Precursors
Authors: Goetze Brigitte , Herrschaft Bernhard , Auner Norbert 
Published: July 2007

AbstractReactions of dichlorodivinylsilane (2) and LitBu in a molar ratio 1/1 and 1/2 lead to highly reactive intermediates, which can be trapped by suitable reagents. In the presence of trimethylmethoxysilane, norbornadiene, anthracene and diphenylacetylene, products are formed that provide evidence for the intermediate formation of both the neopentylsilene H2C=CH(Cl)Si=CHCH2tBu (3) and the 2‐silaallene tBuCH2CH=Si=CHCH2tBu (4). The formation of double cycloadducts from 4 is particularly interesting as a preparatively facile route for the synthesis of silaspirocycles such as 13, 16 and 17, which have been characterized by single‐crystal X‐ray structure analysis.
Kinetics and Mechanism of the Reactions of Amine Boranes with Carbenium Ions
Authors: Funke Marcus‐ A. , Mayr Herbert 
Published: July 2007

AbstractThe kinetics of the reactions of trialkylamine boranes and pyridine boranes with benzhydryl cations have been determined photometrically. Second‐order rate laws are obeyed, first‐order with respect to amine borane concentration and first‐order with respect to carbocation concentration. As for other reactions of carbocations with neutral nucleophiles, the rates of these reactions are only slightly affected by solvent polarity. The structure—reactivity relationships and kinetic isotope effects are in accord with a polar mechanism proceeding through a transition state where the migrating hydride is partly bound to the entering carbon and to the leaving boron atom. The rate constants correlate linearly with the electrophilicity parameters E of the carbenium ions. It is therefore possible to use the linear free enthalpy relationship logk2 = s(E+N) for determining nucleophilicity parameters N for the amine boranes and to compare their hydride‐donating abilities with those of other non‐charged hydride donors (silanes, germanes, stannanes, and dihydropyridines).
1,2‐cis‐C‐glycoside synthesis by samarium diiodide‐promoted radical cyclizations
Authors: Skrydstrup Troels , Mazéas Daniel , Elmouchir Mohamed , Doisneau Gilles , Riche Claude , Chiaroni Angèle , Beau Jean‐Marie 
Published: July 2007

AbstractThe samarium diiodide reduction of glycosyl pyridyl sulfones bearing a silicon‐tethered unsaturated group at the C2–OH position leads to the stereo‐specific synthesis of 1,2‐cis‐C‐glycosides in good yield after desilylation. These reactions proceed via an anomeric radical with subsequent 5‐exo cyclization. Unlike the corresponding glycosyl phenyl sulfones, the pyridyl derivatives react instantaneously with samarium diiodide and do not require a cosolvent such as hexamethylphosphoramide (HMPA). Under these reaction conditions radical cyclization precedes the second reduction step. Examples of 5‐exo‐trig and ‐dig ring closures are given. The synthetic utility of this method was demonstrated by a short synthesis of methyl C‐isomaltoside.
A Highly Convergent Total Synthetic Route to Glycopeptides Carrying a High‐Mannose Core Pentasaccharide Domain N‐linked to a Natural Peptide Motif
Authors: Danishefsky Samuel J. , Hu Shuanghua , Cirillo Pier F. , Eckhardt Matthias , Seeberger Peter H. 
Published: July 2007

AbstractN‐Linked glycopeptides were synthesized by condensation of a highmannose anomeric amine bearing a pentasaccharide with aspartic‐acid‐containing tri‐ and pentapeptides through the agency of IIDQ. The pentasaccharide portion, corresponding to the „core”︁ region of all asparagine‐linked glycoproteins, was assembled by means of glycal‐derived thioethyl donors and glycal acceptors. The central mannose residue was established by inversion of the C2 hydroxyl of a glucosyl precursor in the pentasaccharide. The protecting‐group scheme employed allows the extension of the pentasaccharide through the terminal mannose units. While a fully convergent coupling of the high‐mannose carbohydrate to the peptide domain has thus been accomplished for the first time with a fully synthetic sugar, the stereochemical integrity of the anomeric center of the carbohydrate domain was not maintained and a mixture of glycopeptides was obtained.
[(TptBu, Me)CrR]: A New Class of Mononuclear, Coordinatively Unsaturated Chromium(II) Alkyls with cis‐Divacant Octahedral Structure
Authors: Kersten Jörg L. , Kucharczyk Robin R. , Yap Glenn P. A. , Rheingold Arnold L. , Theopold Klaus H. 
Published: July 2007

