Conformational and electronic interaction studies of α-substituted carbonyl compounds. Part 9. ω-Hetero-substituted acetophenones

Abstract

ν_CO frequencies and intensities, n→π*_CO transition energies and α-methylene carbon chemical shifts were measured for some ω-hetero-substituted acetophenones (φCOCH₂X: X = F, OMe, NMe₂, NEt₂, Cl, Br, SEt, and I), and their conformations were estimated with the help of molecular-mechanics calculations. The stability of the gauche rotamers is discussed in terms of hyperconjugative interactions between σ_CX and π_CO* orbitals. The carbonyl frequency shifts of the cis rotamers are interpreted as due to the substituent field effect, while the corresponding shifts for the gauche rotamers have been ascribed to an interplay of the inductive and hyperconjugative effects. The abnormal negative ν_COgauche shifts and the n→π*_CO band bathochromic shifts of the ω-fluoro- and ω-methoxy-acetophenones, and the observed shielding effect on the α-methylene carbon atom of the studied acetophenones, are discussed in terms of π*_CO/σ_CX and π*_CO/X_1p orbital overlapping. The higher stabilization of the ω-ethylthioacetophenone gauche rotamer, the largest negative ν_COgauche shift, and the lower shielding effect for its methylene carbon have been interpreted as due to the simultaneous occurrence of π*_CO/σ_CS and π_CO/σ_CS orbital interactions.