NMR studies on intramolecular mobility of ortho-substituted push-pull phenyl butadienes†

Abstract

The ¹H and ¹³C NMR spectra of a series of 3-aryl-2-cyano-5,5-bis(methylthio)penta-2,4-dienenitriles (3) and 3-aryl-2-cyano-5-dimethylamino-5-methylthiopenta-2,4-dienenitriles (4) with different ortho-phenyl substituents were recorded. The NMR data are compared with those of the corresponding para-substituted compounds 1 and 2. Dynamic ¹H NMR measurements showed rotation processes about the C-2,C-3, C-3,C-4, C-4,C-5, and C-5,N bonds. The free energies of activation ΔG _c^‡ are discussed with respect to electronic and steric effects of the substituents. The magnitude of the rotation barriers correlates with the variation in bond lengths and angles as determined by X-ray structure analyses.