Copper Catalysts for Selective C-C Bond Cleavage of β-O-4 Lignin Model Compounds

The reactivity of homogeneous copper catalysts towards the selective C-C bond cleavage of both phenolic and non-phenolic arylglycerol beta-aryl ether lignin model compounds has been explored. Several copper precursors, nitrogen ligands, and solvents were evaluated in order to optimize the catalyst system. Using the optimized catalyst system, copper(I) trifluoromethanesulfonate [Cu(OTf)]/L/TEMPO (L=2,6-lutidine, TEMPO=2,2,6,6-tetramethyl-piperidin-1-yl-oxyl), aerobic oxidation of the non-phenolic beta-O-4 lignin model compound proceeded with good selectivity for C-alpha-C-beta bond cleavage, affording 3,5-dimethoxybenzaldehyde as the major product. Aerobic oxidation of the corresponding phenolic beta-O-4 lignin model proceeded with different selectivity, affording 2,6-dimethoxybenzoquinone and alpha, beta-unsaturated aldehyde products resulting from cleavage of the C-alpha-C-aryl bond. At low catalyst concentrations, however, a change in selectivity was observed as oxidation of the benzylic secondary alcohol predominated with both substrates.