Acetic Anhydride → (E)-1-(2-pyridyl)-2-[2-(2-pyridyl)ethenyl]-1H-benzimidazole

Participants

Reaction Conditions

Reaction Steps

A stirred mixture of 2-methyl-1-(2-pyridyl)-1H-benzimidazole (0.50 g, 2.39), pyridine-2-carboxaldehyde (0.50 g, 4.67 mmol) and acetic anhydride (5 ml) was heated to 80° C. for 1.5 h and then to reflux for about 6 h. After cooling, the mixture was diluted with water (15 ml), basified with a 10% aqueous solution of sodium hydroxide (100 ml) and extracted with dichloromethane (2×100 ml). The combined organic layers were washed with water (100 ml) and extracted with 2 N hydrochloric acid (3×50 ml). The combined aqueous extracts were basified with a 50% aqueous solution of sodium hydroxide (3×50 ml) and extracted with dichloromethane (4×50 ml). The combined organic extracts were washed consecutively with water (100 ml) and brine (100 ml), dried (magnesium sulfate) and concentrated to dryness. Column chromatography (silica gel, 125 g; ethyl acetate) gave 0.18 g of free base (E)-1-(2-pyridyl)-2-[2-(2-pyridyl)ethenyl]-1H-benzimidazole as a brown oil. This product was dissolved into methanol (5 ml), treated with a 10% methanol solution of hydrogen chloride (5 ml) and concentrated to dryness. Recrystallization from 2-propanol afforded 0.15 g (21%) of the titled compound. MW: 371.27; mp: 148.0-155.0° C.; 1H-NMR (DMSO-6) δ: 8.84-8.78 (1H, m), 8.74-8.69 (1H, m), 8.30-8.22 (1H, m), 8.19-8.10 (2H, m), 8.02-7.95 (1H, m), 7.91-7.84 (2H, m), 7.79-7.70 (2H, m), 7.65-7.40 (4H, m).

Provenance & Evidence