Pyridine-triazole ligands for copper-catalyzed aerobic alcohol oxidation

Abstract

© The Royal Society of Chemistry 2015. A series of Cu(NN′)<inf>2</inf>(OTf)<inf>2</inf> complexes containing pyridine-triazole ligands [OTf = OSO<inf>2</inf>CF<inf>3</inf>; NN′ = NN′<inf>Ph</inf> (1), NN′<inf>hex</inf> (2), NN′<inf>py</inf> (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′<inf>py</inf> > NN′<inf>hex</inf> ∼ NN′<inf>Ph</inf>. The catalyst systems consisting of 5 mol% Cu(OTf)<inf>2</inf>/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′<inf>py</inf> ligand at room temperature afforded the corresponding aldehydes in >99% and 46% yields, respectively after 24 h.