Reduction of Functionalized Tertiary Phosphine Oxides with BH₃

A direct stereoselective conversion of tertiary hydroxyalkylphosphine oxides to the corresponding tertiary hydroxyalkylphosphine–boranes involving facile reduction of the PO bond by BH3 under mild conditions has been developed. The unprecedented facility of reduction of the strong PO bond by BH3, a mild reducing agent, has been achieved through an intramolecular PO···B complexation directed by proximal α- or β-hydroxy groups present in the phosphine oxide structures. As established by two chemical correlations, the developed transformation of hydroxyalkylphosphine oxides into hydroxyalkylphosphine–boranes takes place with complete inversion of configuration at P.