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

Abstract

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 0C, Pd(PPh3)4, K2CO3 (or Et3N)] 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.