AbstractReaction of CrCl2 with TptBu, MeK yielded [TptBu, MeCr(3‐tBu, 5‐MepzH)Cl] (1) and [TptBu, MeCrCl] (2), while the same reaction in the presence of pyridine gave 1 and [TptBu, MeCr(py)Cl] (3). Alkylation of 3 with Grignard reagents produced the chromium(II) alkyls [TptBu, MeCrR] (4, R = Et; 5, R = Ph; 6, R = CH2SiMe3), which reversibly added to pyridine to form the five‐coordinate adducts [TptBu, MeCr(py)R] (7, R = Et; 8, R = Ph; 9, R = CH2SiMe3). 1, 2, 4, and 5 were structurally characterized by X‐ray diffraction. The four‐coordinate molecules 2, 4, and 5 adopt a highly unusual cis‐divacant octahedral coordination geometry, while 1 is the first five‐coordinate TptBu, Me‐complex of a first‐row transition metal. Despite their coordinative unsaturation, chromium alkyls 4‐6 do not polymerize ethylene, or even react with it. This observation is inconsistent with the catalytic activity commonly ascribed to divalent chromium in heterogeneous polymerization catalysts. Attempts to oxidize 4‐6 (e.g., with [Cp2Fe]BPh4) to cationic chromium(II) alkyls failed, yielding [TptBu, MeCr(thf)][BPh4] (10) instead.
Tetracenomycin M, a Novel Genetically Engineered Tetracenomycin Resulting from a Combination of Mithramycin and Tetracenomycin Biosynthetic Genes
Authors: Künzel Eva , Wohlert Sven‐Eric , Beninga Claus , Haag Sabine , Decker Heinrich , Hutchinson C. Richard , Blanco Gloria , Mendez Carmen , Salas Jose A. , Rohr Jürgen 
Published: July 2007

AbstractThe hybrid strain Streptomyces glaucescens Tü49 (pGB7) contains recombined genes of the tetracenomycin C (4) and the mithramycin (6) biosynthesis cluster. It was designed by the trans formation of plasmid pGB7 into the tetracenomycin producer Streptomyces glaucescens Tü49. Plasmid pGB7 carries the minimal polyketide synthase (PKS) genes of mithramycin biosynthesis (mtmP, mtmK, mtmS) along with its upstream (mtmX, which encodes a gene product of unknown function) and downstream flanking genes (the ketoreductasecoding mtmT1 and fragments of mtmO 1, which encodes an oxygenase). It was assumed that early intermediates of the wellknown biosynthesis of 4, such as tetracenomycin F2 (3) or very similar molecules, are likely to also serve as intermediates of the biosynthesis of aureolic acid antibiotics, such as 6. Thus, the enzymes of both parent biosynthetic pathways should be able to act on such intermediates, and several hybrid molecules were expected. Although the experiment resulted in new products, only the novel hybrid natural product tetracenomycin M (1), whose constitution was determined unambigously by spectroscopic methods, was obtained in larger amounts. The formation of 1 can be explained, if a combination of enzymes of both parent biosynthetic pathways is taken into consideration. When plasmid pGB7 is transformed into Streptomyces lividans TK21, that is, a strain which does not produce any secondary metabolites under our laboratory conditions, the production of SEK15 (2) is observed. The latter is well known as the product of the minimal PKS of decaketides, and its exclusive production indicates that the aureolic acid antibiotics are constructed via a single decaketide chain and that the enzyme products of the flanking genes mtmX, mtmT1, and mtmO1 cannot contribute in this second experiment.
Two New Closely Related MoV Hydroxymonophosphates Built Up of Cd[Mo6P4O25(OH)6]2 And Cd[Mo6P4O26(OH)5]2 Clusters
Authors: Guesdon A. , Borel M. M. , Leclaire A. , Raveau B. 
Published: July 2007

