General procedure for the synthesis of 2-chloro-1H-indoles from 2-indolones: at room temperature, phosphorus oxychloride (POCl3, 1 mL, 10.82 mmol, 2 eq.) was slowly added to a well-stirred suspension of N,N-diethylaniline (1 mL, 5.41 mmol, 1 eq.) and 1,3-dihydro-2H-indol-2-one (0.72 g, 5.41 mmol , 1 equiv) in a suspension in anhydrous toluene (15 mL). The reaction mixture was heated to reflux for 3 hours. After completion of the reaction, it was cooled to room temperature and the reaction was quenched by careful addition of water (15 mL). The organic layer was separated and washed with water (3 x 25 mL). The organic layer was dried over anhydrous magnesium sulfate (MgSO4), filtered and concentrated under reduced pressure to give 2-chloro-1H-indole (0.76 g, 93% yield) as an orange solid. The product was confirmed by LC-MS analysis ([M-H]-: 150/152).
