Synthesis, molecular structure, conformational, and intramolecular hydrogen bond strength of ethyl 3-amino-2-butenoate and its N-Me, N-Ph, and N-Bn analogs; An experimental and theoretical study

Abstract

Amino derivatives of ethyl acetoacetate like ethyl (Z)- 3-(amino)but-2-enoate (EAB), ethyl (Z)- 3-(methylamino)but-2-enoate (Me-EAB), ethyl (Z)- 3-(phenylamino)but-2-enoate (Ph-EAB), and ethyl (Z)- 3-(benzylamino)but-2-enoate (Bn-EAB) were synthesized. Their structures, conformations, and intramolecular hydrogen bonding (IHB) is characterized using computational analysis (density functional theory, DFT, methods) at the B3LYP/6-311++G(d,p) level and spectroscopic techniques)IR, UV, and NMR). All the mentioned theoretical and experimental results were compared to those of (Z)-methyl 3-aminobut-2-enoate (MAB). The vibrational spectra of EAB are fully assigned. Vibrational spectroscopy results confirmed the existence of two conformers of title molecules. DFT calculations at the B3LYP/6-311++G(d,p) level show the calculated NH bond lengths in EAB, Me-EAB, Ph-EAB, Bn-EAB, and MAB are 1.015, 1.017, 1.022, 1.018, and 1.014 Ǻ, respectively. Also, according to the natural bond orbital (NBO) results, the σN-H10↔LP(2)O7 interactions for EAB, Me-EAB, Ph-EAB, Bn-EAB, and MAB are 5.21, 6.77, 8.65, 7.18, and 5.11 kcal/mol, respectively. So, the trend in IHB strength is: Ph-EAB > Bn-EAB ∼ Me-EAB > EAB ∼ MAB, which also agrees with the mentioned experimental results. The mentioned trend also is confirmed by the LP(O7)↔σNH steric exchange interaction.