Michael addition of -nucleophiles to azoalkenes provides simple access to phosphine oxides bearing an alkylhydrazone moiety.

β-Hydrazonophosphine oxides are precursors of useful organophosphorus compounds, including phosphorylated -heterocycles, α-aminophosphonates, and vinylphosphonates. In this work, a general transition metal-free synthesis of β-hydrazonophosphine oxides was developed. The method relies on the Michael addition of phosphine oxides RP(O)H to reactive azoalkenes (1,2-diaza-1,3-butadienes), which are generated from α-halohydrazones and Hunig's base.