NMR Study Of The Tautomerisms Of 2-Propionyl-5-ethyl Formic-1, 3-cyclohexanedione

The keto-enol tautomerism and the enol-enol tautomerism of 2-Propionyl-5-ethyl formic-1,3-cyclohexanedione were studied by Nuclear Magnetic Resonance(NMR). Experiments such as ¹H NMR,¹H-¹H COSY,¹³C NMR,¹³C DEPT,¹H-¹³C HSQC and ¹H-¹³C HMBC were carried out both in the chloroform(CDCl₃)solvent and the dimethylsulfoxide(DMSO)solvent.The results show that 2-Propionyl-5-ethyl formic-1,3-cyclohexanedione is only observed in the enol form.The compound can exist as four different enol forms,viz.the external and the internal tautomers.Both the experiments of variable temperature and the two dimensional(2D)exchange spectroscopy(EXSY)experiment show no exchange between the external tautomeric pairs in the chloroform solution.But in the dimethylsulfoxide solution which has larger polarity,the experiments of variable temperature show exchange between the internal tautomeric pairs.The polarity of solvents has great influence on the exchange rate.Both the deuterium isotope effects on ¹³C chemical shifts and the long range ¹H-¹³C coupling constants show fast exchange between the external tautomeric pairs. It has been found that an endo-tautomer prevails.Also,the equilibrium constant of the fast exchange was estimated.The results of this study show that NMR technology has special application in the structural analysis of the tautomerisms.