CONFORMATIONAL STUDIES OF ALPHA-SUBSTITUTED CARBONYL-COMPOUNDS BY IR SPECTROSCOPY .7. ETHYL ALPHA-(ALKYLTHIO)-ESTERS

Carbonyl stretching frequencies and apparent molar absorptivities are reported for some α-(alkylthio)-acetates, -propionate and -isobutyrate, in the fundamental and 1st overtone regions and compared to those for the corresponding unsubstituted esters. The I.R. data for α-(alkylthio)-esters indicate the cis-gauche rotational isomerism. Both the unusual solvent effect and the (formula Omitted) frequency shifts for the title compounds indicate that the cis rotamers arc less polar than the gauche ones. The gauche rotamer (G,) for the cr-(ethylthio)-propionate corresponds to the gauche rotamer (G) for the α-(ethylthio)-acetate while the more crowded gauche rotamer (G'2) for the α-(ethylthio)-isobutyrate corresponds to the gauche rotamer (G2) for the α'-(ethylthio)-propionate. The larger (formula Omitted) values for the cis (C) and gauche (G'2) rotamers of α-(ethylthio)-isobutyrate in comparison to the cis (C) and gauche (G1) rotamers of the α-(ethylthio)-propionate are explained in terms of variation in the α-carbonyl angle, changes in mechanical coupling and hybridization. Charge transfer from (formula Omitted) to 3d(s) orbitals and (formula Omitted) hyperconjugative interaction are also invoked to explain the obtained results. © 1990, Taylor & Francis Group, LLC. All rights reserved.