bluuebunny/crossref_metadata_2025_split huggingface dataset:
Cyclobis(Paraquat‐4,4′‐Biphenylene)–an Organic Molecular Square
Authors: Asakawa Masumi , Ashton Peter R. , Menzer Stephan , Raymo Françisco M. , Stoddart J. Fraser , White Andrew J. P. , Williams David J. 
Published: September 2007

AbstractTemplate‐directed syntheses of cyclobis(paraquat‐4,4′‐biphenylene) (1)– a Molecular square–have been achieved by use of π‐electron‐rich macrocyclic hydroquinone‐based and acyclic ferrocene‐based templates. In particular, the use of a polyether‐disubstituted ferrocene derivative as a template permits synthesis of 1 (which is accessible only in very low yields without templates) on a preparative scale. Furthermore, the use of a macrocyclic hydroquinone‐based polyether template in corporating an ester function in one polyether chain–an (oriented) macrocycle–affords a 1 : 1 mixture of two topologically stereoisomeric [3]catenanes. Ester hydrolysis of the π‐electron‐rich macrocyclic components mechanically interlocked with 1 within the catenated structures releases the tetracationic cyclophane in quantitative yield as a result of the degradation of the [3]catenanes. The molecular square has been characterized by X‐ray crystallography, FAB mass spectrometry, 1H NMR and 13C NMR spectroscopies, and elemental analysis. The binding properties of 1 and of the smaller cyclophane cyclobis(paraquat‐p‐phenylene) toward a series of π‐electronrich guests have also been investigated with the above techniques and UV/VIS spectroscopy. The self‐assembly of the resulting supramolecular complexes in solution and in the solid state is driven mainly by π–π stacking interactions and hydrogen‐bonding interactions, as well as by edge‐to‐face T‐type interactions. In particular, the complexation of ferrocene or a ferrocene‐based derivative within the cavity of 1 suggests the possibility of constructing functioning ferrocene‐based molecular and supramolecular devices that can be controlled electrochemically in the form of catenanes, rotaxanes, and pseudorotaxanes.
Diastereoselective Zwitterionic Aza‐Claisen Rearrangement: Synthesis of Nine‐Membered Ring Lactams and Transannular Ring Contraction
Authors: Diederich Michel , Nubbemeyer Udo 
Published: September 2007

AbstractThe zwitterionic aza‐Claisen rearrangement of optically active 3‐pyrrolidine acryl esters and various acid chlorides to generate optically active azoninones proceeds with high simple diastereoselectivity (internal asymmetric induction) and a complete 1,3‐chirality transfer. The reaction path observed depends on the subsitution pattern of the allylic system: while the more electron‐rich alkylated allyl amine fromed predominantly von Braun type products, the α,β‐unsaturated esters could be rearranged with high yields. The azoninones thus obtained were treated with electrophiles, inducing regio‐ and diastereoselective transannular ring contractions. The resulting indolizidinones should be useful key intermediates in alkaloid synthesis.
Observing a Homogeneous Ziegler Catalyst Precursor at Work: Insertion Reactions into the Zirconium – Carbon Bond of the (Butadiene)ZrCp2/B(C6F5)3 Addition Product
Authors: Temme Bodo , Karl Jörn , Erker Gerhard 
Published: September 2007

Abstract(Butadiene)zirconocene adds to B(C6F5)3 to form the metallacyclic metallocene–boron betaine system 1, which contains a C–F → Zr interaction. Addition of tert‐butylisocyanide or tert‐butyl‐cyanide leads to cleavage of the zirconium – fluorine linkage and formation of the adducts 4 and 7, respectively. With additional tert‐butylisocyanide, 4 reacts further to yield the corresponding η2‐imino‐acyl betaines 5 and 6 (2 stereoisomers), whereas 7 inserts additional tert‐butyl‐cyanide into the Zr–C bond to give a chiral metallacyclic N‐zirconaketimine complex that is probably stabilized by an internal borate → Zr interaction. Ethene and propene insertion reactions into the Zr ‐ C bond of 1 follow a similar course: the mono‐insertion products 9 and 10, respectively, generated in situ and characterized by NMR spectroscopy at – 35°C in [D8]toluene solution, are chiral and are also stabilized by internal borate → Zr coordination. Complexes 9 and 10 are likely to be intermediates in the formation of active homogeneous Ziegler polymerization catalysts.
Resonance‐Assisted O‐H ⃛ O Hydrogen Bonding: Its Role in the Crystalline Self‐Recognition of β‐Diketone Enols and its Structural and IR Characterization
Authors: Bertolasi Valerio , Gilli Paola , Ferretti Valeria , Gilli Gastone 
Published: September 2007

