1,721,848 research outputs found
Preparation and Catalytic Enantioselective Reactions of C3-Symmetric Tris(Oxazoline)s Derived from Kemp's Triacid
No full textSimple Synthesis of Enantiomerically Pure C2-Symmetric Bisoxazolidines from Amino Alcohols and Formaldehyde
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Studies on the Zinc-mediated Phenyl and Alkynyl addition to Carbon-heteroatom double bonds
No full textThis thesis describes the work carried out by the author in the field of enantioselective dialkylzinc-mediated carbon-carbon bond forming reactions. In particular, the asymmetric addition of C(sp2) and C(sp) nucleophiles to carbon-heteroatom double bonds was the subject of this research. In the first part of the thesis, the synthesis of new chiral compounds to be used as ligands for the enantioselective phenyl addition to aldehydes is described. Novel alfa-hydroxy oxazolines derived from such precursors as mandelic acid, benzoylformic acid and ethyl oxamate were synthesized and used as chiral promoters for the reaction of a mixture of triphenylborane and diethylzinc with aromatic and aliphatic aldehydes. As a result, chiral secondary alcohols were obtained in good yields with up to 81% enantiomeric excess. In the second part of this work, the synthesis of propargylic amines by the reaction of a mixture of a terminal acetylene and dimethylzinc with imines was developed. Whereas it was found that the reaction can proceed also in the absence of ligands, providing an easy and high yielding way to prepare racemic propargylamines, the addition to the reaction mixture of a tertiary amino alcohol, having a norephedrine-based structure, as a chiral promoter allowed to obtain the products in good to high yields with up to 97% ee
Studies on the Zinc-mediated Phenyl and Alkynyl addition to Carbon-heteroatom double bonds
No full textThis thesis describes the work carried out by the author in the field of enantioselective dialkylzinc-mediated carbon-carbon bond forming reactions. In particular, the asymmetric addition of C(sp2) and C(sp) nucleophiles to carbon-heteroatom double bonds was the subject of this research. In the first part of the thesis, the synthesis of new chiral compounds to be used as ligands for the enantioselective phenyl addition to aldehydes is described. Novel alfa-hydroxy oxazolines derived from such precursors as mandelic acid, benzoylformic acid and ethyl oxamate were synthesized and used as chiral promoters for the reaction of a mixture of triphenylborane and diethylzinc with aromatic and aliphatic aldehydes. As a result, chiral secondary alcohols were obtained in good yields with up to 81% enantiomeric excess. In the second part of this work, the synthesis of propargylic amines by the reaction of a mixture of a terminal acetylene and dimethylzinc with imines was developed. Whereas it was found that the reaction can proceed also in the absence of ligands, providing an easy and high yielding way to prepare racemic propargylamines, the addition to the reaction mixture of a tertiary amino alcohol, having a norephedrine-based structure, as a chiral promoter allowed to obtain the products in good to high yields with up to 97% ee
New C 1- and C 2-symmetric sulfoximines : synthesis and application in enantioselective metal catalysis
No full textIn this dissertation, the preparation of new C1- and C2-symmetric sulfoximines (N,N- and P,N-ligands) for enantioselective metal catalysis is described. Guided by the excellent level of enantioselectivity observed using aminosulfoximine in the Cu-catalyzed Mukaiyama-aldol reaction, modifications of this class of potential ligand by substitution of the aryl bridge was effected. Gratifyingly, a methyl-substituted aminosulfoximine proved to be the best ligand of our screening in the Cu-catalyzed carbonyl-ene reaction between pyruvate esters and olefins. Moreover, aminosulfoximines were applied in the Cu-mediated vinylogous Mukaiyama-aldol reaction between pyruvate esters and dienol silanes, affording alpha,beta-unsaturated delta-hydroxy diesters in good yield with enantioselectivities of up to 99% ee
Neue Phospholan-basierte P,N-Chelatoren zur Katalyse
