bluuebunny/crossref_metadata_2025_split huggingface dataset:
Regioselectivity in the Gas‐Phase Nucleophilic Attack on O‐Protonated 3‐Methyl‐2‐cyclohexen‐1‐ol and 1‐Methyl‐2‐cyclohexen‐1‐ol
Authors: Dezi Emanuela , Lombardozzi Antonietta , Renzi Garbriele , Pizzabiocca Adriano , Speranza Maurizio 
Published: July 2007

AbstractOur radiolytic study of the occurrence in the gas phase of concerted SN2′ reactions on several open‐chain allylic oxonium ions generated in the gas phase from the attack of gaseous GA+ acids (GA+CnH+5 (n = 1, 2), iC3H+7, and (CH3)2F+) on suitable substrates is now continued with cyclic allylic alcohols, such as 3‐methyl‐2‐cyclohexen‐1‐ol (1) and 1‐methyl‐2‐cyclohexen‐1‐ol (2), with both MeOH and NMe3 as neutral nucleophiles. With MeOH as the nucleophile, the substitution reaction exclusively takes place on 1 as the starting compounds, whereas when the substrate is 2 it is accompanied by extensive elimination. With NMe3, only the elimination reaction is observed in the same systems. The analysis of the isomeric distribution of the substitution and elimination products allows definition of the corresponding reaction patterns. As for open‐chain oxonium ions, the nucleophilic attack on O‐protonated 1 and 2 is preceded by significant intramolecular interconversion. Partial unimolecular dissociation of the same ionic intermediates also takes place. After careful evaluation of the extent of these side processes, it is demonstrated that the O‐protonated 1 undergoes the concerted SN2 process with MeOH almost exclusively (≥ 99%). With O‐protonated 2, how ever, the concerted SN2′ pathway (84–95%) prevails over the classical SN2 one (6–17%). Concomitant [1,2] (E2) and [1,4] elimination (E2′) pathways involve attack of the selected nucleophiles on the oxonium ions from 1 and 2. Their relative extent (E2′/E2:1.78–1.96 (1); 1.43‐1 70 (2)) appears only slightly dependent on the nature of the ionic substrate, the nucleophile (whether MeOH or NMe3), and the leaving group (whether H2O or MeOH). The effects of both intrinsic structural factors and experimental conditions in determining the SN2′/SN2 and E2′/E2 branchings in the selected oxonium ions is discussed and compared with related gas‐phase data.
Unexpected Basicity of a Hexacoordinate Silicon Compound, {2,6‐Bis[ (dimethylamino)methyl]phenyl}bis(1,2‐benzenediolato)silicate
Authors: Chuit Claude , Corriu Robert J. P. , Mehdi Ahmad , Reyé Catherine 
Published: July 2007

AbstractThe high basicity of {2,6‐bis‐[(dimethylamino) methyl]phenyl} bis (1,2‐benzenediolato)silicate (1) was demonstrated by its quantitative protonation in methanol to give the zwitterion 2. It was found that 1 is much more basic (pKa in CH2Cl2 = 16.7) than the “proton sponge” 1,8‐bis(dimethylamino)naphthalene (3) and also much more basic than {2,6‐bis‐[(dimethylamino) methyl]phenyl} bis (1,2‐benzenediolato)phosphorane (4). The stability of the zwitterion 2, the geometry of which corresponds to a more perfect octahedron than 1, can explain the high basicity of 1. Dynamic NMR studies of 2 in solution show that at low temperature the hydrogen‐bonding interaction with one oxygen atom observed in the solid state is maintained. On raising the temperature, firstly the hydrogen bond breaks, then dynamic coordination occurs, in which the NMe2 groups displace each other in conjunction with transprotonation.
A 2,3‐Connected Tellurium Net and the Cs3Te22Phase
Authors: Liu Qiang , Goldberg Norman , Hoffmann Roald 
Published: July 2007

