Amino Acid Ionic Liquids Catalyzed d-Glucosamine into Pyrazine Derivatives: Insight from NMR Spectroscopy

Using environment-friendly catalysts to convert biomass into compounds with high values is one of the central topics of green chemistry. In this work, [Ch][Pro] (cholinium as the cation and l-proline as the anion) ionic liquid was synthesized and applied as a model catalyst in the production of deoxyfructosazine (DOF) and fructosazine (FZ) from d-glucosamine (GlcNH₂). The ¹³C NMR chemical shift titration experiments and the diffusion-ordered NMR spectroscopy (DOSY) measurements showed that, when the [Ch][Pro] interacted with GlcNH₂, the l-proline anion ([Pro]^-) played a major catalytic role instead of cholinium cation ([Ch]⁺). The effects of the reaction temperature and the amount of [Ch][Pro] on the product yields were surveyed. The experimental results showed that the highest DOF yield (33.78%) was obtained after 30 min at 100 °C when the molar ratio of [Ch][Pro]/GlcNH₂ was 1. Moreover, in situ ¹H NMR and in situ ¹³C NMR experiments were applied to monitor the reaction process with [Ch][Pro] as the catalyst. The reactive intermediate, dihydrofructosazine, was clearly detected by these in situ techniques. Accordingly, a possible reaction pathway was proposed. By applying other amino acids as the anions, we also prepared five other [Ch][AA] ionic liquids, and they showed different catalytic activities and selectivity in the GlcNH₂ self-condensation reaction.