C2-Symmetric Pyrazole-Bridged Ligands and Their Application in Asymmetric Transition-Metal Catalysis
by Torben Böhnisch
Date of Examination:2015-07-23
Date of issue:2016-06-03
Advisor:Prof. Dr. Franc Meyer
Referee:Prof. Dr. Franc Meyer
Referee:Prof. Dr. Guido Clever
Referee:Prof. Dr. Thomas Waitz
Referee:Prof. Dr. Ulf Diederichsen
Referee:Prof. Dr. Dietmar Stalke
Referee:Prof. Dr. Konrad Koszinowski
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Name:Dissertation Torben Böhnisch 2015_online version.pdf
EnglishWithin the scope of this thesis the synthesis of a plethora of examples of in total three types of C2-symmetric pyrazole-bridged ligand systems (namely PyrBOX, PyrBPyBOX and ProPyrazole) could be established (Chapter 4).
Subsequently, the PyrBOX ligands proofed to be versatile ligands for asymmetric allylic substitution reactions (Chapter 7). Starting from three different palladium allyl precursors and four PyrBOX ligands in total twelve different complexes could be isolated. The investigations of these intermediate dinuclear allyl palladium complexes in solution (e.g., EXSY, VT-NMR) and in solid state further allowed gaining a deeper understanding of the catalytic process. These studies furthermore revealed the presence of solvation and cooperative effects in such dinuclear palladium catalysts.
Combination of the same PyrBOX ligand scaffold with other transition metal ions gave rise to variety of helical supramolecular structures (Chapter 9). For instance, the diastereoselective formation of a pentanuclear cluster helicate could be observed. Further investigations lead to the isolation of, to the best of knowledge, first examples of helical mesocate complexes of the general formula [(Lx)3M2]+ (M = Mn(II), Cd(II)). While potential applications of these and related chiral, multinuclear structures remain to be explored, the introduction of a rigid bridging group and the resulting properties certainly extended the class of helical multimetallic supramolecular compounds.
Finally, a very promising catalyst system, based on the ProPyrazole ligand scaffold, for the atropselective oxidative coupling of 2-naphthols was introduced (Chapter 11). Sophisticated kinetic and ESI-MS studies were performed, which allowed the targeted optimization of the catalyst systems and revealed new key insights into the mechanism of such transformations.
The obtained results therefore may offer a glimpse of the potential of C2-symmetric pyrazole-bridged ligands for cooperative asymmetric catalysis.
Keywords: Pyrazole-Bridged Ligands; Asymmetric Transition-Metal Catalysis; Cooperative Asymmetric Catalysis; Enantioselective Catalysis; Two-Center Catalysis; Pyrazole; PyrBOX; PyrBPyBOX; ProPyrazole; Tsuji-Trost; Allyl Palladium; Asymmetric Allylic Substitutions; Asymmetric Allylic Alkylations; Solvation Effects; Cooperative Effects; EXSY; Helicate; Mesocate; Cluster Helicate; Chiral Supramolecular Coordination Compounds; Atropisomerism; Atropselective Oxidative Coupling; 2-Napthol; Reaction Progress Kinetic Analysis (RPKA)