(Diazomethyl)dimethylphosphine Oxide - A Diazoalkane Reagent for [3+2] Cycloadditions.

A safe and efficient method for the in-situ preparation of (diazomethyl)dimethylphosphine oxide - a hereto unexplored diazoalkane reagent - is developed. The method is based on the diazotization of the corresponding P(O)Me-substituted amine (readily available in multigram quantities) in non-aqueous media. The protocol provides the target product as ca. 1.5 M CHCl solution which is stable at -18 °C. The utility of the synthesized diazoalkane is illustrated by its [3+2] cycloaddition with electron-poor alkynes and alkenes providing the corresponding P(O)Me-substituted pyrazoles and pyrazolines with moderate to good efficiency. In this view, the title compound represents and an important extension of medicinally relevant phosphine oxide reagents.