AbstractThe bonding in the recently reported Cs3Te22phase, which contains both Te8rings and remarkable Te6sheets, is studied by approximate molecular orbital theory. Our focus is on the geometric and electronic features of the unique 2,3‐connected Te net found as a substructure in this phase. The calculations show that both the linear and T‐shaped Te geometries in the 2,3‐connected Te net of Cs3Te22are determined by their particular electron count. Both types of tellurium atoms are hypervalent; we make connections to other well known hypervalent molecules, such as XeF2, I3−, and BrF3. Several possible variations and distortions of this net are discussed, all of which are found to be less stable. The discrete crown‐shaped Te8units that appear in the phase show normal covalent bonding and should occur in smaller molecular entities, too. According to our computations, Cs3Te22should be metallic. Two structurally related phases, CsTe7and Cs2Te15, are suggested.
1,3‐Alternate Calix[4]arenecrown‐5 Conformers: New Synthetic Ionophores with Better K+/Na+ Selectivity than Valinomycin
Authors: Casnati Alessandro , Pochini Andrea , Ungaro Rocco , Bocchi Carlo , Ugozzoli Franco , Egberink Richard J. M. , Struijk Helma , Lugtenberg Ronny , De Jong Feike , Reinhoudt David N. 
Published: July 2007

AbstractNew 25,27‐dialkoxycalix‐[4]arenecrown‐5 conformers 8, 10, and 11 have been synthesized and studied. The compounds 8a and 8b, fixed in 1,3‐alternate structure, have been obtained in 57 and 40% yield, respectively, by reaction of the corresponding 25,27‐dialkoxycalix[4]arenes 7a–b with tetraethylene glycol di‐p‐toluenesulfonate in the presence of Cs2CO3. The cone 10a and 10b and the partial cone 11 conformers were obtained by selective demethylation of the 25,27‐dimethoxycalix[4]arenecrown‐5 (6a) and subsequent dialkylation with NaH/DMF and KOtBu/THF, respectively. In the solid state (X‐ray), compound 6a adopts a flattened cone conformation, which is also found to be most abundant in CD3CN and CD3OD solution. Upon complexation with potassium picrate compound 6a was converted quantitatively into the 1,3‐alternate conformation. All new ligands synthesized were used in the extraction of alkali metal cations from H2O into CHCl3, and as active components in supported liquid membranes and in chemically modified field effect transistors. Results were compared to those obtained with with the natural antibiotic valinomycin 1. All ligands showed high selectivity for potassium. Ligand 8a, fixed in the 1,3‐alternate conformation, is more selective than valinomycin and shows the highest K+/Na+ selectivity known so far.
Redox Properties of the Diatomic Bare Iron Chalcogenides FeO and FeS in the Gas Phase
Authors: Harvey Jeremy N. , Heinemann Christoph , Fiedler Andreas , Schröder Detlef , Schwarz Helmut 
Published: July 2007

AbstractThe electron‐transfer behavior of the binary iron chalcogenides FeO and FeS has been examined by means of mass spectrometry and ab initio calculations using the averaged coupled‐pair functional (ACPF) method. The experimental and theoretical results are in good agreement with each other and also with previous studies. The ionization energies (IE) of the diatomic species are found to be IE(FeO) = 8.8±0.2 eV, IE(FeO+) = 17.9±0.4 eV, IE(FeS) = 8.3±0.3 eV, and IE(FeS+) = 16.3±0.5 eV. Two new diatomic dications, FeS2+ and FeO2+, are shown to exist as metastable minima on the corresponding potential‐energy surfaces. The data enable an evaluation of the intrinsic gas‐phase redox properties of FeS and FeO, and the comparison demonstrates that iron sulfide is more prone to undergo facile reduction and oxidation than iron oxide.
Processing of Blue Boron Nitride Thin Films with a Solid–Gas Reaction
Authors: Mokhtari Mohamed , Park Hyung S. , Roesky Herbert W. , Johnson Stephen E. , Bolse Wolfgang , Conrad Jens , Plass Winfried 
Published: July 2007

