Ruthenium Catalysis
The ruthenium-catalysed 1,3-dipolar azide-alkyne cycloaddition (RuAAC) gives the 1,5-triazole. Unlike CuAAC in which only terminal alkynes reacted, in RuAAC both terminal and internal alkynes can participate in the reaction. This suggests that ruthenium acetylides are not involved in the catalytic cycle.
The proposed mechanism suggests that in the first step, the spectator ligands undergo displacement reaction to produce an activated complex which is converted, via oxidative coupling of an alkyne and an azide to the ruthenium containing metallocyle (Ruthenacycle). The new C-N bond is formed between the more electronegative and less sterically-demanding carbon of the alkyne and the terminal nitrogen of the azide. The metallacycle intermediate then undergoes reductive elimination releasing the aromatic triazole product and regenerating the catalyst or the activated complex for further reaction cycles.
Cp*RuCl(PPh3)2, Cp*Ru(COD)and Cp* are commonly used ruthenium catalysts. Catalysts containing cyclopentadienyl(Cp) group are also used. However, better results are observed with the pentamethylcyclopentadienyl(Cp*) version. This may be due to the sterically demanding Cp* group which facilitates the displacement of the spectator ligands.
Read more about this topic: Azide-alkyne Huisgen Cycloaddition