Dynamic NMR Investigation of the Annular Tautomerism in Dihydropyrimidines

Article abstract of DOI:10.1021/ja00338a004

1H and 13C NMRspectra of 6-methyl-2,4-diphenyl-1,4-dihydropyrimidine (MDHP) in several organic solvents are reported.In dilute solutions of purified solvents, separate NMR signals from the two tautomeric forms, 1,4-MDHP (A) and 1,6-MDHP (B), are observed.From the relative intensities of the peaks, the equilibrium constant K / has been determined in CDCl3, dioxane, Me2SO, and HMPA.In impure solvents and/or a high concentration of MDHP there is fast tautomeric equilibrium between forms A and B, and only a single, average set of peaks is observed.The kinetics of the tautomeric process and its dependence on temperature and MDHP concentration in purified CDCl3 solvent was quantitatively studied by 1H NMR line-shape analysis.The results indicate that two mechanisms contribute to the reaction, (i) a monomolecular process characterized by the kinetic parameters k₁(300 K) = 31 +/- 4 s^-1 and ΔE^* = 4.5 +/- 0.6 kcal/mol and (ii) a bimolecular reaction with k₂(300 K) = (1.0+/- 0.2)10³ M^-1 s^-1 and ΔE^* = 3.3 +/- 0.7 kcal/mol.