bluuebunny/crossref_metadata_2025 huggingface dataset:
Silicon‐Bridge Effects on Photophysical Properties of Silafluorenes
Authors: Shimizu Masaki , Tatsumi Hironori , Mochida Kenji , Oda Katsunari , Hiyama Tamejiro
Published: July 2008

The preparation of 4,5‐dimethylsilylene‐ or 4,5‐tetramethyldisilylene‐bridged 9‐silafluorenes was achieved by lithiation of 2,2′,6,6′‐tetrabromobiphenyls followed by silylation with dichlorodimethylsilane or 1,2‐dichloro‐1,1,2,2‐tetramethyldisilane, respectively. X‐ray analysis of the silylene‐bridged silafluorene revealed that the molecular framework was perfectly planar and four SiC(methyl) σ bonds were completely orthogonal to the plane. Both the silicon atoms and the benzene rings were significantly deformed from the normal tetrahedral and hexagon shapes, respectively. The silicon bridge at the 4,5‐positions was found to induce a red shift of the absorption and fluorescence spectra measured in cyclohexane, compared with 9‐silafluorenes. It is remarkable that the disilylene‐bridged silafluorene emitted blue light (λem=450 nm) with a large Stokes shift. The emission maxima of the silicon‐bridged silafluorenes in thin films were similar to those measured in cyclohexane solution. DFT calculations suggested that introduction of the silicon bridge led to increases in both the HOMO and LUMO levels compared with 9‐silafluorene.
Conjugated Polyelectrolytes for Protein Assays and for the Manipulation of the Catalytic Activity of Enzymes
Authors: An Lingling , Wang Shu , Zhu Daoben
Published: July 2008

A new method has been developed to discriminate between proteins with different isoelectric points by using fluorescent conjugated polyelectrolytes. Charged water‐soluble polyfluorenes that contain 2,1,3‐benzothiadiazole (BT) units demonstrate intramolecular energy transfer from the fluorene units to the BT sites when oppositely charged proteins are added to the mixture. This results in a shift in emission color from blue to green and a change in the emission intensity of conjugated polyelectrolytes, which makes it possible to assay proteins. The formation of conjugated polyelectrolyte/enzyme complexes by electrostatic interactions can be utilized to manipulate the activity of enzymes by means of local alteration of enzyme charge density. The oppositely charged substrate binds to conjugated polyelectrolyte and reduces the distance between the enzyme and the substrate, leading to an increase in the cleavage reaction rate. The new method has three important features: 1) it offers a convenient “mix‐and‐detect” continuous approach for protein assays and rapid detection of enzyme activity; 2) the use of water‐soluble conjugated polyelectrolytes imparts the sensor with a high sensitivity; 3) this method does not require fluorescent labels on the targets, which should significantly reduce the cost.
Synthesis of Bulky Arylphosphanes by Rhodium‐Catalyzed Formal [2+2+2] Cycloaddition Reaction and Their Use as Ligands
Authors: Kobatake Takayuki , Kondoh Azusa , Yoshida Suguru , Yorimitsu Hideki , Oshima Koichiro
Published: July 2008

Treatment of 1‐alkynylphosphane sulfides with 1,6‐ or 1,7‐diynes in the presence of a cationic rhodium catalyst results in a formal [2+2+2] cycloaddition reaction to afford the corresponding aromatic phosphane sulfides. The aromatic rings formed in the cycloaddition naturally bear one or two substituents at the ortho positions to the phosphorus atom, which creates a sterically hindered environment around the phosphorus atom. The following desulfidation of the products is facile under radical conditions or with the aid of tris(dimethylamino)phosphane, providing the corresponding bulky phosphanes. Dicyclohexyl(2,6‐diphenylaryl)phosphane, which is available through this sequence, proves to serve as an efficient ligand in palladium‐catalyzed cross‐coupling amination reactions.
Chiral Schiff Bases as Highly Active and Enantioselective Catalysts in Catalytic Addition of Dialkylzinc to Aldehydes
Authors: Tanaka Takanori , Sano Yuki , Hayashi Masahiko
Published: July 2008

We have developed chiral Schiff base catalysts based on the ONO‐tridentate ligand for the catalytic enantioselective addition of dialkylzinc to aldehydes. Various aldehydes were smoothly converted into corresponding optically active secondary alcohols with high enantioselectivity (up to 98 % ee) in high yield. Furthermore, we have succeeded in decreasing the catalyst loading minimum to 0.1 mol % in the chiral Schiff base‐catalyzed enantioselective alkylation of aldehydes.
Marked Deuterium Isotope Effects on the Enantioselectivity in Rhodium‐Catalyzed Asymmetric Hydrogenation of Enamides
Authors: Imamoto Tsuneo , Itoh Takuma , Yoshida Kazuhiro , Gridnev Ilya D.
Published: July 2008

Deuterium isotope effects on the enantioselectivity in the rhodium‐catalyzed asymmetric hydrogenation of enamides and related substrates have been studied. Distinct deuterium isotope effects were observed in the hydrogenation of aryl‐substituted enamides having an ortho substituent that is capable of forming a hydrogen bond. The observed isotope effects are interpreted in terms of the competitive reactions of two dihydride intermediates and dideuteride intermediates that exist in equilibrium in the catalytic cycle.
A General Preparation of Polyfunctional Benzylic Zinc Organometallic Compounds
Authors: Metzger Albrecht , Schade Matthias A. , Manolikakes Georg , Knochel Paul
Published: July 2008

A new method for the preparation of highly functionalized benzylic zinc chlorides by the direct insertion of zinc dust into the corresponding benzylic chlorides in the presence of LiCl is described without the formation of homocoupling products (<5 %). Various reactions of these benzylic zinc reagents with a broad range of electrophiles, which lead to polyfunctionalized products, are reported. In particular, the cross‐coupling reactions of the benzylic zinc chlorides with aromatic chlorides, bromides, and tosylates in the presence of [Ni(acac)2] (acac=acetylacetonate) and PPh3 proceeded in good to excellent yields to give the corresponding diaryl and heterodiaryl methanes.
Catalytic Stereoselective Glycosidation with Glycosyl Diphenyl Phosphates: Rapid Construction of 1,2‐cis‐α‐Glycosidic Linkages
Authors: Koshiba Miyuki , Suzuki Noritoshi , Arihara Ryoichi , Tsuda Toshifumi , Nambu Hisanori , Nakamura Seiichi , Hashimoto Shunichi
Published: July 2008

