Sc(OTf) 3 and Ambient Air Promote Radical‐Based Cross‐Dehydrogenative Coupling Reactions: Reactivity of α‐Enaminone toward Donor–Acceptor Cyclopropanes

A new cross‐dehydrogenative coupling reaction for tetrahydroquinolinones formation is achieved using α‐enaminones and donor–acceptor cyclopropanes. Traditional cross‐dehydrogenative coupling reactions require the presence of a redox‐active metal, and an additional oxidant. This work introduces a new type of catalytic system, which only involves a redox‐inactive rare earth metal and ambient air serving as the oxidant. Mechanistic investigation has revealed a radical pathway for the cross‐dehydrogenative C sp2 ‐C sp3 coupling reaction and Sc(OTf) 3 combined with 3 O 2 are found to facilitate the critical SET process. When 3 mol% of Sc(OTf) 3 is used, α‐enaminone served as nitrogen nucleophile to react with the donor–acceptor cyclopropanes, resulting in new ring‐opening products. Increasing the catalyst loading to 30 mol%, tetrahydroquinolinones are formed via cross‐dehydrogenative C sp2 ‐C sp3 coupling reaction. Both reactions give good to excellent yields with broad substrate scopes. Enantiospecific formation of tetrahydroquinolinones is also investigated with high retention of enantiomeric purity (up to 97% ee). The discovery that Sc(OTf) 3 and air can promote the formation of α‐enaminone radical and malonic ester radical is unprecedented and is expected to initiate their broad applications in developing novel organic transformations.