Lewis acids can be used as catalysts in different reactions, but the term Lewis acid catalysts often refers to metal salts. Metal complexes have been widely used for asymmetric catalysis. Asymmetric synthesis can however be performed in a metal-free way by using organocatalysis. New Lewis acid catalysts that are more effective, enantioselective and environmental friendly is of interest. This new type of Lewis acid catalysts could for example be of carbocation based character. The aim of this project was to synthesize chiral carbocations with different degree of sterical hindrance and investigate their catalytic ability in Diels-Alder reactions. It was presumed that the Diels-Alder reactions were going to be performed in an asymmetric way since the carbocation catalysts were achiral. Two chiral carbocations were synthesized successfully. The first synthesized carbocation, the less sterical hindered compound 8, was formed as a racemic mixture. The second carbocation, compound 16, could be formed as an enatiomeric pure compound. Both carbocations showed catalytic ability in Diels-Alder reactions and compound 8 was comparable with some common Lewis acid catalysts. In general, when using compound 8 as catalyst, higher catalyst amount gave higher conversions. Higher concentrations also gave higher conversions, but up to a certain level. No trend between polarity of different solvents and conversions could be seen. However, an increased temperature leads to faster reactions. The more rigid and sterical hindered compound 16 catalyzed the reactions slower than compound 8. The longer reaction time may indicate that the reaction occurs with higher selectivity, but no method to measure the ee of the product was found. An attempt to synthesize a third even more sterical hindered chiral carbocation, compound 19, resulted in a product contaminated by impurities that showed a catalytic ability lower than compound 8 and compound 16 in Diels-Alder reactions. The synthesis and the use of carbocations as Lewis acid catalysts in Diels-Alder reactions seem promising as a new type of catalysts even though there are questions that are still unanswered, e.g. counter ions effects, possible side reactions, selectivity etc.

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第17773号 / 理博第3896号 / 新制||理||1562(附属図書館) / 30580 / 京都大学大学院理学研究科化学専攻 / (主査)教授 丸岡 啓二, 教授 大須賀 篤弘, 教授 時任 宣博 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DGAM

This thesis describes the use of 2H-azirines as reactivesubstrates in Lewis acid catalysed nucleophilic additions andin the Diels-Alder reaction.A number of carbon nucleophiles have been added to aseries of 2H-azirines in the presence and absence ofBF3·Et2O. 3-(2-Naphthyl)-2H-azirine has been used as amodel substrate in the enantioselective addition oforganolithium reagents to an 2H-azirine.A selection of Lewis acids has been screened for theirpossible use in the normal electron demand Diels-Alder reactionbetween 3-alkyl-, 3-aryl-, and 3- carboxyl-2H-azirines and avariety of dienes. Lewis acid activation was found to shortenreaction times and facilitate lower reaction temperatures.These cycloadditions proceeded with endo selectivity providinga single diastereoisomeric product.DFT calculations of Lewis acid activated 2H-azirineshave been carried out. Keywords:2H-azirines, Lewis acid activation, chiralligands, organolithium reagents, Diels-Alder reactions,DFT-calculations / <p>NR 20140805</p>

Wood is often combined with other materials such as thermoplastics, adhesives and coatings. In general, combinations of wood and polymers especially in outdoor exposure have poor long-term durability. This behaviour can be related to an insufficient wood-polymer adhesion due to the low intrinsic compatibility between the wood substance and the polymers used. Another source for woodpolymer de-bonding is the high hygroscopicity of wood and great difference in hygro-thermal properties between the components. The basic conceptual idea related to this work is to reduce the hygrosensitivity of wood by applying different wood modification methods, in particular, acetylation, furfurylation and heat treatment. The effects of such chemical modifications of wood, also accompanied with ageing effects, on its adhesion properties with commonly used synthetic polymers are, however, not well understood. In this context, the over-all purpose of this thesis is to achieve a better understanding of wood-polymer adhesion and interfacial forces which also may guide us to tailor the interaction between modified wood and e.g. thermoplastics and adhesives. The main focus of this thesis is therefore to apply contact angle analysis based on the Chang-Qin-Chen (CQC) Lewis acid-base model in order to estimate the work of adhesion (Wa) between the wood, modified wood and certain polymers. Contact angle measurements on wood samples were performed based on the Wilhelm plate principle. Related to this, an effort was also made to characterize the studied modified wood surfaces according to morphology and chemical composition. The methods that have been used are low vacuum scanning electron microscopy (LV-SEM), X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). Results show that so-called interaction parameters can be successfully estimated for prediction of Wa between wood and polymers using the applied CQC model. Furthermore, such wetting analysis was successfully related to spectroscopic findings of the chemical composition of the wood samples surface. Ageing effects, i.e. the time after preparation of the wood surface, play a central role for the surface characteristics. In most cases, ageing resulted in a significant decrease of Wa between wood and water and a moderate decrease between wood and thermoplastics. The surface characteristics of acetylated wood were, however, more stable over time compared to unmodified, furfurylated and heat treated wood. The predicted Wa with the adhesives for heat treated and acetylated wood was increased due to ageing. Future work is planned to involve studies in order to relate such predicted adhesion properties with the actual performance of various wood-polymer systems. / QC 20101108

No description available.

