Cu^II -Catalyzed Oxidative Formation of 5-Alkynyltriazoles

In an alcoholic solvent under the catalysis of Cu(OAc)₂ ⋅H₂ O, organic azide and terminal alkyne could oxidatively couple to afford 5-alkynyl-1,2,3-triazole (alkynyltriazole) at room temperature under an atmosphere of O₂ in a few hours. The involvement of 1,5-diazabicyclo[4.3.0]non-5-ene (DBN) is essential, without which the redox neutral coupling instead proceeds to produce 5-H-1,2,3-triazole (protiotriazole) as the major product. Therefore, DBN switches the redox neutral coupling between terminal alkyne and organic azide, the copper-catalyzed "click" reaction to afford protiotriazole, to an oxidation reaction that results in alkynyltriazole. The organic base DBN is effective in accelerating the copper(II)-catalyzed oxidation of terminal alkyne or copper(I) acetylide, which is intercepted by an organic azide to produce alkynyltriazole. The proposed mechanistic model suggests that the selectivity between alkynyl- and protiotriazole, and other acetylide or triazolide oxidation products is determined by the competition between copper(I)-catalyzed redox neutral cycloaddition and copper(II)/O₂ -mediated acetylide oxidation after the formation of copper(I) acetylide.