mechanism revolves around a palladium cycle which is in agreement with the "classical" cross-coupling mechanism, and a copper cycle, which is less well known. The palladium cycle Palladium precatalyst species are activated under reaction conditions to form a reactive Pd0 compound, A. The exact identity of the catalytic species depends strongly upon reaction conditions. With simple phosphines, such as PPh3 (n=2), and in case of bulky phosphines (i.e., ) it was demonstrated that monoligated species (n=1) are formed. Furthermore, some results point to the formation of anionic palladium species, [L2Pd0Cl]− , which could be the real catalysts in the presence of anions and halides. The active Pd0 catalyst is involved in the oxidative addition step with the aryl or vinyl halide substrate to produce PdII species B. Similar to the above discussion, its structure depends on the employed ligands. This step is believed to be the rate-limiting step of the reaction. Complex B reacts with copper acetylide, complex F, in a transmetallation step, yielding complex C and regenerating the copper catalyst. The structure of complex C depends on the properties of the ligands. For the facile reductive elimination to occur, the substrate motifs need to be in close vicinity, i.e. cis-orientation, so there can be trans-cis isomerisation involved. In reductive elimination the product tolane is expelled from the complex and the active Pd catalytic species is regenerated. The copper cycle The copper cycle is not entirely well described. It is suggested that the presence of a base results in the formation of a π-alkyne complex E. This increases the acidity of the terminal proton and leads to the formation of copper acetylide, complex F, upon deprotonation. Acetylide F is then involved in the transmetallation reaction with palladium intermediate B. The mechanism of a copper-free Sonogashira variant 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