bluuebunny/crossref_metadata_2025_split huggingface dataset:
Mechanistic Investigations into the Asymmetric Transfer Hydrogenation of Ketones Catalyzed by Pseudo‐Dipeptide Ruthenium Complexes
Authors: Wettergren Jenny , Buitrago Elina , Ryberg Per , Adolfsson Hans 
Published: April 2009

AbstractLithium‐powered: A kinetic investigation into the asymmetric transfer hydrogenation of non‐activated aryl alkyl ketones, catalyzed by N‐Boc‐protected α‐amino acid hydroxyamide ruthenium–arene complexes, has revealed that the reactions proceed through an unprecedented bimetallic outer‐sphere mechanism. Under optimized conditions, these catalysts provide access to secondary alcohols in high yields and with excellent enantioselectivities (>99 % ee).magnified imageThe combination of N‐Boc‐protected α‐amino acid hydroxyamides (pseudo‐dipeptides) and [{Ru(p‐cymene)Cl2}2] resulted in the formation of superior catalysts for the asymmetric transfer hydrogenation (ATH) of non‐activated aryl alkyl ketones in propan‐2‐ol. The overall kinetics of the ATH of acetophenone to form 1‐phenylethanol in the presence of ruthenium pseudo‐dipeptide catalysts were studied, and the individual rate constants for the processes were determined. Addition of lithium chloride to the reaction mixtures had a strong influence on the rates and selectivities of the processes. Kinetic isotope effects (KIEs) for the reduction were determined and the results clearly show that the hydride transfer is rate‐determining, whereas no KIEs were detected for the proton transfer. From these observations a novel bimetallic outer‐sphere‐type mechanism for these ATH process is proposed, in which the bifunctional catalysts mediate the transfer of a hydride and an alkali metal ion between the hydrogen donor and the substrate. Furthermore, the use of a mixture of propan‐2‐ol and THF (1:1) proved to enhance the rates of the ATH reactions. A series of aryl alkyl ketones were reduced under these conditions in the presence of 0.5 mol % of catalyst, and the corresponding secondary alcohols were formed in high yields and with excellent enantioselectivities (>99 % ee) in short reaction times.
Cooperative Metal Binding and Helical Folding in Model Peptides of Treble‐Clef Zinc Fingers
Authors: Sénèque Olivier , Bonnet Estelle , Joumas France Lyne , Latour Jean‐Marc 
Published: April 2009

AbstractSynergy in zinc fingers: The comparison between peptide folding and metal binding properties of two model peptides of treble‐clef zinc fingers presenting high affinities for zinc and cobalt reveals a cooperative effect: the metal folds the peptide into a α‐helix, which in turn strengthens the metal core.magnified imageTwo peptides, LTC and ${{\rm L}{{{\rm T}\hfill \atop {\rm TC}\hfill}}}$ have been synthesised to model the treble‐clef zinc fingers encountered in many Zn(Cys)4‐site‐containing proteins. Both are cyclic peptides with a linear tail grafted on a glutamate side chain of the cycle. They differ by the length of this tail, which lacks five amino acids in ${{\rm L}{{{\rm T}\hfill \atop {\rm TC}\hfill}}}$ compared to LTC. Both peptides bind Zn2+ and Co2+ in 1:1 metal/peptide ratio and the structure of these complexes have been characterised by NMR, UV/Vis and CD spectroscopy. Both peptides fold the same way around the metal ion and they fully reproduce the classical fold of treble‐clef zinc fingers and display an extended hydrogen‐bond network around the coordinating sulfur atoms. The structures of the M⋅LTC complexes reveal that the linear tail forms a short two‐turn α‐helix, present in the metallated form only. The formation of this helix constitutes a rare example of metal‐induced folding. The second turn of this helix is composed of the five amino acids that are absent in ${{\rm L}{{{\rm T}\hfill \atop {\rm TC}\hfill}}}$. The study of the pH‐dependence of the Zn2+ binding constants shows that the metal ion is bound by four cysteinates above pH 5.2 and the binding constants are the highest reported so far. Interestingly, the binding constant of Zn⋅LTC is about tenfold higher than that of Zn⋅${{\rm L}{{{\rm T}\hfill \atop {\rm TC}\hfill}}}$. This difference clearly indicates that the helix, present in Zn⋅LTC only, stabilises the Zn2+ complex by about 1.2 kcal mol−1. The origin of this stabilisation is ascribed to an electrostatic interaction between the [ZnS4]2− centre and the helix. This reveals a cooperative effect: zinc binding allows the folding of the tail into a helix which, in turn, strengthens the zinc complex.
Fluorinated Carbohydrates as Lectin Ligands: Versatile Sensors in 19F‐Detected Saturation Transfer Difference NMR Spectroscopy
Authors: Diercks Tammo , Ribeiro João P. , Cañada F. Javier , André Sabine , Jiménez‐Barbero Jesús , Gabius Hans‐Joachim 
Published: April 2009

