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

Abstract

The direct anodic oxidation of non-phenolic lignin model compounds was investigated to understand their basic behaviors. The results of cyclic voltammetry (CV) studies of monomeric model, such as 1-(4-ethoxy-3-methoxyphenyl)ethanol, are interpreted as the oxidation for C a -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 a -carbonyl compounds in high yields (60-80%). It is suggested that deprotonation at C a -H in the ECEC mechanism (Eselectron transfer and Cschemical step) is important for C a -carbonylation. In the uncatalyzed bulk electrolysis of a b-O-4 model dimeric compound, 4-ethoxy-3-methoxyphenylglycerol-b-guaiacyl ether, the corresponding C a -carbonyl compound was not detected but as a result of C a -C b 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 b-O-4 models. The conclusion is that direct electrooxidation is unsuitable for C a -carbonylation of lignin.

Studies on electro-oxidation of lignin and lignin model compounds. Part 2: N-Hydroxyphthalimide (NHPI)-mediated indirect electro-oxidation of non-phenolic lignin model compounds

“…The corresponding C a -carbonyl compounds 2G, 2S, and 2P arose in 97.2, 96.3, and 84.8% yields, respectively. These yields are similar but higher than those in direct EO (Shiraishi et al 2011). In other words, C a -carbonylation in NHPI-mediated indirect EO is more efficient.…”

“…Non-phenolic lignin model compounds 1G, 1S, 1P, and 3G were prepared as described by Shiraishi et al (2011). Other chemicals were used as obtained from Nacalai Tesque Inc. (Kyoto, Japan).…”

“…Cyclic voltammetry (CV) measurements were performed according to the method in the previous paper (Shiraishi et al 2011). Bulk electrolyses of monomers (1G, 1S, or 1P) were performed using NHPI (0.1-0.2 eq.…”

“…of the substrate) at 0.7 V vs. Ag/Ag q . Other reaction conditions were described in the previous paper (Shiraishi et al 2011).…”

“…Oxidation peak (a) at 1.8 V was found, which is thought to be the peak by several electrons withdrawing, because a shoulder peak at 1.3 V is present in CV of NHPI in 0.1 M tetrabutylammonium perchlorate (TBAP)/CH 3 CN (data not shown), and a broad peak at 1.3 V was reported in CV of NHPI in 0.1 M TEAP/CH 3 CN (Kishioka and Yamada 2005). The oxidation potentials (1.3-1.8 V) are higher than those of non-phenolic lignin model compounds (for example, 1.1 V for compound 1G, see Shiraishi et al 2011). Accordingly, NHPI does not work as a mediator in the system without a catalyst.…”