Step 1: Preparation of 3-(4-bromophenyl)-1H-pyrrole-2,5-dione
4-Bromoaniline (7.48 g, 43.52 mmol) was dissolved in a mixed solution of hydrochloric acid (37%, 13 mL) and water (5.5 mL) and stirred vigorously for 30 min at room temperature to form a precipitate. The reaction mixture was cooled to 0 °C and a solution of sodium nitrite (3.30 g, 47.87 mmol) in water (9 mL) was added slowly and dropwise until the reaction solution changed to a clarified yellow solution, indicating completion of the diazotization reaction. Subsequently, a solution of maleimide (8.45 g, 87.05 mmol) in acetone (35 mL) was added dropwise at 0 °C and the pH of the reaction solution was adjusted to 3-3.5 by the addition of sodium acetate. copper(II) chloride (0.88 g, 6.57 mmol) was added to the vigorously stirred reaction mixture. The reaction mixture was stirred at 0°C for 1 hr and then transferred to room temperature to continue stirring overnight. After confirming the completion of the reaction by thin layer chromatography (TLC), the solvent was removed by distillation under reduced pressure to give the crude product. The crude product was purified by column chromatography (silica gel, eluent ratio of 4:6 ethyl acetate:petroleum ether) to afford the target compound 3-(4-bromophenyl)-1H-pyrrole-2,5-dione as a yellow solid (5.8 g, 57% yield). Electrospray ionization mass spectrometry (ESIMS) showed m/z of 252.3 ([M+H]+).
