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.