Olefin cis-dihydroxylation is an important chemical transformation for the synthesis of both natural products and pharmaceuticals that typically involves the use of osmium reagents. Unfortunately, osmium has some unavoidable drawbacks due to its high toxicity and low earth abundance. However, Nature has evolved a family of enzymes called Rieske dioxygenases to degrade polyaromatic molecules that can accomplish this transformation using an iron active site. This active site consists of a mononuclear iron center coordinated by two histidines and one aspartate in a facial array. Intrigued by the biological systems and the need to replace toxic and precious osmium reagents, I have designed and developed novel synthetic bio-inspired iron catalysts for C=C cis-dihydroxylation. The central theme of my thesis is to determine how ligands affect the electronic and structural properties of the iron catalysts. To achieve this goal, I have investigated two ligand families: facial tridentate NNO ligands and tetradentate tripodal N4 ligands. The NNO ligands are structurally more related to the enzymatic active sites but only a few complexes are catalytically active. A series of newly designed and structurally related ligands has been synthesized and their reactivities have been compared. Mechanistic results point to a novel active oxidant (Chapters 2 and 3). On the other hand, a comparison of two pairs of catalysts supported by closely related N4 ligands demonstrate the importance of available cis-sites on the metal center to carry out cis-dihydroxylation (Chapters 4 and 5). Lastly, the first example of naphthalene cis-dihydroxylation by a synthetic iron catalyst, mimicking the action of naphthalene 1,2-dioxygenase, is described (Chapter 6).
University of Minnesota Ph.D. dissertation. August 2011. Major: Chemistry. Advisor: Prof. Lawrence Que, Jr. 1 computer file (PDF); xxii, 241 pages.
Bio-inspired nonheme iron catalysis for olefin oxidation - ligand design and mechanistic studies.
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