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Date of Award
Doctor of Philosophy (PhD)
ABSTRACT OF THE DISSERTATION
Oxidative C–C and C–Heteroatom Reactivity of High-Valent Nickel Complexes
Sofia M. Smith
Doctor of Philosophy in Chemistry
Washington University in St. Louis, 2019
Professor Liviu M. Mirica, Chair
Professor Kevin D. Moeller, Co-Chair
Nickel catalysts are commonly used for cross-coupling reactions such as Negishi, Kumada and Suzuki couplings. While Ni(0), Ni(I), and Ni(II) intermediates are most relevant in these transformations, Ni(III) and Ni(IV) species have also been recently proposed to play a role in catalysis. The formation of C–C and C–heteroatom bonds plays a fundamental role in organic transformations, and today cross-coupling reactions are one of the most powerful tools for the construction of new C–C bonds. However, limited examples exist of Ni-mediated C–heteroatom bond formation reactions, likely due to the difficulty of accessing high-valent organometallic Ni species that can undergo reductive elimination.
One way to expand our knowledge of these C–C and C–heteroatom atom bond formation reactions is to look at the stabilization and destabilization effects of the high-valent nickel complexes. Studying these effects will help us determine the most efficient way in performing C–C and C–heteroatom atom bond formation reactions. Tetradentate pyridinophane ligands have been to stabilize uncommon high-valent organometallic nickel complexes. By varying the N-substituents, we are now able to probe the stabilization and destabilization effects that might play a crucial role in the C–C and C–heteroatom atom bond formation reactions.
A series of nickel complexes were synthesized to probe the C–C and C–heteroatom atom bond reactivity using the Ni-dimethyl and -metallacycle complexes. These complexes were fully characterized and their reactivity was tested. The oxidative reactivity was studied by looking at a variety of oxidants which include dioxygen and hydrogen peroxide to probe any C–O bond formation. Interestingly, while the oxidation of the NiII metallacycle complexes with various oxidants led to exclusive C–C bond formation in very good yields, the use of O2 or H2O2 as oxidants led to appreciable amounts of C–O bond formation products, especially for an asymmetric pyridinophane ligand with one tosyl N-substituent. Moreover, cryo-ESI-MS studies strongly support the formation of several high-valent nickel species as key intermediates in this unprecedented aerobic Ni-mediated oxygenase-type chemistry.
Lastly, the tetradentate pyridinophane ligands were used in two catalytic oxidation reactions, a hydroxylation and a chlorination reaction for unactivated alkanes. These catalytic oxidation reactions use mild oxidants such as hypohalites which are commercially available. All these reactions could have important implications in organic transformations.
Chair and Committee
Liviu M. Mirica
Kevin D. Moeller, John R. Bleeke, Sophia E. Hayes, Nigam P. Rath,
Smith, Sofia Marie, "Oxidative C–C and C–Heteroatom Reactivity of High-Valent Nickel Complexes" (2019). Arts & Sciences Electronic Theses and Dissertations. 1833.
Available for download on Sunday, April 19, 2020