bluuebunny/crossref_metadata_2025 huggingface dataset:
λ5‐Phosphetes, Benzo‐λ5‐Phosphetes, Naphtho‐λ5‐Phosphetes: Four‐π‐, Eight‐π‐, and Twelve‐π‐Electron Systems
Authors: Heim Udo , Pritzkow Hans , Fleischer Ulrich , Grützmacher Hansjörg , Sanchez Muriel , Réau Régis , Bertrand Guy
Published: July 2007

A number of possible strategies for the preparation of λ5‐phosphetes were tested as follows: cyclopropenium 6 was treated with the lithium salt of diphenylphosphine to give phosphinocyclopropene 7, but 7 did not undergo ring expansion upon photolysis or thermolysis. P‐chloro‐Ctrimethylsilyl‐substituted ylide 8b reacted with two equivalents of dimethyl acetylenedicarboxylate to afford phosphinine 13 via a transient λ5‐phosphete 12. Addition of aluminum trichloride to P‐halogenated ylides 17a–b led to dihydrophosphetium salts 19a–b, which, upon treatment with pyridine, isomerized into the 1, 2‐dihydrophosphet‐2‐ium salts 20a–b. Hydrolysis of derivatives 20a–b cleanly afforded phosphoniums 21 a–b, which reacted with NaN(SiMe3)2 to give rise to the corresponding λ5‐phosphetes 22a–b. The benzo‐λ5‐phosphete 22a underwent ring expansion reactions with dimethyl acetylenedicarboxylate and acetonitrile, leading to benzo‐λ5‐phosphinine and benzo‐1, 4λ5‐azaphosphinine in good yields. Derivative 22b was characterized by X‐ray crystal structure analysis. Ab initio SCF calculations, IGLO‐13C chemical shifts and Δχ for various benzannulated derivatives and phosphorus heterocycles are presented.
Bimetallic Sesquifulvalene Complexes–Compounds with Unusually Large Hyperpolarizability β
Authors: Behrens Ulrich , Brussaard Hugo , Hagenau Ute , Heck Jürgen , Hendrickx Eric , Körnich Jan , Van Der Linden Johannes G. M. , Persoons André , Spek Anthony L. , Veldman Nora , Voss Bea , Wong Hans
Published: July 2007

In order to construct bimetallic organometallic donor–acceptor complexes with nonlinear optical (NLO) properties, the bimetallic sesquifulvalene complexes [{CpFe(η5‐C5H4)}Z{η7‐C7H6)Cr‐(CO)3}]BF4 (2BF4) have been synthesized with different bridging functions Z (2a: Z = ‐ 2b: Z = C2; 2c: Z  (E)‐C2H2). X‐ray structure determinations show that 2a BF4 and 2c BF4 crystallize in acentric space groups (P41, P212121), whereas crystalline 2b BF4 has the centric space group P21/c. The organometallic fragments in 2a adopt a transoid conformation, those in 2b and 2c are cisoid. The cyclo‐C5 and cyclo‐C7 rings of the sesquifulvalene ligands are almost coplanar; this allows an optimum electronic interaction between the ferrocenyl donor and the [η7‐C7H6)Cr(CO)3]+ acceptor moiety in the crystalline state. Cyclovoltammetric studies show an electrochemically reversible one‐electron oxidation ( > 0 mV vs. FeH/FcH+, FcH = ferrocene) and an irreversible one‐electron reduction (≤–900 mV), which are assigned to the ferrocenyl and [(η7‐C7H6)Cr(CO)3]+ units, respectively. UV/vis spectra reveal low‐energy absorptions for 2a–c at about λ=600 nm, showing strong negative solvatochromism. These absorption maxima are related to a charge‐transfer excitation between the ferrocenyl and the [(η7‐C7H6)Cr(CO)3]+ groups, indicating considerable nonlinear optical properties. Determination of the first hyperpolarizability β by means of hyper Rayleigh scattering yields exceptionally large β values: β(2b) = 570 × 10−30 esu and β(2c) = 320 × 10−30 esu. These unexpectedly large β values are explained by resonance enhancement.
Studies on the Topography of Biomembranes: Regioselective Photolabelling in Vesicles with the Tandem Use of Cholesterol and a Photoactivable Transmembrane Phospholipidic Probe
Authors: Nakatani Yoichi , Yamamoto Masakuni , Diyizou Yvonne , Warnock William , Dollé Valérie , Hahn Wolfgang , Milon Alain , Ourisson Guy
Published: July 2007

2H NMR, DSC and UV studies of DMPC (1,2‐dimyristoyl‐sn‐glycero‐3‐phosphocholine) vesicles have demonstrated the favourable physicochemical properties of the transmembrane phospholipid probes 1 a and 1 b for membrane topographical studies. In particular, in the presence of a physiological amount of cholesterol, only one transmembrane conformation is observed. The use of 1 a and cholesterol together for photolabelling experiments in DMPC vesicles led to a remarkable improvement in the regioselectivity of cross‐linking between 1 a and DMPC, and between 1 a and cholesterol: the myristoyl chains functionalized at C11, C12 and C13 made up 95% of the total photolabelled myristates, and cholesterol was principally functionalized at the C25 position on the side‐chain. This indicates the formation of a highly ordered bilayer structure and proves directly the orientation of cholesterol perpendicular to the membrane plane with its chain terminal buried in the middle of the bilayer.
The Pre‐Reactive Complex of H2S and BrCl; Observation and Characterisation by Rotational Spectroscopy
Authors: Bloemink Hannelore I. , Legon A. C.
Published: July 2007

By using a fast‐mixing nozzle in a Fourier transform microwave spectrometer, any chemical reaction accompanying mixing of H2S and BrCl was prevented, thus allowing the observation of the pre‐reactive complex H2S ··· BrCl. The rotational spectra of eight isotopomers of the complex were recorded. The analysis of the determined spectroscopic constants shows that the atoms S ··· BrCl are collinear or nearly so and that the H2S plane is approximately perpendicular to the S ··· BrCl axis with r(S ··· Br) = 3.094 (7) Å. This geometry is in agreement with previously stated rules for B ··· XY complexes, where B is a Lewis base and XY is an (inter)halogen molecule. The intermolecular interaction is shown to be relatively weak, both in terms of the intermolecular stretching force constant kσ and the intramolecular electric change redistribution δ within the BrCl subunit that accompanies formation of H2S ··· BrCl.
Synthesis and Properties of 4,4,9,9‐Tetramethyl‐1‐oxa‐cycloundecane‐5,6,7,8‐tetrone and 5,5,10,10‐Tetramethyl‐1‐oxa‐cyclotridecane‐6,7,8,9‐tetrone
Authors: Gleiter Rolf , Ackermann Uwe , Oeser Thomas , Irngartinger Hermann
Published: July 2007

