Glucose Activation by Transient Cr2+ Dimers
Glucose Activation by Transient Cr2+ Dimers
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DOI：
10.1002/anie.201000250
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发表时间：
2010-01-01
影响因子：
16.6
通讯作者：
Hensen, Emiel J. M.
中科院分区：
文献类型：
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作者：
Pidko, Evgeny A.;Degirmenci, Volkan;Hensen, Emiel J. M.
对全球变暖和能源安全的担忧促使人们探索化石碳氢化合物资源的替代品，以供应化学品和能源。[1]生物质是一种很有前途的可再生原料。碳水化合物是生物质的主要成分，需要有效的途径将其转化为燃料和化学品。[2，3]5-羟甲基呋喃甲醛(HMF)被认为是一个关键的生物可再生平台分子。[3，4]虽然利用Brnsted和Lewis酸催化剂可以高产率地从果糖中获得HMF，但[5，6]葡萄糖的选择性转化仍然是一个挑战。直到最近，在离子液体(IL)1-乙基-3-甲基咪唑氯(EMIMCL)中氯化铬(II)催化葡萄糖脱水的产率才被报道。[6]路易斯酸催化的离子液体中的转化被认为涉及单核金属络合物。[6-9]为了将葡萄糖脱水成HMF，提出了一种CrCl3+阴离子来配位二异氰酸酯中间体，该中间体经历了葡萄糖异构化为果糖所需的氢转移(H位移)。[6，8]通常，认为离子-液体介质的作用是提供一个非配位的极性环境来稳定催化活性的低配位金属物种。[8]然而，缺乏金属-碳键形成的证据。除了这种化学催化系统，一些酶很容易促进葡萄糖的转化。典型的能够高选择性地将葡萄糖异构化为果糖的酶包含一个涉及两个金属中心的活性中心。[10-14]在这项研究中，我们结合动力学实验、原位X射线吸收光谱(XAS)和密度泛函理论(DFT)计算来揭示在离子-液体介质中氯化铬(II)对选择性葡萄糖脱水的独特反应活性的分子水平细节。作为一个起点，我们研究了1008℃下，在EMIMCl2中，CrCl2催化葡萄糖脱水的动力学。该反应通过葡萄糖异构化为果糖，然后脱水得到HMF(图1)。反应3h，羟甲基纤维素得率为62%。用果糖进行实验，在相同的条件下，HMF的得率为59%。在这两种情况下，起始糖几乎完全转化。因此，CrCl2/EMIMClCata-
Concerns about global warming and energy security have led to the exploration of alternatives to fossil hydrocarbon resources to supply chemicals and energy.[1] Biomass is a promising renewable feedstock. Efficient routes are required for the conversion of carbohydrates, the main constituents of biomass, into fuels and chemicals.[2, 3] 5-Hydroxymethylfurfural (HMF) is considered a key biorenewable platform molecule.[3, 4] Although HMF can be obtained from fructose in high yield by using Brønsted and Lewis acid catalysts,[5, 6] the selective transformation of glucose, the dominant sugar in cellulosic biomass, remains a challenge. Only recently, unprecedented HMF yields were reported for glucose dehydration by chromium (II) chloride in the ionic liquid (IL) 1-ethyl-3-methylimidazolium chloride (EMIMCl).[6] Lewis acid catalyzed transformations in ionic liquids are thought to involve mononuclear metal complexes.[6–9] For the dehydration of glucose to HMF, a CrCl3 À anion was proposed to coordinate an enediolate intermediate, which undergoes the hydrogen transfer (H shift) required for the isomerization of glucose to fructose.[6, 8] Usually, it is assumed that the role of the ionic-liquid medium is to provide a noncoordinating polar environment to stabilize catalytically active low-coordinate metal species.[7] The direct involvement of the organic cations of the ionic liquid in the catalytically active complex has also been mentioned;[8] however, evidence of the formation of metal–carbon bonds is lacking. Besides this chemocatalytic system, some enzymes readily promote glucose conversion. Typical enzymes capable of isomerizing glucose to fructose with high selectivity contain an active site involving two metal centers.[10–14]In this study, we combined kinetic experiments, in situ X-ray absorption spectroscopy (XAS), and density functional theory (DFT) calculations to unravel the molecular-level details of the unique reactivity of chromium (II) chloride towards selective glucose dehydration in an ionic-liquid medium. As a starting point, we investigated the kinetics of glucose dehydration by CrCl2 in EMIMCl at 1008C. The reaction proceeds through glucose isomerization to fructose, followed by dehydration to give HMF (Figure 1). The HMF yield after a reaction time of 3 hours is 62%. An experiment with fructose gave HMF in 59% yield under identical conditions. In both cases, the starting sugar underwent almost complete conversion. Thus, the CrCl2/EMIMCl cata-