NMR studies on the axial chirality of ortho-substituted push–pull phenyl butadienes

1H NMR measurements of a series of ortho-phenyl substituted 3-aryl-2-cyano-5,5-bis(alkylthio)- (3h), 3-aryl-2-cyano-5-alkylthio-5-dialkylamino- (4h, 5) and 3-aryl-2-cyano-5,5-bis(dialkylamino)penta-2,4-dienenitriles (6) with prochiral groups showed the rotation about the C-3–C-aryl bond to be hindered within the NMR timescale. The activation parameters of this atropisomerization process are discussed with respect to steric effects of substituents. The rotation barriers correlate with bond lengths and angles as determined by X-ray structure analyses.