Visible-Light Mediated Reaction of Indole with Aryl Halides Using Porphyrin Photocatalyst: A Feasible Technique to Afford N-Functionalized Indoles

• Published in: Catalysis letters 2024-08, Vol.154 (8), p.4176-4200
• Main Authors: Patil, Vijay Shivaji, Bhagat, Pundlik Rambhau
• Format: Article
• Language: English
• Subjects: Catalysis; Chemistry; Chemistry and Materials Science; Industrial Chemistry/Chemical Engineering; Organometallic Chemistry; Physical Chemistry
• ISSN: 1011-372X; 1572-879X
• DOI: 10.1007/s10562-024-04613-1

The expansion of novel techniques for the construction of C–N bond is a crucial task in synthetic chemistry, owing to involvement of nitrogenous scaffolds including drug moieties and naturally occurring compounds. The formation of N-functionalized indoles is an enormous challenge in pharmaceutical industries and fine chemicals at ambient conditions. Metal/Base free N-arylation methods are rated one of the best favourable protocols for avoiding metal contamination in the finished products. Herein, ionic liquid entangled sulphonic acid functionalized porphyrin-based photocatalyst (ILSAFPc) was prepared and characterized by proton NMR, FT-IR. The band gap and proton level (Hammett acidity) were determined by UV-Vis. spectrophotometer were found to be 1.18 eV and 0.351 respectively. Fluorescence emission spectroscopy showed notable intense Soret band near 424 nm and four Q-bands in the 420, 460, 602, 653 nm region revealed the formation of metal free porphyrin. ILSAFPc was successively employed as a photocatalyst for the N-functionalization of indole with aryl halides to provide the desired N-aryl-indoles in good to excellent yields 62 to 78%. The C–N bond-coupling reactions of aryl halides with indole and other nucleophiles achieved in a Lab-made photoreactor under visible-light (5W LED) irradiation at room temperature in 20h. Mechanistic experiments endorsed the formation of aryl radical using TEMPO scavenger, due to electron generated by ILSAFPc. Moreover, the catalyst could be reused for six consecutive runs with appreciable yields (51%) illustrating its heterogeneous nature. The stability of the ILSAFPc was further confirmed by leaching test and supported by proton NMR and FT-IR studies. These features of the protocol promote the N-arylation of indole, a very precious for the synthesis of fine chemicals and intermediates of pharmaceutical scaffolds. Graphical Abstract