Uncharted Pathways for CrCl3 Catalyzed Glucose Conversion in Aqueous Solution

Abstract

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Mechanism and malleability of glucose dehydration to HMF: entry points and water-induced diversions

“…Quantification of the isotopomers indicates that approximately 95% of glucose reacts through a 1,2 H-shift, while 4% react though a 1,5 H-shift and only 1% reacts through a 1,2 C-shift under reaction conditions to HMF. Both [6- 13 C] sorbose 2,19 and [2- 13 C] mannose (Fig. S2) were found in the reaction mixture, thus supporting the conversion of glucose to HMF through the depicted pathways.…”

“…28 Some recent studies have nevertheless emphasised a neutral or even beneficial effect of 15-50% water on HMF formation, 18 as water suppresses the sequestration of glucose into unreactive disaccharides and anhydroglucose forms. 18,19 The effect of water in the Cr III -catalysed glucose conversion to HMF was thus scrutinised in more detail using in situ NMR.…”

“…The conversion of glucose to HMF in these solvents is limited by the formation of anhydrosugars or disaccharides. 18,19 These undesired side reactions can be counteracted in the presence of water. Some disagreement persists in the literature, whether the overall selectivity for HMF formation from glucose is improved or impaired by the addition or accumulation of water in the dehydration reaction.…”

“…At millimolar concentrations, paramagnetic catalysts constitute paramagnetic cosolutes that accelerate the T 1 relaxation of analyte 13 C nuclear spins. 19,20 Such conditions permit sensitivity improved, quantitative in situ NMR spectroscopy with higher time resolution than experiments in the absence of paramagnetic catalysts could have. Here, this improvement on in situ spectroscopy was employed to gain insight into the entry points, kinetics and water effect in the Cr III -catalysed conversion of glucose to HMF in DMSO.…”

Kinetic variations in acid-catalyzed monosaccharide conversion

“…This mixture takes advantage of the ability of NMR to track the provenance of molecular species through isotope labelling under identical reaction conditions: [1- 13 C] glucose and [2- 13 C] fructose were predominantly converted to the respective [1-13 C] and [2- 13 C] HMF products and these reactions to the same chemical could be distinguished by producing isotopic isomers with the 13 C isotope in different positions. Besides functioning as a catalyst, the chromium salt also acted as a relaxation agent, which rapidly restored nuclear magnetization to its equilibrium and thus permitted the rapid accumulation of high-quality NMR spectra [27].…”