NMR and DFT study of the epimerization of Xylose to Lyxose promoted by calcium chloride with triethylamine
As a renewable resource, the catalytic conversion of xylose has attracted much attention. The epimerization of xylose to lyxose is an important way to obtain the rare sugar lyxose. Current approaches to xylose epimerization encounter several challenges, including the requirement for high temperatures, the use of costly catalysts and a propensity for side reactions that favor the formation of ketoses. CaCl 2 is an environmentally friendly and naturally abundant metal salt that can act as a Lewis acid in catalytic processes. Herein, we demonstrate that CaCl 2 and Et 3 N work synergistically to achieve 68 % lyxose yield under mild conditions. Compared to conventional approaches, which often require harsh conditions or expensive catalysts, our system demonstrates a cost-effective and selective strategy. Moreover, through a combined experimental and computational approach (NMR, isotope tracing and DFT), we provide unprecedented mechanistic insights into the role of calcium complexation in facilitating epimerization. Complexation between xylose and calcium ion was confirmed by NMR titration and Diffusion Ordered NMR Spectroscopy (DOSY-NMR). Isotope experiments further demonstrated that the C1-C2 migration caused the turnover of the hydroxyl group at C2 in the presence of calcium ions. The frontier orbital analysis and deprotonation energy calculation showed that the hydroxyl group at C1 had the lowest deprotonation energy, which initiated the ring-opening process. The DFT calculations reveal that the calcium ions preferentially complex with the hydroxyl groups of xylose, thereby reducing the energy barrier for the C1 proton dissociation and facilitating the epimerization. These findings offer a new perspective on xylose epimerization with Ca-based catalyst.
危险品化学品经营许可证（带存储）