Light-Promoted Dearomative Cross-Coupling of Heteroarenium Salts and Aryl Iodides via Nickel Catalysis

Partially saturated nitrogen heterocycles are versatile building blocks for the preparation of other nitrogen heterocycles. For example, dihydropyridines can be converted to pyridines, tetrahydropyridines, and piperidines through oxidation, reduction, and functionalization reactions, respectively. Dearomatization of heteroarenes is an attractive approach for the synthesis of partially saturated heterocycles such as dihydropyridines due to the wide availability of heteroarenes. Significant research efforts have been dedicated to the addition of nucleophiles to various heteroarenium salts in this direction using organoboron or organometallic reagents. The availability of organoboron and organometallic coupling partners has been an important limitation to this chemistry. Direct coupling of electrophiles with heteroareniums could significantly improve the scope of these dearomatization reactions due to the wider availability of electrophiles compared to nucleophiles such as organoboron and organometallic reagents. Herein, we report the coupling of aryl iodides with pyridinium and related heteroarenium salts catalyzed by Ni/bpp and an Ir photocatalyst using Zn as a terminal reductant. This methodology tolerates a wide range of functional groups and allows the coupling of aryl and heteroaryl iodides, thus significantly expanding the scope of nitrogen heterocycle scaffolds that could be prepared through dearomatization of heteroarenes. The reaction products have been further functionalized to prepare various nitrogen heterocycles. Initial mechanistic studies indicate that the reaction described herein goes through a unique mechanism involving dimers of dihydroheteroarenes.