This thesis describes the use of 2H-azirines, three-membered unsaturatednitrogen-containing heterocycles, as reactive intermediates ina number of Lewis acid promoted alkylations and Diels-Alderreactions providing synthetically useful aziridines. In order to carry out this investigation a new generalprocedure for the ring closure of vinyl azides, forming theresultant 3-substituted-2H-azirines, was developed applying low boiling solventsin closed reaction vessels at elevated temperatures. The addition of organolithium reagents in the presence ofcommercially available chiral ligands, to the 3-(2-naphthyl)-2H-azirine was studied, which gave the correspondingaziridines. Several Lewis acids were shown to catalyze the normalelectron-demand Diels-Alder reaction between 3-alkyl-,3-aromatic-, and 3-ester-substituted 2H-azirines and various dienes. These reactions gave theexpected cycloadducts in moderate yields. Using a chiral auxiliary high diastereoselectivity wasobtained in the addition of alkyl radicals to a8-phenylmenthyl-substituted 2H-azirine-3-carboxylate. The alkyl radicals weregenerated from the corresponding trialkyl borane and molecularoxygen. Hydroborations and transmetallations were used toprepare these trialkylboranes. Catalytic amounts of CuClincreased the diastereoselectivity in the radical additionreactions. Attempts were made to explain how the coordination of aLewis acid to the azirine nitrogen atom affects thereactivity/stability of the azirine. DFT calculations and NMRexperiments involving Lewis acid-azirine complexes wereperformed. Keywords:Enantioselective, diastereoselective, vinylazide, 2H-azirines, aziridines, Lewis acid, chiral ligand,chiral auxiliary, organolithiums, Diels-Alder reaction, alkylradicals, triethylborane.

This thesis deals with the development of new methods for efficient synthesis and analysis in asymmetric catalysis. It focuses on the preparation of chiral cyanohydrins by enantioselective addition of cyanide to prochiral aldehydes. The initial part of the thesis describes the development of a dual Lewis acid– Lewis base activation system for efficient synthesis of chiral O-acylated and Ocarbonylated cyanohydrins. This system was used for the preparation of a variety of cyanohydrins in high isolated yields and with up to 96% ee. Activation of the cyanide by nucleophilic attack of the Lewis base at the carbonyl carbon atom was supported experimentally. Secondly, convenient procedures for the synthesis of polymer-bound chiral YbCl3-pybox and Ti-salen complexes are described. The polymeric complexes were employed in cyanation of benzaldehyde. A T-shaped microreactor was used for screening of reaction conditions for the enantioselective cyanation of benzaldehyde using trimethylsilyl cyanide and acetyl cyanide as cyanide sources. A microreactor charged with the polymeric Tisalen complex was used for enantioselective cyanation of benzaldehyde. Finally, an enzymatic method for high throughput analysis of ee and conversion of products from chiral Lewis acid–Lewis base-catalysed additions of α- ketonitriles to prochiral aldehydes was developed. The method could be used for the analysis of a variety of O-acylated cyanohydrins. Microreactor technology was successfully combined with high throughput analysis for efficient catalyst optimisation. / QC 20100809

In the first part of this study, four new chiral ligands (FAM) were synthesized and used in catalytic amounts in asymmetric 1,3-dipolar cycloaddition reactions of azomethine ylides. This method leads to the synthesis of chiral pyrrolidines, which are found in the structure of many biologically active natural compounds and drugs. It was found that using 10 mol% of one of these chiral ligands with different dipolarophiles (dimethyl maleate, dimethyl fumarate, methyl acrylate, tert-butyl acrylate, and Nmethylmaleimide), pyrrolidine derivatives could be synthesized in up to 94% yield and 95% ee. In the second part of the study, the catalytic activity of these chiral ligands were tested with titanium in asymmetric alkynylzinc addition reactions to aldehydes. By this method, chiral propargylic alcohols, which are important precursors for the natural products and pharmaceuticals can be synthesized. Using our catalyst, chiral propargylic alcohols were obtained in up to 96% yield and 98% ee. Although, most of the catalyst systems in the literature worked only with aromatic or aliphatic aldehydes and phenylacetylene, the catalyst system developed in this study worked with four different types of aldehydes (aromatic, aliphatic, heteroaromatic and a,b-unsaturated) and two different aliphatic acetylenes very successfully. Additionally, chiral ligand can be recovered in more than 90% yield and reused without losing its activity.

<p>This thesis describes the use of 2<i>H</i>-azirines, three-membered unsaturatednitrogen-containing heterocycles, as reactive intermediates ina number of Lewis acid promoted alkylations and Diels-Alderreactions providing synthetically useful aziridines.</p><p>In order to carry out this investigation a new generalprocedure for the ring closure of vinyl azides, forming theresultant 3-substituted-2<i>H</i>-azirines, was developed applying low boiling solventsin closed reaction vessels at elevated temperatures.</p><p>The addition of organolithium reagents in the presence ofcommercially available chiral ligands, to the 3-(2-naphthyl)-2<i>H</i>-azirine was studied, which gave the correspondingaziridines.</p><p>Several Lewis acids were shown to catalyze the normalelectron-demand Diels-Alder reaction between 3-alkyl-,3-aromatic-, and 3-ester-substituted 2<i>H</i>-azirines and various dienes. These reactions gave theexpected cycloadducts in moderate yields.</p><p>Using a chiral auxiliary high diastereoselectivity wasobtained in the addition of alkyl radicals to a8-phenylmenthyl-substituted 2<i>H</i>-azirine-3-carboxylate. The alkyl radicals weregenerated from the corresponding trialkyl borane and molecularoxygen. Hydroborations and transmetallations were used toprepare these trialkylboranes. Catalytic amounts of CuClincreased the diastereoselectivity in the radical additionreactions.</p><p>Attempts were made to explain how the coordination of aLewis acid to the azirine nitrogen atom affects thereactivity/stability of the azirine. DFT calculations and NMRexperiments involving Lewis acid-azirine complexes wereperformed.</p><p><b>Keywords:</b>Enantioselective, diastereoselective, vinylazide, 2<i>H</i>-azirines, aziridines, Lewis acid, chiral ligand,chiral auxiliary, organolithiums, Diels-Alder reaction, alkylradicals, triethylborane.</p>

No description available.