Understanding the role of ethylene glycol in a remarkable catalyst-free Strecker reaction of a-CF₃ ketoimine: A theoretical study

Density functional theory (DFT) calculations have been performed to understand the role of Ethylene Glycol (EG) in a remarkable catalyst-free Strecker reaction of α-CF₃ ketoimine. The fluorine hydrogen bonds involved in titled reaction have been studied. To find the suitable calculation methods for the weak C–F⋯H interaction, we chose seventeen density functionals and MP2 method for calculation of the structures of three typical C–F⋯H hydrogen-bonded systems. Their structures were computed and compared with the available experimental data. The results show that the M05-2X and M06-2X functionals are better than the other methods. Based on these facts, the interactions between the α-CF₃ ketoimine and EG were investigated at the M06-2X/6-31G(d,p) level. And the results indicate the presence of strong C–F⋯H–O hydrogen bond in the complexes between α-CF₃ ketoimine and solvent EG. Moreover, we also studied the reaction mechanism of the titled reaction with or without the assistance of EG at the M06-2X/6-31G(d,p) level. The results show that the two possible pathways without the aid of EG have very high free energy barriers and the reaction can't proceed, which is consistent with the experiment fact. However, with the assistance of EG, the reaction activation free energy barrier can decrease greatly. For the two-EGs model with two EG molecules, the reaction free energy barrier decreases from 47.5 kcal mol⁻¹ to 24.7 kcal mol⁻¹. The two EGs play an important role to stabilize the transition state and decrease the activation barrier greatly. The present theoretical calculations imply the role of solvent EG in the titled interesting catalyst-free Strecker reaction of α-CF₃ ketoimine. Our theoretical calculations provide a satisfactory explanation for the experiment.