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Trisoxazolines: Synthesis and Application in Enantioselective Palladium- and Copper-Catalysed Reactions

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Trisoxazolines: Synthesis and Application in Enantioselective Palladium- and Copper-Catalysed Reactions Trisoxazolines: Synthesis and Application in enantioselective Palladium- and Copper-catalysed Reactions INAUGURAL-DISSERTATION zur Erlangung der Doktorwürde der Naturwissenschaftlich-Mathematischen Gesamtfakultät der Ruprecht-Karls-Universität Heidelberg vorgelegt von Diplom-Chemikerin Carole Foltz-César aus Beinheim, Frankreich durchgeführt am Anorganisch-Chemischen Institut der Ruprecht der Ruprecht-Karls-Universität Heidelberg und am Institut de Chimie der Université Louis Pasteur de Strasbourg Tag der mündlichen Prüfung: 30. November 2007 Trisoxazolines: Synthesis and Application in enantioselective Palladium- and Copper-catalysed Reactions Gutachter (Heidelberg): PD Dr. Markus Enders Prof. Dr. Günter Helmchen Gutachter (Strasbourg): Prof. Kilian Muñiz Acknowledgements Am ertser Stelle möchte ich Herrn Prof. L. H. Gade für die wertvolle Unterstützung und Diskussionsbereitschaft danken. Je remercie également Stéphane Bellemin-Laponnaz pour la confiance qu’il m’a accordée tout au long de ce travail de thèse. Malgré la distance, il a toujours su rester disponible pour partager son expérience et ses conseils. Herrn Prof. H. Wadepohl danke ich für die Durchführung und Auswertung der Röntgenstrukturanalysen und seine Diskussionsbereitschaft. Ich danke auch Herrn Dr. M. Enders für seine stete Hilfsbereitschaft. I am grateful to the Université Franco-Allemande and the European Doctoral College for funding. Irene Meyer und Stefan Schweizer möchte ich für ihre Mitarbeit im Rahmen eines Forschungspraktikums danken. Ein besonderer Dank geht an Beate, Dana und Lorenz für die grosse Hilfsbereitschaft und gute Zusammenarbeit. I would like to thank the members of Lab 2.11, to whom I am indebted for a friendly environment within which to work: Adeline, Felix and Nathanaëlle. I would also like to express my thanks to Ben for his endless help. Für die schnelle Dursicht des Manuskriptes danke ich Susanne. Ein grosser Dank gilt den ehemaligen und gegenwärtigen Kollegen des Arbeitskreises für die gute Zusammenarbeit und die angenehme Arbeitsatmosphäre. Ein spezieller Dank geht an das FFF-Team: Heike, Katharina, Michaela und Susanne für die vielen Dinge, die über den normalen Arbeitsalltag hinausgingen. Je ne saurais également oublier celles et ceux qui sont sur l’autre rive du Rhin et qui ont accompagné dans la bonne humeur et l’amitié mes études ou ma thèse, notamment Aline, Clémence, Hélène, Sophie et Véronique. Un grand merci à Felix pour son amitié et son aide précieuse tout au long de ces trois années passées côte à côte. Merci à Nadia et Natha pour leurs bons petits plats concoctés pendant notre WG ! Mais surtout merci pour leur soutien et leur amitié. Leur présence durant ces trois années m’aura beaucoup aidée. Mention particulière à Vincent : merci pour tout … Enfin, cette thèse n'aurait peut être pas vu le jour sans le soutien inconditionnel de mes parents, de Claire et Clémence et de ma famille. Par ces quelques mots, je tiens à les remercier chaleureusement. Abstract This thesis describes the coordination chemistry and catalytic applications of the 1,1,1- tris(oxazolinyl)ethane (“trisox”) family of ligands. The studies described herein are primarily concerned with the effect of the threefold rotational symmetry of the ligands, as well as the role of the third oxazoline arm in catalytic reactions in which there are intermediates that possess a bidentate coordination of the trisox ligand. The syntheses of highly symmetrical chiral 1,1,1-tris(oxazolinyl)ethane ligands bearing phenyl, benzyl or indanyl substituents, and of mixed bis- and trisoxazolines is described. The isomerisation of the 2-bromooxazolines was observed, in which the thermally induced rearrangement generates the corresponding α-bromo-isocyanate derivatives. Reaction of the latter with phenylethylamine led selectively to the N-cyclised aziridines or to the O-cyclised 2- aminooxazolines, depending on the reaction conditions. The coordination chemistry of the trisoxazoline ligands with palladium is then described. Palladium(II) chloride and allyl complexes and a number of palladium(0) complexes were successfully synthesised. The dynamic behaviour of these complexes in solution was studied and activation parameters were determined for the exchange of the oxazoline moieties. The systematic comparison of the catalytic efficiency of trisox- and bisox-palladium systems in allylic substitution is described. It was demonstrated that the trisoxazoline-based complexes are superior catalysts in direct comparison to the corresponding bisoxazoline-based catalysts. The study showed that the additional donor function appears to play a role in