Preparation of isomeric hydrazones: Role of lone electron pair of nitrogen in controlling the stability

Isomeric hydrazones (E and Z) derived from N-aminoimide of anthracene-maleic anhydride adduct and alkyl aryl ketones have been isolated and characterised by H-1, C-13 NMR and IR spectral data. The dissymmetric cage moiety has demonstrated restricted rotation about N-N bond and the ketimino group (R1R2C=N-) in orthogonal geometry to the succinimidyl plane. The electronic interaction of the lone electron pair of ketimino nitrogen with the cage phenyl ring restricts inversion of nitrogen lone pair and provides stability to the stereoisomers. The phenomenon supports the lateral shift mechanism reported for the less enthalpy of activation for E-Z isomerization. Sterically crowded configuration in case of alkyl aryl hydrazones has been found to be thermodynamically more stable as compared to its isomeric product. Conformational energy analysis also suggested the Z-isomer to be the lower energy conformer.