Ternary deep eutectic solvents catalyzed d-glucosamine self-condensation to deoxyfructosazine: NMR study

Ternary deep eutectic solvents (TDESs) comprising choline chloride (ChCl), glycerol and l-arginine were synthesized as catalysts and solvents for the conversion of d-glucosamine (GlcNH₂) into deoxyfructosazine (DOF). The interactions between these three components in the prepared TDESs were studied by ¹H-, ³⁵Cl-NMR spectra and ¹H diffusion-ordered spectroscopy (DOSY) measurements. The chemical shift changes of active hydrogen in the ¹H-NMR spectra of TDES system and widening of signals in the ³⁵Cl-NMR spectra confirmed the hydrogen bonding interaction between the components, which was further supported by the decrease of diffusion coefficients (D) of the TDES components according to ¹H DOSY measurements. The influences of reaction temperature and l-arginine content in the TDESs on the yield of DOF were also studied. The experimental results have shown that when the molar ratio of ChCl, glycerol, and l-arginine was 1:2:0.1, DOF was the major product with a yield of 22.6% at 90 °C for 120 min. The chemical shift titration indicated that the carboxyl group of l-arginine in the TDES is the catalytical active site, so the mechanism of the catalytic reaction between GlcNH₂ and the TDES was proposed. Moreover, a reaction intermediate, dihydrofructosazine, was identified in the self-condensation reaction of GlcNH₂ by an in situ ¹H NMR technique.