Exploring a metal/base-free porphyrin involving a carboxyl-functionalized pyridine moiety for photocatalytic N-arylation of benzamide validated using RSM

Abstract

A porphyrin comprising a carboxyl-functionalized pyridine moiety was synthesized and characterized using ¹H NMR, ¹³C NMR, FT-IR, powder-XRD, BET, ICP-MS, SEM and EDAX. The proton level (H₀ = 1.19) and energy band gap (1.39 eV) were determined via UV-Vis spectrophotometry. The UV-visible and fluorescence emission spectra indicated the absorption window of the porphyrin photocatalyst with a distinct Soret band at 424 nm and four Q-bands at 517, 558, 595, and 649 nm. The existence of four Q-bands, the powder XRD data and the ICP-MS analysis supported the absence of metal in the porphyrin photocatalyst. The best photocatalytic conditions generated using Box–Behnken design of RSM (0.2 mol% PcCFP, 5 W LED, 1 : 1.2 ArX : ArCONH₂, 24 h) were confirmed through the model reaction of benzamide and 1-bromo-4-nitrobenzene. The N-arylation of benzamide was achieved in a custom-built photoreactor at ambient conditions under exposure to 5 W LED light. Different ArX compounds comprising electron-repelling and electron-attracting groups were assessed to test the potential of the photocatalyst. The porphyrin was found to exhibit significant catalytic activity for C–N bond formation, resulting in 21–73% yields of the substituted benzanilide products. The N-arylated benzamide formation was confirmed using ¹H NMR, ¹³C NMR, HR-MS and SC-XRD. Additionally, heteroaryl halides such as 2-bromo-, 3-bromo-, and 4-bromo-pyridine, as well as 2-chloro-4-methylpyridine, were also found to be compatible and provided admirable yields (28–67%). The stability and heterogeneous nature of the porphyrin photocatalyst were confirmed using FT-IR. The stability of the photocatalyst after the sixth run was demonstrated by the slight decline in the yield of the product from 71 to 67%. The formation of an aryl radical was detected using the scavenger TEMPO, which led to the achievement of N-arylated benzamides containing intermediates of industrial drugs.