Additive- and Oxidant-Free Expedient Synthesis of Benzimidazoles Catalyzed by Cobalt Nanocomposites on N-Doped Carbon

^◊ These authors contributed equally.

Abstract

A one-pot direct synthesis of a wide range of biologically active benzimidazoles through coupling of phenylenediamines and aldehydes catalyzed by a highly recyclable nonnoble cobalt nanocomposite was developed. A broad set of benzimidazoles can be efficiently synthesized in high yields and with good functional-group tolerance under additive- and oxidant-free mild conditions. The catalyst can be easily recycled for successive uses, and the process permits gram-scale syntheses of benzimidazoles.

Key words

cobalt catalysis - heterogeneous catalysis - benzimidazoles - phenylenediamines - aldehydes - coupling reaction

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• 11 2-Phenyl-1H-benzimidazole (3a); Typical Procedure A 25 mL sealed tube equipped with a magnetic stirrer bar was charged with catalyst CoO _x @NC-800 (20 mg, 10 mol%) then sealed with a rubber septum and evacuated to remove air. Benzene-1,2-diamine (0.2 mmol), PhCHO (0.24 mmol), and THF (5 mL) were injected into the tube from a syringe, and the tube was placed in a preheated oil bath at 100 °C for 8 h. When the reaction was complete, the mixture was filtered and the organic layers were collected, combined, dried (Na₂SO₄), and concentrated under vacuum. The residue was purified by passage through a column of silica gel (200–300 mesh) to give a white solid; yield: 38.1 mg (98%). ¹H NMR (400 MHz, DMSO-d ₆): δ = 8.19 (s, 2 H), 7.59 (m, 5 H), 7.22 (s, 2 H). ¹³C NMR (101 MHz, DMSO-d ₆): δ = 151.7, 139.8, 130.5, 130.4, 129.4, 126.9, 122.6, 115.7.
• 12 Gram-Scale Synthesis of 2-Phenyl-1H-benzimidazole (3a) A 500 mL round-bottomed flask equipped with a condenser and a magnetic stirrer bar was charged with benzene-1,2-diamine (1 g, 9.25 mmol), PhCHO (13.3 mmol), and catalyst CoO _x @NC-800 (925 mg, 10 mol%), then sealed with a rubber septum and evacuated to remove air. THF (200 mL) was added to the flask from a syringe, and the mixture was refluxed for 8 h. When the reaction was complete, the mixture was filtered, and the filtrate was concentrated in a rotary evaporator. The product was then purified by column chromatography; yield: 1.54 g (86%). The structure of the product was confirmed by NMR spectroscopy.