1. In a flask containing 50% aqueous diethylamine (500 mmol) and K2CO3 (50 mmol), a solution of 4-bromobenzyl bromide (50 mmol) in THF (90 mL) was added slowly and dropwise. The reaction mixture was stirred at 60 °C under Ar atmosphere for 24 hours. After completion of the reaction, water (60 mL) was added and extracted with CHCl3 (100 mL x 3). The organic layers were combined, dried with Na2SO4, filtered and concentrated under reduced pressure to give 4-bromobenzyl dimethylamine in 99% yield (boiling point 55 °C/1 mmHg).
2. Mg (40 mmol) was dried with a vacuum pump at 70 °C for 20 min. To the dried Mg was added a solution of 4-bromobenzyldimethylamine (30 mmol) in anhydrous THF (70 mL) and stirred for 1 h at room temperature under Ar atmosphere. Subsequently, anhydrous THF (30 mL) solution of chlorodiphenylphosphine (30 mmol) was slowly added at 0°C and stirred at room temperature for 2 hours. At the end of the reaction, the mixture was filtered and washed with dichloromethane. To the filtrate was added a 30% aqueous solution of H2O2 (60 mmol) at 0 °C and stirred for 1 h at room temperature. After completion of the reaction, the reaction was quenched with water (100 mL) and extracted with chloroform (50 mL x 3). The organic layer was washed with brine, dried over Na2SO4, filtered and concentrated under reduced pressure. To the residue, diethyl ether (300 mL) was added, stirred for 0.5 h to dissolve the product, filtered and concentrated under reduced pressure to give 4-(diphenylphosphinyl)benzyldimethylamine in 95% yield.
3. To a flask containing anhydrous 1,2-dimethoxyethane (200 mL) solution of 4-(diphenylphosphinoyl)benzyldimethylamine (60 mmol) was added dimethyl sulfate (120 mmol) at 0 °C. The mixture was mixed with a mixture of 2,2-dimethoxyethane (200 mL) and 4-(diphenylphosphinoyl)benzyldimethylamine (60 mmol). The mixture was stirred at 50 °C under Ar atmosphere for 1 hour. Subsequently, LiAlH4 (180 mmol) was added at 0 °C and stirred at room temperature for 2 hours. Upon completion of the reaction, it was quenched with ice, then quenched with 1 M aqueous HBr solution (200 mL) and washed with ether (200 mL x 2). The aqueous phase was extracted with dichloromethane (100 mL x 3), the organic layers were combined, dried with Na2SO4, filtered and concentrated under reduced pressure. To the residue, diethyl ether (300 mL) was added, stirred under Ar atmosphere for 0.5 h. 4-(Diphenylphosphino)benzyltrimethylammonium bromide was obtained by filtration in 79% yield.
