Synthesis
General procedure for the synthesis of 2-bromo-5-methylthiophene from 2-methylthiophene: A bromine solution was prepared by dissolving 48.0 g of bromine (0.3 mol) in 260 cm3 dioxane. This bromine solution was slowly added dropwise under stirring conditions to a solution of 29.4 g 2-methylthiophene (0.3 mmol) dissolved in 140 cm3 dioxane. The reaction mixture was stirred continuously for 3 hours at room temperature. Subsequently, the reaction system was cooled to 0 °C and saturated aqueous sodium bicarbonate solution was carefully added to neutralize the reaction. The mixture was extracted with ethyl acetate and the organic layer was separated. The organic layer was dried over anhydrous sodium sulfate and evaporated under reduced pressure to remove the solvent. The residue was purified by distillation to give 37.2 g 2-bromo-5-methylthiophene (0.21 mol, 70% yield). Boiling point: 68-70 °C (20 mbar). 1H NMR (400 MHz, CDCl3): δ = 2.42 (s, 3H), 6.51 (d, 1H, J = 3.6 Hz), 6.83 (d, 1H, J = 3.6 Hz) ppm. LC-MS: retention time tR = 1.65 min; MS: m/z = 176, 178 ([M+ 1]+, [M+3]+).
References
[1] Journal of Organic Chemistry, 2014, vol. 79, # 4, p. 1836 - 1841
[2] Synthesis, 2002, # 9, p. 1133 - 1135
[3] Green Chemistry, 2018, vol. 20, # 19, p. 4448 - 4452
[4] Synthesis, 2011, # 2, p. 207 - 209
[5] European Journal of Organic Chemistry, 2018, vol. 2018, # 20, p. 2592 - 2602
