טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentBuchman Yekaterina
SubjectOxygen Atom Transfer Catalysis by Manganese Corroles
DepartmentDepartment of Chemistry
Supervisor Professor Zeev Gross
Full Thesis text - in Hebrew Full thesis text - Hebrew Version


Abstract

Cyctochrome P450 is a class of heme-containing enzymes that catalyzes the transfer of one oxygen atom from molecular oxygen to substrates in living organisms.  The overall oxidation process includes formation of an (oxo)iron complex, called Compound I and considered as the main complex responsible for oxygen atom transfer to substrates. There are many types of reactions performed in this manner, including epoxidation, hydroxylation and sulfoxidation.

Extensive research in this field focused on synthetic models that mimic the natural system, using synthetic porphyrin and corrole metal complexes. It has been shown that the corresponding iron, manganese and chromium derivatives can activate external oxidants for substrate oxidation. In most cases, the research revealed that intermediates responsible for oxygen atom transfer are (oxo)metal species that are analogous to enzymatic Compound I.

When this issue was checked for the (oxo)manganese(V) corrole complex of 5,10,15-tris(pentafluorophenyl)corrole (abr. (tpfc)Mn(O)), it was found that there was no stoichiometric oxygen atom transfer from the complex to styrene. On the other hand, the (tpfc)MnIII complex served as well catalyst for the oxidation of styrene by iodosylbenzene. These findings raised many questions regarding the role of (tpfc)MnV(O) as the reactive intermediates in catalytic oxidations. The main goal of this work was to provide additional information for resolving these questions. 

Hammett r values were elucidated for both stoichiometric and catalytic reactions, using two manganese corrole catalysts: (tpfc)Mn and the b-pyrrole brominated analogue (tpfc-Br8)Mn, and two types of reactions: sulfoxidation and epoxidation.  The results revealed quite different r values for stoichiometric and catalytic reactions, with the former being larger. The immediate conclusion was that the active intermediate during catalysis is of different structure, reactivity, and selectivity than (oxo)manganese(V) corrole.

Analysis of the results obtained in this study were consistent with disproportionation of two (tpfc)MnV(O) molecules to [(oxo)MnIV]- and [(oxo)MnVI]+ species, with the latter as the very active oxidant. Other mechanistic proposals were checked as well, but the results did not support the hypothesis behind them. Specific experiments were designed for further critical examination of the disproportionation process. For that purpose, less electron withdrawing MnIII corroles and MnIV corroles were prepared. In accord with the hypothesis, the new complexes were indeed better catalysts than (tpfc)Mn.

Concluding, this work provided new evidence in favor of the disproportionation mechanism, and highlighted the importance of [(oxo)MnVI]+ intermediates.