Ytterbium acetate promoted asymmetric reductive amination: Significantly enhanced stereoselectivity

Reductive amination of prochiral unhindered 2-alkanones 1 with (R)- or (S)-alpha-MBA in the presence of Yb(OAc)(3) (50-110 mol %), Raney-Ni, and hydrogen (120 psi) results in increased diastereoselectivity for the amine products 2 (80-89% de) with good yield (80-87%). The increased de is based on comparison with the best previously reported de's when using (R)- or (S)-alpha-MBA, regardless of the strategy employed [stepwise (isolation of ketimines) or one-pot (reductive amination)], reducing agent examined, or achiral Lewis acid or Bronsted acid examined. An in situ cis- to trans-ketimine isomerization mechanism, promoted by Yb(OAC)(3), has been proposed to account for the observed increase in diastereoselectivity and suggests a new entry into the control of ketimine geometry.