In der vorliegenden Arbeit werden M(II)-Formiat-Komplexe mit M=Cu, Ni, Zn beschrieben. Das themogravimetrische Verhalten dieser Komplexe wird vorgestellt. TG-MS-Untersuchungen geben erste Hinweise auf das Metallisierungsverhalten obiger Spezies. Die Charakterisierung der entsprechenden Komplexe erfolgte durch die Elementaranalyse, IR-Spektroskopie sowie in einzelnen Fällen durch die Einkristallröntgendiffraktometrie.

This thesis describes our physical organic and mechanistic investigations into N Heterocyclic Carbene (NHC) mediated organocatalytic transformations, through a collaboration with the research group of Dr AnnMarie O'Donoghue and PhD student Richard Massey at Durham University. Initial research focused upon the determination of kinetic acidities and associated pKₐ values for a range of triazolium salts using C(3) H/D exchange, monitored by ¹H NMR spectroscopy. Estimates for pKₐ values in the range 16.6 17.4 were obtained, which are some ~2 and ~3 5 pK units lower than analogous imidazolium and thiazolium species respectively, with modest N substituent (0.3 pK units) effects observed. At lower pD values, an altered pD dependence indicates a dicationic triazolium species is formed (through N(1) protonation) with an estimated pKₐᴺ¹ of -0.2-0.5 and C(3) H pKₐ values at least 2 units lower than their monocationic analogues. This methodology was subsequently extended to mesoionic NHCs, where pKa values of 23.0 27.1 for a range of triazolium and 30.2 31.0 for a range of imidazolium salts were estimated. A detailed study of the NHC catalysed intramolecular Stetter reaction was also undertaken using ¹H NMR spectroscopy. A range of 3 (hydroxybenzyl)azolium salts (adducts), formed from the addition of NHC to aldehyde were isolated, enabling the generation of reaction profiles and the determination of rate constants. The reaction proceeds via rapid and reversible adduct generation, followed by rate limiting Breslow intermediate formation, with electron withdrawing N aryl substituents increasing the rate of product formation. Consistent with rate limiting deprotonation, deuterium exchange studies of O methylated adduct analogues found electron withdrawing N-aryl units gave faster exchange. Examination of the equilibrium constants for adduct formation revealed that both in the case of NHCs bearing 2,6 disubstituted N aryl units and aldehydes bearing a 2 ether substituent, the equilibrium position is significantly shifted towards adduct. Finally, studies at sub-stoichiometric NHC concentrations, monitored by HPLC, imply the reaction is first order with respect to NHC precursor, but zero order in aldehyde, again indicative of rate limiting deprotonation.

This thesis deals with secondary interactions in asymmetric catalysis and their impact on the outcome of catalytic reactions. The first part revolves around the metal-catalyzed asymmetric allylic alkylation reaction and how interactions within the catalyst affect the stereochemistry. An OH–Pd hydrogen bond in Pd(0)–π-olefin complexes of hydroxy-containing oxazoline ligands was identified by density functional theory computations and helped to rationalize the contrasting results obtained employing hydroxy- and methoxy-containing ligands in the catalytic reaction. This type of hydrogen bond was further studied in phenanthroline metal complexes. As expected for a hydrogen bond, the strength of the bond was found to increase with increased electron density at the metal and with increased acidity of the hydroxy protons. The second part deals with the use of hydroxy- and methoxy-containing phosphinooxazoline ligands in the rhodium- and iridium-catalyzed asymmetric hydrosilylation reaction. The enantioselectivities obtained were profoundly enhanced upon the addition of silver salts. This phenomenon was explained by an oxygen–metal coordination in the catalytic complexes, which was confirmed by NMR studies of an iridium complex. Interestingly, the rhodium and iridium catalysts nearly serve as pseudo-enantiomers giving products with different absolute configurations. The final part deals with ditopic pyridinobisoxazoline ligands and the application of their metal complexes in asymmetric cyanation reactions. Upon complexation, these ligands provide catalysts with both Lewis acidic and Lewis basic sites, capable of activating both the substrate and the cyanation reagent. Lanthanide and aluminum complexes of these ligands were found to catalyze the addition of the fairly unreactive cyanation reagents ethyl cyanoformate and acetyl cyanide to benzaldehyde, whereas complexes of ligands lacking the Lewis basic coordination sites failed to do so. / QC 20100709

