Methods of Carbohydrate Functionalization and Defunctionalization Public Deposited

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  • March 19, 2019
Creator
  • Adduci, Laura
    • Affiliation: College of Arts and Sciences, Department of Chemistry
Abstract
  • Transformations of naturally abundant carbohydrates offer the potential to generate valuable chemicals from inexpensive chiral materials. Initial work described herein focuses on the oxidative addition of glycosyl halides to palladium(0) centers. This process models the first step of a non-radical cross-coupling cycle targeting the installation of a carbon-based fragment at the anomeric center of a carbohydrate, generating a C-glycoside. Although carbohydrates, as secondary alkyl halides, are particularly challenging substrates for oxidative addition, certain conditions provided glycosyl-palladium oxidative addition products. Subsequent investigations probed the stability and further reactivity of the isolated glucosyl palladium complexes to assess their suitability for further functionalization; they were found to be quite prone to elimination processes, which hindered attempts to elaborate this work to a cross-coupling cycle. A second area of research focused on defunctionalization reactions to develop methods for biofeedstock syntheses. Initially, a hydrosilylative approach to carbohydrate deoxygenation was employed that involved catalysis by an iridium complex supported by a pincer ligand. Under iridium catalysis, the very active diethylsilane reduced all of the C-O bonds in a monosaccharide, resulting in the generation of n-hexane and, presumably through alkyl shift processes, 2-methylpentane and 3-methylpentane. The ratio of alkane product isomers varied for different carbohydrate starting materials. Subsequently, complete hydrosilylative reduction of carbohydrates was catalyzed by commercially available Lewis acidic tris(pentafluorophenyl)borane. Several differences between the borane and iridium systems were observed. Notably, reactions catalyzed by borane proceeded more quickly than those catalyzed by iridium. This increased catalytic activity allowed the use of less active tertiary silanes, such as dimethylethylsilane and triethylsilane, as the reductive equivalent. Hydrosilylative reduction using these tertiary silanes proved to be selective for certain sites on the carbohydrate, affording partially deoxygenated compounds while retaining some of the carbohydrate stereochemistry. Close observation of the product stereochemistry revealed that certain substrates were epimerized during the course of the reaction. These data led to a proposed mechanism involving intramolecular cyclization to give a cyclic silyl oxonium species as an intermediate. The proposed cyclic intermediate was then independently generated and characterized. These intermolecular cyclization processes provide an explanation for an array of observations, suggesting that they may be quite prevalent in deoxygenation reactions of carbohydrates.
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  • In Copyright
Advisor
  • Brookhart, Maurice
  • Gagne, Michel
  • Schauer, Cynthia
  • Waters, Marcey
  • Miller, Alexander
Degree
  • Doctor of Philosophy
Degree granting institution
  • University of North Carolina at Chapel Hill Graduate School
Graduation year
  • 2014
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  • Chapel Hill, NC
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  • This item is restricted from public view for 2 years after publication.
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