Cu-porphyrin-mediated photocatalytic system for C-N coupling: Base-free N-arylation ullmann-type approach to benzotriazole

Abstract

Heterogeneous photocatalytic C-N cross coupling is an inspiring synthetic approach that proposes environmentally benign way to achieve a range of pharmaceutically important N-arylated heterocyclic derivatives from the corresponding substrates. Nevertheless, despite abundant protocols in catalyst strategy and progress of reaction parameters, C-N coupling of N-H azoles with inactivated aryl halides continues an unsolved obstacle in synthesis. We report a Cu-porphyrin catalyzed visible light-assisted photocatalytic technique for the N-arylation of benzotriazole and other N-heterocyclic compounds with activated and inactivated Ar-X in a photo-chamber at ambient conditions. A novel porphyrin was synthesized from IL based precursor and characterized by ¹H NMR, ¹³C NMR, SEM-EDX. The C-N bond formation advances with high N-position selectivity under optimized photocatalytic conditions under exposure of 5 W LED lights. The ambient conditions and tolerance of diverse activated/inactivated functionalities of the ArX facilitated a library of N-arylated heterocycles with admirable yields (60 %-85 %). The mechanism of C-N bond formation was aided via Ar* formation, trapped by the TEMPO scavenger-adduct (m/z = 261.19) from the GC–MS spectral analysis. Ease of separation and steadiness of the photo-catalyst with respect to efficiency found appreciable by maintaining its structural similarity and reused for five times with yield reduction from 85 % to 74 % under standard conditions. The stability of Cu-PcBILSAM photocatalyst was assessed by proton NMR, FT-IR spectra and concluded that no structural changes found of after 5th run. The heterogeneous characteristic of the porphyrin was further endorsed by seeping experiment during the N-arylation of benzotriazole and a productive protocol to synthesize pharmaceutical precursors.