bluuebunny/crossref_metadata_2025_split huggingface dataset:
Asymmetric Control of 1,3‐Dipolar Cycloaddition Reactions with Azomethine Ylides by Means of Proline Esters as Chiral Auxiliary Groups
Authors: Waldmann Herbert , Bläser Edwin , Jansen Martin , Letschert Hans‐Peter 
Published: July 2007

AbstractUpon treatment with triethylamine or DBU in the presence of LiBr, aromatic and aliphatic imines of amino acid esters are converted to N‐metalated azomethine ylides. These 1,3‐dipoles undergo highly stereoselective cycloadditions with N‐acryloyl‐(S)‐proline esters in THF at −78 to −40°C to afford highly substituted pyrrolidines with complete regiocontrol and good to excellent diastereomeric ratios. The chiral auxiliary groups can readily be removed from the cycloadducts by simple acid hydrolysis. To rationalize the observed stereoselectivity a transition‐state model is proposed in which the lithium cation is coordinated to both the 1,3‐dipole and the dipolarophile.
A Dual‐Mode Molecular Switching Device: Bisphenolic Diarylethenes with Integrated Photochromic and Electrochromic Properties
Authors: Kawai Stephen H. , Gilat Sylvain L. , Ponsinet Rachel , Lehn Jean‐Marie 
Published: July 2007

AbstractThe bisphenolic dithienylethene molecules 1a and 1b were synthesized in overall yields of 45% from 4‐bromoanisole and 44% from 2.6‐di‐tert‐butyl‐4‐iodophenol, respectively. The corresponding extended quinones 3a and 3b were also prepared. Photochemical studies showed that compounds 1 are photochromic; the open forms 1 could be converted with UV light of 312nm to the closed coloured forms 2 with photostationary states lying at essentially complete conversion (> 98%). The 1a‐2a system was found to exhibit good resistance to photofatigue and thermal stability for both photoisomers. Cyclic voltammetry studies involving the 2/3 couples showed that whereas 2b undergoes irreversible oxidation at + 0.85 V (vs. SCE in THF), the hydroquinone 2a is reversibly oxidized at an E1/2 of + 0.72V (in MeCN, quasi‐reversibly in THF at + 0.81 V); this reflects the differences in deprotonation behaviour of the generated QH2/2+ species. The large difference in oxidation potential between 1a and 2a allows the photochemical switching of redox properties. In a complementary fashion, redox switching of the photochromic properties within the 2a‐3a pair is possible since 3a is stable to visible light. Owing to this unique behaviour, the triad consisting of 1‐3a represents a novel molecular device with mutually regulating photo‐ and electrochromic behaviour. In addition, the ability to interconvert between the three stable states makes the system well‐suited as the basis for an optical memory system with multiple storage and nondestructive readout capacity through a write‐lock‐read‐unlock‐erase cycle.
Total Synthesis of Rapamycin
Authors: Nicolaou K. C. , Piscopio Anthony D. , Bertinato Peter , Chakraborty Tushar K. , Minowa Nobuto , Koide Kazunori 
Published: July 2007

AbstractDetails of the total synthesis of rapamycin (1) are reported. The synthesis required the preparation of intermediates 4 – 9 in nonracemic form; key coupling reactions included a chromium‐mediated addition of vinyl iodide 8 to aldehyde 7 and an Evans aldol reaction to couple fragments 62 and 9. Intermediates 4 and 6 were joined through an amide bond formation to afford advanced intermediate 71. Swern oxidation of the diol in 71 was followed by a selective removal of the TES groups and a second Swern oxidation. Finally, removal of the remaining silyl protecting groups provided fully deprotected, penultimate intermediate 2 in which all carbons were in their proper oxidation state. Macrocyclization was achieved through a tandem inter/intramolecular palladium‐mediated Stille coupling reaction between distannylethene 3 and bis(vinyl iodide) 2. This latter process accomplished in one step the installation of the remaining two carbons of the natural product and the completion of its total synthesis.
Mono‐ and Bis(pentaisopropylcyclopentadienyl) Cobalt and Rhodium Sandwich Complexes and Other Decabranched Cyclopentadienyl Complexes
Authors: Buchholz Dirk , Gloaguen Bruno , Fillaut Jean‐Luc , Cotrait Michel , Astruc Didier 
Published: July 2007