A commercially available 0.1 M solution of HClO4 in dioxane has been shown to catalyze the glycosidation of glycosyl diphenyl phosphates. The per‐O‐benzyl‐protected glucosyl and galactosyl donors and the 3,4,6‐tri‐O‐acetyl‐2‐azido‐2‐deoxygalactosyl donor each react with a range of acceptor alcohols in the presence of 0.05–0.2 equiv of HClO4 in dioxane/Et2O (1:1) to afford glycosides in good yields with good to excellent α selectivities. The synthetic utility of this glycosidation method was demonstrated by a stereoselective synthesis of the α‐galactosylceramide KRN7000, an activator of natural killer (NK) T cells through CD1d molecules.
Synthesis and Application of Easily Recyclable Thiomorpholines for Use in Sulfur Ylide Mediated Asymmetric Epoxidation of Aldehydes
Authors: Hansch Markus , Illa Ona , McGarrigle Eoghan M. , Aggarwal Varinder K.
Published: July 2008

Chiral nonracemic thiomorpholines have been synthesized in four to six steps from limonene or achiral alkenes using α‐methylbenzylamine to control absolute stereochemistry. These aminosulfides have been used to generate sulfur ylides, which have been applied in the asymmetric epoxidation of aldehydes as easily recoverable catalysts. Excellent yields (up to 98 %), enantioselectivities (up to 97:3 e.r.), and diastereoselectivities (≥98:2 trans/cis) were achieved in these epoxidations and the sulfides were easily recovered in high yield (up to 97 %) by simple acid/base extraction.
The effect of consequential thinking on trust game behavior
Authors: Kugler Tamar , Connolly Terry , Kausel Edgar E.
Published: July 2008

Contrary to rational Expected Monetary Value (EMV) predictions that no money will be transferred in Trust Games, in experiments players make positive transfers. Theorists have proposed modifying the Sender's utility function while retaining utility‐maximization assumptions to account for this behavior. Such accounts assume that Senders can grasp the possible outcomes of their choices, their probabilities, and utilities. In reality, however, Senders' choices are unexpectedly complex, and the assumption that they approximate expected utility maximization is highly implausible. Instead, we suggest that Senders are guided by general propensities to trust others. Two experiments examine the effect of inducing consequential thought on Sender behavior. One induced consequential thought directly; the other did so indirectly. The amount sent was significantly reduced following either manipulation. This suggests that models of Sender behavior in Complex Trust Games should not assume that participants routinely engage in consequential thinking (CT) of the depth that would be required for utility maximization. Copyright © 2008 John Wiley & Sons, Ltd.
Enhanced Complexity and Catalytic Efficiency in the Hydrolysis of Phosphate Diesters by Rationally Designed Helix‐Loop‐Helix Motifs
Authors: Razkin Jesus , Lindgren Johan , Nilsson Helena , Baltzer Lars
Published: July 2008

HJ1, a 42‐residue peptide that folds into a helix‐loop‐helix motif and dimerizes to form a four‐helix bundle, successfully catalyzes the cleavage of “early stage” DNA model substrates in an aqueous solution at pH 7.0, with a rate enhancement in the hydrolysis of heptyl 4‐nitrophenyl phosphate of over three orders of magnitude over that of the imidazole‐catalyzed reaction, k2(HJ1)/k2(Im)=3135. The second‐order rate constant, k2(HJ1) was determined to be 1.58×10−4 M−1 s−1. The catalyst successfully assembles residues that in a single elementary reaction step are capable of general‐acid and general‐base catalysis as well as transition state stabilization and proximity effects. The reactivity achieved with the HJ1 polypeptide, rationally designed to catalyze the hydrolysis of phosphodiesters, is based on two histidine residues flanked by four arginines and two adjacent tyrosine residues, all located on the surface of a helix‐loop‐helix motif. The introduction of Tyr residues close to the catalytic site improves efficiency, in the cleavage of activated aryl alkyl phosphates as well as less activated dialkyl phosphates. HJ1 is also effective in the cleavage of an RNA‐mimic substrate, uridine‐3′‐2,2,2‐trichloroethyl phosphate (leaving group pKa=12.3) with a second‐order rate constant of 8.23×10−4 M−1 s−1 in aqueous solution at pH 7.0, some 500 times faster than the reaction catalyzed by imidazole, k2(HJ1)/k2(Im)=496.
Modular Assembly Using Sequential Palladium Coupling Gives Easy Access to the SMoC Class of Cellular Transporters
Authors: Rebstock Anne‐Sophie , Visintin Cristina , Leo Elisabetta , Garcia Posada Cristina , Kingsbury Sarah R. , Williams Gareth H. , Stoeber Kai , Selwood David L.
Published: July 2008

The transducing ability of the third helix of transcription factor homeodomains is effectively mimicked by a biphenyl system displaying guanidine groups. The biphenyl class of small molecule carriers (SMoCs) can carry biomolecules into a wide variety of cell types. A “combinatorial” approach to the synthesis of SMoCs is described using sequential Pd0 coupling chemistry to assemble the molecules from highly functionalized building blocks. SMoCs coupled to the DNA licensing repressor protein geminin can inhibit DNA replication in vitro. We conducted a structure–activity investigation utilizing a range of SMoC–geminin conjugates and demonstrate that both electrostatic and structural features are important for efficient uptake and functional activity. The best analogue was more efficient than either (Arg)4 or (Arg)8 linked to geminin. Effective inhibition of DNA synthesis was achieved in fibroblasts and osteosarcoma cell lines.
Blue‐Light Induced Interaction of LOV Domains from Chlamydomonas reinhardtii
Authors: Kutta Roger J. , Hofinger Edith S. A. , Preuss Hendrik , Bernhardt Günther , Dick Bernhard
Published: July 2008

