Silicone polymers and network-materials have proven extremely useful in a variety of applications owing to their superb properties when compared to carbon-based polymers. Polysiloxanes containing functional groups other than simple alkyl moieties have allowed for further manipulations of pendant groups along the polymer backbone leading to a greater range of possible chemical transformations, as well as changes in physical/interfacial properties. One aspect of functional polymers that has yet to be explored with respect to primarily silicone-based systems is that of stimuli-responsive materials. In order for this unique application to work, silicones must be functionalized with a group or groups that can influence the polymer’s properties based on that group’s response to specific external stimuli. Boronic acids represent one such group, wherein the most common stimuli used to affect changes in ionization state and solubility are pH and diol-binding. Boronic acids are also capable of forming weak hydrogen-bonded dimers with other boronic acids, and dynamic covalent bonds with Lewis bases. It is proposed that the covalent attachment of boronic acids and their derivatives onto silicones could lead to stimuli-responsive silicone materials. Herein, the synthesis of silicone-boronic acids and their protected boronic esters is described. The simple two-step method involving boronic acid protection followed by hydrosilylation has led to a variety of molecules differing in molecular weight and three-dimensional geometry through the use of commercially available hydride-functional silicones. Initial results regarding saccharide binding selectivity and the impacts on silicone solubility are provided. The unique interfacial behaviour of silicone-boronic esters and their propensity to form self-assembled, crosslinked films at an air/water interface are also reported. Using several different diol protecting groups and a variety of aqueous sub-phases, the mechanism for changes in physical properties as well as crosslinking were revealed. Finally, the production of new thermoplastic silicone elastomers from silicone-boronic esters and amine-containing molecules is discussed. The Lewis acid/Lewis base complexation that occurs between nitrogen and boron can provide enough strength to produce robust, yet recyclable, silicone elastomers without the use of catalyst or solvent. Elastomers can be easily dissolved and reformed through the introduction and removal of a mono-functional Lewis base. The impact of crosslink density, controlled by the quantities and molecular weights of each polymer component used, on physical characteristics is reported. / Thesis / Doctor of Philosophy (PhD)

In der vorliegenden Arbeit wurde die Reaktivität von außergewöhnlich Lewis-sauren festen Katalysatoren: Aluminiumchlorofluorid (ACF) und high-surface Aluminiumfluorid (HS-AlF3) unter moderaten Reaktionsbedingungen untersucht. Die Katalysatoren konnten erfolgreich für die Aktivierung von C-F- und C-Cl-Bindungen (CH3F, CH2F2, CHF3, CH3Cl, CH2Cl2 und ClCH2-CH2Cl) in der Anwesenheit von Silanen eingesetzt werden. Bei Hydrodehalogenierungsreaktionen in Batch-Reaktoren (NMR-Röhrchen) wurden höhere Umsätze mit ACF als mit HS-AlF3 erreicht. Dieser Befund lässt sich mit sterischen Effekten erklären. Die kleineren Mikroporen des ACFs sind besser geeignet für die Aktivierung von kleinen Molekülen als die Mesoporen von HS-AlF3. Die sterischen Aspekte wurden mit Hilfe von FTIR-Experimenten ausführlich untersucht. Unterschiedliche Beobachtungen wurden gemacht, wenn ein zusätzliches Substrat anwesend war, z.B. C6D6 zur Synthese des Friedel-Crafts-Produktes. HS-AlF3 führte zu höheren Umsätzen bei fluorierten Substraten, während bei chlorierten Substraten ACF sich als der bessere Katalysator erwies. Die Herausforderung bestand darin, die Katalysatoren unter sehr anspruchsvollen Bedingungen wie kurze Kontaktzeiten im Flow-Reaktor zu testen. Die Aktivierung von fluorierten und chlorierten Substraten war möglich. Der Unterschied in der Reaktivität der Katalysatoren war nicht so ausgeprägt wie bei Reaktionen in Batch-Reaktor. Aufgrund der kurzen Kontaktzeiten können nur kurze Diffusionswege ausgenutzt werden, was zu ähnlichen Reaktivitäten führt. Mit Hilfe von Experimenten im Flow-Reaktor als auch MAS NMR-, EA-, NH3-TPD-, XRD-, FTIR-, PulseTA®-Experimenten konnte der Mechanismus aufgeklärt werden. Der Katalysator muss zunächst mit Silan gesättigt werden, welcher eine Schutzfunktion hat und die Fluorierung des Katalysators unterdrückt. Ausserdem konnten die Katalysatoren erfolgreich für die Hydrosilylierungsreaktionen und weiterhin zu Deoxygenierung von