30
S.C. Pan, B. List
the reaction with significant enantiomeric excess (ee) values
(Scheme 24). After a thorough screening of various amines we identified morpholine salt 28 as a highly enantioselective catalyst (Mayer
and List 2006).
Treating aromatic, trisubstituted α,β-unsaturated aldehydes 20 with
a slight excess of dihydropyridine 21 and a catalytic amount of salt 28
at 50 °C in dioxane for 24 h, the corresponding saturated aldehydes 22
were obtained in high yields and in enantioselectivities of 96–99% ee
(Scheme 25).
Significantly, the previously developed chiral amine based catalysts
that we and MacMillan and co-workers have studied have not been of
use for sterically nonhindered aliphatic substrates. For example, citral
(29), of which the hydrogenation product citronellal (30) is an intermediate in the industrial synthesis of menthol and used as a perfume
ingredient, could not readily be used (Scheme 26, Eq. 41). We could
Scheme 25. Asymmetric counteranion-directed catalysis: transfer hydrogenation of enals