On the chirality of 2-hydroxy-NN-dialkythiobenzamides. Demonstration of three consecutive conformational processes

Abstract

The geminal anisochronism observed in the ¹H n.m.r. spectra of some of the title compounds has previously been ascribed to different causes. It has now been unequivocally shown that the geminal anisochronism is a consequence of the molecular chirality arising from slow rotation of the aryl ring with respect to the thioamide group. The free-energy barrier to this process has been found to lie in the range 11.2–13.4 kcal mol^–1, increasing with the size of the N-alkyl groups. A lower energy process (ΔG^‡= 8.6 kcal mol^–1) in 2-hydroxy-NN-di-isobutylthiobenzamide has been identified as the exchage of the isobutyl groups between two anti-periplanar positions. The third process with the highest barrier in each case (ΔG^‡= 13.4–15.6 kcal mol^–1) is the E–Z exchange of the alkyl groups by rotation around the C(S)^–N bond.