The synthesis of 4,4,9,9‐tetramethyl‐1‐oxa‐cycloundecane‐5,6,7,8‐tetrone (9) and 5,5,10,10‐tetramethyl‐1‐oxa‐cyclotridecane‐6,7,8,9‐tetrone (10) has been achieved in a multistep procedure. The key steps in this synthesis were the ring closure of 23 and 24 to 25 and 26, respectively, and the oxidation of the triple bond with RuO2/NaIO4 to the dihydroxydiketones 31 and 32. Compound 9 is the first cyclic tetraketone for which an intramolecular donor‐acceptor stabilization has been found. A strong transannular interaction between the ether oxygen and the C4O4 unit in 9 was detected by X‐ray studies on single crystals of 9. The transannular distance is 2.7–2.8 Å. Further evidence for a strong transannular interaction was obtained from the comparison of the reduction potential and the first band in the UV/Vis spectrum with the corresponding values from other openchain tetraketones. These findings were substantiated by PE investigations on 9. The crystal structures of the dihydroxy‐diketones 31 a and 32 a showed that, in the case of the eleven‐membered ring (31 a), there are also short transannular distances between the ether oxygen and the C2O2 moiety (2.5 Å and 2.9 Å). In the case of the thirteen‐membered ring (32 a), no transannular interactions were found in the solid state.
Highly Selective Formation of tert‐Butyl Hydroperoxide from the Reaction of Isobutane and O2 in a Zeolite under Visible Light
Authors: Blatter Fritz , Sun Hai , Frei Heinz
Published: July 2007

Isobutane and oxygen gas loaded into zeolite BaY react upon irradiation with green or blue light to yield tert‐butyl hydroperoxide in a single‐photon process. This was discovered when monitoring the reaction at room temperature in situ by FT‐infrared spectroscopy. Selectivity was 98%, even upon conversion of more than half of the reactants loaded into the zeolite. Diffuse reflectance spectra revealed a visible absorption tail which originates from an isobutane · O2 collision complex. It is attributed to the isobutane · O2 contact charge‐transfer absorption, whose onset is shifted from the UV into the visible region by the high electrostatic field of the zeolite.
Zirconium(II)‐ and Hafnium(II)‐Assisted Reductive Coupling of Coordinated Carbonyl Groups Leading to Ketenylidene Complexes of Zirconium(IV) and Hafnium(IV)
Authors: Calderazzo Fausto , Englert Ulli , Guarini Alessandro , Marchetti Fabio , Pampaloni Guido , Segre Annalaura , Tripepi Giovanna
Published: July 2007

The biscyclopentadienyldicarbonyl derivatives of zirconium(II) and hafnium(II) ([MCp2(CO)2]; MZr, Hf) promote the reductive coupling of coordinated carbon monoxide to give, in the presence of N,N‐dialkylcarbamtes of the tetravalent metals [M(O2CNR2)4], the ketenylidene complexes [M3Cp2(μ2‐CCO)‐(μ3‐O)(O2CNR2)6] (1 a: MZr, REt; 1 b: MZr, RiPr; 2: MHf, RiPr). The yields of the isolated zirconium complexes are as high as 60%, while that of the hafnium derivative is 40%. The X‐ray crystal structure analysis of 1 b shows that it consists of trinuclear molecules, with the three zirconium atoms held together by the bidentate C2O ligand, by the tridentate bridging oxide and by the bidentate carbamato groups. Labelling experiments with [ZrCp2(13CO)2] show that the ketenylidene ligand originates from the coordinated CO groups. These ketenylidene complexes, which are rare examples of compounds containing a CCO but no CO ligand, are characterized by an intense IR band at about 2015 cm−1, associated with the bridging C2O ligand. The ketenylidene group of 1 b was readily removed and replaced by a bidentate μ‐oxo ligand of the same hapticity by reaction with carbon dioxide or acetone or by thermal decomposition. The resulting product [Zr3Cp2(μ‐O)(μ3‐O)‐(O2CNiPr2)6] (3) crystallizes in the same space group as 1 b and with similar cell constants and bond parameters.
Conformational Preference of 2‐(Halomethyl)‐ and 2‐(Oxymethyl)Pyridines: Microwave Spectrum, Ab Initio, and MM3 Studies of 2‐(Fluoromethyl)Pyridine
Authors: Moberg Christina , Adolfsson Hans , Wärnmark Kenneth , Norrby Per‐Ola , Marstokk Karl‐Magnus , Møllendal Harald
Published: July 2007

One single conformer was assigned from the microwave spectrum of 2‐(fluoromethyl)pyridine, investigated in the gas phase in the 26.0–39.0 GHz spectral region at about –10°C. Its Cα–F bond was found to be coplanar with the ring and anti to the N–C2 bond (syn to the C2–C3 bond). There was no indication in the microwave spectrum of the presence of other rotameric forms of the molecule.

The results of the spectroscopic study were backed up by ab initio calculations at the MP2/6‐31 G** (frozen core) level. These calculations predict that the assigned conformer is the only stable form of the molecule. The transition state was calculated to have the CH2F group 180° from the stable anti conformation. The energy of the transition state was computed to be 20.2 kJ mol‐1 higher than the energy of the anti rotamer.

