|M.Sc Student||Nigel-Etinger Izana|
|Subject||Advanced Catalysis by New Corrole Metal Complexes|
|Department||Department of Chemistry||Supervisor||Professor Zeev Gross|
|Full Thesis text|
The two main
approaches for achieving enantioselective functionalization of organic
molecules are organocatalysis and utilization of metal-based catalysts. A less
frequently adopted methodology is to rely on easily accessible non-chiral metal
complexes for the catalysis and on biomolecules for inducing chirality, thus
resembling Nature’s style for addressing the issue.
The main research goals of this research are: synthesis and characterization of new chiral and non-chiral metallocorroles, investigating the ability of these new complexes in asymmetric catalytic oxidation of sulfides, and comparison of two different approaches for achieving asymmetric sulfoxidation: a) Utilization of metal complexes chelated by chiral corroles. b) Catalysis by bioconjugated non-chiral metallocorroles.
We demonstrate that 2,6-dibromophenylcorroles could be utilized as versatile synthons for modular construction of new chiral corroles via palladium-catalyzed C-Br amidation reactions with a chiral amide. Successful insertion of manganese(III) and iron(III) produced chiral metallocorrole complexes that could be utilized in asymmetric catalysis, in what we termed the covalent approach.
Non-chiral metallocorroles suitable for utilization in the non covalent approach were prepared by selective electrophilic substitutions that led to a new series of amphiphilic corroles. The spontaneous and non-covalent conjugation of the corresponding metal complexes with serum albumins was used for inducing asymmetric catalysis in a biomimetic fashion: the metal complex being responsible for catalysis and the protein for providing a chiral environment.
The covalent approach was found to be difficult due to long reaction times and low yields of the chiral corrole; and the metal complexes were eventually found to be not stable under the reaction conditions. Results obtained with hydrogen peroxide were superior relative to iodosylbenzene, mainly because of a better survival of the catalysts.
On the contrary, the non-covalent approach was found to be much more promising, easier to apply, more flexible and characterized by much more promising results. The amphiphilic corroles were obtained in reasonable yields, within a short time and under very flexible reaction conditions. Hydrogen peroxide was used as an oxidant, which is both economical and an environmentally benign. In addition, most accessible proteins were used for supplying a chiral environment; and water was used as solvent for the reactions. Another important finding was the good stability of the amphiphilic corrole metal complexes under the applied reaction conditions.
The current examinations revealed that the non-covalent approach was much more successful: it provided enantioselectivity of up to 65%, while the covalent approach was limited to 24-27% ee.