Mechanism-Based Enantiodivergence in Manganese Reduction Catalysis: A Chiral Pincer Complex for the Highly Enantioselective Hydroboration of Ketones

A manganese alkyl complex containing a chiral bis(oxazolinyl-methylidene) isoindoline pincer ligand is a pre-catalyst for a catalytic system of unprecedented activity and selectivity in the enantioselective hydroboration of ketones, thus producing preparatively useful chiral alcohols in excellent yields with up to greater than 99% ee. It is applicable for both aryl alkyl and dialkyl ketone reduction under mild reaction conditions (TOF > 450 h(-1) at -40 degrees C). The earth-abundant base-metal catalyst operates at very low catalyst loadings (as low as 0.1 mol%) and with a high level of functional-group tolerance. There is evidence for the existence of two distinct mechanistic pathways for manganese-catalyzed hydride transfer and their role for enantiocontrol in the selectivity-determining step is presented.