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3.2.1 Catalysts Used in RuAAC

DOI: 10.1055/sos-SD-235-00118

Paterson, A. J.; Beke-Somfai, T.; Kann, N.Science of Synthesis: Click Chemistry, (20211293.

Although a number of ruthenium(II) complexes catalyze the cycloaddition between an azide and an alkyne, catalysts containing both pentamethylcyclopentadienyl and chloro ligands are the most efficient in terms of conversion and high regioselectivity for the 1,5-disubstituted isomer.[‌15‌] [Ru(Cp*)Cl(PPh3)2] and Ru(Cp*)Cl(cod) are most commonly used, with the latter showing higher catalytic activity, allowing reactions to be performed at ambient temperature. Other catalysts such as Ru(Cp*)Cl(nbd), {Ru(Cp*)Cl}4, and {Ru(Cp*)Cl2}n have also been applied with good results.[‌21‌] The Ru(Cp*)Cl fragment is considered to be the active catalytic species, where the role of the electron-rich pentamethylcyclopentadienyl ligand is to stabilize the high formal oxidation states of ruthenium.[‌15‌] Replacing the chloro ligand by bromo or iodo significantly lowers the activity, and removing the chloro ligand altogether using silver(I) results in an inactive catalyst. Ruthenium complexes lacking the Ru(Cp*)Cl unit, such as Ru(OAc)2(PPh3)2 and RuH2(CO)(PPh3)3, can be used to produce the 1,4-disubstituted 1,2,3-triazole isomer, but in general with substantially lower catalytic activity,[‌15‌] and CuAAC is a better choice in that case.