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Phosphonate-Directed Catalytic Asymmetric Hydroboration: Synthesis of Functionalized Chiral Secondary and Tertiary Boronic Esters and Mechanistic Insights
Over the past 30 years, catalytic asymmetric hydroboration (CAHB) of alkenes has emerged as a leading methodology to access chiral primary and secondary boronic esters. However, it wasn’t until 2015 that directed-CAHB was for the first time shown to efficiently access chiral tertiary boronic esters. The latter are excellent precursors to synthetically challenging structural motifs such as chiral tertiary alcohols, carbinamines and all-carbon quaternary stereocenters via stereospecific C-B bond substitutions. This dissertation focuses on phosphonate-directed CAHB of diverse alkene substrates, including challenging stereodefined trisubstituted alkenes, to efficiently access multifunctional chiral secondary and tertiary boronic esters. Mechanistic insights obtained via deuterium labelling experiments indicate that CAHB reactions proceed via tertiary-alkyl rhodium intermediates for substrates that lead to chiral tertiary boronic esters. The applications of phosphonate-functionalized chiral secondary and tertiary boronic ester products are discussed in the context of stereospecific C-B bond transformations, through phosphonate unmasking methodologies via thiophosphonate and oxophosphonate intermediates, and through synthesis of the cytotoxic natural product bakuchiol.
Chakrabarty, Suman, "Phosphonate-Directed Catalytic Asymmetric Hydroboration: Synthesis of Functionalized Chiral Secondary and Tertiary Boronic Esters and Mechanistic Insights" (2019). ETD collection for University of Nebraska - Lincoln. AAI22592183.