Base metal iron catalyzed sustainable oxidation of vanillyl alcohol to vanillic acid in deep eutectic solvents and implementation of vanillic acid for fine-chemical synthesis

Abstract

In the modern era, sustainable development for the production of fine chemicals from abundant biomass by utilizing various chemical transformations has become a strong trend of research in the scientific community. This may provide a sustainable alternative to petrochemicals as the major source of fine chemicals. Lignin is an alternative major source of monomeric phenolic compounds, and the syntheses of fine chemicals by oxidising lignin-based monomeric phenolics are gaining serious attention. For instance, biomass derived vanillin or vanillyl alcohol can be oxidized to vanillic acid which has been employed as a new building block for the syntheses of various value-added products. In this context, an air-stable iron(II) complex has been synthesized and utilized as an excellent base-metal catalyst for the selective oxidation of vanillyl alcohol to vanillic acid. We used hydrogen peroxide and tert-butyl hydroperoxide as green oxidants. These peroxidative oxidations of vanillyl alcohol to vanillic acid were performed in metal-free type-III deep eutectic solvents as green and sustainable reaction media. After the first set of oxidation reactions, the catalyst and the reaction medium were recycled five times without any noticeable change in catalytic performance. The CHEM21 green metrics toolkit was also used to examine the sustainable and green features of the optimized oxidation protocol for the conversion of vanillyl alcohol to vanillic acid. Low E-factors (4.65) suggest waste minimized sustainable oxidations of vanillyl alcohol to vanillic acid. Finally, vanillic acid was used as a starting material for the syntheses of several fine chemicals with various (potential) applications such as flavorants, odorants, surfactants and bio-based plasticizers.