Synthesis and computational investigation of novel antioxidants prepared by oxidative depolymerization of lignin and aldol condensation of aromatic aldehydes.

Abstract

Novel antioxidants were synthesized by CuSO4 catalyzed oxidative depolymerization of lignin to form aromatic aldehydes followed by aldol condensation with methyl ethyl ketone (MEK). Aldol condensation was found to greatly improve the antioxidation ability of lignin depolymerized products. Three lignin monomeric aromatic aldehydes, i.e. p-hydroxybenzaldehyde, vanillin, and syringaldehyde were further employed for aldol condensation with MEK, resulting in successful synthesis of new antioxidants, 1-(4-hydroxyphenyl)pent-1-en-3-one (HPPEO), 1-(4-hydroxy-3-methoxyphenyl)pent-1-en-3-one (HMPPEO) and 1-(4-hydroxy-3,5-dimethoxyphenyl)pent-1-en-3-one (HDMPPEO), respectively. Kinetic modeling illustrated that p-hydroxybenzaldehyde had the largest rate to react with MEK, followed by vanillin and then syringaldehyde, which was probably affected by the presence of methoxyl groups. Syringaldehyde derived product (HDMPPEO) displayed the best antioxidation ability. As revealed by density functional theoretical calculation, electron-donating group such as methoxyl group and conjugated side chain could effectively improve the antioxidation ability. Hydrogen atom transfer (HAT) mechanism was prone to occur in non-polar solvents while the sequential proton loss electron transfer (SPLET) mechanism was favored in polar solvents. This work thus can inspire new pathways for valorization of lignin to produce high value-added products.