Oxidation of non-phenolic β-O-aryl-lignin model dimers catalysed by lignin peroxidase. Comparison with the oxidation induced by potassium 12-tungstocobalt(III)ate

Abstract

The H₂O₂-promoted oxidations of the non-phenolic β-O-aryl-lignin model dimers 1-(3,4-dimethoxyphenyl)-2-phenoxyethanol (1) and 2-(4-methoxyphenoxy)-1-phenylethanol (2) catalysed by LiP at pH = 4.0 have been studied. The oxidation of 1 mainly leads to the corresponding ketone, indicating that the prevailing reaction of the intermediate radical cation 1⁺˙ is Cα–H deprotonation. The oxidation of 2 forms 2-(4-methoxyphenoxy)-2-phenylethanol (7, an isomer of 2), 2-phenyl-1,4-dioxaspiro[4.5]deca-6,9-dien-8-one (8) and products coming from the cleavage of the C–C bond γ to the more electron rich ring. The formation of all these products can be rationalised by assuming that the main reaction of the intermediate 2⁺˙ is a nucleophilic attack of the alcoholic OH group on the ring bearing the positive charge. This leads to a spirocyclohexadienyl radical, which either is then oxidised to the dioxaspirodecadienone 8 or undergoes ring opening to give an alkoxyl radical from which the isomer of 2 and the C–C bond cleavage products may form. Support for this mechanism has been provided by a study of the oxidation of 4-MeOC₆H₄OCH₂CD₂OH and by comparing the results with those obtained when the alkoxyl radical 4-MeOC₆H₄OCH₂CD₂O˙ was generated from 4-MeOC₆H₄OCH₂CD₂OOt-Bu. The oxidation of 1 induced by the genuine one-electron oxidant potassium 12-tungstocobalt(III)ate at pH = 4.0 confirms the results obtained with LiP. However, under the same conditions, no fragmentation products were observed in the oxidation of 2, probably due to a fast oxidation, by potassium 12-tungstocobalt(III)ate, of the spirocyclohexadienyl radical.