The phototropin from Chlamydomonas reinhardtii is a 120 kDa blue light receptor that plays a key role in gametogenesis of this green alga. It comprises two light‐sensing domains termed LOV1 and LOV2 (light oxygen and voltage) and a serine/threonine kinase domain. The post‐translationally incorporated chromophore is flavin mononucleotide (FMN). Upon absorption of blue light, LOV domains undergo a photocycle that activates a Ser/Thr kinase. The mechanism of this activation is still unknown. We studied the oligomerization of the recombinant LOV1 domain (amino acids 16–133) of C. reinhardtii by means of UV/Vis spectroscopy, size‐exclusion chromatography (SEC), and chemical cross‐linking with glutardialdehyde. The thermal back‐reaction of LOV1 from the signaling state to the dark state as monitored by UV/Vis spectroscopy after an intensive blue light pulse could not be explained by a monoexponential model, although the spectra did not indicate the presence of an additional species. Therefore, we investigated the quaternary structure of the LOV1 domain by size‐exclusion chromatography in the dark. This revealed an equilibrium between dimers and higher oligomers (MW>200 kDa) under native conditions. No monomers were detected by SEC. However, by analysis of the equilibrium by cross‐linking of the protein with glutardialdehyde and subsequent SDS‐PAGE, monomers and dimers were identified. Exposure of LOV1 to blue light resulted in a decrease in the monomer/dimer ratio, followed by re‐equilibration in the dark. Calculation of the solvent‐accessible surface area and the Conolly surfaces of the LOV1 dimers present in the crystal structure support the experimental observation that no mononomers are detected in the native state. A model is presented that accounts for a blue‐light‐driven change in the quaternary structure of the LOV1 domain and gives hints to the molecular basis of light activation and regulation in LOV‐containing proteins.
Dynamic Combinatorial Self‐Assembly of Cyclophilin hCyp‐18 Ligands through Oxorhenium Coordination
Authors: Clavaud Cécile , Le Gal Julien , Thai Robert , Moutiez Mireille , Dugave Christophe
Published: July 2008

The dynamic combinatorial assembly of independent modules A and B through oxorhenium(V) coordination by a NS2+S motif in the presence of cyclophilin hCyp‐18—an important peptidyl‐prolyl isomerase—was investigated. Increasing glutathione (GSH) concentrations were used to dissociate [A⋅ReVO⋅B] complexes that displayed low affinity for hCyp‐18. Conversely, coordinates that displayed submicromolar affinities for hCyp‐18 were protected against thiol exchange and could be detected by LC‐MS. Determination of the GSH concentration that decreased the extracted ionic current of the complex by 50 % (CC50) enabled the selection of three oxorhenium coordinates that were shown to bind to the active site of hCyp‐18 and to inhibit its peptidyl–prolyl isomerase activity in the micromolar to submicromolar range.
Multivalent Carbohydrate Recognition on a Glycodendrimer‐Functionalized Flow‐Through Chip
Authors: Branderhorst Hilbert M. , Ruijtenbeek Rob , Liskamp Rob M. J. , Pieters Roland J.
Published: July 2008

Dendrimers were fitted out with up to eight mannose moieties by “click” chemistry. They were subsequently attached to aluminum oxide chips via a spacer that was linked to the dendrimer core; this resulted in a microarray of glycodendrimers. Binding of the glycodendrimers to the fluorescent lectins ConA and GNA was observable in real time. In a single experiment it was possible to observe the multivalency enhancement or cluster effect in the binding event. This effect was small for ConA, in agreement with its widely spaced binding sites, whereas it was large for GNA, with its twelve much more closely spaced binding sites. The dendrimer‐fitted chip represents a valuable screening tool for multivalency effects. Furthermore kinetic and thermodynamic data on binding events can be deduced. Inhibition experiments are also possible with the system as was shown for ConA with α‐methyl mannose as the inhibitor.
A Gold(I)‐Catalyzed Intramolecular Reaction of Propargylic/Homopropargylic Alcohols with Oxirane
Authors: Dai Lun‐Zhi , Shi Min
Published: July 2008

The gold(I)‐catalyzed cycloisomerization of epoxy alkynes in the presence of a nucleophile is an efficient protocol to provide ketal skeletons with high stereoselectivity. An intramolecular reaction of propargylic/homopropargylic alcohols with oxirane to produce ketal/spiroketals in moderate yields under mild conditions has been reported. Moreover, the mechanism of this kind of reaction has been discussed on the basis of a series of control and 18O tracer experiments.
Spectroscopic Properties of Orthometalated Platinum(II) Bipyridine Complexes Containing Various Ethynylaryl Groups
Authors: Clark Meaghan L. , Diring Stéphane , Retailleau Pascal , McMillin David R. , Ziessel Raymond
Published: July 2008

