A Combined-Redox Synthesis of 2-Alkylbenzimidazoles from 2-Nitroanilines by Semiconductor Photocatalysis

Abstract

One-pot photocatalytic synthesis of 2-substituted benzimidazoles from 2-nitroanilines with different alcohols by combined redox cyclization reaction using TiO₂-P25 is reported. The reaction, when performed with ethanol, gives 2-methylbenzimidazole as a single product. However, in a mixture of ethanol and water (1:1), it gives both 2-methylbenzimidazole and 1-ethyl-2-methylbenzimidazole. The products have been characterized by Fourier transorm–infrared, ¹H NMR, ¹³CNMR, and gas chromatography–mass spectrometry analysis. TiO₂-P25 behaves as a combined redox photocatalyst by reducing 2-nitroaniline and oxidizing alcohols simultaneously under ultraviolet light. Easy product isolation under mild conditions and good yield make this one-pot synthesis ecofriendly.

GRAPHICAL ABSTRACT

Keywords:

• Combined redox reaction
• 1-ethyl-2-methylbenzimidazole
• 2-methylbenzimidazole
• photocatalytic cyclization
• TiO₂-P25

ACKNOWLEDGMENTS

K. Selvam is thankful to the Council of Scientific and Industrial Research, New Delhi, for the award of a senior research fellowship. The authors thank the Ministry of Environment and Forests (MOEF), New Delhi, for the financial support through Research Grant Nos. 315-F-36 and 19/9/2007-RE.

Notes

Notes. [o-nitroaniline] = 14 mg / 30 mL; catalyst = 50 mg/30 mL; time of irradiation = 3 h; I = 1.381 × 10⁻⁶ Einstein L⁻¹ s⁻¹.

Notes. [o-nitroaniline] = 14 mg/30 mL; catalyst = 50 mg/30 mL; acetonitrile–ethanol (1:1); time of irradiation = 3 h; I = 1.381 × 10 ⁻⁶ Einstein L⁻¹ s ^−1..

Notes. [o-nitroaniline] = 14 mg/30 mL; catalyst suspended = 50 mg/30 mL; time of irradiation = 3 h; I = 1.381 × 10 ⁻⁶ Einstein L⁻¹ s ⁻¹ .

Notes. [Substituted o-nitroaniline] = 14 mg/30 mL; catalyst suspended = 50 mg/30 mL; acetonitrile-ethanol (1:1); time of irradiation = 3 h; I = 1.381 × 10 ⁻⁶ Einstein L⁻¹ s⁻¹.