Toward Understanding Base-Catalyzed Isomerization of Saccharides

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.