Insights into degradation pathways of oxidized anhydroglucose units in cellulose by β-alkoxy-elimination: a combined theoretical and experimental approach
Depolymerization of cellulose starting from an oxidized anhydroglucose unit through balkoxy- elimination, triggered by alkaline media, is one of the key reactions responsible for cellulose aging. This study investigates the detailed mechanisms for the chain cleavage by a combination of experimental and quantum chemical methods. Three model compounds for oxidized anhydroglucose units in cellulose were employed: C2-keto, C3-keto-, and C6-aldehyde 4-O-methyl methyl b-D-glucosides, representing anhydroglucose units of cellulose that have been oxidized at C2, C3, and C6, respectively. The alkali-induced b-alkoxy elimination from the model compounds started from the corresponding enolates and followed first order kinetics. While methanol is being released in the case of the model compounds, the analogous process effects chain cleavage in the case of the polymer cellulose. The kinetic rate constants for the C6-aldehyde compound 2, the 2-keto compound 3 and the 3-keto counterpart 4 had a ratio of 1:5:22, indicating the 3-keto compound to be the least stable one. Elimination from an oxidized 6-position (6-aldehyde) was thus more than 20 times slower than that from an oxidized C-3 (3-keto). A 6-carboxyl group is completely innocent with regard to belimination. MP4(SDQ)//DFT(M06-2X) calculations indicated that the degradation pathway starting from the 3-keto enolate had the smallest activation barrier because of stabilization of the transition state by charge transfer from O-5 to C-1. The 3-keto enolate path was consequently more favorable than the alternative ones involving the 2-keto and the 6-keto enolates, which do not exhibit this transition state stabilization. Experimental and computational data thus agreed very well. In polymeric cellulose, also leaving group effects of the O-4 and O-1 glucopyranosyl anions come into play. Calculations indicated the O-4 anion to be more stable, and hence the better leaving group. In actual cellulose, the degradation starting from 3-keto units will become even more dominant than in the model compound, suggesting that carbonyls at C-2 and C-3, both of which afford the C-2 enolate due to the rapid interconversion between the C-2 and C-3 enolates, are chiefly responsible for alkali-induced chain cleavage in oxidatively damaged cellulose, while an aldehyde at C-6 is more innocent.