Contribution of the γ-hydroxy group to the β-O-4 bond cleavage of lignin model compounds in a basic system using tert-butoxide

Abstract

The most common non-phenolic β-O-4-type lignin model compounds with or without the γ-hydroxymethyl group (C6-C3- or C6-C2-type, respectively) were treated in a 0.5 mol/L potassium tert-butoxide in DMSO solution at 30 °C to examine the effects of presence of the group. The β-O-4 bond of the C6-C3-type cleaved more rapidly than the C6-C2-type, indicating that the γ-hydroxy group contributes to the cleavage, in contrast to their reactions in alkaline pulping processes. Furthermore, the β-O-4 bond of the threo isomer of the C6-C3-type cleaved more rapidly than that of the erythro isomer. This result can be attributed to the fact that the erythro isomer has the hydrogen bond between a generated alkoxide and the other hydroxy group at its α- and γ-positions in its most-preferential conformer, interfering with the β-O-4 bond cleavage. It was also suggested in treatments of their methyl-etherified derivatives at the α- or γ-hydroxy group that the contribution of the γ-hydroxy group of the threo isomer is greater than that of the erythro isomer. Detailed examination of the distribution profile of reaction products supported this greater contribution of the γ-hydroxy group of the threo isomer.