AbstractThe acidity of methyl protons in the 18‐electron cationic complexes [MCp*Cp]1PF and [MCp]+PF (M = Co or Rh, Cp = η5‐C5H5, Cp* = η5‐ C5Me5) has been used for novel syntheses of starburst organometallic complexes. [CoCp*Cp]+PF was deprotonated at −30 °C with Si2Me6NK (1 equiv) in THF to give the fulvene complex [CoICp(η4‐C5Me4CH2)]. This was not stable at 20°C, but was characterized by 1H and 13C NMR at −30 °C. The complexes [MCp*Cp]+PF (M = Co or Rh) reacted with excess base (KOH or tBuOK) and alkyl halides (RX = CH3I, C2H5I, CH2CHCH2Br, or PhCH2Br) to give decasubstituted complexes (i.e., two hydrogens of each methyl group are replaced by two R groups). Distinct endo and exo alkyl groups are observed in the new complexes by NMR at room temperature. Coalescence occurs at higher temperatures; this indicates that the bulky alkyl groups are rotating. The calculated activation parameters (ΔG≠) are 71.3±0.8, 70.3±0.8, and 81.0±0.8 kJmol−1 for [Co(C5iPr5)Cp]+PF, [Rh(C5iPr5)‐Cp]+PF (in C6D5NO2), and [Co{C5‐(CHEt2)5}Cp]+PF6 (in o‐C6H4Cl2), respectively. The single directionality of these ligands is confirmed by the X‐ray crystal structure of [Co(C5iPr5)Cp]+PF6. This structure clearly shows the “paddle wheel” conformation adopted in the C5iPr5 ligand, due to steric hindrance between adjacent isopropyl groups and the staggered conformation of the two rings. The relative E° values measured by cyclic voltammetry show that the electron‐donating properties of the new C5(CHR2)5 ligand are between those of C5H5 and C5Me5‐ With the decamethylcobalticinium salt [CoCp]+PF, the permethylation reactions with base and CH3 I did not go to completion, even under forcing conditions and with repeated reaction (the maximum number of methyl groups introduced was 14). On the other hand, [RhCp]+PF, in which the distance between rings is larger than in the Co analogue, reacted with excess KOH and CH3I to give [Rh(C5iPr5)2]+PF6 in 55% yield; only one diastereoisomer was detected by NMR (ΔG≠ = 85.5±0.8 kJ mol−1 in C6D5NO2). The C5iPr5 sandwich complexes are extremely robust, even at the 19‐electron (CoII) stage. The 20‐electron K+ salt of the CoI anion was regiospecifically protonated by H2O at the Cp ligand to give [Co(C5iPr5)(η4‐C5H6)] and was decomplexed at 190°C to K+(C5iPr5)−.
Ab initio Study of the Coordination Modes of the Tetrahydroborato Ligand: Structure of the [Cu(BH4)(PH3)n] (n = 1, 2, 3) Complexes
Authors: Jarid A. , Lledos A. , Jean Y. , Volatron F. 
Published: July 2007

AbstractAll‐electron ab initio calculations (MP4/MP2 level) have been performed on [Cu(BH4)(PH3)n] (n = 1, 2, 3) complexes. Full‐geometry optimizations were carried out in each case, and the stationary points were characterized by the diagonalization of the analytically calculated Hessian matrix. The η2 coordination mode, with a tetrahedral arrangement around the copper atom, is the most stable structure for n = 2, while for n = 3 a strongly nonlinear η1 coordination mode is preferred. These results are in agreement with the experimental data available on related complexes. For n = 1, for which there is no experimental data, the η3 structure turns out to be the most stable. The energy differences associated with some changes in the coordination mode (η1 → η2 for n = 3 and η3 → η2 for n = 1) are small. Finally, a mechanism for the exchange between terminal and bridging hydrogen atoms is proposed for each complex under study.
The Alkenyl Mechanism for Fischer‐Tropsch Surface Methylene Polymerisation; the Reactions of Vinylic Probes with CO/H2 over Rhodium Catalyst
Authors: Turner Michael L. , Long Helen C. , Shenton Adele , Byers Peter K. , Maitlis Peter M. 
Published: July 2007

