Mechanistic Study of Glucose-to-Fructose Isomerization in Water Catalyzed by Al(OH)2(aq)+

The nature of the active aluminum species and their interaction with glucose in water are studied to establish a detailed mechanism for understanding AlCl3-catalyzed glucose-to-fructose isomerization. The combination of activity results with electrospray ionization tandem mass spectrometry (ESI-MS/MS) reveal that Al(OH)2(aq)+ species contribute a lot to the isomerization. Attenuated total reflection infrared spectroscopy (ATR-IR) results show that glucose undergoes a ring-opening process which is accelerated by the Al(OH)2(aq)+ species. The binding of acyclic glucose with Al(OH)2(aq)+ species occurs at the C1–O and C2–O positions of glucose, which initiates the hydride shift of the aldose-to-ketose isomerization. The in situ 27Al NMR data elucidate the maintenance of the hexa-coordinated form of Al species throughout the reaction. An obvious kinetic isotope effect occurs with the C2 deuterium-labeled glucose, confirming that the intramolecular hydride shift from the C2 to C1 positions of glucose is the rate-limiting step for the isomerization. The apparent activation energy (Ea) of the AlCl3-catalyzed glucose-to-fructose isomerization reaction is estimated to be 110 ± 2 kJ·mol–1.