Cleavage of P=O in the Presence of P-N: Aminophosphine Oxide Reduction with In Situ Boronation of the PIII Product

In contrast to tertiary phosphine oxides, the deoxygenation of aminophosphine oxides is effectively impossible due to the need to break the immensely strong and inert PO bond in the presence of a relatively weak and more reactive PN bond. This long-standing problem in organophosphorus synthesis is solved by use of oxalyl chloride, which chemoselectively cleaves the PO bond forming a chlorophosphonium salt, leaving the PN bond(s) intact. Subsequent reduction of the chlorophosphonium salt with sodium borohydride forms the PIII aminophosphine borane adduct. This simple one-pot procedure was applied with good yields for a wide range of PN-containing phosphoryl compounds. The borane product can be easily deprotected to produce the free PIII aminophosphine. Along with no observed PN bond cleavage, the use of sodium borohydride also permits the presence of ester functional groups in the substrate. The availability of this methodology opens up previously unavailable synthetic options in organophosphorus chemistry, two of which are exemplified.