Neutral orthometalated platinum(II) complexes of the deprotonated 6‐phenyl‐2,2′‐bipyridine ligand (bearing a trialkoxygallate, tolyl, ethynyltrialkoxygallate, or ethynyltolyl substituent) and a σ‐bonded Cl, ethynyltolyl, or ethynyltrialkoxygallate coligand have been prepared by a stepwise procedure based on copper‐promoted cross‐coupling reactions. The X‐ray structure of the [2‐(p‐tolyl)ethynyl][4‐{2‐(p‐tolyl)ethynyl}‐6‐phenyl‐2,2′‐bipyridyl)]platinum(II) complex revealed a coplanar arrangement of all residues bound to platinum, although the tolylethynyl groups exhibit position‐dependent bending in the solid state. The complexes exhibit charge‐transfer absorption in the visible region. All except two of the complexes also exhibit charge‐transfer emission, typically from an excited state that has a submicrosecond lifetime at room temperature in deoxygenated dichloromethane solution. In accordance with the presence of a carbometalated polypyridine ligand, the emitting state is assumed to have a mixture of metal‐to‐ligand charge‐transfer (MLCT) and intra‐ligand charge‐transfer (ILCT) character. However, spectral comparisons and electrochemical data suggest that the emissive state also exhibits interligand charge‐transfer (LLCT) character when an electron‐rich ethynylaryl group is bound to platinum. In keeping with altered orbital parentage in the latter systems, the emission occurs at longer wavelength. The excited‐state lifetime is also shorter, evidently due to vibronic interactions. The decay is so efficient when an ethynyltrialkoxygallate group binds to platinum that there is no detectable emission in fluid solution, although the complexes do emit in a frozen glass. The excited states are subject to associative (exciplex) quenching by Lewis bases, but the admixture of ILCT and/or LLCT character diminishes efficiency, except for relatively strong bases like dimethyl sulfoxide and dimethylformamide.
Cyclophanes of Perylene Tetracarboxylic Diimide with Different Substituents at Bay Positions
Authors: Feng Junqian , Zhang Yuexing , Zhao Chuntao , Li Renjie , Xu Wei , Li Xiyou , Jiang Jianzhuang
Published: July 2008

Cyclophanes of perylene tetracarboxylic diimides (PDIs) with different substituents at the bay positions, namely four phenoxy groups at the 1,7‐positions (1), four piperidinyl groups at the 1,7‐positions (2), and eight phenoxy groups at the 1,6,7,12‐positions (3) of the two PDI rings, have been synthesized by the condensation of perylene dianhydride with amine in a dilute solution. These novel cyclophanes were characterized by 1H NMR spectroscopy, MALDI‐TOF mass spectrometry, electronic absorption spectroscopy, and elemental analysis. The conformational isomers of cyclophanes substituted with four piperidinyl groups at the 1,7‐positions (2 a and 2 b) were successfully separated by preparative TLC. The main absorption band of the cyclophanes shifts significantly to the higher energy side in comparison with their monomeric counterparts, which indicates significant π–π interaction between the PDI units in the cyclophanes. Nevertheless, both the electronic absorption and fluorescence spectra of the cyclophanes were found to change along with the number and nature of the side groups at the bay positions of the PDI ring. Time‐dependent DFT calculations on the conformational isomers 2 a and 2 b reproduce well their experimental electronic absorption spectra. Electrochemical studies reveal that the first oxidation and reduction potentials of the PDI ring in the cyclophanes increase significantly compared with those of the corresponding monomeric counterparts, in line with the change in the energy of the HOMO and LUMO according to the theoretical calculations.
Ultrafast Excited State Dynamics Controlling Photochemical Isomerization of N‐Methyl‐4‐[trans‐2‐(4‐pyridyl)ethenyl]pyridinium Coordinated to a {ReI(CO)3(2,2′‐bipyridine)} Chromophore
Authors: Busby Michael , Hartl František , Matousek Pavel , Towrie Mike , Vlček Antonín
Published: July 2008

Two multifunctional photoactive complexes [Re(Cl)(CO)3(MeDpe+)2]2+ and [Re(MeDpe+)(CO)3(bpy)]2+ (MeDpe+=N‐methyl‐4‐[trans‐2‐(4‐pyridyl)ethenyl]pyridinium, bpy=2,2′‐bipyridine) were synthesized, characterized, and their redox and photonic properties were investigated by cyclic voltammetry; ultraviolet–visible–infrared (UV/Vis/IR) spectroelectrochemistry, stationary UV/Vis and resonance Raman spectroscopy; photolysis; picosecond time‐resolved absorption spectroscopy in the visible and infrared regions; and time‐resolved resonance Raman spectroscopy. The first reduction step of either complex occurs at about −1.1 V versus Fc/Fc+ and is localized at MeDpe+. Reduction alone does not induce a trans→cis isomerization of MeDpe+. [Re(Cl)(CO)3(MeDpe+)2]2+ is photostable, while [Re(MeDpe+)(CO)3(bpy)]2+ and free MeDpe+ isomerize under near‐UV irradiation. The lowest excited state of [Re(Cl)(CO)3(MeDpe+)2]2+ has been identified as the Re(Cl)(CO)3→MeDpe+ 3MLCT (MLCT=metal‐to‐ligand charge transfer), decaying directly to the ground state with lifetimes of ≈42 (73 %) and ≈430 ps (27 %). Optical excitation of [Re(MeDpe+)(CO)3(bpy)]2+ leads to population of Re(CO)3→MeDpe+ and Re(CO)3→bpy 3MLCT states, from which a MeDpe+ localized intraligand 3ππ* excited state (3IL) is populated with lifetimes of ≈0.6 and ≈10 ps, respectively. The 3IL state undergoes a ≈21 ps internal rotation, which eventually produces the cis isomer on a much longer timescale. The different excited‐state behavior of the two complexes and the absence of thermodynamically favorable interligand electron transfer in excited [Re(MeDpe+)(CO)3(bpy)]2+ reflect the fine energetic balance between excited states of different orbital origin, which can be tuned by subtle structural variations. The complex [Re(MeDpe+)(CO)3(bpy)]2+ emerges as a prototypical, multifunctional species with complementary redox and photonic behavior.
Evaluation of Electron Population Terms for 〈rSe−3〉4p, 〈rS−3〉3p, and 〈rO−3〉2p: How Do HOMO and LUMO Shrink or Expand Depending on Nuclear Charges?
Authors: Nakanishi Waro , Hayashi Satoko , Narahara Kenji , Yamaki Daisuke , Hada Masahiko
Published: July 2008

