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Reaction of pentafluorophenyl(diphenyl)phosphine with nitrilimines is a multistep process leading to formation of unstable fused heterocycles with a fluorophosphorane moiety as primary products. Their hydrolysis involves opening of the heteroring and subsequent cleavage of the N-N bond in intermediate phosphorylated hydrazones. The final products are diphenyl(6-arylamino-2,3,4,5-tetrafluorophenyl)phosphine oxides.