This thesis deals with method development in asymmetric catalysis and specifically syntheses of enantioenriched O-functionalized cyanohydrins. The first part describes the development of a method for the synthesis of O‑alkoxycarbonylated and O-acylated cyanohydrins. Ethyl cyanoformate and acyl cyanides were added to aldehydes in a reaction catalyzed by a chiral dimeric Ti-salen complex together with a tertiary amine. High yields and enantioselectivities were in most cases obtained. Mechanistic studies were performed and a reaction mechanism was proposed. ­ The second part describes a method in which the undesired minor enantiomer in a Lewis acid–Lewis base-catalyzed acylcyanation is continuously recycled into prochiral starting material. Close to enantiopure O‑acylated cyanohydrins were obtained in high yields. The third part deals with asymmetric acylcyanations of ketones. Acetyl cyanide was found to add to α‑ketoesters in a reaction catalyzed by a chiral Lewis base. Yields up to 77% and 82% ee were obtained. The final part describes an enzymatic method for high-throughput analysis of O‑acylated cyanohydrins. The enantiomeric excess and conversion were determined for products obtained from a number of aromatic and aliphatic aldehydes. / QC 20100818

Gegenstand dieser Arbeit war die Untersuchung verschiedener Systeme Chlorosilan / Amin bezüglich deren elektronischer Struktur, Molekülstruktur und Reaktivität. Einen Schwerpunkt bildete dabei die Untersuchung der LEWIS-BASE-katalysierten Disproportionierung unterschiedlich chlorierter Disilane mit elektronenreichen Alkenen, wie Tetrakis-(dimethylamino)-ethylen (TDAE) oder N,N,N’,N’-Tetramethyl-1,4-phenylendiamin (TPDA). Die Alkene sollten auf mögliche Elektronenübertragungsreaktionen, aber auch Chelatbildung mit den eingesetzten Disilanen bzw. intermediär gebildeten Silylenen getestet werden. Ein weiterer Schwerpunkt lag bei der Synthese und Charakterisierung neuer hetero- und homocyclischer Oligosilane. Die erhaltenen Cyclooligosilane wurden auf ihre Donorwirkung gegenüber elektronendefizienten π-Alkenen und eine damit verbundene Charge-Transfer-Komplexbildung untersucht. In die Auswertung wurden auch ab-initio-Berechnungen einbezogen, die mit den experimentellen Ergebnissen (u.a. NMR, IR, Röntgeneinkristallstrukturanalyse) vergleichend diskutiert wurden.

Biomass is increasingly used as a feedstock in global energy production. This may present operational challenges in energy conversion processes which are related to the inorganic content of these biomasses. As a larger variety of biomass is used the need for a basic understanding of ash transformation reactions becomes increasingly important. This is not only to reduce operational problems but also to facilitate the use of ash as a nutrient source for new biomass production. Ash transformation reactions were examined in the present work using the Lewis acid-base concept. The model presented in Paper I was further extended and discussed, including the definition of tertiary ash transformation reactions as reaction steps where negatively charged molecular ions, Lewis bases, other than hydroxides are present in the reactants. The effect of such reactions for bonding of various metal ions, Lewis acids, were discussed. It was found that the formation of various phosphates through secondary and tertiary ash transformation reactions is important for the behaviour of biomass ash in combustion. The suggested model was supported by findings in Papers II-VIII. The experimental findings in Papers II-VIII were discussed in terms of ash transformation reactions. The fuel design choices made to investigate the effect of phosphorus in particular on ash transformation reactions were high-lighted. Addition of phosphoric acid to woody-type and agricultural biomasses showed that phosphate formation has a large influence on the speciation of Si, S, and Cl. Co-combustion of a problematic agricultural residue with other biomasses showed that the relation between phosphorus, alkali and alkaline earth metal content is important. Co-combustion of biosolids with wheat straw was shown to greatly improve the combustion properties of wheat straw. It was suggested that fuel analyses should be presented using molar concentration (mole/kg) in diagrams based on ash transformation reactions and elements forming Lewis acids or bases. This may facilitate the assessment of the combustion behaviour of a fuel. Some comments were made on fuel design and additives, specifically pointing out that phosphorus content should always be carefully considered in relation to alkali and alkaline earth metals in fuels and fuel blends.