AbstractAs a novel approach for studying carbohydrate–lectin interactions spectroscopically, we combine the resolution and specificity of 19F‐detected NMR spectroscopy with the sensitivity of the saturation transfer difference (STD) technique. The resulting background‐free 19F‐STD spectra open a promising perspective for broad application with medical relevance, for example, in drug discovery.magnified image
Stepwise Conversion of Two Pyrrole Moieties of Octaethylporphyrin to Pyridin‐3‐ones: Synthesis, Mass Spectral, and Photophysical Properties of Mono and Bis(oxypyri)porphyrins
Authors: Ryppa Claudia , Niedzwiedzki Dariusz , Morozowich Nicole L. , Srikanth Rapole , Zeller Matthias , Frank Harry A. , Brückner Christian 
Published: April 2009

AbstractModified porphyrins: New, mild conditions are described for the conversion of free‐base octaethylporphyrin (OEP) into the corresponding monooxypyriporphyrin. The same conditions can be used to convert the monooxypyriporphyrin into the unprecedented bis(oxypyri)porphyrin (see figure).magnified imageFree‐base octaethylporphyrin (OEP) was converted in two steps (β,β′‐dihydroxylation and oxidative diol cleavage with concomitant aldol condensation) to the corresponding oxypyriporphyrin. This conversion was previously described to be applicable only to the NiII complex of OEP. Modified diol cleavage conditions made this reaction sequence now applicable to free‐base OEP. The single‐crystal structure of the resulting free‐base oxypyriporphyrin was determined, proving its near‐perfect planarity. The reaction sequence can also be applied to oxypyriporphyrin itself, generating the unprecedented bacteriochlorin‐type bis(oxypyri)porphyrin as two separable isomers. The ground‐state (UV/Vis and fluorescence spectroscopies) and excited‐state (transient triplet–triplet absorption, triplet lifetimes, and triplet EPR spectroscopy) photophysical properties of all chromophores are compared with those of OEP, chlorins, and oxochlorins. The pyridone‐modified porphyrins possess unique spectroscopic signatures that distinguish them from regular porphyrins or chlorins. The presence of the pyridone moiety alters the ESI+ collision‐induced fragmentation properties of these oxypyriporphyrins only to a minor degree when compared with those of OEP or chlorins, confirming their stability.
Versatile Reactivity of a Zwitterionic Isolable Silylene toward Ketones: Silicon‐Mediated, Regio‐ and Stereoselective CH Activation
Authors: Xiong Yun , Yao Shenglai , Driess Matthias 
Published: April 2009

Abstract1,4‐Dipolar nature is the key: A metastable de‐aromatized siloxindane 2 has been obtained from the regio‐ and stereoselective addition of silylene LSi (1) (L=CH[(CCH2)CMe][N(Ar)]2, Ar=2,6‐iPr2C6H3) to benzophenone at low temperature, in which the 1,4‐dipolar nature of 1 is proposed to play a key role. At room temperature, silicon‐mediated CH activation occurs and the initial product 2 rearranges stereoselectively under re‐aromatization to the corresponding siloxindane 3.magnified imageReactions of the isolable, ylide‐like silylene LSi: (1) (L=CH[(CCH2)CMe][N(Ar)]2, Ar=2,6‐iPr2C6H3) with different ketones (benzophenone, benzylideneaceton, acetophenone) have been investigated. The metastable dearomatized siloxindane 2, which was suggested previously as reaction intermediate but could not be detected in related reactions, has been obtained for the first time from the regio‐ and stereoselective addition of silylene 1 to benzophenone at low temperature. By variation of the reaction temperature, the initial product 2 rearranges under re‐aromatization stereoselectively to the corresponding siloxindane 3, which has also been isolated in high yield. In contrast, reactions of 1 with benzylidenacetone and acetophenone furnish the [4+1] cycloaddition product 4 and the silyl enol ether 5, respectively. It is proposed that in all reactions the 1,4‐dipolar nature of 1 plays key role, leading to highly reactive donor–acceptor adducts, which undergo rearrangement (tautomerization) to the final products. The new compounds 2, 3, 4, and 5 were characterized by 1H, 13C, and 29Si NMR spectroscopy, EI‐MS, and elemental analysis, and their structures determined by single‐crystal X‐ray diffraction analysis.
“Catalysis in a Tea Bag”: Synthesis, Catalytic Performance and Recycling of Dendrimer‐Immobilised Bis‐ and Trisoxazoline Copper Catalysts
Authors: Gaab Manuela , Bellemin‐Laponnaz Stéphane , Gade Lutz H. 
Published: April 2009

