Istasse Thibaut, Richel Aurore
Laboratory of Biomass and Green Technologies, University of Liege - Gembloux Agro-Bio Tech Passage des Déportés 2, B-5030 Gembloux Belgium
RSC Adv. 2020 Jun 22;10(40):23720-23742. doi: 10.1039/d0ra03892j. eCollection 2020 Jun 19.
The conversion of abundant hexoses ( glucose, mannose and galactose) and pentoses ( xylose and arabinose) to 5-hydroxymethylfurfural (5-HMF) and 2-furfural (2-F) is subject to intensive research in the hope of achieving competitive production of diverse materials from renewable resources. However, the abundance of literature on this topic as well as the limited number of studies systematically comparing numerous monosaccharides hinder progress tracking. Herein, we compare and rationalize reactivities of different ketoses and aldoses. Dehydration mechanisms of both monosaccharide types are reviewed regarding the existing experimental evidence. Ketose transformation to furan derivatives likely proceeds through cyclic intermediates and is hindered by side-reactions such as isomerization, retro-aldol reactions and polymerization. Different strategies can improve furan derivative synthesis from ketoses: limiting the presence of water, improving the dehydration rate, protecting 5-HMF and 2-F reactive moieties with derivatization or solvent interactions and extracting 5-HMF and 2-F from the reaction medium. In contrast to ketoses, aldose conversion to furan derivatives is not favored compared to polymerization reactions because it involves their isomerization or a ring contraction. Enhancing aldose isomerization is possible with metal catalysts ( CrCl) promoting a hydride shift mechanism or with boric/boronic acids promoting an enediol mechanism. This catalysis is however far more challenging than ketose dehydration because catalyst activity depends on numerous factors: Brønsted acidity of the medium, catalyst ligands, catalyst affinity for monosaccharides and their accessibility to several chemical species simultaneously. Those aspects are methodically addressed to support the design of new monosaccharide dehydration systems.
将丰富的己糖（葡萄糖、甘露糖和半乳糖）和戊糖（木糖和阿拉伯糖）转化为5-羟甲基糠醛（5-HMF）和2-糠醛（2-F）是当前深入研究的课题，旨在实现从可再生资源中竞争性生产多种材料。然而，关于这一主题的文献数量众多，且系统比较多种单糖的研究数量有限，这阻碍了进展跟踪。在此，我们比较并阐明了不同酮糖和醛糖的反应活性。结合现有实验证据，综述了这两种单糖类型的脱水机制。酮糖向呋喃衍生物的转化可能通过环状中间体进行，并受到诸如异构化、逆羟醛反应和聚合等副反应的阻碍。可以通过不同策略改善从酮糖合成呋喃衍生物的过程：限制水的存在、提高脱水速率、通过衍生化或溶剂相互作用保护5-HMF和2-F的反应性部分，以及从反应介质中提取5-HMF和2-F。与酮糖不同，醛糖转化为呋喃衍生物与聚合反应相比并不占优势，因为这涉及它们的异构化或环收缩。使用促进氢化物转移机制的金属催化剂（CrCl）或促进烯二醇机制的硼酸/硼酸盐可以增强醛糖异构化。然而，这种催化比酮糖脱水更具挑战性，因为催化剂活性取决于众多因素：介质的布朗斯特酸度、催化剂配体、催化剂对单糖的亲和力以及它们同时与多种化学物质的可及性。对这些方面进行了系统探讨，以支持新型单糖脱水系统的设计。
