[Synthesis]
N-Bromosuccinimide (NBS, 2.66 g, 14.9 mmol) was slowly added to a solution of N,N-dimethylformamide (DMF, 50 mL) of 2-ethyl-6-methylaniline (2.03 g, 15 mmol) at 0 °C. The reaction mixture was stirred at room temperature for 10 min and then poured into saturated aqueous sodium chloride solution. The mixture was extracted with ethyl acetate (EtOAc) and the organic phase was washed twice with saturated aqueous sodium chloride solution, concentrated and purified by a Biotage fast chromatography system (40 M silica gel column, 15% ethyl acetate/heptane as eluent) to give 4-bromo-2-ethyl-6-methyl aniline as a reddish brown liquid (3.21 g, 100% yield). 4-Bromo-2-ethyl-6-methylaniline (3.21 g, 15 mmol) was dissolved in acetic acid (50 mL) and stirred for 3 h. 2M aqueous sodium nitrite (11 mL, 22.5 mmol) was added dropwise. The reaction mixture was stirred at room temperature overnight. The reaction solution was concentrated and the resulting solid was dissolved in ethyl acetate and washed three times with saturated aqueous sodium chloride solution. The organic layer was dried over anhydrous sodium sulfate, filtered and concentrated, and the crude product was purified by a Biotage fast chromatography system (40 M silica gel column, 15-30% ethyl acetate/heptane gradient elution) to afford 5-bromo-7-ethyl-1H-indazole (1.11 g, 33% yield) and 5-bromo-3,7-dimethyl-1H-indazole (0.84 g, 25% yield), respectively. 5-Bromo-7-ethyl-1H-indazole (225 mg, 1.00 mmol), molybdenum hexacarbonyl (132 mg, 0.50 mmol), Herrmann's catalyst (trans-bis(acetic acid)bis[o-(di-o-tolylphosphino)benzyl]dipalladium, 46.9 mg, 0.05 mmol), and aqueous sodium carbonate (318 mg, 3.00 mmol, dissolved in 2 mL of water) were added to 1,4-dioxane (1.5 mL). The suspension was sealed and irradiated in a microwave reactor at 165 °C for 15 min (high absorption setting). Upon completion of the reaction, the reaction vial was opened, filtered through diatomaceous earth, washed with ethyl acetate and concentrated to afford the target compound (140 mg, 74% yield). |