AbstractResults consistent with the participation of vinyls in the initiation and of alkenyl species in the propagation steps of the Fischer–Tropsch reaction are reported. Substantial incorporation of 13C2 into the alkene and alkane (C3–C7) hydrocarbon products occurred when doubly labelled vinyls (13C2H3Br. (13C2H3)4Si, or 13C2H4) were added as molecular probes to the hydrogenation of carbon monoxide over rhodium/ceria/silica catalysts (1 atm, 220°C). There was, by contrast, no significant incorporation of 13C1 into any of the organic products; thus cleavage of the C2 probe did not occur. The degree of 13C2 incorporation decreased with increasing molecular mass of the hydrocarbon; this indicates that the probe molecule initiated but did not propagate. A mathematical model based on polymerisation of surface methylenes initiated by a vinyl, propagated by alkenyls and terminated by reaction with a surface hydrogen or by coupling has been developed to explain the 13C2 incorporation data. Under the conditions of the experiments, the relative ability of the probes to initiate is: vinyl bromide (60%)>tetravinylsilane (30%)>ethene (15%). Substantial formation of 13C4 products also occurred when vinyl bromide or tetravinylsilane were used as probes; this arises from a dimerisation of the vinyl on the surface, a process which has been modelled in homogeneous systems and also by other workers in studies on single crystal surfaces. There was no significant 13C incorporation into the oxygenates (methanol, ethanol, acetaldehyde); these products are formed by a different path.
The Solution Structure of Oxidized HiPIP I from Ectothiorhodospira halophila; Can NMR Spectroscopy Be Used to Probe Rearrangements Associated with Electron Transfer Processes?
Authors: Bertini Ivano , Eltis Lindsay D. , Felli Isabella C. , Kastrau Dieter H. W. , Luchinat Claudio , Piccioli Mario 
Published: July 2007

AbstractIn the 1H NMR spectrum of the oxidized form of the high‐potential iron‐sulfur protein (HiPIPI) from Ectothiorhodospira halophila, 91% of the total proton resonances and 100% of the residues have been assigned. The standard COSY, NOESY, and TOCSY sequences have been optimized for the paramagnetism of the molecule. Extensive assignment of the 15N NMR spectrum has been obtained through HMQC spectra. With 1437 dipolar connectivities, of which about 10% involved fast‐relaxing protons, a family of 18 structures was generated with an RMSD of 0.65 Å by using the programs developed by Wüthrich. The family of structures was further refined by various calculation steps; the final RMSD was 0.48 Å. The structures appear to be very similar but not equal to the structures of the reduced protein. Despite the similarity in structure, significant variations in the chemical shifts are observed. A similar behavior was observed for the homologous protein from Chromatium vinosum. It is concluded that NMR is a sensitive tool to monitor differences between oxidized and reduced proteins; however, the detailed structural variations should be evaluated with caution at the present level of resolution, which roughly corresponds to a resolution of 2.5 Å in an X‐ray structure.
Ab Initio ECP/DFT Calculation and Interpretation of Carbon and Oxygen NMR Chemical Shift Tensors in Transition‐Metal Carbonyl Complexes
Authors: Kaupp Martin , Malkin Vladimir G. , Malkina Olga L. , Salahub Dennis R. 
Published: July 2007

