A Selenourea-Thiourea Brønsted Acid Catalyst Facilitates Asymmetric Conjugate Additions of Amines to α,β-Unsaturated Esters
Yingfu Lin, William J. Hirschi, Anuj Kunadia, Anirudra Paul, Ion Ghiviriga, Khalil A. Abboud, Rachael W. Karugu, Mathew J. Vetticatt, Jennifer S. Hirschi, Daniel Seidel
Abstract
β-Amino esters are obtained with high levels of enantioselectivity via the conjugate addition of cyclic amines to unactivated α,β-unsaturated esters. A related strategy enables the kinetic resolution of racemic cyclic 2-arylamines, using benzyl acrylate as the resolving agent. Reactions are facilitated by an unprecedented selenourea-thiourea organocatalyst. As elucidated by DFT calculations and 13C kinetic isotope effect studies, the rate-limiting and enantiodetermining step of the reaction is the protonation of a zwitterionic intermediate by the catalyst. This represents a rare case in which a thiourea compound functions as an asymmetric Brønsted acid catalyst.