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