AbstractTrapped in a “tea bag”: Membrane bags, fabricated from a dialysis membrane, have been employed as semipermeable containers for dendrimer‐immobilised oxazoline–copper(II) Lewis acid catalysts. Dipping them into reactant solutions leads to the catalytic conversion of the substrates, which penetrate the membrane as does the product in the reverse direction (see scheme).magnified imageBis‐ and trisoxazolines (BOX and trisox), containing a linker unit in the ligand backbone that allows their covalent attachment to carbosilane dendrimers, have been employed as polyfunctional ligands for recyclable CuII Lewis acid catalysts that were immobilised in a membrane bag. The oxazolines contained an alkynyl unit attached to their backbone that was deprotonated with LDA or BuLi and then reacted with the chlorosilyl termini of zeroth‐, first‐ and second‐generation carbosilane dendrimers in the presence of TlPF6. The functionalised dendritic systems were subsequently separated from excess ligand by way of dialysis. The general catalytic potential of these systems was assessed by studying two benchmark reactions, the α‐hydrazination of a β‐keto ester as well as the Henry reaction of 2‐nitrobenzaldehyde with nitromethane. For both reactions the bisoxazoline‐based catalysts displayed superior selectivity and, in particular, catalyst activity. The latter was interpreted as being due to the hindered decoordination of the third oxazoline unit, the key step in the generation of the active catalyst, in the immobilised trisox–copper complexes. Solutions of the second‐generation dendrimer catalysts were placed in membrane bags, fabricated from commercially available dialysis membranes, with the purpose of catalyst recycling based on dialysis. Overall, the supported BOX catalyst gave good and highly reproducible results throughout the study, whereas the performance of the trisox dendrimer system decreased monotonically. The reason for the different behaviour is the markedly lower activity of trisox‐based catalysts relative to those based on the BOX ligand. This necessitated an increased reaction time for each cycle of the trisox derivatives, resulting in higher levels of catalyst leaching, which was attributed to a modification of the structure of the membrane by its exposure to the solvent trifluoroethanol at 40 °C.
Anion‐Templated Formation of Supramolecular Multinuclear Assemblies
Authors: Wezenberg Sander J. , Escudero‐Adán Eduardo C. , Benet‐Buchholz Jordi , Kleij Arjan W. 
Published: April 2009

AbstractAnion meets host(s): Lewis acidic zinc(II)‐based salphen complexes have been found to interact strongly with various anions both in solution as well as in the solid state. A biologically relevant model anion (acetate, shown in red) has been exploited to serve as a molecular template for the synthesis of multinuclear Zn assemblies (Zn shown in green) with a short metal–metal distance, and consequently, resembles in part the metal‐ion proximity found in a range of multimetallic enzymes.magnified imageThe use of anions as supramolecular building motifs is reported herein. The interaction of various anions with [Zn(salphen)] complexes (salphen=N,N′‐phenylenebis(salicylideneimine)) has been studied, with a particular focus on assembly formation in the presence of acetate anions. Strong association of the latter with the Zn metal centre of the salphen complex was quantified by UV/Vis titration studies. The [Zn(salphen)]–OAc assemblies were further characterized by NMR spectroscopy and X‐ray analysis and revealed that the acetate anions can function as bridging modules between two [Zn(salphen)] units. The binding strength of a second complex to the acetate anion may be regulated by the substitution pattern of the salphen ligand. The anion‐templated assembly formation was further demonstrated by the supramolecular synthesis of a trinuclear Zn3 assembly. These results hold promise for the directed assembly formation of [Zn(salphen)] complexes with biologically relevant carboxylates, such as amino acids, and potentially new bio‐sensing materials.
The First Magnetic Nanoparticle‐Supported Chiral DMAP Analogue: Highly Enantioselective Acylation and Excellent Recyclability
Authors: Gleeson Oliver , Tekoriute Renata , Gun'ko Yurii K. , Connon Stephen J. 
Published: April 2009

AbstractRecyclable organocatalysts: The first chiral DMAP analogue immobilized on magnetic nanoparticles is a highly active catalyst that is capable of the acylation of racemic sec‐alcohols under convenient, process‐scale friendly conditions (room temperature, 5 mol % loading, with acetic anhydride as the acylating agent) with excellent enantiodiscrimination. The catalyst is easily recovered and possesses unprecedented recyclability—in this study it was demonstrated to retain excellent activity and selectivity over 32 iterative cycles.magnified image
CD‐sensitive Zn‐porphyrin tweezer host‐guest complexes, part 1: MC/OPLS‐2005 computational approach for predicting preferred interporphyrin helicity
Authors: Petrovic Ana G. , Chen Yihui , Pescitelli Gennaro , Berova Nina , Proni Gloria 
Published: April 2009

