A Boron-Oxygen Transborylation Strategy for the Asymmetric Reduction of Ketones

The asymmetric reduction of ketones with pinacolborane (HBpin) has been catalysed by an enantioenriched borane catalyst, formed in situ from 9-borabicyclo[3.3.1]nonane (H-B-9-BBN) and β-pinene. While previously only used stoichiometrically, this borane has now been rendered catalytic using a new turnover mechanism, B-O transborylation. Thermodynamic and kinetic parameters of the key B-O/B-H transborylation showed this is a sub-class of σ-bond metathesis.