Synthesis
The general procedure for the synthesis of 4-bromo-5-methylthiophene-2-boronic acid from 3,5-dibromo-2-methylthiophene was as follows: 3,5-dibromo-2-methylthiophene (12.8 g, 50 mmol) was dissolved in anhydrous ether (240 mL) under argon protection and degassed. The solution was cooled to -78 °C and n-butyllithium (n-BuLi, 20.4 mL, 50 mmol, 2.5 M hexane solution) was added slowly and dropwise. The reaction mixture was stirred continuously at -78 °C for 30 min. Subsequently, tributyl borate (12.68 g, 50 mmol) was added and the reaction mixture was stirred overnight at room temperature. Upon completion of the reaction, 1 M HCl aqueous solution (200 mL) was added for acidification, and the ether phase was separated and extracted. The ether phase was washed three times with 1 M NaOH aqueous solution (3 x 70 mL). The NaOH phases were combined, concentrated and the pH was adjusted with HCl to 1. Under this acidic condition, the target product 4-bromo-5-methylthiophene-2-boronic acid precipitated as a beige solid in 76% yield.
References
[1] European Journal of Organic Chemistry, 2011, # 18, p. 3301 - 3312
[2] Acta Crystallographica Section C: Crystal Structure Communications, 2005, vol. 61, # 10, p. o599-o601
[3] Canadian Journal of Chemistry, 2007, vol. 85, # 1, p. 12 - 20
[4] Journal of Molecular Structure, 2007, vol. 833, # 1-3, p. 23 - 29
[5] Dyes and Pigments, 2010, vol. 84, # 1, p. 25 - 35