AbstractThe fact that hydrogen bonding is normally stronger than other nonbonding attractive forces can be exploited for the rational design of molecular crystals with known packing features and specific physical properties (crystal engineering). In the present paper the problem of obtaining homodromous molecular chains controlled by strong O–H ⃛ O interactions is investigated, particular attention being paid to β‐chains, that is, infinite hydrogen‐bonded chains of β‐diketone enol fragments ⃛ OCCCOH ⃛, which are linked by stronger‐than‐usual resonance‐assisted hydrogen bonds (RAHBs) and are intrinsically interesting as prototypes of a large family of switching proton bistate molecular devices. Accordingly, the crystal and molecular structures of thirteen new compounds containing the 1,3‐cyclopentanedione and 1,3‐cyclohexanedione fragment (or their heterocyclic analogues) were determined, and most of them were found to give the expected β‐chain packing pattern. Comparison with literature data makes it possible to identify seven fundamental β‐chain patterns, which can be shown to be selected by reason of the relative encumbrances of the substituents. Furthermore, a general analysis of all functional groups able to form strong OH ⃛ O bonds reveals a semiquantitative correspondence between the OH ⃛ O measurable parameters (O ⃛ O, H ⃛ O and OH distances, and ṽ(OH) IR stretching frequencies) and the hydrogen bond energy EHB, and a hierarchy of chemical functionalities that are well characterized by limited EHB ranges and that, in decreasing order of energy, can direct the crystal packing process.
Opening and Closure of the Fullerene Cage in cis‐Bisimino Adducts of C60: The Influence of the Addition Pattern and the Addend
Authors: Schick Georg , Hirsch Andreas , Mauser Harald , Clark Timothy 
Published: September 2007

AbstractThe synthesis, isolation, and spectroscopic characterization of the bisimino[60]fullerenes C60(NCOOR)2 (1a: R = Et, 1b. R = tBu) with a cis‐1 addition pattern as well as of their regioisomeric analogues 2–7 with different addition patterns are described. Whereas compounds 2–7 are typical fulleroaziridines and obey the rule of the minimization of [5,6] double bonds, the cis‐1 isomers 1a and 1b represent the first examples of fullerene derivatives with open transannular [6,6] bonds. Characteristic features within the fullerene framework of these valence isomers VI are the presence of a doubly bridged open 14‐membered ring with a phenanthrene perimeter as well as of an 8‐membered 1,4‐diazocine heterocycle. Moreover, it is shown that, by transforming cis‐1‐C60(NCOOtBu)2 (1b) into cis‐1‐C60(NH)2 (1c), the fullerene cage can be closed in an intraring 2π→2s̀ isomerization to valence isomer V. These are the first chemical modifications of the fullerene core that allow the synthesis of both open and closed valence isomers with the same addition pattern. Density‐functional as well as AM1 calculations corroborate the experimental findings that it depends on the addition pattern as well as on the nature of the addends whether or not the transannular [6,6] bonds are closed. Only in the cis‐1 adducts C60(NR)2 that prefer planar imino bridges (e.g., carbamates or amides) are the open forms VI more stable than the closed isomers V.
4,4‐Dimethylbicyclo[3.1.0]Hexa‐1(6),2‐Diene— A Highly Strained 1,3‐Bridged Cyclopropene
Authors: Albers Reinhard , Sander Wolfram , Ottosson Carl‐Henrik , Cremer Dieter 
Published: September 2007

AbstractThe photochemical rearrangement of 4,4‐dimethylcyclohexa‐2,5‐dienylidene (3) to 4,4‐dimethylbicyclo‐[3.1.0]hexa‐1(6),2‐diene (5) was investigated by means of the matrix isolation technique. Carbene 3 was generated in argon matrices at 10 K by photolysis of the corresponding diazo compound γ (>550 nm) and was characterized by IR and UV/Vis spectroscopy and its characteristic thermal reaction with triplet oxygen. Long‐wavelength irradiation (γ > 515 nm) induced an irreversible rearrangement of carbene 3 to the highly strained cyclopropene 5. This is in contrast to the previously investigated 1 H‐bicyclo[3.1.0]hexa‐3,5‐dien‐2‐one (1), which is thermally labile even at 10 K and rearranges back to 4‐oxo‐2,5‐cyclohexa‐dienylidene. The experimental findings were confirmed by ab initio calculations at the MP 2/6‐31 G(d) level. MP 2 and experimental IR frequencies are in good agreement. The strain energies of 5 and 1 are 75 and 78 kcalmol‐1, more than 20 kcal mol‐1 larger than that of cyclopropene. The somewhat greater stability of 5 than 1 is a consequence of better π delocalization in the diene unit of the bicyclic system.
X‐Ray Structural Studies of Highly Enantioselective Mn(salen) Epoxidation Catalysts
Authors: Pospisil Paul J. , Carsten Douglas H. , Jacobsen Eric N. 
Published: September 2007

