Structures, spectroscopic analysis, herbicidal activities and enamine–aminone tautomerism of new β-diketone derivatives modified with glycylglycine methyl ester

• New β-diketone derivatives modified by glycylglycine ester are synthesized.
• Optimized geometries and theoretical vibrational frequencies can well reproduce the experimental data.
• The enamine–aminone tautomerism is caused by proton transfer.
• The tested compounds own higher inhibition ability to monocotyledon than to dicotyledon.

New β-diketone derivatives modified with glycylglycine methyl ester have been synthesized and characterized by IR, UV, 1H NMR, 13C NMR, Elemental analysis and single-crystal X-ray diffraction, the analytical results show that compound 1 and compound 2a exist in enamine form while compound 2b exists in aminone form. The optimized geometries and theoretical vibrational frequencies of the compounds calculated by using DFT/B3LYP with 6-31g (d, p) basis set in the ground state can well reproduce the experimental data. The results of herbicidal activity tests indicate that all the tested compounds own higher inhibition ability to monocotyledon than to dicotyledon, especially to green-bristlegrass with the inhibitory rates about 100%. Theoretical enamine–aminone tautomerism study at DFT/B3LYP/6-31g (d, p) shows that tautomerism between compound 2a and 2b is mainly caused by the proton transfer.