期刊
ACS CATALYSIS
卷 3, 期 7, 页码 1469-1476出版社
AMER CHEMICAL SOC
DOI: 10.1021/cs400273c
关键词
fructose; glucose; hydride shift; isomerization; Lewis acid; sorbose; stereospecific; titanium-Beta
资金
- Catalysis Center for Energy Innovation, an Energy Frontier Research Center
- U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences [DE-SC0001004]
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.
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