Sonogashira Coupling - Catalysts

Catalysts

Typically, two catalysts are needed for this reaction: a zerovalent palladium complex and a halide salt of copper(I). Examples of such palladium catalysts include compounds in which palladium is ligated to phosphines (Pd(PPh₃)₄). A common derivative is Pd(PPh₃)₂Cl₂, but bidentate ligand catalysts, such as Pd(dppe)Cl, Pd(dppp)Cl₂, and Pd(dppf)Cl₂ have also been used. The drawback to such catalysts is the need for high loadings of palladium (up to 5 mol %), along with a larger amount of a copper co-catalyst. PdII is often employed as a pre-catalyst since it exhibits greater stability than Pd0 over an extended period of time and can be stored under normal laboratory conditions for months. The Pd II catalyst is reduced to Pd0 in the reaction mixture by either an amine, a phosphine ligand, or a reactant, allowing the reaction to proceed. The oxidation of triphenylphosphine to triphenylphosphine oxide can also lead to the formation of Pd0 in situ when catalysts such as bis(triphenylphosphine)palladium(II) chloride are used.

Copper(I) salts, such as copper iodide, react with the terminal alkyne and produce a copper(I) acetylide, which acts as an activated species for the coupling reactions. Cu(I) is a co-catalyst in the reaction, and is used to increase the rate of the reaction.