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

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/n_x 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 π_co/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 Non Additivity Effect of the α-methylene carbon chemical shifts for the majority of the title compounds in comparison with the other hetero-substituted carbonyl compounds are further support for the larger contribution of the π*_co/[sgrave]_c-x and π*_co/n_x orbital interactions in the thioesters than the same interactions for the other carbonyl compounds. This has been attributed to a great stabilization of the π*_co orbital in the carboxythioethyl group of the thioester due to its interaction with a 3d sulfur orbital.

Key Words:

• S-ethyl α-heterosubstituted thioacetates
• conformational studies
• molecular mechanics calculations
• IR spectra
• ¹³C NMR spectra
• electronic interactions