AbstractTwo new molybdenum(v) hydroxyphosphates have been synthesized hydrothermally, Na2Cd3(Mo2O4OH)6‐(PO4)2(PO3OH)6[N(CH3)4]4·10H20 (1) and Cd9(Mo2O4OH)12(PO4)6(PO3OH)10‐[N(CH3)4]8·15H2O (2). Their structures have been determined from single‐crystal X‐ray diffraction. The water molecules and hydroxyl groups have been deduced from valence calculations. Both compounds crystallize in the triclinic space group P1, with the cell parameters for 1 a = 12.340(2), b = 12.596(1), c = 14.717(2) Å, α = 107.24(1)°, β = 89.83(1)°, γ = 114.31(1)°, V = 1972.3(4) Å3, and for 2 a = 11.942(1), b = 13.339(2), c = 26.765(3) Å, α = 85.33(1)°, β = 86.87(1)°, γ = 64.08(1)°, V = 3821.3(9) Å3. The two frameworks can be described on the basis of similar [Mo6P4X31]n‐ (X = O, OH) anionic clusters, but 1 is a tridimensional structure, whereas 2 exhibits a monodimensional structure.
Stoichiometric and Catalytic Reactions of the Polysiloxane‐Bound (Ether‐Phosphine)Rhodium(I) Complex [ClRh(PO)(P ∼ O)] in Interphases
Authors: Lindner Ekkehard , Schneller Theodor , Auer Friedrich , Wegner Peter , Mayer Hermann A. 
Published: July 2007

AbstractThe reaction of four equivalents of the monomeric trimethoxysilyl(T)‐functionalized ether‐phosphine ligand CyP(CH2CH2OCH3)(CH2)3SiR3 (R = OMe [1 a(T0)], Me [1 b]) with [{μ‐ClRh(COE)2}2] yielded the monomeric pseudo 14 electron rhodium(I) complexes [ClRh(PO)(P∼O)] (2a(T0)2, 2 b). For the sol‐gel process the complex 2 a(T0)2 was protected by introduction of the volatile, reversibly binding ligand pyridine. Thus, the monomeric compound 2 a(T0)2 was co‐condensed with two and eight equivalents of the co‐condensation agent MeSi(OMe)2(CH2)6(MeO)2SiMe (D0‐C6‐D0) to give the polysiloxane‐bound congeners 2(Tn)2(Di‐C6‐Di)y (y = 2 and 8, respectively; i = 0–2; n = 0–3). The polysiloxane‐bound complex 2(Tn)2‐(Di‐C6‐Di)2 was treated with a variety of small molecules in the gas/solid and liquid/solid interphases. It was shown that a facile cleavage of the Rh‐O bond in the ether‐phosphine chelate occurred even in the solid state. The reaction of 2(Tn)2‐(Di‐C6‐Di)2 with carbon monoxide, carbon disulfide, and diphenylacetylene resulted in the irreversible coordination of the molecule to the metal. In the presence of pyridine, the polysiloxane‐bound complex 2(Tn)2‐(Di‐C6‐Di)2 oxidatively added hydrogen to give the octahedrally configurated complex [ClRhH2(Py)(P ∼ O)2] [6(Tn)2(Di‐C6‐Di)2]. Treatment of dry 2(Tn)2‐(Di‐C6‐Di)2 with ethene led to the reversible formation of the corresponding complex. Although the materials display low surface areas, at least 75% of the rhodium centers within the matrix are accessible to the rather bulky tolan molecules. The complexes 2(Tn)2(Di‐C6‐Di)y (y (y = 2, 8) show high activities and selectivities in the hydrogenation of tolan. The conversion was found to depend markedly on the amount of co‐condensate (D0‐C6‐D0) and on the polarity of the solvent. The polysiloxane‐bound complexes 2(Tn)2(Di‐C6‐Di)y are more active than their monomeric congener 2 a(T0)2.