[Ru(bpy)3]2+ aided photocatalytic synthesis of 2-arylpyridines via Hantzsch reaction under visible irradiation and oxygen atmosphere
A rapid photocatalytic synthesis of unusual Hantzsch products—highly substituted 2-arylpyridines—via Hantzsch reaction has been achieved in the presence of a catalytic amount of [Ru(bpy)3]2+ and molecular oxygen under visible irradiation, using a household white lamp (45 W CFL). Here, the photoredox reaction occurs without the addition of any sacrificial donor. The 1,2-dihydropyridine formed in situ plays the role of a sacrificial donor, and superoxide radicals generated from molecular oxygen help in the aromatization of 1,2-dihydropyridine to yield the corresponding 2-arylpyridine selectively as the sole product. Further, the merging of the Fenton reaction with photoredox catalysis for the aromatization of 1,2-dihydropyridine was found to be efficient enough to enhance the rate of the oxidation, but the selectivity of the process was reduced under such conditions, yielding multiple products as detected by LC-MS analysis.