Aerobic oxidation of vanillyl alcohol to vanillin catalyzed by air-stable and recyclable copper complex and TEMPO under base-free conditions

Abstract

The need of sustainable development in the modern era has driven the transformation of biomass for the production of fine chemicals into a strong trend of research. The aerobic oxidation of lignin-derived monomeric phenolics for the synthesis of various value-added products has attracted significant attention of the scientific community. In particular, enormous research interest has been devoted to the aerobic oxidation of vanillyl alcohol to vanillin, which is an important aroma chemical with wide applications. Four copper(II) complexes were synthesized for this purpose. The coordination of the Schiff base ligand L₁ containing pendant thioether arm with copper(II) perchlorate and copper(II) chloride yielded homoleptic complex 1a and dinuclear complex 2a, respectively. Similar coordination of the NNS amine ligand L₂ with copper(II) perchlorate and copper(II) chloride resulted in the formation of homoleptic complex 1b and mononuclear complex 2b, respectively. All four air stable complexes were tested for the aerobic oxidation of vanillyl alcohol to vanillin at ambient conditions in the presence of a catalytic amount of the TEMPO radical. Mononuclear complex 2b displayed the best catalytic activity. Species 2b is highly selective for the conversion of vanillyl alcohol to vanillin in various green solvent mixtures. Catalyst 2b is highly recyclable and did not show any reduction of catalytic activity after three cycles. Finally, the green and sustainable credentials of various catalytic protocols under various reaction conditions were compared with the help of CHEM21 green metrics toolkit. A plausible catalytic pathway is proposed based on the published reports and experimental evidences.