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

摘要

Ternary deep eutectic solvents(TDESs) comprising choline chloride(Ch Cl), 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 Ch Cl, 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 Glc NH_(2) and the TDES was proposed. Moreover, a reaction intermediate, dihydrofructosazine, was identified in the self-condensation reaction of Glc NH_(2) by an in situ ^(1)H NMR technique.