Investigation of Kinetically Labile Supramolecu-lar System Addressing Substrate Selectivity in the Catalytic Epoxidations of Olefins
Sammanfattning: This thesis is devoted to the development and investigation of a supramolecular system used to catalyze Jacobsen-Katsuki epoxidations with the aim to attain substrate selectivity without product inhibition. The supramolecular catalyst is composed of two well-defined entities or units: a catalytic and a receptor. The keyword for the design was flexibility. The interactions foming the supramolecular catalyst were designed to give the system enough flexibility for it to be able to release the product as soon as it is formed avoiding product inhibition. This could be obtained by using the 2-pyridone hydrogen-bonding motif. The catalyst unit is a derivative of Jacobsen's (Salen)MnIII, where 2-quinolones are attached to the 5- and 5´-positions of the salen ligand. The receptor unit is a zinc-porphyrin with 2-pyridone groups attached to it. The spatial arrangement of the 2-pyridone motifs was designed to favor the formation of a cavity between the catalyst and the receptor unit. The receptor can select a pyridine-appended olefin via coordination to its zinc-porphyrin core. The supramolecular catalyst was shown to be partly selective for such olefins over phenyl appended one. Three different substrates were synthesized and investigated pairwise. Two substrates contained the pyridine required for it to be selected by the supramolecular catalyst. These substrates were differentiated by their size. The third one lacks the anchor; the pyridyl group is exchanged for a phenyl group. The synthesis of the receptor and investigation of the substrate selectivity is discussed. The epoxidations were run at different concentrations of receptor and catalytic units, varying the composition of the system like the presence of different substrates and additives. The effect of both the receptor and substrate on the stability, reactivity, substrate- and enantio-selectivity of the catalyst unit were addressed and are presented in addition to the above-mentioned substrate selectivity. A complete mathematical model is also presented which helped us to estimmate the association constants characterizing the system.
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