Reactions of phosphines with acetylenes. Part IV. A stable 1,2-diphosphorane. Restricted rotation in a stable alkylidene diphosphorane

Abstract

The reaction of dimethyl acetylenedicarboxylate with an excess of triphenylphosphine gives a stable 1,2-diphosphorane whose structure [Ph₃P:C(CO₂Me)·C(CO₂Me):PPh₃] has been proved by a combination of physical and chemical methods. The n.m.r. spectrum of the diphosphorane at room temperature in deuteriochloroform solution establishes the compound to exist as a mixture of three slowly interconverting conformers, in which there is an energy barrier to rotation about the bond between the ester carbonyl carbon atom and the adjacent carbon atom. Dimethyl acetylenedicarboxylate and diphenylmethylphosphine give an analogous 1,2-diphosphorane with similar properties.