Mechanism of Diphenylprolinol Silyl Ether Catalyzed Michael Addition Revisited—but Still Controversial

摘要

The stereoselective conjugate addition of chiral enamines to prochiral nitro alkenes to provide γ-nitro carbonyl derivatives is widely recognized as a powerful carbon-carbon bond-forming process. The discovery that the cnamine can be produced in situ from the carbonyl compound and a catalytic amount of a chiral amine led several research groups to search for efficient amine catalysts. Among the different catalysts applied to the reaction, the silylated prolinol 1, introduced by Hayashi and co-workers, proved to be particularly efficient (Scheme 1). The reaction shown in Scheme 1 exhibits high facial selectivity with respect to both reaction partners, thus allowing two adjacent stereogenic centers to be installed with high enantio- and diastereoselectivity using a wide range of substrates. Its synthetic versatility has been demonstrated by cascade reaction sequences leading to complex biologically active compounds. This Michael addition also serves as a benchmark for assessing new catalysts. Although this catalytic process has attracted high interest