The Development of Iron Photoredox Strategies in Organic Transformations for the Construction of C–N and C–C Bonds
| dc.contributor.advisor | West, Julian G. | en_US |
| dc.creator | Kao, Shih-Chieh | en_US |
| dc.date.accessioned | 2024-08-30T18:35:05Z | en_US |
| dc.date.created | 2024-08 | en_US |
| dc.date.issued | 2024-08-09 | en_US |
| dc.date.submitted | August 2024 | en_US |
| dc.date.updated | 2024-08-30T18:35:05Z | en_US |
| dc.description | EMBARGO NOTE: This item is embargoed until 2025-08-01 | en_US |
| dc.description.abstract | Organic transformations via photoredox strategies have emerged as a powerful tool for the synthesis of valuable chemical products; however, prevailing strategies heavily relied on the use of precious transition-metal catalysts such as iridium, ruthenium or silver, impeding their general applications. The motivation of this work was to develop new catalytic systems using cheap and sustainable earth-abundant element metals that allow efficient synthesis of otherwise challenging molecules via photoredox mechanisms. Vicinal diamines are prevalent in bioactive molecules, pharmaceuticals, and molecular catalysts, underscoring their significance. Olefin diazidation emerges as a promising strategy for synthesizing these motifs due to the ability to reduce azides to amines and the availability of structurally diverse olefins. Traditional diazidation methods often involve harsh conditions and have limited substrate scopes. Recent advancements using azidobenziodoxolone (ABX, Zhdankin reagent) or electrochemical methods, overcome some of these challenges; however, they introduce different limitations such as high costs and procedural complexity. In chapter 1, we introduce an innovative photochemical diazidation approach utilizing cheap iron salts, which leverages visible light-induced homolysis (VLIH) and radical ligand transfer (RLT). This methodology, which eschews the need for additional oxidants or complex reaction apparatus, offers sustainability and economic benefits while demonstrating compatibility with continuous flow chemistry, providing an efficient and practical route for the synthesis of organic diazides. Preliminary mechanistic studies support the radical nature of the cooperative process in photochemical diazidation, demonstrating this approach as a highly effective method for olefin difunctionalization. Decarboxylative functionalization is a potent strategy for synthesizing diverse products, with ligand-to-metal charge transfer (LMCT) involving earth-abundant 3d metals emerging as a prominent method for reaction design. While recent advancements in coppermediated decarboxylative C–N bond formation via a LMCT/radical polar crossover (RPC) mechanism have been demonstrated, they face limitations in catalytic function and substrate scope with unactivated alkyl carboxylic acids, challenging their general applicability. In chapter 2, we present a novel photochemical, nucleophilic decarboxylative azidation using iron-catalyzed visible light-induced homolysis (VLIH) and radical ligand transfer (RLT). Our proposed iron-catalyzed approach leverages inexpensive iron nitrate and simple azide sources to convert a variety of carboxylic acids into organic azides under mild conditions. This method avoids the need for external oxidants, complex ligands, or pre-activation of carboxylic acids. Mechanistic studies suggest a radical pathway with nitrate acting as an internal oxidant, offering a “redox-neutral” transformation. This new methodology provides a straightforward and efficient route for synthesizing aliphatic azides, expanding the toolkit for C-N bond formation in pharmaceutical and organic synthesis. Hydroalkylation, the addition of a carbon fragment and hydrogen across an alkene, is an optimal method for forming C(sp3)–C(sp3) bonds from readily available starting materials. Despite notable advancements in branch-selective hydroalkylation via transition metal catalysis, a general strategy for linear-selective hydroalkylation remains underdeveloped, with certain reactions, such as hydroethylation, remaining largely elusive. In chapter 3, we demonstrate that these challenging reactions can be achieved catalytically through traceless radical polarity reversal (TRPR), which employs a removable electron-withdrawing group to facilitate radical alkene addition, followed by in situ removal under reaction conditions. This approach facilitates a variety of previously unattainable hydroalkylation reactions, such as hydromethylation, hydroethylation, and hydrocyclobutylation, all with high linear selectivity using simple malonic acids as alkyl donors. Additionally, it allows for the rapid and efficient synthesis of bioactive molecules, exemplified by a GPR119 agonist, which was produced in good yield and efficiency. To further showcase the practical utility of our approach, we demonstrated that mono- or difluoromalonic acids can act as novel mono- or difuoromethylene linchpins for accessing gem-mono- or gem-difuoroalkyl skeletons from abundant feedstock chemicals. Importantly, our unified iron/thiol dual catalytic system manages both the alkene addition and the removal of the polarity reversal group, offering a sustainable and straightforward method for these transformations. Preliminary mechanistic studies suggest a dual-catalytic radical mechanism involving decarboxylation via ironmediated visible light-induced homolysis (VLIH) and hydrogen atom transfer steps. Overall, traceless radical polarity reversal provides a versatile solution for alkene hydroalkylation in both simple and complex settings. In general, these new iron photoredox strategies have shown great advantages and improved sustainability compared to the previous work using noble and expensive transition metals and/or harsh conditions. In addition, these strategies allowed us to study the unprecedent catalytic properties of earth-abundant elements and synthesize a wide variety of valuable and traditionally inaccessible molecules, advancing fundamental knowledge in the fields of homogeneous catalysis and synthesis. | en_US |
| dc.embargo.lift | 2025-08-01 | en_US |
| dc.embargo.terms | 2025-08-01 | en_US |
| dc.format.mimetype | application/pdf | en_US |
| dc.identifier.citation | Kao, Shih-Chieh. The Development of Iron Photoredox Strategies in Organic Transformations for the Construction of C-N and C-C Bonds. (2024). PhD diss., Rice University. https://hdl.handle.net/1911/117834 | en_US |
| dc.identifier.uri | https://hdl.handle.net/1911/117834 | en_US |
| dc.language.iso | eng | en_US |
| dc.rights | Copyright is held by the author, unless otherwise indicated. Permission to reuse, publish, or reproduce the work beyond the bounds of fair use or other exemptions to copyright law must be obtained from the copyright holder. | en_US |
| dc.subject | Iron | en_US |
| dc.subject | Photocatalysis | en_US |
| dc.subject | Visible light | en_US |
| dc.title | The Development of Iron Photoredox Strategies in Organic Transformations for the Construction of C–N and C–C Bonds | en_US |
| dc.type | Thesis | en_US |
| dc.type.material | Text | en_US |
| thesis.degree.department | Chemistry | en_US |
| thesis.degree.discipline | Natural Sciences | en_US |
| thesis.degree.grantor | Rice University | en_US |
| thesis.degree.level | Doctoral | en_US |
| thesis.degree.name | Doctor of Philosophy | en_US |