Disaccharides as substrates and mechanistic probes for efficient carbohydrate conversion to formic acid in water near room temperature

Abstract

Formate is a hydrogen carrier that can be obtained by the oxidation of carbohydrates with hydrogen peroxide under aqueous alkaline conditions near room temperature. The most relevant route among various conceivable pathways for glucose degradation has only recently been clarified. The conversion of biomass-derived disaccharides such as maltose from starch and cellobiose from cellulose into formate could further support the green production of formate. The mechanism, intermediates, side products, and effect of the substrate structure (such as α- vs. β-linkages or the presence of reducing vs. non-reducing ends) remain poorly understood for the conversion of O-glycosidically linked carbohydrates. Here, we close these gaps and show that stoichiometric amounts of base and a surplus of hydrogen peroxide can lead to a near-quantitative and surprisingly rapid conversion of disaccharides to >97% organic carbon in formate. Real-time observations show that glucose is the main intermediate, indicating that the accessibility of the aldehyde groups in stable glucopyranosyl rings is a limiting factor. Side products include glycosylated aldonic acids, which derive from aldose-to-ketose isomerization near the reducing end. This transformation, known as the Lobry de Bruyn–Van Ekenstein transformation, facilitates partial oxidation pathways leading to stable glycosylated C10 and C11 acids. Higher concentrations of H₂O₂ suppress isomerization by favoring direct oxidative cleavage, thus minimizing these side products. The absence of disaccharide byproducts, the near-complete conversion of reducing and non-reducing glucopyranosyl residues in disaccharides, and the effect of radical scavengers provide further mechanistic understanding. The combination of quantitative NMR, isotope labeling, and real-time reaction tracking thus provides novel insight into the efficient conversion of disaccharides to formate.