Synthesis of Enantioenriched Aryl‐ tert ‐Butylphenylphosphine Oxides via Cross‐Coupling Reactions of tert ‐Butylphenylphosphine Oxide with Aryl Halides

A series of enantiomerically enriched tertiary phosphine oxides have been prepared via the Pd‐catalyzed cross‐coupling reactions of enantiomerically pure tert ‐butylphenylphosphine oxide, with a variety of aryl iodides and bromides. This new protocol under optimized reaction conditions [toluene, 110 0 C, Pd(PPh 3 ) 4 , K 2 CO 3 (or Et 3 N)] afforded highly functionalized P‐chiral phosphine oxides with a yield of 78% to 95% and with enantiomeric excesses above 98%. The stereoretentive outcome of the cross‐coupling reactions was proved by X‐ray crystallography of selected phosphine oxides: ( S )‐(2‐aminophenyl)( tert ‐butyl)(phenyl)phosphine oxide ( 3a ) and ( S )‐anthracen‐9‐yl( tert ‐butyl)(phenyl)phosphine oxide ( 3i ). When attempting to convert the enantiomerically pure phosphine oxide 3a to the corresponding borane by the treatment with the borane dimethyl sulfide complex partial stereoerosion at a stereogenic phosphorus atom was observed. Racemic tert ‐butyl (2‐(dimethylamino)phenyl)(phenyl)phosphine ( 7a ) was isolated in a quantitative yield upon deprotection of the corresponding borane ( 8a ) and converted to a palladium crystalline complex ( 9 ), the structure of which has been proved by X‐ray crystallography.