AbstractFor the first time, solid–gas reaction techniques have been used for the synthesis and processing of thin films of boron nitride. Clear to intensely colored blue BN films were grown on Si(100) substrates by the transformation of borazine (B3N3H6) with a titanium complex as initiator under flowing nitrogen gas. The thickness of the films ranged from 70 to 100 nm, as determined by Rutherford backscattering (RBS) and atomic force microscopy (AFM) analyses. The intensity of the blue color of the thin film can be correlated to its thickness. The composition of the film determined by RBS studies corresponds to the stoichiometric formula B0.49N0.45O0.06, and N/B and O/B ratios are found to be 0.92 and 0.12, respectively. Nitrogen contents determined by nuclear reaction analysis agree well with the RBS results. Moreover, Auger electron spectroscopy (AES) measurements show that no titanium is present in the films and confirm the composition determined by RBS studies. X‐ray photoelectron spectroscopy (XPS) shows the presence of boron and nitrogen in the blue BN film. Electron spin resonance (ESR) experiments at 293 and 12 K indicate a single broad signal with a g value (g = 2.005) close to that of a free electron. This synthetic approach provides opportunities for the preparation of new thin‐film materials and for the fundamental study of solid‐gas reactions.
Synthesis, Crystal Structure, Magnetism, and Magnetic Anisotropy of Cyclic Clusters Comprising six Iron(III) Ions and Entrapping Alkaline Ions
Authors: Caneschi Andrea , Cornia Andrea , Fabretti Antonio C. , Foner Simon , Gatteschi Dante , Grandi Romano , Schenetti Luisa 
Published: July 2007

AbstractThe reaction of ferric chloride and β‐diketones (HL) in alkaline methanol solution represents a good synthetic route to hexairon(III) clusters [MFe6‐(OCH3)12(L)6]+ (M = Na, Li), which exhibit an unusual sixfold molecular symmetry. Single‐crystal X‐ray diffraction showed that the six octahedrally coordinated iron(III) ions define a ring and are linked by twelve bridging methoxide ligands. The resulting [Fe6(OCH3)12] skeleton has the remarkable property of acting as a host for an alkali‐metal ion both in the solid state and in organic solution, as demonstrated by 23Na and 7Li NMR experiments. The magnetic behavior of these systems is consistent with the presence of a nonmagnetic S = 0 ground state and of antiferromagnetic exchange interactions between the high‐spin ferric ions. The energy of the excited states was studied in detail by high‐field DC and pulsed‐field differential magnetization experiments at 0.7 and 1.5 K. Single‐crystal susceptibility measurements at variable temperature revealed a sizeable magnetic anisotropy, which has been successfully analyzed in terms of single‐ion and dipolar contributions. The results are relevant to research into the origin of superparamagnetic‐type behavior in transition‐metal clusters.
Photoswitched and Functionalized Oligothiophenes: Synthesis and Photochemical and Electrochemical Properties
Authors: Tsivgoulis Gerasimos M. , Lehn Jean‐Marie 
Published: July 2007

AbstractThe extended dithienylethene compounds 1–3 have been synthesized. They undergo photochromic and electrochemical changes by photoinduced interconversion between open and closed forms of type A and B. In addition, both forms of the N‐methylated derivatives 1b and 2b have absorption bands in which excitation results in very large differences in fluorescence between the two forms with very little effect on the opening/closing state, a feature of interest for optical memory data systems. The compounds 2 and 3 contain six and eight conjugated thiophene units in the closed forms, respectively, and thus represent oligothiophenes endowed with a photoactivated switch; they are of special interest in this respect in view of the potential use of switched oligothiophenes in molecular electronic devices.
The [Fe(etz)6](BF4)2 Spin‐Crossover System—Part Two: Hysteresis in the LIESST Regime
Authors: Hauser Andreas , Hinek Roland , Spiering Hartmut , Gütlich Philipp 
Published: July 2007

