CONFORMATIONAL STUDIES OF α-SUBSTITUTED CARBONYL COMPOUNDS BY I.R. SPECTROSCOPY. VII. ETHYL α-(ALKY LTHI0)-ESTERS

Abstract

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 Avco frequency shifts for the title compounds indicate that the cis rotamers arc less polar than the gauche ones. The gauche rotamer (G₁) for the a-(ethylthio)-propionate corresponds to the gauche rotamer (G) for the α-(ethylthio)-acetate while the more crowded gauche rotamer (G₂′) for the a-(ethylthio)-isobutyrate corresponds to the gauche rotamer (G₂) for the α-(ethylthio)-propionate. The larger Δμ_CO values for the cis (C) and gauche (G₂′) 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 |Gp_CO to 3d_(s orbitals and |Gp_CO/σ_C-S hyperconjugative interaction are also invoked to explain the obtained results.

Key Words:

• Conformational studies
• electronic interactions
• I.R. spectroscopy
• ethyl α-(alkylthio)-esters.