Toward Understanding Base-Catalyzed Isomerization of Saccharides

Abstract

This Perspective addresses recent advances in isomerization of aldoses into ketoses catalyzed by bases. In recent years, catalytic activity of a number of bases was reported, but the structure–performance correlations are typically missing or remain poorly understood. In this Perspective, the reaction mechanism and kinetics of the base-catalyzed isomerization are discussed. An overview of products obtained via base-catalyzed transformations of d-glucose (Glc), d-lactose (Lac), d-galactose (Gal), and aldopentoses d-ribose (Rib), d-arabinose (Ara), d-xylose (Xyl), and d-lyxose (Lyx) is provided. Side reactions that occur under base conditions are summarized. Catalysis by soluble metal hydroxides and general bases is considered. Moreover, structure–selectivity relationships observed for soluble bases as well as methods for stabilization of ketoses via complexation are discussed. Influences of organic (co-)solvents on reaction kinetics and acceleration of the reaction rate in aqueous phase in the presence of soluble salts–also referred to as salt effect–are addressed. Next, solid bases applied as catalysts for isomerization are considered with a focus on contributions of homogeneous and heterogeneous catalysis into the isomerization processes. The applicability of a filtration test and a contact test as tools to estimate impact of leached species onto catalysis is critically discussed.

Highly Selective Isomerization of Glucose to Fructose through a Biphasic and Recyclable Mimetic Enzyme Catalyst

“…The extraction of hydrogen from C 2 by the phenoxy anion and the subsequent proton transfer from the imine positive ion leads to the formation of the intermediate cis-diol C, which is regarded as a rate-determining step in the isomerization. [47] The second rotation of the C 3 À C 4 chain leaves the substrate in a more favorable state for the closed-loop step. After the extraction of hydrogen from C 5 À OH by phenoxy ion and the proton transfer of imine group to the oxygen on C 2 carbonyl results in the formation of intermediate D. Finally, the closed loop of D generates the product fructose.…”

Insight into the alkaline earth metal salt promotion for alkali-catalyzed glucose isomerization

“…9 Currently, the isomerization of glucose to fructose is mainly carried out by enzymatic or chemical catalytic processes. [10][11][12] However, enzymatic methods are costly and have a narrow operating window, while chemical catalysis often suffers from low efficiency and fructose selectivity. 10 In this context, the development of a fast and selective process for glucose-to-fructose is urgent and essential.…”

“…Alkali-catalyzed glucose isomerization into fructose is one of the important chemical catalytic isomerization processes. 10,12,13 Lots of alkali catalysts such as inorganic Brønsted alkalis, organic alkalis, organic amines, basic amino acids, solid alkali catalysts, alkali ionic liquids, etc. have been studied.…”

“…have been studied. 12,14,15 Generally, high fructose selectivity can be obtained in an alkaline system when the glucose conversion is relatively low, but it will drop significantly if the fructose yield exceeds 30%. 16 The main reason is that ketoses typically exhibit lower stabilities than aldoses due to a series of extremely complex side reactions under alkali conditions.…”

Dynamic Cu(I)/Cu(II) Redox Shuttle for Maltose and Lactose Isomerization under Pulsed Electric Field

“…15 However, alkaline catalysis suffers from unideal selectivity of target ketose derived from an undesirable side reaction of hydrolysis. 10 As for electroactivation approach, the low yield of lactulose for example was hard to break through the bottleneck of 40% due to the limitation of electrode unreversible decomposition derived from DC-regulated power. 16−18 useful for ensuring its lifespan.…”