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Greener Chemistry Using Boronic Acids as Organocatalysts and Stoichiometric Reaction Promoters
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- Author / Creator
- Zheng, Hongchao
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Catalysis is crucial for society in view of producing pharmaceutical agents and commodity chemicals. Industry and academia alike are constantly searching for organic transformations which can not only efficiently produce important pharmaceuticals and commodity chemicals, but can also do so in a manner that is environmentally sound. During the past five years, arylboronic acids have been emerging as a promising new class of organocatalysts for the direct activation of carboxylic acids, alcohols and diols through reversible formation of boronate adducts. Such unique activation modes have resulted in the application of boronic acid catalysis toward the development of more efficient and milder protocols for existing transformations. Boronic Acid Catalysis (BAC) provides such a platform for the pursuit of “green chemistry” by providing a mild means to achieve reactions that would otherwise require harsh or wasteful conditions. To this end, several new methods employing diversely substituted arylboronic acids as organocatalysts and stoichiometric reaction promoters were developed. ortho-Substituted arylboronic acids activate unsaturated carboxylic acids presumably through the formation of an active monoacyl boronate species, which can provide electrophilic activation of the carboxylate group through H-bonding. Chapter 2 describes the application of this activation concept to a variety of cycloadditions and nucleophilic conjugate additions involving unsaturated carboxylic acids. Due to their Lewis acidic character, electron-deficient arylboronic acids exhibit excellent catalytic activity for the activation of hydroxyl groups by facilitating the complete and partial ionization of the C–O bond. This strategy allows the direct activation of hydroxyl groups without recourse to prior activation operations, thus permitting direct functionalizations in a step- and atom-economical manner. The successful application of these catalytic systems to a broad range of classical chemical transformations is also discussed in Chapter 3. Owing to their remarkable binding ability to 1,2- or 1,3-diol frameworks, benzoboroxoles have the potential to serve as transient masks or organocatalysts to control regioselectivity in the glycosylation of fully unprotected sugars. Chapter 4 describes initial attempts in this important area. In search of a milder preparation of 2-aryl-1,3,2-aryldioxaborins, which are stable o-quinomethane precursors, an efficient ZrCl4 catalyzed ortho-hydroxyalkylation of phenols with aldehydes promoted by 3,5-bis(trifluoromethyl)phenylboronic acid was investigated and optimized. This methodology is presented in Chapter 5.
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- Graduation date
- Fall 2012
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- Type of Item
- Thesis
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- Degree
- Doctor of Philosophy
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- License
- 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.