Electron population terms 〈${r{{- 3\hfill \atop N\hfill}}}$〉 are evaluated for N=Se, S, and O. Calculations are performed on HOMO and LUMO constructed by pure atomic 4p(Se), 3p(S), and 2p(O) orbitals, employing the 6‐311+G(3d) and/or 6‐311++G(3df,3pd) basis sets at the HF, MP2, and DFT (B3 LYP) levels. Se4+, Se2+, Se0, and Se2− with the Oh symmetry are called G(A: Se) and HSe+, H2Se, and HSe− with the C∞h or C2v symmetry are named G(B: Se), here [G(A+B: Se) in all]. HOMO and LUMO in G(A+B: N) (N=Se, S, and O) satisfy the conditions of the calculations for 〈${r{{- 3\hfill \atop N\hfill}}}$〉. The 〈${r{{- 3\hfill \atop {\rm Se}\hfill}}}$〉4p, 〈${r{{- 3\hfill \atop {\rm S}\hfill}}}$〉3p, and 〈${r{{- 3\hfill \atop {\rm O}\hfill}}}$〉2p values correlate well with the corresponding MO energies (εN) for all calculation levels employed. Plots of 〈${r{{- 3\hfill \atop N\hfill}}}$〉HOMO and 〈${r{{- 3\hfill \atop N\hfill}}}$〉LUMO versus Q(N) (N=Se, S, and O) at the HF and MP2 levels are analyzed as two correlations. However, the plots at the DFT level can be analyzed as single correlation. A regression curve is assumed for the analysis. Behaviors of 〈${r{{- 3\hfill \atop N\hfill}}}$〉 clarify how valence orbitals shrink or expand depending on Q(N). The applicability of 〈${r{{- 3\hfill \atop N\hfill}}}$〉 is examined to establish a new method that enables us to analyze chemical shifts with the charge effect separately from others. A utility program derived from the Gaussian 03 (NMRANAL‐NH03G) is applied to evaluate 〈${r{{- 3\hfill \atop N\hfill}}}$〉 and examine the applicability to the NMR analysis.
A Rigid, Chiral, Dendronized Polymer with a Thermally Stable, Right‐Handed Helical Conformation
Authors: Zhang Afang , Rodríguez‐Ropero Francisco , Zanuy David , Alemán Carlos , Meijer E. W. , Schlüter A. Dieter
Published: July 2008

First‐ and second‐generation dendronized polymethacrylates PG1 and PG2 carrying chiral 4‐aminoproline‐based dendrons were obtained on the half‐gram scale in high molar masses (PG1: Mn=5×106 g mol−1, PG2: Mn=1×106 g mol−1) by spontaneous (radical) polymerization of the corresponding vinyl macromonomers. NMR spectroscopic studies on PG2 together with its unprecedented high glass transition temperature (Tg>200 °C, decomp) and structural parameters provided by atomistic MD simulations show this polymer to be rather rigid. Optical rotation and CD measurements revealed that PG2 adopts a helical conformation that remains unchanged over wide ranges of temperature and solvent polarity. It is also retained when the polymer is deprotected (and thus positively charged, de‐PG2) at its terminal amino groups, by which the mass and steric demand of the dendrons is reduced by roughly 50 %. Molecular dynamics simulations on models of PG2 reveal its helical conformation to be right‐handed, irrespective of backbone tacticity, and initial results also indicate that de‐PG2 retains the right‐handedness.
Laves Phases, γ‐Brass, and 2×2×2 Superstructures: A New Class of Quasicrystal Approximants and the Suggestion of a New Quasicrystal
Authors: Berger Robert F. , Lee Stephen , Johnson Jeffreys , Nebgen Ben , So Adrian Chi‐Yau
Published: July 2008

Of the most common cubic intermetallic structure types, several (MgCu2, Cu5Zn8, Ti2Ni, and α‐Mn) have superstructures with unusual symmetry properties. These superstructures (Be5Au, Li21Si5, Sm11Cd45, and Mg44Ir7) have the unusual property of pairs of perpendicular pseudo fivefold axes, most apparent in their X‐ray diffraction patterns. The current work shows that an 8D to 3D projection method cleanly describes most (and in one case, all) of the atomic positions in the four superstructures mentioned above. This type of projection, which maps the E8 lattice (a mathematically simple 8D crystal) into 3D space, combines the desired higher dimensional point group's perpendicular fivefold rotations with 3D translational symmetry—exactly what we see in the experimental crystal structures. The projection method successfully accounts for all heavy atom positions in the four superstructures, and at least 60–70 % of the light atom positions. The results suggest that all of these structures, previously known to be connected only by qualitative similarities in their atomic “clusters”, are approximants of a single, as‐yet unknown, class of quasicrystal.
Oriented Immobilization of a Fully Active Monolayer of Histidine‐Tagged Recombinant Laccase on Modified Gold Electrodes
Authors: Balland Véronique , Hureau Christelle , Cusano Angela Maria , Liu Yingli , Tron Thierry , Limoges Benoît
Published: July 2008

The formation of a dense monolayer of histidine‐tagged recombinant laccase on gold electrodes by using a short thiol‐NTA linker is described, as well as a kinetic analysis of the process by cyclic voltammetry. From a detailed analysis of the catalytic reduction of dioxygen by laccase in the presence of a one‐electron redox mediator it can be concluded that the immobilized enzyme remains as active as in homogeneous solution.
Ligand‐Free Sonogashira Coupling Reactions with Heterogeneous Pd/C as the Catalyst
Authors: Mori Shigeki , Yanase Takayoshi , Aoyagi Satoka , Monguchi Yasunari , Maegawa Tomohiro , Sajiki Hironao
Published: July 2008

A variety of aryl iodides were coupled with aromatic and aliphatic terminal alkynes to give the corresponding 1,2‐disubstituted aromatic alkynes in good yields by using only 0.4 mol % of the heterogeneous 10 % Pd/C as the catalyst without a ligand, copper salt, or amine in an aqueous medium.
Non‐natural Amino Acids as Modulating Agents of the Conformational Space of Model Glycopeptides
Authors: Fernández‐Tejada Alberto , Corzana Francisco , Busto Jesús H. , Jiménez‐Osés Gonzalo , Peregrina Jesús M. , Avenoza Alberto
Published: July 2008

