Amino acid-functionalized cyclopentadienyl molybdenum tricarbonyl complex and its use in catalytic olefin epoxidation

The complex [(N-benzyloxycarbonylprolyl)cyclopentadienyl]-Mo(CO)₃Me was prepared by the reaction of the amino acid-functionalized cyclopentadienyl-lithium reagent with Mo(CO)₆, and subsequent methylation with CH₃I. The complex was characterized by FTIR, ¹H and ¹³C NMR, and elemental analysis. A single-crystal X-ray diffraction study showed that the complex is chiral, crystallizing in the orthorhombic P2₁2₁2₁ space group with the Flack parameter refining to –0.007(16), which unequivocally confirms the presence of an enantiomerically pure compound. The complex was examined as a catalyst precursor in the liquid-phase epoxidation of trans-β-methylstyrene at 280–330 K, using either tert-butylhydroperoxide (tert-BuOOH), cumylhydroperoxide or urea-hydrogen peroxide adduct as oxidant, and, optionally, a co-solvent. With chloroform and tert-BuOOH, the catalytic activity surpasses that previously reported for chiral complexes of the type CpMo(CO)₃X, giving the epoxide isomers ((R,R)-(+) and (S,S)-(?)-1-phenylpropylene oxide) in excellent selectivity, albeit with negligible enantiomeric excess. FTIR spectroscopy showed that the oxidative decarbonylation of the tricarbonyl complex with tert-BuOOH is fast under the reaction conditions used, and that the structural integrity of the amino acid-functionalized cyclopentadienyl group is retained during this process. The use of the ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate as a solvent for the catalyst generates a biphasic liquid–liquid reaction system that enables, at the end of a catalytic cycle, the separation and reutilization of the catalyst.     

Spontaneous assembly of a nine-vertex lithium framework encapsulating the peroxide dianion

Investigation of the structure of the lithiated bicyclic guanidine 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine (hppH) has led to the reproducible isolation of compounds containing the 'Li9(hpp)7(O2)' core, in which a peroxide dianion has been encapsulated within a mono-capped, cubic array of lithium ions.     

Selective monoprotection of 1,n-terminal Diols in supercritical carbon dioxide: a striking example of solvent tunable desymmetrization

The reaction between 1,n-terminal diols (n = 3 or 6) with simple alcohols (MeOH, EtOH, and n-PrOH) in supercritical CO(2) over an acid catalyst (Amberlyst 15) leads to two possible products, a mono- and a bis-ether. At 150 degrees C, the selectivity of the reaction with 1,6-hexanediol and MeOH can be switched from 1:20 in favor of the bis-ether at 50 bar to 9:1 in favor of the desymmetrized mono-ether at 200 bar. It is demonstrated that the switch in selectivity is associated with the phase state of the reaction mixture, with monophasic conditions favoring the mono-ether and biphasic conditions favoring the bis-ether. A rationalization of this effect is also presented.     

Measurements and modeling of long range 3He diffusion in the lung using a "slice-washout" method

In healthy lung tissue, pulsed-gradient-spin-echo (PGSE) methods reveal apparent diffusion coefficients (ADC) of the order 0.20 cm2 s(-1); for diffusion times of approximately 2 ms. For these short diffusion times the ADC is only sensitive to structures approximately (2Dt)1/2 approximately 0.6mm in size. Recent work, using magnetic tagging of the longitudinal magnetization has revealed much smaller ADC values for longer length scales. In this work, the in vivo ADC from within the air-spaces, was measured using a new technique. The signal from a series of images was analyzed from a slice that was repeatedly imaged. Diffusion tends to "top-up" the non-renewable polarization within the slice, which leads to a non-exponential decay in image signal. Image data were compared to 1D finite-difference simulations of diffusion to calculate a long range ADC value. The results yield values of the order 0.034 cm2 s(-1), which are nearly an order of magnitude smaller than those reported by PGSE measurements at shorter diffusion times.     

Comparison of e+e− annihilation with QCD and determination of the strong coupling constant

We have analyzed 1113 events of the reaction e⁺e^? → hadrons at CM energies of 12 and 30 GeV in order to make a detailed comparison with QCD. Perturbative effects can be well separated from effects depending on the quark and gluon fragmentation parameters to yield a reliable measurement of the coupling constant α_S. At 30 GeV, the result is α_S = 0.17 ± 0.02 (statistical) ± 0.03 (systematic). QCD model predictions, using the fragmentation parameters determined along with α_S, agree with both gross properties of the final states and with detailed features of the three-jet states.     

Infinite median graphs, (0, 2)-graphs,and hypercubes

Hypercubes are characterized among connected bipartite graphs by interval conditions in several ways. These results are based on the following two facts: (i) connected bipartite graphs are median provided that all their intervals induce median graphs, and (ii) median (0, 2)-graphs are hypercubes. No cardinality restrictions are made.     

An experimental study of the K+ → πoμ+ν decay dalitz plot

A sample of K⁺_μ3 events, detected in the CERN 1.1 m³ heavy-liquid bubble chamber, has been used to investigate the q² dependence of the form factors, giving ξ(0) = ?1.1 ± 1.0 and ξ(6.6m²_π) = ?0.34 ± 0.20, for λ₊ = 0.027. A three parameter fit gives a value for λ₊ = 0.025 ± 0.017 in good agreement with the preceding K_e3 analysis.     

Two-body hadronic decays of the 3.1 GeV resonance

Hadronic two body decays of the 3.1 GeV resonance were measured and the following branching ratios were obtained: Γ_π⁺_π^?/Γ_μμ <0.0046, Γ_K⁺_K^?/Γ_μμ <0.0084, and $\text{Γ}{}_{\mathrm{p}}\text{p}\text{/ Γ}{}_{\mathrm{\mu }}{}_{\mathrm{\mu \mu }}\text{= 0.036 ± 0.010}$ assuming the p$\text{p}$ decay distribution is ≈ + cos²θ.     

IR spectroscopy for unstable molecules

Summary Infrared spectroscopy is a rather old-fashioned analytical technique; however, developments over the past few years in both Fourier Transform IR and IR laser techniques have permitted IR spectroscopy to be applied increasingly successfully to the detection, characterisation and monitoring of unstable molecules. Three experimental techniques, each of which employs principally IR spectroscopy, are described: matrix isolation; liquid noble gases as solvents; time-resolved studies. Examples given concentrate on organometallic intermediates of possible relevance to homogeneous catalysis.

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IR-Spektroskopie für instabile Moleküle

Zusammenfassung Infrarot-Spektroskopie ist eine ziemlich altmodische analytische Technik; jedoch haben es die Entwicklungen der letzten Jahre in der Fourier-Transform-IR-Interferometrie und in der IR-Laser-Technik ermöglicht, die IR-Spektroskopie mit wachsendem Erfolg beim Nachweis, bei der Charakterisierung und bei der Beobachtung instabiler Moleküle einzusetzen. Drei experimentelle Techniken, von denen jede auf der IR-Spektroskopie basiert, werden beschrieben: Matrix-Isolation, flüssige Edelgase als Lösungsmittel und zeitaufgelöste Untersuchungen. Die vorgestellten Beispiele konzentrieren sich auf möglicherweise für die homogene Katalyse wesentliche metallorganische Zwischenprodukte.