Study of the π-electron distribution in push-pull alkenes by 1H and 13C NMR spectroscopy. Part 4. The conformation and dynamic behaviour of substituted N-phenyl-2,3-dihydro-4(1 H)-pyridones

Abstract

The 1H and 13C NMR spectra of a number of differently substituted N-phenyl-2,3-dihydro-4(1 H)-pyridones (integrated push-pull alkenes) were unequivocally assigned by means of a whole arsenal of 1D and 2D NMR spectroscopic methods. From the extracted NMR parameters and the results of accompanying quantum chemical calculations the preferred conformation of the compounds studied was found to be half-chair with the 1- and 2-substituents in pseudo-axial and axial conformations, respectively. By means of dynamic 13C NMR spectroscopy a dynamic process present was studied (restricted N-aryl rotation); the barriers to rotation were determined approximately at the coalescence temperature and in one case ( 1d) by complete line-shape analysis (CLSA). The dependence of the barrier to C,N rotation on the electron density polarization along the push-pull moiety (push-pull character) is discussed critically.