Sonogashira coupling reaction copper-free mechanism

Bohm, V. P. W., Herrmann, W. A. Coordination chemistry and mechanisms of metal-catalyzed C-C coupling reactions, 13 a copper-free procedure for the palladium-catalyzed Sonogashira reaction of aryl bromides with terminal alkynes at room temperature. Eur. J. Org. Chem. 2000,3679-3681. 

In copper-free Sonogashira coupling the competition of ligand and amine base determines the reaction mechanism. The oxidative addition of Arl with (Ph3P)4Pd is faster when amine is present. With the proposed mechanisms the efficiency of PhsP > PhsAs is explained. 

For the copper-free Sonogashira reaction, the mechanistic study reported in this thesis revealed that, just like in other cross-coupling reactions (i.e. Stille, Negishi), there are several competing reaction pathways and a change on the reaction conditions (e.g. solvent, ligands, substrates, base) might favor one over the other ones. Moreover, a new mechanism in which the acetylide (formed by deprotonation of the alkyne) directly reacts with the catalyst was also proposed. 

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