General procedure for the synthesis of 4-bromo-4',4''-dimethyltriphenylamine from 4,4'-dimethyltriphenylamine: 4,4'-dimethyltriphenylamine (10 g, 36.6 mmol) was dissolved in dichloromethane (200 mL) at room temperature, followed by the addition of N-bromosuccinimide (NBS) (6.764 g, 38 mmol). The reaction mixture was stirred for about 3 hours. Upon completion of the reaction, the solid insoluble material was removed by filtration and the filtrate was purified by passing through a silica gel column and subjected to fast column chromatography using a gradient elution (hexane to hexane/dichloromethane = 4:1). The fraction containing the target product was collected and concentrated under reduced pressure to remove the solvent to afford the white solid product 4-bromo-4',4''-dimethyltriphenylamine (compound 25) (12.86 g, 99.7% yield).