AbstractThis article describes a computational study on dimeric zinc porphyrin tweezer complexes with primary/secondary amines and secondary alcohols that validates the use of Optimized Potential for Liquid Simulations (OPLS‐2005) as the lead computational choice for assisting the tweezer methodology in the absolute configurational assignment of organic compounds. A supramolecular, microscale approach known as the tweezer method has been widely applied in the past decade for determining the absolute configuration of chiral substrates that are difficult to study by other readily available methods. The method relies on a host/guest complexation mechanism between a porphyrin tweezer moiety and a substrate, after its conversion into a bidentate conjugate. The formation of 1:1 complexes is a stereodifferentiating process: upon complexation, the originally achiral tweezer adopts a preferential interporphyrin helicity, dictated by the absolute configuration of the chiral substrate. By correctly predicting the sign of the interporphyrin helicity in the complex, OPLS‐2005 provides a correlation between the observed circular dichroism (CD) signal and the absolute configuration of the substrate. It also offers a great degree of insight into the structural factors responsible for chiral recognition and the amplitude of exciton couplets. Moreover, the preferential binding sites between the Zn‐tweezer and secondary amine conjugates were revealed by using the new computational approach. Chirality, 2010. © 2009 Wiley‐Liss, Inc.
CD‐sensitive Zn‐porphyrin tweezer host‐guest complexes, part 2: Cis‐ and trans‐3‐hydroxy‐4‐aryl/alkyl‐β‐lactams. A case study
Authors: Chen Yihui , Petrovic Ana G. , Roje Marin , Pescitelli Gennaro , Kayser Margaret M. , Yang Yan , Berova Nina , Proni Gloria 
Published: April 2009

AbstractThis article describes an application of the host‐guest chiral recognition approach called tweezer methodology for the determination of the absolute configuration of 3‐hydroxy‐β‐lactams. These substrates represent challenging cases due to their chemical reactivity, the presence of multiple stereogenic centers and several functional groups which offer various possibilities of binding to the Zn‐porphyrin host. OPLS‐2005, the force field used in this work to predict the interporphyrin twist, modeled correctly the host‐guest complexation mechanism and revealed conformational details of the bound substrates. The computational study also suggested that in cases where an increase in the magnitude of the stereodifferentiation and an intense experimental CD are observed, the bound conformation of the conjugates are hydrogen bonded. The present investigation provides evidence that when the tweezer method is assisted by the OPLS‐2005 based computational approach, it can be successfully applied to the configurational and conformational elucidation of multi‐functional compounds with multiple stereogenic centers. Chirality, 2010. © 2009 Wiley‐Liss, Inc.
Near‐Infrared Emission Quantum Yield of Soluble Short Single‐Walled Carbon Nanotubes
Authors: Aprile Carmela , Martín Roberto , Alvaro Mercedes , Scaiano Juan C. , Garcia Hermenegildo 
Published: April 2009

AbstractSoluble short single‐walled carbon nanotubes in aerated D2O emit in the near‐infrared (NIR) region (see picture) with a quantum yield of (3.9±0.5) ×10−3 and a half‐life of 7.65 μs. This emission is quenched by electron‐ acceptor molecules.magnified imageSoluble short single‐walled carbon nanotubes (sSWNT) are prepared by means of chemical fragmentation and purification of commercial SWNTs. The average length of the nanotubes, as estimated by microscopy, is 400 nm. Infrared spectroscopy studies reveal the presence of a significant population of carboxyl groups. The thermogravimetric profile of an sSWNT shows that this material is hydrophilic, contains carboxyl groups, and is almost free from inorganic impurities. In D2O solution, the obtained sSWNTs emit in the near‐infrared (NIR) region with a quantum yield of (3.9±0.5)×10−3, determined using the 1270 nm singlet oxygen emission as a reference standard. The temporal profile can be fitted with first‐order kinetics and a half‐life of 13.9 μs. The NIR emission is quenched through a static mechanism by 2,4,6‐triphenylpyrylium (TP+), a typical electron acceptor.
CSR in business start‐ups: an application method for stakeholder engagement
Authors: Retolaza Jose Luis , Ruiz Maite , San‐Jose Leire 
Published: April 2009

