Studies on electrooxidation of lignin and lignin model compounds. Part 1: Direct electrooxidation of non-phenolic lignin model compounds

摘要

The direct anodic oxidation of non-phenolic lignin model compounds was investigated to understand their basic behav iors. The results of cyclic voltammetry (CV) studies of mon omeric model, such as l-(4-ethoxy-3-methoxyphenyl)etha nol, are interpreted as the oxidation for C_α-carbonylation did not proceed in the reaction without a catalyst, but a base promotes this reaction. Indeed, the bulk electrolyses of the monomeric lignin model compounds with 2,6-lutidine afforded the corresponding C_α-carbonyl compounds in high yields (60-80%). It is suggested that deprotonation at C_α-H in the ECEC mechanism (E = electron transfer and C= che mical step) is important for C_α-carbonylation. In the unca talyzed bulk electrolysis of a β-O-4 model dimeric com pound, 4-ethoxy-3-methoxyphenylglycerol-β-guaiacyl ether, the corresponding C_α-carbonyl compound was not detected but as a result of C_α-C_β cleavage 4-O-ethylvanillin was found in 40% yield. In the electrolysis reaction in the presence of 2,6-lutidine (as a sterically hindered light base), the reaction stopped for a short time unexpectedly. These results indicate the different electrochemical behavior of simple monomeric model compounds and dimeric β-O-4 models. The conclu sion is that direct electrooxidation is unsuitable for C_α-car bonylation of lignin.