Sequential Visible-Light Photoactivation and Palladium Catalysis Enabling Enantioselective [4+2] Cycloadditions.

Abstract

Catalytic asymmetric cycloadditions of reactive ketene intermediates provide new opportunities for the production of chiral heterocyclic molecules. Though known for over 100 years, ketenes still remain underexplored in the field of transition-metal (TM)-catalyzed asymmetric cycloadditions because (1) ketenes, as highly electron-deficient species, are possibly unstable to low-valence TMs (i.e., decarbonylation or aggregation) and (2) the conventional thermal synthesis of ketenes from acyl chlorides and amines may be incompatible with TM catalysis (i.e., reactive acyl chloride and amine hydrochloride byproducts). Herein, we detail the unprecedented asymmetric [4+2] cycloaddition of vinyl benzoxazinanones with a variety of ketene intermediates via sequential visible-light photoactivation and palladium catalysis. It is well demonstrated that the traceless and transient generation of ketenes from α-diazoketones through visible-light-induced Wolff rearrangement is important for the success of present cycloaddition. Furthermore, chiral palladium catalysts with a new, chiral hybrid P, S ligand enable asymmetric cycloaddition with high reaction selectivity and enantiocontrol.