Asymmetric Hydrogenation of Enamides

Significance

An asymmetric Hantzsch ester mediated transfer hydrogenation of aromatic enamides catalyzed by a combination of chiral phosphoric acid 1 and acetic acid is reported. Mechanistic assumptions involve the acid-dependent generation of iminium ions 2 and 3, wherein the phosphoric acid activated intermediate 2 exclusively undergoes further reaction. The role of the achiral acid apparently lies in the perpetuation of a sufficient quantity of the iminium species, as well as in the neutralization of the basic pyridine byproduct.