Thermodynamic Insights into MgBr₂-Mediated Glucose Interconversion to Fructose Undertaking Multiple Reaction Pathways by Applying the Macro- and Micro-Kinetic Principles

Abstract

While many reports on the kinetics and mechanism of glucose isomerization to fructose via a chemical pathway have been published, thermodynamic insights into the interconversion reaction are rarely reported. Here, we report the temperature-dependent characteristics of the cation- and anion-mediated glucose conversion to fructose. The counter ions of MgBr2 enabled the reaction via different pathways in water; e.g., Mg2+ influenced the reaction by undertaking the 1,2-hydride shift mechanism and accounted for up to 50% of the conversion. However, the counter ion (Br–) promoted the conversion via the proton transfer mechanism, which contributed to the remaining 50%, based on the results of the isotopic labeling experiments. This selective transformation (32% wt fructose yield and 76% selectivity) aided by MgBr2 is attributed to the formation of a weak water shell around Mg2+ (due to a lower ratio of MgBr2 to water), which permitted the cation to expose its catalytic activity and affected the administering activity by Br–, which induces maximum side reactions. By applying the principles of transition-state Eyring and Marcus theories to the elementary steps involving a proton transfer and electron transfer, respectively, the temperature-dependent characteristics of the corresponding pathways were determined. The Eyring model exhibited a linear trend in the ln(k/T) vs 1/T plot with an activation energy barrier of 70.25 kJ/mol (comparable to the value of the collision-based Arrhenius model). The semi-classical Marcus model disclosed that the hydride shift is a normal electron transfer rate based on the localization of k ET in the λ< -ΔG o >0 region.

Fructose Epimerization to l-Sorbose in Water over Molybdenum Oxide: Reaction Kinetics and Mechanism Insights

“…l -Sorbose formation using fructose was confirmed via 13 C NMR analysis by comparison with reference l -sorbose and d -fructose spectra . As shown in Figure , the reference l -sorbose exhibited six intense peaks.…”

“…However, the hydride shift is regarded as a rate-limiting step . As per the literature, the formation of open-ring fructose involves a different reaction mechanism, , leading to the synthesis of tagatose and allulose side products. Unlike sorbose formation, the synthesis of these products involves structurally dissimilar intermediates.…”

“…L-Sorbose formation using fructose was confirmed via 13 C NMR analysis by comparison with reference L-sorbose and D-fructose spectra. 50 As shown in Figure 4, the reference L-sorbose exhibited six intense peaks. The peak at 94.0 ppm under a more deshielded region corresponded to the carbonyl carbon.…”