AbstractIn the [Fe(etz)6](BF4)2 spincrossover system the iron(II) complexes occupy two nonequivalent lattice sites, sites A and B. Complexes on site A show a thermal high‐spin (HS) → low‐spin (LS) transition at 105 K, whereas complexes on site B remain in the HS state down to 10 K. Complexes on both sites exhibit light‐induced spin state conversions (LIESST) at 20 K: LS → HS on site A with λ = 514.5 nm, and HS → LS on site B with λ = 820 nm. The relaxation processes subsequent to the HS ⇌ LS conversion on site B reveal a light‐induced HS⇌LS bistability for the complexes on site B at 70 K. The bistability as well as the absence of a thermal spin transition on site B are attributed to a thermal hysteresis for the B‐site complexes with a critical temperature T↑c≈︁77 K on heating. This hysteresis can be interpreted in terms of strong cooperative effects of elastic origin, which, in addition, cause characteristic deviations of the relaxation on site B from first‐order kinetics (self‐acceleration). In contrast, the HS → LS relaxation at 60 K on site A after irradiation with λ = 514.5 nm shows an unusual self‐retardation.
New Ruthenium(II) Complexes of Functionalized Monoanionic Aryldiamine N,C,N'‐Terdentate Ligands: Syntheses of [RuII{2,6‐(Me2NCH2)2‐4‐R‐C6H2}‐(terpy)]+Cl‐; X‐ray Structure of a Dimeric Organolithium Compound, [Li{2,6‐(Me2NCH2)2‐4‐Ph‐C6H2}]2
Authors: Steenwinkel Pablo , James Stuart L. , Grove David M. , Veldman Nora , Spek Anthony L. , van Koten Gerard 
Published: July 2007

AbstractThe new anionic functionalized aryldiamine ligands [2,6‐(Me2NCH2)2‐4‐R‐C6H2]‐ (R = Me3SiCC, C6H5, Me3‐Si), formally derived from [2,6‐(Me2‐NCH2)2C6H3]‐, have been prepared as their lithium compounds. The compound [Li{2,6‐(Me2NCH2)2‐4‐Ph‐C6H2}]2 crystallizes in the monoclinic space group C2/c (no. 15) with a = 13.1225(5), b = 13.5844(7), c = 18.9859(12) Å, β = 105.329(5)°, V = 3264.0(3) Å3. Z = 4. The structure refinement converged to R1 = 0.0374 for 2037 observed reflections [Fo>4σ(Fo)] and wR2 = 0.0922 for 2560 unique data. The organolithium compounds have been used in transmetalation reactions to give the corresponding functionalized organoruthenium(II) complexes [RuII{2,6‐(Me2NCH2)2‐4‐R‐C6H2}‐(terpy)]+Cl‐ (terpy = 2,2′;6′,2′‐terpyridine). The RuII species with R = HC°C has also been synthesized.
Studies Towards the Total Synthesis of Epothilones: Asymmetric Synthesis of the Key Fragments
Authors: Schinzer Dieter , Limberg Anja , Böhm Oliver M. 
Published: July 2007

AbstractMembers of a new class of macrolide—the so‐called epothilones (1)—showing a taxol‐like biological activity have recently been isolated. A convergent approach to1is presented, and the asymmetric syntheses of the three key intermediates3, 4, and8are reported.
Ruthenium(II)‐Catalyzed Oppenauer‐Type Oxidation of Secondary Alcohols
Authors: Almeida Maria L. S. , Beller Marion , Wang Guo‐Z. , Bäckvall Jan‐E. 
Published: July 2007

AbstractHighly efficient ruthenium‐catalyzed Oppenauer‐type oxidations of secondary alcohols to ketones have been developed. The catalytic system consists of [(PPh3)3RuCl2] (1) and K2CO3 or [(C4Ph4COHOCC4Ph4)(μ‐H)(CO)4Ru2] (2) in refluxing acetone. The catalytic reaction is of high efficiency and permits a catalyst: substrate ratio of 1:1000 at 56 °C. In some cases the initial turnover rate exceeds 1500 h‐1. The reaction was found to be general and compatible with double bonds and oxidation‐sensitive aromatics. With some allylic alcohols, isomerization to saturated ketones took place. The reaction proceeds by ruthenium‐catalyzed dehydrogenation of the alcohol and subsequent hydrogen transfer to acetone. No primary kinetic isotope effect was observed for the catalytic reaction when α‐deuterated 1‐phenylethanol was employed as substrate (kH/kD=1.1); this shows that β‐hydride elimination from a ruthenium alkoxide intermediate is not the rate‐limiting step.
Ab Initio Quantum Mechanical Model Calculations on the Catalytic Mechanism of Aspartylglucosaminidase (AGA): A Serine Protease‐Like Mechanism with an N‐terminal Threonine and Substrate‐Assisted Catalysis
Authors: Peräkylä Mikael , Rouvinen Juha 
Published: July 2007

