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  • Nitrogen-based ligands : synthesis, coordinationchemistry and transition metal catalysisCitation for published version (APA):Caipa Campos, M. A. (2005). Nitrogen-based ligands : synthesis, coordination chemistry and transition metalcatalysis Eindhoven: Technische Universiteit Eindhoven DOI: 10.6100/IR594547

    DOI:10.6100/IR594547

    Document status and date:Published: 01/01/2005

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    https://doi.org/10.6100/IR594547https://research.tue.nl/en/publications/nitrogenbased-ligands--synthesis-coordination-chemistry-and-transition-metal-catalysis(a6707de5-ddfb-40ee-ab0e-ad51fd30f07c).html
  • Nitrogen-Based Ligands

    Synthesis, Coordination Chemistry and Transition Metal Catalysis

  • Nitrogen-Based Ligands

    Synthesis, Coordination Chemistry and Transition Metal Catalysis

    PROEFSCHRIFT

    ter verkrijging van de graad van doctor aan de Technische Universiteit Eindhoven, op gezag van de

    Rector Magnificus, prof.dr.ir. C.J. van Duijn, voor een commissie aangewezen door het College voor

    Promoties in het openbaar te verdedigen op maandag 12 september 2005 om 16.00 uur

    door

    Mabel Andrea Caipa Campos

    geboren te Bogot, Colombia

  • Dit proefschrift is goedgekeurd door de promotor: prof.dr. D. Vogt Copromotor: dr. C. Mller

    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 / cordinatieverbindingen / 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

    General Introduction

    Chapter 1

    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.

    120 d

    C6

    v

    h

    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

    MacrocyclicExocyclic

    LL

    L

    Bicyclic

    LLL

    Acyclic

    L

    L L

    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).

    H H

    HH

    H

    CH3H

    H3C

    Homotopic methyl groups

    CH3

    CH3

    O

    H

    OH

    H

    Diastereotopic methyl groups

    a) b)

    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 oc

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