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First-Row Transition Metal-Mediated Reactions: Catalytic Ester Hydrogenolysis, Aerobic Alkane Oxidation, and [5+2] Ring-Expansion Reactions
- Author / Creator
- Yu, Shaohui
The research described in this dissertation focuses on first-row transition metal catalyzed and mediated organic reactions. The first part of the thesis involves the exploration of the catalytic reactivity of phosphoranimide-supported tetrametallic nickel and cobalt clusters, while the second part focuses on the development of stoichiometric cobalt/iron-mediated ring-expansion reactions.
The ester hydrogenolysis reactions catalyzed by the new nickel cluster proceeded under remarkably mild conditions. Both nickel and cobalt catalysts show unique selectivity for ester sp3 C–O bond activation, affording the corresponding carboxylic acids and alkanes by simple hydrogenolysis. Other relevant reactions catalyzed by this cluster include ketone hydrogenation and an unprecedented new reduction of benzyl aldehyde, in which sequential Tishchenko and ester reduction reactions are involved.
The catalytic aerobic alkane oxidation reactions were developed, remarkably, using the same nickel and cobalt clusters. Despite the air-sensitive nature of the clusters, the air-oxidized catalysts mediate aerobic oxidation, activating sp3 C–H bonds in hydrocarbons and functionalized organics to afford mainly ketones at elevated temperatures. The investigated substrates include alkyl arenes, ethers and non-functionalized hydrocarbons. Oxidation occurs selectively at the methylene positions α to the activating groups, which include either phenyl groups or oxygen atoms. For alkanes, tertiary C–H bonds appear to be more reactive than secondary C–H bonds, suggesting a free radical process, at least at some point(s) in the overall transformation.
For cobalt-mediated [5+2] cyclopentenyl/alkyne ring-expansion reactions, the narrow substrate scope defined in previous studies limits the potential for developing applications. Two synthetic routes for preparing unsymmetrical cobalt(I) cyclopentadiene precursors were examined, including reduction of unsymmetrical cobaltocenium compounds and replacement of ethylene ligands with cyclopentadiene in cobalt(I) bisethylene complexes. The attempted [5+2] ring-expansion reactions afford unsymmetrical cobaltocenium compounds as the major products, instead of the desired seven-membered ring formation.
Based on DFT calculations conducted by Salai and Nakamura, a new strategy for [5+2] ring-expansion reaction was proposed. Instead of η4-cyclopentadiene cobalt (I) complexes, an α-diimine iron(0) alkyne complex is protonated in the presence of cyclic alkene. However, the reaction generates an ammonium derivative from ligand reduction and no ring-expansion products are obtained. New ligand design is required for the further investigations.
- Graduation date
- Spring 2014
- Type of Item
- Doctor of Philosophy
- This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.