Leitfähigkeitsmessungen in flüssigem Ammoniak

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A Fujiwara-Moritani-Type Alkenylation Using a Traceless Directing Group Strategy: A Rare Example of C−C Bond Formation towards the C2-Carbon of Terminal Alkenes

Herein we report, a rhodium-catalyzed Fujiwara-Moritani-type reaction of unactivated terminal alkenes and benzoic acid derivatives bearing electron donating residues under mild conditions. The acid functionality acts as a traceless directing group delivering products alkenylated in meta-position to the electron donating substituent in contrast to the usually obtained ortho- and para-substitution in Friedel-Crafts-type reactions. Remarkably, the new C−C bond is formed to the C2 of the terminal olefin, in contrast to similar reported transformations. Initially formed mixtures of exo- and endo-double bond isomers can be efficiently isomerized to the more stable endo-products.     

Conformational Effects of Regioisomeric Substitution on the Catalytic Activity of Copper/Calix[8]arene C−S Coupling

Functionalization of the phenolic rim of p-tert-butylcalix[8]arene with phenanthroline to create a cavity leads to formation of two regioisomers. Substitution of positions 1 and 5 produces the known C_2v-symmetric regioisomer 1,5-(2,9-dimethyl-1,10-phenanthroyl)-p-tert-butylcalix[8]arene ( L^1,5 ), while substitution of positions 1 and 4 produces the C_s-symmetric regioisomer 1,4-(2,9-dimethyl-1,10-phenanthroyl)-p-tert-butylcalix[8]arene ( L^1,4 ) described herein. [ Cu(L^1,4)I ] was synthesized from L^1,4 and CuI in good yield and characterized spectroscopically. To evaluate the effect of its cavity on catalysis, Ullmann-type C−S coupling was chosen as proof-of-concept. Selected aryl halides were used, and the results compared with the previously reported Cu(I)/ L^1,5 system. Only highly activated aryl halides generate the C−S coupling product in moderate yields with the Cu(I)/ L^1,4 system. To shed light on these observations, detailed computational investigations were carried out, revealing the influence of the calix[8]arene macrocyclic morphology on the accessible conformations. The L^1,4 regioisomer undergoes a deformation that does not occur with L^1,5 , resulting in an exposed catalytic center, presumably the cause of the low activity of the former system. The 1,4-connectivity was confirmed in the solid-state structure of the byproduct [ Cu(L^1,4 − H) (CH₃CN)₂] that features Cu(I) coordinated inside a cleft defined by the macrocyclic framework.     

Temperature stability of reversed phase and normal phase stationary phases under aqueous conditions

In this study the temperature stability of several normal phase and RP columns was investigated using a water-only mobile phase. The temperature was adjusted to 120 degrees C for the bare silica stationary phases and to 185 degrees C for the metal oxide and carbon stationary phases. It could be shown that metal oxide stationary phases exhibited excellent thermal stability over the duration of the test period and are therefore suitable for high temperature LC applications.     

Electrochemical detection of osmium tetroxide-labeled PCR-products by means of protective strands

This communication reports about how single-stranded 136 base polymerase chain reaction (PCR) products labeled with electrochemically active osmium tetroxide bipyridine can be detected voltammetrically by hybridization with probe strands immobilized on gold electrodes. These electroactive ssDNA targets have been obtained by means of Lambda Exonuclease treatment of the double-stranded PCR products followed by hybridization of the remaining single strands with short protective strands and covalent labeling with osmium tetroxide bipyridine. Square-wave voltammetric signals of these osmium labels have been obtained only upon hybridization with the immobilized probe strands. An optimal 50 °C hybridization temperature has been found with a saturation of the probe layer at 30 min hybridization time and 7.5 nmol/l target concentration. The blank capture probe layer alone did not yield any signal. Unprotected strands produced almost no interference. Such double-selective switch-on electrochemical hybridization assays hold great promise for the specific detection of PCR products.     

