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
作者:Enrico Baciocchi、Massimo Bietti、Maria Francesca Gerini、Osvaldo Lanzalunga、Simona Mancinelli
DOI:10.1039/b101362i
日期:——
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+Ë is CαâH 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 CâC
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+Ë 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 CâC 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-MeOC6H4OCH2CD2OË was generated from 4-MeOC6H4OCH2CD2OOt-Bu.
The oxidation of 1 induced by the genuine one-electron oxidant potassium 12-tungstocobalt(III)ate at pHÂ =Â 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.
研究了 pH= 4.0 条件下 LiP 催化的 H2O2 促进的非酚类δ-O-芳基木质素模型二聚体 1-(3,4-二甲氧基苯基)-2-苯氧基乙醇 (1) 和 2-(4-甲氧基苯氧基)-1-苯乙醇 (2) 的氧化作用。1 的氧化主要生成相应的酮,表明中间自由基阳离子 1+Ë 的主要反应是 CαâH 去质子化。2 的氧化反应生成 2-(4-甲氧基苯氧基)-2-苯乙醇(7,2 的异构体)、2-苯基-1,4-二氧杂螺[4.5]癸-6,9-二烯-8-酮(8)以及由 CâC 键 δ³ 裂解到电子更丰富的环上的产物。假设中间体 2+Ë 的主要反应是带有正电荷的环上的醇羟基发生亲核反应,则所有这些产物的形成都是合理的。这将产生一个螺环己二烯基,然后将其氧化为二氧杂螺癸二烯酮 8,或者进行开环反应,产生一个烷氧基基,并由此形成 2 的异构体和 CâC 键裂解产物。通过研究 4-MeOC6H4OCH2CD2OH 的氧化过程,并将结果与从 4-MeOC6H4OCH2CD2OOt-Bu 生成烷氧基自由基 4-MeOC6H4OCH2CD2OË 时的结果进行比较,为这一机理提供了支持。 在 pH= 4.0 的条件下,真正的单电子氧化剂 12-钨钴(III)ate钾诱导的 1 的氧化过程证实了使用 LiP 得到的结果。不过,在相同的条件下,2 的氧化过程中没有观察到碎片产物,这可能是由于 12-钨钴(III)ate钾快速氧化了螺环己二烯基。