The synthesis and conformational analysis in aqueous solution of different α‐methyl‐α‐amino acid diamides, derived from serine, threonine, β‐hydroxycyclobutane‐α‐amino acids, and their corresponding model β‐O‐glucopeptides, are reported. The study reveals that the presence of an α‐methyl group forces the model peptides to adopt helix‐like conformations. These folded conformations are especially significant for cyclobutane derivatives. Interestingly, this feature was also observed in the corresponding model glucopeptides, thus indicating that the α‐methyl group and not the β‐O‐glucosylation process largely determines the conformational preference of the backbone in these structures. On the other hand, atypical conformations of the glycosidic linkage were experimentally determined. Therefore, when a methyl group was located at the Cβ atom with an R configuration, the glycosidic linkage was rather rigid. Nevertheless, when the S configuration was displayed, a significant degree of flexibility was observed for the glycosidic linkage, thus showing both alternate and eclipsed conformations of the ψs dihedral angle. In addition, some derivatives exhibited an unusual value for the ϕs angle, which was far from a value of −60° expected for a conventional β‐O‐glycosidic linkage. In this sense, the different conformations exhibited by these molecules could be a useful tool in obtaining systems with conformational preferences “à la carte”.
Half‐Sandwich Bis(tetramethylaluminate) Complexes of the Rare‐Earth Metals: Synthesis, Structural Chemistry, and Performance in Isoprene Polymerization
Authors: Zimmermann Melanie , Törnroos Karl W. , Sitzmann Helmut , Anwander Reiner
Published: July 2008

The protonolysis reaction of [Ln(AlMe4)3] with various substituted cyclopentadienyl derivatives HCpR gives access to a series of half‐sandwich complexes [Ln(AlMe4)2(CpR)]. Whereas bis(tetramethylaluminate) complexes with [1,3‐(Me3Si)2C5H3] and [C5Me4SiMe3] ancillary ligands form easily at ambient temperature for the entire LnIII cation size range (Ln=Lu, Y, Sm, Nd, La), exchange with the less reactive [1,2,4‐(Me3C)3C5H3] was only obtained at elevated temperatures and for the larger metal centers Sm, Nd, and La. X‐ray structure analyses of seven representative complexes of the type [Ln(AlMe4)2(CpR)] reveal a similar distinct [AlMe4] coordination (one η2, one bent η2). Treatment with Me2AlCl leads to [AlMe4] → [Cl] exchange and, depending on the Al/Ln ratio and the CpR ligand, varying amounts of partially and fully exchanged products [{Ln(AlMe4)(μ‐Cl)(CpR)}2] and [{Ln(μ‐Cl)2(CpR)}n], respectively, have been identified. Complexes [{Y(AlMe4)(μ‐Cl)(C5Me4SiMe3)}2] and [{Nd(AlMe4)(μ‐Cl){1,2,4‐(Me3C)3C5H2}}2] have been characterized by X‐ray structure analysis. All of the chlorinated half‐sandwich complexes are inactive in isoprene polymerization. However, activation of the complexes [Ln(AlMe4)2(CpR)] with boron‐containing cocatalysts, such as [Ph3C][B(C6F5)4], [PhNMe2H][B(C6F5)4], or B(C6F5)3, produces initiators for the fabrication of trans‐1,4‐polyisoprene. The choice of rare‐earth metal cation size, CpR ancillary ligand, and type of boron cocatalyst crucially affects the polymerization performance, including activity, catalyst efficiency, living character, and polymer stereoregularity. The highest stereoselectivities were observed for the precatalyst/cocatalyst systems [La(AlMe4)2(C5Me4SiMe3)]/B(C6F5)3 (trans‐1,4 content: 95.6 %, Mw/Mn=1.26) and [La(AlMe4)2(C5Me5)]/B(C6F5)3 (trans‐1,4 content: 99.5 %, Mw/Mn=1.18).
Enhanced Cooperativity through Design: Pendant CoIIISalen Polymer Brush Catalysts for the Hydrolytic Kinetic Resolution of Epichlorohydrin (Salen=N,N′‐Bis(salicylidene)ethylenediamine Dianion)
Authors: Gill Christopher S. , Venkatasubbaiah Krishnan , Phan Nam T. S. , Weck Marcus , Jones Christopher W.
Published: July 2008

The CoIIIsalen‐catalyzed (salen=N,N′‐bis(salicylidene)ethylenediamine dianion) hydrolytic kinetic resolution (HKR) of racemic epoxides has emerged as a highly attractive and efficient method of synthesizing chiral C3 building blocks for intermediates in larger, more complex molecules. HKR reaction rates have displayed a second order dependency on the concentration of active sites, and thus researchers have proposed a bimetallic transition state for the HKR mechanism. Here we report the utilization of pendant CoIIIsalen catalysts on silica supported polymer brushes as a catalyst for the HKR of epichlorohydrin. The novel polymer brush architecture provided a unique framework for promoting site–site interactions as required in the proposed bimetallic transition state of the HKR mechanism. Furthermore, the polymer brushes mimic the environment of soluble polymer‐based catalysts, whereas the silica support permitted facile recovery and reuse of the catalyst. The polymer brush catalyst displayed increased activities over the soluble Jacobsen Cosalen catalyst and was observed to retain its high enantioselectivities (>99 %) after each of five reactions despite decreasing activities. Analysis indicated decomposition of the salen ligand as an underlying cause of catalyst deactivation.
Analysis of Linear Free‐Energy Relationships Combined with Activation Parameters Assigns a Concerted Mechanism to Alkaline Hydrolysis of X‐Substituted Phenyl Diphenylphosphinates
Authors: Um Ik‐Hwan , Han Jeong‐Yoon , Hwang So‐Jeong
Published: July 2008