AbstractCarbon and oxygen NMR chemical shift tensors for Group 6 hexacarbonyl complexes M(CO)6 (M  Cr, Mo, W) have been calculated by using a combination of quasirelativistic metal effective‐core potentials and density‐functional theory. Comparison with high‐resolution solid‐state shift tensors indicates excellent agreement between theory and experiment. The sensitivity of the shifts to the W‐C distance in W(CO)6 is discussed. A breakdown of the shielding tensor components into contributions from localized molecular orbitals allows the detailed interpretation of the trends on going down Group 6, and of differences to free CO. Group trends in the carbon shielding tensors are related largely to contributions from M‐C σ‐bonding orbitals. The presence of occupied metal (n–1)p and (n–1)d orbitals is partly responsible for the changes on going from free to metalbound CO. The origin of the less pronounced trends in the oxygen shielding tensors is more complicated. The influence of scalar relativistic effects on the shift tensors has been studied for W(CO)6 and is found to be relatively small, in spite of considerable changes in the W‐C distance.
Bis[2]catenanes and a bis[2]rotaxane–Model Compounds for Polymers with Mechanically Interlocked Components
Authors: Ashton Peter R. , Huff Jürgen , Menzer Stephan , Parsons Ian W. , Preece Jon A. , Stoddart J. Fraser , Tolley Malcolm S. , White Andrew J. P. , Williams David J. 
Published: July 2007

AbstractThe self‐assembly of three bis[2]catenanes and a bis[2]rotaxane, by two complementary strategies, is reported. A synthetic route to derivatives of bis‐para‐phenylene[34]crown‐10 (BPP34C10) and 1,5‐naphtho‐para‐phenylene[36]‐crown‐10 (1/5NPP36C10) containing a fused five‐membered ring with a secondary amine function is described. These intermediate N‐allylimido macrocyclic polyethers undergo template‐directed reactions with 1,1′‐[1,4‐phenylenebis‐(methylene)]bis‐4,4′‐bipyridinium bis‐(hexafluorophosphate) and 1,4‐bis(bromo‐methyl)benzene to produce [2]catenanes containing an N‐allyl functionality. The N‐allylimido macrocyclic polyethers have also been reduced and deprotected to afford macrocycles possessing a free NH group, which are then linked through a 4,4′‐biphenyldicarbonyl spacer to produce bis(crown ether)s, in which each crown ether moiety has two recognition sites. These ditopic BPP34C10 and 1/5NPP36C10 derivatives are capable of sustaining self‐assembly reactions at both recognition sites to yield bis[2]catenanes. The self‐assembly of a complementary bis[2]catenane, in which two tetracationic cyclophanes are linked together with a flexible hexyl chain, has also been achieved by treating 1,1′‐[1,4‐phenylenebis(methylene)]bis‐4,4′‐bipyridinium bis‐(hexafluorophosphate) with a compound containing two linked 1,4‐bis(bromomethyl)benzene units in the presence of BPP34C10. Replacing BPP34C10 with a dumbbell‐shaped compound containing a linear polyether unit intercepted by a naphthalene residue and terminated by two bulky adamantoyl groups has led to the self‐assembly of a bis[2]rotaxane. The X‐ray crystal structures of one of the catenanes and its associated crown ether component are reported, together with solution state dynamic 1H NMR spectroscopic studies, showing that there is substantial degree of order characterizing the molecular structure of the catenanes.
Fluorescent Sensors for Transition Metals Based on Electron‐Transfer and Energy‐Transfer Mechanisms
Authors: Fabbrizzi Luigi , Licchelli Maurizio , Pallavicini Piersandro , Perotti Angelo , Taglietti Angelo , Sacchi Donata 
Published: July 2007

AbstractFluorescent sensors for 3d divalent metal ions have been designed by means of a supramolecular approach: an anthracene fragment (the signalling subunit) has been linked to either a cyclic or a noncyclic quadridentate ligand (the receptor). Occurrence of the metal‐receptor interaction is signalled through the quenching of anthracene fluorescence. When the receptor (i.e., the dioxotetramine subunit of sensors 2 and 3) is able to promote the one‐electron oxidation of the metal, quenching takes place through a photoinduced metal‐to‐fluorophore electron‐transfer mechanism. In the case of sensors containing a tetraamine binding subunit (4 and 5), quenching proceeds by an energy‐transfer process. Selective metal binding and recognition can be achieved by varying the pH, and metal ions can be distinguished (e.g., CuII from NiII) by spectrofluorimetric titration experiments in buffered solutions. Whereas systems 2, 3 and 5 show reversible metal binding behaviour, the cyclam‐containing system 4 irreversibly incorporates transition metals (due to the kinetic macrocyclic effect) and cannot work properly as a sensor.
Substrate Uptake and Activation by Dimolybdenum and Ditungsten Hexaalkoxides–Factors Influencing the Cleavage of C–X Multiple Bonds in the Reactions Between [Mo2(OR)6] (R  tBu or CH2tBu) and Ar2CS, Et2NCN, and P(nBu)3
Authors: Budzichowski T. A. , Chisholm M. H. , Folting K. 
Published: July 2007

