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

• Published in: ACS catalysis 2013-12, Vol.3 (12), p.3111-3122
• Main Authors: Sedai, Baburam, Díaz-Urrutia, Christian, Baker, R. Tom, Wu, Ruilian, Silks, L. A. “Pete”, Hanson, Susan K
• Format: Article
• Language: English
• ISSN: 2155-5435; 2155-5435
• DOI: 10.1021/cs400636k

The reactivity of homogeneous oxovanadium and copper catalysts toward aerobic oxidation of phenolic and nonphenolic β-1 lignin model compounds has been investigated. Aerobic oxidation of diastereomeric, nonphenolic β-1 lignin models (1T, 1E) using the six-coordinate vanadium complex (HQ)2VV(O)(O i Pr) (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 Cα–Cβ 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)2VV(O)(O i Pr) or a stoichiometric amount of CuOTf/2,6-lutidine/TEMPO yielded 2,6-dimethoxybenzoquinone (10) as the major product, arising from cleavage of the Caryl–Cα bond. Different selectivity was observed in the oxidation of 2T, 2E using the five-coordinate complex (dipic)VV(O)(O i Pr) (dipic = dipicolinate), with α,β-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 β-1 lignin models are discussed.