Reinvestigation of the Reactions of 1-Phenyl-3,4-dimethylphosphole with Dimethyl Acetylenedicarboxylate

In contrast with an earlier report, the reaction of 1-phenyl-3,4-dimethylphosphole with an excess of dimethyl acetylenedicarboxylate (DMAD) at room temperature affords the phosphole + 2 DMAD ylide 10, whose structure was established by a careful analysis of the 13C NMR spectrum. Its stability is surprising, since the analogous Ph3P + 2 DMAD adduct spontaneously rearranges to other products and has never been fully characterized. When the reaction of phosphole with DMAD was carried out in the presence of AgNO3, the intermediate 1:1 adduct was oxidized to the stable oxo-ylide 13, whose X-ray crystal structure shows the highly zwitterionic character.