Aerobic Oxidation of 2-Phenoxyethanol Lignin Model Compounds Using Vanadium and Copper Catalysts

Lignin is the most abundant renewable aromatic-containing macromolecule in Nature. Intensive research efforts are underway to obtain additional value from lignin beyond current low-value heating. Aerobic oxidation has emerged as one promising alternative for the selective depolymerization of lignin, and a variety of models for the most abundant beta-O-4 linkage have been employed. In this work, aerobic oxidation of the simple beta-O-4 lignin models 2-phenoxyethanol (2) and 1-phenyl-2-phenoxyethanol (3) were investigated using the oxovanadium complex (HQ)(2)V-v(O)(O'Pr) (HQ = 8-oxyquinolinate) and CuCI/TEMPO/2,6-lutidine as catalysts in several different solvents at 100 degrees C (TEMPO = 2,2,6,6-tetramethylpiperidine-l-oxyl). Using the vanadium catalyst, reactions proceed more readily in pyridine (vs dimethyl sulfoxide) presumably via an initial base-assisted alcohol dehydrogenation followed by oxidative C-C and C-O bond cleavage to afford phenol, formic acid and CO2. In contrast, the copper-catalyzed reactions suffer from extensive formylation of the substrate and radical coupling to give TEMPO-functionalized products. These results suggest that use of more complex beta-O-4 lignin models is required for accurate comparison of selective oxidation catalysts.