Restricted rotation of the amino group and ring inversion in substituted thienopyridines. A dynamic NMR and quantum mechanical study

A series of sixand eight-membered ring-fused aminothiophenes 1-8 was investigated in terms of the height of the rotational barrier of the amino group by dynamic H NMR spectroscopy and quantum chemical methods. The restricted rotation around the N-C(sp) bond depends only on steric influences of the fused ring. The compounds with the eight-membered fused ring show a decreased barrier to rotation. This is caused by the increased steric energy of their ground states and similar energies of all calculated transition states. In these cases, the steric hindrance leads to a wider deviation of the amino group from planarity. Thus, the π-conjugation to the thiophene ring is more disturbed and the energy difference between ground and transition state is lowered. An additional dynamic process of the eight-membered rings was observed and could be classified as ring inversion which is sterically hindered by the neighboured protons of the amino group.