Metal-free Methods Utilizing Single-electron Oxidations
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Redox transformations are critically important steps in organic syntheses; however, they often require stoichiometric reagents or metal catalysts. Growing interest in “greener” alternatives to traditional processes motivated our group to develop methods that achieve single-electron oxidations under metal-free conditions. This dissertation describes our work toward developing metal-free methods that avoid the use of stoichiometric oxidants, high cell potentials, toxic metal reagents, and produces minimal byproducts. Towards the goal of increasing atom efficiency and reducing waste, we developed a method that integrates organocatalysis with electro-organic synthesis to achieve efficient aldehyde transformations. This method was successfully demonstrated for the one-pot conversion of aldehydes to esters and thioesters. The optimization of the thioesterification reaction emphasized the importance of taking the redox potentials of azolium precatalysts into consideration when developing N-heterocyclic carbene (NHC) catalyzed reactions. This led us to systematically characterize the redox properties of a series of azolium and azolinium salts, including benzothiazolium, thiazolinium, thiazolium, triazolium, imidazolium, and imidazolinium salts. The series includes a broad range of N-aryl thiazolium salts that collectively demonstrate the ability to fine tune the reduction potential of the thiazolium ring via electronic modification to the N-aryl moiety. Additionally, a novel class of N-aryl thiazolinium salts has been synthesized and characterized. In contrast to what has been observed from imidazolium and imidazolinium counterparts, saturation of the thiazolium backbone to give thiazolinium salts results in more facile electrochemical reduction. Finally, we developed a method to achieve ring-opening metathesis polymerization (ROMP) mediated by oxidation of organic initiators in the absence of any transition-metals. Radical cations, generated via one-electron oxidation of vinyl ethers, were found to react with norbornene to give polymeric species with microstructures essentially identical to those traditionally obtained via metal-mediated ROMP. We found that vinyl ether oxidation could be accomplished under mild conditions using an organic photoredox mediator. This led to high yields of polymer and generally good correlation between Mn values and initial monomer to catalyst loadings. Moreover, temporal control over reinitiation of polymer growth was achieved during on/off cycles of light exposure. This method demonstrates the first metal-free method for controlled ROMP.
- Chemistry