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 P-D-glucopyranoside (MGP beta) 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 O-2 95 degrees C). The simulation involved in situ AOS generation by reaction of 2,4,6-trimethylphenol with O-2, so that various types of AOS were operative in the system. MGP beta was degraded slightly more than methyl alpha-D-glucopyranoside (MGP alpha) 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 MGP beta and methyl beta-D-(1-H-2)glucopyranoside (MGP beta-1D) were jointly reacted with AOS, MGP beta was degraded slightly more than MGP beta-1D. This difference results from the kinetic isotope effect, indicating that the anomeric hydrogen of MGP beta is undoubtedly abstracted by AOS, although the contribution of this abstraction to the overall degradation of MGP beta has not been clarified yet. MGP beta and MGP beta-1D were also jointly subjected to alkaline hydrogen peroxide treatment (0.5 mol l(-1) NaOH, 0.36 mmol l(-1) Fe3+, 95 degrees C) in which the oxyl anion radical (O-center dot), the conjugate base of hydroxyl radical (HO center dot), is likely only one of the AOS to degrade these compounds. Since no difference in degradation was observed, some AOS other than O-center dot must participate in anomeric hydrogen abstraction from MGP beta in these oxygen-alkali treatments. When MGPa and methyl alpha-D-(1-H-2)glucopyranoside were jointly reacted with AOS, almost no difference in degradation was observed. This result suggests that abstraction of the anomeric hydrogen from MGPa by AOS is no more than a very minor reaction. It was confirmed that the anomeric hydrogen of MGP beta is activated towards AOS reaction owing to the anomeric effect, which is in fact a stereoelectronic effect.