Role of the Ligand and of the Size and Flexibility of the Palladium−Ancillary Ligand Cycle on the Reactivity of Substituted Alkynes toward Palladium(0) Complexes Bearing Potentially Terdentate Nitrogen−Sulfur−Nitrogen or Nitrogen−Nitrogen−Nitrogen Ligands: Kinetic and Structural Study

The reaction between palladium(0) complexes bearing potentially terdentate ligands and dimethyl
acetylenedicarboxylate (DMA) to give the corresponding palladacyclopentadiene complexes was studied
under kinetic conditions. The reactivity of the complexes was markedly influenced by the nature of the
ancillary ligand. Thus, when pyridyldithioether (SNS) and dipyridylthioether (NSN) ligands are used,
the reactivity and the rate law of the corresponding derivatives are similar to those of the unsubstituted
bidentate pyridylthioether substrates and, therefore, a marked rate increase can be obtained only by
reduction of the olefin steric requirement. When terdentate NNN ligands are used, an apparent difference
in reactivity between the derivatives bearing the pyridine−amine−pyridine and pyridine−amine−quinoline
ligands is observed. On the basis of a detailed structural study (NMR, X-ray) and on kinetic investigations,
an interpretation which takes into account the flexibility of the cycle formed between the ligand and
palladium is proposed. Thus, irrespective of the size of the cycle, the complexes in which the ligand
forms flexible cycles undergo ring opening less easily, with a consequent reduction of reactivity.
Conversely, rigid rings cannot undergo associative attack without companion ring opening, this
phenomenon being crucial in favoring the alkyne attack.