Synthesis, Characterization, and Structure of Quinoline-based Benzimidazole Derivatives

Quinoline-based benzimidazole compounds have been successfully synthesized and characterized by various spectroscopic techniques like FT-IR, H-1 NMR, C-13 NMR, and mass spectral analysis, and the structures have been authenticated by single crystal X-ray diffraction method. Here, we report an economical, mild, and efficient procedure that involves condensation of 8-hydroxyquinoline-2-carbaldehyde with various diamines as substrates to give bis-imines. A systematic study towards both aliphatic and aromatic bis-imines has been conducted to investigate the ring-closure reaction that generates the benzimidazole moiety in the heterocyclic compounds discussed in this study. Aliphatic bis-imines does not undergo cyclization; however, the bis-imines derived from o-phenylenediamine and derivatives generates heterocyclic compounds with benzimidazole moiety. In contrast to synthetic procedures reported earlier for benzimidazoles, the reaction conditions herein reported are simpler. Path for reactions holds initial condensation with one equivalent of 8-hydroxyquinoline-2-carbaldehyde to form mono-imine followed by immediate intramolecular ring closure. The subsequent nupleophilic attack to another equivalent of 8-hydroxyquinoline-2-carbaldehyde and migration of hydride generates the benzimidazole moiety and the active methylene group. The CH2 group was confirmed from H-1 and C-13 NMR, wherein the two hydrogens appeared at around 6.40-6.52 ppm and the carbon center appeared at 51.54-51.77 ppm. The single crystal X-ray diffraction also confirmed the formation of benzimidazole moiety and the active methylene group in the heterocyclic compounds discussed in this study.