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

Abstract

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 situ1H NMR technique.