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Asymmetric Hydrogenation of Enamides
G. Li, J. C. Antilla*
University of South Florida, Tampa, USA
23 March 2009 (online)
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.