Immobilisierung von Katalysatoren für die asymmetrische Transferhydrierung von Arylketonen

Mithilfe der asymmetrischen Transferhydrierung (ATH) lassen sich chirale Substanzen katalytisch auf einfachem, sicherem und umweltfreundlichem Wege herstellen. Übergangsmetallkomplexe bestehend aus Rhodium, Ruthenium oder Iridium und einem chiralen Liganden werden dabei in den meisten Fällen als Katalysatoren eingesetzt. Die hohen Reinheitsanforderungen an die entstehenden Produkte, insbesondere wenn sie als Zwischenstufen für die Produktion von Pharmaka Verwendung finden, sowie die hohen Kosten für die Katalysatoren selbst erfordern deren vollständige Abtrennung aus dem Reaktionsgemisch und machen eine Mehrfachverwendung wünschenswert. Mit dem Ziel einer Effizienzsteigerung der ATH sollten in dieser Arbeit quantitativ abtrennbare sowie vielfach wiederverwendbare Katalysatoren durch Immobilisierung hergestellt werden. Zu diesem Zweck wurden Rhodium- und Rutheniumkomplexe mit funktionellen Gruppen versehen, über die eine Anbindung an oberflächenfunktionalisierte Polymerträger möglich war. Funktionsfähige ATH-Katalysatoren konnten so aus einem modifizierten Rhodium(III)-p-toluensulfonyl-1,2-diphenylethylendiamin-Komplex mit fixierter η5-Tetramethylcyclopentadienyl-Einheit, der an Polymerchips und Polymerpartikeln immobilisiert wurde, gewonnen werden. Die geträgerten Katalysatoren wurden in der asymmetrischen Transferhydrierung von Arylketonen in wässriger Natriumformiatlösung eingesetzt und wiesen eine hohe Enantioselektivität sowie die Möglichkeit zur Wiederverwendung auf. Das Katalysatorsystem wurde eingehend im Hinblick auf optimale Reaktionsbedingungen untersucht, wobei insbesondere der pH-Wert als entscheidender Faktor für katalytische Aktivität und Wiederverwendbarkeit identifiziert wurde; eine deutliche Verbesserung der Leistungsfähigkeit wurde durch die Wahl eines sauren Reaktionsmediums erzielt. Kinetische Untersuchungen zeigten, dass die enantioselektive Reaktion von Acetophenon zu Phenylethanol sowohl unter basischen als auch unter sauren Bedingungen durch ein einfaches Modell zweiter Ordnung beschrieben werden kann. Der in Gegenwart von Säure deutliche Anstieg der Reaktionsgeschwindigkeit bzw. die Änderung der Aktivierungsparameter ohne Verlust an Enantioselektivität könnte jedoch auf einen veränderten Mechanismus hindeuten. Das Katalysatorsystem erwies sich bei einfacher Handhabung als effizient und robust. Durch Upscaling-Experimente konnte zudem gezeigt werden, dass es großes Potenzial für die technische Anwendung in einer ökologisch und ökonomisch effizienten Produktion enantiomerenreiner Komponenten birgt.Asymmetric transfer hydrogenation (ATH) is an operationally simple, safe, and environmentally benign catalytic method for the generation of chiral compounds. The catalysts used are generally transition metal complexes based on chiral ligands and either rhodium, ruthenium, or iridium. The high purity requirements on chiral substances, especially when they are used as intermediates in the production of pharmaceuticals, as well as the high catalyst costs make a complete catalyst separation essential and the reuse of the catalysts desirable. Thus, in order to increase the overall efficiency of ATH processes, in this thesis quantitatively separable and highly reusable catalysts were established via immobilization. New strategies were developed to synthesize linker-containing rhodium and ruthenium complexes, which could then be attached to surface-functionalized polymer supports. Functional heterogeneous ATH catalysts were generated from a modified version of a rhodium(III)-p-toluenesulfonyl-1,2-diphenylethylenediamine complex with tethered cyclopentadienyl unit immobilized on polymer chips and polymer beads. The supported catalysts were applied in the asymmetric transfer hydrogenation of aryl ketones in an aqueous solution of sodium formate, and excellent enantioselectivity and reusability were achieved. In determining the optimal reaction conditions, the pH of the solution was found to play a particularly decisive role in determining the activity and reusability of the catalysts, and a significant improvement was achieved when the reaction was run in an acidic medium. The results of kinetic experiments indicated that a second-order model describes the enantioselective conversion of acetophenone to phenylethanol under both basic and acidic conditions. However, significantly different rates and activation parameters suggested different mechanisms; acid may accelerate the reaction by changing the mode of the proton transfer. The catalytic system proved very simple to use and robust, and through upscaling of the reaction it was shown that there is a high potential for technical application to the ecologically and economically rational production of enantioenriched building blocks

