Date of Award
Spring 2023
Document Type
Dissertation
Degree Name
Doctor of Philosophy (PhD)
Department
Chemistry
First Advisor
Holland, Patrick
Abstract
Chapter 1 describes the field of metal-mediated hydrogen atom transfer (MHAT) reactions. An overview of synthetic methods which use MHAT is discussed. Two classes of metal hydrides are defined and their properties and reactivity are contrasted. The current understanding of the mechanisms in MHAT catalysis is discussed, including the steps of metal hydride formation, hydrogen atom transfer to alkene, alkyl radical trapping, and catalyst turnover. From these discussions, a general mechanistic framework is proposed for catalytic MHAT reactions.Chapter 2 describes (salen)cobalt(III) complexes proposed to be catalytic intermediates in oxidative MHAT reactions. These complexes are shown to be rapidly reduced by a silane to cobalt(II), and the silyl byproducts suggest a mechanism involving hydride transfer to cobalt, and X-type ligand exchange from cobalt to silane. This reduction reaction is found to occur with a 2:1 cobalt-to-silane stoichiometry. In the presence of both a silane and an alkene, these cobalt(III) complexes are demonstrated to produce a cobalt(III) sec-alkyl, consistent with the formation of a transient cobalt hydride and subsequent hydrogen atom transfer to the alkene. The selectivity for HAT to an alkene was found to be dependent on the concentration of cobalt(III). This finding informs ways to improve the selectivity for HAT and reduce the need for excess oxidant and silane in synthetic methods. The collection of experimental results are consistent with prior claims from the MHAT literature on the mechanisms of metal hydride formation and hydrogen atom transfer reactions. Chapter 3 describes kinetic studies and in situ spectroscopic monitoring of a (salen)cobalt-catalyzed MHAT hydroalkoxylation. The Variable Time Normalization Analysis (VTNA) technique was used to determine a rate dependence proportional to [cobalt]2 [silane]1. This result is consistent with metal hydride formation being the turnover- limiting step in catalysis, and furthermore suggests a mechanism with a pre-equilibrium involving two cobalt complexes. A cobalt(III) alkyl complex and a bimetallic cobalt(III) fluoride complex are demonstrated to catalyze this reaction at rates equal to those of the original conditions, supporting the claim that they are on-cycle intermediates. The catalytic reaction mixture was also monitored by UV-Vis spectroscopy, which gave limited evidence for the presence of a cobalt(III) alkyl and a cationic cobalt(III) complex. Mechanistic claims from the literature are discussed in light of our findings. We rule out the prior literature claim that the turnover-limiting step in catalysis is the reaction between a cobalt(III) alkyl and cobalt(III) nucleophile complex. Based on our obtained rate law, we offer an alternate explanation for the observed second-order dependence on catalyst concentration. Chapter 4 describes the synthesis, characterization, and reactions of a (salen)cobalt(IV) sec-alkyl complex. EPR and ENDOR spectroscopy, obtained and analyzed in collaboration with the Hoffman group, support the assignment of a cobalt(IV) oxidation state as well as the presence of a Co–C bond. The oxidation of a cobalt(III) alkyl to the formally cobalt(IV) state was found to occur at unusually low potentials, and we suggest that this cobalt(IV) alkyl complex is accessible under catalytic conditions. Lastly, the cobalt(IV) alkyl was demonstrated to react with nucleophilic alcohols to produce ether products, as proposed to occur in MHAT catalysis. These experiments support the claim that cobalt(IV) alkyls are the catalytic intermediate which enable the observed radical- polar crossover in oxidative MHAT catalysis.
Recommended Citation
Wilson, Conner Venn, "Mechanistic Studies on (Salen)cobalt-Catalyzed Hydrogen Atom Transfer Reactions" (2023). Yale Graduate School of Arts and Sciences Dissertations. 1083.
https://elischolar.library.yale.edu/gsas_dissertations/1083