AbstractThe reaction between diarylthiones and [Mo2(OCH2tBu)6] in hydrocarbon solvents yielded [Mo2(OCH2tBu)6‐(μ‐S)(CAr2)] by cleavage of the CS double bond. The Lewis base adduct [Mo2 (OCH2tBu)6(μ‐S) (CPh2) (PMe3)] (2) has been crystallographically characterized; it contains six‐ and five‐coordinate Mo atoms linked through μ‐S and μ‐OR groups. The rate of the cleavage of the CS bond has been studied by variable‐temperature 1H NMR in [D8]toluene. A Hammett plot shows that both electron‐donating and electron‐releasing substituents in the aryl groups enhance the rate relative to Ph2CS. The activation parameters for cleavage of the CS bonds in Ph2CS, (p‐MeOC6H4)2CS, and (m‐CF3C6H4)2CS exhibit essentially identical values for ΔS‡, while ΔH‡ is somewhat smaller for the thiones bearing electron‐donating or ‐withdrawing substituents. We also studied the reversible binding of Et2NCN to [Mo2(OtBu)6] as a model system wherein the CN bond is reduced but not cleaved and the (MM)6+ center oxidized. Finally, the reversible binding of P(nBu)3 to [Mo2(OCH2tBu)6], wherein the product exhibits no net oxidation of the metal centers, has been similarly studied. The activation parameters for these three classes of reactions are compared with previous studies of the reversible binding of CN− to [M2(OR)6] compounds and the reaction 2[W2(OiPr)6] → [W4(OiPr)12]. We propose that the activation parameters associated with CS cleavage are associated with the formation of a μ‐η1,η2‐SCAr2 reactive complex similar to others described elsewhere,[23] and that they result from a) the bimolecular nature of the reaction and b) reorganizational energy. The greater facility of CX multiple‐bond cleavage in reactions involving [W2(OR)6] compounds relative to [Mo2(OR)6] compounds, the substituent effects, and the dependence on R are discussed in terms of the orbital energy match between the MM and CX π and π* orbitals, and an analogy is made with neutral Diels‐Alder cycloaddition reactions.
1,2,3‐Triphosphetenes, First Examples of a New Class of Phosphorus Heterocycles
Authors: Pucknat Heike , Grobe Joseph , Le Van Duc , Broschk Burkhardt , Hegemann Marianne , Krebs Bernt , Läge Mechtild 
Published: July 2007

AbstractHitherto unknown 1,2,3‐triphosphetenes RCPPCF3PCF3 (3a–d) are formed as main products in reactions of phosphaalkynes RCP [R = iPr2N (1a), tBu (1b), Me2EtC (1c), 1‐methylcyclohexyl (1d)] with the cyclotetraphosphane (PCF3)4 (2). According to NMR results the CF3 groups in 3a–d have a trans disposition; an X‐ray diffraction study of 3a confirms this structure. The PP bond lengths in 3a are equal [2.201 (2) and 2.204 (2) Å] and correspond to single bonds. A considerable shortening is observed for the sp2‐C–N bond (1.336 Å) which, together with the elongation of the PC bond (1.746 Å), indicates effective π donation of the lone pair on nitrogen. Surprisingly, 3a can be prepared in quantitative yields by reaction of the PP ylide Me3PPCF3 (6) with 1 a (molar ratio: 2:1). In contrast, the corresponding reactions of 6 with the alkyl‐substituted phosphaalkynes 1b–d lead to the novel phosphorus ylides Me3PC(R)PPCF3PCF3PCF3 [R = tBu (10a), Me2EtC (10b), 1‐methylcyclohexyl (10c)] in good yields. In their molecular groundstate structures, determined by X‐ray diffraction, the lone pair on the phosphano P atom prefers the syn position with respect to the ylidic PC bond. An unusual lengthening of the sp2‐C–C bond [1.553 (4) (10a), 1.543 (6) (10b), 1.551 (4) Å (10c)] to values typical for sp3‐C/sp3‐C distances is observed.
Zintl Anions as Starting Compounds for the Synthesis of Polynuclear Transition Metal Complexes
Authors: Ahlrichs Reinhart , Fenske Dieter , Fromm Katharina , Krautscheid Harald , Krautscheid Ulrike , Treutler Oliver 
Published: July 2007