A kinetic study is reported for alkaline hydrolysis of X‐substituted phenyl diphenylphosphinates (1 a–i). The Brønsted‐type plot for the reactions of 1 a–i is linear over 4.5 pKa units with βlg=−0.49, a typical βlg value for reactions which proceed through a concerted mechanism. The Hammett plots correlated with σo and σ− constants are linear but exhibit many scattered points, while the corresponding Yukawa–Tsuno plot results in excellent linear correlation with ρ=1.42 and r=0.35. The r value of 0.35 implies that leaving‐group departure is partially advanced at the rate‐determining step (RDS). A stepwise mechanism, in which departure of the leaving group from an addition intermediate occurs in the RDS, is excluded since the incoming HO− ion is much more basic and a poorer nucleofuge than the leaving aryloxide. A dissociative (DN + AN) mechanism is also ruled out on the basis of the small βlg value. As the substituent X in the leaving group changes from H to 4‐NO2 and 3,4‐(NO2)2, ΔH ≠ decreases from 11.3 kcal mol−1 to 9.7 and 8.7 kcal mol−1, respectively, while ΔS ≠ varies from −22.6 cal mol−1 K−1 to −21.4 and −20.2 cal mol−1 K−1, respectively. Analysis of LFERs combined with the activation parameters assigns a concerted mechanism to the current alkaline hydrolysis of 1 a–i.
A Multilateral Mechanistic Study into Asymmetric Transfer Hydrogenation in Water
Authors: Wu Xiaofeng , Liu Jianke , Di Tommaso Devis , Iggo Jonathan A. , Catlow C. Richard A. , Bacsa John , Xiao Jianliang
Published: July 2008

The mechanism of aqueous‐phase asymmetric transfer hydrogenation (ATH) of acetophenone (acp) with HCOONa catalyzed by Ru‐TsDPEN has been investigated by stoichiometric reactions, NMR probing, kinetic and isotope effect measurements, DFT modeling, and X‐ray structure analysis. The chloride [RuCl(TsDPEN)(p‐cymene)] (1), hydride [RuH(TsDPEN)(p‐cymene)] (3), and the 16‐electorn species [Ru(TsDPEN‐H)(p‐cymene)] (4) were shown to be involved in the aqueous ATH, with 1 being the precatalyst, and 3 as the active catalyst detectable by NMR in both stoichiometric and catalytic reactions. The formato complex [Ru(OCOH)(TsDPEN)(p‐cymene)] (2) was not observed; its existence, however, was demonstrated by its reversible decarboxylation to form 3. Both 1 and 3 were protonated under acidic conditions, leading to ring opening of the TsDPEN ligand. 4 reacted with water, affording a hydroxyl species. In a homogeneous DMF/H2O solvent, the ATH was found to be first order in the concentration of catalyst and acp, and inhibited by CO2. In conjunction with the NMR results, this suggests that hydrogen transfer to ketone is the rate‐determining step. The addition of water stabilized the ruthenium catalyst and accelerated the ATH reaction; it does so by participating in the catalytic cycle. DFT calculations revealed that water hydrogen bonds to the ketone oxygen at the transition state of hydrogen transfer, lowering the energy barrier by about 4 kcal mol−1. The calculations also suggested that the hydrogen transfer is more step‐wise in nature rather than concerted. This is supported to some degree by the kinetic isotope effects, which were obscured by extensive H/D scrambling.
Belizeanic Acid: A Potent Protein Phosphatase 1 Inhibitor Belonging to the Okadaic Acid Class, with an Unusual Skeleton
Authors: Cruz Patricia G. , Fernández José Javier , Norte Manuel , Daranas Antonio Hernández
Published: July 2008

Belizeanic acid (BA), a novel metabolite belonging to the okadaic acid class of protein phosphatase inhibitors, was isolated from artificial cultures of the dinoflagellate Prorocentrum belizeanum. The structure and conformational behaviour of BA was elucidated by a combination of NMR spectroscopy and conformational analysis. The isolation of this metabolite, which possesses a simplified version of the okadaic acid skeleton, supports the biogenetic pathway previously reported for this class of compounds. BA showed potent inhibitory activity against protein phosphatase 1 (PP1) within the nM range. A plausible model for the interaction of BA with the PP1 binding pocket was derived from computational docking studies.
Rigid–Flexible Block Molecules Based on a Laterally Extended Aromatic Segment: Hierarchical Assembly into Single Fibers, Flat Ribbons, and Twisted Ribbons
Authors: Lee Eunji , Huang Zhegang , Ryu Ja‐Hyoung , Lee Myongsoo
Published: July 2008

Self‐assembling rigid–flexible block molecules consisting of a laterally extended aromatic segment and different lengths of hydrophilic coils were synthesized and characterized. The block molecule based on a long poly(ethylene oxide) coil (1), in the melt state, shows an unidentified columnar structure, whereas the molecule with a shorter poly(ethylene oxide) coil (2) self‐organizes into an oblique columnar structure. Further decrease in the poly(ethylene oxide) coil length as in the case of 3, on heating, induces a rectangular columnar structure in addition to an oblique columnar mesophase. In diethyl ether, 1 and 2 were observed to self‐assemble into uniform nanofibers with bilayer packing. Remarkably, these elementary fibers were observed to further aggregate in a lateral way to form well‐defined flat ribbons (1) and twisted ribbons (2) with solvent exchange of diethyl ether into methanol. Furthermore, the ribbons formed in methanol dissociated into elementary fibers in response to the addition of aromatic guest molecules. This transformation between ribbons and single fibers in response to the addition of guest molecules is attributed to the intercalation of aromatic substrates within the rigid segments and subsequent loosening of the aromatic stacking interactions. These results demonstrate that the introduction of a laterally extended aromatic segment into an amphiphilic molecular architecture can lead to the hierarchical formation from elementary fibers of nanoribbons with a tunable twist through controlled lateral interactions between aromatic segments.
The First Example of Molecularly Imprinted Nanogels with Aldolase Type I Activity
Authors: Carboni Davide , Flavin Kevin , Servant Ania , Gouverneur Veronique , Resmini Marina
Published: July 2008