Optimization of GaAs solar cell performance and growth efficiency at MOVPE growth rates of 100 μm/h

III-V devices outperform all other solar cells in terms of efficiency. However, the manufacturing of these cells is expensive and prevents their use in a number of applications, which would benefit from the high efficiency. A major contribution to the cost is the metal-organic vapor phase epitaxy process for the III-V compounds. Increasing growth rates and, hence, machine throughput, as well as the growth efficiency, are important steps toward reducing the cost of III-V solar cells. We demonstrate the growth of GaAs solar cells at extremely high growth rates of 100 μm/h and achieve a VOC of 1.028 V, a base diffusion length of 6.5 μm, and an efficiency of 23.6% under AM1.5g conditions. Furthermore, we show reactor adjustments leading to growth rates up to 140 μm/h and reach conditions where more than half of the Ga from the precursor is incorporated into the solar cell layers. The results are encouraging and demonstrate a pathway toward lower cost III-V solar cell manufacturing

Compartmentalization of Incompatible Catalytic Transformations for Tandem Catalysis

In Nature, incompatible catalytic transformations are being carried out simultaneously through compartmentalization that allows for the combination of incompatible catalysts in tandem reactions. Herein, we take the compartmentalization concept to the synthetic realm and present an approach that allows two incompatible transition metal catalyzed transformations to proceed in one pot in tandem. The key is the site isolation of both catalysts through compartmentalization using a core–shell micellar support in an aqueous environment. The support is based on amphiphilic triblock copolymers of poly­(2-oxazoline)­s with orthogonal functional groups on the side chain that can be used to cross-link covalently the micelle and to conjugate two metal catalysts in different domains of the micelle. The micelle core and shell provide different microenvironments for the transformations: Co-catalyzed hydration of an alkyne proceeds in the hydrophobic core, while the Rh-catalyzed asymmetric transfer hydrogenation of the intermediate ketone into a chiral alcohol occurs in the hydrophilic shell

Compartmentalization of Incompatible Catalytic Transformations for Tandem Catalysis

In Nature, incompatible catalytic transformations are being carried out simultaneously through compartmentalization that allows for the combination of incompatible catalysts in tandem reactions. Herein, we take the compartmentalization concept to the synthetic realm and present an approach that allows two incompatible transition metal catalyzed transformations to proceed in one pot in tandem. The key is the site isolation of both catalysts through compartmentalization using a core–shell micellar support in an aqueous environment. The support is based on amphiphilic triblock copolymers of poly­(2-oxazoline)­s with orthogonal functional groups on the side chain that can be used to cross-link covalently the micelle and to conjugate two metal catalysts in different domains of the micelle. The micelle core and shell provide different microenvironments for the transformations: Co-catalyzed hydration of an alkyne proceeds in the hydrophobic core, while the Rh-catalyzed asymmetric transfer hydrogenation of the intermediate ketone into a chiral alcohol occurs in the hydrophilic shell

Towards Recyclable NAD(P)H Regeneration Catalysts

Rh(III)-TsDPEN, an immobilized analog of the well-known [Cp*Rh(bpy)(H2O)]2+ was evaluated as a heterogeneous, recyclable regeneration catalyst for reduced oxidoreductase cofactors [NAD(P)H]. Repeated use of this catalyst was established and the catalytic properties were initially investigated. Apparently, Rh(III)-TsDPEN is prone to severe diffusion limitations, necessitating further developments. Overall, a promising concept for chemoenzymatic redox catalysis is proposed, which may overcome some of the current limitations such as catalyst cost and incompatibility of Rh with some biocatalysts

Towards recyclable NAD(P)H regeneration catalysts

Rh(III)-TsDPEN, an immobilized analog of the well-known [Cp*Rh(bpy)(H2O)]2+ was evaluated as a heterogeneous, recyclable regeneration catalyst for reduced oxidoreductase cofactors [NAD(P)H]. Repeated use of this catalyst was established and the catalytic properties were initially investigated. Apparently, Rh(III)-TsDPEN is prone to severe diffusion limitations, necessitating further developments. Overall, a promising concept for chemoenzymatic redox catalysis is proposed, which may overcome some of the current limitations such as catalyst cost and incompatibility of Rh with some biocatalysts.BiotechnologyApplied Science

