CONFORMATIONAL AND ELECTRONIC INTERACTION STUDIES OF α-SUBSTITUTED CARBONYL COMPOUNDS. XII. S-ETHYL α-HETERO-SUBSTITUTED THIOACETATES

Abstract

Abstract The analysis of the v co bands in the IR spectra of the S-ethyl α-heterosubstituted thioacetates helped by Molecular Mechanics Calculations indicated the existence of the cis/gauche rotational isomerism. In the thioacetates series the gauche rotamers predominate over the cis ones. This has been ascribed to the large stabilization of the π∗co orbital leading to a stronger π∗co/[sgrave]c-x and π∗co/nx orbital interactions and to a larger stabilization of the gauche rotamers. The positive carbonyl cis shifts (Δv c) ascribed to the Repulsive Field Effect are smaller than the ones for the heterosubstituted-acetones, -acetophenones, -methyl acetates and -N,N-diethylacetamides. This trend is discussed in terms of the lower carbonyl dipole moment for the thioesters which is mainly due to the πco/3d(s) interaction in the carboxythioethyl group. The larger negative carbonyl gauche shifts (Δv g) for the heterosubstituted thioacetates in relation to the other referred carbonyl compounds along with the higher...