The molecular‐imprinting approach was used to obtain a nanogel preparation capable of catalysing the cross‐aldol reaction between 4‐nitrobenzaldehyde and acetone. A polymerisable proline derivative was used as the functional monomer to mimic the enamine‐based mechanism of aldolase type I enzymes. The diketone template used to create the cavity was designed to imitate the intermediate of the aldol reaction and was bound to the functional monomer using a reversible covalent interaction prior to polymerisation. By using a high‐dilution polymerisation method, soluble imprinted nanogels were prepared with dimensions similar to those of an enzyme and with the advantage of solubility and flexibility previously unattainable with monolithic polymers. Following template removal and estimation of active‐site concentrations, the kinetic characterisation of both imprinted and non‐imprinted nanogels was carried out with catalyst concentrations between 0.7 and 3.5 mol %. Imprinted nanogel AS147 was found to have a kcat value of 0.25×10−2 min−1, the highest value ever achieved with imprinted polymers catalysing CC bond formation. Comparison of the catalytic constants for both imprinted nanogel AS147 and non‐imprinted nanogel AS133 gave a ratio of kcat 147/kcat 133=18.8, which is indicative of good imprinting efficiency and highlights the significance of the template during the imprinting process. This work represents a significant demonstration of the superiority of nanogels, when the molecular‐imprinting approach is used, over “bulk” polymers for the generation of catalysts.
Synthesis of a New Seleninic Acid Anhydride and Mechanistic Studies into Its Glutathione Peroxidase Activity
Authors: Yu Sun‐Chol , Borchert Astrid , Kuhn Hartmut , Ivanov Igor
Published: July 2008

Starting from low toxic salicyloylglycine, a new seleninic acid anhydride 7 that lacks Se⋅⋅⋅N or Se⋅⋅⋅O non‐bonded interactions was synthesized. This compound exhibits a fourfold higher glutathione peroxidase‐like (GPx‐like) activity than ebselen and inhibits plant and mammalian 12/15‐lipoxygenases at lower micromolar concentrations. Because of these pharmacological properties, 7 may constitute a new lead compound for the development of anti‐inflammatory low‐molecular‐weight seleno‐organic compounds. Analyzing the redox products of 7 with glutathione (GSH) and tBuOOH, we identified three potential catalytic cycles (A, B, C) of GPx‐like activity that are interconnected by key metabolites. To study the relative contribution of these cycles to the catalytic activity, we prepared selected reaction intermediates and found that the activity of seleninic acid anhydride 7 and of the corresponding diselenide 11 and selenol 14 compounds were in the same range. In contrast, the GPx‐like activity of monoselenide 9 was more than one order of magnitude lower. These data suggested that cycles A and B may constitute the major routes of GPx‐like activity of 7, whereas cycle C may not significantly contribute to catalysis.
Ring‐Closing Metathesis in the Synthesis of BC Ring‐Systems of Taxol
Authors: Ma Cong , Schiltz Stéphanie , Le Goff Xavier F. , Prunet Joëlle
Published: July 2008

BC ring‐systems of taxol with different or no protecting group for the C1,C2‐diol moiety have been efficiently synthesized. The eight‐membered B ring is formed by a ring‐closing metathesis reaction (RCM) between the C10 and C11 carbon atoms. The influence of the 1,2‐diol protecting group on the RCM reaction has been studied in detail.
A General Synthetic Route to Mixed NHC–Phosphane Palladium(0) Complexes (NHC=N‐Heterocyclic Carbene)
Authors: Fantasia Serena , Nolan Steven P.
Published: July 2008

Mixed NHC–phosphane palladium(0) complexes [(NHC)Pd(PR3)] (NHC: N‐heterocyclic carbene) are synthesized directly from commercially available reagents, with the possibility to tune the nature of both the NHC and the phosphane. Reaction of [(NHC)Pd(allyl)Cl] (palladium source) and PR3, in the presence of a base afforded, in isopropanol, [(NHC)Pd(PR3)] in good yields. We found that the nature of the solvent played a key role in the efficient reduction of the PdII precursor to Pd0. Supported by experimental evidence we propose that the reduction step is driven by the isopropoxide anion formed in situ from isopropanol and a base. Detection of acetone in the reaction mixture confirms that the isopropoxide anion acts as the reducing agent. Moreover, different bases proved efficient for the reaction. The structures of the complexes were unambiguously confirmed by X‐ray analysis. Exposure of these complexes to air does not lead to decomposition, but to the oxo‐complex [(NHC)Pd(PR3)(O2)], which is stable both in the solid state and in solution.
ChemInform Abstract: Theoretical Investigation of Clusters of Phosphorus and Arsenic: Fascination and Temptation of High Symmetries.
Authors: Nava Paola , Ahlrichs Reinhart
Published: July 2008

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.
ChemInform Abstract: Synthesis and Crystal Structures of α‐ and β‐Modifications of Cr2IrB2 Containing 4‐Membered B4 Chain Fragments, the τ‐Boride Cr7.9Ir14.1B6 and Orthorhombic Cr2B.
Authors: Kotzott Dominik , Ade Martin , Hillebrecht Harald
Published: July 2008

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.
ChemInform Abstract: On the Structure and Twinning of PtAl4.
Authors: Woerle Michael , Krumeich Frank , Chatterji Tapan , Kek Stefan , Nesper Reinhard
Published: July 2008

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.
ChemInform Abstract: The Structure of LiMg(AlD4)3.
Authors: Grove H. , Brinks H. W. , Heyn R. H. , Wu F.‐J. , Opalka S. M. , Tang X. , Laube B. L. , Hauback B. C.
Published: July 2008

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.
ChemInform Abstract: Luminescence Properties of a New Red Long‐Lasting Phosphor: Mg2SiO4: Dy3+, Mn2+.
Authors: Lin Lin , Yin Min , Shi Chaoshu , Zhang Weiping
Published: July 2008

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.
ChemInform Abstract: Structural and Magnetocaloric Properties of the New Ternary Silicides Gd6M5/3Si3 with M: Co and Ni.
Authors: Gaudin Etienne , Tence Sophie , Weill Francois , Rodriguez Fernandez Jesus , Chevalier Bernard
Published: July 2008

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.