Pyridine–triazole ligands for copper-catalyzed aerobic alcohol oxidation

Abstract

A series of Cu(NN′)₂(OTf)₂ complexes containing pyridine–triazole ligands [OTf = OSO₂CF₃; NN′ = NN′_Ph (1), NN′_hex (2), NN′_py (3)] with different substituents at the triazole N4 position or 2,2′-bipyridine (bpy; 4) have been synthesized. Crystal structures of 1 and 3 reveal a trans-isomer with strong preference for regular-type triazole coordination (for 3) whereas the Cu–bipyridine complex 4 is more stable in a cis-form. Cyclic voltammetry of 1–4 suggest that the electron-donating strength follows the trend: bpy > NN′_py > NN′_hex ∼ NN′_Ph. The catalyst systems consisting of 5 mol% Cu(OTf)₂/NN′/TEMPO (TEMPO = (2,2,6,6-tetramethylpiperidin-1-yl)oxy) in the presence of 2 × 2.0 cm² Cu⁰ sheets as a reducing agent and 10 mol% N-methylimidazole (NMI) exhibit good activities for aerobic oxidation of benzyl alcohol to benzaldehyde. Catalytic studies have shown that the activities were higher with more electron-rich N-based ligands. Furthermore, oxidation of aliphatic alcohols such as 1-hexanol and 2-methyl-1-pentanol using the Cu catalyst system with the NN′_py ligand at room temperature afforded the corresponding aldehydes in >99% and 46% yields, respectively after 24 h.