Highly Enantioselective Synthesis of Propargyl Amides through Rh-Catalyzed Asymmetric Hydroalkynylation of Enamides: Scope, Mechanism, and Origin of Selectivity

Chiral propargyl amides are particularly useful structural units in organic synthesis. The enantioselective synthesis of propargyl amide is highly desirable. Conventional approach involves the use of a stoichiometric amount of metal reagent or chiral auxiliary. In comparison, direct alkynylation with terminal alkyne is attractive because it avoids the use of stoichiometric organometallic reagent. The asymmetric coupling of aldehyde, amine, and alkyne (A3-coupling) provides an efficient method for the synthesis of N-alkyl and N-aryl-substituted propargyl amines, but this strategy is not amenable for the direct enantioselective synthesis of propargyl amide. We have developed a new strategy and report here a Rh-catalyzed asymmetric hydroalkynylation of enamides. Alkynylations occur regioselectively at the α position of an enamide to produce chiral propargyl amides. High yield and enantioselectivity were observed. Previous alkynylation methods to prepare chiral propargyl amine involve the nucleophilic addition to an electron-deficient imine. In contrast, our current approach proceeds through regioselective hydroalkynylation of an electron-rich alkene. Kinetic studies indicated that migratory insertion of the enamide to the rhodium hydride is turnover limiting. Computational studies revealed the origin of regio- and enantioselectivities. This novel strategy provides an efficient method to access chiral propargyl amides directly from terminal alkynes.