AbstractAb initio quantum mechanical model calculations were used in studying the acylation and deacylation steps of the catalytic mechanism of aspartylglucosaminidase (AGA). AGA catalyses the hydrolysis of an amide linkage between oligosaccharide and asparagine by utilising an N‐terminal threonine as a catalytic amino acid. Results are reported for the model enzyme reaction at the MP2/6–31+G*//HF/6–31 + G* + ΔZPE level. Contribution of aqueous solvation to the relative energies was estimated by using the continuum solvation model of Tomasi. The serine protease‐like catalytic mechanism was found to be feasible for AGA. The protonated α‐amino group of the substrate of AGA was suggested to enhance the catalysis by stabilising the anionic oxygen of the substrate, which is formed in the reaction, and by lowering the pKa of the nucleophilic oxygen of the N‐terminal threonine. Finally, the similarities in the catalytic mechanisms of AGA and other amidohydrolases were discussed.
The Role of Water Exchange in Attaining Maximum Relaxivities for Dendrimeric MRI Contrast Agents
Authors: Tóth Éva , Pubanz Dirk , Vauthey Sylvain , Helm Lothar , Merbach André E. 
Published: July 2007

AbstractMacrocyclic GdIII complexes attached to dendrimers represent a new class of potential MRI contrast agents. They have an extended lifetime in the blood pool, which is indispensable for their application in magnetic resonance angiography, and high relaxivities, which reduce the dose required to produce quality images. We performed a variable‐temperature and ‐pressure 17O NMR study in aqueous solution and at 14.1, 9.4, and 1.4 T on the water exchange and rotational dynamics of three macrocyclic GdIII complexes based on polyamidoamine dendrimers, as well as on the GdIII complex of the monomer unit with the linker group. The water exchange rates k298ex for generation 5 [G5(N{CS}N‐bz‐Gd‐{DO3A}{H2O})52], generation 4 [G4(N‐{CS}N‐bz‐Gd{DO3A}{H2O})30], generation 3 [G3(N{CS}N‐bz‐Gd{DO3A}‐{H2O})23], and the monomer [Gd(DO3A‐bz‐NO2)(H2O)] complexes are 1.5±0.1, 1.3±0.1, 1.0±0.1, and 1.6±0.1 × 106 s‐1, respectively, and the activation volumes ΔV≢ of water exchange on the latter two compounds are + 3.1±0.2 and + 7.7±0.5 cm3 mol‐1, indicating dissociatively activated exchange reactions ({CS}N‐bz‐{DO3A}=1‐(4‐isothiocyanatobenzyl)amido‐4,7,10‐tri(acetic acid)tetraazacyclododecane). The rotational correlation times for the dendrimers are 4 to 8 times longer than for monomeric or dimeric GdIII poly(amino carboxylates). As a consequence of the slow rotation, the proton relaxivities of these dendrimer complexes are considerably higher than those of smaller complexes. However, the low water exchange rates prevent the dendrimer proton relaxivities from attaining the values expected from the increase in the rotational correlation times. Modifications of the chelating ligand may result in a faster water exchange and thus allow the full benefit of slow rotation to be achieved.
Interaction of Acetonitrile with Olefins and Alcohols in Zeolite H‐ZSM‐5: In situ Solid‐state NMR Characterization of the Reaction Products
Authors: Stepanov Alexander G. , Luzgin Mikhail V. 
Published: July 2007

AbstractThe reaction products and intermediates from the interaction of acetonitrile with olefins (oct‐1‐ene) or alcohols (tert‐butyl alcohol) in zeolite H‐ZSM‐5 at 296 K have been characterized with 13C and 15N solid‐state NMR. It has been shown that coadsorption of acetonitrile and olefin on H‐ZSM‐5 gives rise to the intermediate N‐alkylnitrilium cation represents a persistent species inside a zeolite under anhydrous conditions. Upon admittance of water to the pores of the zeolite, the N‐alkylnitrilium cation slowly converts into N‐alkylamide in accordance with the classic Ritter reaction. In the case of acetonitrile and alcohol, just after coadsorption both the intermediate N‐alkylnitrilium cation and the final N‐alkyl‐amide are identified simultaneously; the former slowly disappears over a few days. Thus, 1) it has been shown that the Ritter reaction can occur not only in liquid acidic media but also on a solid acid catalyst, zeolite H‐ZSM‐5; 2) N‐alkylnitrilium cations have been detected and characterized with solid‐state NMR as persistent intermediates in the Ritter reaction for the first time while the reaction proceeds.
An Analytical Study of the Redox Behavior of 1,10‐Phenanthroline‐5,6‐dione, its Transition‐Metal Complexes, and its N‐Monomethylated Derivative with regard to their Efficiency as Mediators of NAD(P)+ Regeneration
Authors: Hilt Gerhard , Jarbawi Tafeeda , Heineman William R. , Steckhan Eberhard 
Published: July 2007