The results are interpreted in terms of a stereoelectronic effect, and the orbital overlaps responsible for the observed effect are discussed. It is shown that 2‐(fluoromethyl)pyridine serves as a good model for 2‐(alkoxymethyl)pyridines, previously found to show the same conformational preference.
Emission from Zeolite‐Occluded Manganese–Diimine Complexes
Authors: Knops‐Gerrits Peter‐Paul H. J. M. , De Schryver Frans C. , Auweraer Mark Van Der , Mingroot Hans Van , Li Xiao‐Yuan , Jacobs Pierre A.
Published: July 2007

Manganese complexes of 2,2′‐bipyridine (bpy) and 1,10‐phenantroline (phen) have been synthesised in the supercages of cubic NaX and NaY and in the hypercages of the hexagonal NaEMT faujasites. The coordination and redox chemistry were studied with ESCA, CA, FT‐IR, FT‐Raman, diffuse reflectance and emission techniques. FT‐IR/FT‐Raman shows cis coordination for all complexes and a high Mn–N stretching frequency in the phen complexes as a result of steric constraints imposed by the ligand. [Mn(bpy)2]2+ in the different zeolites shows metal‐to‐ligand charge transfer (MLCT; at 495 nm); for [Mn(phen)2]2+‐NaY the MLCt is broadened owing to complex distortion. On MLCT excitation [Mn(bpy)2]2+ complexes show an ipsochromic shift in the emission and an increase in quantum yield with increasing steric restrictions imposed by the zeolite. The ipsochromic shift of the emission band of [Mn(phen)2]2+ in NaY results from the combined effect of the ligand field (this suggests emission from a CT state) and of coordinative distortion. The key factor influencing the emission properties is found to be the overall matrix‐induced complex distortion. Cation stabilisation of the ligand anion affects emission indirectly. The decay times for [Mn‐(bpy)2]2+‐NaY are in the millisecond range (7.5–11.5 ms). A proposed model for excitation and emission properties of zeolite‐occluded MnII complexes involves excitation of a quartet CT state, intersystem crossing and subsequent emission. The enhanced stability of the coordination sphere in the zeolite allows complexes to luminesce from a CT state, which is not detected in solution. The zeolite behaves as a supramolecular cryptating agent, protecting complexes from photodissociation.
O‐Atom Transfer to Fe+n Clusters (n = 2–10) from O2, N2O and CO2: “Microoxides of Iron”
Authors: Gehret Oliver , Irion Manfred P.
Published: July 2007

We report on the gas phase reactions of small Fe+n clusters (n = 2–10) with O2, N2O and CO2 in an FT‐ICR mass spectrometer. Under our experimental conditions, clusters of all sizes reacted readily with O2 and all but the dimer reacted with N2O. Only the smallest Fe+n clusters (n = 2–4) appeared to activate CO2. For all X–O molecules (X = O, N2, CO), reaction pathways were observed that include the transfer of O atoms. In addition, the reactions with O2 and N2O were accompanied by the loss of one or two Fe atoms. Thermochemical considerations based upon the well‐known X–O bond energies were used to calculate Fen‐O+ bond dissociation energies (BDEs) for sizes n = 2–6; these amount to roughly 550 kJ mol‐1 and thus are considerably higher than the BDE of the Fe‐O+ ion. All oxidation reactions of the Fe+n clusters (n = 2–6) studied in more detail were terminated by products of FexOx+ stoichiometry (x = 1–4). These “microoxides of iron” are not able to activate any further X–O bonds. Secondary reactions of FexOx+ clusters with C6H6, C2H4 and NH3 were investigated for two selected sizes (x = 2, 3) and compared with reactions of the naked Fe+n clusters.
Fluorescence Redox Switching Systems Operating through Metal Centres: the NiIII/NiII Couple
Authors: Fabbrizzi Luigi , Licchelli Maurizio , De Santis Giancarlo , Sardone Nicola , Velders Aldo Hendrikus
Published: July 2007

The covalently linked two‐component systems 3 and 4 display fluorescence redox switching activity: the NiIII form quenches the fluorescence of the proximate aromatic fluorophore, whereas the NiII form does not. Thus, fluorescence can be switched on and off at will through the reversible NiII/NiIII redox reaction, which is carried out both electrochemically (in MeCN) and chemically (in EtOH). Quenching of the excited fluorophore F* is ascribed to a thermodynamically favoured F*‐to‐NiIII electron transfer mechanism. The more flexible system 5 does not work as a switch, since the fluorescence of the anthracene subunit is quenched in both NilII and NiIII forms (an OFF/OFF situation), through an energy transfer mechanism. The crystal and molecular structure of 4 in its protonated form is also described.
A Theoretical Study of the Additivity of Proton Affinities in Aromatics: Polysubstituted Benzenes
Authors: Eckert‐Maksik Mirjana , Klessinger Martin , Maksii Zvonimir B.
Published: July 2007

An additivity rule for proton affinities (PAS) in polysubstituted benzenes is derived from the MP2(fc)/6‐31 G**//HF/6‐31 G* + ZPE(HF/6‐31 G*) theoretical model by use of the concepts of homodesmic reactions and independent substituents. The performance of the additivity rule of thumb is very good; this is evidenced by the excellent agreement of the estimated PAS with the latest experimental data. We believe that the additivity should work for larger aromatic compounds too. The PA increments, which characterize the influence of each substituent on a particular site of the benzene ring undergoing electrophilic substitution, proved useful in discussing various chemical properties of this family of compounds.
Alkali‐Metal Cation Binding by Self‐Assembled Cryptand‐Type Supermolecules
Authors: Albrecht Markus , Röttele Herbert , Burger Peter
Published: July 2007

Reaction of three equivalents of alkyl‐bridged biscatechol ligands (1‐3)‐H4 with two equivalents of [(acac)2TiO] in the presence of two equivalents of alkalimetal carbonate (M = Na, K) affords helicate‐ or meso‐helicate‐type supramolecular coordination compounds by spontaneous self‐assembly processes. The cryptand‐type structure of the binuclear tetraanions [(1)3Ti2]4‐ or [(2)3Ti2]4‐ permits inclusion of sodium cations; this was observed by 23Na NMR spectroscopy in solution. Competition experiments with potassium or lithium cations revealed that potassium but not lithium easily displaces the sodium ions from the cavity. Titration experiments indicated that the binding constant for potassium is significantly higher than for sodium. On the other hand, for the large tetraanion [(3)3Ti2]4‐ with hexamethylene spacers, no binding of alkali‐metal ions could be observed by NMR spectroscopy in solution. In the solid state, however, two potassium ions could be incorporated in the interior of the cryptand‐type helicate.
Synthesis and Structure of a Bis(Double Helicate) and its Cryptatoclathrate
Authors: Saalfrank Rolf W. , Harbig Roland , Nachtrab Jochen , Bauer Walter , Zeller Klaus‐Peter , Stalke Dietmar , Teichert Markus
Published: July 2007