Language, creoles, varieties: From emergence to transmission

This book offers a selection of papers dealing with second language acquisition, foreign language teaching and creole linguistics inspired by the scientific legacy of Mauritian-born scholar Georges Daniel Véronique (Port-Louis, 1948). An important part of the book is devoted to the description of learner varieties with a focus on sociolinguistic factors, such as the learner situation – from asylum seekers to Erasmus students –, the degree of familiarity with the target language – having or not previous knowledge about a genetically related language –, the degree of literacy, and the type of instruction. Linguistic complexity, case marking, the use of self-positioning pronouns, verbal morphology and aspectual values are among the linguistic phenomena analyzed by the authors having contributed to this part of the volume. Another part of this volume deals with language didactics and addresses the questions of whether manipulating specific constructions from a usage-based perspective and a focus-on-form approach do indeed aid beginner learners to acquire complex forms in L2 German and nominal forms in L2 Polish, respectively. It also explores how some educational policies in Sweden have affected both the offer of French as a Foreign Language and its demand by students. The contributions to creole studies present diachronic analyses targeting the /z/ plural marking in Réunion creole, Fa d’Ambô and spoken French, and a set of NPs found in two speeches pronounced in 1835 on the island of Agaléga by a coconut oil producer whose features are similar to Mauritian creole. Linguistic, social and historical factors are at the center of these contributions

Language, creoles, varieties: From emergence to transmission

This book offers a selection of papers dealing with second language acquisition, foreign language teaching and creole linguistics inspired by the scientific legacy of Mauritian-born scholar Georges Daniel Véronique (Port-Louis, 1948). An important part of the book is devoted to the description of learner varieties with a focus on sociolinguistic factors, such as the learner situation – from asylum seekers to Erasmus students –, the degree of familiarity with the target language – having or not previous knowledge about a genetically related language –, the degree of literacy, and the type of instruction. Linguistic complexity, case marking, the use of self-positioning pronouns, verbal morphology and aspectual values are among the linguistic phenomena analyzed by the authors having contributed to this part of the volume. Another part of this volume deals with language didactics and addresses the questions of whether manipulating specific constructions from a usage-based perspective and a focus-on-form approach do indeed aid beginner learners to acquire complex forms in L2 German and nominal forms in L2 Polish, respectively. It also explores how some educational policies in Sweden have affected both the offer of French as a Foreign Language and its demand by students. The contributions to creole studies present diachronic analyses targeting the /z/ plural marking in Réunion creole, Fa d’Ambô and spoken French, and a set of NPs found in two speeches pronounced in 1835 on the island of Agaléga by a coconut oil producer whose features are similar to Mauritian creole. Linguistic, social and historical factors are at the center of these contributions

Language, creoles, varieties: From emergence to transmission

This book offers a selection of papers dealing with second language acquisition, foreign language teaching and creole linguistics inspired by the scientific legacy of Mauritian-born scholar Georges Daniel Véronique (Port-Louis, 1948). An important part of the book is devoted to the description of learner varieties with a focus on sociolinguistic factors, such as the learner situation – from asylum seekers to Erasmus students –, the degree of familiarity with the target language – having or not previous knowledge about a genetically related language –, the degree of literacy, and the type of instruction. Linguistic complexity, case marking, the use of self-positioning pronouns, verbal morphology and aspectual values are among the linguistic phenomena analyzed by the authors having contributed to this part of the volume. Another part of this volume deals with language didactics and addresses the questions of whether manipulating specific constructions from a usage-based perspective and a focus-on-form approach do indeed aid beginner learners to acquire complex forms in L2 German and nominal forms in L2 Polish, respectively. It also explores how some educational policies in Sweden have affected both the offer of French as a Foreign Language and its demand by students. The contributions to creole studies present diachronic analyses targeting the /z/ plural marking in Réunion creole, Fa d’Ambô and spoken French, and a set of NPs found in two speeches pronounced in 1835 on the island of Agaléga by a coconut oil producer whose features are similar to Mauritian creole. Linguistic, social and historical factors are at the center of these contributions