Aerobic Oxidation of β-1 Lignin Model Compounds with Copper and Oxovanadium Catalysts

The reactivity of homogeneous oxovanadium and copper catalysts toward aerobic oxidation of phenolic and nonphenolic beta-1 lignin model compounds has been investigated. Aerobic oxidation of diastereomeric, nonphenolic beta-1 lignin models (1T, 1E) using the six-coordinate vanadium complex (HQ)(2)V-v(O)((OPr)-Pr-i) (HQ = 8-oxyquinolinate) as a precatalyst in pyridine afforded ketone (3) and dehydrated ketone (4) derived from oxidation. of the secondary alcohol. In contrast, using CuOTf/2,6-lutidine/TEMPO (OTf = trifluoromethanesulfonate, TEMPO = 2,2,6,6-tetramethyl-piperidin-1-yl-oxyl) in toluene for the same reaction afforded 3,5-dimethoxybenzaldehyde (5) and 4-methoxybenzaldehyde (6) as major products resulting from C-alpha-C-beta bond cleavage. Reactions of the corresponding phenolic lignin model compounds (2T, 2E) with 10 mol % CuOTf/2,6-lutidine/TEMPO gave ketone (9) as the major product, whereas 10 mol % (HQ)(2)V-v(O)((OPr)-Pr-i) or a stoichiometric amount of CuOTf/2,6-lutidine/TEMPO yielded 2,6-dimethoxybenzoquinone (10) as the major product, arising from cleavage of the C-aryl-C-alpha bond. Different selectivity was observed in the oxidation of 2T, 2E using the five-coordinate complex (dipic)V-v(O)((OPr)-Pr-i) (dipic = dipicolinate), with alpha,beta-unsaturated aldehyde (14) as the major product (formed from oxidation of the primary alcohol and dehydration). The key differences in chemoselectivity between the vanadium and copper catalysts in the oxidations of these phenolic and nonphenolic beta-1 lignin models are discussed.