General procedure for the synthesis of 3,5-dimethoxyiodobenzene from 3,5-dimethoxyaniline:
A. Synthesis of 3,5-dimethoxyiodobenzene
In a 500 mL three-necked round-bottomed flask equipped with a thermometer, mechanical stirrer, and dosing funnel, 12 M hydrochloric acid (100 mL, 1.2 mol) and crushed ice (100 g) were added. The flask was immersed in a dry ice-acetone cooling bath and 3,5-dimethoxyaniline (15.3 g, 100 mmol) was added with stirring. A 40 mL aqueous solution of sodium nitrite (NaNO2, 8.4 g, 120 mmol) was slowly added dropwise to this cold mixture, and the rate of the dropwise acceleration was controlled so that the temperature of the reaction mixture was maintained between -10 and -5 °C. The temperature of the reaction mixture was maintained between -10 and -5 °C by controlling the rate of the dropwise acceleration. After the dropwise addition, the reaction mixture was continued to be stirred at 0-5 °C for 1 hour.
The resulting dark red diazonium salt solution was slowly added to a 200 mL aqueous solution of potassium iodide (KI, 83 g, 500 mmol) that was well stirred at room temperature. The mixture was stirred for 2 hours and then left to stand overnight. Upon completion of the reaction, the reaction solution was extracted with ether (200 mL x 4). The organic phases were combined, washed sequentially with brine (200 mL × 2) and saturated aqueous sodium thiosulfate (Na2S2O3) (200 mL × 2), dried over anhydrous magnesium sulfate (MgSO4), filtered and concentrated to a small volume. After adding silica gel, the mixture was evaporated to dryness. This pre-adsorbed silica gel was placed on a silica gel column and petroleum ether was used as eluent to give a final 17.5 g (66% yield) of colorless solid 3,5-dimethoxyiodobenzene.
Product characterization: 1H-NMR (CDCl3, 300 MHz) δ ppm: 6.85 (2H, d, J = 2.3 Hz, Ar-H), 6.40 (1H, t, J = 2.3 Hz, Ar-H), 3.76 (s, 6H, 2 × OCH3).
