Additive-Free Oxidative Cleavage of a Simple Lignin Model Compound in Air using Vanadium Complexes

Three vanadium complexes supported by salophan and salan ligands featuring differences in steric, electronic, and oxidation state (IV vs V) of the metal center have been evaluated for their potential in catalyzing the oxidative cleavage of a simple lignin model compound. All complexes were found to be effective, and under optimized conditions (145 °C, 48 h, in air) produced the cleavage products, phenol and benzoic acid, in good yields (62–69%) and selectivity (69–77%); significant differences in reactivity were not observed except at a lower temperature (125 °C). Complex 3c featuring sterically bulky tert butyl groups at ortho/para positions of the phenol arms, a cyclohexyl backbone and a V(V) center resulted in the highest yields and selectivity at 135 °C over 72 h (69–78% yield; 78–89% selectivity). Longer reaction times as well as reaction temperatures were found to compromise yield and selectivity for both cleavage products. An oxidizing atmosphere was found to be crucial for the observed reactivity as reactions attempted under an inert atmosphere did not result in significant conversion. Preliminary mechanistic investigations suggest that the lignin model compound is oxidized prior to undergoing cleavage, and the vanadium(V) complex is more effective at cleaving the oxidized product in comparison to the vanadium(IV) counterparts. Quite significantly, the oxidative cleavage was achieved in the absence of any basic or acidic additives.

Graphical Abstract