Although beneficial for the effectiveness of the reaction, the use of copper salts in "classical"  Sonogashira reaction is accompanied with several drawbacks, such as the application of environmentally unfriendly reagents, the formation of undesirable alkyne homocoupling (Glaser side products), and the necessity of strict oxygen exclusion in the reaction mixture. Thus, with the aim of excluding copper from the reaction, a lot of effort was undertaken in the developments of Cu-free Sonogashira reaction. Along the development of new reaction conditions, many experimental and computational studies focused on elucidation of reaction mechanism. Until recently, the exact mechanism by which the Cu-free reaction occurs was under debate, with critical mechanistic questions unanswered. It was shown in 2018 by Košmrlj et al. that the reaction proceeds along the two interconnected Pd/Pd catalytic cycles.
* Similar to the original mechanism, the Pd cycle begins with the oxidative addition of the aryl halide or triflate to the Pd catalyst, forming complex B and activating aryl halide substrate for the reaction.
* Acetylene is activated in the second, Pd mediated cycle. Phenylacetylene was proven to form Pd monoacetylide complex D as well as Pd bisacetylide complex F under mild reaction conditions.
* Both activated species, namely complexes B and F, are involved in the transmetallation step, forming complex C and regenerating D.
* The resulting products of reductive elimination, disubstituted alkyne product as well as regenerated Pd catalytic species, complete the Pd catalytic cycle.
It was demonstrated that amines are competitive to the phosphines and can also participate as ligands L in the described reaction species. Depending on the rate of the competition between amine and phosphines, a dynamic and complex interplay is expected when using different coordinative bases.