Mechanism and Stereocontrol in Planar-Chiral Organocatalyst-Mediated Enantioselective Additions of Pyrroles to Ketenes: CH···O interactions and Stereoelectronic Effects.
Abstract: Computational study of planar-chiral 4-(pyrrolidino) pyridine (PPY*) catalysis is reported. A chiral-acid resting state and a chiral-base rate determining step are operative in the enantioselective coupling of ketenes and pyrroles. The catalyst controls the enantioselectivity via a combination of stereoelectronic effects and CH···O interactions. The former differentiates the E/Z-catalyst enolates in the transition states while the latter controlthe facial selectivity of the protonation.