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

Abstract

νCO 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 σCX and πCO* 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 νCOgauche 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/σCX and π*CO/X1p orbital overlapping. The higher stabilization of the ω-ethylthioacetophenone gauche rotamer, the largest negative νCOgauche shift, and the lower shielding effect for its methylene carbon have been interpreted as due to the simultaneous occurrence of π*CO/σCS and πCO/σCS orbital interactions.