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

摘要

The N-hydroxyphthalimide (NHPI)-mediated indirect elec tro-oxidation of non-phenolic lignin model compounds has been investigated for selective C_α-carbonylation of lignin. A cyclic voltammogram of NHPI in 0.1 M LiClO_4/CH_3CN with 2,6-lutidine interpreted that NHPI can act as a mediator in the indicated process in the range 0.5-0.8 V vs. Ag/Ag~+. The corresponding C_α-carbonyl compounds was obtained in high yields (85-97%) in the case of the monomer 1-(4-eth-oxy-3-methoxyphenyl) ethanol in 0.1 M LiClO_4/CH_3CN or 0.1 M LiClO_4/(CH_3CN/H_2O=7/3) with a small amount of 2,6-lutidine at 0.7 V vs. Ag/Ag~ + . The processing of the dimeric lignin model compound (4-ethoxy-3-methoxyphe-nylglycerol-β-guaiacyl ether) also gave the corresponding C_α-carbonyl compound in high yield (88-92%). The reaction proceeds through hydrogen atom transfer in the NHPI-medi ated electro-oxidation. On the other hand, the direct electro oxidation and indirect electro-oxidation mediated by ABTS [2,2'-azinobis(3-ethylbenzothiazoline-6-sulfonate)] of the dimeric compound preferentially gave the corresponding C_α-C_β cleavage product in low or moderate yields (5-40%). The conclusion is that NHPI is an excellent mediator for selective C_α-carbonylation of non-phenolic β-O-4 structures in lignin in electronic mediator system.