Enantioselective Hydroalkylation of Alkenylpyridines Enabled by Merging Photoactive Electron Donor–Acceptor Complexes with Chiral Bifunctional Organocatalysis
Jing Guo, Ying Xie, Zemin Lai, Jiang Weng, Albert S. C. Chan, Gui Lu
Abstract
The potential of electron donor–acceptor (EDA) complex photochemistry has recently been recognized in visible-light-induced photocatalyst-free radical reactions. The design of catalytic asymmetric reactions driven by EDA complexes remains a substantial challenge, and existing examples are limited to sole activation modes with aminocatalysts or phase-transfer catalysts. Herein, we demonstrate that chiral bifunctional hydrogen-bonding catalysis can realize the asymmetric reaction of an EDA complex via dual activation modes and afford vicinal tertiary stereocenters at the β,γ-positions of pyridines in high yields with good enantio- and diastereoselectivities. Mechanistic studies suggest that the crucial success factor for this transformation is the use of chiral phosphoric acid (CPA), which not only accelerates the in situ formation of EDA aggregates between redox-active esters (RAEs) and Hantzsch esters (HEs) but also provides proper substrate activation and asymmetric induction.