Coordination chemistry usually applies to transition metals, but has recently been extended to the p-block elements. For the pnictogen atoms (group 15), this type of coordination chemistry has already been applied to antimony and bismuth, where they behave as Lewis acceptor centres. However, complexes with nitrogen and phosphorus as Lewis acidic centres are rare, due to their relatively small atomic radii and inherent basic nature. Instead, these elements (Pn(III)) are typically observed as donor centres because they are better at donating their electron pair, than they are at accepting them. To enhance the Lewis acidity at the phosphorus and nitrogen centres, a cationic charge can be introduced by heterolytically abstracting a halide and replacing it with a weakly coordinating anion, providing more opportunities for new reactivity. The presence of a stereochemically active lone pair at the acceptor site also introduces new reactivity patterns to be explored. The formation of these main group coordination complexes opens doors to potential applications in catalysis, small molecule activation, or as material precursors. 2,2’-bipyridine (bipy) has been a prototypical ligand used in transition metal coordination chemistry due to its high basicity and oxidative resistance. This property has been exploited to enable a comprehensive study of a series of Pn(III) tricationic and dicationic complexes using 2,2’-bipyridine (bipy); 4,4’-di-tert-butyl-2,2’-bipyridine (tBu2bipy); 4-dimethylaminopyridine (DMAP); and other main group containing ligands. / Graduate

No description available.

This work describes the application of a variety of Lewis bases, encompassing predominantly N-heterocyclic carbenes (NHCs), but also the use of imidazoles, aminopyridines, amidines and isothioureas, as effective catalysts in the dearomatisation of heterocyclic carbonates, predominantly the rearrangement of oxazolyl carbonates to their C-carboxyazlactone isomers by means of the Steglich rearrangement. This rearrangement reaction has been investigated extensively, with the development of simplified reaction procedures and the invention of domino cascade protocols incorporating this transformation. In an attempt to understand the mechanism of this O- to C-carboxylation process, a number of interesting observations have been made. Firstly, the class of NHC has an important factor in promoting the rearrangement, with triazolinylidenes being the most effective. Secondly, an interesting chemoselectivity has been delineated using triazolium-derived NHCs, prepared using weak bases (typically Et₃N) or strong metallated bases; both alkyl and aryl oxazolyl carbonates undergo smooth rearrangement with triazolinylidenes derived from strong metallated bases such as KHMDS, while only aryl oxazolyl carbonates undergo rearrangement using Et₃N. Extensive effort has focused towards the development of asymmetric variants of these protocols, primarily towards the design, synthesis and evaluation of chiral NHC precatalysts. To this end, a number of chiral azolium salts have been prepared, encompassing a number of different NHC classes, including C₁- and C₂-imidazolinium salts, C₂-imidazolium salts and a range of triazolium salts. Efforts towards the asymmetric catalysis of the Steglich rearrangement of oxazolyl carbonate substrates have given an optimal 66% ee. Similar rearrangements have been demonstrated with the related furanyl heterocyclic substrate class, producing a mixture of α- and γ-carboxybutenolides. In contrast to the analogous oxazolyl carbonates, the regioselectivity of this rearrangement is dependent upon the nature of the Lewis base employed. Amidines and aminopyridines give a mixture of the α- and γ- regioisomers with generally the α-regioisomer being preferred, while a triazolium-derived NHC gives rise to predominantly the thermodynamically more stable γ-carboxybutenolide. Using amidines or aminopyridines, this rearrangement has been shown to proceed via an irreversible C-C bond-forming process, but in contrast, the rearrangement using the NHC proceeds via an equilibrium process with an optimised regioselectivity of >98:2 for the γ-carboxybutenolide regioisomer over the α-regioisomer. Whilst the asymmetric variant using chiral NHCs has proven unfruitful, rearrangements using a chiral isothiourea have given high levels of regioselectivity towards the α- regioisomer and with excellent levels of enantiodiscrimination (77–95% ee).

Gegenstand dieser Arbeit war die Untersuchung verschiedener Systeme Chlorosilan / Amin bezüglich deren elektronischer Struktur, Molekülstruktur und Reaktivität. Einen Schwerpunkt bildete dabei die Untersuchung der LEWIS-BASE-katalysierten Disproportionierung unterschiedlich chlorierter Disilane mit elektronenreichen Alkenen, wie Tetrakis-(dimethylamino)-ethylen (TDAE) oder N,N,N’,N’-Tetramethyl-1,4-phenylendiamin (TPDA). Die Alkene sollten auf mögliche Elektronenübertragungsreaktionen, aber auch Chelatbildung mit den eingesetzten Disilanen bzw. intermediär gebildeten Silylenen getestet werden. Ein weiterer Schwerpunkt lag bei der Synthese und Charakterisierung neuer hetero- und homocyclischer Oligosilane. Die erhaltenen Cyclooligosilane wurden auf ihre Donorwirkung gegenüber elektronendefizienten π-Alkenen und eine damit verbundene Charge-Transfer-Komplexbildung untersucht. In die Auswertung wurden auch ab-initio-Berechnungen einbezogen, die mit den experimentellen Ergebnissen (u.a. NMR, IR, Röntgeneinkristallstrukturanalyse) vergleichend diskutiert wurden.