NMR Studies on Imidines. V. 1H and 13C Nuclear Magnetic Resonance Study of the Tautomerism and Geometrical Isomerism of 1,3‐Bis(Arylimino)Isoindolines

Abstract

AbstractDetailed 1H and 13C NMR data are given for a series of 1,3‐bis(arylimino) isoindolines [aryl = phenyl (2), p‐tolyl (3), p‐methoxyphenyl (4) and 2‐pyridyl (5)] and for two N‐methyl derivatives: 1,3‐bis(phenylimino)‐2‐methylisoindoline (6) and 3‐N,N‐methyl, p‐tolyamino‐1‐(p‐tolylimino)‐1H‐isoindole (7). The diimino form is shown to be the predominant tautomer in compounds 2 – 5 by 13C chemical shift comparison with model compounds 6 and 7. Geometrical isomerism about the exocyclic C=N (C‐N) bonds is discussed. The imino model 6 adopts the EE configuration exclusively. The amino model 7 is present in a 90:10 ZE:EE ratio with respect to the p‐tolylimino substituent at C‐1. Compounds 2 – 4 show a mixture of ZZ and ZE isomers in a ratio of about 70:30. The bis‐pyridyl derivative 5 is present exclusively as the ZZ geometrical isomer due to an intramolecular hydrogen bond bridging mechanism.