The structures of the hitherto unknown bis(double helicate) 10 and its cryptatoclathrate (10)2·2THF were unequivocally determined by X‐ray diffraction. Bis(double helicate) 10 is formed in a one‐pot synthesis starting from CH‐acidic bis (tetrazolylmethyl ketone) 9 and Zn(OAc)2. The formation of racemic, homochiral 10 from [Zn2L32] fragments, which are formed in a self‐assembly process, is governed by chiral self‐recognition. According to NMR studies only 10 is present in solution. 13C CP/MAS NMR spectroscopy and X‐ray analysis confirm aggregation of 10 with two molecules of THF to yield the inclusion compound (10)2·2 THF in the solid state.
Phosphorus‐Containing Dendrimers: Synthesis of Macromolecules with Multiple Tri‐ and Tetrafunctionalization
Authors: Lartigue Marie‐Laure , Slany Michael , Caminade Anne‐Marie , Majoral Jean‐Pierre
Published: July 2007

A variety of dendrimers up to generation 7 possessing terminal P(S)Cl2, (3‐[G1]‐3‐[G7]) or P(O)Cl2, (7‐[G1]‐7‐[G3], 7‐[G3, 7‐[G7) have been tri‐ and tetrafunctionalized. Selective monosubstitution of P(X)Cl2 (X = S, 0) termini with allylamine or propargylamine gave the trifunctionalized dendrimers 5‐[G1], 5‐[G7], 8‐[G1], 8‐[G3], 10[G1], 11‐[G4], ll‐[G4], 12‐[G1]‐12‐[G3], 12‐[G5], and 12‐[G7]. Reaction of dendrimers 4‐[G1] and 4‐[G4], possessing terminal P(S)[N‐allyl) fragments, with propargylamine afforded trifunctionalized dendrimers l4‐[G1 and l4‐[G4]. Multiply trifunctionalized macromolecules 13‐[G2] and 13‐[G3] with P(S)(NH‐allyl)(NH‐propargyl) moieties at the surface were prepared by treatment of 8‐[G2] and 8‐[G3] with propargylamine. Dendrimers 15‐[G1]‐l5‐[G3] and l6‐[G1]‐l6‐[G3] with P(O)(NH‐allyl)(OC6H4CHO) and P(O)(NH‐propargyl)(OC6H4CHO) termini were also synthesized. Reaction of hydrazine or cyanomethylenetriphenylphosphorane (17) with compounds 15‐[G1], 15‐[G3], and 16‐[G1]‐16‐[G3] led to the multiply tetrafunctionalized dendrimers 18‐[G1], 18‐[G3], 19‐[G1], 20‐[G1]‐20‐[G3], 21‐[G1], and 21‐[G2].
The [Fe(etz)6](BF4)2 Spin‐Crossover System—Part One: High‐Spin ⇌ Low‐Spin Transition in Two Lattice Sites
Authors: Hauser Andreas , Gütlich Philipp , Hinek Roland , Spiering Hartmut , Schollmeyer Dieter
Published: July 2007

The [Fe(etz),](BF,), spin‐cross‐over system (etz = 1‐ethyl‐1 H‐tetrazole) crystallizes in space group P1, with the following lattice constants at 298 K: a 10.419(3), b=15.709(1), c = 18.890(2) Å = α = 71.223(9), β =77.986(10), and γ = 84.62(1)° V = 2862.0(9) Å3 and Z = 3. Two nonequivalent lattice sites, one without (site A) and one with (site B) inversion symmetry, are observed. The population of the two sites nA:nB is 2:l. Iron(II) on site A undergoes a thermal low‐spin (LS) → high‐spin (HS) transition with T1/2I, = 105 K. whereas that on site B remains in the high‐spin state down to cryogenic temperatures. Application of external pressure of up to 1200 bar between 200 and 60 K does not cause formation of the low‐spin state on site B. On site A the high‐spin state can be populated as a metastable state at 20 K by irradiating the sample with λ = 514.5 nm; on site B a light‐induced population of the low‐spin state can be achieved with λ = 820 nm.
First Observation of a [1,7] Boron Shift—Three Independent Fluxional Processes in the Tricarbonyliron Complex of Cycloheptatrienyl(dipropyl)borane
Authors: Gridnev Ilya D. , Tok Oleg L. , Bubnov Yuri N. , Gurskii Mikhail E.
Published: July 2007

The fluxional behavior of [1–4‐η4 ‐ exo ‐ 7 ‐ dipropylborylcyclohepta ‐ 1,3,5‐triene]tricarbonyliron (14) has been studied by dynamic NMR. The 2 D 1H and 13C EXSY NMR experiments carried out at various temperatures revealed that three independent fluxional processes occur in 14. The fastest rearrangement is a [1,7] B sigmatropic shift accompanied by [1,2] Fe migration (EA = 71.2±2.3 kJ mol‐1, ln A = 32±1). Two other rearrangements also take place: [1,3] B sigmatropic and [1,3] Fe haptotropic migrations. The analysis of the observed selectivity towards [1,j] B sigmatropic shifts ([1,3] and [1,7] B shifts are observed, but [1,5] B shifts are not) in terms of theoretical predictions gives reason to assume that the [1,j] boron migrations occur with inversion of configuration at the migrating atom.
Efficient Stereoselective Synthesis of Plasmenylcholines
Authors: Rui Yuanjin , Thompson David H.
Published: July 2007

