Synthesis of DPA-triazole structures and their application as ligand for metal catalyzed organic reactions

In this work, the use of DPA-triazole (DPA = dipicolylamine) molecules as ligands for metal catalyzed organic reactions has been investigated. A small library of ligands has been prepared by a CuAAC (click reaction) between propargyl-DPA and different azides. For a selected ligand the complexation with Zn(II) in solution has been investigated by NMR and MS. DFT calculation supported the structure of the complex as revealed by NMR data. The ligand was then used for metal-catalyzed organic transformation. The Henry reaction between nitromethane and aromatic aldehydes was efficiently catalyzed by the Zn(II)-DPA-triazole complex with only 2 mol% of catalyst. Changing the metal from Zn(II) to Cu(II), the system proved to be effective in the formation of acetals from the reaction of aldehydes and methanol or ethanol, thus revealing the versatility of the DPA-triazole ligand. Finally, a soluble polymer-supported ligand was used in the Zn(II) catalyzed Henry reaction, allowing for the recovery and reuse of the catalyst up to five times without evident loss of activity.