AbstractThe Zintl anion P3−7 reacts with complexes of transition metal halides to form multinuclear metal phosphorus clusters. Reaction of Li3P7·3 DME with [FeCp(CO)2Br] or [NiCl2(PBu3)2] leads to the formation of [P7{FeCp(CO)2}3] (1) and [{Ni(PBu3)2}4P14] (2), respectively. X‐ray structure determinations show that in 1 the P7 cage of Li3P7 remains intact but in 2 a P14 framework is formed by linkage of two norbornadiene‐like P7 units. The P14 skeleton coordinates to four Ni(PBu3)2 groups. LiCp* and CoCl2 react with P7(SiMe3)3 to give [(Cp*Co)3‐(P2)3] (3) with three Cp*Co groups bridged by P2 units. Reaction of FeCl2 and LiCp* with P7(SiMe3)3 yields [(Cp*Fe)3P6][FeCl3(thf)] (4) or [(Cp*Fe)3{n3‐P3)Fe}P6] (5), depending on the reaction conditions. In 4 the structure of the Fe3P6 core of the [(Cp*Fe)3P6]+ cation is comparable to closo‐B9H92−, while in 5 the Fe4P6 core does not obey the Wade rules. The unusual Co‐As cluster [Co6As12(PEt2Ph)6] (6), prepared from K, As, and [CoCl2(PEt2‐Ph)2], can be described as a Co6As6 heteroicosahedron linked to two Co3As3 octahedra by common Co3 faces. A theoretical treatment within the density functional approximation reproduces the experimental structures of 2 and 6 and allows an interpretation of molecular electronic structures. In 2 one finds P‐P double bonds that are delocalized to some extent into vacant Ni AOs. For the cage compound 6 the Co 3 d AOs participate in cage bonding and 38 electrons can be assigned to cage bonds; this is in accord with the (2N + 2) rule for 18‐atom cages but not with the usual electron counting rules.
Thianthrene 5‐Oxide as a Mechanistic Probe in Oxygen Transfer Reactions: The Case of Carbonyl Oxides versus Dioxiranes Revisited
Authors: Adam Waldemar , Golsch Dieter , Görth Felix Christian 
Published: July 2007

AbstractThianthrene 5‐oxide (SSO) constitutes a useful mechanistic tool for the assessment of the electronic character of oxygen transfer agents by means of their Xso values, which reflect the extent of nucleophilic oxidation at the SO site in SSO. Treatment of dioxiranes 1 a–d with the SSO probe confirms that these are electrophilic oxidants (Xso < 0.15). Dioxirane sulfoxidation is sensitive to protic solvents and acids, which implies a polar mechanism (SN2) with nucleophilic attack of the sulfide electron pair on the dioxirane peroxide bond. In contrast, the carbonyl oxides 2 a and b, diethyl persulfoxide (3), and adamantylideneadamantane perepoxide (4) are nucleophilic oxidants (Xso > 0.85). However, the cyclopentadienone carbonyl oxides 2 c and d show low Xso values typical for electrophilic oxygen transfer agents. For these carbonyl oxides, photoisomerization to the respective dioxiranes is proposed under the photooxidation conditions of the diazoalkanes. Additionally, the trans/cis ratio of bissulfoxides (SOSO) formed provides valuable information on the stereochemical course of the oxygen‐transfer process due to the steric requirements of the oxidant. Thus, the electrophilic dioxiranes preferentially attack the axial lone pair of SSO because of repulsion by the peri hydrogen atoms to afford trans‐SO‐SO.