General procedure for the synthesis of 1-methyltribromoimidazole from N-methylimidazole: To a solution of N-methylimidazole (1.64 g, 19.97 mmol) and sodium acetate (25 g, 300 mmol) in acetic acid (180 mL) was added drop-wise a solution of bromine (9.6 g, 60.07 mmol) in acetic acid at room temperature. The reaction mixture was stirred at room temperature for 2.5 hours. Upon completion of the reaction, the acetic acid was removed by distillation under reduced pressure and the residue was suspended in 500 mL of water and stirred for 10 min at room temperature. The precipitate was collected by filtration, washed with water and dried under high vacuum to give 2,4,5-tribromo-1-methyl-1H-imidazole (1.82 g, 29% yield, some of the product was retained in the mother liquor) as a light yellow powder.
The resulting tribromide (1.82 g, 5.71 mmol) was suspended in 45 mL of water, sodium sulfite (13 g, 103 mmol) was added, and the reaction mixture was stirred under rapid reflux conditions for 24 hours. After cooling to room temperature, the organic phase was extracted with ether (3 x 75 mL), the combined organic phases were dried with magnesium sulfate, filtered and concentrated to give 1.61 g of a mixture of tribromo, dibromo and monobromoimidazole.
The mixture was again subjected to reducing conditions using 15 mL of a 3:1 water/acetic acid solvent mixture and the same amount of sodium sulfite and heated in a sealed vessel at 130 °C for 60 hours. After cooling to room temperature, the pH of the reaction mixture was adjusted to 9-10 with 2N sodium hydroxide. the organic phases were extracted with ether (3 x 50 mL), combined and dried over magnesium sulfate, filtered and concentrated to give the crude 4-bromo-1-methyl-1H-imidazole (571 mg, about 62% yield).
Using 4-bromo-1-methyl-1H-imidazole (571 mg, about 3.53 mmol), 4-butyl-1-methyl-1H-imidazole (95 mg, 22% yield) was synthesized by substituting hexylboronic acid with propylboronic acid (372 mg, 4.24 mmol) according to the method of Example 3.1.
Butyl lithium (0.34 mL, 2.5 M hexane solution) was added dropwise to a 2 mL anhydrous tetrahydrofuran solution of diisopropylamine (0.13 mL, 0.918 mmol) at -0 °C. The solution was stirred while warming to -20 °C over 20 min. After cooling to -78 °C, 2 mL of anhydrous tetrahydrofuran solution of 4-butyl-1-methyl-1H-imidazole (95 mg, 0.765 mmol) was added dropwise. The reaction mixture was stirred at -78 °C for 40 min. Dimethylformamide (0.24 mL, 3.06 mmol) was added and the solution was stirred while warming to room temperature. The reaction mixture was poured into 15 mL of 1N hydrochloric acid and stirred for 5 minutes. The pH of the reaction mixture was adjusted to 7-8 by careful addition of saturated sodium bicarbonate solution. the organic phase was extracted with dichloromethane (3 x 20 mL), combined and dried over magnesium sulfate, filtered and concentrated. The crude product was purified by silica gel column chromatography using gradient elution (5-50% hexane solution of ethyl acetate) to afford 1-methyl-4-propyl-1H-imidazole-2-carbaldehyde (9 mg, 8% yield) as an off-white solid.