AbstractIn this paper, we propose a Corporate Social Responsibility (CSR) method to apply in business start‐ups or newly created firms, whose main aim is the engagement of stakeholders. Several different CSR resources have been developed from various initiatives, both public and private. However, these initiatives do not highlight and consider the characteristics of newly created firms; moreover, most CSR theories and methods of applying social responsibility in firms are focused on medium and large firms, whose characteristics are so different, compared to start‐ups and newly created firms. The method proposed in this paper shows the possibility, at least theoretically, to implement a CSR method to tackle all of the interests of future and potential stakeholders in business start‐ups. Copyright © 2009 John Wiley & Sons, Ltd and ERP Environment.
Solid‐State NMR Investigation of Ligand Mobility and Reactivity in Transition Metal Complexes
Authors: Chierotti Michele R. , Gobetto Roberto 
Published: April 2009

AbstractSolid‐state NMR spectroscopy represents a powerful tool to access information on chemical structure, dynamics and reactivity at molecular level on solid compounds and materials. Transition metal complexes represent an unrestricted source of examples for solid‐state dynamics and reactivity due to the presence of highly mobile groups, very reactive ligands and the possibility of expanding the coordination sphere. The key question of the influence of the mobility on the reactivity of a system is the occasion for presenting several selected examples of solid‐state dynamics and reactivity of transition metal complexes. Although only few unambiguous examples are reported in which this correlation has been investigated, this review presents the central role of solid‐state NMR spectroscopy in the investigation of ligand motion, transformation mechanisms, kinetics and products of solid‐state reactions in transition metal complexes. The aim of this microreview is to offer the state‐of‐the‐art on this subject in order to develop new insights and to promote cooperative efforts in this fascinating field.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)
A Zinc Porphyrin Bearing Two Lateral dpp‐Containing Rings and Its [3]Pseudorotaxane (dpp: 2,9‐diphenyl‐1,10‐phenanthroline)
Authors: Roche Cécile , Sour Angélique , Niess Frédéric , Heitz Valérie , Sauvage Jean‐Pierre 
Published: April 2009

AbstractA zinc porphyrin attached to two dpp‐containing rings was prepared from a 31‐membered ring with an attached pendent aldehyde function and 5‐mesityldipyrromethane by using the synthetic method introduced long ago by Lindsey and his group. The obtained porphyrin is of the ABAB type, with two mesityl groups on the 5‐ and 15‐positions and two macrocycles on the 10‐ and 20‐positions. From this compound, a [3]pseudorotaxane was obtained in good yield from the reaction of the porphyrin with two equivalents of a copper(I) salt followed by the addition of two equivalents of 2,9‐dianisyl‐1,10‐phenanthroline.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)
Discovering gorilla
Authors: Conniff Richard 
Published: April 2009

AbstractOn July 16, 1847, a missionary newly arrived in New York City from West Africa packed a collection of bones in a box and shipped them off to a colleague in Massachusetts. In a letter, Thomas S. Savage (Fig. 1), a graduate of Yale College and Yale Medical School, admitted to being “quite unwell,” probably meaning “utterly wretched.” He had already endured tropical diseases in Liberia off and on for more than a decade, and he'd seen his first two wives languish and die there, probably of malaria. He wasn't the sort to complain lightly.
Realization of contact resolving approximate Riemann solvers for strong shock and expansion flows
Authors: Kim Sung Don , Lee Bok Jik , Lee Hyoung Jin , Jeung In‐Seuck , Choi Jeong‐Yeol 
Published: April 2009

AbstractThe Harten–Lax–van Leer contact (HLLC) and Roe schemes are good approximate Riemann solvers that have the ability to resolve shock, contact, and rarefaction waves. However, they can produce spurious solutions, called shock instabilities, in the vicinity of strong shock. In strong expansion flows, the Roe scheme can admit nonphysical solutions such as expansion shock, and it sometimes fails. We carefully examined both schemes and propose simple methods to prevent such problems. High‐order accuracy is achieved using the weighted average flux (WAF) and MUSCL‐Hancock schemes. Using the WAF scheme, the HLLC and Roe schemes can be expressed in similar form. The HLLC and Roe schemes are tested against Quirk's test problems, and shock instability appears in both schemes. To remedy shock instability, we propose a control method of flux difference across the contact and shear waves. To catch shock waves, an appropriate pressure sensing function is defined. Using the proposed method, shock instabilities are successfully controlled. For the Roe scheme, a modified Harten–Hyman entropy fix method using Harten–Lax–van Leer‐type switching is suggested. A suitable criterion for switching is established, and the modified Roe scheme works successfully with the suggested method. Copyright © 2009 John Wiley & Sons, Ltd.