Selective aerobic oxidation of biomass model compound veratryl alcohol catalyzed by air-stable copper(ii) complexes in water

Abstract

Air-stable square pyramidal copper(II) bromide complexes 1, 2 and 3 with NNS, N(NH)S and NNO ligand frameworks, respectively, were synthesized and successfully utilized as effective catalysts for the aerobic oxidation of biomass model compound veratryl alcohol in water as a green and sustainable reaction medium. Complex 2 displayed the best catalytic activity. The oxidation reactions were successfully conducted under ambient conditions (40 °C) in the presence of a catalytic amount of copper catalyst and TEMPO ((2,2,6,6-tetramethylpiperidin-1-yl)oxyl) utilizing air as a sustainable oxidant. Addition of an external base is required to obtain a quantitative yield and complete selectivity towards the desired aldehyde product. The oxidation reaction can also be conducted in the absence of base; however, selectivity declined, with the formation of an over-oxidation product, veratric acid. Acetonitrile is the most commonly used solvent in oxidation reactions, and various green solvents such as methanol, ethanol, acetone and ethyl acetate were also used for the same purpose. Although acetonitrile and water gave similar catalytic efficiency, we selected water after considering the green and sustainable aspects. Optimized reaction conditions in water were utilized for the oxidation of other lignin biomass-derived alcohols as well as various other substituted benzylic alcohols to the corresponding aldehydes. Aldehyde products were easily isolated by simple workup processes (filtration and evaporation). Complex 2 is highly recyclable and quantitative isolated yields of product were obtained even after ten consecutive runs. The present catalytic protocol produced less waste as reflected in the low E-factor (4.16). Finally, the sustainable credentials of various optimized reaction protocols were evaluated with the help of the CHEM21 green metrics toolkit.