General procedure for the synthesis of 5,6-difluoroindolin-2,3-dione from 5,6-difluoroindolin-2-one: TiCl4 (0.7 mL, 6 mmol) was added drop-wise to a stirring suspension of Zn powder (0.78 g, 12 mmol) in freshly distilled anhydrous THF (15 mL) at room temperature and under dry N2 atmosphere. After addition, the mixture was refluxed for 2 hours. The resulting suspension of low-valent titanium reagent was cooled to room temperature. Subsequently, a solution of 5,6-difluoroindoline-2,3-dione (2 mmol) in THF (10 mL) was slowly added. The reaction mixture was stirred at room temperature for about 5 min under N2 protection. After confirming the completion of the reaction by thin layer chromatography (TLC) analysis, the reaction was quenched with 3% HCl (15 mL) and extracted with CHCl3 (3 x 50 mL). The organic phases were combined, washed with water (3 x 50 mL) and dried over anhydrous Na2SO4. After concentration of the solvent under reduced pressure, the crude product was purified by column chromatography (petroleum ether/ethyl acetate=5:1) to afford the target compound 5,6-difluoroindol-2-one.
