High-valent Iron Intermediates in Nonheme Iron Catalytic Systems Designed for Hydrocarbon Oxidations

Abstract

Inspired by nonheme iron enzymes, synthetic chemists have developed iron complexes to catalyze hydrocarbon oxidation reactions. High-valent iron intermediates have been proposed to be the oxidant for both enzymes and synthetic catalysts. For future development of catalysts, it is critical to discover and understand pathways for forming high-valent iron oxidants that can perform difficult oxidative transformations such as alkane and aromatic hydroxylation. Additionally, understanding the pathways to generate iron-based oxidants in model synthetic systems can help in elucidating mechanisms of the enzymes. This thesis describes a new pathway to form reactive high-valent FeV oxidants by utilizing strong Lewis and Brϕnsted acids. The acids facilitate heterolytic cleavage of the O–O bond in FeIII–OOH intermediates generated from the reaction of nonheme FeII complexes and H2O2. This pathway converts an inefficient catalyst for cyclohexane hydroxylation into an efficient catalytic system, forming an FeV oxidant in the catalytic cycle that hydroxylates cyclohexane within seconds at -40 °C. This new oxidant can also perform benzene hydroxylation equally efficiently. FeIII(OTf)3 is one of the Lewis acids that does this chemistry, giving rise to the first synthetic example where a mononuclear FeIII–OOH intermediate is activated by a second iron(III) ion to form an FeV oxidant. This work introduces the idea that the second iron in diiron nonheme enzymes can also act as a Lewis acid to activate O2 and form high-valent iron oxidants like Q in sMMO, which oxidizes methane to methanol. In addition, this thesis explores the importance of ligand topology around the iron center by comparing the effect of Lewis acid on the reactivity of three different catalytic systems. The effect of ligand topology was also investigated in the case of FeV intermediates that were generated stoichiometrically via one-electron oxidation of two topological isomers of an FeIV compound. The properties of the isomeric FeV intermediates, and the effect of Lewis acid in each case were explored.