the reaction the presence of a ligand has its advantages. The ligand protects the Cu ion from interactions leading to degradation and formation of side products and also prevents the oxidation of the Cu(I) species to the Cu(II). Furthermore, the ligand functions as a proton acceptor thus eliminating the need of a base. 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, through oxidative coupling of an alkyne and an azide to the ruthenium containing metallacycle (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*[RuCl4] 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. Silver catalysis Recently, the discovery of a general Ag(I)-catalyzed azide–alkyne cycloaddition reaction (Ag-AAC) leading to 1,4-triazoles is reported. Mechanistic features are similar to the generally accepted mechanism of the copper(I)-catalyzed process. Silver(I)-salts alone are not sufficient to promote the cycloaddition. However the ligated Ag(I) source has proven to be exceptional for AgAAC reaction. Curiously, pre-formed silver acetylides do not react with azides; however, silver acetylides do react with azides under catalysis with copper(I). References Cycloadditions Name reactions ms:Tindak balas klik