Ruthenium-catalyzed synthesis of novel phenoxy(phenyl)-1H-benzo[d]imidazoles: Spectral characterization and DFT studies

The synthesis of a series of novel phenoxy(phenyl)-1H-benzo[d]imidazoles by oxidative cyclization of phenoxybenzaldehyde and diamine in the presence of ruthenium(II)-catalysts is described_. The transformation using a catalytic amount of [RuCl₂(p-cymene)]₂ was found to be highly efficient as the reaction selectively produces monoarylated benzimidazoles in excellent yields (up to 92 %). This result signify high efficacy of the catalyst for the selective synthesis of phenoxy(phenyl)-1H-benzo[d]imidazoles (monoarylated benzimidazole-containing diphenyl ether scaffolds). The use of other ruthenium catalyst viz. N^O chelate ruthenium(II)-complex gave monoaryled benzimidazole and 1,2-diarylbenzimidazole as the major and minor products, respectively. On the other hand, the reaction carry out using excess MgSO₄/TFA produced mono- and 1, 2-diarylbenzimidazole derivative in almost equal amount or diarylated compounds as the major product. Electron withdrawing substituent in the diphenyl ether ring gave higher yields then electron donating substituents. Theoretical study such as structure optimization, HUMO-LUMO, hardness, softness and electronegativity determination of 2-arylbenzimidazole derivatives have been carried out in DFT/B3LYP/6–311⁺⁺G(d,p) quantum chemical method and basis set using the Gaussian 09 W-Gaussian-View 5.0.9 package program. The synthesized compounds were characterized using FTIR, NMR and mass spectral data. The crystal structure of a precursor compound, 2-(2, 4-difluorophenoxy)benzaldehyde, has been determined by a single X-ray crystallography.