the product/substrate exchange step as well as in the initial generation of the active catalyst. Finally, the exploitation of the dynamic coordination of the trisoxazolines to copper(II) in two copper-catalysed asymmetric reactions is described. It has been shown that C3-symmetric trisoxazolines form highly efficient enantioselective copper(II) Lewis acid catalysts, in which their success is based on the concept of a stereoelectronic hemilability of the divalent copper. In a direct comparison with the analogous bisoxazoline systems, the trisox/copper catalysts have proven to be more efficient in an enantioselective Mannich reaction as well as an enantioselective α-amination of prochiral β-ketoesters in presence of low catalyst loadings. To conclude the implications of the use of chiral tridentate podands in stereoselective catalysis compared to the more established bidentate chelates have been highlighted. Kurzfassung Diese Arbeit beschreibt die Koordinationschemie und die katalytische Anwendung der Familie der 1,1,1-Tris(oxazalinyl)-ethanliganden („Trisox“). Die hier beschriebenen Untersuchungen behandeln in erster Linie den Einfluß der dreizähligen Rotationssymmetrie der Liganden sowie die Rolle des dritten Oxazolinarms in katalytischen Reaktionen, deren Intermediate einen bidentaten Koordinationsmodus des Trisoxliganden beinhalten. Zu Beginn wird die Synthese hochsymmetrischer chiraler 1,1,1-Tris- (oxazalinyl)ethanliganden, die Phenyl-, Benzyl- oder Indanylsubstituenten tragen, sowie nichtsymmetrischer Bis- und Trisoxazoline beschrieben, die gemischte Substituenten an der 4- Position des Oxazolins tragen. Bei den bromierten Monooxazolin-Zwischenstufen wurde eine Isomerisierung beobachtet, bei der durch thermisch induzierte Umlagerung aus den 2- Bromoxazolinen die korrespondierenden α-Bromo-isocyanat-Derivate entstehen. Diese reagieren mit Phenylethylamin in Abhängigkeit von den Reaktionsbedingungen selektiv entweder zu den N-zyklisierten Aziridinen oder den O-zyklisierten 2-Aminooxazolinen. Anschließend wird die Koordinationschemie der Trisoxazolinliganden mit Palladium beschrieben. Es konnten erfolgreich Palladium(II)chlorid- und -allyl-Komplexe und Palladium(0)-Komplexe synthetisiert werden. Im Zuge der Untersuchung des dynamischen Verhaltens dieser Komplexe in Lösung wurden die Aktivierungsparameter des Austauschs der Oxazolinreste bestimmt. Ein sys-tematischer Vergleich der katalytischen Effizienz von Trisox- und Bisox-Palladium-Systemen zeigte, daß die Trisox-basierten Komplexe den jeweils korrespondierenden Bisox-Systemen überlegen sind. Die zusätzliche Donorfunktion des dritten Oxazolinarms scheint eine wichtige Rolle beim Produkt-Substrat-Austausch und der Bildung der aktiven Katalysatorspezies zu spielen. Abschließend werden zwei kupferkatalysierte Katalysen beschrieben, bei denen die dynamische Koordination der Trisoxazoline an Kupfer(II) ausgenutzt wird. Es konnte gezeigt werden, daß C3-symmetrische Trisoxazoline hocheffiziente Kupfer(II)-Komplexe für die enantioselektive Lewissäurekatalyse bilden, deren Aktivität auf der Hemilabilität des divalenten Kupfers basiert. Im direkten Vergleich mit den analogen Bisoxazolinsystemen erwiesen sich die Trisox/Kupfer-Katalysatoren insbesondere bei niedrigen Katalysatorbeladungen als effizienter in der enantioselektiven Mannich-Reaktion und der enantioselektiven α-Aminierung prochiraler β- Ketoester. Zusätzlich wurden die Auswirkungen der Nutzung chiraler tridentater Podanden im Vergleich zu den besser etablierten bidentaten Bisox-Chelatliganden untersucht. Résumé Le but de ce travail a été de concevoir de nouveaux ligands trisoxazolines de haute symétrie et de déterminer leur intérêt par rapport aux ligands de symétrie C2 bien établis dans la littérature. Une étude directe de l’influence de la symétrie C3 en catalyse asymétrique par comparaison directe entre des systèmes trisoxazoline/métal et bisoxazoline/métal a donc été effectuée. Dans un premier temps, trois nouvelles trisoxazolines de symétrie C3 ont été synthétisées avec, sur les cycles oxazoline, les substituants chiraux phenyl, benzyl et indanyl. Des bisoxazolines et trisoxazolines à groupements mixtes ont également été obtenues. Il a été démontré que les 2-bromooxazolines, intermédiaires clé de la synthèse des trisox, se réarrangent thermiquement pour donner les dérivés α-bromo-isocyanate correspondants. Ces derniers, après réaction avec la phenylethylamine, génèrent sélectivement des aziridines, produits de N- cyclisation, ou des aminooxazolines, produits de O-cyclisation, selon les conditions réactionnelles employées La chimie de coordination des trisoxazolines avec le palladium a ensuite été explorée. Des complexes de palladium(II) et de palladium(0) ont été synthétisés. Il a été montré que ces ligands présentent un mode de coordination bidentate en phase solide. En solution, le processus dynamique de ces complexes a été observé
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