Ketonen eingesetzt werden. / This thesis investigated the reactivity of extraordinarily lewis-acidic solid catalysts under moderate reaction conditions: aluminium chorofluoride (ACF) und high-surface aluminium fluoride (HS-AlF3).The catalysts could be successfully applied in the activation of C-F and C-Cl bonds (CH3F, CH2F2, CHF3, CH3Cl, CH2Cl2 und ClCH2-CH2Cl) in the presence of silanes. Higher conversions were reached for hydrodehalogenation batch-reactions (NMR tubes) with ACF than for HS-AlF3. This finding can be explained by steric effects. The smaller micropores of ACF are better suited for the activation of small molecules than mesopores of HS-AlF3. Steric aspects were examined extensively by FTIR experiments. Different observations were made in cases where an additional substrate was present, for example C6D6 led to the formation of the Friedel-Crafts product. In this case HS-AlF3 showed higher conversions of fluorinated substrates, while for chlorinated substrates ACF was found to be more efficient. The challenge of this work was to optimize conditions under which the catalysts are not only efficient in batch reactors with long contact times but also under short contact times in flow reactors. Experiments demonstrated that fluorinated and chlorinated substrates can be activated under such conditions. The difference in reactivity of both catalysts was not as extensive as for reactions in a batch reactor. Short contact times require short diffusion pathways, thus more similar reactivities. Experiments in a flow reactor, supported by further analyses by MAS NMR, EA, NH3-TPD, XRD, FTIR and PulseTA® led to the elucidation of the catalytic mechanism. The catalyst must first be saturated by silane, which inhibits and protects the catalyst from fluorination. The catalysts were also successfully applied for hydrosilylation reactions and deoxy-genation of ketones.

No presente trabalho foram realizados estudos de regiosseletividade das reações de abertura de 2,3-epoxipropanoato de etila (1) utilizando diferentes nucleófilos de selênio e algumas dessas reações foram desenvolvidas com a adição do ácido de Lewis trifluoreto de boro dietil éter (BF3·Et2O). A abertura desse oxirano ao utilizar os nucleófilos MeSeMgCl e MeSeLi-BF3·Et2O ocorreu seletivamente no Carbono C-3 formando o composto de interesse (3-metilseleno 2-hidroxipropanoato de etila), já ao utilizar MeSeLi (em ausência ácido de Lewis) a abertura procedeu-se seletivamente no carbono C-2 formando 2-metilseleno-3-hidroxipropanoato de etila. A reação com o nucleófilo (Na[PhSeB(OEt)3]) levou à mistura desses regioisômeros. O ácido de Lewis BF3·Et2O em presença do selenolato levou a inversão de regiosseletividade da reação de abertura do epóxido 1 e a razão estequiométrica de BF3·Et2O adicionada ao meio reacional correspondeu, proporcionalmente, a porcentagem de obtenção do produto de abertura em C-3 (Tabela 1). Os resultados obtidos sugeriram que BF3·Et2O altera a nucleofilicidade do selenolato (RMN de 77Se) a partir de uma interação selênio-boro. A formação da ligação Se-B pode ocorrer com ou sem a liberação de fluoreto e esse mecanismo foi investigado por meio do emprego de uma sonda fluorescente seletiva desse haleto. O mecanismo dessas reações também foram investigados por cálculos teóricos, os quais mostram-se totalmente coerentes com os resultados experimentais. / In the present work was studied reactions of regioselective opening of 2,3-epoxyester using different selenolatos and some of this reactions were developed by adding Lewis acid BF3·Et2O. The opening reaction of this oxirane using the nucleofilms MeSeMgCl and MeSeLi-BF3·Et2O occurred selectively in carbon C-3 forming the compound of interest (ethyl 3-methylselene 2-hydroxypropanoato of ethyl), already using MeSeLi (in Lewis acid absence) the reaction was selectively on C-2 carbon to form ethyl 2-methylselene-3-hydroxypropanoate. The reaction with the nucleophile (Na[PhSeB(OEt)3]) formed a mixing of these regioisomers. The Lewis acid BF3·Et2O in presence of selenolate reverses the regioselectivity of opening epoxide (1) reaction and the stoichiometric value of BF3·Et2O added in the reaction corresponded proportionally with the percentage of C-3 product (Table 1). The results suggested that BF3·Et2O alters the nucleophilicity of selenolate (77Se NMR) from a selenium-boron interaction. Se-B bond formation may occur with or without fluoride release and this mechanism has been investigated by the use of a selective fluorescent probe of that halide. The mechanism of these reactions was also investigated by theoretical calculations, which are fully consistent with the experimental results.