No full textThis thesis descibes the synthesis of P,N-chelators bearing a phospholane moiety and a nitrogen coordination site and their application in asymmetric catalysis.(Benz-)imidazole was choosen as a backbone for the chelators due to the likely high solubility in ionic liquids of the resultant ligands, which is important for the heterogenization of the intended catalysts. Substitution of the imidazole with pyridine, phenyl or methyl enabled tuning of the steric and electronic properties as well as the bite-angle of the chelator. After optimizing the synthesis of the (benz-)imidazole derivatives an atom economical phosphorylation through a Lewis-acid mediated reaction with trimethylsilylphospholane was investigated. Additionaly, an unpatented synthesis of the chiral phospholane moiety was sought. Therefore, several acid chlorides and amides were reacted with 2,5-hexanediol to build up analogues to Burk's „Cyclic Sulfate“ which reacts as a phospholane precursor. The cyclic carbonates and silanes were subsequently employed in the synthesis of phospholanes. Furthermore, a method for the direct synthesis of P,N-chelators derived from maleinimide and analogues was pursued. This method opens the door to a whole new class of catalyst precusors. Dichloromaleinimide was treated with trimethylsilylphospholane and a primary or secondary amine to obtain the related P,N-substituted compounds. Simple cyclic amines like piperidine and morpholine could be used, as well as aromatic amines like aniline or chiral compounds like prolinol
Planar-chirale Liganden und Ligandensysteme : stereoselektive Darstellung und Einsatz in der asymmetrischen Katalyse
No full textStudies on transition metal-catalysed oxidations with hydrogen peroxide as terminal oxidant
No full textIn the last few decades, hydrogen peroxide has emerged as an environmentally viable alternative in many oxidative processes. This oxidant does not require the use of a co-reductant and in the presence of non-toxic, inexpensive metals, such as iron, effects homogeneous liquid phase oxidations, producing water as the sole by-product. This "green" combination represents a highly desirable solution to the environmental issue. Following these guidelines, hydrogen peroxide was chosen as terminal oxidant for the oxidation of unreactive substrates and for the oxidative cleavage of double bonds. This thesis work was therefore divided in three sections.In the first part an iron-catalysed oxidation system for saturated hydrocarbons, using hydrogen peroxide as oxidant, has been developed, providing a promising tool for large-scale applications.The method exhibited great advantages: the reaction proceeds under truly catalytic conditions, the catalyst does not require any ligand, and water is the only by-product formed. The presence of an additive, i.e. a carboxylic acid, was found to enhance the system and the effect of different acids was studied. To extend the scope of the reaction, several substrates were tested, such as arylalkanes, cycloalkanes and alcohols. Arylalkanes were easily converted into the corresponding ketones with excellent selectivities and the less reactive and more challenging cycloalkanes gave the corresponding alcohols and ketones with yields comparable to the best reported in literature so far. The system presented peculiar selectivity parameters that are consistent with the involvement of both alkylhydroperoxide or of Fe(OOH) species. In situ IR studies were then conducted to elucidate some mechanistic aspects. In the second part, the applicability of the Fe(ClO4)2/CH3COOH/H2O2 system to the oxidative cleavage of double bonds was demonstrated. The reaction conditions were studied and optimised, showing that the presence of an additive, namely acetic acid, is essential to enhance the feature of the system. Substrates with various substitution pattern were considered, revealing a limitation of the applicability of the protocol to alpha,alpha-disubstituted styrenes, which were easily and selectively transformed into the corresponding acetophenone derivatives in good yields.The third part of this thesis was dedicated to the synthesis and application of new chiral C1-symmetric triazacyclononanes. Amino acids were chosen as readily available source of chirality and the Crab-like methodology proved to be the ideal synthetic approach, granting the desired flexibility. Transition metal complexes of triazazcyclononanes are known to be efficient and selective catalyst for the CH oxidation of unreactive substrate using hydrogen peroxide as oxidant. The new C1-symmetric TACNs were employed in the Mn- and Fe-catalysed oxidation of ethylbenzene with H2O2 as oxidant and acetic acid as additive. Although only low conversions were obtained when iron was used as metal source, the system proved to be able of transferring the chiral information into the catalytic process, thus representing the first example of enantioselective CH-activation catalysed by in situ formed iron-TACN complexes. The chiral C1-symmetric TACN demonstrated activity also in the enantioselective Mn- catalysed epoxidation of simple olefins, with moderate to good conversions and modest ee. A second approach towards the enantioselective CH oxidation of hydrocarbons was also evaluated, using an isolated Mn-TMTACN complex in combination with chiral carboxylic acids
Enantioselective, metal catalyzed oxidations : investigation of the Baeyer-Villiger oxidation and the Kharasch-Sosnovski reaction
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