Nickel(II) complexes bearing pyrazolylimine ligand: synthesis,structure, and catalytic properties for vinyl‐type polymerization of norbornene

Two nickel(II) complexes of {2‐[C₃HN₂(R₁)₂‐3,5]}[C(R₂)?N(C₆H₃ⁱPr₂‐2,6)]NiBr₂ (complex 1 : R₁ = CH₃, R₂ = 2,4,6‐trimethylphenyl; complex 2 : R₁ = R₂ = Ph) were synthesized and characterized. The solid‐state structure of complex 1 has been confirmed by X‐ray single‐crystal analysis. Activated by methylaluminoxane (MAO), complexes 1 and 2 are capable of catalyzing the polymerization of norbornene with moderate activities [up to 10.56 × 10⁵ gPNBE (mol Ni h)^?1] with high molecular weights (M_w?13.56 × 10⁵ g mol^?1) and molecular weight distributions were around 2. The influences of polymerization parameters such as reaction temperature and Al–Ni molar ratio on catalytic activity and molecular weight of the polynorbornene were investigated in detail. The obtained polynorbornenes were characterized by means of ¹H‐NMR and FTIR techniques. The analytical results of polymer structures indicated that the norbornene polymerization is vinyl‐type polymerization rather than ROMP. Copyright © 2009 John Wiley & Sons, Ltd.     

Quantitative sphingosine measurement as a surrogate for total ceramide concentration—preclinical and potential translational applications

Biomarkers are an increasingly important constituent of the drug development process, offering the potential of increased efficiency through reduced compound attrition and earlier proof of mechanism and/or efficacy. Assays developed for compound screening that can be directly translated for clinical trials are especially valuable, but their successful adoption requires a careful balance between assay performance and implementation costs. One such ‘fit‐for‐purpose’ biomarker assay, the indirect measurement of pharmacological modulation of sphingolipid biosynthesis and disposition, is presented here. Among spingolipids, numerous ceramide species are readily detectable in different lipoprotein fractions of mammalian plasma, but their parallel quantification can be prohibitively expensive and time consuming. Ceramides differ in their fatty acid moiety, which is readily removed by hydrolysis, yielding a common sphingosine derivative, the measurement of which serves as an indicator of total ceramide. When followed by liquid chromatography tandem mass spectrometry (LC/MS/MS) for detection, robust analyte quantification becomes relatively straightforward. The practical utility of a method developed to be fit for the purpose of rapidly and quantitatively measuring treatment‐induced variations in total ceramide from hamster plasma and individual lipoprotein fractions is described. With a linear calibration range from 0.003 to 33.4 μm sphingosine, precision and accuracy error in plasma‐based quality controls spiked with ceramides was less than 15%. The specificity of the assay for ceramides was also assessed. The simplicity of the method would allow for its potential translation to other preclinical species, as well as for clinical applications in later‐stage drug development. Copyright © 2009 John Wiley & Sons, Ltd.     

Oxidation of methyl and ethyl butanoates

This paper describes an experimental and modeling study of the oxidation of methyl and ethyl butanoates in a shock tube. The ignition delays of these two esters mixed with oxygen and argon for equivalence ratios from 0.25 to 2 and ester concentrations of 0.5% and 1% were measured behind a reflected shock wave for temperatures from 1250 to 2000 K and pressures around 8 atm. To extend the range of studied temperatures in the case of methyl butanoate, two sets of measurements were also made in a jet‐stirred reactor at 800 and 850 K, at atmospheric pressure, at residence times varying between 1.5 and 9 s and for equivalence ratios of 0.5 and 1. Detailed mechanisms for the combustion of methyl and ethyl butanoates have been automatically generated using a version of EXGAS software improved to take into account these oxygenated reactants. These mechanisms have been validated through comparison of simulated and experimental results in both types of reactor. The main reaction pathways have been derived from reaction flux and sensitivity analyses performed at different temperatures. © 2010 Wiley Periodicals, Inc. Int J Chem Kinet 42: 226–252, 2010     

Crystallographic rationalization of the reactivity and spectroscopic properties of (2R)‐S‐(2,5‐dihydroxyphenyl)cysteine

At 150 K, the title compound, C₉H₁₁NO₄S, crystallizes in the orthorhombic form as a zwitterion and has a low gauche conformation [χ = −46.23 (16)°] for an acyclic cysteine derivative. A difference in bond length is observed for the alkyl C—S bond [1.8299 (15) Å] and the aryl C—S bond [1.7760 (15) Å]. The –NH₃⁺ group is involved in four hydrogen bonds, two of which are intermolecular and two intramolecular. The compound forms an infinite three‐dimensional network constructed from four intermolecular hydrogen bonds. Characterization data (¹³C NMR, IR and optical rotation) are reported to supplement the incomplete data disclosed previously in the literature.     

