32
S.C. Pan, B. List
Scheme 27. Asymmetric counteranion-directed catalysis: transfer hydrogenation of enone
used chiral imidazolidinone catalysts gave satisfying yields or enantioselectivities in the conjugate reduction of 3-methyl cyclohexenone 33 to
the product 34 (Scheme 27).
Hypothesizing that primary amine catalysts, due to their reduced
steric requirements, might be suitable for the activation of ketones, we
studied various salts of α-amino acid esters. (For pioneering use of primary amine salts in asymmetric iminium catalysis involving aldehyde
substrates, see Ishihara and Nakano 2005; Sakakura et al. 2006; for
the use of preformed imines of α,β-unsaturated aldehydes and amino
acid esters in diastereoselective Michael additions, see Hashimot et al.
1977.) We have developed a new class of catalytic salts, in which both
the cation and the anion are chiral. In particular, valine ester phosphate
salt 35 proved to be an active catalyst for the transfer hydrogenation
of a variety of α,β-unsaturated ketones 36 with commercially available
Hantzsch ester 11 to give saturated ketones 37 in excellent enantioselectivities (Scheme 28; Martin and List 2006).