The first practical total chemical synthesis of a plasmenylcholine (1–O‐1'‐(Z)‐hexadecenyl‐2‐hexadecanoyl‐sn‐glycero‐3‐phosphocholine) with pure (Z) olefin stereochemistry is reported. Monopalmitin was doubly protected as the 3‐TBDPS‐2‐TBDMS ethers (tert‐butyldiphenylsilyl‐, tert‐butyldimethylsilyl‐) and converted to the corresponding 1‐O‐1′‐(Z)‐hexadecenyl‐2‐TBDMS‐3‐TBDPS‐glyceryl ether (by the method of ref. [43]). Clean deprotection with tetra‐butylammonium fluoride in the presence of imidazole gave 1‐O‐1′‐(Z)‐hexadecenylglycerol in >90% yield. Resilylation with TBDPSCl followed by acylation of the sn‐2 alcohol with palmitoyl chloride and deprotection of the resulting 3‐TBDPS‐2‐hexadecanoyl‐1–O‐1′‐(Z)‐hexadecenylglycerol at –20°C with Bu4NF gave 2‐hexadecanoyl‐1–O‐1′‐(Z)‐hexadecenylglycerol in 86% yield. The 3‐phosphocholine group was attached by phosphorylating the free hydroxyl with 2‐chloro‐2‐oxo‐1,3,2‐dioxaphospholane in the presence of pyridine, instead of Et3N, as base to avoid acyl migration; the dioxaphospholane triester intermediate was subsequently cleaved with Me3N to give 1‐O‐1′‐(Z)‐hexadecenyl‐2‐hexadecanoyl‐sn‐glycero‐3‐phosphocholine in 18% overall yield from monopalmitin. The efficiency and flexibility of this route makes it well‐suited to the preparation of a wide variety of 1‐, 2‐, and 3‐substituted as well as isotopically labeled plasmenylcholines for biophysical and biochemical studies.
Thermochemistry of Metal Nitrides in the Ca/Zn/N System
Authors: McHale James M. , Navrotsky Alexandra , Kowach Glen R. , Disalvo Francis J.
Published: July 2007

The energetics of ternary oxide formation is reasonably well understood, but little is known of the energetics of ternary nitride formation. We have developed new procedures for high‐temperature oxide melt solution calorimetry on metal nitrides and employed them to study compounds in the Ca/Zn/N system. The calorimetry utilized the easy reduction of MoO3 in the solvent to supply a pathway for rapid oxidation of “N3–” to N2 gas. These experiments represent the first high‐temperature solution calorimetry of ternary metal nitrides. The results indicate that Ca3N2 acts as a basic nitride analogous to CaO in oxide systems, while Zn3N2 acts as an acidic nitride (“N3–” acceptor) in this ternary nitride system.
Organometallic Half‐Sandwich Complexes Promote the Formation of Linear Oligopeptides from Amino Acid Esters
Authors: Robl Christian , Maurus Michael , Sünkel Karlheinz , Beck Wolfgang , Krämer Roland , Polborn K.
Published: July 2007

Organometallic dipeptide ester complexes of the general formula [(L)M(Cl)(K2‐NH2CH2CONCH2CO2R)] (1: LCp*, MRh, 2: LCp*, MIr, 3: Lη6‐C6Me6, MRu) react smoothly with various α‐L‐amino acid esters in the presence of NEt3 to yield the tripeptide ester complexes [(L)M(Cl)(K2‐NH2CHR'CONCH2CONHCH2CO2R)] (5–7). In the same fashion chloro K2‐tetrapeptide ester complexes 10 and 11 are obtained either from tripeptide ester complexes or by subsequent addition of two equivalents of amino acid ester to a dipeptide ester complex. When the strong base NaOMe is used in the reaction of the diglycine ester compounds with amino acid esters. K3‐tripeptide ester complexes 12 and 13 are produced, in which one of the two coordinated peptide nitrogen atoms is pyramidal. The hexamethylbenzene ruthenium complexes 13 with tripeptide ligands are formed with very high diastereoselectivity. A plausible reaction mechanism for the metal‐promoted peptide synthesis is presented. Synthesis and isolation of the peptide esters proceeds without racemization.
Acid‐Base Properties of Adenosine 5′‐O‐Thiomonophosphate in Aqueous Solution
Authors: Song Bin , Sigel Roland K. O. , Sigel Helmut
Published: July 2007

The acidity constants of H2‐(AMPS)± were determined by potentiometric pH titrations in aqueous solution at 25°C and I=0.1M (NaNO3). Titrations with a combined single‐junction glass electrode were hampered in the presence of AMPS by a “poisoning” effect; the problem could be avoided by use of two separated electrodes. The values of the acidity constants PKHH2(AMPS) = 3.72 ± 0.03 and pKHH2(AMPS) = 4.83 ± 0.02 are relatively close to each other; the buffer regions of the two equilibria overlap, and therefore a micro acidity constant scheme was developed and the constants for the various sites calculated. It is concluded that the thiophosphateprotonated species (AMPS⋅H)− dominates at about 75% occurrence, while the form (H⋅AMPS)−, with the proton at the N1 site of the adenine residue, occurs at about 25%. Semiempirical AM1 and PM3 calculations including water as a solvent locate the proton in (AMPS⋅H)− mainly on the terminal oxygen atoms rather than the sulfur. The acid‐base properties of H2(AMPS)± are considerably more complicated than those of the parent nucleotide, H2(AMP)±; for the latter the two (intrinsic) acidity constants are well separated and consequently practically all protons have left the N1 site before deprotonation at the monoprotonated phosphate group occurs. Finally, an estimate for the acidity constants of H2(ATPγS)2‐ is given.
Synthesis and Reactivity of [(η6‐Biphenylene)Mn(CO)3]+
Authors: Dullaghan Conor A. , Carpenter Gene B. , Sweigart D. A.
Published: July 2007

Biphenylene readily coordinates through one of the phenyl rings to form [(η6‐biphenylene)Mn(CO)3]+ (2+), the X‐ray structure of which is reported. The nucleophiles H−, Me−, Me3CC(O)CH−2 and P(OEt)3 add to 2+ at a bridgehead carbon to afford high yields of cyclohexadienyl complexes (3); the X‐ray structure of the product of hydride addition is reported. P(OMe)3 adds to 2+ to give 3, which then undergoes a spontaneous Arbuzov elimination to the dimethyl phosphonate adduct. Nucleophilic additions to 2+ are unusual in that they occur at a substituted (bridgehead) carbon. A kinetic and thermodynamic study of the reaction with P(OEt)3 shows that the bridgehead carbons in 2+ are orders of magnitude more electrophilic than are the arene carbons in unstrained [(arene)Mn(CO)3]+ systems.
Theoretical Investigation of the Mechanism of the Baeyer‐Villiger Reaction in Nonpolar Solvents
Authors: Okuno Yoshishige
Published: July 2007