AbstractThe synthesis as well as the chemical and electrochemical properties of homoleptic and heteroleptic (trispyridyl‐methylamine as coligand) transition‐metal complexes (Ru and Co) of 1,10‐phenanthroline‐5,6‐dione (PD) and of its N‐monomethylated derivative (PDMe+) are described. In particular, their ability to abstract hydride ions was studied. Electrochemical investigations with cyclic voltammetry, rotating disk electrode experiments, and spectroelectrochemical methods at different pH values gave an insight into the complex electrochemistry of the compounds described, which is strongly influenced by a hydration pre‐equilibrium. The electrochemically active quinone form of the transition‐metal complexes can be reduced to the hydroquinone state in acidic solution and to their transition‐metal‐stabilized semi‐quinone states for neutral and basic solutions, whereas PDMe+ is reduced to the hydroquinone state in both acidic and neutral solutions. The compounds can also be reduced chemically, and are efficient catalysts for the indirect oxidation of the enzymatic cofactor NAD(P)H. For the indirect aerobic NAD(P)H oxidation, up to 900 turnovers per hour can be observed, an achievement yet to be reached by other catalyst systems.
Rhodium(I)‐Assisted Stereoselective Coupling of an Alkyl, Aryl or Vinyl Group with a Vinylidene Ligand: A Novel Synthetic Route to π‐Allyl and π‐Butadienyl Rhodium Complexes
Authors: Werner Helmut , Wiedemann Ralf , Steinert Paul , Wolf Justin 
Published: July 2007

AbstractIn the first part of this work, a general method for the preparation of aryl, methyl, vinyl and alkynyl(vinyl‐idene)rhodium(I) complexes trans‐[Rh(R')‐(CCHR)(PiPr3)2] (8–14, 18–22) and trans‐[Rh(R')(CCMe2)(PiPr3)2] (16, 17) from the corresponding chloro(vinyl‐idene) derivatives and Grignard reagents is described. Whilst compounds 8 and 10–13 react with pyridine to give trans‐[Rh(CCR)(py)(PiPr3)2] (23–25) by elimination of R'H, treatment of 8–11, 16, and 18 with carbon monoxide yields the square‐planar η1‐vinyl and η1‐butadienylrhodiumcarbonyl complexes trans‐[Rh{η1‐(Z)‐C(R')CHR}(CO)(PiPr3)2] (27–32). The reaction of 8 or 18 with methyl or tert‐butylisocyanide leads stereoselectively to the isocyaniderhodium(I) compounds trans‐ [Rh{η1‐(Z)‐C(R)CHPh}(CNR') (PiPr3)2] (33–35). Acid‐induced cleavage of the rhodium‐carbon σ bond of 27, 30, or 31 with CH3CO2H gives trans‐[Rh(η1‐O2CCH3)‐(CO)(PiPr3)2] (38) and the corresponding olefin or diene, respectively. In the absence of a Lewis base such as pyridine, CO, or CNR', compounds 18–20 rearrange in benzene at 40–50 °C to afford the isomeric π‐allyl complexes [Rh(η3‐1‐RC3H4)(PiPr3)2] (40–42) almost quantitatively. The vinyl(vinylidene) compounds 11 and 12 also undergo an intramolecular rearrangement that leads to the η3‐2,3,4‐butadienyl‐ or to the alkynyl(ethene)rhodium(I) isomers, depending on the reaction conditions. In an analogous manner to the η1‐vinyl‐and η1‐butadienyl(carbonyl) derivatives 27, 30, and 31, the π‐allyl and π‐butadienyl complexes also react with acetic acid to give [Rh(η1‐O2CCH3)(PiPr3)2] (47) and the respective olefin.