AbstractThe relationship between catalyst structure and enantioselectivity in the asymmetric epoxidation of unfunctionalized olefins by a series of chiral Mn(sa1en) complexes (1 – 10) was examined. The X‐ray structures of 5‐coordinate complexes 5, 8, of 6‐coordinate 9 ([6,6′= ‐tBu; 4,4 = ‐tBu]+ClO4−), and 10 (6,6′= ‐tBu; 4,4′=‐Br) were determined. Catalysts 1 – 9 were derived from (R,R)‐1, 2 diaminocyclohexane and catalyst 10 from (S,S)‐1,2‐diphenylethylenediamine. Catalysts 1–9 differ in the stereoelectronic substitution of the ortho (6,6) and para (4,4) positions of the salicylidene moiety. A comparison between structures 5, 8, and 9 reveals that the ligand geometry around the metal center and the chiral diimine backbone remains remarkably constant in both five‐ and six‐coordinate cyclohexanediamine‐derived complexes; in contrast, the salicylidene regions of the complexes display a wide range of conformations. The asymmetric epoxidation of indene and 6‐cyano‐2,2‐dimethylchromene with NaOCl catalyzed by complexes 1 – 10 was effected. Systematically increasing the steric bulk on the ortho and then the para position in the order 1 (6,6′ = ‐H; 4,4′ = ‐H),2(6,6′ = ‐CH3; 4,4′ = ‐CH3),3(6,6′=‐tBu;4,4′=‐H),4(6,6′=‐tBu; 4,4′ =‐CH3), 5 (6,6′=‐tBu; 4,4′=‐tBu), and 6 (6,6′=‐tBu; 4,4′= ‐trityl), and electronically modifying the para substituents in 7 (6,6 = ‐tBu; 4,4 =‐OMe) and 8 (6.6′ = ‐tBu; 4,4′=‐OTIPS) resulted in enhanced enantioselectivities of the desired epoxides. The conformational variations observed in the solid state are likely to reflect accessible solution conformations and may help explain the high levels of stereoinduction obtained with these catalysts in the asymmetric epoxidation of unfunctionalized olefins.
Structure and Selectivity of a Small Dimeric Encapsulating Assembly
Authors: Valdés Carlos , Toledo Leticia M. , Spitz Urs , Rebek Julius 
Published: September 2007

AbstractThe binding selectivity of a dimeric molecular capsule in solution and its X‐ray crystal structure in the solid state are reported. The capsule is a self‐complementary bisglycoluril molecule, formed through a series of intermolecular hydrogen bonds. The dimeric host is capable of discriminating in CDCl3 solution between methane and ethane as guests, and direct evidence for the selectivity is presented from 1H NMR studies. The thermodynamic parameters for the encapsulation of methane, obtained from a VT 1H NMR (500 mHz) study, are presented.
A Straightforward, Highly Stereoselective Synthesis of Protected Isostatine Derivatives
Authors: Castejón Patricia , Moyano Albert , Pericàs Miquel A. , Riera Antoni 
Published: September 2007

AbstractA novel strategy for the synthesis of isostatine derivatives has been developed. Contrary to previous approaches to isostatine, a non‐proteinogenic amino acid that is an essential component of the didemnins, the present synthesis does not require the intermediacy of the expensive amino acid D‐allo‐isoleucine, the starting material being commercially available enantiopure (S)‐2‐methyl‐1‐butanol. Steps in the sequence include catalytic asymmetric epoxidation, regioselective titanium‐promoted opening of an epoxy alcohol with an ammonia equivalent, stereospecific generation of an N‐Bocamino epoxide and its nucleophilic opening by a cyanide anion. Application of this method has permitted the enantioselective preparation of isostatine methyl ester and, for the first time, of isostatine amide, both in fully protected form.
Synthesis of a Conformationally Constrained Heparin‐like Pentasaccharide
Authors: van Boom Jacques H. , Sakairi Nobuo , van Der Marel Gijsbert A. , Basten Jan E. M. , van Boeckel Constant A. A. 
Published: September 2007

