A H and C NMR study of the prototropic equilibria of 2-indolinone

¹³C NMR data are reported for 0.5 M solutions in CDCl₃ of 2-indolinone and six related compounds. The question of prototropic equilibria is discussed involving three possible tautomers, A, B, C. Form A is found to predominate in the dynamic equilibria established for three of the compounds studied. In the case of a fourth compound slow exchange between forms B and C results in the finding of both of these forms in the approximate ratio 1:1.8. These findings are supported by ¹H NMR measurements. Corresponding ¹H and ¹³H C NMR measurements on methanol solutions are essentially the same.