Bromine → 6-Bromo-5-fluoroindoline

A solution of 5-fluoroindoline (4.65 g, 34 mmole) in conc. H2SO4 acid (60 ml) under argon was treated with silver sulfate (5.5 g, 18 mmole) and stirred for 0.5 h, then cooled to −5° C. and treated dropwise over 15 min with bromine (5.6 g, 35 mmole). The mixture was maintained at −5° C. for 0.5 h, then allowed to warm to room temperature over 1 h, before adding cautiously to well stirred ice/water (600 ml). The mixture was filtered through Kieselguhr, then basified by addition of 40% NaOH solution and extracted with Et2O (2×300 ml). The combined extract was dried (Na2SO4) and concentrated under vacuum to afford the title compound as a beige solid (6.5 g, 89%). MH+ 216/218.