The boron-catalyzed polymerization of dimethylsulfoxonium methylide. A living polymethylene synthesis

Trialkyl and aryl organoboranes catalyze the polymerization of dimethylsulfoxonium methylide (1). The product of the polymerization is a tris-polymethylene organoborane. Oxidation affords linear telechelic alpha-hydroxy polymethylene. The polymer molecular weight was found to be directly proportional to the stoichiometric ratio of ylide/borane, and polydispersities as low as 1.01-1.03 have been realized. Although oligomeric polymethylene has been the most frequent synthetic target of this method, polymeric star organoboranes with molecular weights of 1.5 million have been produced. The average turnover frequency at 120 degrees C in 1,2,4,5-tetrachlorobenzene/toluene is estimated at >6 x 10(6) g of polymethylene (mol boron)(-1) h(-1). The mechanism of the polyhomologation reaction involves initial formation of a zwitterionic organoborane.ylide complex which breaks down in a rate-limiting 1,2-alkyl group migration with concomitant expulsion of a molecule of DMSO. The reaction was found to be first order in the borane catalyst and zero order in ylide. DMSO does not interfere with the reaction. The temperature dependence of the reaction rate yielded the following activation energy parameters (toluene, DeltaH(++) = 23.2 kcal/mol, DeltaS(++) = 12.6 cal deg/mol, DeltaG(++) = 19.5 kcal/mol; THF, DeltaH(++) = 26.5 kcal/mol, DeltaS(++) = 21.5 cal deg/mol, DeltaG(++) = 20.1 kcal/mol).     

Racemic and (+)-ethyl 2-acetamido-2-carboxy-5-oxohexanoate

Racemic ethyl 2-acetamido-2-carboxy-5-oxohexanoate has been isolated by single hydrolysis of the corresponding diethyl ester and resolved essentially quantitatively by diastereomeric salt formation with (–)-quinine. The (+)-isomer was retrieved from the less-soluble quininium salt. Racemate crystals, (plusmn;)-1, are monoclinic, space group P2₁/c, a = 7.609(4)Å, b = 16.731(7)Å, c = 10.746(5) Å, = 99.83(4)° enantiomeric crystals, (+)-1, are monoclinic, space group P2₁, a = 5.857(2)Å, b = 15.282(4)Å, c = 7.618(2)Å, = 95.62(2)°. Molecular packing is similar in the two structures; the enantiomer has a 0.7% higher volume per molecule and the lower fusion temperatures by 3°C. In both structures, carboxylic acids donate hydrogen bonds to the amide carbonyls of adjacent molecules without reciprocation. Amides form three-centered hydrogen bonds to carboxy oxygen of an intramolecular carboxylic acid group and to oxygen of a ketone carbonyl in an adjacent molecule.     

Die Schnellelektrolyse mit station?ren Elektroden

Ohne Zusammenfassung     

Application of matrix-assisted laser desorption/ionization to on-line aerosol time-of-flight mass spectrometry

Matrix-assisted laser desorption/ionization (MALDI) mass spectra were obtained from single biological aerosol particles using an aerosol time-of-flight mass spectrometer (ATOFMS). The inlet to the ATOFMS was coupled with an evaporation/condensation flow cell that allowed the aerosol to be coated with matrix material as the sampled stream entered the spectrometer. Mass spectra were generated from aerosol composed either of gramicidin-S or erythromycin, two small biological molecules, or from aerosolised spores of Bacillus subtilis var niger. Three different matrices were used: 3-nitrobenzyl alcohol, picolinic acid and sinapinic acid. A spectrum of gramicidin-S was generated from approximately 250 attomoles of material using a molar ratio of 3-nitrobenzyl alcohol to analyte of approximately 20:1. A single peak, located at 1224 Da, was obtained from the bacterial spores. The washing liquid and extract solution from the spores were analyzed using electrospray mass spectrometry and subsequent MS/MS product ion experiments. This independent analysis suggests that the measured species represents part of the B. subtilis peptidoglycan. The on-line addition of matrix allows quasi-real-time chemical analysis of individual, aerodynamically sized particles, with an overall system residence time of less than 5 seconds. These results suggest that a MALDI-ATOFMS can provide nearly real-time identification of biological aerosols. Copyright 2000 John Wiley & Sons, Ltd.     

