GENERAL PROCEDURE: Trifluoromethanesulfonic anhydride (Tf2O, 1.2 eq.) was slowly added dropwise to a solution of 4-bromo-N,N-dimethylbenzamide (1.0 eq.) and 2,6-di-tert-butyl-4-methylpyridine (DTBMP, 1.2 eq.) in dichloromethane (CH2Cl2, 5 mL) cooled to -78 °C. The reaction mixture was gradually warmed up to 0 °C over 2 hours. Subsequently, a solution of butylmagnesium bromide (1.0 eq.) in ether (Et2O) was slowly added dropwise to the reaction mixture at -78 °C and stirring was continued at the same temperature for 2 hours. After completion of the reaction, the reaction was quenched with 15% aqueous hydrochloric acid (5 mL). The organic layer was separated and the aqueous phase was extracted with ether (3 x 5 mL). The organic layers were combined, washed with saturated saline, dried over anhydrous sodium sulfate (Na2SO4), filtered and concentrated under reduced pressure. The residue was purified by silica gel fast column chromatography to obtain the target product 4-bromovalerophenone. For the separation of the by-product N,N-dimethyl-4-bromobenzamide, the aqueous phase was alkalized with 15% aqueous sodium hydroxide (6 mL) and extracted with dichloromethane (3 x 10 mL). The organic layers were combined, dried over anhydrous sodium sulfate (Na2SO4), filtered and concentrated under reduced pressure. The residue was purified by silica gel fast column chromatography to give N,N-dimethyl-4-bromobenzamide.
