HINDERED ROTATION ABOUT THE NITROGEN-ARYL CARBON BOND IN 2-ACETAMIDOTHIOPHENE DERIVATIVES

Theoretical calculations based on the MNDO method have been carried ou t for four 2-acetamidothiophenes bearing different substitutents R-5 a nd a bulky ethoxycarbonyle group in the 3 position. The results confir m that the non-equivalence of the N-methylene protons is due to a hind ered rotation about the nitrogen-aryl carbon bond, in the same way as in ortho-disubstituted anilides. Satisfactory correlations are obtaine d between the net electric charge of the sulphur atom, the geometric p arameters of the molecules studied and the activation parameters for t he site exchange studied by NMR as a function of the nature (Hammett s igma(P) value) of the substituent R-5.