6-Bromo-1H-indole-3-carbaldehyde was synthesized as follows:
1) N,N-dimethylformamide (10 mL) was cooled to 0 °C under nitrogen protection and phosphorus trichloride (3.2 mL, 34.6 mmol) was slowly added keeping the reaction temperature between 0 °C and 80 °C. The reaction was carried out by stirring the reaction mixture for 30 min.
2) Stir the reaction mixture at 0 °C for 30 min.
3) Slowly add a solution of N,N-dimethylformamide (28 mL) of 6-bromoindole (5.5 g, 28.1 mmol), keeping the reaction temperature between 0 °C and 10 °C.
4) The ice water bath was removed and the reaction mixture was stirred at room temperature for 2 hours.
5) The viscous reaction mixture was poured into ice water (250 g) and the pH was adjusted to about 7 with 1N sodium hydroxide solution (tested using litmus paper).
6) The mixture was allowed to stand at room temperature overnight.
7) The pink solid was collected by filtration, washed with water, and recrystallized from ethanol to give 6-bromo-1H-indole-3-carbaldehyde (1.6 g, 25% yield) as a light tan solid.
1H NMR (d6-DMSO, 400 MHz): δ 12.20 (br s, 1H), 9.91 (s, 1H), 8.31 (d, J = 3 Hz, 1H), 8.00 (d, J = 9 Hz, 1H), 7.69 (d, J = 2 Hz, 1H), 7.34 (dd, J = 8, 2 Hz, 1H).
