Nitrogen-based ligands : synthesis, coordination chemistry and transition metal catalysis Citation for published version (APA): Caipa Campos, M. A. (2005). Nitrogen-based ligands : synthesis, coordination chemistry and transition metal catalysis. Technische Universiteit Eindhoven. https://doi.org/10.6100/IR594547 DOI: 10.6100/IR594547 Document status and date: Published: 01/01/2005 Document Version: Publisher’s PDF, also known as Version of Record (includes final page, issue and volume numbers) Please check the document version of this publication: • A submitted manuscript is the version of the article upon submission and before peer-review. There can be important differences between the submitted version and the official published version of record. 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Vogt Copromotor: dr. C. Müller CIP-DATA LIBRARY TECHNISCHE UNIVERSITEIT EINDHOVEN Caipa Campos, Mabel A. Nitrogen-Based Ligands : Synthesis, Coordination Chemistry and Transition Metal Catalysis / by Mabel A. Caipa Campos. – Eindhoven : Technische Universiteit Eindhoven, 2005. Proefschrift. – ISBN 90-386-2707-6 NUR 913 Trefwoorden: homogene katalyse / asymmetrische synthese ; katalytische hydrogenering / coördinatieverbindingen / C3-symmetrische liganden / fosfor en stikstof verbindingen / overgangsmetaalcomplexen Subject headings: homogeneous catalysis / asymmetric synthesis ; catalytic hydrogenation / coordination compounds / C3-symmetric ligands / phosphorus and nitrogen compounds / transition metal complexes Design Cover: Mabel A. Caipa, Jos M. J. Paulusse Jan-Willem Luiten, JWL producties Printed at the Universiteitsdrukkerij, Technische Universiteit Eindhoven. The research described in this thesis was financially supported by the National Research School Combination Catalysis (NRSC-Catalysis). Copyright © 2005 by Mabel A. Caipa Campos. It is precisely the possibility of realizing a dream that makes life interesting Paulo Coelho A mis Padres Evelio y Dilia Mabel y a mi hermano Alejandro Voor Jos Contents Chapter 1: General Introduction 1 Chapter 2: Synthesis and Characterization of C3-Symmetric Ligands 19 Chapter 3: Coordination Chemistry of C3-Symmetric Tris(oxazoline) and Tris(imidazoline) Ligands 49 Chapter 4: C3-Symmetric Ligands in the Ruthenium(II)-Catalyzed Transfer Hydrogenation of Ketones 93 Chapter 5: New Concepts: Post-Synthesis Ligand Modification for Asymmetric Catalysis 129 Summary 177 Samenvatting 181 Curriculum Vitae 185 Acknowledgements 187 1 Chapter 1 General Introduction Stereoselectivity - the selective formation of one stereoisomer from a prochiral substrate in the presence of a catalyst - is a fundamental issue in homogeneous catalysis. Selectivity can be steered through catalyst design by changing the properties of the ligand, by choice of the metal and the counterions. In this context, symmetry has proven to be a powerful tool. It can reduce the number of conformations a catalyst can adopt and thereby restrict the number of possible reaction-pathways, which may lead to higher selectivities. C2- symmetric ligands render the available coordination sites in square planar complexes homotopic, which is the reason for their success in many metal-catalyzed asymmetric reactions. In octahedral complexes, however, C2 symmetry is less effective. Homotopicity of the available coordination sites can only be achieved with C3 symmetry. Successful applications of C3 symmetry in asymmetric, biomimetic and polymerization catalysis will be described, as well as applications in molecular recognition. A different approach to catalyst design involves the non-covalent modification of a site close to the metal center. Easily accessible N-containing phosphorus ligands can be tuned by quaternization of the nitrogen atom. Structural variation in both the backbone of the ligand and the counterions provides a wealth of opportunities. Chiral counteranions can in principle make an achiral catalyst enantioselective. The different types of ion-pairs and the strong influence of solvents will be presented and examples of catalyst modification by ion- pairing will be given. Chapter 1 1.1 Symmetry Symmetry can be found everywhere around us, from flowers to man-made objects, from ancient monuments to modern inventions, but also on a scale not visible to the human eye, the molecular scale. In the common bacterium Escherichia coli, a highly symmetric Verotoxin 1 is produced (Figure 1.1). Five monomers self-assemble to form a pentameric antibody, enhancing binding affinity dramatically.[1] Figure 1.1 Symmetry in the Verotoxin 1 B-subunit The symmetry of a free molecule can be described completely in terms of symmetry elements that entail specific rotations and reflections.[2] Benzene for example, has a C6 symmetry axis, horizontal and vertical reflection planes. C3 symmetry can be observed in the painting by M.C. Escher as depicted in Figure 1.2. When the figure is rotated over 120°, an identical situation is obtained. σd 120° σv C6 σh Figure 1.2 Symmetry elements Symmetry is closely related to topology. Topology is the area of mathematics that analyzes how geometric objects can be deformed or preserved upon rotation, reflection, 2 General Introduction etc. Molecules can be considered topological objects. For example, a tri-functional molecule with C3 symmetry can have four different topologies. It may be acyclic, exocyclic, macrocyclic or bicyclic as illustrated in Figure 1.3. L L L L L L L L L L L L Acyclic Exocyclic Macrocyclic Bicyclic Figure 1.3 Topologies of C3 symmetric structures Furthermore, topicity describes the symmetry relationships between two or more groups (or atoms) in a molecule that have identical connectivities, i.e. they are connected to the molecule in the same way. Two classes are distinguished: homotopic, if groups are in identical environments and diastereotopic, if they are in different environments. Additionally, two groups are enantiotopic when apart from their connectivity; their chirality is equal as well. Homotopic groups are related to one another by a bond rotation, reflection, or an axis of rotation in the complex, while diastereotopic groups are not related by any symmetry element operation (Figure 1.4). HH OH H CH3 O CH3 a) b) H H3C H H CH3 H H Homotopic methyl groups Diastereotopic methyl groups Figure 1.4 Topicity for a) 2,6-dimethylnaphthalene and b) 2-hydroxy-3-methyl butanal 1.2 C2 and C3 Symmetry In order to explain how symmetry may be advantageous in coordination chemistry, two different coordination environments with C2- and C3-symmetric ligands will be considered (square planar and octahedral). In a square planar environment, two coordination sites are occupied by the C2-symmetric ligand (Figure 1.5). Moreover, the 3 Chapter 1 remaining free coordination sites (A and B) are homotopic, i.e. their environment is identical. L L A B Figure 1.5 Coordination of a C2-symmetric ligand in a square planar environment The combination of C2 symmetry and chirality has led to highly enantioselective catalysts.[3] The success of these systems is attributed to the homotopicity of the available coordination sites. Coordination of a substrate to either of the two sites will lead to the same intermediates. Thereby the number of reaction pathways is limited and better control over selectivity can be obtained. However, in an octahedral environment the situation is different. Once the C2- symmetric ligand coordinates to the metal center, four coordination sites will remain available (A, B, C, D in Figure 1.6a). From these remaining sites, A and D are homotopic, as are B and C; while the other four possible