In a coordinating solvent such as DMF, the fast oxidative addition of vinyl triflates to Pd0(PPh3)4 performed under stoichiometric conditions gives the cationic complexes [(η1-vinyl)Pd(PPh3)2(DMF)]+TfO-, which have been characterized by conductivity measurements, electrospray mass spectrometry, and NMR spectroscopy, before their decomposition to vinylphosphonium salts [vinyl-PPh3]+TfO- and Pd0 complexes. [(η1-vinyl)Pd(PPh3)2(DMF)]+TfO- complexes are less stable than [(aryl)Pd(PPh3)2(DMF)]+TfO- formed in the oxidative addition of aryl triflates to Pd0(PPh3)4. The rate constant of the oxidative addition of vinyl triflates and bromides to Pd0(PPh3)4 has been determined and compared to that of aryl triflates and halides. The following reactivity orders are established in DMF: vinyl-OTf ≫ vinyl-Br > PhBr and vinyl-OTf ≫ PhOTf.

Amines used as bases in copper-free, palladium-catalyzed Sonogashira reactions play a multiple role. The oxidative addition of iodobenzene with [Pd(0)(PPh(3))(4)] is faster when performed in the presence of amines (piperidine>morpholine). Amines also substitute one ligand L in trans-[PdI(Ph)(L)(2)] (L=PPh(3), AsPh(3)) formed in the oxidative addition. This reversible reaction, which gives [PdI(Ph)L(R(2)NH)], is favored in the order AsPh(3)>PPh(3) and piperidine>morpholine. Two mechanisms are proposed for Sonogashira reactions, depending on the ligand and the amine. When L=PPh(3), its substitution by the amine in trans-[PdI(Ph)(PPh(3))(2)] is less favored than that of the alkyne. A mechanism involving prior coordination of the alkyne is suggested, followed by deprotonation of the ligated alkyne by the amine. When L=AsPh(3), its substitution in trans-[PdI(Ph)(AsPh(3))(2)] by the piperidine is easier than that by the alkyne, leading to a different mechanism: substitution of AsPh(3) by the amine is followed by substitution of the second AsPh(3) by the alkyne to generate [PdI(Ph)(amine)(alkyne)]. Deprotonation of the ligated alkyne by an external amine leads to the coupling product. This explains why the catalytic reactions are less efficient with AsPh(3) than with PPh(3) as ligand.