The Baeyer‐Villiger reaction of p‐anisaldehyde with peroxyacetic acid in nonpolar solvents to give p‐anisylformate was examined on the basis of ab initio molecular orbital calculations. To explain the experimental observations, the free‐energy change was evaluated for each case in the absence and in the presence of an acid catalyst. It was found that, without catalysts, the rate‐determining step corresponds to the carbonyl addition of peroxyacetic acid to p‐anisaldehyde and the reaction hardly occurs. Acetic acid was found to catalyze the carbonyl addition and change the rate‐determining step from the carbonyl addition to the migration of the carbonyl‐adduct intermediate. Trifluoroacetic acid was observed to catalyze both the carbonyl addition and migration, and the carbonyl addition was demonstrated to be a rate‐determining step. The results provided a convincing explanation of the complex kinetics seen experimentally. Further calculations were performed for the reaction of benzaldehyde with peroxyacetic acid to give phenylformate. Migratory aptitude was found to depend on the catalyst. Isotope effects were also investigated, and the exceptional isotope effect observed experimentally was shown to be due to the rate‐determining carbonyl addition caused by autocatalysis. It is concluded that the mechanism of the reaction varies with catalysis or substituent effects.
Syntheses, Crystal Structures, and Properties of Novel Heterooctametallic Clusters Na2M'2[M3O4(O2CEt)8]2 (M' = Fe, Cr, Mo; M3 = Mo3, MoW2, W3)
Authors: Xu Li , Li Zhaohui , Liu Huang , Huang Jinshun , Zhang Qianer
Published: July 2007

A mixture of Na2MO4, M(CO)6, FeCl3·6H2O, and (EtCO)2O was heated at 120°C to produce the heterooctanuclear clusters Na2Fe2[M3O4‐(O2CEt)8]2 (M = Mo, 1; W, 2). The bioxo‐capped clusters Na[M3O2(O2CEt)9] (M3 = MoW2, W3) were treated with Fe‐(CO)5, Cr(CO)6, and Mo(CO)6 in (EtCO)2O at 120°C to afford Na2M'2‐[MoW2O4(O2CEt)8]2 (M' = Fe, 3; Cr, 4; Mo, 5) and Na2Mo2[W3O4(O2CEt)8]2 (6), respectively. The isomorphous clusters 1, 2, 5, and 6 were characterized by X‐ray crystallography. The structures, similar to those of the analogues Na2Cr2[M3O4‐(O2CEt)8] (M = Mo, 7; W, 8) and Na2V2[M3O4(O2CEt)8]2 (M = Mo, 9; W, 10), consist of two incomplete cubane‐type [M3O4(O2Et)8]4‐ units centrosymmetrically bridged by two M'(III) metal ions through μ‐oxo and propionate groups; the resulting cluster dianions are linked by Na+ ions into infinite chains. Clusters 1–10 do not dissolve in water and common organic solvents at room temperature. Unlike the chromium(III) clusters 7 and 8, the iron(III) and vanadium(III) clusters 1 and 9 are soluble in aqueous acid solution, as a result of their decomposition into isolated [Mo3O4]4+ cluster units. Temperature‐dependent magnetic susceptibilities of 1, 2, and 7–10 were measured over the range 2–200 K and modeled by means of the spin‐Hamiltonian H = – 2JS1S2 to give spin exchange coupling constants J/k of –0.60, –0.72, –1.76, –1.31, –4.80, and –1.46 K, respectively. These figures show that antiferromagnetic spin exchange coupling interactions between the magnetic, bridging M'(III) ions are very weak. These species show characteristic bands in the IR spectra at 740–820 cm−1, which can probably be assigned to ṽ(M‐μ‐O).
Screening of Various Catalysts for the Asymmetric Cyclopropanation of Structurally Typical Silyl Enol Ethers—Scope and Limitations of Evans's Bisoxazoline Copper Catalyst
Authors: Schumacher Ralf , Reißig Hans‐Ulrich , Dammast Franziska
Published: July 2007

The scope and limitations of asymmetric syntheses of 2‐siloxycyclopropanecarboxylates 3 were examined by combination of structurally typical silyl enol ethers and methyl diazoacetate with various chiral catalysts. It was found that the Schiff base complex 5ṁCu(OAc)2 and the bisoxazoline complex 6ṁCuOTf gave the highest stereoselectivities (e.g., 72% ee for cis‐3d with 5ṁCu(OAc)2 as catalyst or 76% and 73% ee for cis‐ and trans‐3a, respectively, with 6‐CuOTf as catalyst). High enantioselectivities or diastereoselectivities were obtained. Other catalysts based on copper, rhodium or ruthenium complexes afforded significantly lower values. In further investigations, Evans's bisoxazoline complex 6‐CuOTf proved to be limited to 1,1‐disubstituted silyl enol ethers (e.g. 1a and 1e) for high enantiomeric excesses (up to > 95%, poor diastereoselectivities) and to 1,2‐substituted enol ethers (e.g. 1e) for high diastereoselectivities (cis:trans < 3:97, eetrans = 49%).
Functionalized Lipid Tubules as Tools for Helical Crystallization of Proteins
Authors: Ringler Philippe , Müller Wolfgang , Ringsdorf Helmut , Brisson Alain
Published: July 2007

The development of functional supramolecular devices built by self‐assembly of elementary molecules and with bioactive properties arouses considerable interest in the field of nanotechnology and new materials. We report here the formation of a new class of lipid tubules exhibiting both properties of molecular recognition and crystal formation for the protein streptavidin. These lipid tubules, made of biotin‐containing dioctadecylamine molecules, are straight hollow cylinders with a constant diameter of 27 nm and variable length up to several micrometers. They are unilamellar with an inner diameter of about 16 nm, as shown by cryoelectron microscopy. Streptavidin binds to the biotinylated tubules and assembles spontaneously into ordered helical arrays at the tube surface. These crystals exhibit regular order up to about 1.5 nm resolution. In addition, the helical streptavidin arrays act as functionalized supramolecular devices that bind a wide variety of biotinylated objects, as demonstrated here with proteins and liposomes.
Manipulation of Electric Charge on Vesicles by Means of Ionic Surfactants: Effects of Charge on Vesicle Mobility, Integrity, and Lipid Dynamics
Authors: Yaroslavov Alexander A. , Udalyk Oleg Y. , Kabanov Viktor A. , Menger Fredric M.
Published: July 2007

