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Origin of Stereocontrol in Photoredox Organocatalysis of Asymmetric α-Functionalizations of Aldehydes

Li, Man, Sang, Yueqian, Xue, Xiao-Song, Cheng, Jin-Pei
Journal of organic chemistry 2018 v.83 no.6 pp. 3333-3338
aldehydes, catalysts, catalytic activity, enamines, enantioselectivity, models, moieties, organic chemistry, redox reactions
The merger of the common photoredox catalyst Ru(bpy)₃Cl₂ with an imidazolidinone organocatalyst by MacMillan et al. has enabled a series of highly enantioselective α-functionalizations of aldehydes, a landmark discovery in photoredox organocatalysis. Herein, we present the theoretical investigation into the origin of enantioselectivity in asymmetric radical additions to the MacMillan imidazolidinone enamines, the key stereocontrolling step in photoredox organocatalysis of asymmetric α-functionalizations of aldehydes. The calculations reveal a hidden but crucial role of E-cis enamine in enantiocontrol. The enantioselectivity in the radical additions is mainly determined by steric effects. A model based on the pseudo C₂-symmetric arrangement of the methyl and tert-butyl moieties on the catalyst is proposed. This rationalizes the stereoselective outcome of these reactions and provides a good model to understand MacMillan’s imidazolidinone/photoredox dual catalysis. The insights obtained from this study should be valuable in future efforts toward the design and development of new enantioselective catalytic radical reactions.