Kationische Zinkkomplexe werden als katalytisch aktive Spezies zahlreicher Lewis-Säure-katalysierter Reaktionen vermutet. Die diesen Katalysereaktionen zugrunde liegenden Mechanismen sind jedoch unzureichend verstanden. Das Ziel der vorliegenden Dissertation ist die Synthese strukturell definierter kationischer Zinkorganyle als Modellverbindungen für katalytisch aktive Spezies zinkkatalysierter Reaktionen. Der Fokus liegt auf Zinkverbindungen mit Cyclopentadienylliganden (Cp-Liganden), da Cp-Liganden ungewöhnliche Bindungssituationen stabilisieren können. In dieser Arbeit wird die erfolgreiche Synthese und Charakterisierung mehrerer kationischer Zinkkomplexe des Pentamethylcyclopentadienylliganden (Cp*-Liganden) beschrieben. Ein gemeinsames Strukturmerkmal dieser Komplexe besteht in der Koordination des Zinkatoms durch einen Cp*-Liganden sowie durch Neutralliganden. Die hohe Elektrophilie dieser Verbindungen ließ sich durch Reaktivitätsuntersuchungen belegen. Ein weiterer Ansatz zur Stabilisierung kationischer Zinkverbindungen besteht in der Verwendung funktionalisierter Cp-Liganden, die über eine neutrale Donorgruppe in einer Seitenkette verfügen. Es wurden neutrale und kationische Zinkkomplexe amino- sowie thiofunktionalisierter Cp-Liganden synthetisiert und charakterisiert. Für die kationischen Komplexe konnte eine intramolekulare Stabilisierung des Zinkatoms durch die Donorgruppe nachgewiesen werden. Einige der in dieser Arbeit beschriebenen Zinkkomplexe wurden als Präkatalysatoren intermolekularer Hydroaminierungsreaktionen eingesetzt, wobei teilweise hohe katalytische Aktivitäten erreicht wurden. Untersuchungen zum Mechanismus der Katalysereaktionen zeigten, dass eine hohe Elektrophilie des Zinkzentralatoms für eine effektive Katalyse entscheidend ist. / Cationic zinc complexes are assumed to act as catalytically active species in various Lewis acid catalyzed reactions. However, the mechanisms of these reactions are poorly understood. The aim of this dissertation is to synthesize structurally well-defined cationic zinc organyls as model compounds for catalytically active species in zinc catalyzed reactions. The thesis focuses on zinc complexes bearing cyclopentadienyl (Cp) ligands, as Cp ligands can stabilize unusual bonding situations. The successful synthesis and characterization of several cationic zinc complexes with pentamethylcyclopentadienyl (Cp*) ligands is described. As a common structural feature of these complexes, the zinc center is coordinated by a Cp* ligand and additional neutral ligands. The highly electrophilic character of these compounds was proven in reactivity studies. A further approach to stabilize cationic zinc compounds is to apply functionalized Cp ligands featuring a donor group in a side chain. Neutral and cationic zinc complexes bearing amino- and thio-functionalized Cp ligands were synthesized and characterized. The cationic donor-functionalized complexes were shown to exhibit an intramolecular stabilization of the zinc atoms by the donor groups. Several zinc complexes which are described in this thesis were examined as precatalysts in intermolecular hydroamination reactions. In some cases high catalytic activities were achieved. Studies on the mechanism of the catalysis reactions revealed that the presence of a highly electrophilic zinc center is crucial for good catalytic performance.

