Investigation of hydrogen abstraction from methyl glucoside by active oxygen species under oxygen delignification conditions. Part 1: Study on the anomeric position

We investigated whether active oxygen species (AOS) abstract the anomeric hydrogen from a carbohydrate model compound, methyl β-d-glucopyranoside (MGPβ) under conditions simulating oxygen delignification in pulp and paper mills (0.5 mol l-1 NaOH, 0.36 mmol l-1 Fe3+, 1.1 MPa O2, 95°C). The simulation involved in situ AOS generation by reaction of 2,4,6-trimethylphenol with O2, so that various types of AOS were operative in the system. MGPβ was degraded slightly more than methyl α-d-glucopyranoside (MGPα) when they were jointly reacted with AOS. Thus, we suggest that the configurational difference in their anomeric positions has some effects on their reactions with AOS. When MGPβ and methyl β-d-(1-2 H)glucopyranoside (MGPβ-1D) were jointly reacted with AOS, MGPβ was degraded slightly more than MGPβ-1D. This difference results from the kinetic isotope effect, indicating that the anomeric hydrogen of MGPβ is undoubtedly abstracted by AOS, although the contribution of this abstraction to the overall degradation of MGPβ has not been clarified yet. MGPβ and MGPβ-1D were also jointly subjected to alkaline hydrogen peroxide treatment (0.5 mol l-1 NaOH, 0.36 mmol l-1 Fe3+, 95°C) in which the oxyl anion radical (O-·), the conjugate base of hydroxyl radical (HO·), is likely only one of the AOS to degrade these compounds. Since no difference in degradation was observed, some AOS other than O-· must participate in anomeric hydrogen abstraction from MGPβ in these oxygen-alkali treatments. When MGPα and methyl α-d-(1-2 H)glucopyranoside were jointly reacted with AOS, almost no difference in degradation was observed. This result suggests that abstraction of the anomeric hydrogen from MGPα by AOS is no more than a very minor reaction. It was confirmed that the anomeric hydrogen of MGPβ is activated towards AOS reaction owing to the anomeric effect, which is in fact a stereoelectronic effect.