Publication Date

Spring 2021

Advisor(s) - Committee Chair

Dr. Rui Zhang (Director), Dr. Kevin Williams, and Lawrence Hill

Degree Program

Department of Chemistry

Degree Type

Master of Science

Abstract

Numerous transition metal catalysts have been designed as biomimetic model compounds for the active site of metalloenzymes found throughout Nature, most notably cytochrome P450 monooxygenases that carry out the oxidative transformations of organic substrates with near-perfect chemo-, regio-, and stereo-selectivity. The primary active oxidants in catalytic and enzymatic cycles are fleeting high-valent metal-oxo intermediates where the oxo ligand can transfer to an organic substrate in a process known as oxygen atom transfer (OAT).

In the present work, porphyrin-manganese(III), salen-chromium(III), and salenmanganese( III) derivatives were successfully synthesized and spectroscopically characterized using 1H NMR and UV-Vis spectroscopies. A facile photochemical approach was applied for the successful production of porphyrin-manganese(IV)-oxo, salenchromium( V)-oxo, and salen-manganese(V)-oxo intermediates. The photochemistry in all circumstances was rationalized through the cleavage of the oxygen-halogen bond in bromate and chlorate photo-liable precursors under visible light irradiation. The results serve as a ‘proof-of-concept’ that photolysis reactions are not exclusive to porphyrin or corrole systems. Meanwhile, conventional chemical oxidation methods were applied to the generation of identical high-valent metal-oxo species using terminal oxidants, such as iodobenzene diacetate and m-chloroperoxybenzoic acid. The kinetics of OAT reactions of these generated metal-oxo intermediates with various organic substrates were studied, providing a direct comparison for their reactivities.

In addition, the present catalytic studies demonstrated that porphyrinmanganese( III), and salen-chromium(III), and salen-manganese(III) complexes showed excellent activity and selectivity for the oxidation of sulfides, alkenes, and activated alkanes. Furthermore, kinetic and competition studies along with Hammett analyses were conducted on the chemically- and photo-generated metal-oxo species, providing detailed mechanistic insights into the potential reaction pathways and active intermediates. With porphyrin-manganese complexes, a direct oxygen atom transfer event occurs in the presence of reactive nucleophiles such as sulfides and porphyrin-manganese(IV)-oxo species; while a disproportionation mechanism in the case of weak nucleophiles such as hydrocarbons where the premier oxidant is manganese(V)-oxo species, although the manganese(V)-oxo species was spectroscopically detectable. In the instance of salenderivatives, there was no observed rate acceleration effect towards sulfides in the presence of electrophilic chromium(V)-oxo and manganese(V)-oxo cationic species. Presumably, the electrophilicity of these metal-oxo species was hampered due to the strong outer-sphere coordination of sulfoxide to the metal ion. The mechanistic studies imply that the observed chromium(V)-oxo and manganese(V)-oxo species are unlikely to serve as the primary oxidants under catalytic turnover conditions.

Disciplines

Biochemistry | Inorganic Chemistry | Organic Chemistry

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