Insights into degradation pathways of oxidized anhydroglucose units in cellulose by β-alkoxy-elimination: a combined theoretical and experimental approach

Cellulose, May 2018

Depolymerization of cellulose starting from an oxidized anhydroglucose unit through β-alkoxy-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 β-d-glucosides, representing anhydroglucose units of cellulose that have been oxidized at C2, C3, and C6, respectively. The alkali-induced β-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 β-elimination. 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. Graphical Abstract Open image in new window

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Insights into degradation pathways of oxidized anhydroglucose units in cellulose by β-alkoxy-elimination: a combined theoretical and experimental approach

Cellulose Insights into degradation pathways of oxidized anhydroglucose units in cellulose by b-alkoxy-elimination: a combined theoretical and experimental approach 0 T. Elder USDA-Forest Service, Southern Research Station , 521 Devall Dr., Auburn, AL 36849 , USA 1 M. Bacher A. Potthast T. Rosenau (&) Division of Chemistry of Renewables, Department of Chemistry, University of Natural Resources and Life Sciences Vienna , Muthgasse 18, 1190 Vienna , Austria 2 T. Hosoya Graduate School of Life and Environmental Sciences, Kyoto Prefectural University , 1-5 Hangi-cho, Shimogamo, Sakyo-ku, Kyoto 606-8522 , Japan 3 T. Rosenau Johan Gadolin Process Chemistry Centre, A ̊ bo Akademi University , Porthansgatan 3, 20500 Turku , Finland 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. Graphical Abstract the pulp and paper industries are increasingly regarded as a business in which high-tech and innovation are very well present. The emergence of biorefinery concepts has also newly stressed the recycling, biomineralization and aging aspects of (ligno)celluloses. Sustainability in a material science sense— Introduction The pulp and paper industries are an important mainstay of many national economies worldwide. This is somewhat contrasting with the general perception of cellulosics as being conventional, relatively low-cost bulk products. Cellulosic products are widely seen as ‘‘being there anyway’’, as commodities that are produced in huge amounts, having been around already for decades, if not centuries. Cellulosics are usually not perceived as high-tech materials and are rarely linked to cutting-edge research in the mind of the users and customers. Novel products usually do not intrigue customers as fancy cell phones, the newest cars or advanced computer technologies do. Only recent developments, being connected to increased environmental awareness worldwide, recognition of global problems, and the advent of bioeconomies and biorefineries, have brought back cellulosics into public perception as valuable biomaterials. In this context, aging, degradation, durability, properties changing over time—became hot topics. Many studies dealing with cellulose degradation, damage, yellowing and aging, coming from ‘‘classical’’ pulping and bleaching chemistry or from conservational science in the second half of the twentieth century, have thus been ‘‘re-discovered’’ or repeated, and several new ones have been added. It is a long and well established fact that cellulose oxidation chiefly influences its properties (Lewin 1997; Potthast et al. 2006) . While ‘‘cellulose oxidation’’ is, in principle, well-defined and can be related to precise chemical structures and struc (...truncated)


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Takashi Hosoya, Markus Bacher, Antje Potthast, Thomas Elder, Thomas Rosenau. Insights into degradation pathways of oxidized anhydroglucose units in cellulose by β-alkoxy-elimination: a combined theoretical and experimental approach, Cellulose, 2018, pp. 3797-3814, Volume 25, Issue 7, DOI: 10.1007/s10570-018-1835-y