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21.
Ute Henniges Elisabeth Kloser Anjan Patel Antje Potthast Paul Kosma Martin Fischer Klaus Fischer Thomas Rosenau 《Cellulose (London, England)》2007,14(5):497-511
The oxidative effect of carbanilation mixtures containing dimethylsulfoxide (DMSO) was demonstrated by means of alcohol model
substances in which competitive carbanilation was prevented due to steric hindrance of the hydroxyl function, rendering those
compounds specific probes for oxidation effects. Dimethylsulfonium ions and derived ylide species were shown to be the actually
oxidizing species according to trapping methodology using lipophilic olefins which were converted into the corresponding cyclopropane
and epoxide derivatives. The experimental data were in good agreement with DFT computations carried out on the B3LYP/6-311+G(d,p)
level of theory. The direct interaction of cellulose and sulfoxide solvent was proven by means of methyl-(2-naphthyl)sulfoxide
(MNSO) as a model for DMSO, which caused introduction of UV-detectable methylthionaphthyl ether moieties into the cellulose,
formed in Pummerer-type side reaction paralleling the chemical behavior of DMSO. A facile color test—responding to sulfoxide-derived
oxidizing species—was developed to assess the suitability of carbanilation conditions with regard to cellulose oxidation and
degradation. DMSO-based carbanilation systems have to be used with great caution for determination of molecular weight parameters
and for similar purposes which require complete maintenance of the cellulose integrity. Cellulose oxidation/degradation by
DMSO-derived intermediates upon carbanilation can be minimized but cannot be avoided completely. Thus, if cellulose integrity
is an issue as it is in cellulose analytics, it is recommended to replace DMSO by solvent components of similar solution behavior
but without the inherent danger of generating oxidants, such as pyridine or DMAc, whenever possible.
相似文献
Thomas RosenauEmail: |
22.
Axel Rußler Thomas Lange Antje Potthast Thomas Rosenau Elena Berger-Nicoletti Herbert Sixta Paul Kosma 《Macromolecular Symposia》2005,223(1):189-200
Analytical monitoring of xanthation in the viscose process along with xanthate group analysis in the viscose material is a long-debated problem in cellulose chemistry. The task is rendered extremely intricate by the lability of the starting material and the harshness of the reaction medium, which adds to a lack of suitable analytical approaches. In a four-years' endeavor in our lab, a method is being developed which allows to analyze the distribution of xanthate groups in viscoses relative to the anhydroglucose units and along the cellulose chain. In a first step the xanthate groups are stabilized by alkylation, which was optimized towards quantitative conversion. In a second step, the remaining free hydroxyl groups are protected by carbanilation, followed by selective removal of the stabilized xanthate groups. Steps two and three thus generate an inverse image of the initial xanthate pattern. In the forth and fifth step, the liberated hydroxyl groups are methylated, and the carbanilates are removed, so that in the overall process the xanthates were replaced by methyl groups. All reaction steps have been comprehensively tested with regard to completeness of conversion and orthogonality of the protecting groups. 相似文献
23.
We propose and study a new variant of the Burgers equation with dissipation fluxes that saturate as the gradients become unbounded. If the upstream-downstream transition is above a critical threshold, the corresponding Riemann problem admits a weak solution wherein part of the transit is accomplished by a jump. It is shown that the solution to a Cauchy problem with sufficiently small compact or periodic initial data preserves its initial smoothness. © 1997 John Wiley & Sons, Inc. 相似文献
24.
25.
Sakakibara K Nakatsubo F French AD Rosenau T 《Chemical communications (Cambridge, England)》2012,48(62):7672-7674
Right-handedness derived from bisporphyrins attached to a cellotriose backbone at O-6 and O'-6 positions is revealed for the first time. This cellotriose is proposed as a model of alternatingly functionalized cellulosics, which have promising properties for applications in optoelectronics and molecular receptors owing to the chirality and rigid backbone effects. 相似文献
26.
It is shown that in a current carrying extrinsic semiconductor, a quasi-neutral wave of carrier concentrations evolves into a shock wave(s). Inhomogeneous distribution of impurities causes either amplification (an absolute instability) or depression of these waves. 相似文献
27.
Reaction of 2-hydroxy-para-benzoquinones with DMSO/Ac2O produced dimethylsulfonium ylides, of which crystal structures as well as solid and liquid state NMR spectra were recorded. The ylides react with tertiary methylamine N-oxides in a one-pot, multi-step process to 3-methylamino-substituted benzoquinones. The mechanism starts with a deoxygenative deprotonation of the amine N-oxides, followed by a formal electrophilic displacement of DMSO by the resulting carbonium-iminium ion. 相似文献
28.
29.
Reaction of tocopheryl bromide 2a or chromanyl bromide 2b with triphenyl phosphine produced phosphomium salt intermediates (3a-b), which further reacted with acyl chlorides to novel furotocopherol compounds 4-11 in good yields. The cyclization proceeded according to a two step esterification-Wittig mechanism. Similarly, furotocopheryl dimer 12 was prepared starting from oxalyl chloride. The coupling of tocopheryl phosphonium salt 3a onto modified polystyrene provided a new, vitamin E-loaded resin. 相似文献
30.