Reactions of β-O-4-type Non-phenolic Lignin Model Compounds in Various Basic Systems using tert-Butoxide and Other Bases under Mild Conditions

Abstract

Non-phenolic C₆-C₂- and C₆-C₃-type lignin model compounds with the β-O-4 bond were treated in tert-butyl alcohol (tBuOH) or dimethyl sulfoxide (DMSO) containing potassium tert-butoxide (KOtBu) or various other bases under mild conditions (at a base concentration of 0.5 mol/L and 30ºC) to examine how the reactions differ between these systems. The β-O-4 bond cleavage in KOtBu/tBuOH was slower than that in KOtBu/DMSO owing to the greater solvation of tBuO¯ in tBuOH than in DMSO. The β-O-4 bond cleavage of the erythro isomer of the C₆-C₃-type compound was slower than that of the threo isomer in all the reactions. This is explained by the preferred formation of a strong hydrogen bond between the α-hydroxy and the dissociated γ-alkoxy groups (or vice versa) of the former, which interferes with the cleavage. The rates of the β-O-4 bond cleavages in DMSO were in the order of the systems containing: NaOtBu ＞ KOtBu ≫ LiOtBu, which seems to relate to whether each base dissolves as an ion pair or free ions in DMSO. Those in DMSO were in the order of the systems containing KOtBu ≫ potassium hydride (KH) ≫ potassium iso-propoxide ＞ potassium ethoxide, which is consistent with their basicities except for KH.