A combination of electrophoresis, dynamic light scattering, conductometry, and fluorescence spectroscopy was applied to investigate vesicles (both in the “solid” and “liquid” states) that had been imparted with electric charge through the incorporation of ionic amphiphiles. These amphiphilic compounds comprised cardiolipin (with two negative charges), sodium dodecyl sulfate (with one negative charge), and cetylpyridinium bromide (with one positive charge). By this means it was discovered that negative vesicles could be converted into neutral vesicles, and then into positive vesicles, by the addition of a cationic surfactant. The amount of cationic surfactant required for the conversion depended upon the mobility of the surfactant within the bilayer. Vesicles were found to be capable of absorbing large amounts of surfactant, both cationic and anionic, before ultimately disintegrating and releasing their contents. Mixtures of cationic and anionic vesicles were able to exchange surfactant, and thereby neutralize each other's charges, without any concurrent vesicle fusion. This phenomenon is reliable only if the vesicles are in the liquid state. Finally, a biphasic exchange process was observed in which a surfactant rapidly departs from one bilayer and then enters another, while a fluorescently labeled lipid travels the reverse path only slowly.
A Packing Model for Interpenetrated Diamondoid Structures—an Interpretation Based on the Constructive Interference of Supramolecular Networks
Authors: Hirsch Keith A. , Wilson Scott R. , Moore Jeffrey S.
Published: July 2007

The ability to predict and subsequently control solid‐state structure has been identified as a major challenge in the field of crystal engineering. Here we suggest the concept of constitutive models as a tool for understanding crystal packings and for designing new solid‐state structures. Such models are intended to relate molecular interactions and their geometrical constraints with solid‐state organization. These models will most likely be of greatest use for crystals consisting of supramolecular networks, that is, infinite assemblies of small molecules associating through strong, directional, and selective noncovalent interactions. The concept of the constitutive packing model is illustrated for interpenetrated diamondoid coordination networks based on crystalline adducts of 4,4′‐biphenyldicarbonitrile with silver(I) salts. Observed structural deformations induced by counterions of varying size may be understood in terms of the interference of two supramolecular networks within this system: the diamondoid metal–ligand coordination network and face‐to‐face aromatic stacks of the organic ligand. The constitutive model developed here has been applied to other diamondoid coordination networks in the literature and is found to be general.
A Density Functional Study of [Fe(PR3)4H3]+ Isomers: Comparing Model Compounds (r = H) with Real Molecules (R = CH3)
Authors: Jacobsen Heiko , Berke Heinz
Published: July 2007

The structures, energetics, and dynamics of various isomers of [Fe‐(PR3)4H3]+ (R = H, Me) were studied by density functional theory. The conformations considered were derived from a square planar (P), tetrahedral (T) and C2v‐butterfly (C) arrangement of the phosphine ligands. For PH3, the stability ranking PC>T was obtained, whereas in the PMe3 case the P isomer was least stable, T>C>P. Mechanisms for the hydrogen exchange in the C and T isomers are discussed. For the latter, this process might be described as a tetrahedral jump, but could also be viewed as a dodecahedral distortion of a cubic arrangement. The theoretical findings are compared with the results of experimental studies of [Fe(PR3)4H3]+ (R = Me, Et).
Enantioselective Hydrogenation of Imines with Chiral (Phosphanodihydrooxazole)iridium Catalysts
Authors: Schnider Patrick , Koch Guido , Prétôt Roger , Wang Guozhi , Bohnen Frank Michael , Krüger Carl , Pfaltz Andreas
Published: July 2007

Cationic iridium(I) complexes of chiral phosphanodihydrooxazoles were used as catalysts for the enantioselective hydrogenation of prochiral N‐alkyl and N‐aryl imines. The complexes are air‐stable crystalline solids that can be readily prepared and are easy to handle. The structures of two complexes were determined by X‐ray analysis. For N‐alkyl imines of acetophenone, enantiomeric excesses of up to 79% were obtained. Dialkyl ketimines and cyclic imines showed lower reactivity and selectivity. A remarkable dilution effect was observed for the hydrogenation of the N‐phenyl imine of acetophenone: decreasing the substrate and catalyst concentration led to a significant improvement of the enantioselectivity. Thus, up to 89% ee could be achieved using 0.1 mol% of catalyst. The highest enantioselectivities were obtained in weakly coordinating solvents such as CH2Cl2. Additives such as halides, imides, or amines were found to poison the catalyst. Hydrogen pressures of 100 bar were usually employed, but in some cases identical results were achieved with only 1 bar H2.
Control of the Helical Chirality in Octahedral Complexes by a Chiral Macrobicyclic Cavity Possessing Six Convergent Hydroxyl Groups
Authors: Baret Paul , Beaujolais Virginie , Gaude Didier , Coulombeau Christian , Pierre Jean‐Louis
Published: July 2007

The (aR, aR, aR) and (aS, aS, aS) enantiomers of a chiral macrobicyclic ligand with a bicapped tris(binaphthol) structure were synthesized. Complexation of gallium(III), chromium(III) and iron(III) centres in the chiral cavities of these two ligands yielded exclusively one octahedral configuration in each case: the (aR, aR, aR) enantiomer gave a A complex and the (aS, aS, aS) enantiomer a complex. These assignments were established by CD spectroscopy for chromium and iron centres and by molecular modelling.
Twelve‐Membered O‐Bridged Cyclic Ethers of Red Seaweeds in the Genus Laurencia Exist in Solution as Slowly Interconverting Conformers
Authors: Guella Graziano , Chiasera Giuseppe , Mancini Ines , Öztunç Aysel , Pietra Francesco
Published: July 2007

