Electronic effects of heterocyclic ring systems as evaluated with the aid of 13C and 15N NMR chemical shifts and NBO analysis

The electronic effects of the 5- and 6-membered heterocyclic rings on the C=N-N unit of five different hydrazone 0 derivatives of pyridine-2-, -3- and -4-carbaldehydes, pyrrole-2-carbaldehyde, furan-2- and -3-carbaldehydes and thiophene-2- and -3-carbaldehydes have been studied with the aid of C-13 and N-15 NMR measurements together with the natural bond orbital (NBO) analysis. As model compounds are used the corresponding substituted benzaldehyde derivatives. The polarization of the C=N unit of the hydrazone functionality of the heteroaryl derivatives occurs in an analogous manner with that of phenyl derivatives. The electron-withdrawing heteroaryl groups destabilize and the electron-donating groups stabilize the positive charge development at the C=N carbon while the effect on the negative charge development is opposite. The N-15 NMR chemical shift of the C=N and C=N-N nitrogens and the NBO charges at C=N-N unit can be correlated with the replacement substituent constants sigma of the heteroaryl groups. C-13 NMR shifts of the C=N carbon of N,N-dialkylhydrazones of the heteroarenecarbaldehydes can be correlated with a dual parameter equation possessing the polar substituent constant sigma* of the heteroaryl group and the electronegativity of the heteroatom as variables. Copyright (c) 2008 John Wiley & Sons, Ltd.