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 (GlcNH 2 ) into deoxyfructosazine (DOF). The interactions between these three components in the prepared TDESs were studied by 1 H-, 35 Cl-NMR spectra and 1 H diffusion-ordered spectroscopy (DOSY) measurements. The chemical shift changes of active hydrogen in the 1 H-NMR spectra of TDES system and widening of signals in the 35 Cl-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 1 H 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 GlcNH 2 and the TDES was proposed. Moreover, a reaction intermediate, dihydrofructosazine, was identified in the self-condensation reaction of GlcNH 2 by an in situ 1 H NMR technique.