The twelve‐membered O‐bridged cyclic ether obtusallene IV (1), a new isolate from the red seaweed Laurencia obtusa from Kaş in the Turkish Mediterranean, revealed temperature‐dependent NMR signals attributable to a major conformer in equilibrium with a minor conformer by 180° flipping of the trans olefinic bond. This prompted us to reexamine the known congeners obtusallene I (2), 10‐bromoobtusallene I (3), obtusallene II (4), and obtusallene III (5), isolated both from the same and taxonomically related seaweeds, as well as their semisynthetic derivative peracetylobtusallene III (6). Two conformers could in fact be directly observed at room temperature for 2–3 and at low temperature for 4. Marked cross‐saturation‐transfer effects between the couples of conformers confirmed these observations. Activation energies for processes involving 10‐membered subunit rings (2–3) are higher than for 11‐membered (4–5) analogues, where faster conformational motion occurs too resulting in mediated vicinal J couplings. 1,3‐Dihydroxy substituents in 5 form an intramolecular hydrogen bond in low‐polarity, non‐H‐bonding solvents; this results in the existence of two further conformers, giving more complex NMR spectra. Descriptions in the literature of single conformers for obtusallenes 2–5 must have resulted from overlooking minor NMR signals or attributing them to impurities.
New Strategies for the Generation of Large Heteropolymetalate Clusters: The β‐B‐SbW9 Fragment as a Multifunctional Unit
Authors: Bösing Michael , Loose Ina , Pohlmann Heinrich , Krebs Bernt
Published: July 2007

Proceeding from the α‐B‐SbW9O33 fragment 1, various novel large heteropolyanions ([Na2Sb8W36O132(H2O)4]22– (2), [Sb2W22O74(OH)2]12− (3) and [Sb2W20M2O70(H2O)6](14−2n)− (4); Mn+ = Fe3+, Co2+, Mn2+, Ni2+) have been obtained in crystalline form by systematic synthesis. Their unexpected structures have been studied by X‐ray diffraction: 2, 3 and 4 contain β‐SbW9 units connected to each other by Sb3+ ions, WO2/WO2OH groups, and WO2/Mn+ (H2O)3 groups, respectively. Structural details and especially the systematic synthesis strategy for the formation of these interesting polytungstates are discussed in order to explain the unusual behaviour of 1 in aqueous solution under different reaction conditions.
Self‐Assembly of Bisurea Compounds in Organic Solvents and on Solid Substrates
Authors: van Esch Jan , De Feyter Steven , Kellogg Richard M. , De Schryver Frans , Feringa Ben L.
Published: July 2007

New low molecular weight gelators based on the structure RNHCONHXNHCONHR have been synthesized and tested for their ability to cause gelation of organic solvents. Compounds 2 (R = n‐dodecyl, X = ‐(CH2)9‐), 3 (R = n‐dodecyl, X = ‐(CH2)12‐), 4 (R = n‐dodecyl, X = 4, 4′‐biphenyl), and 5 (R = benzyl, X = ‐(CH2)9‐) form thermoreversible gels with a wide range of organic solvents, at concentrations well below 10 mgmL−1. Depending on the nature of the R and X groups, the solvents that undergo gelation include hexadecane, p‐xylene, 1‐octanol, n‐butyl actetate, cyclohexanone, and tetralin. The gels are stable up to temperatures well above 100°C, but are easily disrupted by mechanical agitation. Light microscopic investigations revealed that compounds 2–5 spontaneously aggregate to form thin flat fibers, which can be several hundreds of micrometers long and only 2–10 μm wide. Depending on the solvent, multiple twists in the fibers are observed. In the gels, these fibers form an extended three‐dimensional network, which is stabilized by multiple mechanical contacts between the fibers. Electron microscopy and X‐ray powder diffraction revealed that the fibers consist of stacks of sheets. The thickness of the sheets is 3.65 and 3.85 nm for 2 and 3, respectively. Scanning tunneling microscopic investigations of 2 absorbed on graphite showed that 2 forms long ribbons with a width of 5.0 nm. In the ribbons the molecules have a parallel arrangement, with the long molecular axis perpendicular to the long ribbon axis. The two urea groups within a given molecule are each part of mutually parallel extended chains of hydrogen bonds. Based on these observations a model is proposed for the arrangement of the molecules in the fibers. In this model the bisurea molecules aggregate through hydrogen‐bond formation into long ribbons, which assemble into sheets. In these sheets the ribbons are tilted. Finally, the sheets stack to form long thin fibers. This model is supported by molecular dynamics simulations.
New Evidence for Conformational Flexibility in Cyclodextrins from Vibrational Raman Optical Activity
Authors: Bell Alasdair F. , Hecht Lutz , Barron Laurence D.
Published: July 2007

Conformational flexibility in cyclodextrins (CDs) as a function of methylation, solvent interaction and the extent of inclusion complex formation has been studied by using vibrational Raman optical activity (ROA). The work exploited the sensitivity of ROA to skeletal mobility by comparing the intensity of the glycosidic ROA couplet between about 850 and 970 cm−1in maltoheptose (MH), β‐cyclodextrin (β‐CD), heptakis(2,6‐di‐O‐methyl)‐β‐cyclodextrin (DM‐β‐CD) and heptakis(2,3,6‐tri‐β‐methyl)‐β‐cyclodextrin (TM‐β‐CD) in buffered aqueous solution, in DMSO and with sodium benzoate and benzoic acid as guests in buffered aqueous solution. Increases in couplet signal strength were interpreted in terms of a reduction in conformational flexibility of the CD ring. In buffered aqueous solution the ROA intensity order MH < TM‐β‐CD < β‐CD < DM‐β‐CD was observed. The linear molecule MH is expected to be the most flexible of the four oligosaccharides studied, while the changes registered for the three CD macrocycles may be related to the degree of intramolecular hydrogen bond formation and its influence on conformational flexibility. In DMSO, the same ROA intensity order is observed, but with an approximately constant increase relative to the values obtained in aqueous solution. This can be explained by the tighter binding of DMSO in the CD cavities compared with H2O. For the inclusion complexes, our results indicate that the tighter the guest is bound, the larger is the reduction in the conformational flexibility of the CD macrocycle. The residual mobility sensed by ROA in CDs is similar to that sensed in proteins; this provides further insight into their analogous ligand‐binding and catalytic properties.