2,3,3-Trimethylindolenine (Sigma Aldrich)

Iodomethane (Sigma Aldrich)

2,3,3-Trimethylindolenine (0.48 g, 3.00 mmol), iodomethane (0.43 g, 3.00 mmol) and acetonitrile (3 mL) were added to a 35 mL CEM microwave reaction vessel and irradiated at 120^oC for 10 minutes. The precipitate formed was isolated at the pump and washed with ethyl acetate (20 mL) and cold diethyl ether (10 mL) and dried in vacuo to give 1,2,3,3-tetramethyl-3H-indolium iodide  (0.76 g, 84%) as a pink solid.

The authors found the above synthesis to not require purification.  The microwave used for this synthesis was a CEM Discover.  Note the huge signal around 3.33ppm in the 1H & COSY NMR is due to water within the d6-DMSO solvent.

¹H NMR (400 MHz, DMSO-d₆) δ 7.90 – 7.84 (m, 1H), 7.83 – 7.76 (m, 1H), 7.66 – 7.57 (m, 2H), 3.94 (s, 3H), 2.74 (s, 3H), 1.51 (s, 6H). ¹³C NMR (101 MHz, DMSO) δ 196.50, 142.50, 142.02, 129.83, 129.31, 123.70, 115.53, 54.39, 35.01, 22.13, 14.39. LCMS m/z: 174.2 [M – I^-].

Angela J. Winstead, Grace Nyambura, Rachael Matthews, Deveine Toney and Stanley Oyaghire. Synthesis of Quaternary Heterocyclic Salts, Molecules 2013, 18, 14306-14319