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The direct asymmetric conjugate addition of α-cyanoacetates to enones generating densely functionalized α-amino acid precursors with adjacent quaternary and tertiary stereocenters is described comparing mono- and bis-palladacycle catalysts. This edge article features the complementary value of mono- and bimetallic catalysis in a case study using related catalyst systems. Different major diastereomers of the 1,4-addition products are formed by the use of the planar chiral mono- and bimetallic catalyst systems and provide access to epimeric amino acid derivatives. Both catalyst types require the use of a Brønsted acid (HOAc) as a co-catalyst to avoid an undesired β-hydride elimination. Kinetic studies show that the C–C bond forming step takes place almost instantaneously with the bis-palladium complex after productive substrate coordination. This extraordinarily high reactivity for an elementary step generating a sterically demanding linkage of a quaternary and a tertiary stereocenter stresses the cooperativity of both metal centers.
Eitel et al. (Tue,) studied this question.