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
Document status and date:Published: 01/01/2005
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Download date: 01. Aug. 2019https://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
Synthesis, Coordination Chemistry and Transition Metal Catalysis
Synthesis, Coordination Chemistry and Transition Metal Catalysis
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
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
A mis Padres Evelio y Dilia Mabel
y a mi hermano Alejandro
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
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.
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.
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. 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.
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,
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.
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).
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 oc