Indole-5-carboxylic acid 5.00g [Combiblocks] (31.0 mmol) solution in ethanol 99% 120mL and tert-butanol 180mL in a 1L RB flask was cooled on ice bath to +5 C. Meanwhile, a solution of lithium bromide 9.0g (103.6 mmol) in neat acetic acid 60mL was placed into an addition funnel. Neat bromine 5.0mL (16.0g; 100.1 mmol) was then charged to this LiBr solution and the addition funnel was briefly swirled by hand to mix the reagents. The resulting bromine+LiBr solution was dropwise added into the vigorously stirred indolecarboxylic acid solution at +5 C over a 90 min period. (After a complete addition the addition funnel was then washed with EtOH 2 x 5 mL and the washings were added to the reaction mix). At the end of the bromine addition the cooling bath was let to expire and the reaction mix was stirred at +5 to +15 C bath for 1 hour and at 15 C for additional 15 min. The reaction mixture was then diluted with additional acetic acid 100mL. Zn dust 20g [Aldrich; <10 micron] (306 mmol) was added in one portion (gas evolution!) and the mixture was stirred in an open flask on ambient water bath overnight (16 hours).
The next day, the precipitated solids were collected by filtration, washed with ethanol and dried by suction. The solid (containing a mix of the product, unreacted Zn metal and Zn salts) was transferred into a large beaker on a hotplate, suspended in boiling methanol (300mL) and filtered. The extraction with boiling methanol was repeated twice more, to separate the unreacted Zn metal from the product. The combined methanolic filtrates were evaporated to dryness. Separately, the acetic acid+LiBr – containing filtrates from the reaction mix were concentrated to a small volume on rotovap and the produced salt-rich residue was diluted with water 0.6L and acidified with 6M HCl to about pH= 1.5. This mixture was then combined with the evaporation residue obtained from the methanolic filtrates. The solids in the flask were re-suspended by a brief sonication (5 min) and the slurry was cooled down on ice bath, then placed into a freezer (-20C) for 4 hours. The precipitated product was collected by filtration, washed with ice-cold water, dried by suction and on highvac. Y=5.23g (95%) of a light tan solid.
1H(d6-DMSO, 400MHz): 12.58 (br s, 1H), 10.72(s, 1H), 7.82 (dd, 8.3Hz, 1.6Hz, 1H), 7.75 (s, 1H), 6.88 (d, 8.3Hz, 1H), 3.54 (s, 2H)
Note: Skin contact with bromine produces excruciating burns: Heavy duty protective gloves with extended sleeves are required. Make sure that the used syringe does not leak, work in the hood, do not wash bromine-contaminated glassware with acetone (use water or alcohol).