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

beta-Hydrazonophosphine oxides are precursors of useful organophosphorus compounds, including phosphorylated N-heterocycles, a-aminophosphonates, and vinylphosphonates. In this work, a general transition metal-free synthesis of beta-hydrazonophosphine oxides was developed. The method relies on the Michael addition of phosphine oxides R2P(O)H to reactive azoalkenes (1,2-diaza-1,3-butadienes), which are generated in situ from a-halohydrazones and Hunig's base. The reaction stereoselectively leads to Z-isomers of beta-hydrazonophosphine oxides that are stabilized by intramolecular hydrogen bonding. The conversion of the products thus obtained into potential chelating ligands was showcased.

Automated summary

A general transition metal-free synthesis of beta-hydrazonophosphine oxides was developed using the Michael addition of phosphine oxides R2P(O)H to reactive azoalkenes. The Z-isomers of beta-hydrazonophosphine oxides formed through this reaction are stabilized by intramolecular hydrogen bonding. The conversion of the products into potential chelating ligands was demonstrated.