Detecting Elusive Intermediates in Carbohydrate Conversion: A Dynamic Ensemble of Acyclic Glucose–Catalyst Complexes

The role of acyclic carbohydrates in pathways toward value-added chemicals has remained poorly characterized due to the low population of acyclic forms, and due to their instability under reaction conditions. We conduct steady-state and pre-steady-state measurements by direct reaction progress monitoring with sensitivity-optimized NMR spectroscopy in the molybdate-catalyzed epimerization of glucose to mannose. We detect an exchanging pool of at least 5 acyclic glucose–catalyst complexes under near-optimum reaction conditions. In the presence of catalyst, the acyclic glucose population increases within few seconds prior to reaching a steady state. Exchange between the acyclic intermediates increases under conditions that favor epimerization. Species accounting for less than 0.05% of total glucose can be monitored with subsecond time resolution to allow kinetic analysis of intermediate formation and catalytic conversion. Epimerization occurs 2–3 orders of magnitude faster than the binding of acyclic glucose to the catalyst under near-optimum reaction conditions. The current study brings insight into the nature of acyclic intermediate-catalyst complexes of very low population and into experimental strategies for characterizing very minor intermediates in carbohydrate conversion to value-added compounds.