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Oxime-directed Catalytic Asymmetric Hydroboration and Hydrogenation
Both catalytic asymmetric hydroboration (CAHB) and catalytic asymmetric hydrogenation (CAH) have been studied for more than 30 years. However, previously developed methods still have significant limitations. A multitude of primary and secondary boronic esters have been successfully prepared via CAHB, but the synthesis of chiral tertiary boronic esters via CAHB is limited to aryl substituted structures only. This thesis will discuss oxime ether-directed CAHB of methylidene and trisubstituted alkene substrates leading to chiral tertiary boronic esters with up to 87% yield and 96:4 er. In addition, regiodivergent hydroboration of aryl substituted methylidenes will be presented. The utility of the CAHB methods is demonstrated through the several stereospecific transformations of resulting boronic esters. Studies into the oxime ether-directed CAHB led to another unexpected discovery. The addition of a proton source or H2 to the reaction mixture leads to the exclusive formation of the hydrogenation product. An efficient borane promoted CAH method, which works very well for various aryl and alkyl di-, tri- and tetrasubstituted alkenes, was developed as a result. Two small natural products (–)-lasiol and (–)-enterodiol were prepared using this new hydrogenation methodology.
Shoba, Veronika, "Oxime-directed Catalytic Asymmetric Hydroboration and Hydrogenation" (2018). ETD collection for University of Nebraska-Lincoln. AAI10846217.