Scope and Mechanism of Tandem Aza-Michael Reaction/Enantioselective Protonation Using a Pd-μ-Hydroxo Complex under Mild Conditions Buffered with Amine Salts

The tandem aza-Michael reaction/enantioselective protonation of a-substituted alpha,beta-unsaturated carbonyl compounds is described in detail. The key to success is the combined use of a Bronsted basic palladium-mu-hydroxo complex and amine salts, which allows for the controlled generation of active catalyst and nucleophilic free amines. This catalytic system was applicable to various acceptors and aromatic amines, and the desired beta-amino acid derivatives with a chiral center at the alpha position were produced in good yield with excellent enantioselectivity (up to 98% ee). For electron-deficient amines, the introduction of free amine as an additive was effective in promoting the reaction. The results of mechanistic studies, including determination of the absolute configuration of the product, are discussed.