AbstractAn octasulfated pentasaccharide 1 having an L‐iduronic acid moiety in a fixed 1C4 conformation was synthesized by the coupling of a triosyl donor 3 with a disaccharide acceptor 4 followed by deprotection and O‐sulfation. The acceptor 4 was prepared from the fully acetylated 5‐C‐allyl‐β‐D‐glucose building block 7 by means of a TMSOTf‐promoted glycosylation, intramolecular substitution and ozonolysis of the olefinic bond as the key reactions. Compound 1 showed very low activity in an antithrombin 111‐mediated anti‐Xa assay; this reflects the importance of the presence of a flexible L‐iduronic acid moiety in heparin‐like antithrombotics.
A Novel Catalytic Cycle for the Synthesis of Epoxides Using Sulfur Ylides
Authors: Aggarwal Varinder K. , Abdel‐Rahman Hesham , Fan Li , Jones Ray V. H. , Standen Mike C. H. 
Published: September 2007

AbstractA novel, neutral catalytic cycle for the synthesis of epoxides from carbonyl compounds and diazo compounds using catalytic quantities of transition metal salts and sulfides has been developed. In this catalytic cycle, the diazo compound is decomposed by the transition metal salt to give a metallocarbene, and this is picked up by the sulfide to give a sulfur ylide, which then reacts with the aldehyde to give an epoxide and returns the sulfide back into the catalytic cycle. To obtain good yields of epoxides it is necessary to maintain a low concentration of the diazo compound (by slow addition), otherwise dimerisation of the diazo compound is the dominant reaction. Factors affecting the outcome of the reaction were studied. The reactions are relatively insensitive to solvent, but are sensitive to the structure of the sulfide, the metal salt and the concentration. Unhindered sulfides give good yields of epoxides with any metal salt, but with hindered sulfides higher yields are obtained with Cu(acac)2 than with Rh2(OAc)4. The yields of epoxides are sensitive to sulfide concentration especially when using substoichiometric amounts of sulfides. Higher concentration leads to faster rates of formation and subsequent reaction of the sulfur ylides, and consequently to higher yields. This novel catalytic cycle has also been applied to base‐sensitive aldehydes. We found that our new catalytic cycle for epoxidation gives much improved yields of epoxides compared to those obtained by traditional sulfur ylide chemistry and is tolerant to a wide variety of sensitive functional groups. Ketones also participate in the catalytic cycle, although they give reduced yields of epoxides compared to aldehydes and require a slightly elevated temperature.
The Aminium Salt and Photoinduced Electron Transfer Initiated Diels‐Alder Cycloaddition of Electron‐rich Allenes: Evidence for a Stepwise Mechanism and the Importance of Steric and Electronic Effects for the Reactivity of Distonic Radical Cation Intermediates
Authors: Schmittel Michael , Wöhrle Clemens , Bohn Ingo 
Published: September 2007

AbstractSeveral novel electron‐rich aryl‐allenes have been synthesized, characterized, and used as dienophiles in the radical‐cation‐catalyzed cycloaddition with 1,2,3,4,5‐pentamethylcyclopentadiene, which affords, in most cases, the Diels‐Alder products in 5 min at 0°C with a high peri‐, chemo‐, facial, and stereoselectivity. In line with oxidation‐potential considerations it is concluded that the electron‐transfer‐induced reaction proceeds along a [3 + 2] pathway by cycloaddition of the diene radical cation to a neutral allene with a rather short chain length. The low cycloaddition yields from some of the allenes are interpreted as evidence for a stepwise mechanism involving distonic radical cations as key intermediates. We discuss the tendency of the distonic radical cations to undergo ring closure to the Diels‐Alder‐product radical cations in terms of enthalpy considerations, which, we suggest, offer a novel criterion for the design of stepwise radical cation reactions.
Anion–Cation Redox Competition and the Formation of New Compounds in Highly Covalent Systems
Authors: Rouxel Jean 
Published: September 2007