O objetivo deste trabalho foi investigar o uso de NbCl5 como ácido de Lewis em reações de cicloadição. Foram estudadas algumas reações de cicloadição [2 + 2], Reações de Diels-Alder entre enonas e ciclopentadieno e Reações da aza-Diels-Alder com bases de Schiff, avaliando o efeito da temperatura e da concentração molar do NbCl5. A comparação dos rendimentos, dos produtos formados e do tempo de reação com NbCl5, também foi um ponto fundamental da pesquisa. As reações de cicloadição [2 + 2] foram realizadas entre ésteres propiólicos e diferentes tipos de alcenos (éteres enólicos de silício e alcenos alifáticos). Nas reações com os éteres enólicos de silício não foi verificada a formação dos respectivos adutos de ciclobuteno, pois o NbCl5 promove a quebra da ligação oxigênio-silício, não levando à formação do produto desejado. Nas reações com os alcenos alifáticos foi possível obter o respectivo aduto, porém com rendimentos menores que com outros ácidos de Lewis. Quanto às reações de Diels-Alder, foram investigadas as reações entre diferentes ciclo-enonas (dienófilos de baixa reatividade) com ciclopentadieno (dieno) na presença de NbCl5. Os resultados obtidos indicam que o NbCl5 é um bom ácido de Lewis para ativar reações de Diels-Alder, apresentando tempos reacionais menores e alta diastereosseletividade a temperaturas mais baixas, quando comparado com outros ácidos de Lewis. A possibilidade de efetuar reações do Diels-Alder a -78 0C é um dos aspectos de destaque neste trabalho, pois, além de demonstrar a forte ativação do sistema enona exercido pelo NbCl5, possibilita obter produtos com alta seletividade. Paralelamente aos estudos de sistemática reacional foram realizados estudos de elucidação estrutural completa de alguns dos compostos sintetizados, utilizando-se diversas técnicas de RMN (uni e bidimensionais), e o auxílio de cálculos teóricos. Nas reações de aza-Diels-Alder entre bases de Schiff e di-hidropirano, o NbCl5 se mostrou um ótimo catalisador para a síntese de derivados de piranoquinolinas. Estes derivados formam uma importante classe de produtos naturais que apresentam ampla atividade biológica. As reações foram conduzidas com baixas concentrações de nióbio e em tempos relativamente curtos, obtendo-se rendimentos variando de 72 a 96 %. Outro fator a se destacar é a alta diastereosseletividade encontrada nestas reações. Também foram realizados estudos da atividade tripanocida dos derivados de piranoquinolinas preparados através das reações de aza-Diels-Alder catalisadas por NbCl5. / The aim of this work was to investigate the use of NbCl5 as Lewis acid in cycloadittion reactions. We have studied [2 + 2] cycloaddition reactions, Diels-Alder reactions between enonas and cyclopentadiene and aza-Diels-Alder reactions with Schiff bases. The effects of the temperature and of the molar concentration of NbCl5 were also evaluated. Comparasion of reaction yields, obtained products as well as reation time with NbCl5 were also a key point on this work. The [2 + 2] cycloaddition reactions were performed using propiolic ester and different types of alkenes (silyl enol ethers and aliphatic alkenes). In the reactions with silyl enol ethers, formation of the corresponding cyclobutene aductts was not verified, since the NbCl5 promotes the rupture of oxygen-silicon bonds, and the desired product is not obtained. In the reactions with aliphatic alkenes, it was possible to obtain the adduct, however, in lower yields as compared to those obtained with others Lewis acids. Regarding the Diels-Alder reactions, we have investigated reactions using different cycloenones (dienophiles of low reactivity) with cyclopentadiene (diene) with NbCl5. The obtained results indicate that NbCl5 is a good Lewis acid to activate these Diels-Alder reactions, resulting in shorter reaction times and higher diastereoselectivity at lower temperatures than other Lewis acids. The possibility of carrying out Diels-Alder reaction at -78 0C is another remarkable aspect of this work. Besides demonstrating the strong activation of the enone system by NbCl5, it opens the possibility of obtaining high stereoselectivity. We have also performed studies of complete structural elucidation of some compounds by using different NMR techniques (uni and bidimensional), with the help of theoretical calculations. In the aza-Diels-Alder reactions between Schiff bases and dihydropyran, the NbCl5 was an excellent catalyst for the synthesis of pyranoquinoline derivatives. These derivatives are an importante class of natural products that exhibit wide range of biological activity. The reactions were carried out at low concentration of niobium and in relatively short times, resulting in yields varying from 72 to 96 %. Another aspect that should be remarked is the high diastereoselectivity found in these reactions. We have also carried out studies of tripanocydal activity of pyranoquinoline derivatives prepared through aza-Diels-Alder reactions catalyzed by NbCl5.

