The general procedure for the synthesis of 4,4'-biphenyl diboronic acid from 4,4'-dibromobiphenyl was as follows: to a solution of 4,4'-dibromo-1,1'-biphenyl (5.00 g, 16.03 mmol) in anhydrous THF (100 mL) was added slowly and dropwise n-butyllithium (1.0 M hexanes solution, 40.1 mL, 64.1 mmol) at -78 °C, and was stirred for 1 hour. Subsequently, the reaction mixture was slowly warmed to -30 °C and trimethyl borate (4.83 mL, 43.3 mmol) was added dropwise and then stirring was continued for 1 hour at room temperature. Upon completion of the reaction, the reaction mixture was acidified to pH=2 with 1.5 N HCl. The resulting precipitate was filtered to afford 4,4'-biphenyldiboronic acid in the form of an off-white solid (2.5 g, 10.34 mmol, 64.5% yield). The structure of the product was confirmed by NMR (DMSO-d6, δ=2.50 ppm, 400 MHz): δ 8.06 (broad peak, 4H), 7.88 (double peak, J=8.53 Hz, 4H), 7.65 (double peak, J=8.4 Hz, 4H).
