Ternary Electron Donor–Acceptor Complex Enabled Enantioselective Radical Additions to α, β-Unsaturated Carbonyl Compounds

Abstract

An electron donor–acceptor (EDA) complex is an association between a Lewis base (electron donor) and a Lewis acid (electron acceptor) in the ground state. The EDA complex has received widespread attention and been recognized as an attractive synthetic method because this complex can generate active radical ion pairs by light irradiation in mild reaction conditions and does not require any additional photosensitizer. Herein, we successfully developed the first example of visible-light-induced enantioselective radical reaction to α, β-unsaturated carbonyl compounds using a ternary EDA complex in the presence of chiral oxazaborolidinium ions as a chiral Lewis acid catalyst. When the α, β-unsaturated aldehydes were applied in this reaction system, the 1,2-addition products were obtained in high yields (up to 93%) with high enantioselectivities (up to 95% ee). Compared to α, β-unsaturated aldehydes, α, β-unsaturated ketones gave the 1,4-addition products in excellent yields (up to 99%) with high enantioselectivities (up to 94% ee). Various mechanistic experiments, such as UV/vis absorption spectroscopy, supported the generation of the ternary EDA complex between α, β-unsaturated carbonyl compounds coordinated to the chiral oxazaborolidinium ion catalyst and α-silyl amines.