Copper catalysed aerobic oxidation of benzylic alcohols in an imidazole containing N4 ligand framework

Abstract

The catalytic aerobic oxidation of benzylic alcohols to corresponding aldehydes has been investigated employing a tetradentate copper(II) complex that incorporates N-methyl imidazole (NMI) as an integral part of an N₄-ligand framework. The complex [L₃-Cu]²⁺ [(L₃ = 1-methyl-4,5-dihydro-1H-imidazol-2-yl)methyl(2-ethyl)amine] has been synthesized, characterized by spectroscopic and structural techniques, and its catalytic activity towards the aerobic oxidation of benzylic alcohols studied under ambient conditions. Catalytic turnover required addition of the nitroxyl radical initiator TEMPO. No oxidation was observed with hydroxyl radical initiators H₂O₂ or tetrabutylammonium hydroperoxide. Additional NMI enhanced turnover with yields up to 99% within 4 hours. The relative quantities of substrate, catalyst, TEMPO, and NMI were systematically varied in a series of experiments to determine optimal conditions. The effect of oxygen at various flow rates as an alternate to ambient aerobic conditions was also investigated.