Après avoir pu développer une nouvelle méthode de déracémisation de biphényles iodés permettant, pour la première fois, d’influencer la diastéréosélectivité de la réaction de piégeage du réactif d’Andersen par dédoublement cinétique, nous avons obtenu de nombreuses informations et développé des méthodes permettant de lever différents verrous synthétiques pour accéder à des paires de Lewis frustrées basées sur le squelette biphénylique et portant simultanément ou non un groupement acide de Lewis et une base de Lewis. Par la fonctionnalisation de ces biphényles, nous avons pu accéder à de nouvelles biphénylphosphines énantiopures pouvant jouer le rôle de base de Lewis dans le domaine des paires de Lewis frustrées (FLP) mais aussi d'organocatalyseur nucléophile ou de ligand pour la catalyse organométallique. Différentes méthodes ont ensuite été (re)développées, pour accéder à des acides de Lewis : d'une part la synthèse de boranes par fonctionnalisation de sels d'organotrifluoroborate de potassium comme précurseurs polyvalents d’acides de Lewis chiraux et énantiopurs et d'autre part l’utilisation de silanes électrophiles. L’ensemble des informations et méthodes découlant de ce travail pourront être appliquées à la synthèse de molécules ambiphiles, nouvelles paires de Lewis atropo-frustrées. / After having developed a new method of deracemization of iodinated biphenyls allowing us, for the first time, to influence the diastereoselectivity of the trapping by the Andersen reagent, we have turned our attention to the use of this molecular scaffold in the design of new frustrated Lewis pairs bearing either a Lewis acidic group, a Lewis basic group or simultaneously both groups. We first accessed a new series of enantiopure biphenylphosphines that can find applications as Lewis base (including the field of frustrated Lewis pairs), as nucleophilic organocatalyst or as ligand in organometallic catalysis. Then, various methods were (re)developed to access chiral biphenyl-based Lewis acids: firstly, the functionalization of biphenyltrifluoroborate salts as chiral and enantiopure borane precursors and secondly the use of electrophilic silanes. The whole information and methodologies developed herein may be applied to the synthesis of new ambiphilic compounds as new atropo-frustrated Lewis pairs.

Chapter one; “(-)-Rhazinilam and (-)-Rhazinal: Alkaloids with Anti-mitotic Properties Derived from Kopsia teoi”, provides the background information behind the motives that initiated this research project. The plant alkaloid (-)-rhazinilam [(-)-1] and its naturally-occurring derivative (-)-rhazinal [(-)-13] both exhibit potent anti-mitotic activities and, as such, are interesting targets for total synthesis. Chapter one is a review of the literature regarding these two compounds and discusses the occurrence, proposed biosynthetic origins, structural elucidation and biological activites of compound (-)-1 and that of its analogues including alkaloid (-)-13. Previous total syntheses of these two compounds are then examined, concluding with the only reported total synthesis of compound (-)-13. Developed within the Banwell research group, this total synthesis produced the racemic modification of alkaloid (-)-13 due to a lack of any stereocontrol in the key intramolecular Michael addition step. This unprecedented key step, involving cyclisation of the C2 of pyrrole onto an N-tethered and ?,?-disubstituted acrylate to produce a quaternary-carbon stereogenic centre, would be of greatly enhanced utility if it could be achieved in a catalytic-enantioselective fashion. The realisation of this goal is the central aim of the research conducted within this thesis. ¶ Chapter two; “Investigating Asymmetric Induction in the Intramolecular Michael Addition of pyrrole to N-Tethered Acrylates and Related Species”, introduces the model study used to direct research towards achieving the goal of asymmetric induction in the title process. The model is a somewhat simplified version of the original process used in the total synthesis of compound (-)-13 involving cyclisation of the C2 of pyrrole onto an N-tethered and ?