Tautomerism in carbohydrate-derived salicylidene schiff bases: Solution, solid-state, and theoretical investigations

Abstract

Enol-imine and keto-amine tautomerism in a series of carbohydrate-derived Schiff bases were investigated in gas, liquid, and solid-state by theoretical calculation, NMR spectroscopies, and single-crystal XRD, respectively. Two of these sugar-modified Schiff-bases derived from benzyl 2-deoxy-2-salicylideneamino-α-D-glucopyranoside (H2L3-tBu and H2L3-OMe) were synthesized and crystallized from methanol and their crystal structures determined. The results show that H2L3-tBu crystallizes in the enol-imine form while H2L3-OMe adopts the keto-amine form. The crystal packing of the latter is characterized by hydrogen bonding via the co-crystallized methanol molecules. Calculations were performed to shed light as to what parameters govern these behaviors. In this regard, eight different salicylideneamino substituted sugar-modified Schiff-bases (considering electronic and steric effects) were studied theoretically. Separate calculations were performed both in the gas phase as well as in methanol as solvent. It was found that the enol-amine form of the Schiff base is the stable tautomer in the gas phase. The electronic of the substituents was also seen to influence this tendency as electron releasing groups were found to increase the energy difference between keto and enol forms. In contrast the opposite trends are seen for electron-withdrawing groups in both gas phase and solution. Moreover, these energy differences between both tautomers in methanol are considerably lower than those obtained in the gas phase. Molecular structures of selected Schiff bases in solution were also studied experimentally by utilization of variable temperature (VT) 1H NMR and low temperature 1H−1H COSY. The results indicate that the presence of the other tautomer can be confirmed at lower temperatures. Based on these findings, it is believed that the participation of methanol in the stabilization of tautomers is influenced by both the position and nature of the substitution.