Titanium-Beta Zeolites Catalyze the Stereospecific Isomerization of D-Glucose to L-Sorbose via Intramolecular C5-C1 Hydride Shift

Pure-silica zeolite beta containing Lewis acidic framework Ti4+ centers (Ti-Beta) is shown to catalyze the isomerization of D-glucose to L-sorbose via an intramolecular C5-C1 hydride shift. Glucose-sorbose isomerization occurs in parallel to glucose-fructose isomerization on Ti-Beta in both water and methanol solvents, with fructose : formed as the predominant product in water and sorbose as the predominant product in methanol (at 373 K) at initial times and over the course of >10 turnovers. Isotopic tracer studies demonstrate that C-13 and D labels placed respectively at the Cl and C2 positions of glucose are retained respectively at the C6 and C5 positions of sorbose, consistent with its formation via an intramolecular C5-C1 hydride shift isomerization mechanism. This direct Lewis acid-mediated pathway for glucose sorbose isomerization appears to be unprecedented among heterogeneous or biological catalysts and sharply contrasts indirect base-mediated glucose sorbose isomerization via 3,4-enediol intermediates, or via retro-aldol fragmentation and recombination of sugar fragments. Measured first-order glucose sorbose isomerization rate constants (per total Ti; 373 K) for Ti-Beta in methanol are similar for glucose and glucose deuterated at the C2 position (within a factor of similar to 1.1), but are a factor of similar to 2.3 lower for glucose deuterated at each carbon position, leading to H/D kinetic isotope effects expected for kinetically relevant intramolecular C5-C1 hydride shift steps. Optical rotation Measurement's Show that isomerization of D-(+)-glucose (92% enantiomeric purity) with Ti-Beta in water (373 K) led to the formation of L-(-)-sorbose (73% enantiomeric purity) and D-(-)-fructose (87% enantiomeric purity) as the predominant stereoisomers, indicating that stereochemistry is preserved at carbon centers not directly involved in intramolecular C5-C1 or C2-C1 hydride shift steps, respectively:. This new Lewis acid-mediated rearrangement of glucose to sorbose does not appear to have a metalloenzyme analog.