Mechanistic and Kinetic Investigation on the Formation of Palladacyclopentadiene Complexes. A Novel Interpretation Involving a Bimolecular Self-Reaction of a Monoalkyne Intermediate

The stoichiometric reaction between the complex [Pd(η2-dmfu)(BiPy)] (dmfu = dimethylfumarate; BiPy = 2,2′-bipyridine) and the deactivated alkynes dmbd (dimethyl-2-butynedioate) and pna (methyl (4-nitrophenyl)propynoate), providing the respective palladacyclopentadienes, was investigated. The mechanism leading to the palladacyclopentadiene derivative involves a bimolecular self-rearrangement of the monoalkyne intermediate [Pd(η2-alk)(BiPy)] (alk = dmbd, pna), followed by the customary attack of the free alkyne on the intermediate [Pd(η2-alk)(BiPy)] itself and on the elusive and highly reactive “naked palladium” [Pd(BiPy)(0)] formed. The alkyne pna proved to be less effective in the displacement of dmfu than dmbd. The reaction under stoichiometric equimolar conditions of the latter with [Pd(η2-dmfu)(BiPy)] allows the direct determination of the bimolecular self-reaction rate constant k c and consequently the assessment of all the rate constants involved in the overall mechanistic network.