-monosubstituted Michael acceptor, to produce a tertiary-carbon stereogenic centre. This simplification allows the rapid synthesis of a broad range of potential substrates for use in the title process, thus enabling the investigation of various different approaches to inducing asymmetry therein. High levels of asymmetric induction are observed with the use of chiral substrates or catalysts, facilitating the synthesis of both 6- and 7-membered rings annulated to pyrrole with construction of the relevant tertiary-carbon stereogenic centre in enantio-enriched form. For the reactions producing a 6-membered ring annulated to pyrrole, unambiguous proof of the absolute sense of asymmetric induction observed in the intramolecular Michael addition event is established using a chemical correlation study involving elaboration of a key indolizine-type cyclisation product, to the plant alkaloid of known absolute stereochemistry, (-)-tashiromine [(-)-75]. For the reaction producing a 7-membered ring annulated to pyrrole, the same information is obtained via X-ray crystallographic analyses of a dibrominated derivative of a key pyrroloazepine-type cyclisation product. ¶ Chapter three “An Enantioselective Total Synthesis of the Alkaloid (-)-Rhazinal: An Anti-mitotic Agent Isolated from Kopsia teoi.”, focuses on the application of methodology developed in the previous chapter, to the original goal of inducing asymmetry in the intramolecular Michael addition reaction, involving cyclisation of the C2 of pyrrole onto an N-tethered and ?,?-disubstituted acrylate to produce a quaternary-carbon stereogenic centre. This is ultimately achieved in a catalytic-enantioselective fashion, resulting in the first such total synthesis of the anti-mitotic alkaloid (-)-rhazinal [(-)-13]. ¶ Chapter four “Extending the Reaction Manifold to the Syntheses of Related Natural Products: A Formal Total Synthesis of (+)-Aspidospermidine and Syntheses of (-)-Rhazinilam and (-)-Leuconolam from (-)-Rhazinal”, describes three extensions to the reaction manifold used in the enantioselective total synthesis of alkaloid (-)-13: The acquisition in an enantioselective manner, of an intermediate previously obtained in racemic form, en route to the racemic modification of the natural product (±)-aspidospermidine [(±)-134], constitutes a formal and enantioselective total synthesis of (+)-aspidospermidine [(+)-134]. The direct deformylation of (-)-rhazinal [(-)-13], is carried out, to produce the parent alkaloid (-)-rhazinilam [(-)-1]. The pyrrole ring present in (-)-rhazinilam [(-)-1] is oxidised, to produce the related natural product (-)-Leuconolam [(-)-12] which has not, hitherto, been prepared by total synthesis. ¶Chapter five contains the experimental procedures and characterisation data associated with compounds described in chapters two to four.

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

The thesis describes the realization of an asymmetric [2,3]-sigmatropic rearrangement of achiral allylic amines. It is divided into two parts; the first part deals with the development of a Lewis acid-mediated [2,3]-sigmatropic rearrangement and the second the asymmetric version thereof. Quaternization of an -amino amide with various Lewis acids established BBr3 and BF3 to be the most appropriate ones. Various allylic amines were subsequently rearranged into the corresponding homoallylic amines in good to excellent syn-diastereoselectivities, revealing the endo-transition state to be the preferred pathway. The structures of the intermediate Lewis acid-amine complexes were confirmed by NMR spectroscopy studies and DFT calculations. Based on this investigation a chiral diazaborolidine was chosen as Lewis acid and was shown to efficiently promote the asymmetric [2,3]-sigmatropic rearrangement furnishing homoallylic amines in good yields and excellent enantiomeric excesses. In contrast to the achiral rearrangement mediated by BBr3 and BF3, the asymmetric version gave the opposite major diastereomer, revealing a preference for the exo-transition state in the asymmetric rearrangement. To account for the observed selectivities, a kinetic and thermodynamic pathway was presented. On the basis of a deuterium exchange experiment on a rearranged Lewis acid-amine complex and an NMR spectroscopic investigation, the kinetic pathway was shown to be favored. / QC 20100927