2,3,3-trimethyl-5-nitro-indole (See Synthetic Pages 911)

Tin (II) Chloride (Sigma Aldrich)

Hydrochloric Acid (Fisher Scientific)

To a 250 mL wide necked conical flask was added 2,3,3-trimethyl-5-nitro-indole (5.24 g, 27 mmol), and hydrochloric acid (6M, 150 mL). The solution was stirred until all the solid had dissolved, then tin (II) chloride dihydrate (33.3 g, 148 mmol) was added slowly with constant stirring, producing a white precipitate. Upon total addition of the tin (II) chloride dihydrate, the reaction was continued to stir at 100°C for 3 hours*. After this time the mixture was allowed to cool and then the pH was adjusted to pH=8** via the slow addition of sodium hydroxide pellets. The solid precipitate produced on neutralisation was isolated by vacuum filtration and washed with ethyl acetate (300 mL), the mixture was extracted with ethyl acetate (800 mL) and the combined organic phases were dried with sodium sulphate, filtered***, and evaporated to dryness to afford 2,3,3-trimethylindol-5-amine (3.28 g, 70%) as a brown solid.****

*A conical flat bottomed flask was fitted with a reflux condenser.  The temperature was controlled using the sensor on the hot plate.  Stirring was accomplished using a stirring bar, which was controlled from the hot plate.  The whole reaction was accomplished in the open air without the need of dry/inert conditions.

** Universal indicator paper was used to monitor the change in pH to pH=7.

*** Filtration was used to remove any impurities.

**** The brown solid can be recrystallised to give a pure compound, however we wanted to react on the amine and were happy to proceed with the crude NMR which is shown with minor impurities.

¹H-NMR (300 MHz, DMSO-d₆): δ 7.05 (d, J = 8.1 Hz, 1H, Ar-H), 6.56 (d, J = 2.0 Hz, 1H, Ar-H), 6.42 (dd, J = 8.1, 2.2 Hz, 1H, Ar-H), 4.99 (s, 2H, Ar-NH₂) 2.10 (s, 3H, C-CH₃), 1.15 (s, 6H, 2C-CH₃). ¹³C-NMR (300 MHz, DMSO-d₆): δ 181.97, 147.59, 146.95, 144.33, 119.81, 112.56, 108.19, 53.00, 23.48, 15.20. IR (ATR): 3334, 3192, 2958, 2923, 2862, 1670, 1617, 1576, 1460, 1422, 1377, 1344, 1286, 1221, 1113, 1062, 945, 859, 820. GC-MS (EI) m/z: 174.09 (M⁺).

Demeter, O., Kormos, A., Koehler, C., Mezö, G., Németh, K., Kozma, E., Takács, L.B., Lemke, E.A., Kele, P. Bisazide Cyanine Dyes as Fluorogenic Probes for Bis-Cyclooctynylated Peptide Tags and as Fluorogenic Cross-Linkers of Cyclooctynylated Proteins (2017) Bioconjugate Chemistry, 28 (5), pp. 1552-1559.