A-21·CuI as a catalyst for Huisgen’s reaction: about iodination as a side-reaction

Abstract The polymer-supported catalyst Amberlyst A-21·CuI has found many applications in copper(I)-catalyzed Huisgen’s (CuAAC) and other reactions. This catalyst was found to be efficient and reusable for the formation of 1,4-disubstituted 1,2,3-triazoles. It was used with success in several usual procedures, as well as in automated and continuous-flow approaches. However, in a few instances, this heterogeneous catalyst gave a few percentages of the 5-iodotriazole derivative, like it was observed under homogeneous conditions. Since the iodination process was scarcely encountered and that we did not, to the best of our knowledge, observed this side-product before, we became interested in studying the possible causes of this side-reaction. The preparation of the catalyst, and to a lesser extent the purity of the reagents, as well as the substrate and the amount of catalyst used were found to be the cause of iodination up to 12% but kept below 0.5% with the right procedure.