Electrophoresis on a microfluidic chip for analysis of fluorescence-labeled human rhinovirus

We report the analysis of human rhinovirus serotype 2 (HRV2) on a commercially available lab-on-a-chip instrument. Due to lack of sufficient native fluorescence, the proteinaceous capsid of HRV2 was labeled with Cy5 for detection by the red laser (lambda ex 630 nm) implemented in the instrument. On the microdevice, electrophoresis of the labeled virus was possible in a BGE without stabilizing detergents, which is in contrast to conventional CE; moreover, analysis times were drastically shortened to the few 10 s range. Resolution of the sample constituents (virions, a contaminant present in all virus preparations, and excess dye) was improved upon adaptation of the separation conditions, mainly by adjusting the SDS concentration of the BGE. Purity of fractions from size-exclusion chromatography after labeling of virus was assessed, and affinity complex formation of the labeled virus with various recombinant very-low-density lipoprotein receptor derivatives differing in the number of concatenated V3 ligand binding repeats was monitored. Virus analysis on microchip devices is of particular interest for experiments with infectious material because of easy containment and disposal of samples. Thus, the employment of microchip devices in routine analysis of viruses appears to be exceptionally attractive.     

Operationally unsaturated pincer/rhenium complexes form metal carbenes from cycloalkenes and metal carbynes from alkanes

Operationally unsaturated (i.e., 16/18-electron) (PNPR)Re(H)4, where PNPR is N(SiMe2CH2PR2)2, is reactive at 22 degrees C with cyclic olefins. The first observed products are generally (PNPR)Re(H)2(cycloalkylidene), with hydrogenated olefin as the product of hydrogen abstraction from the tetrahydride. The tetrahydride complex with R = tBu generally fails to react (too bulky), that with R = cyclohexyl suffers a (controllable) tendency to abstraction of 3H from one ring, forming an eta3-cyclohexenyl compound, and that with R = iPr generally gives the richest bimolecular reactivity. The cyclic monoolefins studied show distinct reactivity, C6 giving first the carbene and then coordinated cyclohexadiene, C5 giving carbene, then diene, and then eta5-C5H5, C8 giving carbene and then eta2-cyclooctyne, and C12 giving an eta3-allyl. Norbornene gives a pi-complex of the norbornene in thermal equilibrium with its carbene isomer; at 90 degrees C, hydrocarbon ligand Calpha-Cbeta bond cleavage occurs to give, for the first time, a carbyne complex from an internal olefin. Two compounds synthesized here have the formal composition "(PNPR)Re + olefin", and each of these is capable of dehydrogenating the methyl group of a variety of alkanes at 110 degrees C to form (PNP)ReH triple bond (CR).     

Microwave spectra of O(2)-HF and O(2)-DF: hyperfine interactions and global fitting with infrared data

Spectra of the open shell complexes O(2)-HF and O(2)-DF were recorded using Fourier transform microwave spectroscopy. A complete analysis of the hyperfine structure and a global fit including microwave and infrared frequencies [W. M. Fawzy, C. M. Lovejoy, D. J. Nesbitt, and J. T. Hougen, J. Chem. Phys. 117, 693 (2002)] are reported. The Fermi contact interaction between the electron and nuclear spins, the electron spin-nuclear spin dipolar interaction, the nuclear spin-nuclear spin dipolar interaction, and the nuclear electric quadrupole interaction (for O(2)-DF) were considered in the analysis. The correspondence between the magnetic hyperfine constants and the two nuclei of the H(D)F is unambiguously established. In both O(2)-HF and O(2)-DF, the Fermi contact parameter is larger for the fluorine than for the hydrogen, while for the nuclear spin-electron spin dipolar hyperfine constants, the reverse is true. The effective angle between the HF bond and the a axis of the complex, determined from the nuclear spin-nuclear spin interaction constant, is 38(4) degrees. The same angle for the DF complex, derived from the deuterium nuclear quadrupole coupling constant, is 31(4) degrees.