π-Ligand Exchange on Phosphenium Ions:  Reversible Exchange between Free and Coordinated Alkynes in Phosphirenium Salts

The rate of reversible alkyne exchange between 1-methyl-1,2,3-triphenylphosphirenium triflate and ethylphenylacetylene, and that of the reverse reaction between 2-ethyl-1-methyl-1,3-diphenylphosphirenium triflate and diphenylacetylene, are independent of alkyne concentration, which suggests a mechanism that involves rate-determining elimination of alkyne from the phosphirenium triflate to give a transient phosphenium triflate. The mechanism is supported by the observation of crossover products upon mixing phosphirenium triflates containing different substituents.