A Versatile Protocol for the Quantitative and Smooth Conversion of Phosphane Oxides into Synthetically Useful Pyrazolylphosphonium Salts

A convenient protocol for the smooth conversion of the resistant P?O bond in phosphane oxides into a reactive P?N bond of synthetically useful pyrazolylphosphonium salts is described. A highly charged, oxophilic, phosphorus-centered trication is employed and the reactions are conducted at room temperature with quantitative yields. The resulting pyrazolylphosphonium cations are valuable synthetic intermediates and are used for the synthesis of a variety of organophosphorus compounds. This represents a new approach towards the transformation of the rather inert phosphoryl group under very mild reaction and workup conditions and aims towards alternatives to existing reduction methods for phosphane oxide functionalization.