Chiral induction from solvents-lactic acid esters in the asymmetric hydroboration of ketones

The hydroboration of acetophenone in the chiral solvent (S)-methyl lactate exhibits moderate enantioselectivities. A six-membered transition state involving the ketone, the borane, and the lactate as the only chiral source is proposed. Molecular modeling explains the experimentally observed enantioselectivities. Calculated ee-values are in accordance with those experimentally observed. Improved ee-values (up to 60%) can be obtained in the presence of stoichiometric amounts of Lewis acid at lower reaction temperatures.