1H NMR spectra and structure of safranines.: Hindered rotation of the 3-dialkylamino group in 7-azo derivatives

The H-1 NMR spectra of safranine derivatives are reported. Semiempirical calculations indicate that the high shielding of the 4- and 6-proton adjacent to an amino or hydroxyl group (resonating between 5.5 and 6.0 ppm) is due to accumulation of negative charge on the 4-and 6-C atoms, augmented by the magnetic anisotropy effect of the orthogonal 5-phenyl ring. The ortho protons of the latter resonate upfield to the meta and para protons. Hindered rotation of dialkylamino groups, altogether unexpected in view of the rather low barriers in similar anilines, is observed only in the azo derivatives. AMI suggest that this is due to destabilization of the transition state when an amino group is substituted for an azo group. (C) 1997 Elsevier Science Ltd.