Direct conversion of sec-phosphine oxides into phosphinous acid-boranes

Two straightforward and general procedures for the synthesis of phosphinous acid-boranes directly from sec-phosphine oxides and borane reagents under basic and under neutral conditions have been developed. The conversion is stereoselective and occurs with predominant inversion of configuration at P. The protonation constants of a representative series of phosphinous acid-phosphinous acid-boranes have been determined.