Photocatalytic Approach Towards Benzimidazole Synthesis and Oxidation of Indoles by Porphyrins

This work is focused on the photocatalytic applications of three porphyrins having electron-withdrawing and electron-donating groups at their meso-positions. Porphyrins were tested for the diamine coupling reaction and chemoselective oxidation of indole derivatives under visible light. The porphyrin with meso-tetrakis-p-cyanophenyl groups found to be the best photoredox catalyst as compared to the rest of them. For diamine coupling, the method utilizes 0.25 mol% of the porphyrin catalyst and wide variety of aldehydes and diamine were converted to benzimidazole derivatives in high yields (up to 99%). Computational studies were done to optimize the geometry of porphyrin catalysts and provide mechanistic support for the benzimidazole synthesis. Also, oxidation of indole derivatives was achieved in decent to high yields (up to 90%) with just 0.1 mol% of porphyrin, demonstrating the traits of an efficient, and versatile photocatalyst for organic transformations.