AbstractThe increase in energy of the sp anionic band on going from oxides to the less electronegative sulfides, selenides, or tellurides (with a similar trend in neighboring columns) facilitates anion–cation redox interactions involving the d levels of transition metal cations and the sp levels of anionic species. The interactions can induce phase transitions or gradual change within a given structural model. When a cation is reduced by electron transfer to its d levels from the sp band, holes appear at the top of the latter. Interesting soft chemistry can be carried out based on redox processes that neutralize the holes with electrons. This approach also allows particular structural types to be stabilized. Three structural domains can be recognized amongst the transition elements: 1) on the left‐hand side of the periodic table layered structures are observed involving M4+ and (chalcogen)2‐ ions; 2) formation of sets of metal‐metal bonds is then observed, the geometry of which depends on both the initial electron population and the electron transfer to the metal; 3) on the right‐hand side the metals in their highest oxidation state are no longer active, and the holes at the top of the sp band are taken up by a catenation of the anions, which can lead to full polymerization of the anionic sublattice.
Synthesis of Enantiopure Homoallylic Alcohols and Ethers by Diastereoselective Allylation of Aldehydes
Authors: Tietze Lutz F. , Schiemann Kai , Wegner Christoph , Wulff Christian 
Published: September 2007

AbstractEnantiopure homoallylic alcohols 5, which are important building blocks in organic synthesis, are obtained with an ee of greater than 99% and a yield of 75–95% by cleavage of the secondary homoallylic ethers 4 using sodium in liquid ammonia. The ethers 4 are formed with excellent diastereoselectivity and in 52–89% yield by treatment of the aldehydes 1 with the trimethylsilyl ether of N‐trifluoroacetylnorpseudoephedrine (2) in the presence of a catalytic amount of TMS triflate or TMS borontriflate, followed by addition of allylsilane 3. Nearly all achiral aliphatic aldehydes employed gave a diastereoselectivity of over 99:1. With the chiral aldehydes 24, the difference between matched and mismatched pairs was low; this reveals that there is strong reagent control.
A Systematic Review of Gender Differences in Mortality after Coronary Artery Bypass Graft Surgery and Percutaneous Coronary Interventions
Authors: Kim Catherine , Redberg Rita F. , Pavlic Tiana , Eagle Kim A. 
Published: September 2007

AbstractGender differences exist in outcomes, particularly early mortality, for percutaneous interventions (PCI) and coronary artery bypass graft surgery (CABG). Better understanding of this issue may target areas for improvement for all patients undergoing revascularization. Therefore, we summarized the evidence on gender differences in PCI and CABG outcomes, particularly early mortality, and mediators of this difference. Using the key terms “women” or “gender,” “revascularization,” “coronary artery bypass,” “angioplasty,” “stent,” and “coronary intervention,” we searched MEDLINE from 1985 to 2005 for all randomized controlled trials (RCTs) and registries reporting outcomes by gender. Bibliographies and the Web sites of cardiology conferences were also reviewed. The literature was examined to identify gender differences in outcomes and mediators of these differences. We identified 23 studies reporting outcomes by gender for CABG and 48 studies reporting outcomes by gender for PCI. The majority of studies noted greater in‐hospital mortality in women than in men, with mortality differences resolving with longer follow‐up. Early mortality differences were reduced but not consistently eliminated after adjustment for comorbidities, procedural characteristics, and body habitus. Power to detect gender differences after multivariate adjustment was limited by declining mortality rates and small sample size. Gender was an independent risk factor for complications after both CABG and PCI. Women experience greater complications and early mortality after revascularization. Future exploration is needed of gender differences in quality of care and benefit from combinations of stenting and antiplatelet, and anticoagulant medications in order to optimize treatment. Copyright © 2007 Wiley Periodicals, Inc.
An integrated parameter identification method combined with sensor placement design
Authors: Li Yiqian , Xiang Zhihai , Zhou Masheng , Cen Zhangzhi 
Published: September 2007

AbstractMaterial parameter identification based on in situ measurements plays a very important role in engineering. Since noises are inevitable in measurements and the parameter identification is inherently ill posed, it is necessary to consider some methods to stabilize the identification procedure. One choice is properly designing the sensor placement, which has attracted much attention over the past 30 years. Most existing sensor placement design methods are based on certain sensitivity analysis, which commonly requires evaluating Jacobian matrix for given parameters. However, the ‘true’ values of parameters are unknown at that moment, because sensor placement design is carried out before the parameter identification. Consequently, roughly estimated ‘true’ parameters have to be used and less optimal sensor placement could be obtained. To solve this problem, this paper presents an integrated method in which the parameter identification and sensor placement design are carried out alternatively. The validity of the proposed method is illustrated by two simple academic examples. Then it is implemented to detect damages in a real highway bridge. Copyright © 2007 John Wiley & Sons, Ltd.