Modulating the Photochemistry of Bipyridylic Compounds by Symmetric Substitutions

A quantum electronic study of the effect of substituents on (2,2′‐bipyridyl)‐3,3′‐diol and (2,2′‐bipyridyl)‐3,3′‐diamine is presented. A large difference in the photochemical behavior between the original and the substituted selected systems is expected. For the sake of simplicity, the study is restricted to the symmetrically bi‐substituted compounds: fluorine, the more electronegative atom and thus a strong σ‐acceptor but also a weak π‐donor group, and NO₂, a strong π‐acceptor substituent. Among the large set of compounds studied, two receive special attention: 5,5′‐dinitro‐(2,2′‐bipyridyl)‐3,3′‐diamine and 6,6′‐difluoro‐(2,2′‐bipyridyl)‐3,3′‐diol. While in the former case the nitro substitution transforms (2,2′‐bipyridyl)‐3,3′‐diamine, previously suggested to behave as a photomemory material, into a simple fluorescent species, the latter substitution turns (2,2′‐bipyridyl)‐3,3′‐diol into a fresh new candidate for a photomemory device.     

Trace-level analysis of polychlorinated biphenyls,organochlorine pesticides and polycyclic aromatic hydrocarbons in human plasma or serum by dispersive liquid–liquid microextraction and gas chromatography–tandem mass spectrometry

A method to determine 8 polychlorinated biphenyls (PCBs), 23 organochlorine pesticides (OCPs) and 16 polycyclic aromatic hydrocarbons (PAHs) was described using dispersive liquid–liquid microextraction (DLLME) of a small amount of plasma or serum sample and gas chromatography–tandem mass spectrometry (GC–MS/MS). The appropriate selection of the extraction solvent and dispersing solvent contributes to a high extraction yield and a clean extract. To verify the developed method, the interference, linearity of the calibration curve, detection limit, precision and accuracy were evaluated. The calibration curves were linear by 2–3 orders of magnitude with correlation coefficients above 0.997 in all cases. The LODs of PCBs, OCPs and PAHs were measured in the ranges of 0.0006–0.0029, 0.001–0.029 and 0.0002–0.012 ng/mL. The intraday precision achieved by this method was 2.19–10.3% (PCBs), 1.65–14.3% (OCPs) and 0.91–12.8% (PAHs), and the intraday accuracy 1.56–7.37% (PCBs), 2.34–19.6% (OCPs) and 1.49–15.7% (PAHs). The advantage of this method is that the analysis of PCBs, OCPs, and PAHs can be performed in a single chromatographic run, and the low detection limit enables monitoring of target substances in low exposure general public samples, and the analysis procedure is relatively simple and fast.     

Reduced formation of peroxide and radical species stabilises iron-based hybrid catalysts in polymer electrolyte membrane fuel cells

The incorporation of Pt into an iron-nitrogen-carbon(Fe NC)catalyst for the oxygen reduction reaction(ORR)was recently shown to enhance catalyst stability without Pt directly contributing to the ORR activity.However,the mechanistic origin of this stabilisation remained obscure.It is established herein with rotating ring disc experiments that the side product,H₂O₂,which is known to damage FeNC catalysts,is suppressed by the presence of Pt.The formation of reactive oxygen species is additionally inhibited,independent of intrinsic H₂O₂ formation,as determined by electron paramagnetic resonance.Transmission electron microscopy identifies an oxidised Fe-rich layer covering the Pt particles,thus explaining the inactivity of the latter towards the ORR.These insights develop understanding of Fe NC degradation mechanisms during ORR catalysis,and crucially establish the required properties of a precious metal free protective catalyst to improve Fe NC stability in acidic media.