Wolf Hiller

Nuclear-Magnetic-Resonance Measurements Reveal the Origin of the Debye Process in Monohydroxy Alcohols

Monohydroxy alcohols show a structural relaxation and at longer time scales a Debye-type dielectric peak. From spin-lattice relaxation experiments using different nuclear probes, an intermediate, slower-than-structural dynamics is identified for n-butanol. Based on these findings and on translational diffusion measurements, a model of self-restructuring, transient chains is proposed. The model is demonstrated to explain consistently the so-far puzzling observations made for this class of hydrogen-bonded glass forming liquids.

One-pot oxidation/isomerization of Z-allylic alcohols with oxygen as stoichiometric oxidant.

A method for generating (E)-α,β-unsaturated aldehydes from Z-allylic alcohols or E/Z-mixtures is described. The one-pot procedure involves a Cu-catalyzed oxidation followed by an organocatalytic Z/E-isomerization with N,N-dimethylaminopyridine (DMAP).

Access to Skipped Polyene Macrolides through Ring-Closing Metathesis: Total Synthesis of the RNA Polymerase Inhibitor Ripostatin B†

Rip-Roaring! A convergent total synthesis of antibiotic ripostatin B was developed. A key step in the synthesis is a metathesis reaction allowing for a ring closure to the labile doubly skipped triene macrolide.

Investigation of the tacticity of oligostyrenes by on-line h.p.l.c./1H n.m.r.

Abstract Oligostyrenes can effectively be analysed with respect to chemical structure and tacticity by on-line h.p.l.c./ 1 H n.m.r.. The separation of the oligostyrenes into oligomers is conducted on a reversed phase column using acetonitrile as the eluent. Via on-line coupling the chromatographic peaks are directly transferred into the n.m.r. spectrometer and analysed on-flow. Information on the chemical structure of the endgroups, the degree of polymerization and the tacticity of the polymer chain is obtained. The experiments have been conducted under conditions which are common for h.p.l.c. separations, i.e. sufficiently high flow rate, moderate sample concentration and on-flow detection. For the first time, the experiments have been carried out using conventional h.p.l.c.-grade acetonitrile and no deuterium lock.

Debye relaxation and 250 K anomaly in glass forming monohydroxy alcohols

A previous dielectric, near-infrared (NIR), and nuclear magnetic resonance study on the hydrogen-bonded liquid 2-ethyl-1-hexanol [C. Gainaru et al., Phys. Rev. Lett. 107, 118304 (2011)] revealed anomalous behavior in various static quantities near 250 K. To check whether corresponding observations can be made for other monohydroxy alcohols as well, these experimental methods were applied to such substances with 5, 6, 7, 8, and 10 carbon atoms in their molecular backbone. All studied liquids exhibit a change of behavior near 250 K, which is tentatively ascribed to effects of hydrogen bond cooperativity. By analyzing the NIR band intensities, a linear cluster size is derived that agrees with estimates from dielectric spectroscopy. All studied alcohols, except 4-methyl-3-heptanol, display a dominant Debye-like peak. Furthermore, neat 2-ethyl-1-butanol exhibits a well resolved structural relaxation in its dielectric loss spectrum, which so far has only been observed for diluted monohydroxy alcohols.

Catalytic and Stereoselective ortho-Lithiation of a Ferrocene Derivative†

N,N-Dimethylferrocenylmethylamine (2) is an inexpen-sive, achiral analogue of Ugis amine. Therefore, its desym-metrization by ortho-lithiation has to be mediated by chiralauxiliaries. Herein lies a principal problem that has limited itsuse as a prochiral building block to date: procedures for theasymmetric lithiation of 2 suffer from low conversion and/orstereoselectivities,

Two‐dimensional chromatography of complex polymers, 8. Separation of fatty alcohol ethoxylates simultaneously by end group and chain length

A comprehensive two-dimensional liquid chromatography system was developed to precisely describe the molecular heterogeneity of fatty alcohol ethoxylates. The end-group functionality was analyzed by gradient HPLC while ethylene oxide oligomer distributions were characterized by liquid adsorption chromatography. A baseline separation of all functionality fractions irrespective of the ethylene oxide oligomer chain length was achieved on nonpolar X-Terra C(18) with a methanol-water gradient, whereas an isocratic flow of isopropanol-water on a polar Chromolith Si column gave a separation according to the oligomer chain length without interference of the end-group distribution. The combination of these two methods to conduct online two-dimensional liquid chromatography experiments resulted in a comprehensive two-dimensional picture on the molecular heterogeneity of the sample.

On-line coupling of gradient-HPLC and 1H NMR for the analysis of random poly[(styrene)-co-(ethyl acrylate)]s

Random poly[(styrene)-co-(ethyl acrylate)]s can effectively be analysed with respect to the chemical composition distribution by on-line coupled HPLC/ 1 H NMR. The separation of the copolymers is conducted on a reversed-phase column using acetonitrile/tetrahydrofuran as the eluent mixture. Via on-line coupling, the chromatographic peaks are directly transferred into the NMR spectrometer and analysed on-flow. Information on the chemical composition and the chemical heterogeneity of copolymers with high conversion is obtained. The experiments have been carried out using conventional HPLC-grade solvents and no deuterium lock. The results of the on-line HPLC/ 1 H NMR investigation have been correlated with a HPLC procedure based on calibration with narrow-distributed copolymer standards.

Diluting the hydrogen bonds in viscous solutions of n-butanol with n-bromobutane: II. A comparison of rotational and translational motions

Mixtures of the monohydroxy alcohol n-butanol with n-bromobutane are investigated via dielectric and nuclear magnetic resonance (NMR) techniques. Static- and pulsed-field gradient proton NMR yielded self-diffusion coefficients as a function of concentration and temperature. To monitor reorientational motions, broadband dielectric and (13)C-spin relaxation time measurements were carried out. The latter demonstrate that the structural relaxation stems from the motion of the alkyl chains. By combining data from translational diffusion coefficients with published shear viscosities, hydrodynamic radii were determined that compare favorably with the van der Waals radii of single molecules. The results for the neat alcohol and for the binary mixtures are discussed with respect to a recent transient chain model. The approach of Debye and structural relaxation times at high temperatures, identified as a general feature of monohydroxy alcohols, is also discussed within that framework.

Online size exclusion chromatography-NMR for the determination of molar mass distributions of copolymers.

A general approach of size exclusion chromatography (SEC)-NMR is introduced for the determination of the classical molar mass parameters M(W), M(N), and M(P). It can be used for the determination of molar mass distributions of homopolymers and copolymers. The main advantage of SEC-NMR of copolymers is the possibility of detecting each monomer unit simultaneously with NMR as a quantitative concentration detector. Therefore, it is possible to provide the chemical compositions of copolymers at any elution volume without calibrations. In this respect, a new method will be presented for getting correct signal quantities of onflow data with sufficient NMR sensitivities. As the consequence, the chemical composition of copolymers can be correctly quantified under typical chromatographic conditions with respect to sample concentration and flow rate. Finally, the molar mass calibrations of the copolymers can be easily adjusted according to their chemical compositions. The methods were applied to polystyrene-b-poly(methyl methacrylate) block copolymers of different molar masses. The results of the molar mass distributions and the chemical composition distributions obtained by SEC-NMR are in very good agreement with the complex SEC multidetector analysis.