General procedure for the synthesis of 2-oxo-2H-indole-6-carbonitrile from zinc cyanide and 6-bromo-2-indolone: Commercially available 6-bromo-2-indolone (656 mg), zinc cyanide (288 mg), and tetrakis(triphenylphosphine)palladium(0) (175 mg) were suspended in anhydrous N,N-dimethylformamide (6 mL). The resulting mixture was degassed by three argon pump/exhaust cycles and subsequently transferred to a preheated oil bath (80°C). After stirring the reaction mixture at a constant temperature for 15 h, it was cooled to room temperature, diluted with deionized water (60 mL) and extracted with ethyl acetate (3 x 60 mL). After combining the organic phases, it was washed with deionized water (2 x 60 mL), dried over anhydrous magnesium sulfate, filtered and concentrated under reduced pressure. The residue was purified by fast column chromatography (silica gel, dichloromethane/methanol gradient elution) to afford the target product 2-oxo-2H-indole-6-carbonitrile (385 mg, 81% yield). Mass spectrometry analysis showed [M+H]+ peak of 159.
