93 search hits
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Synthesis of New Alkyn-1-ylsilanes. 1,1-Organoboration, 1,2-Hydroboration
(2009)
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Ezzat Khan
- New alkyn-1-ylsilanes were prepared, characterized and their use in 1,2-hydroboration, 1,1-organoboration reactions or combination of these two was demonstrated. Although the silanes were in general obtained as mixtures, they could be purified by fractional distillation or crystallization. Many of these silanes were fully characterized for the first time by modern NMR spectroscopic techniques and in some cases by X-ray structural analysis. The monoalkyn-1-yl(chloro)silanes were treated with BEt3 at 373-393 K for prolonged periods of time (up to 30 d), and the reactivity of the products was explored. For the first time the 1,2-hydroborating behavior of BEt3 was discovered in a series of reactions. Silanes containing two or more than two (up to four) alkyn-1-yl groups were used for the synthesis of different heterocyclic systems e.g., 1-silacyclobutenes, 1-silacyclopent-2-enes, siloles, borolenes, etc. Intermediates of sufficient stability and final products were studied by multinuclear NMR spectroscopy (1H, 11B, 13C, 29Si and 119Sn NMR). Numerous molecular structures were determined by X-ray structural analysis. Alkenes bearing Si-Cl function(s) are useful precursors for further transformations. They were converted into 1-silacyclobutene derivatives. Various intermediates on the way to 1-silacyclobutenes were discovered aiding in the understanding of the reaction mechanism. The aforementioned heterocycles were also prepared by reacting dialkyn-1-ylsilanes with 9-BBN at 353-373 K. In a first step, alkenyl(alkyn-1-yl)silanes are formed as a result of intermolecular 1,2-hydroboration followed by intramolecular 1,1-vinylboration at the same temperature to afford 1-silacyclobutene derivatives. The first molecular structure of one example was determined by X-ray structural analysis. Corresponding tetraalkyn-1-ylsilanes afforded 4-silaspiro[3,3]hepta-1,5-diene derivatives. The hydroborating reagent 9-BBN, on reaction with alkyn-1-yl(vinyl)silanes, leads to the 1-silacyclopent-2-ene derivatives at ambient temperature. Dialkyn-1-yl(divinyl)silanes gave 5-silaspiro[4,4]nona-1,6-diene derivatives. X-ray structures for representative examples 1-silacyclopent-2-ene as well as for spirosilanes were determined for the first time. Many dialkynylsilanes react with BEt3 and 9-Et-9-BBN under harsh reaction conditions (at 373-393 K for several days) to afford siloles as the result of a slow intermolecular 1,1-alkylboration followed by a fast intramolecular 1,1-vinylboration. This method is very simple and useful and fairly pure siloles were isolated. Protodeborylation using acetic acid as protic reagent was successful for a large number of novel heterocycles. New protodeborylated compounds were prepared and studied. The boron was found as part of a boron-oxygen bicyclic compound which was for the first time isolated and characterized by single crystal X-ray structural analysis.
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Theory and Practice of Reconciliation in Rwanda
(2009)
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Marcus Grohmann
- During recent years, scholars working on the peacebuilding process in Rwanda have often tended to single out specific aspects, for instance judicial responses to the genocide. Little research has been done, however, on the diversity of approaches that constitute the “reconciliation landscape” in Rwanda today. Basing itself on data from field research in 2006, this paper seeks to shed some light on the many programmes carried out in Rwanda related to reconciliation work. Emphasis is put on two case studies. While establishing a theoretical framework of the reconciliation process in the first part of the paper, the following chapters attempt to explain how this relates to the practice of reconciliation in the Rwandan context. The data collected suggest that in the face of political constraints, the Rwandan government must in part rely on civil society actors for the achievement of their goals of “unity and reconciliation”. The multitude of initiatives from actors with a wide range of motivations and approaches should be seen as complementary, while some may have to make up for the shortcomings and constraints of others.
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Perfluorooctanoic Acid (PFOA) and Perfluorooctane Sulfonate (PFOS) in an Aquatic Ecosystem - Distribution and Fate
(2009)
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Anna Maria Becker
- Perfluorooctanoate (PFOA) and perfluorooctane sulfonate (PFOS) belong to the family of perfluorinated surfactants (PFSs). They are widely distributed and persistent in the environment. For over 50 years, they have been used in numerous applications including paper and textile treatment, production of fluoropolymers, cosmetics and insecticides formulations, and fire fighting foams. They can enter the environment via direct and indirect emission sources such as manufacturing processes, use of commercial products containing PFSs, release of waste waters or degradation of precursor substances. Due to their physical-chemical properties, i.e. relatively good solubility, low volatility and stability under environmental conditions, water bodies are important sinks for these chemicals. The aim of this doctoral thesis was to elucidate sources, distribution and fate of PFOA and PFOS in an aquatic ecosystem that is not directly affected by fluorochemical production activity. The presented study was mainly focused on the river Roter Main, Bayreuth, Germany, and gives a good picture of the behaviour of PFOA and PFOS in such an aquatic ecosystem. In order to achieve the main goal, suitable analytical procedures for reliable quantification of trace amounts of the target analytes in different environmental matrices such as water, liquid and solid wastes, sediments, biological tissues, were developed or optimised. Each method included a solid-phase extraction step for analytes’ preconcentration and removal of interfering matrix, followed by quantitative determination via high performance liquid chromatography coupled to electrospray ionisation tandem mass spectrometery (HLPC-ESI-MS). Due to the possibility of ionisation suppression, isotope dilution or standard addition method was applied. Analysis of waste waters collected from four different waste water treatment plants (WWTPs) located in Upper Franconia, Bavaria, Germany, showed that the largest plant (Bayreuth) receiving waste waters of mostly commercial and industrial origin released the highest amount of PFOA and PFOS, whereas the smallest plant (Himmelkron) treating waste waters of only domestic source released the least. The monitoring of waste waters from the WWTP Bayreuth enabled to estimate the typical mass flows of PFOA and PFOS into river waters as about 1 and 5 g/day, respectively, showing that a plant of a medium-size, moderately industrialised city can be a major source of river pollution. Detailed investigation of PFOA and PFOS concentrations in liquid and solid wastes collected at different stages of the treatment process showed additional fluxes of these compounds inside the plant, likely due to the decomposition of their precursors. Analysis of sediments collected from the river receiving treated waste waters showed a significant increase in concentrations of both analytes downstream the outlet of the plant (up to 3- and 4-fold for PFOA and PFOS, respectively). PFOS concentrations were up to 40-fold higher in sediments than in river water, showing its higher adsorption potential in comparison to PFOA (max. sediment/water = 6). Once in the river, PFOS, and to a lower extent PFOA, can bioaccumulate in aquatic organisms. Although partially removed from water, they are still bioavailable for benthic organisms inhabiting the river thus entering the food chain. This was reflected in higher levels found in river goby - in comparison to chub - feeding on invertebrates living in the sediment. At the starting point of this doctoral thesis little information was available about environmental contamination with PFOA and PFOS in Germany, and it is the first study performed in Bavaria giving such a detailed picture of sources and fate of PFSs in a river ecosystem