Practical approaches to the ESI‐MS analysis of catalytic reactions

Electrospray ionization mass spectrometry (ESI‐MS) is a soft ionization technique commonly coupled with liquid or gas chromatography for the identification of compounds in a one‐time view of a mixture (for example, the resulting mixture generated by a synthesis). Over the past decade, Scott McIndoe and his research group at the University of Victoria have developed various methodologies to enhance the ability of ESI‐MS to continuously monitor catalytic reactions as they proceed. The power, sensitivity and large dynamic range of ESI‐MS have allowed for the refinement of several homogenous catalytic mechanisms and could potentially be applied to a wide range of reactions (catalytic or otherwise) for the determination of their mechanistic pathways. In this special feature article, some of the key challenges encountered and the adaptations employed to counter them are briefly reviewed. Copyright © 2014 John Wiley & Sons, Ltd.     

Alkylation of [Pt2(μ-S)2(PPh3)4] with boronic acid derivatives

The reactivity of the metalloligand [Pt₂(μ-S)₂(PPh₃)₄] with the boron-functionalized alkylating agents BrCH₂(C₆H₄)B(OR)₂ (R = H or C(CH₃)₂) was investigated by electrospray ionization mass spectrometry (ESI-MS) in real time using pressurized sample infusion (PSI). The macroscopic reaction of [Pt₂(μ-S)₂(PPh₃)₄] with one mole equivalent of alkylating agents BrCH₂(C₆H₄)B{OC(CH₃)₂}₂ and BrCH₂(C₆H₄)B(OH)₂ gave the dinuclear monocationic μ-sulfide thiolate complexes [Pt₂(μ-S){μ-SCH₂(C₆H₄)B{OC(CH₃)₂}₂}(PPh₃)₄]⁺ and [Pt₂(μ-S){μ-S⁺CH₂(C₆H₄)B(OH)(O^?)}(PPh₃)₄]. The products were isolated as the [PF₆]^? salt and zwitterion, respectively, and fully characterized by ESI-MS, IR, ¹H and ³¹P NMR spectroscopy, and single-crystal X-ray structure determinations.     

Substituent-controlled reactivity in the Nazarov cyclisation of allenyl vinyl ketones

Alkyl substitution α to the ketone of an allenyl vinyl ketone enhances Nazarov reactivity by inhibiting alternative pathways involving the allene moiety and through electron donation and/or steric hindrance. This substitution pattern also accelerates Nazarov cyclisation by increasing the population of the reactive conformer and by stabilising the oxyallyl cation intermediate. Furthermore, α substitution by an alkyl group does not alter the regioselectivity of interrupted Nazarov reactions when the oxyallyl cation intermediate is intercepted by addition of an oxygen nucleophile, or by [4+3] cyclisation with acyclic dienes. The regioselectivity of the Nazarov process for allenyl vinyl ketones was determined to be a result of an electronic bias in the oxyallyl cation intermediate. Computational data are consistent with this observation.     

Application of the Floquet theory to multiple quantum NMR of dipolar-coupled multi-spin systems under magic angle spinning

A new analytical Liouville-space representation of the time-propagator under magic angle spinning (MAS) is introduced using the formalized quantum Floquet theory. This approach has the advantage that it is applicable to the analysis of any type of NMR experiment where MAS is combined with multiple-pulse excitation. General relationships describing the spectral parameters in multiple-quantum (MQ) MAS spectra are derived in this representation. Their use is illustrated with an application to double-quantum (DQ) NMR spectra of dipolar-coupled multi-spin systems. Corresponding to the separation of the MAS time-propagator into a rotor modulated and a dephasing component, two distinct mechanisms for DQ excitation are identified. One of them exploits the rotor-modulated component to excite DQ coherences through dipolar-recoupling techniques, which are familiar for spin pairs. Analytical expressions of the integral intensities and linewidths in the resulting DQ sideband pattern are derived in the form of power series expansions of the inverse rotor frequency, of which coefficients depend on structural parameters. In a multi-spin system they can most reliably be extracted in the fast spinning regime. The other mechanism exploits the dephasing component, which is characteristic to multi-spin systems only. This is shown to give rise to DQ coherences by free evolution at full rotor periods. The possibility to exploit it for selective excitation of higher order MQ coherences is discussed. In either case, the dephasing component also leads to residual broadening. The main results of the theoretical developments are demonstrated experimentally on adamantane.     

A mathematical model of aging-related and cortisol induced hippocampal dysfunction

Background  

The hippocampus is essential for declarative memory synthesis and is a core pathological substrate for Alzheimer's disease (AD), the most common aging-related dementing disease. Acute increases in plasma cortisol are associated with transient hippocampal inhibition and retrograde amnesia, while chronic cortisol elevation is associated with hippocampal atrophy. Thus, cortisol levels could be monitored and managed in older people, to decrease their risk of AD type hippocampal dysfunction. We generated an in silicomodel of the chronic effects of elevated plasma cortisol on hippocampal activity and atrophy, using the systems biology mark-up language (SBML). We further challenged the model with biologically based interventions to ascertain if cortisol associated hippocampal dysfunction could be abrogated.