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

Isomeric hydrazones (E and Z) derived from N-aminoimide of anthracene-malei c anhydride adduct and alkyl aryl ketones have been isolated and characteri sed 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 (R1 R2C=N-) in orthogonal geometry to the succinimidyl plane. The electronic in teraction of the lone electron pair of ketimino nitrogen with the cage phen yl ring restricts inversion of nitrogen lone pair and provides stability to the stereoisomers. The phenomenon supports the lateral shift mechanism rep orted for the less enthalpy of activation for E-Z isomerization. Sterically crowded configuration in case of alkyl aryl hydrazones has been found to b e thermodynamically more stable as compared to its isomeric product. Confor mational energy analysis also suggested the Z-isomer to be the lower energy conformer.